Reliability considerations in simple paralleling applications

When a decision is made to use paralleled generator sets, many considerations need to be addressed to ensure a reliable system.

04/10/2013


Learning objectives

  1. Understand advantages and disadvantages associated with using paralleled generator sets as opposed to a single, larger generator set.
  2. Know the codes that govern generator on-site power generation systems.
  3. Identify best practices to maximize system reliability in simple paralleling applications.

Reliability in power generation systems, defined as the probability that power will be available at any point in time, is the primary reason standby generator sets are purchased. Using paralleled redundant generator sets is one method commonly used to enhance system reliability. Redundancy traditionally has been a requirement only in critical applications such as data centers and hospitals where an extended loss of power could result in loss of life or a substantial financial loss, as these were the only scenarios where the cost of a redundant generator and the associated paralleling switchgear could be justified. In recent years, however, the availability of lower cost power transfer devices and paralleling control systems has made redundant paralleled generators an attractive option in less critical standby power applications. 

The decision of whether to use a single generator set or multiple paralleled generator sets typically is based on reliability and cost. After a decision is made to use paralleled generator sets, many considerations must be addressed to ensure a reliable system. 

Reliability and redundancy

The purpose of redundancy is to eliminate a single point of failure from a system. While it is well documented that having redundant systems makes the overall system more reliable, this is based on the assumptions that single points of failure are truly eliminated and not just moved to another part of the system and that the controls enabling redundancy don’t introduce new failure modes that compromise reliability. Paralleled generator sets that rely on a single master control for signals to start and to close to a paralleled bus actually replace one failure point with two, as the master control and the communication link between the master and the generator sets each represent single points of failure. 

Investing in a reliable standby generator set and a robust maintenance program so the generator doesn’t fail is often a better investment than installing a more complex system to compensate for a failed generator set. 

Total system cost

In some instances, the cost of two small generator sets is less than the cost of one larger generator set. The total installed cost of the system is often overlooked in basic standby applications. Beyond the cost of the generator sets, the following factors must be evaluated: 

  • Foundation: A larger generator set may require additional structural support as its weight will be concentrated on one spot; however, smaller generator sets may require pouring multiple concrete slabs. 
  • Space requirements: Multiple generator sets and their associated switchgear will take up more space than a single larger generator set, although the smaller generator sets offer greater flexibility as they can be maneuvered into smaller spaces. 
  • Cabling: Smaller generator sets enable the use of smaller cables and easier termination. However, paralleled generator sets require additional cable runs, which is labor-intensive, particularly if cable is run underground. 
  • Commissioning costs: Start-up and testing costs of paralleled generator sets are substantially higher than those for a single generator set. 
  • Maintenance costs: Replacement parts for smaller generators are less expensive than replacement parts for a larger generator. However, that difference is more than offset by the labor costs of maintaining two generators and switchgear rather than one single generator set. 
  • Capital investment: When a facility’s power demand is expected to increase in the future, in some cases initial capital investment can be minimized by installing a smaller generator with the intent of adding paralleled generators in the future as demand increases. This will need to be balanced against the future investment required to add generators and switchgear and other required facility modifications. 

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