IEEE Power & Energy Magazine - May/June 2021 - 66

Cold-load pickup schemes for microgrids are called loadreacceleration systems or black-start systems, and they add
loads incrementally over time to prevent overloading IBRs.
voltage and/or frequency of the voltage waveform. For example, some inverters linearly ramp the voltage and frequency at
a constant ratio. Others fix the frequency but ramp the voltage.
These techniques work fine with inverters from the same supplier designed to start together, but coordination is required
for large grid-connected infrastructure where several IBRs
may be attempting to simultaneously re-energize a grid. In
sizing inverters for flux management, a conservative estimate
is to size them from one to two times the transformer rating.
Modern PPRs are commonly used to isolate sensitive
loads during these flux-managed start-ups. Voltage and frequency deviate from nominal during the reenergization of an
islanded microgrid using a flux-management method. This
may cause contactors to chatter and power electronic power
supplies and uninterruptible power supplies to go into alarm
mode for off-nominal conditions. Older protective relays
will not track frequencies and voltages and their protection
elements may not function as desired.

Point-on-Wave Switching
The point-on-wave technique attempts to " remember " where
the flux in one or more of the steel legs of the core was left at
the last de-energization. The point-on-wave technique uses a
PPR to control a special circuit breaker with individual closing of each phase; this enables each phase to be closed independently at voltage zero crossings to limit inrush. There
are, however, two main challenges of this system:
✔✔ how best to properly estimate the flux from indirect
voltage and current measurements when de-energization occurs
✔✔ how best to maintain the millisecond-accuracy close
timing of the point-on-wave closing system with conventional circuit breakers.
These systems incur the cost of the specialty single-phase
circuit breaker and require regular human recalibration as

circuit breaker wear and closing times change. Certainly,
these methods can be improved through direct flux measurement and power electronic closing circuits; however, this is
a costly solution.

Bypass Systems
As shown in Figure 9, a bypass system balances the flux
before energization through a tertiary (third) winding. For
this style of black-start mitigation, PPRs should be programmed to provide full transformer protection during this
sequence of events. This technique has proven to be more
robust and requires less maintenance than the point-on-wave
technique. On the downside, this method is costly due to the
extra circuit breaker, custom transformer, and reactor.

Synchronous Condensers or Generators
Synchronous condensers or generators can be added to provide magnetic inrush demand over the inverter overload
capability. This is a reliable and simple solution that can
correct magnetic inrush problems. Additional PPRs must be
purchased to start, control, and protect the condensers and/
or generators from damage. For some islanded microgrids,
synchronous condensers have been used to improve the stability margins and fault ride-through capability of inverterdominated microgrids.

Oversized Inverters
Although inverters can energize large transformers using a
flux-management method, voltage and frequency will likely
deviate from nominal during a black start. In scenarios
where this voltage ramp is not acceptable, inverters can be
oversized to overcome magnetic inrush challenges. Inverters can be sized to provide reactive power beyond the realpower limit of the battery cells.

Sequenced Technique
PPR
CB1

CB2

Bypass System

figure 9. The bypass system of transformer energization.
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ieee power & energy magazine

This technique involves distributing the transformer inrush
among multiple inverters and sequencing the energization of
downstream transformers. With this method, each inverter is
controlled by a PPR via a time-synchronized start.
In the simultaneous method, high-speed communication is not required among PPRs because events are preprogrmmed to start at a certain time, either from the relay
front panel, remotely, or from a supervisory control and
data acquisition (SCADA) system. Note that SCADA system communication can be performed at a very low data
rate and thus at a low cost. Downstream PPR automatically
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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