POWER May 2020 - 16

O&M
Factory Increases Reliability,
Cuts Costs with Distributed
Generation and Microgrid
Control System
Six times during the previous year the
local utility supplying power to a large
soap manufacturing plant in the Midwest
suffered power outages, shutting down
the plant's production lines. Each time
the lines went quiet, product hardened
requiring the process to be flushed,
cleaned, and reset. Even momentary
outages caused downtime that could
last hours. Productivity halted, costing
time and money. Something had to be
done about power reliability at the plant.
Previously, in pursuit of increased efficiency,
management had installed an
engine-generator in a combined heat
and power (CHP) application. This unit
provided supplementary electricity and
waste heat to the process.
To address the reliability issues,
management investigated the possibility
of expanding the generator's role
from an electricity supplement to an
earnest backup power supply for critical
loads. This distributed generation
intended to improve efficiency came
with a possibility of improving reliability
as well, a win-win.
The project came with a catch, however.
The maximum output of the generator
is 4.4 MW, but the peak plant
load is 6.2 MW. How could the plant
run on lower power supply without disrupting
manufacturing?
Automation in the form of a microgrid
control system is one way. This system
decouples the plant's power system
from the utility, switches the generator's
control mode to islanded operation, and
deenergizes the non-essential loads to
ensure the essential loads remain online.
All this must occur reliably and within a
very tight timeframe, which necessitates
careful planning and engineering.
Microgrid Control System Design
Due to the proliferation of communication
networks and microprocessor-based
devices in the industrial environment,
automation systems have a vast range
of capabilities. Microgrid control systems
are no different. These systems
can perform functions such as automatic
grid decoupling and recoupling, load
shedding (Figure 1) and adding, power
16
microgrid control system.
The controls had to be programmed
to shed non-critical loads immediately,
allowing loads critical to the factory's primary
processes to remain operational.
Non-critical loads include building support
systems, such as HVAC and shop
air compressors. Critical loads directly
support the manufacturing process. The
team studied metering data to determine
how often critical load levels were
less than the generator's capacity. It was
found that this was always the case.
Automatic load shedding takes many
1. This graphic shows the power and signal
flow for a load-shed control scheme. Courtesy:
Stanley Consultants
system control during islanded configurations,
and many other protection, metering,
and diagnostic functions.
Commensurate with this range of
capability, these systems also come
with a wide range of cost. A study was
performed to strike an appropriate balance
between capital investment and
system capability.
First, it was determined that automatic
grid decoupling was required. When a
disturbance occurs on the utility's system,
the plant must be disconnected
in milliseconds to ensure it maintains
dynamic stability. The plant must be isolated
from the problem before electrical
parameters such as voltage and frequency
drift outside of acceptable tolerances.
Because this must occur so quickly, in
practice, this cannot be done via manual
intervention-automation is required.
Next, the capacity of the engine-generator
was originally selected based on
space constraints and economic optimization,
not based on total plant load. The
chosen capacity of the generator was
4.4 MW, but the plant load was greater
than 5 MW more than 60% of the time.
Therefore, some form of load shedding
was required to automatically disconnect
less-critical loads when the plant's
power system was decoupled from the
utility. This could be accomplished in the
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forms. First, one must determine where
the loads will be disconnected. This can
be done at the medium-voltage level,
where disconnecting breakers removes
large sections of the distribution system.
Alternatively, it can be done at the lowvoltage
part of the system, allowing for a
more discerning system.
In a retrofit application, such as this,
the benefit of performing the operations
higher up in the system, at the
medium-voltage level, is that electrical
breaker control is standard. This
means that the switching devices can
be opened and closed by a remotecontrol
system. The disadvantage of
this location is that only coarse control
can be achieved. This increases the risk
of over- or under-shedding.
Over-Shedding, Under-Shedding
Issues and Solutions
Over-shedding occurs when too many
loads are disconnected from their power
source. At a minimum, this problem
disables more equipment in the factory
than required. Worse, if the amount of
load shed exceeds the load step-change
capability of the generator, the entire
system can become unstable. This is a
greater problem for spark-ignited gas engines
than for diesel engines, which can
be more tolerant to load step changes.
Under-shedding occurs when not
enough load is removed from the system,
which will cause the generator to
trip due to a number of reasons, including
overload or under-speed. These risks
can be mitigated by shedding power lower
in the system, at the low-voltage level.
At that level, the system has finer control.
The issue here is that the switching
devices may have manual-only control,
requiring modification to afford them remote
control. Additionally, interconnecPOWER
| May 2020
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POWER May 2020

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