Reliability considerations in simple paralleling applications



Paralleling systems are frequently expanded after they are put into service to accommodate increasing power demands. The ability to add a generator set and the associated switchgear in the future should always be considered. The system should have the flexibility to allow generator sets from a different manufacturer to be added in the future. Being locked in to a certain manufacturer limits the flexibility for future expansions. Additional question to ask concerning expansion include: 

  • Are the generators properly isolated so that new generators can be added without taking the facility off line?
  • What is involved in modifying the control for expansion? If a different manufacturer’s generators are used for the expansion, what will be required for the generators to parallel properly?
  • Does the manufacturer have experience in implementing field expansions, including expansions that include generators from other manufacturers?
  • How can the system be modified to support utility paralleling if that is required in the future? 


The decision on whether to provide backup power with a single generator set or with redundant paralleled generator sets will be based on reliability and cost. The key question is whether the redundancy coupled with the added complexity of a paralleling system increase the system reliability enough to justify the additional cost. When a decision has been made to parallel generator sets, the following should be addressed to maximize the reliability of the system:

  • The control system should be designed with critical functions distributed to the individual generator controls to minimize single points of failure.
  • The controls should have fault tolerance provisions such as load shed and manual operating modes.
  • The installation must meet code requirements for coordination, separation of circuits, and 10-second service to emergency loads and must allow proper isolation of the generator sets.
  • The system must be supported by an organization with a proven track record for servicing complete paralleling systems.  

Rich Scroggins is a technical specialist at Cummins Power Generation. Scroggins has been with Cummins for 20 years in a variety of engineering and product management roles and is active in IEEE 1547 and NEMA SC 16. He received his bachelor’s degree in electrical engineering from the University of Minnesota and an MBA from the University of St. Thomas.

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Anonymous , 05/08/13 10:25 AM:

Excellent Article.
VISH , CA, United States, 05/08/13 12:53 PM:

Some of the increased cost of the parallel system can be off-set by Peak Load shaving technique. Illustration of Pros and cons of the system will greatly help the engineers to make the right decision.
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