IEEE Power & Energy Magazine - May/June 2021 - 76

be extinguished on the main grid. It also guarantees that a
now-isolated portion of the utility feeder is not inadvertently
energized by the DERs.
The relaying scheme uses directional and nondirectional
overcurrent elements, communications, voltage elements,
and frequency elements to quickly, selectively, and securely
detect and isolate faults under both grid-connected and
islanded microgrid operation. This relaying scheme protects
the microgrid during faults within the microgrid and during
faults on the utility system.

Protection Response to Faults
Within the Microgrid
When the microgrid is operating in islanded mode, the
primary protection of the microgrid, including DERs, is
based around a combination of the directional overcurrent,
voltage, and frequency elements in the low-voltage switchgear relays.
While islanded, fault-current levels are substantially
reduced from the fault-current levels during grid-connected operation, so some of the overcurrent elements
intended to identify and isolate these faults are set more
sensitively than when in grid-connected mode. To navigate this wide range of fault currents, different protection
elements, which depend on whether the microgrid is in
grid-connected or islanded mode, are applied. The relays
are programmed to automatically change which protection
elements are used based on whether the microgrid is grid
connected or islanded. If either interconnection breaker
(CB1 or CB2) is closed, then the microgrid is grid tied. If
both interconnection breakers are open, then the microgrid
is islanded.
When faults occur on the 600-V microgrid system while
islanded and with the CHP generators online, the fault current supplied by the microgrid's sources is high enough to
selectively coordinate, resulting in the fault clearing and
isolating of only the faulted circuit segment. The CHP generators and battery energy storage system are expected to
ride through faults and remain online. The PV inverters
are expected to trip offline based on their voltage and frequency protection functions during fault conditions in the
microgrid while islanded.
When the microgrid is connected to the utility source,
the general protection principles typically enforced in small
generator interconnects are applied. The utility provides a
certain amount of available fault current, and the primary
protection of the utility system and microgrid, including
DERs, is based around overcurrent elements. When faults
occur, whether on the utility system or within the microgrid,
protective relays are programmed to respond to these fault
conditions to protect the microgrid and utility equipment
and personnel.
When faults occur while grid connected, selective coordination and fault isolation are achieved through different
operations. On the microgrid's interconnection breakers and
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DER breakers, power and fault current may flow in both
directions depending on the operating conditions. Therefore, the system detects and isolates faults using a directional
blocking scheme, which utilizes directional overcurrent and
communication-based blocking signals to minimize faultclearing time while maintaining selectivity. The CHP generators are protected using directional overcurrent, voltage,
and frequency protection elements. The load feeders and
other equipment are primarily protected using phase and
ground overcurrent protection.
For faults downstream of the main load feeder breakers while grid connected, the CHP generators and battery
energy storage system are expected to ride through the
fault and remain online. The PV inverters may trip because
of the inverters' voltage- and frequency-based anti-islanding protection.

Protection Response to Faults
on the Utility System
When grid connected, microgrid DERs can contribute to
utility system short circuit faults. The microgrid's DERs
can also unintentionally island sections of the utility system outside of the microgrid during a loss-of-source event.
In Figure 5 is an illustration of the possibility of unintentional islanding.
Loss of source refers to a utility outage condition with
no coincident short circuit fault. The microgrid protection
system is designed to interrupt DER contributions to utility
system faults not cleared by other utility system protection
devices and to avoid unintentionally islanding utility system
circuits outside of the microgrid.
When a utility fault or loss-of-source condition is detected,
the relays will trip either the interconnection breaker (CB1
or CB2) or the DER breakers depending on the microgrid's
operating conditions at the time of the event. The protection
response to utility system events and the determination of
which breaker(s) to trip is part of the microgrid's seamless,
unplanned islanding protection scheme.

Seamless, Unplanned Islanding
Seamless, unplanned islanding refers to the ability of a
microgrid to maintain power to the three customer circuits
(i.e., Thomson Park, the YMCA, and Memorial Gardens)
during an unplanned utility outage or disturbance. To achieve
this, the microgrid must transition to island mode as soon
as a utility event is detected, without interruption. Essentially, the microgrid functions similarly to an uninterruptible power source.

The Traditional Microgrid Response
to Utility System Events
As presented in Figure 6, the normal state of the CEP
microgrid is grid connected, with the CHPs supplying most
of the load. A small amount of power is consistently imported
from the utility as a buffer to avoid exporting power.
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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