Page 221 - Sustainable On-Site CHP Systems Design, Construction, and Operations

194 De s i g n

particular facility’s needs. One potential problem with packaged relays is that the entire
relaying system is a single point of failure. Fortunately, however, packaged relay failure
is rare.
While the older style induction-disk relays are also still an option, they are becoming
more and more obsolete. While these induction-disk types have an excellent history,
they do have some inherent disadvantages as compared to the electronic type. The
rotating dick has inertia and must rotate to its original position to reset after pickup,
resulting in substantial reset time. In contrast, the electronic digital relays can reset
instantaneously, providing immediate repeatability of operation. In addition, since
electronic digital relays are nonmechanical they have no springs to adjust or calibrate.
Another consideration in the design of a CHP protection system is whenever
generators must energize transformers, then the overcurrent, differential, and current-
balance relays, along with the integral trip units of low-voltage breakers, must be
carefully selected and set. The relays will need to withstand the high magnetizing in-
rush currents required for transformers without tripping, yet provide proper protection
to the energized transformer. This is one more reason to look carefully at electronic
digital relays.

Specific Protection Requirements
As noted earlier, the protective functions required by a utility are not to protect the CHP
generating facility, but rather to protect the utility distribution system. There are a
number of protective functions that are typically required by all utilities for all intercon-
nected generating facilities. For example, all interconnections will require over/under
voltage trip and over/under frequency trip, which ensures that the voltage and frequency
of the two sources are in synchronization while connected.
The CHP generators must be separated from the utility service grid immediately
upon the occurrence of a power system disruption or disturbance that could result in an
unsafe, undesirable or objectionable operation. Typically, the fastest reestablishment of
a utility service, taking into account the operating time of the utility relaying and reclosers,
is approximately 12 cycles. When the utility service is returned, there is no assurance
that it will be synchronized with the CHP system, so the protection system must ensure
that there would be no possible chance that the two sources will still be connected at the
time the utility service is restored. Therefore, the CHP generator system must be discon-
nected from the utility, via the protective relaying system, within a maximum of 8 to
9 cycles. Once disconnected, the generators may provide emergency and standby
power, with load-shedding as necessary to stay within generator capacity. When the
utility service returns and is stable, then the generators could once again be paralleled
with the utility grid. To accomplish this, it is imperative that a protective relaying and
control system scheme be designed to perform these necessary functions automatically
in a logical, safe, and sequential manner. For system test, maintenance, and recalibra-
tion, certain manual controls may also need to be incorporated, such as a manual
synchronizer. These should be limited to keep everything less complicated, but must be
configured and interlocked to prevent and prohibit any sequence of operation or com-
bination of connections that could result in an unsafe and undesirable operation.
Therefore, a typical protective function required is voltage and frequency sensing,
with time delay. This will ensure that a CHP generating facility currently disconnected
from the utility does not reconnect with the distribution system unless its voltage and
frequency are within a set range, and have been in that set range for a certain period of