**2. Conventional distribution system protection**

Any protection system must be simple, fast, reliable and consistent apart from being selective and sensitive to the faults. Any protection system should not operate under normal conditions and must operate under abnormal conditions ensuring security and dependability of the protective system. These are the two important reliability indices which need to be optimized always. The two main classes are the radial distribution system and the meshed system.

For a radial feeder, fault current flows in one direction only as there is a single source of power. Relay setting in this case is relatively easy. This makes designing of strategies for protection become very straightforward for distribution systems typically. Simple devices such as reclosers, fuses and over current relays are used for protection. As a thumb rule fuses are set to operate for permanent fault and reclosers are set for temporary fault clearance. This is done as a part of fuse to recloser coordination with the intention of saving the fuse and also allow for the temporary faults to clear themselves with fast recloser action. Fuse to fuse coordination, relay to relay coordination and relay to fuse coordination are also required to be done. This is to ensure that minimum number of consumers connected to the distribution system are affected. Generally the fuse to fuse coordination is done from characteristic curves or selectivity tables supplied by manufacturer. In relay to relay coordination, time graded/current graded/combination of time and current grading is employed. Definite time, inverse time O/C relays are used. Inverse definite minimum time relays allow the protection engineer for flexible settings of the relay. Discrimination time of 0.5–0.3 s is possible with the fast acting relays and circuit breakers. In relay to fuse coordination, time margin is computed by taking into consideration, the operating time of the upper fuse for proper relay setting. It is essential that for proper coordination, fault current flowing through the protective

devices must be between the set minimum and maximum fault current that is possible and the fault current through all protective devices are almost equal. It is important to note that in case of a radial feeder, ensuring continuity of supply to maximum possible number of consumers after clearance of sustained fault is not possible. Ring main distribution system is an alternative [7].

In ring main system, each load can be supplied power from two different paths. In case of a fault in one feeder, the other feeder continues to supply whole or a percentage of total loads. Directional O/C relays are used along with non-directional O/C relays to minimize the number of consumers affected. Grading of the directional O/C relays starts from the load end to the source in the ascending order of the time, whereas for the non-directional O/C relays time discrimination is from the source side to the load side. In case the ring main is supplied by more than one source, coordination among the relays is not that easy. If two sources are present, the ring is opened at one end usually at one of the sources and the grading is done by presuming the other source as a single source. Employing differential protection for the section between the two sources is another practice. In this case, the rest of the system is treated as being fed from a single source. If more than two sources are present, then the design of protection system becomes more involved [8].

Most of the conventional protective systems are designed for the radial distribution systems where there is only one directional power flow. Design of a proper protection system that can be adapted for radial or ring main system with more than two sources has challenges posed due to bidirectional power flows that are encountered. Dependability of the usage of conventional protection scheme which is suitable only for radial system is very low and is therefore not recommended for the modern meshed distribution systems with DERs. It would be economical if the existing protection system can be modified or upgraded to match the protection needs of the modern system rather than discarding the old systems and going for an altogether new protection system. It is very expensive and is not advisable. Ways and means of using the existing protection system without losing the important aspects of protection is highly desirable [9]. Lot of research is focused in this direction to find out effective utilization of the existing infrastructural facilities of the convention distribution protection systems.

## **3. Need of renewable energy sources (RES)**

In order to meet the increased demand and to reduce the transmission/distribution losses, generation at the load points is being done using the RES. There is a pressing need to redesign the conventional protection systems incorporating the RES. Another point worth mentioning is that the conventional protection systems are designed for radial systems expecting large values of fault currents. With the introduction of the RES, there are two possible modes of operation namely grid connected mode and islanded mode. In case of grid connected operation, there is a possibility of large magnitude fault currents but may not be always true in islanded operation and it poses serious concerns related to protection. Inclusion of a DER will causes bidirectional power flows in the distribution system. It reduces the possible upper and lower limits of fault current along with reduction in the fault current through protective devices posing serious threat to the conventional protection coordination [10].

DERs and their associated control, communication and protection devices have become an integral part of modern distribution systems. In any distribution system, if the penetration of these DERs is more in any area then that geographical area is being referred as a micro grid. Micro grid is a part of the main distribution grid and it operates independently to some extent. Major element of a micro grid is a

**93**

*Microgrid Protection Systems*

*DOI: http://dx.doi.org/10.5772/intechopen.86431*

when compared to conventional system are listed below.

ensured even in islanded mode of operation.

to feed the fault through the healthy feeder.

temporary fault to appear as a permanent fault.

in islanded mode frequency and voltage control is used [11].

**4. Micro grid and DER integration**

or non-detection of the fault.

and O/C relays.

DER. DER can be a PV cell, fuel cell, wind turbine, diesel generator, energy storage system mainly based on Battery, etc. There are many advantages of DERs like reduction of transmission and distribution losses, eco-friendly power generation reducing the carbon emission, possible reduction in congestion in the networks, enhance the energy efficiency by proper utilization of the solar and wind energy. Major differences between a conventional distribution system and micro grid can be categorized into three parts namely interfacing of the inverter fed DGs, grid connected and islanded operation and bi directional power flows. A micro grid is expected to operate successfully and independently even when there is a disturbance in the main grid. The main challenges posed in the protection of micro grids

1.Sensitivity and selectivity of the protective relays gets affected due to the local generation by the DG. Settings should be done in such a way that protection is

2.Due to the presence of DGs interfaced to the grid through inverters, fault current seen by the relays is reduced during islanded operation. It affects the protective action by the relays in terms of, either delay in the protective action

3.DGs will also affect the maximum and minimum fault currents through a feeder and it results in serious coordination problems between recloser, fuse

4.Undesired tripping of the non-directional O/C relay in a healthy feeder when the DG feeds a fault outside the healthy feeder. It happens because the DG tries

5.An auto recloser clears a temporary fault by fast opening and reclosing of the circuit. If a synchronous DG is present in the micro grid, during this auto recloser action, it might experience a slight shift in synchronism. In that case, recloser will be connecting two systems which are not synchronous causing a serious threat to the entire system. Also the DG will be trying to maintain the system voltage and in turn the arc at the fault location. It might make the

Integration of DER is an important aspect of micro grid operation. There are different control strategies applied in micro grid operation. Basically these can be classified as overall network control and DER control. Supervisory control of the network is done in centralized and decentralized mode using distribution management systems (DMS). DER control is normally chosen depending on the circumstances considering the network operation scenarios and the interaction with other DERs. In grid connected mode real and reactive power control is adopted where as

In a micro grid, apart from DER, there are many other types of equipment such as data interfaces, monitoring devices, communication protocols, protective devices etc. Communication is another important element of modern distribution systems. Effective communication protocols have been established and standardized for use in substations. IEC 61850 is a global standard communication protocol which plays

#### *Microgrid Protection Systems DOI: http://dx.doi.org/10.5772/intechopen.86431*

*Micro-Grids - Applications, Operation, Control and Protection*

possible. Ring main distribution system is an alternative [7].

devices must be between the set minimum and maximum fault current that is possible and the fault current through all protective devices are almost equal. It is important to note that in case of a radial feeder, ensuring continuity of supply to maximum possible number of consumers after clearance of sustained fault is not

In case of a fault in one feeder, the other feeder continues to supply whole or a percentage of total loads. Directional O/C relays are used along with non-directional O/C relays to minimize the number of consumers affected. Grading of the directional O/C relays starts from the load end to the source in the ascending order of the time, whereas for the non-directional O/C relays time discrimination is from the source side to the load side. In case the ring main is supplied by more than one source, coordination among the relays is not that easy. If two sources are present, the ring is opened at one end usually at one of the sources and the grading is done by presuming the other source as a single source. Employing differential protection for the section between the two sources is another practice. In this case, the rest of the system is treated as being fed from a single source. If more than two sources are

present, then the design of protection system becomes more involved [8].

convention distribution protection systems.

**3. Need of renewable energy sources (RES)**

Most of the conventional protective systems are designed for the radial distribution systems where there is only one directional power flow. Design of a proper protection system that can be adapted for radial or ring main system with more than two sources has challenges posed due to bidirectional power flows that are encountered. Dependability of the usage of conventional protection scheme which is suitable only for radial system is very low and is therefore not recommended for the modern meshed distribution systems with DERs. It would be economical if the existing protection system can be modified or upgraded to match the protection needs of the modern system rather than discarding the old systems and going for an altogether new protection system. It is very expensive and is not advisable. Ways and means of using the existing protection system without losing the important aspects of protection is highly desirable [9]. Lot of research is focused in this direction to find out effective utilization of the existing infrastructural facilities of the

In order to meet the increased demand and to reduce the transmission/distribution losses, generation at the load points is being done using the RES. There is a pressing need to redesign the conventional protection systems incorporating the RES. Another point worth mentioning is that the conventional protection systems are designed for radial systems expecting large values of fault currents. With the introduction of the RES, there are two possible modes of operation namely grid connected mode and islanded mode. In case of grid connected operation, there is a possibility of large magnitude fault currents but may not be always true in islanded operation and it poses serious concerns related to protection. Inclusion of a DER will causes bidirectional power flows in the distribution system. It reduces the possible upper and lower limits of fault current along with reduction in the fault current through protective devices posing serious threat to the conventional protection coordination [10].

DERs and their associated control, communication and protection devices have become an integral part of modern distribution systems. In any distribution system, if the penetration of these DERs is more in any area then that geographical area is being referred as a micro grid. Micro grid is a part of the main distribution grid and it operates independently to some extent. Major element of a micro grid is a

In ring main system, each load can be supplied power from two different paths.

**92**

DER. DER can be a PV cell, fuel cell, wind turbine, diesel generator, energy storage system mainly based on Battery, etc. There are many advantages of DERs like reduction of transmission and distribution losses, eco-friendly power generation reducing the carbon emission, possible reduction in congestion in the networks, enhance the energy efficiency by proper utilization of the solar and wind energy.

Major differences between a conventional distribution system and micro grid can be categorized into three parts namely interfacing of the inverter fed DGs, grid connected and islanded operation and bi directional power flows. A micro grid is expected to operate successfully and independently even when there is a disturbance in the main grid. The main challenges posed in the protection of micro grids when compared to conventional system are listed below.

