IEEE Power & Energy Magazine - May/June 2021 - 80

The rate of change of frequency settings for the CEP
microgrid was selected based on power system simulations.
They were validated by performing actual microgrid system
tests in which the utility was intentionally disconnected by
opening an upstream device while the CHPs were running
grid tied.

system based on the fault-current direction. If the microgrid
is transfer ready, the relay trips the interconnection breaker
and sends blocking signals to the generator breakers (CB3
and CB4) to prevent them from tripping.
The protection schemes described previously are supplemented by loss‑of‑source protection in the relay on the
interconnection breaker (CB1 or CB2). The loss-of-source
protection scheme intends to detect conditions in which the
utility feeder is disconnected without a coincident short
circuit fault on the utility system.
To detect a loss-of-source event quickly, the microgrid
cannot simply use undervoltage protection because the
microgrid sources may continue to energize the local utility
system for a brief time after the loss of the utility connection.
In our solution, the relays detect loss-of-source conditions
primarily using the rate-of-change-of-frequency protection. When the utility is disconnected, the rate of change of
frequency works because the DERs must pick up the additional load formerly being supplied by the utility. Initially,
this power comes from the inertia of the CHP's rotational
mass, causing the CHP's rotational speed to decrease. Consequently, the microgrid's frequency also decreases.

Successful Seamless, Unplanned Transition Event

A few months after the microgrid was successfully commissioned, a utility event occurred, and the microgrid successfully transitioned seamlessly from grid tied to islanded
operation. On 15 July 2019, a fault occurred on the subtransmission circuit to which the microgrid is connected. The
microgrid DERs supplied fault current, which was detected
promptly by the directional overcurrent element in the interconnection relay. Figure 9 depicts the relay current and voltage measurements during the fault, including the measurements before the event and after the microgrid separated
from the utility system. The current shown is the output of
one CHP generator as recorded by the relay on CB3. Both
the microgrid and utility voltages were measured by the
relay on interconnection breaker CB1.

Utility
Fault
R
44 kV

Legend:

12 kV
R

CB1 (Point of Interconnection) CB2

R
Fault Current
(Trip)

CB3

Fault
Current
CHP
265 kW

Key Interlock

12 kV
600 V

600 V

Multifunction Relay
(Microprocessor Based)

52 CB52-1

12 kV

Block
Tripping

Microgrid

K
600-V Switchgear Bus
CB4

Fault
Current
CHP
265 kW

CB5

CB7 R

CB8

CB9

ac
dc

ac
dc
Battery
System
250 kW

CB6 R

Solar
8 kW

Memorial
Thomson
YMCA
Gardens
Park
Customer Loads

figure 8. A utility fault-detection example.
80

ieee power & energy magazine

may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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