IEEE Power & Energy Magazine - May/June 2021 - 61

negative-sequence elements are excellent for detecting fault
direction and for the supervision of high-speed tripping elements. I2 levels in a circuit are an effective way to identify
unbalanced faults. Protection engineers have depended on
I2 for decades in their designs at transmission-level voltages.

must be confirmed. Modern inverters are digitally controlled,
switch in the tens of kilohertz, and are therefore capable of
millisecond response times. However, actual inverter fault
response behavior is a function of supplier firmware, and
behavior may also be impacted by site-specific configuration and dynamically adjustable parameters. The authors

Battery State-of-Charge Influence
I2
Negative-Sequence Current (p.u.)

Fault current delivered by a generator is primarily a function
of physical electromagnetic interactions, whereas inverter
behavior is governed by software, and firmware and may be
limited under certain conditions. Figure 4 gives an example of a
battery system with batteries offline or mostly discharged.
Under these conditions, the operational capability of the
inverter is reduced. Thermal issues, battery problems, a low
state of charge, and having parallel inverters out of service
are all reasons why inverter fault current may be reduced.
Note that the high-power factor fault condition shown in Figure 4 is of high-resistive faults and/or low-inductive power
systems associated with microgrids. Transmission and distribution systems have mostly reactive current during faults,
lower voltage systems like microgrids can have high power
factor (highly resistive) faults.
One option to prevent the reduction of fault currents is to
ensure that batteries do not enter low-charge states so that the
current limiting displayed in Figure 4 does not occur. This
charge buffer must be considered in the sizing of batteries for
both economic and fault-related limitations. It is important to
monitor steady-state and transient current-delivering capabilities of each IBR. PPRs are often used to directly control and
monitor each IBR system and generator to make certain that
effective protection and control margins are maintained.

Generator
Currents

Inverter
Currents
I1
1
5
Positive-Sequence Current (p.u.)

figure 3. A relative comparison of the positive- and
negative-sequence components of fault current: inverter
versus generator.

Current
Capability
Region

Reactive
Current
Fault
Current

Inverter Response Time
Another consideration is the inverter response time. Table 1
summarizes the typical response time of conventional controls
and protection. Governors and engine fuel systems respond to
a power system's frequency decay within approximately 1 s.
Generator automatic voltage regulators typically respond in
fewer than 0.2 s to voltage disturbances; time-overcurrent
protection operates in 0.1-7 s load-shedding systems prevent
power system blackouts in a few power system cycles. Distance protective relays identify faults and command circuit
breakers to trip in under a cycle, and traveling-wave relays
detect and identify fault locations in only a few milliseconds.
This timescale of controls and protection is not an accident;
these scales were designed into the power system to ensure a
smooth coordination between the controls and the protection
equipment, which allows protective relays to isolate faults
before controllers overreact or shut down.
Inverter designs are relatively new, whereas utility power
protection and controls are a complex, interwoven set of
functionalities designed and perfected during the last 100
or more years. Although inverter behavior is not constrained
by the same physical limitations as generators, the coordination of inverter time constants with other system components
may/june 2021

Active
Current

Battery SoC Dynamic Limits

figure 4. Inverter fault current restrictions. SoC: state
of change.
table 1. The device times.
Device

Typical Response Time (s)

Governor

Automatic voltage regulator

0.2

Time-overcurrent protection

0.1

Load-shedding protection

0.04

Distance protection

0.01

Traveling-wave protection

0.002

Inverter

Variable
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IEEE Power & Energy Magazine - May/June 2021

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