**5. Hybrid renewable generation units in grid connected mode**

The aspects concerning the introduction of small renewable generation units dispersed on the territory into the low voltage (or medium voltage grid) is analysed in the literature under the general subject of "Dispersed Generation". The main problem concerning the dispersed generation is how to integrate a growing number of dispersed renewable generation units with the grid, without causing problems to the grid stability and to the grid regulation and protection system. The main solution to this problem is to adopt the Smart Grid configuration. Several definition exist in literature of the Smart Grid concept, depending on the aim of the author and on its point of view on the system. In general one Smart Grid can be considered as small low voltage (rarely medium voltage) grids including some dispersed generation units, some loads and at least a storage system, equipped with a communication system that are able to be connected to the low voltage (or medium voltage) grid or, possibly, to work in stand alone configuration. Whether they are integrated in a Smart Grid or connected to the low voltage grid, dispersed renewable generation units can support the Power Quality of the grid supply. To achieve this purpose, the main condition is to equip the renewable generation units with storage systems. Secondarily, to improve further the benefits of dispersed generation on power quality of supply, it can be needed to provide the renewable generation units with a communication system.

In the following sections, for generality purpose, hybrid generation units equipped with a battery storage system will be analysed. In particular the effects of this dispersed renewable generation units on the different power quality requirements will be examined and some design solutions allowing to improve the performances of this systems will be presented.

In the following the main concepts of Power Quality regulation in grid connected systems are analyzed, focusing on how small hybrid generation units, connected on the LV (or MV) grid, can affect them.

Most of the solutions reported below are currently not feasible for LV and MV public networks, that aren't designed to accept active loads, such as renewable generation units. Anyway, as many studies and test sites are being put in place and it seems to be a common will to modify the LV and MV networks structure and management in order to make them able to accept an increasing number of diffused generation, the following analysis will be carried on considering that the MV and the LV network are able to accept and to actively interact with an indefinite number of small hybrid generation units.

Currently, diffused generation units connected to MV and LV networks are required to maintain a "passive behavior", in the sense that they must inject in the grid only the active power available from the sources and separate themselves from the grid whenever a deviation from the nominal values of grid voltage or frequency occurs, that is whenever a problem on the grid appears. In this conditions, diffused generation units can't improve the existing power quality level, they can only avoid to worsen it, by controlling their injected current harmonic content and, eventually, by reducing their output power variations. To increment the Power Quality level of the LV and MV grids, diffused renewable generation

Integration of Hybrid Distributed Generation Units in Power Grid 21

Looking at the voltage regulation operated nowadays on the big power plants connected to the HV network, one can imagine that the voltage regulation on the MV and LV diffused generators could be organized in, at least, two steps: primary voltage regulation and

Each generator participating to the **primary voltage regulation** reacts to any node voltage variations with a change in its output reactive power. The relation between the output power variation and the voltage variation that causes it is called *statism*, and has a different value for each generator. A solution to apply the primary voltage regulation on the diffused generators could be to introduce a regulation statism also in each renewable generation unit, in such a way that its output reactive power changes depending on the grid voltage variations (droop control for microgrids). The droop control is implemented directly in the interface converter and the statism value can be calculated by the generation unit control system or can be imposed by an eternal higher level supervisor. The maximum current value of the interface converters limits the global complex power that the generation unit can inject in the grid. Thus, the reactive power set point should be coordinated with the active power set point resulting from the frequency regulation, not to exceed the current limit of the converter interface. If the reactive power statism is settled directly by the generation unit controller, it will be its duty to coordinate the active and reactive power set points. If the statism is imposed by an external supervisor, the hybrid power plant must send to it the information regarding its actual state (actual current, current limits of the converter, sensible component temperature,…), and the external supervisor will correct consequently the statism value of the power plant, taking into account the information received by all the diffused generators of a certain area. In this case a communication system is needed to implement the primary regulation, but the hybrid generation units of an area can be coordinated together and a more precise primary voltage regulation can be achieved. The **secondary voltage regulation** is implemented by the external supervisor, after an action of the primary regulation, by sending to each power plant the variation in the reactive power production. The secondary voltage regulation has the aim to avoid the circulation of

In this case a communication system is necessary between the central supervisor and the hybrid generation units of an area. The hybrid power plants send to the supervisor the information regarding their actual state and the external controller calculate consequently

The reactive power that can be injected or absorbed by a hybrid generation unit doesn't depend on the energy availability from the sources nor on the state of charge of the battery bank, as the energy available from the renewable generators and from the storage is active and not reactive energy. To supply a certain value of reactive power, the hybrid generation unit needs to absorb a minimal amount of active power sufficient to cover its global internal losses (losses in the converters, on the wirings, in the passive components, …) and the supply of the electronic control system (electronic boards, sensors, relays,… ). This minimum active power can be provided by the renewable sources or by the battery bank or even, if the interface converter allows it, can be absorbed by the grid. In this third case, the hybrid generation units can provide the reactive power required by the grid even if there is not at all availability of active power neither from the renewable sources nor from the

If the minimum active power needed for the power plant operation is guaranteed, the reactive power injected (or absorbed) by the hybrid generation unit is limited only by the

secondary voltage regulation.

reactive power fluxes between generation units.

storage system.

the reactive power injection variation of each generation unit.

units should be enabled by the standards and by appropriate changes on the grid structures, to have an active role. For example small generation units should be allowed to produce also reactive power and to support the grid in case of grid faults. To achieve this result many studies are being done on the network, to individuate possible problems correlated to the injection of active and reactive power in MV and LV nodes, and to solve them by an adjustment of the grid regulation (voltage and frequency regulations should be coordinated with the diffused generators) and protection system (the presence of diffused generators can cause the inversion of some current fluxes and may generate problems during the automatic reclosure operation of MV breakers). In the following the solutions to be adopted on MV and LV network to include diffused generators in their regulation structures will not be considered, but it will be assumed that this solutions will be implemented. The attention will be focused on the diffused generators side, trying to understand which are the principal features that must have diffused hybrid generation units in order to support and increase the Power Quality level of modified MV and LV networks. From these considerations some design constraints will follow, and they will be analyzed in the next section.

It must be noted that all the following considerations are referred only to small diffused hybrid generation units, to be connected on LV and MV network. The big renewable power plants connected to HV, usually big wind power plants, are not considered.

#### **5.1 Requirements on voltage waveform**

Usually the voltage regulation on public grids is made at the High Voltage and possibly even at the Medium Voltage level. In Italy, for example, the last step in voltage regulation is made in the primary substation (HV/MV substation) by setting the tap changers of the HV/MV transformers. Because of the voltage drops in the MV and LV lines, the voltage level on LV distribution network can vary from values close to the rated one near the secondary substations, to values close to the minimal admitted threshold at the end of long LV lines in high load conditions.

Usually, because of the predominantly inductive behaviour of the line impedances, the voltage and frequency regulations can be decoupled in such a way that grid nodes voltage levels are controlled by acting on the reactive power fluxes and the grid frequency is controlled by acting on the active power injections in the grid nodes.

Diffused renewable generation units connected to the low voltage network can cause unexpected local increments of the voltage level. The effect of the diffused generation units on the grid voltage depends on many aspects such as the generation unit size and position along the LV line and the power factor of the complex power injected in the grid. Nowadays in Italy there is no possibility to regulate the power factor of small renewable diffused generation connected to the LV grid, as it's stated that all the generation units connected to the LV grid must have an output power factor equal to 1 (only active power injection). Independently form the grid national rules, that will evolve together with the technical developments, the only way for diffused generation units to assist the voltage regulation in the LV network is to control the reactive power injected in the grid.

The hybrid power plants analyzed in this chapter can contribute to the improvement of voltage profiles on the LV and MV networks and can be actively involved in the grid voltage regulation only if they are able to regulate their reactive power injection in the grid.

units should be enabled by the standards and by appropriate changes on the grid structures, to have an active role. For example small generation units should be allowed to produce also reactive power and to support the grid in case of grid faults. To achieve this result many studies are being done on the network, to individuate possible problems correlated to the injection of active and reactive power in MV and LV nodes, and to solve them by an adjustment of the grid regulation (voltage and frequency regulations should be coordinated with the diffused generators) and protection system (the presence of diffused generators can cause the inversion of some current fluxes and may generate problems during the automatic reclosure operation of MV breakers). In the following the solutions to be adopted on MV and LV network to include diffused generators in their regulation structures will not be considered, but it will be assumed that this solutions will be implemented. The attention will be focused on the diffused generators side, trying to understand which are the principal features that must have diffused hybrid generation units in order to support and increase the Power Quality level of modified MV and LV networks. From these considerations some design constraints will follow, and they will be analyzed in the next

It must be noted that all the following considerations are referred only to small diffused hybrid generation units, to be connected on LV and MV network. The big renewable power

Usually the voltage regulation on public grids is made at the High Voltage and possibly even at the Medium Voltage level. In Italy, for example, the last step in voltage regulation is made in the primary substation (HV/MV substation) by setting the tap changers of the HV/MV transformers. Because of the voltage drops in the MV and LV lines, the voltage level on LV distribution network can vary from values close to the rated one near the secondary substations, to values close to the minimal admitted threshold at the end of long

Usually, because of the predominantly inductive behaviour of the line impedances, the voltage and frequency regulations can be decoupled in such a way that grid nodes voltage levels are controlled by acting on the reactive power fluxes and the grid frequency is

Diffused renewable generation units connected to the low voltage network can cause unexpected local increments of the voltage level. The effect of the diffused generation units on the grid voltage depends on many aspects such as the generation unit size and position along the LV line and the power factor of the complex power injected in the grid. Nowadays in Italy there is no possibility to regulate the power factor of small renewable diffused generation connected to the LV grid, as it's stated that all the generation units connected to the LV grid must have an output power factor equal to 1 (only active power injection). Independently form the grid national rules, that will evolve together with the technical developments, the only way for diffused generation units to assist the voltage regulation in

The hybrid power plants analyzed in this chapter can contribute to the improvement of voltage profiles on the LV and MV networks and can be actively involved in the grid voltage regulation only if they are able to regulate their reactive power injection in the

plants connected to HV, usually big wind power plants, are not considered.

controlled by acting on the active power injections in the grid nodes.

the LV network is to control the reactive power injected in the grid.

**5.1 Requirements on voltage waveform** 

LV lines in high load conditions.

section.

grid.

Looking at the voltage regulation operated nowadays on the big power plants connected to the HV network, one can imagine that the voltage regulation on the MV and LV diffused generators could be organized in, at least, two steps: primary voltage regulation and secondary voltage regulation.

Each generator participating to the **primary voltage regulation** reacts to any node voltage variations with a change in its output reactive power. The relation between the output power variation and the voltage variation that causes it is called *statism*, and has a different value for each generator. A solution to apply the primary voltage regulation on the diffused generators could be to introduce a regulation statism also in each renewable generation unit, in such a way that its output reactive power changes depending on the grid voltage variations (droop control for microgrids). The droop control is implemented directly in the interface converter and the statism value can be calculated by the generation unit control system or can be imposed by an eternal higher level supervisor. The maximum current value of the interface converters limits the global complex power that the generation unit can inject in the grid. Thus, the reactive power set point should be coordinated with the active power set point resulting from the frequency regulation, not to exceed the current limit of the converter interface. If the reactive power statism is settled directly by the generation unit controller, it will be its duty to coordinate the active and reactive power set points. If the statism is imposed by an external supervisor, the hybrid power plant must send to it the information regarding its actual state (actual current, current limits of the converter, sensible component temperature,…), and the external supervisor will correct consequently the statism value of the power plant, taking into account the information received by all the diffused generators of a certain area. In this case a communication system is needed to implement the primary regulation, but the hybrid generation units of an area can be coordinated together and a more precise primary voltage regulation can be achieved.

The **secondary voltage regulation** is implemented by the external supervisor, after an action of the primary regulation, by sending to each power plant the variation in the reactive power production. The secondary voltage regulation has the aim to avoid the circulation of reactive power fluxes between generation units.

In this case a communication system is necessary between the central supervisor and the hybrid generation units of an area. The hybrid power plants send to the supervisor the information regarding their actual state and the external controller calculate consequently the reactive power injection variation of each generation unit.

The reactive power that can be injected or absorbed by a hybrid generation unit doesn't depend on the energy availability from the sources nor on the state of charge of the battery bank, as the energy available from the renewable generators and from the storage is active and not reactive energy. To supply a certain value of reactive power, the hybrid generation unit needs to absorb a minimal amount of active power sufficient to cover its global internal losses (losses in the converters, on the wirings, in the passive components, …) and the supply of the electronic control system (electronic boards, sensors, relays,… ). This minimum active power can be provided by the renewable sources or by the battery bank or even, if the interface converter allows it, can be absorbed by the grid. In this third case, the hybrid generation units can provide the reactive power required by the grid even if there is not at all availability of active power neither from the renewable sources nor from the storage system.

If the minimum active power needed for the power plant operation is guaranteed, the reactive power injected (or absorbed) by the hybrid generation unit is limited only by the

Integration of Hybrid Distributed Generation Units in Power Grid 23

frequency variations (droop control for microgrids). The droop control is implemented directly in the interface converter and the statism value can be calculated by the generation unit control system or can be imposed by an eternal higher level supervisor. It could be useful to adapt the statism value to the energy availability of the system: in case of a grid frequency reduction, an hybrid generation unit with a low energy availability will react with a lower increase of the injected power than a hybrid generation unit that have the battery fully charged. If the statism is set directly by the generation units controller, it will be its duty to calculate the energy availability of the system and adapt consequently the statism and no communication system is required in this case for the primary frequency regulation. If the statism is imposed by an external supervisor, the hybrid power plant must send to it the information regarding its actual state (battery state of charge, current availability of renewable sources, weather forecast,…), and the external supervisor will correct consequently the statism value of the power plant, taking into account the information received by all the diffused generators of a certain area. In this case a communication system is needed to implement the primary regulation, but the hybrid generation units of an area can be coordinated together and a better exploitation of the renewable sources can be

The **secondary frequency regulation** is implemented by the external supervisor, who send to each power plant the variation in the active power production necessary to bring the grid frequency to the nominal value after an action of the primary regulation. In this case a communication system is necessary between the central supervisor and the hybrid generation units of an area. The hybrid power plants send to the supervisor the information regarding their actual state and the external controller calculate consequently the power

It must be noted that unlike the traditional power plants, the hybrid generation units can also absorb active power from the grid. This working condition is possible if the interface converter is bidirectional and if the control system is enabled to manage this condition. One could think to exploit this characteristic, as it allow a larger range of active power regulation. For example it could useful to set two different value for the positive statism, that regulates the positive output power variations, and the negative statism, that regulates the variations in the active power absorbed by the generation unit. In this way generation units with a low battery state of charge can be controlled to react with a low injected power increment if the grid frequency decreases, and with a high absorbed power increment of the grid frequency raises. This kind of control can improve the battery management and the overall system coordination, but need a very strong coordination of the diffused generation unit, to avoid the condition where some generation units are discharging their batteries in the grid and some other generation units are absorbing active power to recharge their storage system. This exchange of active power between the storage systems is an unwanted condition because it leads to an great waste of energy caused by the charging and

To guarantee the capability to participate to the primary and secondary frequency regulations, the generation units should keep a **primary and secondary active energy reserve**. This energy reserve can be stored in the battery bank of the hybrid generation units, by setting some threshold on the battery state of charge. The entity of the frequency regulation reserve must be fixed during the designing process together with the network

performed.

injection variation of each generation unit.

discharging efficiencies of the storage systems.

operator, depending on the size of the power plant.

sizing of its components. The current circulating in each component should not over goes the maximal design value, thus the output active and reactive power of the hybrid system are limited in order to respect this condition.

In the design of the grid connected hybrid power plants it should be taken into account the maximum output current that can be supplied, considering both the active and the reactive contribute.

What explained about the voltage regulation concerns the hybrid generation units with a **DC bus configuration**. When this generation units are connected in parallel to the grid, they have only one interface point (interface converter) that can supply reactive power to the grid.

If the hybrid generation units have an **AC bus configuration**, all the source converters and the storage converters of all the power plants are connected in parallel on the grid, that can be seen as a common extension of all the AC buses. In this case all the grid connected converters can supply reactive power to the grid, within the limits of their maximum rated current. It's a design choice to decide which converters are enabled to participate to the voltage regulation, and if the converters will receive directly the reactive power set point form the external regulator or if the external regulator will send a global reactive power set point for the entire generation units and then the generation nit internal controller will split the reference reactive power value to each converter.

#### **5.2 Requirements on voltage frequency**

Unlike the voltage amplitude, the voltage frequency is a global variable, that is the same for all the grid nodes. As said before the voltage regulation is correlated to the active power, in particular, if the total active power injection in the grid nodes is equal to the total active power absorbed by the loads, the grid frequency remains constant. If the active power injected in the grid nodes is higher than the active power absorbed, the grid frequency increases, and vice versa. Hybrid distributed generation units could contribute to the grid frequency regulation, as they are able to regulate the active power injected in the grid and they can also absorb active power and store it in the battery bank. Actually, the contribution of a single hybrid generation units is irrelevant among the total amount of active power exchanged in the grid nodes, thus a variation in the active power supplied by a single hybrid generation unit wouldn't have any effect on the grid frequency. Only if the number of diffused generators is relevant and if all the generation units are coordinated by a higher level supervisor, the diffused generation can have an impact on the grid frequency regulation. Each generation unit must be equipped with a communication system that enables it to receive from the grid operator the active power reference values, and it must guarantee a certain active power availability to follow the reference set point. Looking at the frequency regulation operated nowadays on the big power plants connected to the HV network, one can imagine that the frequency regulation on the MV and LV diffused generators could be organized in at least two steps: primary regulation and secondary regulation.

Each generator participating to the **primary frequency regulation** reacts to any frequency variations with a change in its output active power. The relation between the output power variation and the frequency variation that causes it is called *statism*, and has a different value for each generator. A solution to apply the primary frequency regulation on the diffused generators could be to introduce a regulation statism also in each renewable generation unit, in such a way that its output power changes depending on the grid

sizing of its components. The current circulating in each component should not over goes the maximal design value, thus the output active and reactive power of the hybrid system

In the design of the grid connected hybrid power plants it should be taken into account the maximum output current that can be supplied, considering both the active and the reactive

What explained about the voltage regulation concerns the hybrid generation units with a **DC bus configuration**. When this generation units are connected in parallel to the grid, they have only one interface point (interface converter) that can supply reactive power to the

If the hybrid generation units have an **AC bus configuration**, all the source converters and the storage converters of all the power plants are connected in parallel on the grid, that can be seen as a common extension of all the AC buses. In this case all the grid connected converters can supply reactive power to the grid, within the limits of their maximum rated current. It's a design choice to decide which converters are enabled to participate to the voltage regulation, and if the converters will receive directly the reactive power set point form the external regulator or if the external regulator will send a global reactive power set point for the entire generation units and then the generation nit internal controller will split

Unlike the voltage amplitude, the voltage frequency is a global variable, that is the same for all the grid nodes. As said before the voltage regulation is correlated to the active power, in particular, if the total active power injection in the grid nodes is equal to the total active power absorbed by the loads, the grid frequency remains constant. If the active power injected in the grid nodes is higher than the active power absorbed, the grid frequency increases, and vice versa. Hybrid distributed generation units could contribute to the grid frequency regulation, as they are able to regulate the active power injected in the grid and they can also absorb active power and store it in the battery bank. Actually, the contribution of a single hybrid generation units is irrelevant among the total amount of active power exchanged in the grid nodes, thus a variation in the active power supplied by a single hybrid generation unit wouldn't have any effect on the grid frequency. Only if the number of diffused generators is relevant and if all the generation units are coordinated by a higher level supervisor, the diffused generation can have an impact on the grid frequency regulation. Each generation unit must be equipped with a communication system that enables it to receive from the grid operator the active power reference values, and it must guarantee a certain active power availability to follow the reference set point. Looking at the frequency regulation operated nowadays on the big power plants connected to the HV network, one can imagine that the frequency regulation on the MV and LV diffused generators

could be organized in at least two steps: primary regulation and secondary regulation.

Each generator participating to the **primary frequency regulation** reacts to any frequency variations with a change in its output active power. The relation between the output power variation and the frequency variation that causes it is called *statism*, and has a different value for each generator. A solution to apply the primary frequency regulation on the diffused generators could be to introduce a regulation statism also in each renewable generation unit, in such a way that its output power changes depending on the grid

are limited in order to respect this condition.

the reference reactive power value to each converter.

**5.2 Requirements on voltage frequency** 

contribute.

grid.

frequency variations (droop control for microgrids). The droop control is implemented directly in the interface converter and the statism value can be calculated by the generation unit control system or can be imposed by an eternal higher level supervisor. It could be useful to adapt the statism value to the energy availability of the system: in case of a grid frequency reduction, an hybrid generation unit with a low energy availability will react with a lower increase of the injected power than a hybrid generation unit that have the battery fully charged. If the statism is set directly by the generation units controller, it will be its duty to calculate the energy availability of the system and adapt consequently the statism and no communication system is required in this case for the primary frequency regulation. If the statism is imposed by an external supervisor, the hybrid power plant must send to it the information regarding its actual state (battery state of charge, current availability of renewable sources, weather forecast,…), and the external supervisor will correct consequently the statism value of the power plant, taking into account the information received by all the diffused generators of a certain area. In this case a communication system is needed to implement the primary regulation, but the hybrid generation units of an area can be coordinated together and a better exploitation of the renewable sources can be performed.

The **secondary frequency regulation** is implemented by the external supervisor, who send to each power plant the variation in the active power production necessary to bring the grid frequency to the nominal value after an action of the primary regulation. In this case a communication system is necessary between the central supervisor and the hybrid generation units of an area. The hybrid power plants send to the supervisor the information regarding their actual state and the external controller calculate consequently the power injection variation of each generation unit.

It must be noted that unlike the traditional power plants, the hybrid generation units can also absorb active power from the grid. This working condition is possible if the interface converter is bidirectional and if the control system is enabled to manage this condition. One could think to exploit this characteristic, as it allow a larger range of active power regulation. For example it could useful to set two different value for the positive statism, that regulates the positive output power variations, and the negative statism, that regulates the variations in the active power absorbed by the generation unit. In this way generation units with a low battery state of charge can be controlled to react with a low injected power increment if the grid frequency decreases, and with a high absorbed power increment of the grid frequency raises. This kind of control can improve the battery management and the overall system coordination, but need a very strong coordination of the diffused generation unit, to avoid the condition where some generation units are discharging their batteries in the grid and some other generation units are absorbing active power to recharge their storage system. This exchange of active power between the storage systems is an unwanted condition because it leads to an great waste of energy caused by the charging and discharging efficiencies of the storage systems.

To guarantee the capability to participate to the primary and secondary frequency regulations, the generation units should keep a **primary and secondary active energy reserve**. This energy reserve can be stored in the battery bank of the hybrid generation units, by setting some threshold on the battery state of charge. The entity of the frequency regulation reserve must be fixed during the designing process together with the network operator, depending on the size of the power plant.

Integration of Hybrid Distributed Generation Units in Power Grid 25

following three main aspects of the design of renewable hybrid power plants for grid connected operation will be analyzed, trying to point which are the possible choices that

The sizing of renewable generation units is the aspect that mostly affects their capability to participate to the frequency regulation (primary and secondary regulation reserve) and the voltage regulation (high current injection in case of voltage dips), to supply grid services (peak shaving and load leveling) and to guarantee the preferred loads continuity of supply during gird black outs. In grid connected systems the sizing of hybrid diffused generation units is based on technical-economic optimization. Unlike islanded systems, in grid connected generation units it's possible to evaluate the price of the active power (and possibly also reactive power) produced, depending on the kind of services the generation unit offers. One can imagine that in the future the produced energy will be differently paid, in function of its contribution to the grid power quality: for example energy produced in peak shaving operation should be rewarded at a higher price than energy produced in low load hors. Moreover, also grid services not directly correlated to the production of active power, such as the valley filling and the reactive injection to regulate the grid voltage, should be paid, in agreement with the network operator. The initial investment cost should be compared to the revenues coming from the active energy sale and from the supplying of

As said before, the sizing of gird connected hybrid generation units must be done comparing the overall cost of the hybrid system with the revenues coming from the sale

As for the islanded systems, the overall cost of the system is made up of two contributions: the initial capital investment and the maintenance cost. The initial cost is strictly related to the size of the renewable generators and of the battery bank. The main contribution to the maintenance cost is due to the battery replacements during the lifetime of the system, that is usually much longer than the battery lifetime. Globally, the battery cost, that depends on the battery nominal capacity, has a great impact on the total cost of the system, as it affects both

Together with the network operator its should be decided the active energy price in normal conditions (normal supply to the grid) and in case of grid services (active energy supplied during for peak shaving, for load leveling, to guarantee de supply of privileged loads during grid break down, ...). Then, the hybrid system designer should decide which services

A possible strategy for the hybrid system sizing, considering the different grid services to

The grid services requiring an injection (or absorption) of active power affect the renewable generators and battery bank sizing. To the design of the hybrid system it's necessary to get an estimated curve of the active power required to the generation unit by the grid, in order to comply the grid services performances established at the beginning of the project. The estimation of this curve is the most difficult part of the project and is a necessary step, as it allows to correctly size the storage system and the renewable generators. For the peak shaving, load leveling and frequency regulation services, it can be planned, together with

active energy and of grid services to the network operator.

provide to the grid and the system sizing is calculated consequently.

the initial investment and the maintenance cost.

supply is presented in the following.

leads to a higher power quality of supply.

**6.1 Sizing** 

all grid services. **Sizing constraints** 

The value of the active energy reserve that must be guaranteed in the hybrid generation units in order to participate to the primary and secondary frequency regulation affects the size of the storage system and the size of the renewable generators. The storage system must be able to contain an energy at least equal to the sum of the two regulation reserves and the renewable generators must be able to supply enough active power for the ordinary operation and for recharge the storage system with a periodicity depending on the frequency of the frequency regulation operations.

What explained about the frequency regulation concerns the hybrid generation units with a **DC bus configuration**. When this generation units are connected in parallel to the grid, they have only one interface point (interface converter) and they can maintain the same internal power management algorithm used for the islanded operation. So, the interface converter follows the active power set points coming from the frequency regulation and the generation converters extract the maximum power available from the renewable sources (or limit this power to a set point value) and the battery converter maintains the energy balance on the DC.

If the hybrid generation units have an **AC bus configuration**, all the source converters and the storage converters of all the power plants are connected in parallel on the grid, that can be seen as a common extension of all the AC buses. In this case the renewable generator converters operate as current generators, injecting in the grid the maximum power available from the sources, or, if they can work in RPPT mode, limiting the injected active power to a certain set point given by the generation unit supervisor. The storage system converters perform the voltage and frequency regulation on the gird, as the interface converter does for generation units with DC bus configuration. The external regulator has the same functions than in the case of DC bus configuration plants. The coordination of the renewable generators and the battery bank of each power plant is demanded to the power plant general supervisor, which, for example, sends active power set points to each renewable source converters when the RPPT operation is needed. Moreover each power plant general supervisor should send to the external regulator information concerning the present availability of the renewable source and, possibly, the weather forecasts. Thus, even if in AC configuration all the converters of the power plants seems to be independently connected to the grid, the interface between each hybrid power plant and the external supervisor is made only by the power plant general controller, and not by each converter separately.

In the case of AC bus configuration the problem of active power circulation between storage systems is much more relevant than for Dc bus configuration. When all the storage systems are connected in parallel to the grid, they must exchange active power in both direction with the grid, which means that the battery can absorb power indistinctly form any other generator connected to the grid, even form the other storage system operating in discharging mode. The only way to avoid the exchange of active power between storage systems is that the external supervisor disables the charging of all the batteries when there is at least a storage system in discharging operation. This solution is, not feasible, or, at least very restrictive, thus, a rate of active power exchange between storage systems have to be accepted in parallel operation of AC bus generation units.

#### **6. Design aspects of HGDU in grid connected mode**

As highlighted previously, the Power Quality in grid connected systems can be supported and improved by an active participation of the diffused renewable generation units. On the following three main aspects of the design of renewable hybrid power plants for grid connected operation will be analyzed, trying to point which are the possible choices that leads to a higher power quality of supply.

#### **6.1 Sizing**

24 Electrical Generation and Distribution Systems and Power Quality Disturbances

The value of the active energy reserve that must be guaranteed in the hybrid generation units in order to participate to the primary and secondary frequency regulation affects the size of the storage system and the size of the renewable generators. The storage system must be able to contain an energy at least equal to the sum of the two regulation reserves and the renewable generators must be able to supply enough active power for the ordinary operation and for recharge the storage system with a periodicity depending on the

What explained about the frequency regulation concerns the hybrid generation units with a **DC bus configuration**. When this generation units are connected in parallel to the grid, they have only one interface point (interface converter) and they can maintain the same internal power management algorithm used for the islanded operation. So, the interface converter follows the active power set points coming from the frequency regulation and the generation converters extract the maximum power available from the renewable sources (or limit this power to a set point value) and the battery converter maintains the energy balance

If the hybrid generation units have an **AC bus configuration**, all the source converters and the storage converters of all the power plants are connected in parallel on the grid, that can be seen as a common extension of all the AC buses. In this case the renewable generator converters operate as current generators, injecting in the grid the maximum power available from the sources, or, if they can work in RPPT mode, limiting the injected active power to a certain set point given by the generation unit supervisor. The storage system converters perform the voltage and frequency regulation on the gird, as the interface converter does for generation units with DC bus configuration. The external regulator has the same functions than in the case of DC bus configuration plants. The coordination of the renewable generators and the battery bank of each power plant is demanded to the power plant general supervisor, which, for example, sends active power set points to each renewable source converters when the RPPT operation is needed. Moreover each power plant general supervisor should send to the external regulator information concerning the present availability of the renewable source and, possibly, the weather forecasts. Thus, even if in AC configuration all the converters of the power plants seems to be independently connected to the grid, the interface between each hybrid power plant and the external supervisor is made

only by the power plant general controller, and not by each converter separately.

accepted in parallel operation of AC bus generation units.

**6. Design aspects of HGDU in grid connected mode** 

In the case of AC bus configuration the problem of active power circulation between storage systems is much more relevant than for Dc bus configuration. When all the storage systems are connected in parallel to the grid, they must exchange active power in both direction with the grid, which means that the battery can absorb power indistinctly form any other generator connected to the grid, even form the other storage system operating in discharging mode. The only way to avoid the exchange of active power between storage systems is that the external supervisor disables the charging of all the batteries when there is at least a storage system in discharging operation. This solution is, not feasible, or, at least very restrictive, thus, a rate of active power exchange between storage systems have to be

As highlighted previously, the Power Quality in grid connected systems can be supported and improved by an active participation of the diffused renewable generation units. On the

frequency of the frequency regulation operations.

on the DC.

The sizing of renewable generation units is the aspect that mostly affects their capability to participate to the frequency regulation (primary and secondary regulation reserve) and the voltage regulation (high current injection in case of voltage dips), to supply grid services (peak shaving and load leveling) and to guarantee the preferred loads continuity of supply during gird black outs. In grid connected systems the sizing of hybrid diffused generation units is based on technical-economic optimization. Unlike islanded systems, in grid connected generation units it's possible to evaluate the price of the active power (and possibly also reactive power) produced, depending on the kind of services the generation unit offers. One can imagine that in the future the produced energy will be differently paid, in function of its contribution to the grid power quality: for example energy produced in peak shaving operation should be rewarded at a higher price than energy produced in low load hors. Moreover, also grid services not directly correlated to the production of active power, such as the valley filling and the reactive injection to regulate the grid voltage, should be paid, in agreement with the network operator. The initial investment cost should be compared to the revenues coming from the active energy sale and from the supplying of all grid services.

#### **Sizing constraints**

As said before, the sizing of gird connected hybrid generation units must be done comparing the overall cost of the hybrid system with the revenues coming from the sale active energy and of grid services to the network operator.

As for the islanded systems, the overall cost of the system is made up of two contributions: the initial capital investment and the maintenance cost. The initial cost is strictly related to the size of the renewable generators and of the battery bank. The main contribution to the maintenance cost is due to the battery replacements during the lifetime of the system, that is usually much longer than the battery lifetime. Globally, the battery cost, that depends on the battery nominal capacity, has a great impact on the total cost of the system, as it affects both the initial investment and the maintenance cost.

Together with the network operator its should be decided the active energy price in normal conditions (normal supply to the grid) and in case of grid services (active energy supplied during for peak shaving, for load leveling, to guarantee de supply of privileged loads during grid break down, ...). Then, the hybrid system designer should decide which services provide to the grid and the system sizing is calculated consequently.

A possible strategy for the hybrid system sizing, considering the different grid services to supply is presented in the following.

The grid services requiring an injection (or absorption) of active power affect the renewable generators and battery bank sizing. To the design of the hybrid system it's necessary to get an estimated curve of the active power required to the generation unit by the grid, in order to comply the grid services performances established at the beginning of the project. The estimation of this curve is the most difficult part of the project and is a necessary step, as it allows to correctly size the storage system and the renewable generators. For the peak shaving, load leveling and frequency regulation services, it can be planned, together with

Integration of Hybrid Distributed Generation Units in Power Grid 27

energy absorbed from then grid will be dissipated in the breaking resistance of the hybrid power plant and is, actually, wasted energy. If no breaking resistance is included in the hybrid generation unit, the valley filling service can be provided only if the battery state of charge is low enough to enable the storage of all the active energy that is required to be

The considerations written above refers to the energy sizing of the renewable generation unit. The power (or thermal) sizing is calculated considering the maximum active power peak and the maximum reactive power peak that the hybrid generation unit will be required to supply to the grid, and their contemporary probability. The maximum active power peak is presumably due to the peak shaving operation, while the maximum reactive power peak is usually caused by the voltage regulation in case of a significant voltage amplitude

There are many different power plants configuration, that can differ by the nature of the renewable sources, by the type of the hybrid storage system and by the kind of converters used. The different hybrid power plants configuration are usually classed in function of the nature of the power bus (AC bus at frequency line, DC bus or high frequency AC bus). The main choice to be made regarding the hybrid plant topology, as for islanded systems, concern first of all the type common power sharing bus (DC or AC bus) and, secondly the

Some of the aspects considered for the islanded system regarding the DC bus or the Ac bus consideration are still valid also for grid connected systems, such as the consideration regarding the modularity and the overall costs. Some other considerations, listed below,

**Kind of loads and sources.** Grid connected generation units must supply grid frequency AC voltage (or current) both in grid connected and in transient islanded operation in case of grid break down. If the sources are AC generators (doubly fed or synchronous generators) and the storage system is made by electrochemical batteries, then a line frequency microgrid may be a better system since the grid can be directly connected to the AC bus, thus

**Control system.** For the internal control system of each hybrid power plant, the same consideration made for the islanded systems can be referred also for the grid connected configurations. Converters on a DC bus system tend to be simpler than those on a AC bus system, and the control for a DC bus system is simplified since frequency control and reactive power internal fluxes control are not a consideration. In AC bus systems both active and reactive power internal flow should be controlled, monitoring both the amplitude and the frequency of the AC bus voltage. Moreover, the phase shifting between voltage and

Some more considerations should be done regarding the external control system needed to coordinate the operations of several hybrid generation units grouped in a so called "smart grid" and connected in parallel to the grid. As explained previously, this coordination control is necessary in order to enable the diffused generation units to participate to the grid

For **DC bus configuration** systems connected in parallel with the grid, the only interface toward the grid and the other generation units is always the interface converter. The hybrid

typology of the static converters dedicated to the sources and to the battery bank.

should be modified and adapted to the grid connected hybrid power plants.

currents should be monitored in order to control the power flow direction.

voltage and frequency regulation and to provide grid services.

absorbed from the grid.

reduction (voltage dip).

**6.2 Plant configurations** 

eliminating the DC/AC interface converter.

the grid operator, the expected curve of active power required to the generation unit. This curve can be considered as the equivalent of the load demand profile for the islanded systems, as it represents the minimum active power supply that should be guaranteed by the generation unit. An index can be calculated for the grid connected systems, analogue to the LPSP for the islanded systems, in order to define the capability of the hybrid generation unit to satisfy the grid services guaranteed at the beginning of the project. In the following this index will be referred as LGSP (Loss of Grid Service Probability). As for the islanded systems, the LGSP can be calculated as:

$$\text{LGSP} = \left(\sum\_{i=1}^{N} \int\_{\Delta\_{\text{SI},i}} \mathbf{P}\_{\text{gs\\_NS,i}}(\mathbf{t}) \cdot \mathbf{dt} \right) / \to\_{\text{gs\\_tot}} \tag{11}$$

where N is the number of failed service supply to the grid in one year;

Egs\_tot is the total active energy required to fulfill the grid services guaranteed to the grid operator;

Pgs\_NS,i is the power not supplied during the failed ith service supply period

SI,i is the duration of each service supply interruption.

The design value of LGSP is chosen by the hybrid generation unit designer, on the basis of the economical compromise between the overall cost of the system, the revenue from the grid service supplying, and the penalties to be paid in case of loss of grid service supply. It should be noted that in grid connected systems, the over sizing of the hybrid generation unit is not critical, as, except for special conditions of very low load demand, the excessive energy produced by the generation units can always be sold to the grid, even if its price can decrease when the energy need of the grid is low. Consequently, a reasonable design choice could be to fix the LGSP desired value to zero. From this condition the minimal size of the renewable generators and of the storage system can be derived.

The second step is to verify the need of adding a high power storage system to supply high active power peaks to the grid, for example in case of peak shaving service.

If the renewable generation unit must guarantee the continuity of supply to some privileged loads, it should be verified if the energy capability of the hybrid system is enough to guarantee the loads supply during the maximum expected duration of grid interruptions. If the energy capability of the system is not enough, the renewable generators and in particular the storage system sizes should be increased.

It should be noted that the concept of wasted energy changes from islanded systems to grid connected systems. In grid connected systems the produced energy can almost always be sold to the grid, and so the rate of wasted energy can be considered always zero. A real waste of energy can occur when the hybrid system must provide a valley filling service. In this case, when the load is low the hybrid generation units must absorb power from the grid and store it into the battery bank, and the renewable generators must be shut down. During the valley filling period, al the energy available from the renewable sources is not converted and is wasted energy. It must be said that usually the load demand is very low during the night, that is when the photovoltaic source is not available, so the wasted energy in grid connected system is very low and the economical drawbacks of a system over sizing are less relevant than in the case of islanded systems. It must be noted that if the hybrid generation units must provide a valley filling service, the storage system must be properly managed, in order to be able to store the energy absorbed by the grid during the valley filling operation. If the battery are fully charged when the valley filling is required, the energy absorbed from then grid will be dissipated in the breaking resistance of the hybrid power plant and is, actually, wasted energy. If no breaking resistance is included in the hybrid generation unit, the valley filling service can be provided only if the battery state of charge is low enough to enable the storage of all the active energy that is required to be absorbed from the grid.

The considerations written above refers to the energy sizing of the renewable generation unit. The power (or thermal) sizing is calculated considering the maximum active power peak and the maximum reactive power peak that the hybrid generation unit will be required to supply to the grid, and their contemporary probability. The maximum active power peak is presumably due to the peak shaving operation, while the maximum reactive power peak is usually caused by the voltage regulation in case of a significant voltage amplitude reduction (voltage dip).

#### **6.2 Plant configurations**

26 Electrical Generation and Distribution Systems and Power Quality Disturbances

the grid operator, the expected curve of active power required to the generation unit. This curve can be considered as the equivalent of the load demand profile for the islanded systems, as it represents the minimum active power supply that should be guaranteed by the generation unit. An index can be calculated for the grid connected systems, analogue to the LPSP for the islanded systems, in order to define the capability of the hybrid generation unit to satisfy the grid services guaranteed at the beginning of the project. In the following this index will be referred as LGSP (Loss of Grid Service Probability). As for the islanded

( )

gs \_ NS,i gs \_ tot

(11)

SI ,i

LGSP P t dt /E <sup>Δ</sup> <sup>=</sup> = ⋅ 

Egs\_tot is the total active energy required to fulfill the grid services guaranteed to the grid

The design value of LGSP is chosen by the hybrid generation unit designer, on the basis of the economical compromise between the overall cost of the system, the revenue from the grid service supplying, and the penalties to be paid in case of loss of grid service supply. It should be noted that in grid connected systems, the over sizing of the hybrid generation unit is not critical, as, except for special conditions of very low load demand, the excessive energy produced by the generation units can always be sold to the grid, even if its price can decrease when the energy need of the grid is low. Consequently, a reasonable design choice could be to fix the LGSP desired value to zero. From this condition the minimal size of the

The second step is to verify the need of adding a high power storage system to supply high

If the renewable generation unit must guarantee the continuity of supply to some privileged loads, it should be verified if the energy capability of the hybrid system is enough to guarantee the loads supply during the maximum expected duration of grid interruptions. If the energy capability of the system is not enough, the renewable generators and in

It should be noted that the concept of wasted energy changes from islanded systems to grid connected systems. In grid connected systems the produced energy can almost always be sold to the grid, and so the rate of wasted energy can be considered always zero. A real waste of energy can occur when the hybrid system must provide a valley filling service. In this case, when the load is low the hybrid generation units must absorb power from the grid and store it into the battery bank, and the renewable generators must be shut down. During the valley filling period, al the energy available from the renewable sources is not converted and is wasted energy. It must be said that usually the load demand is very low during the night, that is when the photovoltaic source is not available, so the wasted energy in grid connected system is very low and the economical drawbacks of a system over sizing are less relevant than in the case of islanded systems. It must be noted that if the hybrid generation units must provide a valley filling service, the storage system must be properly managed, in order to be able to store the energy absorbed by the grid during the valley filling operation. If the battery are fully charged when the valley filling is required, the

N

i 1

Pgs\_NS,i is the power not supplied during the failed ith service supply period

where N is the number of failed service supply to the grid in one year;

SI,i is the duration of each service supply interruption.

renewable generators and of the storage system can be derived.

particular the storage system sizes should be increased.

active power peaks to the grid, for example in case of peak shaving service.

systems, the LGSP can be calculated as:

operator;

There are many different power plants configuration, that can differ by the nature of the renewable sources, by the type of the hybrid storage system and by the kind of converters used. The different hybrid power plants configuration are usually classed in function of the nature of the power bus (AC bus at frequency line, DC bus or high frequency AC bus).

The main choice to be made regarding the hybrid plant topology, as for islanded systems, concern first of all the type common power sharing bus (DC or AC bus) and, secondly the typology of the static converters dedicated to the sources and to the battery bank.

Some of the aspects considered for the islanded system regarding the DC bus or the Ac bus consideration are still valid also for grid connected systems, such as the consideration regarding the modularity and the overall costs. Some other considerations, listed below, should be modified and adapted to the grid connected hybrid power plants.

**Kind of loads and sources.** Grid connected generation units must supply grid frequency AC voltage (or current) both in grid connected and in transient islanded operation in case of grid break down. If the sources are AC generators (doubly fed or synchronous generators) and the storage system is made by electrochemical batteries, then a line frequency microgrid may be a better system since the grid can be directly connected to the AC bus, thus eliminating the DC/AC interface converter.

**Control system.** For the internal control system of each hybrid power plant, the same consideration made for the islanded systems can be referred also for the grid connected configurations. Converters on a DC bus system tend to be simpler than those on a AC bus system, and the control for a DC bus system is simplified since frequency control and reactive power internal fluxes control are not a consideration. In AC bus systems both active and reactive power internal flow should be controlled, monitoring both the amplitude and the frequency of the AC bus voltage. Moreover, the phase shifting between voltage and currents should be monitored in order to control the power flow direction.

Some more considerations should be done regarding the external control system needed to coordinate the operations of several hybrid generation units grouped in a so called "smart grid" and connected in parallel to the grid. As explained previously, this coordination control is necessary in order to enable the diffused generation units to participate to the grid voltage and frequency regulation and to provide grid services.

For **DC bus configuration** systems connected in parallel with the grid, the only interface toward the grid and the other generation units is always the interface converter. The hybrid

Integration of Hybrid Distributed Generation Units in Power Grid 29

valley filling service is not required. To avoid the exchange of active power between storage systems, the external smart grid controller should prevent the charging of any storage system when there is at least one storage system in discharging operation. This control technique is very binding and requires a continuous monitoring of the storage system operation in every working condition of the generation units, and not only during the valley filling operation as for DC bus systems. The only alternative several for AC bus parallel connected generation units is to accept a rate of active power exchange between storage

From what explained above, It can be stated that the DC configuration is preferable regarding the external coordination of several hybrid generation units connected in parallel, as the interface converter ensures a complete decoupling between the internal system of the generation unit and the external system (grids and other generation units connected in

**Efficiency.** As for islanded systems, the global efficiency of the grid connected generation units depends on many aspects: renewable source peak power related to the loads rated power, number and typology of converters, and power flow management. The considerations made for the islanded systems can be referred also for grid connected

One of the main cause of power losses in the system is the storage battery bank, thus, the overall efficiency of an hybrid system is affected by the rate of active energy that has to be stored in the battery bank before being delivered to the grid. The rate of the stored energy can be reduced by optimizing the internal power flow management algorithm and by the

In AC bus configurations transformers can be used to adapt generating units and storage system voltage level to the LV or MV grid voltage. In DC systems, additional conversion stages could be required to raise the output voltage of generation units and the storage system, or the system can be managed at low DC voltage, with no additional conversion stage but with an output transformer between the DC/AC interface converters and the grid. The efficiencies of the additional conversion stages and of the transformer should be

Generally, for medium size generation units connected to the MV grid, the transformer is necessary in both the configurations, to rise the power plant output voltage to the MV grid

**Reliability.** In AC bus systems the critical component is the battery converter. Usually the battery converter implements the active and reactive power regulation to provide the grid services (peak shaving, load leveling,…) and to participate to the voltage and frequency

If the battery converter breaks down, the renewable generator converters can go on feeding active power into the grid, but the active power can no longer be regulated by the storage system, as require provide grid services and to participate to the power and voltage

In DC bus systems there are two critical components: the interface converter and the battery

If the interface converters breaks the hybrid generation units is separated from the grid and

The battery converter maintains the power balance between the active power injected into the grid and the active power generated by the renewable sources If the battery converter

generations units, by adding the following comments.

choice of the grid services that the renewable generation units provides.

evaluated for different operating conditions of the generation units.

the loads and can no longer supply neither reactive nor active power.

systems.

parallel).

level.

regulations.

regulation.

converter.

generation units receives active (P) and reactive (Q) power set points from the external smart grid regulator or from the internal droop control implemented for the voltage and frequency regulations. The interface converters implements the P and Q set points and the generation unit internal energy balance is maintained by the storage converter, by controlling the DC bus voltage. The internal control of the hybrid power plant is not affected by the external system, but it goes on working with the same logic both in grid connected operation and in back up islanded operation. Moreover, when the generation unit is working in back up islanded operation it makes no difference for the energy balance regulator if the generation unit is in single islanded configuration or in parallel with other generation units forming an islanded microgrid configuration. Thanks to the interface converter and its regulator, there is a complete decoupling between the internal hybrid power plants system and the external system that is the grid, in grid connected operation, or other generation units and the privileged loads, in back up islanded operation.

In **AC bus systems**, all the generation units converters are connected in parallel to the grid. The hybrid generation units receives P and Q set points by the external smart grid regulator or by the internal droop control implemented for the voltage and frequency regulations. The renewable generator converters, in normal operating conditions, inject all the active power available into the grid. The battery converter injects the reactive power required by the Q set point and injects or absorb the active power equivalent to the difference between the P set point and the total power produced by the renewable generators. When the AC bus system passes in the back up islanded operation only the storage converter control changes its operation, while the renewable converters are not sensitive to the change in the external system configuration, as for the DC bus configuration.

The main difference between the DC bus and the AC bus configuration concerning the control system is related to the active power exchange between storage systems of different generation units connected in parallel to the grid or forming an islanded smart grid during the back up operation. If the hybrid generation units are not required to provide the valley filling service (that is to absorb active power when the load demand is low) for the DC bus systems the circulation of active power between storage systems can be avoided by using unidirectional interface converter. In such a way the active power flux is always directed toward the grid and the battery bank of each generation unit can be recharged only by the renewable generators of the power plants. If the valley filling operation is required, the interface converters must be bidirectional and the only way to avoid the exchange of active power between storage systems is that the external smart grid controller blocks the charging of any storage system when there is at least one storage system in discharging operation. Usually, all the generation units of a smart grid are coordinated to provide the same grid service, as their single active or reactive contribution would not be perceived by the grid, given the small size of the diffused generation units among the grid inertia. It can be considered that all the hybrid generation units connected to a smart grid, and referring to a single external controller, will be operating in valley filling mode at the same time, thus all the storage systems will be in charging operation at the same time and they will not exchange power. In all other working conditions different form the valley filling, the interface converter will be controlled to allow an active power flux only directed toward the grid, in order to prevent the active power exchange between the battery banks.

In the AC bus configuration all the battery converters are in parallel on the grid and they are necessarily bidirectional, to allow the charge and discharge of the batteries, even if the

generation units receives active (P) and reactive (Q) power set points from the external smart grid regulator or from the internal droop control implemented for the voltage and frequency regulations. The interface converters implements the P and Q set points and the generation unit internal energy balance is maintained by the storage converter, by controlling the DC bus voltage. The internal control of the hybrid power plant is not affected by the external system, but it goes on working with the same logic both in grid connected operation and in back up islanded operation. Moreover, when the generation unit is working in back up islanded operation it makes no difference for the energy balance regulator if the generation unit is in single islanded configuration or in parallel with other generation units forming an islanded microgrid configuration. Thanks to the interface converter and its regulator, there is a complete decoupling between the internal hybrid power plants system and the external system that is the grid, in grid connected operation, or

In **AC bus systems**, all the generation units converters are connected in parallel to the grid. The hybrid generation units receives P and Q set points by the external smart grid regulator or by the internal droop control implemented for the voltage and frequency regulations. The renewable generator converters, in normal operating conditions, inject all the active power available into the grid. The battery converter injects the reactive power required by the Q set point and injects or absorb the active power equivalent to the difference between the P set point and the total power produced by the renewable generators. When the AC bus system passes in the back up islanded operation only the storage converter control changes its operation, while the renewable converters are not sensitive to the change in the external

The main difference between the DC bus and the AC bus configuration concerning the control system is related to the active power exchange between storage systems of different generation units connected in parallel to the grid or forming an islanded smart grid during the back up operation. If the hybrid generation units are not required to provide the valley filling service (that is to absorb active power when the load demand is low) for the DC bus systems the circulation of active power between storage systems can be avoided by using unidirectional interface converter. In such a way the active power flux is always directed toward the grid and the battery bank of each generation unit can be recharged only by the renewable generators of the power plants. If the valley filling operation is required, the interface converters must be bidirectional and the only way to avoid the exchange of active power between storage systems is that the external smart grid controller blocks the charging of any storage system when there is at least one storage system in discharging operation. Usually, all the generation units of a smart grid are coordinated to provide the same grid service, as their single active or reactive contribution would not be perceived by the grid, given the small size of the diffused generation units among the grid inertia. It can be considered that all the hybrid generation units connected to a smart grid, and referring to a single external controller, will be operating in valley filling mode at the same time, thus all the storage systems will be in charging operation at the same time and they will not exchange power. In all other working conditions different form the valley filling, the interface converter will be controlled to allow an active power flux only directed toward the

other generation units and the privileged loads, in back up islanded operation.

grid, in order to prevent the active power exchange between the battery banks.

In the AC bus configuration all the battery converters are in parallel on the grid and they are necessarily bidirectional, to allow the charge and discharge of the batteries, even if the

system configuration, as for the DC bus configuration.

valley filling service is not required. To avoid the exchange of active power between storage systems, the external smart grid controller should prevent the charging of any storage system when there is at least one storage system in discharging operation. This control technique is very binding and requires a continuous monitoring of the storage system operation in every working condition of the generation units, and not only during the valley filling operation as for DC bus systems. The only alternative several for AC bus parallel connected generation units is to accept a rate of active power exchange between storage systems.

From what explained above, It can be stated that the DC configuration is preferable regarding the external coordination of several hybrid generation units connected in parallel, as the interface converter ensures a complete decoupling between the internal system of the generation unit and the external system (grids and other generation units connected in parallel).

**Efficiency.** As for islanded systems, the global efficiency of the grid connected generation units depends on many aspects: renewable source peak power related to the loads rated power, number and typology of converters, and power flow management. The considerations made for the islanded systems can be referred also for grid connected generations units, by adding the following comments.

One of the main cause of power losses in the system is the storage battery bank, thus, the overall efficiency of an hybrid system is affected by the rate of active energy that has to be stored in the battery bank before being delivered to the grid. The rate of the stored energy can be reduced by optimizing the internal power flow management algorithm and by the choice of the grid services that the renewable generation units provides.

In AC bus configurations transformers can be used to adapt generating units and storage system voltage level to the LV or MV grid voltage. In DC systems, additional conversion stages could be required to raise the output voltage of generation units and the storage system, or the system can be managed at low DC voltage, with no additional conversion stage but with an output transformer between the DC/AC interface converters and the grid. The efficiencies of the additional conversion stages and of the transformer should be evaluated for different operating conditions of the generation units.

Generally, for medium size generation units connected to the MV grid, the transformer is necessary in both the configurations, to rise the power plant output voltage to the MV grid level.

**Reliability.** In AC bus systems the critical component is the battery converter. Usually the battery converter implements the active and reactive power regulation to provide the grid services (peak shaving, load leveling,…) and to participate to the voltage and frequency regulations.

If the battery converter breaks down, the renewable generator converters can go on feeding active power into the grid, but the active power can no longer be regulated by the storage system, as require provide grid services and to participate to the power and voltage regulation.

In DC bus systems there are two critical components: the interface converter and the battery converter.

If the interface converters breaks the hybrid generation units is separated from the grid and the loads and can no longer supply neither reactive nor active power.

The battery converter maintains the power balance between the active power injected into the grid and the active power generated by the renewable sources If the battery converter

**2** 

**Optimal Location and Control of** 

*Department of Electrical Engineering, Biskra University* 

**to Enhance Power Quality** 

Belkacem Mahdad

*Algeria* 

**Multi Hybrid Model Based Wind-Shunt FACTS** 

Modern power system becomes more complex and difficult to control with the wide integration of renewable energy and flexible ac transmission systems (FACTS). In recent years many types of renewable source (Wind, solar,) and FACTS devices (SVC, STATCOM, TCSC, UPFC) integrated widely in the electricity market. Wind power industry has been developing rapidly, and high penetration of wind power into grid is taking place, (Bent, 2006), (Mahdad.b et al., 2011). According to the Global Wind Energy Council, GWEC, 15.197 MW wind turbine has been installed in 2006 (Chen et al. 2008), in terms of economic value, the wind energy sector has now become one of the important players in the energy markets, with the total value of new generating equipment installed in 2006 reaching US 23 billion. FACTS philosophy was first introduced by (Hingorani, N.G., 1990), (Hingorani, N.G., 1999) from the Electric power research institute (EPRI) in the USA, although the power electronic controlled devices had been used in the transmission network for many years before that. The objective of FACTS devices is to bring a system under control and to transmit power as ordered by the control centers, it also allows increasing the usable transmission capacity to its thermal limits. With FACTS devices we can control the phase angle, the voltage

In practical installation and integration of renewable energy in power system with consideration of FACTS devices, there are five common requirements as follows (Mahdad. b

3. How to estimate economically the number, optimal size of renewable source and

4. How to coordinate dynamically the interaction between multiple type renewable source, multi type of FACTS devices and the network to better exploit their

5. How to review and adjust the system protection devices to assure service continuity

Optimal placement and sizing of different type of renewable energy in coordination with FACTS devices is a well researched subject which in recent years interests many expert

1. What Kinds of renewable source and FACTS devices should be installed?

performance to improve the reliability of the electrical power system?

and keep the indices power quality at the margin security limits?

**1. Introduction** 

et al., 2011):

magnitude at chosen buses and/or line impedances.

FACTS to be installed in a practical network?

2. Where in the system should be placed?

breaks down, the renewable generator converters can go on feeding active power into the DC bus, but the active power balance is no longer maintained on the DC bus. The generation unit can go on supplying into the grid the active power available from the renewable sources only if the interface converter control changes its operation. If the battery converter doesn't work anymore, the interface converter must maintain the energy balance on the DC bus, by regulating the output active power in order to maintain the DC bus voltage to its nominal value. It's not very reasonable to contemplate this working condition, as the system is not providing grid services and needs to be repaired. Thus, it can be stated that the DC bus generation units can't inject any power into the grid when the battery converter breaks down, even if the interface converter is still working.

### **7. References**

