Paralleling generator systems—part two: switchgear

When designing generator systems, electrical engineers must ensure that the generators and the building electrical systems that they support are appropriate for the specific application. Whether providing standby power for health care facilities or prime power for processing plants, engineers must make decisions regarding generator sizing, load types, whether generators should be paralleled, fuel storage, switching scenarios, and many other criteria.


This article is peer-reviewed.Learning objectives

  • Learn best practices for paralleling generators, touching on dependability, cost savings, efficiency, synchronization, and other aspects.
  • Know the requirements for emergency, standby, and backup power loads.
  • Explain the benefits of parallel power-generation systems.

Editor’s note: Because of the extent of this topic, this article is divided into three parts:

Part 1: The need for backup power, code requirements, why diesel is preferred, generator ratings, and the benefit of paralleling generator systems.
Part 2: Paralleling switchgear, their components, and common paralleling modes.
Part 3: Installation considerations, interconnection with the utility, and generator sizing. Also, two existing parallel generator systems will be presented and their paralleling elements highlighted.

Figure 1: This photo of 480 V paralleling switchgear (PSG) shows generator system control cabinets with an HMI screen, alarm indicating lights, local breaker controls, metering, and mimic panels. Courtesy: JBA Consulting EngineersAs defined in Part 1, “Paralleling generator systems,” in the December 2016 issue of Pure Power, paralleling is the operation in which multiple power sources, usually two or more generators, are synchronized and then connected to a common bus. Paralleling switchgear (PSG) is a combination of protection, metering, control, and switching elements acting as an integrated system to control the distribution of power for the following systems:

  • Emergency systems
  • Legally required standby systems
  • Critical operations power systems (COPS)
  • Business-critical systems, also referred to as optional standby systems.

PSG can be deployed at various voltages. The voltage selected is a function of system capacity, standard voltages for the region, and utility voltages. The systems can be of switchboard construction, metal-enclosed construction, or metal-clad construction. The applicable standards depend on the voltage and type of construction.

For low-voltage PSG:

For medium-voltage PSG:

PSG components

PSG consists of breaker compartments and control compartments (see Figure 1). These can be integrated into one lineup or installed in separate enclosures. The latter is preferred to lower the arc flash hazard risk category. System operators spend most of their time near control compartments; therefore, the recommended approach is to locate the controls compartments outside the arc flash boundary of the breaker compartments.

Paralleling system breakers: Regardless of the layout of the switchgear, the following are used to label basic breaker compartment requirements for parallel switchgear application:

  • Utility main breakers—breakers directly fed from the utility source
  • Generator breakers—breakers directly fed from the generator source
  • Generator main breaker—breakers that tie the generator bus to the load bus and are used for transfer between the two sources (generator source or utility source)
  • Load feeder/distribution breakers—breakers that directly feed loads or transfer switches
  • Tie breakers—breakers between similar source breakers. These could be the load-bus tie or generator-bus tie breakers.

Figure 2: The photo shows a typical 15-kV paralleling switchgear (PSG) lineup. Courtesy: JBA Consulting EngineersProtection: The protection requirements for PSG depend on the size of the generators, system voltage, and the required level of system reliability. Low-voltage circuit breakers with built-in trip units provide the protection necessary for low-voltage feeder circuits. Utility and generator circuits require a more sophisticated protection scheme than low-voltage circuit breakers with integral trip units can provide; therefore, breakers with separate multifunction protective relays are provided. For medium-voltage breakers, relaying systems normally are employed to provide the necessary protective measures for all breakers (see Figure 2).

These protections include:

  • Generator protection. Generators normally require the following protective functions as indicated by ANSI device numbers 25, 27, 32, 40, 46, 49, 50P, 50/51G, 51V, 59, 67G, 81O/U, 87G, and BF (breaker failure). (See Table “ANSI device numbers/description.”)
  • Generator main and generator bus tie breaker protection. Utility circuits normally require the protective functions provided by the device numbers 25, 27, 32, 50/51P, 50/51G, 59, 81O/U, and BF.
  • Incoming utility protection. Utility circuits normally require the protective functions provided by the device numbers 25, 27, 32, 50/51P, 50/51G, 59, 67, 81O/U, and BF.
  • Feeder and load bus tie breaker protection. For feeder circuits in medium-voltage systems, minimum requirements are 50/51P, 50/51G, and BF.
  • Failure to trip. Backup protection must be provided for the case where a breaker fails to operate when required to trip (breaker failure). This protection consists of a current detector in conjunction with a timer initiated by any of the protective relays in the generator zone. If the detector shows that the breaker has not opened by the time the specified time delay has passed, the breaker failure relay will initiate, which trips the backup breakers.

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