Paralleling generator systems

When designing generator systems, electrical engineers must ensure that 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 covers the need for backup power, code requirements, why diesel is preferred, generator ratings, and the benefit of paralleling generator systems.
  • Part 2 covers paralleling switchgear, their components, and common paralleling modes.
  • Part 3 covers installation considerations, interconnection with the utility, and generator sizing. Also, two existing parallel generator systems will be presented and their paralleling elements highlighted.

Expertise in generator power design for emergency, legally required standby, and business critical loads is an essential skill for an electrical engineer to master. When designing generator systems, electrical engineers must ensure that the generators and the building electrical systems can support the critical loads reliably and effectively. Building codes will dictate the prescriptive requirements for these systems (see Table). For business critical loads, the owner or client must be consulted to identify the nonemergency loads that require backup power. When the business needs outlined by the client require increased reliability, a paralleled diesel-generating system and electrical paralleling switchgear (PSG) typically are employed (see Figure 1).

Figure 1: The photo shows a paralleled diesel-generating system and electrical paralleling switchgear (PSG) for a large Las Vegas casino. Courtesy: JBA Consulting EngineersThis article examines standby systems in which generators serve as backup to the main utility source, such as those commonly installed in airports, data centers, hospitality complexes, water-treatment facilities, and in most life safety institutional applications.

The need for backup power

Interruptions of electrical power, even for a short duration, can introduce the potential for situations that could imperil public health and safety. Extreme weather-related disasters often disrupt power to hundreds or thousands of people and businesses, potentially for days. When these situations occur, they call attention to the vulnerability of the nation's electrical grid and the importance of alternatives. Hospitals, airports, data centers, water and sewage facilities, fueling stations, communication, and transportation systems require alternate-power sources to limit the impact and ultimately save lives during times of crisis. The loss of electrical power due to storms, natural disasters, or high-power-demand issues are increasingly common. The loss of business and the associated economic impact from power outages are significant. Emergency generators are necessary to provide the reliable power required to maintain operations during primary supply system failures. 

Why diesel-powered generators are used

Diesel-powered generators are considered among the most reliable approaches to providing backup power. When compared with alternative fuels and technologies, diesel-powered generators provide a steady supply of high-quality power and superior performance for transient or fluctuating power demands due to the high-torque characteristics of diesel engines (see Figure 2). Many international building codes and standards effectively require diesel generators for code compliance because of the need for rapid response time, load-carrying capacity, fuel supply and availability, and reliability. One of the most important and unique features of diesel-powered generators, as compared with other technologies, is quick response time and block-loading capability within seconds of normal source-power failure.Table: Emergency and legally required standby power requirements. Courtesy: JBA Consulting Engineers

NFPA 70: National Electrical Code (NEC), Article 517.30, as well as the California Electrical Code require hospitals and critical care facilities to have standby power systems that start automatically and run at full capacity within 10 seconds of power failure. Natural gas-powered generators generally are not acceptable as a source of power for generators due to fuel-source reliability. During disasters, such as an earthquake, gas lines are immediately turned off to avoid the risk of fire and explosion in case of a rupture. Lastly, diesel generators are available in a range of sizes to meet facility power needs.

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Anonymous , 12/15/16 12:58 PM:

Thank you for your interesting article.
I think it will be useful to also include in the Table the requirements of the NEC, which are less stringent than the IBC and IFC
MIKE , NC, United States, 12/16/16 08:41 AM:

I see an issue with this "10-second" requirement for hospitals. This 10-second requirement was probably in place 30+ years ago, when a small low voltage generator could power all of the emergency load, and all small generators could easily start in that time frame. Now that hospitals are much larger, and the critical/emergency loads are sometimes as high at 5MW or more, hospitals must parallel multiple medium voltage generators together to meet the critical/emergency electrical load. I have not seen MV generators that can parallel in 10 seconds. It seems that the “10 second rule” should be revised to match what a new large MV generator is capable of performing. Either that or hospitals may have to invest in multi-megawatt UPS systems that will come online instantly to bridge the time until the large generators can come online. Would be interested in any feedback on this topic.
Anonymous , 02/14/17 05:57 PM:

Mike, I certainly don't favor increasing the 10 second requirement. If it were me on the table, I'd want the lights back on as soon as possible. Fortunately there are effective strategies for designing large systems to deal with 10 second loads.

For starters, although large medical facilities can have large, multi-generator systems, only the life safety and critical branches are subject to the 10 second rule. These priority loads can be connected to the first generator available. The larger mechanical equipment loads can be added a short time later as additional gensets sync to the bus.

If the combined load of the life safety and critical branches exceeds the capacity of one genset, then the gensets can be subdivided into smaller sub-systems. For example a system of four generators where one is spare, can be broken into two pairs. This approach ensures that two gensets will be up within 10 seconds. After initial startup, the sub-systems can be synced and connected by a tie breaker so the redundant gen will be available to the entire system.
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