IEEE Electrification - December 2020 - 62

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On the premise of reliable location and removal of the

fault device, the microgrid should continue operating
reliably after being disconnected from the distribution
system when a fault occurs in the distribution system.
Furthermore, the microgrid should have the ability to deal
with the internal faults after being cut off from the distribution system.
Specifically, the protection system of the microgrid
mainly includes two aspects, namely, the internal protection of the microgrid and the tie line protection. The internal protection of the microgrid means that, when a fault
occurs within the microgrid, the protection system can
quickly identify, locate and cut off the fault, thereby ensuring the safe and stable operation of the microgrid.
There are two main functions of tie line protection: one
is to quickly and accurately cut the fault off when a fault
occurs on the tie line; the other is when there is a fault in
the distribution system or microgrid, according to the relevant operation principles, to determine whether the
microgrid operates in off-grid or on-grid mode. When a
fault occurs, if the microgrid needs to operate in off-grid
mode, the isolating switch at the point of common coupling operates quickly to isolate the microgrid from the
distribution system. If the microgrid is required to operate
in on-grid mode when there is a fault, the necessary protective measures need to be taken to ensure that the tie
line protection does not malfunction when there is a fault
in the distribution system.
The internal protection strategy of the Xi'an Industrial
Park microgrid is mainly divided into two types: one is the
low-voltage protection strategy, and the other is the current-differential protection strategy. The low-voltage protection strategy is mainly used when the microgrid
operates in the off-grid mode because it can deal with the
low-short-circuit capacity brought by the off-grid operation. The low-voltage protection strategy is suitable for a
microgrid that can be divided into multiple protection

Relay 1
Microgrid
Bus

Protection
Zone 1

DG 1
Load 1

...
Relay n
Protection
Zone n

DG n
Load n

Figure 10. The suitable microgrid structure for low-voltage protection.

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2020

areas, and each protection area is composed of a DG and
load that can achieve power balance.
The suitable microgrid structure for low-voltage protection is shown in Figure 10. When there is a fault in a
microgrid that operates in off-grid mode, the protection
mechanism of the DG inverter begins to run, and the fault
current can be limited to less than twice the rated output
current. According to Ohm's law, the voltage at the DG output terminal will have a large drop, so protection of the
microgrid can be achieved by monitoring the drop of the
DG output terminal voltage.
The current-differential protection strategy can be used
in both the on-grid and off-grid modes, and it is mainly
used to protect the feeders in the microgrid. Specifically,
the differential protection strategy needs to install current
transformers at both ends of each feeder to measure the
current through the two ends of the line. A differential
relay compares the measurements of both ends of the
feeders. Under normal operation and external fault conditions, only a small unbalanced current flows through the
differential relay, and the protection does not operate;
however, when there is a fault on the feeder, the differential relay measures a large unbalanced current. Then, the
circuit breaker at both ends of the feeder operates immediately, and the feeder is cut off from the microgrid.
The tie line protection strategy of the Xi'an Industrial
Park microgrid is divided into two steps. The first is to
determine whether the microgrid operates in off-grid or
on-grid mode after the fault occurs; the second is to determine the microgrid protection strategy.
Before the first step, it is necessary to judge the location of the fault through the negative-sequence power
directional component and the directional element with
memory to determine whether the fault occurs on the distribution network, on the contact line, or within the
microgrid. When a fault occurs on the tie line, the tie line
should be cut off, which causes the microgrid to operate in
off-grid mode. When a fault occurs inside a microgrid that
operates in on-grid mode, the distribution system can provide a large fault current, which is easy to identify. At this
time, the fault device is cut off quickly, and the microgrid
operates in on-grid mode continuously. Due to a large
amount of power exchange between the Xi'an Industrial
Park microgrid and distribution system and the lack of a
sensitive load in the Xi'an Industrial Park microgrid, the
microgrid continues operating in on-grid mode when the
fault occurs on the distribution system.
The second step is to set directional three-step distance protection on the end of the tie line close to the
microgrid side and set a traditional three-step directional
current protection on the end of the tie line away from the
microgrid side. In this way, the tie line is cut off when a
fault occurs on it, and the microgrid turns to operate in
off-grid mode. Meanwhile, the microgrid keeps operating
in on-grid mode when there is a fault on the microgrid or
distribution system.

IEEE Electrification - December 2020

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