Page 220 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Electrical Design Characteristics and Issues 193

completely until the load increases to a high enough level (above the set point). Another
protection possibility is a reverse power relay, which by function in this application
detects power flows from the CHP facility to the utility distribution system. If the power
flow occurs for too long a duration (typical setting is 2 seconds), or the amount of power
being generated is too high (perhaps 0.2 percent of the utility transformer rating), the
CHP controls again will respond by reducing or dropping power generation.
This reverse power relay solution is more common when the CHP facility and
utility have an agreement allowing for incidental power export as it is more likely
that there will be power flowing back onto the utility distribution grid in this mode.
When this type of plant is designed, the generator is often sized with capacity reason-
ably close to the actual facility load. However, the agreement between the facility and
utility is typically such that there is no compensation back to the facility when it gener-
ates power that feeds back into the utility. Therefore, there is no advantage to the facility
to generate more power than it needs (hence the term, “incidental” export).
The advantage for a facility negotiating this type of power export agreement is that
it does not have to respond as quickly to a reverse power situation as it does when it
agrees not to export any power at all. A facility that has frequent sudden reductions in
power load (e.g., large electric power–driven chillers suddenly stopped, or an indus-
trial facility with extremely large motors turned off) may not want to quickly cut gen-
erator power because this may affect other outputs from the CHP plant. A slow reduction
in generation to respond to more than temporary reductions in facility load has the
advantage of allowing a smoother transition combined with as little wasted generation
(i.e., noncompensated energy flowing back to the utility) as possible.
One step beyond this type of agreement is one in which the facility has a net energy
metering system. If a facility has a large load diversity, perhaps on a weekly basis
where the load is much less on weekends, or on a yearly basis when summer and
winter loads are quite different, it may wish to maintain a constant generation capacity
without regard to whether or not the CHP plant is producing slightly more or slightly
less than the facility electric load. In this situation, the utility will track the energy it
provides the facility when it is an importer and the energy it gets from the facility
when it is an exporter. The net difference gets reconciled in energy usage costs either
monthly or yearly. Note that this type of agreement is in general a recent development,
partially born from the recent propagation of alternative energy source exploration
and implementation, the most notable being the solar power generation industry. It
is important that the CHP facility designer investigate whether this opportunity exists
with their local utility before adding the metering and relaying necessary for net
energy metering.

Protective Relays
The major component in utility protection is relays specifically designed for their pur-
pose. Relaying must provide for safety and proper sequence of operation, and must
also protect the utility system and personnel from harm. It is recommended that protec-
tive relays used in CHP systems should be an electronic digital type. Packaged relaying
systems readily available from many companies provide a self-contained single-
component package that can be ordered with any (practically any) combination of pro-
tective relaying requirements. These packaged relaying systems are very reliable, have
infinite repeatability, are compact in size, and do not require recalibration. Simple
programming can turn on or off functions as needed to customize the relay system to a