IEEE Power & Energy Magazine - May/June 2021 - 45

the end points. In distribution systems, differential protection is typically implemented in the form of a fault location,
isolation, and service restoration (FLISR) system. In most
FLISR systems, the end points of the protected zones transmit only a status flag indicating whether a fault current was
detected at that location, reducing the demands on the communications system.
Indications are that this method will work well in IBRsourced microgrids, but FLISR requires a communications
network as well as breakers, relays, and associated transducers at both ends of every protected zone. For this reason, a
FLISR system that provides a high degree of accuracy in
fault location and minimizes the amount of load being interrupted can become quite expensive.
Adaptive Protection

One of the promising solutions to address the protection of
microgrids is adaptive protection. It is a real-time system
that can modify protective actions according to changes in
the system's condition. More specifically, in microgrids,
adaptive protection can change the protection scheme and
settings during mode changes from grid-connected to
islanded operation with updated short circuit current values
depending on if the grid is available to provide higher levels
of fault current.
To this end, the settings of the protection relays should
change immediately after the islanding happens (e.g.,
the pickup of the overcurrent protection relays should
be much lower than the grid-connected mode). Adaptive protection can significantly help with protecting the
microgrid during and after the transition from grid-connected to islanded mode.

Relay

Microgrid
Control Center

Comm
Hub

The conceptual design of microgrid adaptive protection is illustrated in Figure 8. The adaptive protection
relies on the microgrid communication network to 1)
monitor the latest status of the microgrid (e.g., tie-line,
IBR, and local circuit breaker statuses; the latest protection relay settings; and so on) and 2) send the modified
settings to the protection relays once a system condition
change is detected.
Adaptive protection can be added as a module to the
microgrid control center. The adaptive protection module
contains the short circuit model of the microgrid and updates
it in real time using the latest monitored information. Once
the short circuit model is updated, different approaches,
including logic, model, or learning schemes, can be utilized
to find new settings for the protection relays.

Effectively Protecting IBR-Fed Microgrids
Many conventional protection methods, such as coordinated overcurrent, directional relaying, or distance relaying, may not be reliable in microgrids energized by IBRs
or even by multiple distributed rotating machines. Differential protection can work equally well in grid-connected
and islanded modes, even in the presence of distributed load
along feeders, but the cost of upgrade for implementing differential -protection can be prohibitive. Differential protection requires breakers at both ends of every protected zone,
associated current transformers and relays, and high-speed
communications between them. Absorbing this expense is
an issue not addressed in the current financial models used
by utilities.
To alleviate this concern, a suboptimal form of differential protection can be adopted at the cost of selectivity.

Relay

DER

DER
Tie Breaker
DER

Relay

DER

Relay

figure 8. Adaptive protection in microgrids. Comm Hub: communications hub.
may/june 2021

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IEEE Power & Energy Magazine - May/June 2021

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