IEEE Power & Energy Magazine - May/June 2021 - 68

problems, and challenges to conventional protection methods. When microgrids are paralleled to a utility power system, the effects of load composition are reduced due to the
losses associated with traditional power generation, transmission, and distribution. When microgrids are islanded, the
effects of variable-load composition can significantly impact
protection systems. To explain this phenomenon, researchers
have categorized loads into constant impedance (Z), constant
current (I), constant power (P), and motor loads. To dramatize the stark difference in how load types require inverters
and generators to respond, contrast a constant impedance
and constant power load.
As presented in Figure 11, impedance loads increase
their current consumption as voltage increases. Impedance
loads in the power system are the cables, lines, transformers, heating systems, and losses associated with electric
power transport. Inverters, governors, automatic voltage
regulators, and conventional systems are designed and

Constant Impedance Load

Constant Power Load
V

figure 11. Constant impedance and constant power loads.

figure 12. Motor loads.

table 2. Variable-load composition.

Microgrid
Topology

Impedance
Loads

Constant
Power

Motors

Agriculturally
dominated

60%

40%

Data center
dominated

50%

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tuned to work with power systems that are dominated by
impedance loads.
Constant power loads decrease their current as the
voltage increases. Constant power loads behave in an
opposite way in response to constant impedance loads.
Constant power loads thereby commonly destabilize the
gensets and IBRs that were tuned for a constant-impedance power system.
To further demonstrate the effect that variable-load compositions have on inverters, consider how typical motor loads
consume power as a function of frequency (see Figure 12). As
the frequency increases, these loads consume more power.
As the frequency decreases, they reduce power consumption.
DOL loads provide a form of " brakes " to a power system's
frequency excursion and enable slow-acting conventional
generator governors and automatic voltage regulators to stabilize a power system. They offer a similar advantage to any
IBR: IBRs and gensets are much easier to tune with a DOLdominated load composition. However, DOL loads can be
challenging to start with inverters because of inrush associated with cold-load pickup issues.
Load composition is the relative ratio of impedance,
constant power, constant current, and motor loads. Because
constant current loads are rare, microgrid designers focus
on impedance, constant power, and motor loads. Table 2
lists a simplified example of the most common load types.
In this case, load composition varies significantly in the
two formations (topologies) of the same microgrid. This
can occur, for example, if a large data center drops off a
local microgrid, or if large agriculture motor pumps are
added to a circuit.
The effects of variable-load composition have frequently
been observed to challenge controls and protection systems.
Protection engineers working on complex microgrids are
advised to understand and adapt their protection systems to
these variations. Electromagnetic transient hardware-in-theloop studies are a convenient way to explore these interactions; these simulations simulate the power system with a
real-time update typically faster than 80 µs. This high-speed
simulation done in real time allows IBRs and PPRs to be
tested as if they were attached to a real power system.
To understand the effect that variable-load composition has
on protection, take the example of underfrequency elements
that are ubiquitous in PPRs today. Frequency-tripping elements are used for detecting open-circuit conditions, islanded
load-shedding systems, and to meet industry frequency ridethrough requirements at utility interconnection points. Many
microgrids require that frequency pickup times be selected
that can handle multiple load compositions. The most common solution is to adapt the frequency pickup and setpoints
dynamically if there are large load-composition changes.

Testing Microgrid Protection Systems
Testing is required to ensure that systems can safely be energized and meet project-functional requirements. Commissioning
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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