IEEE Power & Energy Magazine - May/June 2021 - 64

Adapting Protection
With Communication
In this example, an industrial facility with eight parallel
gensets and a battery is configured such that no single set of
time-overcurrent curves can distinguish load from faults for
all operational conditions. During low-process load conditions, gensets are sequentially turned off for fuel economy
and reduced emissions; the number of gensets online can
vary from zero to eight. The plant load varies from 200 to
1,800 A, and every genset contributes 1,500 A (transient
reactance) fault duty. The battery inverter is sized at 300
A continuous, produces 600 A of surge current and 300 A
thermal overload current (see Figure 2), and is configured
to stop commutation at 4 s. Eight gensets produce 12,000 A
combined fault current, and with the battery online, the total
fault current is equal to or greater than 12,300 A for 4 s.
By selecting many small gensets that can be turned off,
the operators designed the facility to use minimal fuel (hence,
minimal carbon dioxide emissions) and sized the battery to
allow the facility to operate without gensets at low-load conditions. Although optimally designed for carbon emissions and
minimal environmental impact, the facility cannot be safely
energized because the single-function protective relays chosen
cannot be set to prevent nuisance trips during high-load periods
and also sensitive enough to detect faults when a single genset
or battery inverter is online. This is a case where energizing
this facility would endanger human life or false trip the facility
during high-load conditions at great cost to the operation. This
is a typical microgrid problem that is solved by dynamically
modifying the protection of PPRs.
A common solution to this problem is to replace singlefunction protection devices with multifunction, programmable,
microprocessor-based relays with high-speed communication
Load

Automatic
Transfer Switch
(a)
Standby Power

Load

Circuit
Breakers

Programmable
Protective Relay
(b)

figure 7. Using PPRs to build a microgrid from existing
gensets. (a) Conventional and (b) improved.
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ieee power & energy magazine

(i.e., PPRs). PPRs are placed at all load feeders, at each generator, and at the battery. The genset and battery inverter PPRs
are programmed to control, dispatch, start, stop, and monitor
the gensets and inverters; these same relays communicate their
status to downstream load and feeder relays. Downstream
relays are programmed to dynamically revise their protection
settings as the available fault currents change. By using this
dynamic protection methodology, PPRs enable the plant to
meet both environmental and human safety requirements.

Incorporating Old Switchgear
Into a New Microgrid
Adding renewable or battery-backed generation commonly
requires the modification of existing switchgear and that the
modified gear be updated to the latest regional safety standards. By replacing older single-function relays with PPRs,
end users reduce costs and can make microgrids viable by
eliminating the need to buy new gear.
National Electric Code-sensitive earth ground requirements must be met to ensure human safety. Arc-flash incident
energies are minimized with PPR light-detection elements.
Gear must be retested and approved for reenergization by
regulatory entities. PPRs have onboard light sensors to detect
arc-flash events, substantially reducing energy by tripping
upstream circuit breakers. PPRs also have onboard sensitive earth-ground fault calculation and protection algorithms
not available in single-function relays and possess onboard
oscillography recordings to show inspectors that they are
functioning correctly.

Using Existing Gensets
to Improve Resiliency
If loads in an islanded microgrid exceed renewable production for a prolonged period, the battery system's stored
energy will eventually be depleted. The solution to this problem requires an on-site genset with sufficient stored fuel and/
or a utility connection.
Many facilities have large, stranded investments in
standby gensets with automatic transfer switches, as presented in the conventional solution in Figure 7(a). Large
campuses can have hundreds of gensets and an automatic
transfer switch on each building; however, these switches
inherently isolate gensets from the grid and prevent them
from participating in a microgrid. These gensets are therefore stranded assets.
In the improved PPR solution shown in Figure 7(b), the
genset can be used to participate in energizing a campus
microgrid by eliminating the automatic transfer switches and
installing a PPR to control the generator and grid breaker.
This configuration enables the relay to seamlessly synchronize the generator to a grid to support demand-charge-avoidance techniques and to improve the resilience of a renewable
(inverter-based) microgrid. The relay also commonly has
remote communication with a central microgrid controller and can natively dispatch, start, stop, and monitor the
may/june 2021

IEEE Power & Energy Magazine - May/June 2021

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