**1. Introduction**

Protection is a vital aspect of power system which needs lot of attention everywhere. Majority of the existing protection techniques for distribution systems are developed for radial distribution lines. These techniques will not be directly applicable to the micro grids with meshed network in the presence of distributed energy resources (DER). A CIGRE definition of micro grid is given as *Microgrids are electricity distribution systems containing loads and distributed energy resources, (such as distributed generators, storage devices, or controllable loads) that can be operated in a controlled, coordinated way either while connected to the main power network or while islanded* [1]. A typical CIGRE benchmark LV micro grid is shown in **Figure 1** [2].

The role of DERs in the present and future distribution systems is inevitable. Deployment of distributed generators (DGs) proved to be very effective means of meeting the ever increasing energy needs and concerns for Environment pollution and the depletion of fossil fuels. Employability of proper protection schemes to suit the micro grid environment fed by the renewable energy resources has assumed lot of importance. Protection of micro grids poses several challenges for the utility

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

**Figure 1.**

*CIGRE LV benchmark microgrid.*

engineers. Protection of micro grids opened the doors for various investigations by the researchers across the globe. Some important aspects related to the protection issues of micro grids are presented in this article.

General protection methods applied to the distribution network are designed for radial systems having unidirectional power flow. With DGs power flow is no longer unidirectional and it causes a serious threat when conventional protection methods are used for the micro grid with DGs. Another concern is that the micro grid is expected to operate safely in grid connected or islanded mode. The intermittent nature of the output power from a DG makes the selection of the operating characteristics of the relays to be complicated. Further, most of the DGs are connected to the grid through converters which have independent control strategies. Limited fault current of the inverter based DGs and maintenance of Fault ride through capability should be given due consideration in protection. Locating the fault and proper isolation of the fault are also important [3, 4].

Time graded and current graded over current protective schemes have been in use for the radial distribution systems. Distance and differential protection schemes are also employed. Voltage based protection and THD (total harmonic distortion) based protection are found to be suitable for protection of micro grids with DGs.

**91**

*Microgrid Protection Systems*

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

suitable communication medium [6].

**2. Conventional distribution system protection**

radial distribution system and the meshed system.

the micro grids with DERs.

Adaptability is the need of any protection method used for micro grid. The continuous change in the network configuration due to the addition of DGs and/or future expansions necessitates that the protection equipment must be adaptable as per the requirement [5]. Communication is another aspect of the protection of the micro grid. IEDs (intelligent electronic devices) are being used for this purpose in the grid. Suitable communication protocols are developed and IEC 61850 is being followed. Protection plays a key role in the development of micro grids due to the increase in the number of DGs, IEDs, storage systems and the requirement of a

This review article covers the need for changes to be made to the conventional protection systems when applied to micro grids in general and discusses recent advances made in the field of micro grid protection. Brief and critical review of the recent papers published on this subject is included. It is expected that this review article will provide a bird's eye view of the status of protection systems adapted for

This article comprises of six sections with the introduction as first section. Section 2 discusses conventional distribution system protection along with the deficiencies of the conventional O/C protection systems as applied to typical micro grids. Third section deals with brief description of the renewable energy sources (RES) and the need to replace the conventional generation systems considering environmental considerations. Configuration of micro grids with DERs is explained in Section 4. Problems of interfacing micro grids with conventional grid will be discussed in this section. Section 5 presents critical review of the recent papers dealing with the protection of micro grids. Section 6 concludes the article.

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

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

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

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

engineers. Protection of micro grids opened the doors for various investigations by the researchers across the globe. Some important aspects related to the protection

General protection methods applied to the distribution network are designed for radial systems having unidirectional power flow. With DGs power flow is no longer unidirectional and it causes a serious threat when conventional protection methods are used for the micro grid with DGs. Another concern is that the micro grid is expected to operate safely in grid connected or islanded mode. The intermittent nature of the output power from a DG makes the selection of the operating characteristics of the relays to be complicated. Further, most of the DGs are connected to the grid through converters which have independent control strategies. Limited fault current of the inverter based DGs and maintenance of Fault ride through capability should be given due consideration in protection. Locating the fault and

Time graded and current graded over current protective schemes have been in use for the radial distribution systems. Distance and differential protection schemes are also employed. Voltage based protection and THD (total harmonic distortion) based protection are found to be suitable for protection of micro grids with DGs.

issues of micro grids are presented in this article.

proper isolation of the fault are also important [3, 4].

**90**

**Figure 1.**

*CIGRE LV benchmark microgrid.*

Adaptability is the need of any protection method used for micro grid. The continuous change in the network configuration due to the addition of DGs and/or future expansions necessitates that the protection equipment must be adaptable as per the requirement [5]. Communication is another aspect of the protection of the micro grid. IEDs (intelligent electronic devices) are being used for this purpose in the grid. Suitable communication protocols are developed and IEC 61850 is being followed. Protection plays a key role in the development of micro grids due to the increase in the number of DGs, IEDs, storage systems and the requirement of a suitable communication medium [6].

This review article covers the need for changes to be made to the conventional protection systems when applied to micro grids in general and discusses recent advances made in the field of micro grid protection. Brief and critical review of the recent papers published on this subject is included. It is expected that this review article will provide a bird's eye view of the status of protection systems adapted for the micro grids with DERs.

This article comprises of six sections with the introduction as first section. Section 2 discusses conventional distribution system protection along with the deficiencies of the conventional O/C protection systems as applied to typical micro grids. Third section deals with brief description of the renewable energy sources (RES) and the need to replace the conventional generation systems considering environmental considerations. Configuration of micro grids with DERs is explained in Section 4. Problems of interfacing micro grids with conventional grid will be discussed in this section. Section 5 presents critical review of the recent papers dealing with the protection of micro grids. Section 6 concludes the article.
