**5. Islanding detection and recommended practices for micro grid protection**

An efficient protection scheme must ensure proper protection to the micro grid in its both modes of operation, i.e., grid connected mode and islanded mode. It also should ensure proper functionality during the transition from one mode to another depending on the requirement. The topological network changes due to the transition

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**5.2 Over current protection**

*Microgrid Protection Systems*

micro grid protection.

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

**5.1 Effect of micro grid integration with main grid**

Few points worth mentioning are as follows:

rated current of the interfacing inverter [17].

unbalance between the phases in a three phase system.

assessing the possible fault currents and relay settings.

is connected if the size of the DER is large enough.

makes the fault currents to be significantly different.

the protection of mesh connected micro grid with DERs.

from one mode to other demands for the changes in the settings of the protective relays. Before proceeding further, one must understand the nature of the fault currents during grid connected mode and islanded mode. There are several factors that need to be taken into consideration such as the size of the DERs, type of DER, no. of DERs, how they are integrated to the main grid and the islanding detection methodologies. Many functional differences in the operation of a synchronous DER and inverter based DER calls for alternative protection strategies for them. Initially the effect of micro grid operation on the fault currents is discussed in this section. Later, general categories of O/C protection, distance protection, differential protection along with voltage based methods applied to suit the requirements of micro grid in grid connected and islanded mode are discussed. Adaptive protection is the main ingredient of

Micro grid is integrated with the main grid with an interfacing switch. As per the IEEE standard 1547-2003, a DG should be immediately get disconnected for any type of fault occurrence in the grid [15]. If a fault occurs anywhere in the main grid or micro grid, the static switch connecting the two gets opened and thus the micro grid goes into the islanded mode of operation. Along with opening the static switch, location of the fault also should be detected simultaneously. If incase, the faults happens to be within the micro grid, a suitable protective system should be brought into operation immediately. It should be done online, i.e., detection of the fault location and the initiation of protective action within the micro grid [16]. For faults in the main grid the static switch opens and islands the micro grid so that the DGs do not contribute to the fault current. Then the control system for islanded operation comes into play. For faults in the micro grid the static switch opens to remove the fault current contribution from main grid and the protection system of the micro grid comes into play and clears the fault. Therefore, one should recognize the importance of islanding detection preceding the protective action. In time and accurate detection of islanding is essential for fulfilling adequate protection requirements in micro grid operation.

1.In inverter based DER, fault current gets affected by the limitation of 2 pu

2.If the DER is connected to a single phase load, it might result in considerable

3.Intermittent nature of the power output of a DER throws serious challenges in

4.Short circuit level of the main grid is considerably increased when micro grid

5.Impact of appreciable amount of load being met by voltage sourced converters

Over current protection that has been in use for conventional distribution system protection requires some modifications to be made so that it can be used for

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

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

for the controls and protection are outlined in the next section.

**5. Islanding detection and recommended practices for micro grid** 

An efficient protection scheme must ensure proper protection to the micro grid in its both modes of operation, i.e., grid connected mode and islanded mode. It also should ensure proper functionality during the transition from one mode to another depending on the requirement. The topological network changes due to the transition

a significant role in all aspects of distribution system viz. control, metering and protection. If the state of the micro grid is subjected to frequent changes due to intermittent nature of DGs and changes in load profile, operation strategies of different equipment need to be adjusted accordingly. Thus the system integration efficiency depends on the equipment integration. Further, the conversion of the operating mode of the micro grid from grid connected mode to islanded mode or vice versa also demands the adjustments in operation strategies of different equipment. IEC 61850 provides a flexible architecture, service and service essential for interoperability and upgrading required for various needs of modern distribution systems.

IEDs (intelligent electronic devices) are required as the devices are expected to be intelligent enough for data acquisition, transmission to control centres as well as decision making whenever necessary. These devices are being used extensively and are having the latest technology for sensing. It allows for two way communication and greater awareness on the situation in the power distribution system. These devices can be controlled remotely thus allowing efficient operation during disturbances. Another feature of the IEDs is that they can communicate with other devices present in the system allowing effective fault identification and restoration. With the application of FPGA technology, IEDs are becoming more effective [12]. As the micro grid is interconnected to the main grid, it is essential that the protective system must ensure the safety for faults in micro grid as well as for the faults in main grid. In case of a fault in main grid, micro grid should be isolated such that the consumers supplied by micro grid are not affected. If the fault is in the micro grid itself, then smallest possible percentage of consumers must be disconnected. Under these two circumstances, many challenges are there in the protective system design [13]. Some points to be considered while designing the protective system are (i) intermittent nature of the power generation by DGs due to changes in solar power, wind power, etc., (ii) variations in the load (iii) number of DGs, (iv) type of DGs such as inverter fed DG or synchronous DG, etc., and (v) topology of the network. In the grid connected mode, islanding may result accidentally or incidentally due to faults/human error/intentional opening for servicing/faulty operation of protective devices/natural disasters/and equipment failure. IEDs are employed for control and protection in modern distribution systems. Active management of the network and adaptive protection is possible through IEDs [14]. Inverter based DERs are expected not to get disconnected following a fault or contingency immediately. They should possess the ability to remain connected to the Grid for some time. It is called Fault ride through (FRT) capability. It is necessary to have sufficient fault current for the relays to sense the fault and to maintain the voltage during any contingency. Unlike a synchronously connected DER, inverter based DERs do not possess the FRT capability inherently [3]. FRT requirements in micro grids can be easily accomplished with IEDs by employing suitable controllers for inverters. To change over the protection strategies when the micro grid isolates from the main grid either intentionally or otherwise there is a need to detect quickly such isolation and secure the micro grid. The detection techniques adapted for sensing isolation and taking appropriate action

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**protection**

from one mode to other demands for the changes in the settings of the protective relays. Before proceeding further, one must understand the nature of the fault currents during grid connected mode and islanded mode. There are several factors that need to be taken into consideration such as the size of the DERs, type of DER, no. of DERs, how they are integrated to the main grid and the islanding detection methodologies. Many functional differences in the operation of a synchronous DER and inverter based DER calls for alternative protection strategies for them. Initially the effect of micro grid operation on the fault currents is discussed in this section. Later, general categories of O/C protection, distance protection, differential protection along with voltage based methods applied to suit the requirements of micro grid in grid connected and islanded mode are discussed. Adaptive protection is the main ingredient of micro grid protection.
