IEEE Power & Energy Magazine - May/June 2021 - 79

depending on the transfer-ready signal they receive from the
microgrid controller. If the microgrid is transfer ready, the
relays trip the microgrid interconnection breaker (CB1 or
CB2) and thus initiate a seamless transfer to island mode. If
the microgrid is not transfer ready, the relays trip the DER
breakers. Then, the microgrid controller follows the traditional approach of waiting to see whether the utility comes
back before committing to black start the island.
The faults on the 44-kV utility circuit serving the CEP
substation are detected using several methods. The primary
method uses a communication-based, direct-transfer trip
protection system. When the utility protective relay detects
a fault on the 44-kV utility line supplying the microgrid,
it sends a trip signal to the protective relays in the 600-V
switchgear. The relays route the signal to the appropriate circuit breaker(s) based on the seamless, unplanned islanding
scheme as described previously.
A relay in the 44-/12-kV substation supplying the
microgrid provides backup protection for detecting faults on
the 44-kV utility line. This relay detects faults on the 44-kV

utility line feeding the CEP substation by measuring each
44-kV phase-to-ground voltage. Upon detecting a fault or
other condition requiring the generation sources to be disconnected from the utility, the relay sends a trip signal to the
relays in the microgrid's 600-V switchgear.
Additional protection for detecting 44-kV faults is provided by directional overcurrent protection elements in
the 600-V interconnection breaker relays and the relays on
CHP feeder breakers. When a fault occurs on the 12.47-kV
or 44-kV utility systems, up to 200% of the rated current
may be supplied by the inverter-based DERs and up to 600%
of the rated current may be supplied by CHP generators.
Depending on the location and nature of the fault, DERs
typically supply enough fault current to be dependably and
securely detected by the relays in the low-voltage switchgear.
Short circuit faults on the utility can be detected using a
directional blocking scheme, with the blocking signals sent
over GOOSE messages. Figure 8 shows how the directional
blocking scheme is applied. The relay on the interconnection
breaker (CB1 or CB2) detects that the fault is on the utility

Utility
R
44 kV
Legend:

12 kV

12 kV

R

CB1 (Point of Interconnection)

3) Power Imported
From Utility

R

CHP
265 kW

CB2

Key Interlock

CB4

CB5

R

Battery
System
250 kW

2) Online Generation Capacity

Microgrid

R

K

600-V Switchgear Bus
R

CB6

R

CB7 R

CB8

R

CB9

ac
dc

ac
dc
CHP
265 kW

K

12 kV
600 V

600 V

CB3

Multifunction Relay
(Microprocessor Based)

52 CB52-1

R

R

R

Solar
8 kW

Memorial
Gardens

YMCA

Thomson
Park

1) Total Load

figure 7. The transfer-readiness considerations for the CEP microgrid seamless transition.
may/june 2021	

ieee power & energy magazine 	

79



IEEE Power & Energy Magazine - May/June 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021

Contents
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