Page 210 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 183

provides a connection between the CHP generator and the facility being served, and at
times connection to the utility power grid as well. The switchgear includes more than
just circuit breakers or load interrupter switches. It may also include transformers
depending on the output voltage of the generator and the utilization voltage for the
facility or utility. It will certainly include some level of controls to ensure multiple power
sources are protected or a facility is using the power in the most efficient way possible.

Selection and Design
Before discussing the switchgear selection criteria for each of the operational modes,
this section summarizes the basic fundamentals found in any switchgear design. First,
there are connections into the conductive bus bars within the switchgear from both the
CHP source and the utility source. Whether the connection is via a solid state circuit
breaker, a load interrupter switch, or some other type of switch is dependent on the
whether the connection is low voltage or medium voltage and what type of protection
and coordination characteristics the designer includes. There are also distribution cir-
cuit breakers which again, can be at either low or medium voltage that serve loads
downstream from the main switchgear. Also, there is a feed to the CHP electrical equip-
ment (i.e., source). Depending on the type of prime mover, this may include fuel supply
pumps, water injection pumps, or generator starter motors.
The electrical power generated by a CHP system is utilized in different modes of
operation and each has different switchgear design considerations. The most basic is in
a stand-alone (isolated feed) configuration, where the CHP is the only source of power
for a facility, or portion of a facility. This switchgear design is the simplest, because there
is no paralleling between multiple sources required. The CHP generator is connected to
the switchgear in a manner similar to any utility source, via a load interrupter switch, a
vacuum circuit breaker, or a molded case circuit breaker. The CHP generator may gen-
erate power at medium or low voltage, and the switchgear connected to the generator
may also be at medium or low voltage. Ultimately, of course, the voltage will be trans-
formed to whatever the facility requires, but this is not an atypical installation.
However, several other modes of operation require more specialized distribution
and control. These modes of operation are defined as follows:

• Standby power. If the generator is used only to provide power in event that the
primary power source (typically the utility) is not available.
• Peak shaving. When a facility wishes only to purchase a specific amount of
energy from a utility, either for contractual reasons or economic ones, it will use
energy from its CHP generators to supplement that utility power when its
energy needs exceed that which they receive from the utility.
• Base load. In this case, a facility will use the energy generated by the CHP plant
up to its maximum capability and will only use a separate utility source when
its needs exceed the capacity of the CHP plant. It is fundamentally the opposite
of the peak shaving mode.
• Export. In this case, excess energy generated by the CHP plant not needed to
serve the CHP plant loads will be transported back onto the utility grid.
In any of these applications, the switchgear must be capable of being fed by both a
utility source and the CHP source. Each of the incoming energy sources are capable of