Page 214 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 187

The voltage regulator controls the generator excitation to provide the set-point volt-
age output for a single unit. When generators operate in parallel, and are not connected
to the utility system, the set-point bus voltage is the same for all units. The voltage
regulator current transformers for all parallel generators are connected in series so that
all regulators see the same load-sharing signal, and are called cross-current compensation.
The voltage regulator controls the excitation so that each unit shares proportionately the
total reactive load. This series loop is to the voltage regulator what the load-sharing loop
is to the governor.
However, when the generators operate in parallel with the utility infinite bus, the
terminal voltage (voltage at the junction between generator and utility inputs to the
switchboard) is determined by the utility voltage. In this case, a VAR/power factor
control system becomes critical to regulate reactive power loading for each generator
(preventing excessive generator current). The most effective operational mode is to con-
trol excitation as a function of system power factor, causing reactive power generation
to track real power load, which in turn minimizes the total power (kVA) and generator
current for any facility load. The power factor set point for the VAR/power factor con-
trol system should be between 0.8 and 1.0, with the midpoint of 0.9 recommended for
system optimization.

Environmental Requirements
All electrical equipment is sensitive to the environment in which it operates. Tempera-
ture gradients and moisture have an effect on even the simplest electrical equipment.
For complex switchgear such as that typically found in a CHP installation, environ-
mental requirements for the electrical room are even more critical as noted below.
Most electrical equipment for a CHP system are located in rooms or dedicated
equipment areas and are indoors in close proximity to the prime mover/generator and
related mechanical equipment such as boilers, chillers, or heat exchangers. This means
moisture is likely more prevalent in the atmosphere than within a typical electrical
switchgear room. Moisture is the chief adversary of electrical equipment because over
time it can cause corrosive effects on circuit breaker contacts and wiring connections,
thus leading to potential early and dangerous failure. Additionally, in CHP facility
switchgear there will likely be programmable logic controllers (PLCs), integrated relays,
and other miscellaneous electronic equipment that is also extremely sensitive to mois-
ture and corrosive environments. It is critical that the switchgear be designed and
specified with extra gaskets and seals and that the switchgear be placed in a room
which is also well sealed and/or isolated from the heavy mechanical equipment. Of
course, if the switchgear is placed outdoors, this can be an even greater concern. The
switchgear will need to be specified with a minimum NEMA 3R enclosure as per any
outdoor equipment. However, the designer should give consideration to specifying
NEMA 4X (or IP56) ratings—or preferably, working with the facility designer to find an
indoor location.
Similar considerations must be given to room temperature. The room must be kept
cool enough so that the temperature rating of the sensitive electronic equipment is not
exceeded and so that electrical wiring does not have to be derated due to high tempera-
tures. Often, the switchgear and associated distribution system may include transform-
ers which generate significant heat, so it is even more critical to ensure the room design
includes supplemental cooling and exhaust. However, it is important to note that
care must be taken to ensure temperature gradients within the room are not too large.