along with a status and alarm panel. Mounted inside the enclosure are the automatic starting module, engine governor, automatic synchronizer, protective relays, and breakers. The rear of the enclosure includes the system bus, generator potential, and current transformers. Main generator cables may enter the top or bottom of the cubicle. For medium-voltage switchgear, the breaker and bus section is usually separate from the control section.

• Master control section. The master cubicle houses controllers for engine generator set starting, loading, running, and load shedding. Both automatic and manual controls are provided. A synchronizing panel is usually provided to assist an operator in the manual mode.

• Utility control section. This section may contain the utility breaker, as well as automatic synchronizer, import/export controller (if used), and necessary control circuitry. Utility metering, switches, and protective relays are mounted on the door.

• Distribution. Distribution equipment includes circuit breakers, fuses, automatic transfer switches, and other devices that electrically link the power source to building loads.


Discriminative protection is required in all power systems to quickly isolate faulty circuits, while maintaining continuity of service to unfaulted circuits. The protective relaying and control system must be designed to function automatically in a safe, logical sequence to protect generators, the utility system, and host facility equipment. If the fault remains connected, the whole system may be in jeopardy. Protective systems can be complex, especially if the generators operate in parallel with the utility.

The switchgear contains circuits that sense operating conditions and act to prevent damage to the prime mover. For example, in the case of a reciprocating engine, the circuits would maintain surveillance on such parameters as oil pressure, water temperature, oil level, water level, exhaust temperature, engine speed, and direction of power flow. When any of these parameters goes outside a preset limit, these circuits would initiate immediate disconnect and shutdown of the prime mover. Some of these circuits have two-stage limits. The first stage would cause a warning to be sent to the operator, allowing for sufficient time to intervene before the second stage is reached so that unnecessary service interruptions can be avoided.

Circuits are included for protection of on-site loads and conductors as well. All electric power systems include overcurrent protection of a power feeder at the point where that feeder receives its power. The trip element continuously measures the amount of current flowing in the conductor. When that current exceeds a preset value for a specific time, the circuit breaker opens to disconnect the feeder from its source.

When a generator operates in parallel with a utility-derived power source, the switchgear must incorporate protective circuits that separate the two power sources upon occurrence of an abnormal operating condition. Because the power bus is common to the generator and the utility system, when these systems are operating in parallel, voltage and frequency protective circuits are ineffective. Typically, current and power flow directions and magnitudes are the best means of detecting faults and initiating protective actions.

Most utility system faults are cleared with automatic breaker operation, and the facility system must be protected against the effects of automatic closing. When a very large facility generator remains connected at the time the utility breaker is attempting to reclose, the generator can be connected out of synchronization, with damaging overcur-rents. If there is more than one utility feeder, the controls must ensure that utility tie breakers do not disconnect the generator and reconnect it in an out-of-phase condition. Bus-tie circuit breakers are often used when there is more than one utility feeder.

For synchronous generators, the prime mover governor is controlled to maintain power flow from the generator into the power system and the voltage regulator is controlled to maintain proper VAR sharing. Protective relaying detects changes in current or power magnitude and direction, voltage, frequency, and system impedance. In most cases, when these changes exceed predetermined limits, the relays send a signal to trip the main breaker and disconnect the generating system from the utility system.

Somewhat less protective relaying is required for induction generators. In smaller systems, undervoltage and overvoltage protection may be adequate, though additional relays, such as under- and over-frequency, are sometimes required.

Figure 27-3 is a simplified one-line diagram of a system having the on-site generators capable of paralleling

Fig. 27-3 Line diagram Showing Circuit Breakers on a System with Localized Generators Capable of Paralleling with the Utility Grid. Source: Zenith Controls

with utility. The diagram shows the utility circuit breaker 52-U, the generator circuit breakers, and the building distribution system. For the generator to parallel with the utility system, relaying must be used to trip the utility breaker under loss of power or fault conditions. Figure 27-4 is a one-line diagram illustrating commonly used protective relays for protection and control of the utility breaker. Figure 27-5 is a one-line diagram illustrating typical protective relays used on a medium voltage generator.


25 Synch Check Relay 32 Reverse Power Relay 47 Phase Sequence Undervoltage Relay

51 Timed Overcurrent Relay 81 Frequency Relay

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