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.

12/09/2016


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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