Specifying paralleled generation systems

Learn about important factors related to specifying onsite generation systems, focusing on paralleled generators.


Learning Objectives

  • Gain an awareness of tools available to help the engineer specify paralleled generation systems.
  • Develop a framework for the steps required to specify paralleled generators.
  • Review a few commonly encountered technical specifics related to paralleled generators.



Progress in developing countries, the proliferation of distributed generation, and an ever-increasing focus on power system reliability continue to stimulate demand for onsite power generation. Grid modernization and technological advances in the industry are enabling these systems to be applied in ways not previously common.

From interruptible service agreements to microgrids, the applications of engine-generators have diversified significantly in recent years. As such, sound judgment and fundamentals on the part of the consulting and specifying engineer are as important as ever.

Paralleled generator systems

Benefits of paralleling generators in onsite power systems are well documented. First, multiple generators in a system can increase reliability. Even if a system does not have fully redundant generation, load-prioritization schemes keep critical loads powered if a generator set fails. Next, generators can operate closer to their optimal load with demand schemes that take paralleled units offline when they're not needed. Further, systems with multiple generators are more scalable, facilitating system expansion and capital project flexibility. Also, smaller units can have faster delivery times. Finally, paralleled units offer greater operational flexibility, facilitating routine maintenance.

Creating thoughtful design criteria helps convert the client's goals into high-level technical requirements. These criteria establish design philosophy consensus early in the project, which can dramatically reduce rework down the road. This document is used throughout the project and defines the rest of the design process.

First, the design criteria outline the client's commercial goals. For example, will they use the generation for interruptible service agreements where the utility provides financial incentives in exchange for generation support? Will they be operating in a baseload or peak-shave mode to help minimize demand charges? Or will the system be focused strictly on emergency or standby power?

Answering these questions is critical for the genset supplier and the design of the electrical distribution system. Also, these considerations define code and environmental requirements to which the project will be exposed. For example, NFPA 110: Standard for Emergency and Standby Power Systems has specific requirements for things like start time for emergency power systems. Also, the U.S. Environmental Protection Agency can have more stringent pollution-control requirements for generators that operate while normal power is available.

Next, the engineer should establish the critical loads and the reliability required. In some cases, not all loads are critical. Leveraging this fact can increase the overall reliability of the system, especially with paralleled generators. For example, if a single generator fails, noncritical loads can be shed or not added in the first place. This capability can essentially establish N+1 redundancy for critical loads, even though the overall system isn't fully redundant. Conversely, if every load connected to the generation is essential, it's crucial to know this early so the appropriate design precautions can be taken.

System reliability is a whole science by itself. Tools such as the IEEE 3006 Power System Reliability Standards can be very handy to study and quantify the reliability of different design concepts.

In addition to reliability, anticipated future expansion should be considered. This can establish spatial constraints that may otherwise not exist. For example, does the design need to leave room for an additional generator or does room need to be left for additional sections of switchgear? Does the electrical system need spare capacity for additional generation? Incorporating these things into the design from inception will result in a much cleaner final build-out.

Figure 1: This one-line diagram is configured in an isolated bus configuration. All graphics courtesy: Stanley Consultants. Finally, applicable codes and standards should be compiled. Nothing is worse than finding out that a local code requires something beyond NFPA 70: National Electrical Code (NEC) requirements after construction has begun. Performing a review of applicable codes and standards at the beginning of the project can help the engineer refresh their memory and highlight pitfalls before design work begins in earnest. These can act as valuable guides during design. Also, the client's insurance requirements should be reviewed to see if, for example, FM Global requirements are more stringent than the NEC.

Generation capacity

After the design philosophy has been established, the required capacity for the generation system can be determined. This exercise requires a review of the facility's load profile. If the generators will serve an existing system, actual metering data is very valuable here.

If this data is not available, however, load lists can be compiled to quantify the load. The designer must consider the expected harmonic content of the system, which may require generator derating. Also, starting requirements for large motors must be evaluated to make sure the alternators and exciters can provide the reactive power required. Note a general rule states that a generator should be sized 2.5 times larger than the largest motor it is required to start. Generator manufacturers have online software tools that can aid this effort.

Depending on the type of system, there are NEC requirements that must be considered as well. For example, generation systems are exposed to different requirements when they're supplying emergency, legally required standby or optional standby loads. Specifically, when supplying emergency loads, NEC Article 700.4 states, "An emergency system shall have adequate capacity and rating for all loads to be operated simultaneously." Alternatively, Article 702.4 allows for optional standby-capacity calculations to be made in accordance with Article 220. When demand factors are applied, this can offer some capacity relief.

While the criteria mentioned above will help the designer ensure enough capacity is provided, it is worth noting that having too much capacity can be a problem as well. This can expose the generator to problems such as wet stacking or fuel stagnation.

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