IEEE Power & Energy Magazine - May/June 2021 - 60

PPRs are required to protect life, the environment,
and assets from damage on both utility grids
and microgrids.
This article starts with a summary of the common protection challenges posed by implementing microgrids using some
form of an IBR, and then it provides several common types of
microgrid protection solutions proven on projects worldwide.

Inverter Challenges
This section points out some of the most common challenges that inverter implementation poses for both utility and
microgrid protection and control systems: limited fault current, negative-sequence current contribution, the influence
of battery state of charge, inverter response time, load inrush
current, and frequency tracking. These examples indicate
that the power and flexibility of inverter digital controls can
be harnessed with a clear set of standards to ensure consistent behaviors and coordination with PPRs.

PPR

Inverter

Visualize
Microprocessor
Relay

Control
Monitor

I
Transformer

Protect

Circuit Breaker
Current Transformer
Potential Transformer

Fault Current per Unit of Nameplate

figure 1. PPRs providing protection, control, and monitoring.

X ″ Subtransient Reactance

X ′ Transient Reactance
5

Silicon

Thermal

Stop Commutation
Silicon
Limit Time Time (Optional)
Time

figure 2. The fault current of inverters versus generators.
60

ieee power & energy magazine

Inverter Fault Current Limitations
Figure 2 shows a relative comparison of the total fault current
produced by a synchronous generator versus an equally rated
continuous inverter. The fault current produced by generators
dynamically changes as a function of fault duration. The first
few cycles of a fault are characterized by very high currents,
followed by slightly lower currents. Protection engineers simplify these changes by assuming artificial impedances behind
an ideal voltage source. These impedances are the transient
reactance ( X l ) and subtransient reactance (X m ). The transient
reactance is the value used for coordination of most protective relays to a traditional synchronous generation-driven
power system (from approximately 1900 to today).
As Figure 2 shows, inverter currents during faultedcircuit conditions are radically different from those of synchronous generators. Fault currents produced by inverters
fall into two general categories: 1) silicon short-term limits
and 2) longer-term thermal limits posed by the heat sink and
thermal management control. Inverters reduce their current
from the silicon to the thermal limit after the silicon time.
Some inverters stop commutating after sustained operation
at the long-term thermal limit. Others reduce voltage to
maintain current within limits and only trip if the resulting
voltage dips below acceptable limits.
Power electronic devices are permanently damaged
when silicon current limits are exceeded. Thus, firmware in
the inverters strictly limits this current, and for this reason,
the silicon limit is sometimes referred to as either a hardware or surge limit.
Thermal limits are commonly adapted in inverter firmware using real-time measurements of aluminum heat sinks
and cooling water jackets. Because the hardware associated
with heat removal is expensive, the thermal limit is often the
driving factor in overall inverter construction costs.

Inverter Negative-Sequence
Current Contributions
The totalized inverter fault current limit is calculated as the sum
of positive-sequence current (I1) and negative-sequence current
(I2) (see Figure 3). Zero-sequence currents are most commonly
suppressed in inverters. The level of I2 differs greatly between
inverters and generators. Generators naturally provide significant levels of I2, whereas I2 in the inverters is subject to the control loops and current limits programmed in firmware.
In general, most grid-interactive inverters do not provide I2; however, that is changing. I2 is an important quantity for some conventional protection methods because
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - May/June 2021