Power Generation Paralleling Solutions

Paralleling generators onto a common bus has been offered as a power solution for over 100 years.  The technique used has been the same since inception:  Synchronize an isolated source of energy to a separate (known as common) bus, then control the multiple sources in both kVAR and kW output, normally having each source proportionally support the connected load(s).  In addition, provide a protective scheme, normally through the use of protective relays to allow for safe and continuous operation of the paralleling system as connected to the user distribution system.

While the above explanation may be very basic, the generation and switchgear industries have provided solutions that are considered by many users to be complex, requiring advanced knowledge in control scheme layout, generator control systems and switching systems.  Because of these advanced designs, many users may be hesitant when choosing to use a paralleling system to support their power generation needs.  However, with the advent of digital control over the previous decade, current generator paralleling systems have evolved into cost effective integrated packages, providing users with a simple interface between themselves and what is now offered as a standardized control package, which in many cases is housed in one enclosure, no longer requiring ongoing technical and maintenance support to operate.

Because of this much simpler approach to paralleling, users can now utilize paralleling systems without concerns over complexity and, in many cases, achieve an unexpected benefit: Cost savings.  Now the question becomes “When should paralleling be used?”  This paper will identify some of the questions that should be asked, as well as what the industry can offer today in paralleling systems.

How large is the overall generation requirement? 

One of the advantages of paralleling includes the ability to provide large amounts of power to support the user’s needs.  Most manufacturers can offer between 2 to 2.5mW of power in a single diesel generator design and between 2 to 4mW of power in a single natural gas design, so paralleling is normally a standard approach with requirements above these levels.  So, if the system requirement is above 2mW, then paralleling will likely be offered as a solution.

Is generation redundancy a benefit?

A simple way to determine if this would be advantageous is to review the user’s loads and determine a) Are there loads small enough in size such that a single smaller generator can support the load (or loads) and b) Would generator redundancy provide the user the ability to continue operation, even with reduced capacity.  With modern paralleling systems being cost competitive against larger single engine systems (two 500kW generators with modern paralleling controls can be cost effective against a single 1000kW generator), system costs are no longer a limiting factor in providing paralleling equipment.  The specifying engineer can now weigh the option of paralleling as a viable solution for diesel system designs for 1000kW and larger, and natural gas system designs above 300kW in size. 

Is the budget limited for the generation system? 

The user can take advantage of a paralleling system by limiting the initial size of the system, while designing that smaller system for future growth.  Most manufacturers offer paralleling systems that can be “added” on to the initial generator offering, giving the user the ability to budget their generation project based on current equipment load profiles and monies available.  It is no longer cost effective to provide a large generator during the initial build of a project, with the thinking it would be more expensive to add additional generators in parallel in the future.

Where is the generation system physically located? 

In some metropolitan areas space is a premium, and a large generation system can take a physically large single area to install and support.  If the user has smaller areas (e.g. parking spaces in a parking structure, roof access with natural gas support, smaller unused areas around the facility) smaller generators can be installed and combined in parallel for the larger power need.  This can free up large spaces for other uses, and in some cases provide a large benefit to the user.

Today’s paralleling systems have evolved into integrated systems users can easily operate and maintain, allowing the full benefits of paralleling to be utilized, without the complexity and costs traditionally associated with these systems.

Dave Loba is the application engineer for standby power generation solutions at WPI.

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