IEEE Power & Energy Magazine - May/June 2021 - 67

closes circuit breakers after detecting healthy voltages for a
few seconds. This is the simplest, lowest cost, and most reliable technique because no new equipment is required. It also
allows the mixing of different inverters of differing sizes,
manufacturers, and feature sets.

Helping Inverters With Cold-Load Pickup

PPRs (i.e., PPR2 and PPR3) sufficient time to open their
reclosers. PPR1 issues a start (possibly in coordination with
a magnetic inrush mitigation system). The PPR at each circuit breaker and recloser then sequentially re-energizes its
adjacent device one at a time. This is done without communication as the PPR detects the upstream energized line
and starts an internally programmed timer, which closes the
reclosers in sequence, i.e., PPR2 follows PPR1, and PPR3
follows PPR2.

Cold-load pickup problems are fundamentally different from
magnetic-energization issues. Cold-load pickup is a function
of spinning up inertial components or capacitors, whereas
magnetic inrush is a function of remanent flux. Load pickup is Grid Forming With Droop
in-phase current (real power) and energy consuming, whereas There are two common control modes of grid-scale inverters:
magnetic inrush is out-of-phase currents (reactive power) and grid forming and grid following. The grid-following control
doesn't require energy. For example, DOL motor starting can method uses a frequency-tracking algorithm (see Figure 6)
have magnetic inrush followed by a significant level of cur- and requires attachment to a large power system with signifirent required to speed up the rotating mass of the rotor and of cant inertia and very slow changing frequency. In contrast,
the mechanical load. The time-integrated area of this inrushing the grid-forming control method typically assumes that the
power is the stored kinetic energy of the rotating mass. Cold- inverter is the sole proprietor of frequency control, and no
load pickup for microgrids also includes power electronic utility or large isochronous-driven generators are attached.
loads with inrush currents.
Neither the isochronous grid-forming technique nor the
Whereas magnetic inrush phenomena challenge the grid-following technique work for all power system condisurge capacity of the inverter's silicon, the effects of cold- tions. Attaching a pure isochronous grid-forming inverter
load pickup challenge the thermal withstand capability of to a high-inertia utility power system will invariably cause
the inverter's (Figure 2). Thermal withstand limits are deter- the inverter to trip offline due to under or overcurrent condimined by aluminum heat sinks and the cooling system of tions. Grid-following inverters will not energize an islanded
the inverter. A few options for mitigating inverter cold-load microgrid alone because grid-following methods require a
pickup challenges are to
slow-changing grid frequency. PPRs can certainly be pro✔✔ purchase inverters rated for full inrush current (over- grammed to switch inverters from grid-forming to grid-following modes, but there is a more robust solution than this.
sized, relative to nominal load)
One solution for transitioning between grid-connected
✔✔ supplement inverters with a kinetic energy storage
system, such as synchronous generators or flywheels and islanded operation is to place the inverter permanently
into a grid-forming mode with droop-control. Many IBRs
(incurs additional capital and operating costs)
✔✔ use soft-start or variable-speed drives (expensive) to and generators have inherent droop-control capabilities, but
PPRs can also be used to provide droop functionality. Droop
mitigate load inrush currents
✔✔ program PPRs to lessen the system inrush currents control works in both grid-connected and islanded modes
with reacceleration or pickup schemes (low cost and without changing the inverter modes.
simple).
Cold-load pickup schemes for microgrids are called load- Adapting Relaying
reacceleration systems or black-start systems, and they add to Variable-Load Composition
loads incrementally over time to prevent overloading IBRs. The load composition of modern microgrids can vary conBoth types of systems are fully automatic and programmed into siderably, causing voltage instability, frequency-regulation
PPRs. The systems that bring the
power system from the ground up
are called black-start systems and
commonly rely on fast-acting diesel-reciprocating engines to operate
Inverter
PPR 1
before incrementally adding loads.
In the utility distribution feeder
system (Figure 8), the PPR at
PPR 2
PPR 3
each circuit breaker and recloser
opens its adjacent devices upon deRecloser
Recloser
energization. In Figure 10, PPR1
Load
Load
Load
(or a microgrid site controller) at the
inverter IBR issues a stop command
to the inverter, allowing downstream figure 10. A cold-load pickup problem solved with PPRs.
may/june 2021

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IEEE Power & Energy Magazine - May/June 2021

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