**4.1 Sizing**

m

(4)

for 100% SOC(t) SOC

> >

 

 

10 Electrical Generation and Distribution Systems and Power Quality Disturbances

and number of loads that will be connected to the system. To define the working operations of the battery bank, its rated capacity, *Cnbat*, and its minimum admitted State Of Charge, *SOCm,* should be defined. The value of *SOCm* is usually indicated by the battery manufacturers, as the minimum rate of discharge of each discharging cycle that guarantees the required lifetime of the battery bank. In normal operation the battery maintains the energy balance between the sources and the loads, until its discharging rate doesn't exceed the minimum thresholds. When SOCm is reached, the loads are disconnected and the

To evaluate the design LPSP, it is possible to divide the reference period T (one year) in time steps Δt usually equivalent to one hour: If the energy demand from the loads is higher than the sum of the energy available from the sources and the energy still cumulated in the storage system, there is a of loss of supply and the LPS(t) can be calculated as indicate in (4).

When the battery SOC (t) reaches the threshold SOCm the loads are disconnected, the battery is charged with the energy coming from the renewable sources and the loads are not supplied. The global supply interruption goes on until the battery is recharged at a certain level of charge, SOCreload situated between the fully charge condition and the minimum

More refined and complex strategies can be adopted to reduce as far as possible the LPSP on privileged loads, e.g. it is possible to disconnect the NPL when the battery SOC(t) exceed an imposed threshold, SOCNPL, higher than SOCm. In such way , the remaining energy stored in the battery is used to supply only the PL if the renewable energy is not enough and the

LPS(t) P t P t t P t t SOC(t) SOC C /100 for 100% SOC(t) SOC

LPS(t) P t t P t t P t t SOC(t) SOC C /100 for SOC SOC(t) SOC

= + ⋅Δ − ⋅Δ − − ⋅ > >

= ⋅Δ + ⋅Δ − ⋅Δ − − ⋅ > >

LPS(t) P t P t t for SOC SOC(t) SOC = + ⋅Δ ( ) PL NPL () () <sup>m</sup> < < reload (5)

PL NPL gen NPL nbat NPL

NPL PL gen m nbat NPL m

( ) () () PL NPL Δ⋅+= SOCfor,ttPtP)t(LPS <sup>m</sup> << SOC)t(SOC reload (6)

renewable energy is used only to charge the storage system.

( ) () () () ( )

= + ⋅Δ − ⋅Δ − − ⋅

PL NPL gen m nbat

( ) () () () ( )

design equations (4) became the following.

( ) () () () ( )

PL NPL gen NPL nbat

if P t P t t P t t SOC(t) SOC C /100

PL NPL gen NPL nbat

( ) () () () ( )

() () ( )

PL gen m nbat

if P t t P t t SOC(t) SOC C /100

() () ( )

( ) () () () ( )

if P t P t t P t t SOC(t) SOC C /100 LPS(t) 0

() () () ( )

PL gen m nbat NPL

if P t t P t t SOC(t) SOC C /100 LPS(t) P (t)

+ ⋅Δ > ⋅Δ + − ⋅

+ ⋅Δ ≤ ⋅Δ + − ⋅ =

⋅Δ > ⋅Δ + − ⋅

⋅Δ ≤ ⋅Δ + − ⋅ =

if P

threshold, SOCm.

( ) () () () ( )

LPS(t) P t P t t P t t SOC(t) SOC C /100

PL NPL gen m nbat

t P t t P t t SOC(t) SOC C /100 LPS(t) 0

+ ⋅Δ ≤ ⋅Δ + − ⋅ =

Energy\_demand\_from\_loads Energy\_from\_sources Energy\_available\_in\_storage PL NPL gen m nbat

if P t P t t P t t SOC(t) SOC C /100

+ ⋅Δ > ⋅Δ + − ⋅

The sizing of renewable generation units is the aspect that mostly affects the continuity of supply in stand alone systems. As it has been highlighted previously, the LPSP of a stand alone system depends on the energy available from the renewable generators and on the energy stored in the battery bank. This two variables are closely related to the sizing both of the renewable generators and of the storage systems, and depends on the control strategy used to manage the power flux in the renewable power plant also.

The optimal sizing of renewable hybrid power plants is a quite complex problem, because it concerns the optimisations of several variables.

In this section, some considerations about the optimum sizing methods are presented, in order to underline the main constraints affecting the sizing of the hybrid power plant and to highlight the drawbacks of a sizing choice in the continuity of the loads supply.

To design a hybrid renewable power plant equipped with storage system, the typology and the size of the renewable generators and of the storage system should be fixed.

#### **4.1.1 Renewable generators and storage system**

The choice of the renewable generators type (photovoltaic, wind turbine, hydroelectric, geothermal,…) depends on the renewable sources availability in the location where the system will be installed. The most widespread and easily exploitable renewable source is solar radiation, followed by the wind. Many studies shows that wind and solar radiation distributions have often a complementary behaviour, thus making convenient in many cases to combine wind and solar renewable generators in the hybrid power plants. In the following, this two main renewable sources will be considered, but the analysis can be easily extended also to other renewable generators.

The choice of the storage system should take into account, first of all, that storage systems can be classified into two groups: energy storage systems (batteries, hybrid compressed air systems,…) and power storage systems (super capacitors, flywheels,…). In stand alone systems the first need is to store exceeding energy produced by the renewable sources in order to deliver it to the loads when necessary. For this reason, the more suitable storage

Integration of Hybrid Distributed Generation Units in Power Grid 13

The maximum energy stored in the battery bank is the product of the battery nominal

To guarantee a certain expected lifetime of the battery, expressed in number of charging and discharging cycles, the batteries suppliers indicated a minimum state of charging that it not possible to exceed in the battery lifetime. The minimum state of charge admitted (SOCm) depends on the desired expected lifetime of the battery, expressed in number of cycles. The battery lifetime is affected by many operating conditions, such as the ambient temperature, the amplitude of high frequency ripple in the battery voltage and current, the possibility to

The available capacity of the battery bank Cavbat is lower than its nominal capacity and

The battery bank voltage depends on the battery converter configuration and on the voltage

The number of desired charging/discharging cycles in the battery lifetime depends on economical evaluations that take into account the cost of battery replacements and the expected lifetime of the entire stand alone system. To guarantee the desired number of charging/discharging cycles at specific operating conditions of the battery it is necessary to define a correct value of SOCm. The main design variable that should be defined by the designer of the renewable power plant is the desired available capacity of the battery, which defines the total energy that can be managed (stored or supplied) by the storage system

The two main constraints affecting the sizing of the renewable generators and of the storage

The overall cost of the system is composed by 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 has a great impact on the total cost of

The LPSP is a constraint in the design of renewable power plants that have a big influence on the overall cost. As it can be seen from the LPSP expression (), the availability of loads energy supply increases if both the renewable generators and the battery bank sizes are increased. In some cases, depending on the load power demand and the sources power availability, the LPSP can be increased only using bigger renewable generators. To reach higher values of LSPS especially in case of a low overlap between the source availability and

system are the overall cost of the system and the continuity of supply to the loads.

the system, because it affects both the initial investment and the maintenance cost.

In the same way, the available energy in the storage system, Eavbat, is defined as :

*E CV nbat nbat nbat* = ⋅ [Wh] (8)

( ) 100 /100 *C C SOC avbat nbat* =⋅− *<sup>m</sup>* (9)

( ) 100 /100 *E C V E SOC avbat avbat nbat nbat* = ⋅=⋅ − *<sup>m</sup>* (10)

voltage (Vnbat) and rated capacity (Cnbat):

depends on the value of SOCm:

levels in the renewable power plant.

**4.1.3 Sizing constrains** 

without exceed the minimum state of charge SOCm.

charge the battery following the optimum V-I curves, etc.

system for stand alone renewable energy power plants are batteries. In some cases high power storage systems (usually super capacitors) can be added in parallel to batteries, in order to reduce the electrical stress when high power peak are requested by the loads.

#### **4.1.2 Sizing variables of the renewable generators and of the storage system**

In the case of a hybrid wind/PV power plant, when the renewable generators are working in their maximum power points, the expressions of the power available from the two renewable source are:

power points, the expressions of the power available from the two renewable source are:

$$\mathbf{P\_w(t)} = \mathfrak{n}\_{\mathrm{W}} \cdot \frac{1}{2} \cdot \mathfrak{p}\_{\mathrm{a}} \cdot \mathfrak{c}\_{\mathrm{p}} \cdot \mathbf{v}\_{\mathrm{w}}^{\mathrm{3}}\left(\mathbf{t}\right) \cdot \mathbf{A}\_{\mathrm{w}} \qquad\qquad\text{for the wind turbine}$$

P t JtA PV PV t PV () () =η ⋅ ⋅ for the photovoltaic generator

where:

ηW and ηPV are the total efficiencies of the renewable generators,

ρa is the air density, [kg/m3]

cp is the power coefficient of the wind turbine, depending on its shape and typology,

vw(t) is the instantaneous wind speed at hub height, [m/s]

Jt(t) is the instantaneous solar radiation on the tilted surface of the solar panels, [W/m2]

Aw is the area swept by the wind turbine and APV is the total area covered by the PV panels, [m2]

Aw is the area swept by the wind turbine and APV is the total area covered by the PV panels, [m2]

The two design parameters on which depends the power available from the renewable generators are the two areas Aw and APV.

Some more detailed calculations can be done for the wind generator, considering that the power extracted from the wind turbine is typical limited by three speed thresholds, defining the control strategy of the wind generator controller. When the wind speed is lower than the cut-in threshold (vcut-in) the wind turbine is blocked and no power is extract from the wind generator. When the wind speed is between the cut in threshold and the regulation threshold (vreg) the wind generator is kept in the working point corresponding to the maximum power. When the wind speed is between the regulation threshold and the cut-off threshold (vcut-off), the wind generator is regulated to supply a constant power, equal to its maximum rated power.

Taking into account the wind speed thresholds, the power available from the wind generator is:

$$\mathbf{P}\_{\rm w}(\mathbf{t}) = \begin{cases} \mathbf{0} & \text{for} \quad \mathbf{v}\_{\rm w}(\mathbf{t}) < \mathbf{v}\_{\rm cut-in} \\\\ \eta\_{\rm w} \cdot \frac{1}{2} \cdot \rho\_{\rm s} \cdot \mathbf{c}\_{\rm p} \cdot \mathbf{v}\_{\rm w}^{3}(\mathbf{t}) \cdot \mathbf{A}\_{\rm w} & \text{for} \quad \quad \mathbf{v}\_{\rm cut-in} < \mathbf{v}\_{\rm w}(\mathbf{t}) < \mathbf{v}\_{\rm my} \\\\ \mathbf{P}\_{\rm n,max} & \text{for} \quad \mathbf{v}\_{\rm my} < \mathbf{v}\_{\rm w}(\mathbf{t}) < \mathbf{v}\_{\rm cut-off} \\\\ 0 & \text{for} \quad \mathbf{v}\_{\rm w}(\mathbf{t}) > \mathbf{v}\_{\rm cut-off} \end{cases} \tag{7}$$

system for stand alone renewable energy power plants are batteries. In some cases high power storage systems (usually super capacitors) can be added in parallel to batteries, in order to reduce the electrical stress when high power peak are requested by the loads.

In the case of a hybrid wind/PV power plant, when the renewable generators are working in their maximum power points, the expressions of the power available from the two

power points, the expressions of the power available from the two renewable source are:

( ) ( ) <sup>3</sup> W W ap w w <sup>1</sup> P t cv tA

cp is the power coefficient of the wind turbine, depending on its shape and typology,

Jt(t) is the instantaneous solar radiation on the tilted surface of the solar panels, [W/m2] Aw is the area swept by the wind turbine and APV is the total area covered by the PV panels,

Aw is the area swept by the wind turbine and APV is the total area covered by the PV panels,

The two design parameters on which depends the power available from the renewable

Some more detailed calculations can be done for the wind generator, considering that the power extracted from the wind turbine is typical limited by three speed thresholds, defining the control strategy of the wind generator controller. When the wind speed is lower than the cut-in threshold (vcut-in) the wind turbine is blocked and no power is extract from the wind generator. When the wind speed is between the cut in threshold and the regulation threshold (vreg) the wind generator is kept in the working point corresponding to the maximum power. When the wind speed is between the regulation threshold and the cut-off threshold (vcut-off), the wind generator is regulated to supply a constant power, equal to its

Taking into account the wind speed thresholds, the power available from the wind

−

η ⋅ ⋅ρ ⋅ ⋅ ⋅ < < <sup>=</sup>

−

( )

W ap w w cut in w reg

<sup>1</sup> c v t A for v v t v

( ) ( )

−

−

(7)

( )

3

P for v v t v

< <

0 for v t v

<sup>&</sup>lt;

0 for v t v

<sup>&</sup>gt;

w cut in

( )

n ,max reg w cut off

w cut off

<sup>2</sup> =η ⋅ ⋅ρ ⋅⋅ ⋅ for the wind turbine

P t JtA PV PV t PV () () =η ⋅ ⋅ for the photovoltaic generator

**4.1.2 Sizing variables of the renewable generators and of the storage system** 

ηW and ηPV are the total efficiencies of the renewable generators,

vw(t) is the instantaneous wind speed at hub height, [m/s]

renewable source are:

ρa is the air density, [kg/m3]

maximum rated power.

generator is:

generators are the two areas Aw and APV.

( )

2 P t

W

where:

[m2]

[m2]

The maximum energy stored in the battery bank is the product of the battery nominal voltage (Vnbat) and rated capacity (Cnbat):

$$E\_{\text{ult}} = \mathbb{C}\_{\text{ult}} \cdot V\_{\text{ult}} \text{ [Wh]} \tag{8}$$

To guarantee a certain expected lifetime of the battery, expressed in number of charging and discharging cycles, the batteries suppliers indicated a minimum state of charging that it not possible to exceed in the battery lifetime. The minimum state of charge admitted (SOCm) depends on the desired expected lifetime of the battery, expressed in number of cycles. The battery lifetime is affected by many operating conditions, such as the ambient temperature, the amplitude of high frequency ripple in the battery voltage and current, the possibility to charge the battery following the optimum V-I curves, etc.

The available capacity of the battery bank Cavbat is lower than its nominal capacity and depends on the value of SOCm:

$$\mathcal{C}\_{avbd} = \mathcal{C}\_{ubat} \cdot \left(100 - SOC\_w\right) / 100 \tag{9}$$

In the same way, the available energy in the storage system, Eavbat, is defined as :

$$E\_{wbat} = \mathbf{C}\_{wbat} \cdot V\_{nbat} = E\_{nbat} \cdot \left(100 - SOC\_w\right) / 100 \tag{10}$$

The battery bank voltage depends on the battery converter configuration and on the voltage levels in the renewable power plant.

The number of desired charging/discharging cycles in the battery lifetime depends on economical evaluations that take into account the cost of battery replacements and the expected lifetime of the entire stand alone system. To guarantee the desired number of charging/discharging cycles at specific operating conditions of the battery it is necessary to define a correct value of SOCm. The main design variable that should be defined by the designer of the renewable power plant is the desired available capacity of the battery, which defines the total energy that can be managed (stored or supplied) by the storage system without exceed the minimum state of charge SOCm.

#### **4.1.3 Sizing constrains**

The two main constraints affecting the sizing of the renewable generators and of the storage system are the overall cost of the system and the continuity of supply to the loads.

The overall cost of the system is composed by 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 has a great impact on the total cost of the system, because it affects both the initial investment and the maintenance cost.

The LPSP is a constraint in the design of renewable power plants that have a big influence on the overall cost. As it can be seen from the LPSP expression (), the availability of loads energy supply increases if both the renewable generators and the battery bank sizes are increased. In some cases, depending on the load power demand and the sources power availability, the LPSP can be increased only using bigger renewable generators. To reach higher values of LSPS especially in case of a low overlap between the source availability and

Integration of Hybrid Distributed Generation Units in Power Grid 15

To select the bus configuration of the hybrid power plant several aspects should be taken

**Type of loads and sources.** If the loads operate at DC (computer servers, DC lamps, DC motors,…), the sources are only PV arrays and the storage system is made by electrochemical batteries, which produce power at DC, then the obvious choice is to use a DC bus, in order to avoid an interface DC/AC converter to feed the loads and to reduce the

Fig. 3. AC bus configuration of hybrid wind/PV generation unit with battery bank, in

directly connected, thus eliminating the DC/AC interface converter.

If the loads are industrial machines operating at line frequency, the sources are AC generators (doubly fed or synchronous generators) and the storage system is made by electrochemical batteries, then an AC line may be a better system since the loads can be

No sources and very few loads require high frequency (mainly fluorescent lighting and compressors). For this reason the high frequency AC bus won't be analysed in the following, even if it leads to reduce the size of the passive components, in comparison with grid

**Control system.** Converters on a DC bus system can be simpler than those on a linefrequency AC bus system, and the control for the DC bus is simplified since frequency control is not necessary. In DC bus configurations only active power flow has to be regulated and it is controlled simply acting on the DC voltage level of the common DC bus. Moreover, the flow direction is closely related to current direction. Hence, active power control can be based only on current flow. In AC bus systems active and reactive power flow should be controlled, monitoring both the amplitude and the frequency of the AC bus

complexity of the sources and the battery converters.

into account:

islanded mode

frequency AC bus.

the load profile, it is necessary to increase the battery capacity also in order to improve the supply continuity.

However, the renewable generators size increasing can allow to reach the desired level of LPSP with a less economical effort. A drawback of this design choice is the possibility to have some wasted energy in the system life cycle. The wasted energy is the amount of energy that could be converted from the renewable generators but can't be used to supply the loads nor to charge the batteries, because they are already completely charged. A waste of energy appears when the load demand and the energy availability form the sources are not overlapped and the storage system capacity is too small to treat the whole energy available from the sources and not directly supplied to the load. The wasted energy doesn't implies a waste of money, thus it can be a designer choice to oversize the renewable generators (or to undersize the battery bank) in order to achieve an acceptable LPSP with lower costs than if the same LPSP is achieved avoiding any waste of energy in system.

The size of the renewable generators and of the storage system is the result of a compromise between the total cost of the system and the continuity of supply to the loads, often reached through an iterative procedure.

Usually, no fixed value is set for the LPSP during the designing of the system. The LPSP target value is usually fixed by the designer taking into account the loads typology and profile and the renewable sources availability. In some cases the supply of some loads should be guaranteed in case of complete lack of the renewable sources for a certain period of time. In the telecommunication application, for example, the supply of each telecommunication station is usually guaranteed for three days with no contribution of the renewable sources. When this performances are required, the size of the minimum storage system is directly dependent on the load power and on the time period where the supply must be guaranteed. If it necessary to guarantee some days of supply only by batteries it is possible to have a very expensive storage systems, but also have a very high quality of supply. Considering that the stand-alone loads present a condition of very high quality supply, comparable to the one guaranteed form UPS in grid connected systems, the costs of renewable systems able to guarantee for a certain period the power supply with no renewable sources, are completely justified.

#### **4.2 Plant configurations**

In all hybrid power plants configurations of there is a **power bus**, that is the section of the circuit where all the sources, the interface inverter and the energy storage systems are connected in parallel. Maintaining the energy balance on the power bus of the hybrid power plants is the main condition to maintain the stability of the system.

There are many different power plant configurations, 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. These hybrid power plants configuration are usually classified 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 concerns, 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

#### **4.2.1 Common power bus**

The two typical AC bus and DC bus configurations of a wind/PV hybrid power plant equipped with a battery bank are reported in figure 3 and 4.

the load profile, it is necessary to increase the battery capacity also in order to improve the

However, the renewable generators size increasing can allow to reach the desired level of LPSP with a less economical effort. A drawback of this design choice is the possibility to have some wasted energy in the system life cycle. The wasted energy is the amount of energy that could be converted from the renewable generators but can't be used to supply the loads nor to charge the batteries, because they are already completely charged. A waste of energy appears when the load demand and the energy availability form the sources are not overlapped and the storage system capacity is too small to treat the whole energy available from the sources and not directly supplied to the load. The wasted energy doesn't implies a waste of money, thus it can be a designer choice to oversize the renewable generators (or to undersize the battery bank) in order to achieve an acceptable LPSP with lower costs than if the same LPSP is achieved avoiding any waste of energy in system. The size of the renewable generators and of the storage system is the result of a compromise between the total cost of the system and the continuity of supply to the loads, often reached

Usually, no fixed value is set for the LPSP during the designing of the system. The LPSP target value is usually fixed by the designer taking into account the loads typology and profile and the renewable sources availability. In some cases the supply of some loads should be guaranteed in case of complete lack of the renewable sources for a certain period of time. In the telecommunication application, for example, the supply of each telecommunication station is usually guaranteed for three days with no contribution of the renewable sources. When this performances are required, the size of the minimum storage system is directly dependent on the load power and on the time period where the supply must be guaranteed. If it necessary to guarantee some days of supply only by batteries it is possible to have a very expensive storage systems, but also have a very high quality of supply. Considering that the stand-alone loads present a condition of very high quality supply, comparable to the one guaranteed form UPS in grid connected systems, the costs of renewable systems able to guarantee for a certain period the power supply with no

In all hybrid power plants configurations of there is a **power bus**, that is the section of the circuit where all the sources, the interface inverter and the energy storage systems are connected in parallel. Maintaining the energy balance on the power bus of the hybrid power

There are many different power plant configurations, 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. These hybrid power plants configuration are usually classified 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 concerns, first of all, the type common power sharing bus (DC or AC bus) and, secondly, the typology of the static

The two typical AC bus and DC bus configurations of a wind/PV hybrid power plant

plants is the main condition to maintain the stability of the system.

converters dedicated to the sources and to the battery bank

equipped with a battery bank are reported in figure 3 and 4.

supply continuity.

through an iterative procedure.

renewable sources, are completely justified.

**4.2 Plant configurations** 

**4.2.1 Common power bus** 

To select the bus configuration of the hybrid power plant several aspects should be taken into account:

**Type of loads and sources.** If the loads operate at DC (computer servers, DC lamps, DC motors,…), the sources are only PV arrays and the storage system is made by electrochemical batteries, which produce power at DC, then the obvious choice is to use a DC bus, in order to avoid an interface DC/AC converter to feed the loads and to reduce the complexity of the sources and the battery converters.

Fig. 3. AC bus configuration of hybrid wind/PV generation unit with battery bank, in islanded mode

If the loads are industrial machines operating at line frequency, the sources are AC generators (doubly fed or synchronous generators) and the storage system is made by electrochemical batteries, then an AC line may be a better system since the loads can be directly connected, thus eliminating the DC/AC interface converter.

No sources and very few loads require high frequency (mainly fluorescent lighting and compressors). For this reason the high frequency AC bus won't be analysed in the following, even if it leads to reduce the size of the passive components, in comparison with grid frequency AC bus.

**Control system.** Converters on a DC bus system can be simpler than those on a linefrequency AC bus system, and the control for the DC bus is simplified since frequency control is not necessary. In DC bus configurations only active power flow has to be regulated and it is controlled simply acting on the DC voltage level of the common DC bus. Moreover, the flow direction is closely related to current direction. Hence, active power control can be based only on current flow. In AC bus systems active and reactive power flow should be controlled, monitoring both the amplitude and the frequency of the AC bus

Integration of Hybrid Distributed Generation Units in Power Grid 17

possibility to change suppliers of the system components and to connect additional

**Efficiency.** The global efficiency of the system depends on many aspects: renewable sources peak power related to the loads rated power, number and typology of converters, and power flow management. It's difficult to state which one of the two configurations is the more efficient in a general sense. Without taking into account a specific project, only some

One of the main causes of power losses is the storage battery bank, due to the self-discharge rate and its charging and discharging efficiencies. So, 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 loads. Moreover, the power flow management algorithm and the renewable source peak power related to the loads rated power have a great influence on the

In AC bus configurations transformers can be used to adapt generating units and storage system voltage level to the standard AC low voltage required to the load. 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 loads. Both transformers efficiency and conversion stages efficiency depends on the voltage level of the system (higher voltages mean lower currents and consequently lower losses on conductors and lower switching losses) and they change according to the working condition of the system: usually transformers have the higher efficiency at rated power, while converters have a higher efficiency at low power levels. Even if AC bus configuration has one converter less this does not implies a higher efficiency. It is necessary to analyse the structure of the converters to understand how many conversion stages are necessary to reach the loads voltage levels. In addition, it is necessary to consider that in the DC bus configuration, the DC/DC battery converter can be eliminated, if the voltage level of the battery bank is adequate to the DC bus voltage. This solution can increment the system efficiency, but can lead to a faster ageing of the batteries. **Reliability.** In AC bus systems the critical component is the battery converter. Usually the battery converter is a VSI and it provides the voltage reference on the AC bus. The battery converter act as the grid in grid connected systems: it set the frequency and the amplitude of the AC bus voltage and it maintains the power balance between loads and sources. The power source converters are current controlled inverter, in order to inject on the AC bus the current corresponding to the maximum power available from the sources or to a reduced power level indicated by the power flow control system. If there is a fault on battery converter, the system collapses because there is no longer a voltage reference on the AC bus. Even if there is energy available from the sources, without the battery converter the load are not supplied. If there is a fault on one of the source converters the system can continue to

generation units if required.

preliminary consideration can be made.

overall efficiency of an hybrid generation system.

supply the load with the energy coming from the other sources.

flow control strategy.

In DC bus systems there are two critical components: the interface converter and the battery converter. Clearly, if the interface converters breaks the loads can no longer be supplied. The battery converter maintains the power balance between the loads and the sources by regulating the DC bus voltage level around its rated value. If the battery converter shutdown the system can still work but only if the power available from the sources is higher than the power required form the loads and it is necessary to adopt a different power

voltage. Secondly, the phase shifting between voltage and currents should be monitored in order to control the power flow direction.

Fig. 4. DC bus configuration of hybrid wind/PV generation unit with battery bank, in islanded mode

**Modularity.** In low voltage AC bus hybrid systems, AC bus voltage amplitude and frequency are the same of standard low voltage AC public grid, in order to directly connect the AC loads to the AC bus. Any existing power source unit (PV module + DC/AC converters or wind turbine + doubly fed asynchronous generator, etc.) designed to be connected to the AC grid can be added in parallel on the AC bus of a hybrid system. In this way, the generating units can be acquired from different suppliers and additional generating units can be added if the load demand grows without big complications, paying only attention to not exceed the maximum power capability of wirings, and of the common structures of the hybrid systems.

In DC bus hybrid systems, the DC bus voltage level has not a standard value, as for AC bus systems, because it depends on the voltage level of the sources and of the configuration chosen for the source converters.

For example, In order to avoid the output transformer to adapt the voltage level of the DC/AC interface converter to the AC voltage (220 V) needed to feed standard loads, the DC bus voltage should be fixed around 400 V. In this case it may be necessary to add a DC/DC conversion stage between the renewable generation units (and the battery bank) and the DC bus, to adapt the output voltage level of the sources (and of the battery bank) to the required DC bus voltage level.

Another design choice could be to insert a transformer between the DC/AC interface converter and the loads, and to adapt the DC bus voltage level to the more convenient value for the renewable generation units.

Because the DC bus voltage does not present a standard voltage value only dedicated generation units can be connected in parallel to the DC bus. This condition limits the

voltage. Secondly, the phase shifting between voltage and currents should be monitored in

Fig. 4. DC bus configuration of hybrid wind/PV generation unit with battery bank, in

**Modularity.** In low voltage AC bus hybrid systems, AC bus voltage amplitude and frequency are the same of standard low voltage AC public grid, in order to directly connect the AC loads to the AC bus. Any existing power source unit (PV module + DC/AC converters or wind turbine + doubly fed asynchronous generator, etc.) designed to be connected to the AC grid can be added in parallel on the AC bus of a hybrid system. In this way, the generating units can be acquired from different suppliers and additional generating units can be added if the load demand grows without big complications, paying only attention to not exceed the maximum power capability of wirings, and of the common

In DC bus hybrid systems, the DC bus voltage level has not a standard value, as for AC bus systems, because it depends on the voltage level of the sources and of the configuration

For example, In order to avoid the output transformer to adapt the voltage level of the DC/AC interface converter to the AC voltage (220 V) needed to feed standard loads, the DC bus voltage should be fixed around 400 V. In this case it may be necessary to add a DC/DC conversion stage between the renewable generation units (and the battery bank) and the DC bus, to adapt the output voltage level of the sources (and of the battery bank) to the required

Another design choice could be to insert a transformer between the DC/AC interface converter and the loads, and to adapt the DC bus voltage level to the more convenient value

Because the DC bus voltage does not present a standard voltage value only dedicated generation units can be connected in parallel to the DC bus. This condition limits the

order to control the power flow direction.

islanded mode

structures of the hybrid systems.

chosen for the source converters.

for the renewable generation units.

DC bus voltage level.

possibility to change suppliers of the system components and to connect additional generation units if required.

**Efficiency.** The global efficiency of the system depends on many aspects: renewable sources peak power related to the loads rated power, number and typology of converters, and power flow management. It's difficult to state which one of the two configurations is the more efficient in a general sense. Without taking into account a specific project, only some preliminary consideration can be made.

One of the main causes of power losses is the storage battery bank, due to the self-discharge rate and its charging and discharging efficiencies. So, 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 loads. Moreover, the power flow management algorithm and the renewable source peak power related to the loads rated power have a great influence on the overall efficiency of an hybrid generation system.

In AC bus configurations transformers can be used to adapt generating units and storage system voltage level to the standard AC low voltage required to the load. 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 loads. Both transformers efficiency and conversion stages efficiency depends on the voltage level of the system (higher voltages mean lower currents and consequently lower losses on conductors and lower switching losses) and they change according to the working condition of the system: usually transformers have the higher efficiency at rated power, while converters have a higher efficiency at low power levels.

Even if AC bus configuration has one converter less this does not implies a higher efficiency. It is necessary to analyse the structure of the converters to understand how many conversion stages are necessary to reach the loads voltage levels. In addition, it is necessary to consider that in the DC bus configuration, the DC/DC battery converter can be eliminated, if the voltage level of the battery bank is adequate to the DC bus voltage. This solution can increment the system efficiency, but can lead to a faster ageing of the batteries.

**Reliability.** In AC bus systems the critical component is the battery converter. Usually the battery converter is a VSI and it provides the voltage reference on the AC bus. The battery converter act as the grid in grid connected systems: it set the frequency and the amplitude of the AC bus voltage and it maintains the power balance between loads and sources. The power source converters are current controlled inverter, in order to inject on the AC bus the current corresponding to the maximum power available from the sources or to a reduced power level indicated by the power flow control system. If there is a fault on battery converter, the system collapses because there is no longer a voltage reference on the AC bus. Even if there is energy available from the sources, without the battery converter the load are not supplied. If there is a fault on one of the source converters the system can continue to supply the load with the energy coming from the other sources.

In DC bus systems there are two critical components: the interface converter and the battery converter. Clearly, if the interface converters breaks the loads can no longer be supplied. The battery converter maintains the power balance between the loads and the sources by regulating the DC bus voltage level around its rated value. If the battery converter shutdown the system can still work but only if the power available from the sources is higher than the power required form the loads and it is necessary to adopt a different power flow control strategy.

Integration of Hybrid Distributed Generation Units in Power Grid 19

adopting appropriate power flow control algorithms. Regarding these problems there is not a big difference between the DC or the AC bus configuration. More differences can be found in considering the limitation of the high frequency ripple that can be achieved in the two different configurations, also taking into account the possibility to eliminate the battery

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

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)

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

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

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

provide the renewable generation units with a communication system.

interact with an indefinite number of small hybrid generation units.

converter on DC bus configuration.

grid, can affect them.

**Overall costs.** The overall cost of the system (Lyfe Cycle Cost) can be divided in two parts: initial investment costs and life operation costs. The initial investment depends mainly on the renewable generators peak power and on the battery bank nominal capacity. Also the converters number and design can affect the initial investment costs. Solution with less converters, such as the AC configuration or the DC configuration without battery converter, have smaller initial costs. The use of simpler converters topology with a small number of controlled switching components (usually IGBT) can reduce the initial cost of the system too.

The life operation cost is mainly affected by the number of replacement of the battery bank during the system life. The average life of PV panels is supposed to be at least of 20 years, the same as for the control and conversion equipment. In 20 years of operation the wind generator could require some maintenance operations, especially on the bearings and, if present, on the breaking system (air compressor, pincers, etc). Anyway, the expected life of an hybrid power plants can be considered of 20 years.

The battery expected lifetime is usually indicated by the manufacturers in number of cycle at a certain Depth of Discharge (DOD) and in standard operating conditions. The expected lifetime of batteries increases if the DOD of the cycle is reduced, thus it can often be convenient to oversize the battery bank nominal capacity in order to discharge it always at low DOD and to increase its expected lifetime. It must also be taken into account that if the batteries are working at ambient temperature higher than 20°C their expected life time decreases and approximately it goes halves every 10°C of temperature increment. Also the quality of the electrical voltage and current supplied to the batteries can affect its expected lifetime: a high frequency ripple in the battery current, coming from the battery converter or the others converters in the system can accelerate the battery ageing. If the batteries are charged following the optimal cycles indicated by the manufacturer (V-I curve) and are not periodically treated with a full charge/discharge cycle, their expected lifetime can be shorter than the one indicated by the manufacturer at rated condition.

As the battery lifetime depends on many conditions that can't be known in advanced, it can be seen that it's quite complicated to evaluate the exact number of batteries replacement that must be taken in place during the whole life of the hybrid system. A rough calculation can be done assuming that batteries will undergo a charge/discharge cycle each day with a maximum DOD of 30%. In this case one can expect from gel lead-acid batteries (the most performing technology for sealed lead acid batteries in stationary applications) a lifetime of 4000 cycles in standard conditions, that can be shortened to 2000 cycles considering the high operating temperatures, the high frequency ripple on the battery current and the non optimal charging and discharging operations. Considering one charging/discharging cycle each day, during the entire life of the hybrid system the batteries will need to be replaced at least 3 ÷ 4 times.

As the batteries have a very high cost and are difficult to transport, a very efficient way to reduce the cycle cost of the hybrid system is to adopt solutions that protect the batteries form excessive stress (thermal stress, cycling stress, electrical stress) in order to prevent them from premature ageing. It could be found that more complex battery converters, that can have a higher initial investment cost, are able to reduce the ripple on battery current, so reducing the number of battery replacements required and, consequently, reducing the life cycle costs of the system.

The number of charging/discharging cycles on the batteries can be reduced not only through a proper sizing of the renewable generators and of the storage system, but also by

**Overall costs.** The overall cost of the system (Lyfe Cycle Cost) can be divided in two parts: initial investment costs and life operation costs. The initial investment depends mainly on the renewable generators peak power and on the battery bank nominal capacity. Also the converters number and design can affect the initial investment costs. Solution with less converters, such as the AC configuration or the DC configuration without battery converter, have smaller initial costs. The use of simpler converters topology with a small number of controlled switching components (usually IGBT) can reduce the initial cost of the system

The life operation cost is mainly affected by the number of replacement of the battery bank during the system life. The average life of PV panels is supposed to be at least of 20 years, the same as for the control and conversion equipment. In 20 years of operation the wind generator could require some maintenance operations, especially on the bearings and, if present, on the breaking system (air compressor, pincers, etc). Anyway, the expected life of

The battery expected lifetime is usually indicated by the manufacturers in number of cycle at a certain Depth of Discharge (DOD) and in standard operating conditions. The expected lifetime of batteries increases if the DOD of the cycle is reduced, thus it can often be convenient to oversize the battery bank nominal capacity in order to discharge it always at low DOD and to increase its expected lifetime. It must also be taken into account that if the batteries are working at ambient temperature higher than 20°C their expected life time decreases and approximately it goes halves every 10°C of temperature increment. Also the quality of the electrical voltage and current supplied to the batteries can affect its expected lifetime: a high frequency ripple in the battery current, coming from the battery converter or the others converters in the system can accelerate the battery ageing. If the batteries are charged following the optimal cycles indicated by the manufacturer (V-I curve) and are not periodically treated with a full charge/discharge cycle, their expected lifetime can be shorter

As the battery lifetime depends on many conditions that can't be known in advanced, it can be seen that it's quite complicated to evaluate the exact number of batteries replacement that must be taken in place during the whole life of the hybrid system. A rough calculation can be done assuming that batteries will undergo a charge/discharge cycle each day with a maximum DOD of 30%. In this case one can expect from gel lead-acid batteries (the most performing technology for sealed lead acid batteries in stationary applications) a lifetime of 4000 cycles in standard conditions, that can be shortened to 2000 cycles considering the high operating temperatures, the high frequency ripple on the battery current and the non optimal charging and discharging operations. Considering one charging/discharging cycle each day, during the entire life of the hybrid system the batteries will need to be replaced at

As the batteries have a very high cost and are difficult to transport, a very efficient way to reduce the cycle cost of the hybrid system is to adopt solutions that protect the batteries form excessive stress (thermal stress, cycling stress, electrical stress) in order to prevent them from premature ageing. It could be found that more complex battery converters, that can have a higher initial investment cost, are able to reduce the ripple on battery current, so reducing the number of battery replacements required and, consequently, reducing the life

The number of charging/discharging cycles on the batteries can be reduced not only through a proper sizing of the renewable generators and of the storage system, but also by

an hybrid power plants can be considered of 20 years.

than the one indicated by the manufacturer at rated condition.

too.

least 3 ÷ 4 times.

cycle costs of the system.

adopting appropriate power flow control algorithms. Regarding these problems there is not a big difference between the DC or the AC bus configuration. More differences can be found in considering the limitation of the high frequency ripple that can be achieved in the two different configurations, also taking into account the possibility to eliminate the battery converter on DC bus configuration.
