IEEE Power & Energy Magazine - May/June 2021 - 65

older genset. Distribution utilities, however, may not permit
the paralleling of generators with the utility grid and may
require mechanical interlocks to prevent paralleling.
PPRs are easily justified as the lowest cost solution for
this challenge because the protection must be upgraded to
allow the genset to parallel with the power system. PPRs
are readily programmed for this function. The elimination of
the automatic transfer switches removes a common point of
failure and improves the reliability metrics on the microgrid.

Preventing Outages Caused
By Intermittent Energy
The increasing penetration of nondispatchable (intermittent)
energy reduces inertial ride through (frequency resilience)
and increases the volatility of the power system. For example, if a microgrid is reliant on PV generation to meet load
demand, a cloud-shading event during a moment of high
load can cause an outage. To avoid this, off-grid renewable
microgrids are designed such that PV generation nominal
power is sized from six to eight times the average load to
achieve renewable targets. Similarly, battery-discharge capability is sized from three to four times the peak load.
A low-cost solution to prevent unwanted, wide-scale outages
is to balance the load to the inverter capability by automatically
shedding (tripping off) noncritical loads such as refrigeration,
heating, ventilation, and so forth. This load shedding must be
accomplished within the silicon-limit time of the inverters (Figure 2) to ensure that the facility does not black out. PPRs are
readily programmed to provide this load shedding. PPRs placed
at the PV inverter and at the battery inverter communicate the
overload event to the downstream load relays, which, in turn,
trip the load. PPR load-shedding actions are performed within
one cycle (fewer than 16.7 ms) to ensure that inverters do not
move into a thermal-limiting mode or stop commutating. PPRs
must communicate with the battery systems to understand realtime overload current capability.

Building a Resilient Microgrid
A resilient microgrid can tolerate disturbances and keep critical loads online. Resilient microgrid designs use the most
advanced, high-speed PPR and communication systems to
make certain that critical loads stay online, providing the
highest resilience possible.
Figure 8 provides an example of a resilient power system.
This architecture is used by oil- and gas-producing facilities,
research facilities, and other mission-critical loads. In Figure 8,
there are two points of common coupling (PCCs) sized to ensure
that each can carry full facility loading. Prime power turbinedrive gensets operate on a full-time basis, balancing the PCC
tie flow to near zero, guaranteeing a fully redundant source of
power for the island if the utility is lost. PVs and batteries are
used to reduce fuel costs and offset the utility demand charges
associated with the daily load variations of the facility.
PPRs are placed at each genset, PV, battery, PCC, feeder,
bus coupler, and load. PPRs provide automatic power system
may/june 2021

reconfiguration if any grid section is faulted and isolated.
Differential protection relays are used for all cables, buses,
transformers because differential elements are insensitive to
the varying (and nondeterministic) fault levels of IBRs. The
relays at the PCC automatically open (island) the facility and
automatically resynchronize the PCCs by communicating
with IBR relays via a dispatch message. IBRs are dispatched
for optimal resilience by distributed PPRs. These systems
are designed with layered redundancies and provide the most
reliable microgrid power for critical infrastructure.

Helping Inverters
With Transformer Energization
The magnetic inrush of transformers must be considered
when energizing an islanded microgrid powered by IBRs.
As depicted in Figure 5, this inrush can have a large dc component that falls away as a function of X/R. Depending on
the X/R value, this phenomenon can reach several times the
rated transformer current, which typically ranges from a few
cycles to 0.5 s. This phenomenon challenges the surge limits
of inverters associated with silicon limitations.
Increasing the difficulty of this problem are the U.S. methods of sizing transformers for building service entrances,
where transformers are sized not for load but for the full rating of the downstream conductors and transformer. Therefore, on average, transformers at service entrances are sized
at more than two times the maximum loading of a building.
The most common solutions to the magnetization challenge are
✔✔ flux-management methods
✔✔ point-on-wave switching
✔✔ bypass systems
✔✔ synchronous condensers or generators
✔✔ oversized inverters
✔✔ simultaneous techniques.

Flux Management
By using a flux-management method to reduce the transformer inrush problem of inverters, the inverters adjust the

PCC1

PCC2

Prime
Power
PV

PPR

PPR
PPR

Battery
PPR
PPR

PPR

figure 8. A resilient power system topology. PCC: point of
common coupling.
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IEEE Power & Energy Magazine - May/June 2021

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