Page 212 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 185

upon a step load increase, and conversely with an increase in voltage and frequency
upon a step load decrease. If the step load is large (approximately 20 to 25 percent of the
generator set rating) the transients seen by the generator will be measurable and may
cause a trip of the generator breaker. On the other hand, if the step load is small (less
than 5 percent of the generator rating) the transients will be barely noticeable. When the
CHP generator is operating in parallel with the utility, the step load transients as seen
by the generator will be small, because the utility is, again, practically an infinite bus.
However, if the system is designed such that the CHP generator may see large step
loads (the addition of an electric power–driven chiller, for example) when not in parallel
with the utility, the system must be designed to be able to handle the large transients
that will occur. Alternatively, the distribution system must be designed to limit the size
of step loads, via variable frequency drives or soft starters.

Black start Generator
Switchgear also needs to allow for connection of another special power generation
source in large CHP plants (greater than 1 MW), particularly those using a combustion
turbine generator. In a CHP facility of this size, a potential (and not uncommon) situa-
tion is that the facility is receiving all of its energy from the CHP source because the
utility source is unavailable (perhaps, due to a utility system-wide outage). If a subse-
quent fault in the CHP system temporarily shuts down the plant, the plant operators
will desire to restart the CHP generator(s) as quickly as possible since at that particular
time it is the sole source of power to the facility. However, because the utility source is
unavailable, there is no power for the pumps and starter motors required to restart a
combustion turbine generator. Therefore, a separate black start generator is a critical
design consideration and a must to the electrical system. This generator condition is
called “black start” because its only use is to provide starter power for a separate
generation system in event of a blackout. The black start generator and an automatic
transfer switch connected directly to the CHP plant motor control center is another
aspect of the switchgear design needing consideration.

Controls
To optimize a CHP system in a mode in which both utility and CHP sources feed into
the switchgear, the control system has a major role in controlling the interaction between
utility (purchased) power and internally self-generated power. For example, in a peak
shaving system the demand for utility power must be monitored to ensure that the limit
for a given period does not exceed whatever the contract value set between the utility
and the facility. Accomplishing this may entail a load-shedding scheme, where the
switchgear controls automatically open predetermined circuit breakers (i.e., loads) if
the CHP system does not have enough capacity to carry demand peaks above and
beyond the utility contracted amount.
There are a number of possible components in a CHP switchgear application related
to controls, power quality optimization, and energy management. Tie circuit breakers
and demand controls were mentioned earlier. Another key component is remote breaker
control, which allows an operator in the CHP control room to open or close either input
or distribution output circuit breakers. This allows quick response by a system operator,
when sometimes monitoring the system 24 hours a day, to a potential problem without
requiring maintenance staff to be dispatched to the electrical room. Yet another option
is demand load metering on all of the distribution circuit breakers. Having precise