Page 211 - Sustainable On-Site CHP Systems Design, Construction, and Operations

184 De s i g n

feeding into the switchgear bus at the same time, and because of this there are a number
of considerations and capabilities which should be taken into account for the switch-
gear serving a CHP system.
One such consideration that warrants attention is the type of bussing in the switch-
gear. If the bus is physically separated into two or more discrete sections for various
design reasons (e.g., load distribution), these sections would require connection to each
other via a tie circuit breaker. In this example, the main circuit breakers directly upstream
from the energy sources are both interlocked with a tiebreaker so that the incoming
feeds are physically separated at all times. Another possible method of configuring the
switchgear to accommodate the multiple source input is paralleling switchgear, in
which all incoming sources feed the same physical bus at the same time. To effectively
utilize paralleling switchgear, the switchgear must have a synchronizing system which
ensures that all electric power generated is operating together at the same rated voltage,
frequency, and phase.
In both of these cases, the switchgear and its associated distribution system will
feed all downstream loads if sufficient energy is available from a combination of utility
and CHP sources. Prime mover load control in the switchgear will ensure that power
from all sources is balanced and efficient. However, if one of the sources is lost, or for
some other reason inadequate energy is generated to serve the facility (e.g., voltage
irregularities or low frequency), load-shedding will be required to control the ability of
the switchgear to drop or shed certain lower priority electric loads.
Another capability to be considered in switchgear design includes voltage and
reactive power control, which refers to controlling voltage regulation delivered by each
energy source and automatic adjustments for varying reactive power levels. Remote
overcurrent and protection controls, another design consideration, includes either auto-
matic on/off and reset capabilities of overcurrent devices or manual capabilities from a
remote location. Each of these key elements of switchgear design should be carefully
considered by the CHP designer to ensure maximum optimization and safety of the
CHP system.

Utility Source Characteristics
Utility power grids typically have a capacity that can be as much as 1000 times that of a
CHP facility load, even for an especially large facility. In fact, to the CHP facility the utility
grid appears to be practically infinite in size. Therefore, in the design of power distribu-
tion systems at the utilization point where a building or facility is connected to the grid,
it is common to base the design on an infinite bus characteristic such as constant fre-
quency, constant voltage, and infinite available current. In reality, of course, available
current is not truly infinite but it can be modeled as such to simplify fault current calcula-
tions. However, unlike the utility grid, on-site generators do have finite limitations. Their
available fault current is predictable; the available short-circuit current from a small gen-
erator (in the range of 100 kW) is about 10 times the rated full-load current of the generator
immediately after the initiation of the fault, and drops to about half of that in about
0.1 second. Circuit impedance (resistance of the cabling) further attenuates the available
fault current. The important point is that while the utility grid can be considered an infi-
nite bus, the on-site generators definitely cannot and the CHP designer must take that
into account when selecting and designing key system design components.
Large loads that are applied to a generator (either an increase or a decrease in load)
are called step loads. Any generator will react with a decrease in voltage and frequency