Page 227 - Sustainable On-Site CHP Systems Design, Construction, and Operations

200 De s i g n

ammeter and voltmeter; one selects phase for synchronizer; and one selects kilowatt
or kilovar reading for meter.
47-Phase-sequence voltage relay. This is a very high-speed relay that separates the two
power sources on undervoltage and excessive voltage unbalance. It prevents resyn-
chronizing of the generators with the utility until the utility is at normal voltage on
all phases and has the proper phase sequence.
50/51-Instantaneous and time-delay overcurrent relay. These are for three-phase protec-

tion and disconnection on overcurrent and fault-current flow in either direction.
Curves and settings are selected to provide coordination with other protective
elements in the system and to ensure rapid isolation of the CHP bus from the utility
upon occurrence of a problem not handled by other protective devices.
52-AC breaker. This is to open or close the circuit automatically as directed by the
relays, or manually if necessary.
59-Overvoltage relay. Senses ground fault in the medium-voltage impedance grounded
system (with delta primary) and opens the device 52 AC breaker.
67-Directional overcorrect relay. A very high-speed (within three cycles), three-phase
relay with a primary function of separating the CHP bus from the utility service grid

whenever the instantaneous current flow into the utility system from the generators
exceeds the full-load current of the CHP power system. This prevents a fault in, or
loss of, the utility system from taking more power from the CHP system than it can
safely deliver. After the separation, the protective relaying of the utility and CHP
systems will determine the location and type of malfunction and respond in the
proper manner. The fastest a utility system can reclose is about 12 cycles; this high-
speed relay operation plus the operating time of the circuit breaker should separate
the two systems in less than nine cycles.
81-Frequency relay. This is also a very high-speed relay that separates the utility and
generator sources whenever the bus frequency drops to less than 59.5 Hz (60-Hz
system). The engine governor will vary the speed of the engine to match the bus
frequency, which is determined by the utility system frequency.
The functions of utility connection relays, device functions 52, 50/51, 27, 81, 25, 59, and
43, are for specific protection at the utility and CHP interface bus. In addition, the
following relaying devices are specifically required for the engine-driven generators,
and their functions are as follows:

32-Directional power relay. This device senses the direction of power flow in the gener-
ator circuit. In this application it is usually referred to as a reverse power relay. Reverse
power is power flow into the generator which occurs when the prime mover loses its
driving force. Power into the generator drives it as a motor, which then rotates the
engine. Under these conditions, the generator must be disconnected and shut down.
Since circulating or synchronizing power flow will always occur upon initial connec-
tion of the generator to the bus, some time is required to permit the generator control
system to cause the generator to provide a forward power flow. Therefore, this relay
must have a time-delay function. The trip point of the relay must be set to ignore
synchronizing power, but respond to the lowest reverse power flow and duration
that indicates a motorizing unit. This trip point should be set at 2 seconds at reverse
power of about 8 percent of the forward power rating for diesel and natural gas