Power When You Need It: Parallel Generators Expand Options

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Curt Gibson  has been a PE designing emergency stand-by power systems for over 18 years.

CurtGibson-tnCURT GIBSON
Consulting Engineer

A technology trend is taking place that is increasing reliability and scalability of emergency power systems for data centers. Almost all major generator manufacturers have recently developed controls for their generators that allow them to parallel with each other without needing complex and costly paralleling switchgear. This advancement in controls has allowed smaller, more readily available generators to provide more redundancy and scalability than larger engine generator sets.

Ten years ago, price was no object when considering data center design. The world is now more competitive, and designs are being optimized to achieve the desired reliability at lower costs. By using these new trends, your architecture may change to yield more reliability at a lower cost.

Emergency Power

Emergency power is a complex aspect of data centers. Fear of power loss has always been a focus. Independent studies have shown that the root causes of failures are most often batteries, fuel, operator error and random causes. Overall generator starting reliability is commonly agreed to be 98 percent. Using N+1 would achieve 99.99% reliability which complies with the Uptime Institute’s Tier 3. When higher reliability is desired N+2 would achieve up to 99.9999 percent exceeding Tier 4. Should an engineer use an N+1, N+2, 2N, or 2N+1 system architecture? This should be determined by the resultant probabilities desired,  but can be easily achieved most economically using generators with integrated paralleling control systems that have been well proven and ISO certified in arrangements that optimize the reliability.

Traditionally, the difficulty of paralleling generators was resolved by avoiding it altogether, and choosing single large gen set solutions. Paralleling controls are often provided by third-party vendors in separate cabinets. This made coordination between the generator and controls risky, and integration difficult.

Cost savings and shorter lead times are achieved using commonly available engines and alternators. Larger engines and alternators are not as commonly available, and therefore more expensive. For a 600kW generator the approximate cost is $200/kW. For a 3MW generator the cost is roughly $400/kW. Increased cost of installation should also be considered, but is generally not as large a factor as some would expect.

Planning Generator Capacity

By adding generators modularly, sites can install what they need when they need it. Smaller generators are also easier to move to remedy constrained capacity issues. Some generator manufacturers include the switching on the generator also, making changes to the one-line faster and reduces risks.

Are your engineers considering the new technologies? Specifications and one-lines that are based on traditional paralleling and large generators do not often allow integrated paralleling. Changes in the specifications may be needed to allow an even playing field. Most manufacturers have engineers on staff that would be interested in talking with specifiers to review and suggest changes that might be made to allow these new technologies.

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