Page 213 - Sustainable On-Site CHP Systems Design, Construction, and Operations

186 De s i g n

information for all individual distribution circuits, rather than just a measurement of
overall distribution load, allows the operator or system to make dynamic decisions on
load-shedding or load-addition. Demand load metering can potentially be combined
with control of the CHP generator output. If, for example, a real-time or life-cycle economic
calculation determines that the energy purchased from the utility could be increased
without a large marginal cost, the CHP generation can be decreased to reduce fuel use
or better optimize the CHP output.

Engine/Generator Controls
One significant component of a control system particular to CHP systems is the engine
control system and its interaction with switchgear where it connects to the utility. The
major elements of the engine control system are the governor (ANSI Device #65) and its
accessory load controller (#65C). The major elements of the generator control system
are the voltage regulator (#90) and its accessory volt-ampere reactive (VAR)/power
factor control (#90C).
Most commonly, the CHP engine is driven at a constant speed, maintaining a con-
stant generator output frequency regardless of the load. This is more commonly known
as an isochronous system. In simple terms, the electronic governor senses the load and
varies the fuel supply to the engine to produce the necessary output and maintaining
constant voltage is a function of the amount of excitation current available to the rotating
field. When two or more engines are operated in parallel, the electronic governors will
control the sum of the total load produced by each generator set. In addition, the gen-
erator controls vary the excitation to each generator so that in a paralleled condition
each generator unit shares the total reactive load (measured in kvar) proportionally to
its real load (measured in kVA).
As an example, consider a situation in which two 500-kW generator sets are operating
in parallel with an additional 1000-kW unit, for a total system capacity of 2000 kW.
Each of the 500-kW generator sets will provide 25 percent of the system load, varying
depending upon the downstream requirements. The generator sets will also carry
25 percent of the reactive load. The 1000-kW generator set will carry the remaining
50 percent of the total real and reactive load.
The governor load sensor controls the governor and subsequently, the fuel supplied to
each single engine. This level of control ensures the engines will maintain at constant speed
as the loads vary. When multiple CHP generators are operated in parallel but not connected
to the utility, these governor load sensors are all part of the load-sharing signal loop so
that all generators share the load proportionately. But when the generators are operated in
parallel and connected to the utility, there is an accessory loading control which is switched
into a load-sharing signal loop. This accessory control is continually monitoring the utility
input to the CHP system and this has a significant effect on the generator outputs. The
accessory control function combines with the CHP control system and responds to the total
system demand for both heat and electrical energy. In other words, the load that the CHP
generators share proportionately will vary based on the utility contribution which in turn
varies as determined by the accessory loading control. This functionality may likely add
complexity to the control system. For example, in large systems that are in parallel with a
utility infinite bus, droop control is sometimes used. Droop control is a function which allows
for a slight drop in engine speed with an increase in load. This slight drop in engine speed
allows for optimum operating efficiency of the CHP plant. In this scenario, the engines use
only enough fuel to satisfy the demands for energy programmed into the loading control.