Page 291 - Sustainable On-Site CHP Systems Design, Construction, and Operations

264 Op erations

manual mode, the plant operator watches a dial with a rotating pointer. The pointer will
rotate passing zero at the top of the dial. Zero on the dial indicates when the frequency
of the generator and the line are equal. There are usually two lights located above the dial
that light up when the pointer is at or within a few degrees of zero. The plant operator
uses a knob below the dial to slow the rotation of the pointer. When the pointer is rotat-
ing very slowly and it reaches zero on the dial, the operator will turn a switch which then
locks in the frequency of the generator with the line frequency. It should be noted that on
most control systems, if the operator turns the switch when the frequency is not in sync
and the pointer is not at the zero point on the dial the control system will not allow the
generator to attempt a frequency lock and problems will persist until corrected.

Bootstrapping
In power plants, bootstrapping refers to methods for black starting a main generator. In
large utility plants, there may be a diesel engine generator which allows a small CTG to
start and synchronize to the diesel generator to provide enough power to start the main
generator. In a combined cycle plant, a diesel engine generator may allow a standby
boiler to be fired which supplies steam to the STG. The STG would then be paralleled
to the diesel generator to provide enough power to a CTG. In many cases bootstrapping
will allow for a smaller, more economical diesel engine generator to be utilized.

Restart
Each time the CTG or engine-generator is shutdown or trips off unexpectedly the prime
mover equipment and the internals in the HRSG (or hot water heat recovery unit) are
subjected to thermal stress. This is true for both shutdown and start-up operation.
The plant operator must wait for the unit to go through a warm-up cycle after a start
is initiated. If the engine or CTG trips off, the plant operator must quickly assess
whether there is a serious problem that will cause the unit to be down for an extended
period. The engine will require a rolldown period that is programmed into the control
system. The rolldown period may be 10 minutes or longer depending on the size of the
unit. If the reason for the trip can be reset quickly, the plant operator can gain a permis-
sive to initiate a restart without further delay. If the CTG or engine generator is down for
an extended period, the warm-up cycle will take at least an hour and sometimes much
longer depending on the plant size and on how long equipments/systems have been off.

Decisions on Plant Optimization
Plant optimization opportunities depend on a number of factors including CHP electric
and thermal capacity versus facility loads; the various plant production and use options
for electricity, heating, and cooling; the level and sophistication of plant metering, mon-
itoring, and controls; the data and analysis available (see Chap. 17); and the knowledge
and experience of the plant operators. For example, if a CHP has been designed and
constructed to be fully base loaded thermally and electrically all of the time, then there
are no decisions on plant optimization, as compared to CHP plant that needs to be
adjusted to varying electrical and thermal loads. On the other hand, for example, a
combined cycle CHP plant with duct firing can adjust the amount of steam production
and power produced in an STG. The more computer-tabulated and displayed metering
and monitoring that a CHP possesses, the easier it is to understand and optimize a CHP
plant (see Chaps. 17 and 18).