IEEE Power & Energy Magazine - May/June 2021 - 62

Inverter currents during faulted-circuit
conditions are radically different from those
of synchronous generators.
have found significant advantages in the close cooperation
between protection and inverter control engineers.
Rotating generator sets (gensets) have significant inertial (kinetic) energy storage, whereas inverters do not store
any kinetic energy. Gensets are synchronous generators
that are shaft coupled to an engine (prime mover) and have,
historically, been the primary power-producing equipment
for the utility grid. For example, a 500-kW, reciprocating
gas engine-driven genset has an inertia of roughly 1 s; this
equates to the energy storage of full-rated power for 1 s.
A 500-kW, reciprocating gas genset would therefore have
500 kJ of stored kinetic energy at rated speed. Note, however, that such a generator must slow down to 0 Hz to facilitate the harvesting of this stored inertial energy.

Load Inrush Current
Inverters with batteries store many orders of magnitude more
energy than rotating conventional gensets; however, the rate
at which this energy can be released to the power system
is limited by silicon current and heatsink thermal limits.
Gensets, on the other hand, store limited amounts of kinetic
energy, which is readily available for fast-starting loads. The
result is that gensets can reliably start direct-coupled motor
loads up to 50% of their nameplate power, whereas inverters (unless purposely built with surge capability) can reliably start only direct-coupled motor loads up to 15% of their

nameplate power before voltage sagging occurs. Note that
motor and load rotational speed and load characteristics vary
considerably, and the typical values of 50 and 15%, respectively, are meant to be representative. The nature of motor
loads such as inertia and speed-related load characteristics is
that they can cause large variability in these estimates. Circumstances such as motor pole count, motor X/R (the ratio of
the system reactance to the system resistance) ratios (electrical time constants), mechanical load characteristics, and contactor sensitivity can all affect successful cold-load pickup.
Further modeling can determine more accurate numbers.
Figure 5 depicts the root-mean-square current required for
a direct online (DOL) motor start. After some initial inrush,
the DOL motor gathers kinetic energy as it gains speed. During this time, the DOL absorbs a much larger amount than
rated power; after a start time of a few seconds, the motor
and load are up to speed and the current consumed is the load
power. Inverters limit current during large motor starts that
exceed the overload capability of inverters, which may result
in unwanted voltage sag or unsuccessful load energization.
Focusing on the " Inrush " of the motor start of Figure 5, the
magnetic inrush plot shows that the current waveforms in the
first few cycles have a large dc offset. This dc offset is due to
magnetic remanence in the steel of transformers, motors, and
other magnetic devices; remanence is predicated by the point
on wave of the voltage signal when the circuit breaker (or

Current

Current
Inrush

dc
Kinetic
Energy
Buildup
Load Power

Time

X/R Time Constant
(a)

(b)

figure 5. (a) The motor cold-load pickup and (b) magnetic inrush. X/R: the ratio of the system reactance to the system
resistance.
62

ieee power & energy magazine

may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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