Power Quality and Generators - Part 5: Paralleling Generators in Critical Applications

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Editor's Note

This is the fifth installment in an ongoing series that will cover basic engineering and code issues for standby generators. In a subsequent column, the author will address generator sizing.

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Standby generators play a key role in electrical distribution systems utilized in critical environments. Standby gensets can serve life-safety loads as well as legally required standby and optional loads as defined in Section 700 of the National Electrical Code. Generators can be paralleled together for redundancy or capacity. In addition, generators can be paralleled together to the utility service in either an open (break before make) or a closed (make before break) transition.

Here, I will summarize some of the key points and design criteria involved in designing a paralleled generator system for capacity, redundancy and reliability in critical operations.

Critical installations can be classified using the four-tiered system developed by the Uptime Institute, Santa Fe, N.M.( Tier 1 is the most basic and offers the least amount of redundancy, reliability and uptime. Tier 4 is the most complex, expensive and offers the most uptime, redundancy and reliability.

Tier 1 or 2 is adequate for simple server rooms or small data centers that primarily serve users within a facility or single location. Internet service providers and banking and financial data centers are typically Tier 3 or 4, because they are critical to the company's service continuity and stability, and therefore, must be more redundant and reliable.

With the advent of Tier 3 and Tier 4 data centers in the 1990s and early 2000s, engineers began specifying generator paralleling systems to increase the redundancy of the generators to N+1 and greater as required for these higher tiered data centers. Although a design can achieve the required redundancy from the generator system through standard automatic transfer switches (ATS) and the use of downstream dual-cord servers, or with the use of a closed transition paralleled utility and generator system, I believe a paralleled system offers many advantages.

These advantages include the following:

N+1 achieved. At less than 100% of design load, more than N+1 can be achieved with paralleling gear. For instance, if the total data center load is forecast to be 10 mW and five 2-mW generators are paralleled to achieve the (N) load, six generators would be paralleled together to achieve N+1. If the actual build out and usage of the data center were only 80% of the forecasted peak design load, the total load would be 8 mW. Four 2-mW generators could supply the load, so with six total generators, the system would achieve N+2 redundancy. Independent strings of generator backup systems would not be able to handle an outage of two generators without affecting critical loads even at 80% of design loading.

Required redundancy. Depending on the configuration of the electrical distribution system, a paralleled system can achieve the required redundancy utilizing less total generators than a system with individual strings of electrical components.

Simplified load bank testing. An additional spare breaker can be added to the lineup of generator breakers. The generator tie breaker to the utility board can be opened along with opening all the generator breakers except for the breaker feeding the generator to be tested. By closing only the breaker for the generator to be tested, all of the generators can be load bank tested from a single fixed load bank. This is a much simpler procedure than with a system comprised of individual strings of components.

Load bank testing is an important criterion for ensuring proper operation of critical generators. It is typically recommended to exercise the engine monthly with a minimum load of 30% of the standby power rating or loaded to recommended exhaust stack temperatures. Generators tested at no-load or loads less than 30% of standby rating for long periods of time can develop wet stacking or carboning caused by incomplete combustion and incomplete burning of fuel. This can potentially lead to generator malfunction and failure.

Absence of ATS. There are no distributed automatic transfer switches in the electrical distribution system. This reduces the total valuable floor space that is required for the electrical distribution.

Multiple utility feeders. The paralleling gear can potentially accept more than one utility feeder for additional utility service redundancy and "uptime".

Closed transition. The system can utilize closed transition (make before break). During testing and retransferring from the generators to the utility, the closed transition can eliminate the "bang" on the UPS batteries that can reduce the life of the batteries. The engineer must coordinate with the utility prior to adopting a closed transition system. Most utilities are concerned with these systems and will provide much scrutiny over the intended usage and relay protection of the system.

Soft loading transfer. When utilizing closed transition (make before break), the transition period can be programmed to last for several seconds. The frequency of the generators is matched to the utility under synchronization and paralleled operation. When transferring from the utility to generator, the frequency of the generators can be modified slightly during the parallel operation to slowly increase the load to the generators. This will reduce the heavy step loading on the generators during testing mode or a planned utility outage.

Rotatable. The generators can be rotated during usage. The (+1) unit can be rotated, thereby allowing for equal run times between all the generators.

Programmable priority list. A priority list of most critical to less critical loads can be programmed in to the logic of the paralleling gear. The control logic can be incorporated into downstream automatic transfer switched and/or building automation systems to turn off less critical loads in a situation where more than the redundant generators fail and can't support the entire load. Without this function, and the loss of more than the redundant generator, the entire generator system could go into overload and potentially fail outright.

Paralleling gear can also be utilized to provide additional capacity, by paralleling two smaller generators to meet the load that a larger generator would provide. This system, called "isolated bus," does not incorporate the utility feeder into the gear. The generator distribution is mated up with the utility power downstream at the ATS.

Isolated-bus paralleling gear can be utilized when the total load exceeds the capacity of commonly sized generators. Currently, few manufacturers offer 1,800-rpm, diesel-driven reciprocating engine generators in sizes exceeding 2,000 kW. For total loads in excess of 2,000 kW, a paralleled system can be utilized.

Even at a total facility load that does not exceed the capabilities of a single generator, a two generator system with paralleling gear can offer more reliability. In this situation, a failure of a single generator, will only take out one-half of the systems backup capacity, not the entire generator backup capacity. The control logic of the paralleling gear can be tied to downstream ATS or the building automation system to switch off some of the less critical loads with the loss of a single generator in a two generator paralleled system.

Additionally, one manufacturer indicates that generators sized 400 kW to 600 kW and less utilize standard diesel engines and are thus less expensive in terms of dollars per kW. Generators in excess of 600 kW utilize larger, less common engines and are more expensive in terms of dollars per kW. This manufacturer also claims that the reduction in the cost of the paralleling electronics makes a paralleling system more cost effective than a single larger generator. The same manufacturer's literature indicates a 20% to 30% cost savings and a reduced construction time frame when utilizing the paralleled smaller units. In addition, paralleled systems with smaller generator units can have the flexibility of modular build out with time.

Advantages and disadvantages

Although isolated bus paralleled systems do not offer some of the advantages of full utility- and generator-paralleled closed-transition systems, they offer more flexibility than non-paralleled systems.

Closed-transition paralleled generator systems can provide many advantages, especially for large and power dense applications. There is a legitimate concern in the industry relating to the single point of failure that the utility and generator closed-transition paralleling gear can represent. This concern can be minimized by providing a tie breaker between generator systems that will allow for one of the generator system paralleling boards to feed through the other generator system paralleling board in the event of a single generator system board failure.

Also, for projects incorporating a phased modular build out design, utility and generator paralleling gear can represent extra upfront cost in the first phase of the design.

In addition, there is typically extra expense involved in the planning, engineering coordination and supplementary relay protection that is normally required when adopting a closed transition paralleling system.

To sum up, a closed-transition utility- and generator-paralleled system has its pros and cons over other methods of providing additional generator redundancy. A good understanding of the available systems is required to provide the most cost effective, efficient and reliable system for a specific design application.

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