In the Know

The Case for Reactive Load Bank Testing for Stationary Engines | Part 3

A reactive load bank test of a facility’s power generation system can actually simulate the system’s response to a changing load pattern:

Resistive/reactive combination load banks are used to test the engine generator set at its rated power factor. In most cases this is 0.8 power factor. The reactive component of the load will have a current that “lags” the voltage. The resulting power is described in two terms: the KW - the real power and the KVA - the apparent power. Since the current lags the voltage in the reactive load the total power is not the direct sum of the two, but their vector sum. That vector is the phase angle difference between the voltage and the current. The combination of resistive and reactive current in the load will allow for the full nameplate KVA rating of the generator windings and voltage regulator excitation to be fully tested. Even though the generator set is producing more KVA it is actually not producing more KW. The “real” power, or horsepower, required from the engine is essentially the same.

Unlike resistive testing, a proper resistive/reactive test will create conditions in the equipment being tested that will more accurately simulate those it would experience during a real power failure: a facilities manager can manipulate the power factor to simulate a real power outage. Only reactive load testing can verify the true performance of a genset’s voltage regulator: the voltage regulator is not fully challenged in a resistive only test (the voltage regulator assists the system to recover quickly from large step loads).

The inductive loads developed during reactive testing illustrate how any given system will handle the voltage drop in its regulator, which is paramount when paralleling generators. This inductive load can verify that a system’s voltage regulator is working properly, and only a reactive test can provide it—if the system’s voltage regulator is not working properly, its magnetic field could collapse, rendering the generator useless and preventing other generators in the system to operate in parallel. Resistive/reactive testing can also reveal additional stresses (and predict pending failures) of a system’s switch gear, alternators, and other systems that resistive-only testing cannot.

The Case for Continued Testing

A proper resistive/reactive test of a system’s complete performance with help from a knowledgeable service provider is a facility manager’s only means of truly verifying that his system will operate to an acceptable standard during an emergency. Only after observing and rating a system’s performance, and proving that its various components are able to operate in synch and produce power for a sustained period under the specific stresses produced by the facility, can a facility manager know with confidence that the business is safe from the various threats posed by a sustained interruption of utility power service.

But a proper service regimen doesn’t end there. Like all modern electric systems, emergency power generation equipment must be tested and maintained at proper intervals, and its individual components must also be tested.While a system-wide, reactive load test of a facility’s emergency power generation equipment will determine its ability to function correctly under the stresses and load requirements of actual emergency operation, this testing is not a replacement for testing each component in the system’s manufacturer-recommended service intervals.

Unfortunately, these components still require maintenance and monitoring but when combined with a reactive load, system-wide test, they can accurately forecast a system’s performance and identify its service needs well into the future. But there is no substitute for routine maintenance and testing: it is the responsibility of all personnel charged with monitoring a backup power system to see that all its maintenance needs are consistently met.

A Word to Facilities Managers About Service Providers

Despite what would appear to be proper planning, many facilities managers are taken by surprise when their power generation equipment fails to properly operate during an actual emergency. Although these managers go out of their way to test and maintain their equipment, failure to initiate proper system-wide, resistive/reactive testing of their facility’s equipment leaves them unaware of their system’s weaknesses, and in some cases, these managers are taken by surprise when their equipment fails. Partnering with a responsible and knowledgeable service provider (who is up-to-date on the latest developments in electric power generation technology) is the best way for a facilities manager to prevent such surprises.

Talk to your service provider about your options for a long-term service contract that includes reactive load bank testing of your entire generation system. Your provider should be able to recommend the proper intervals at which you should schedule this test, and may be able to work with you to plan a cost-effective service contract that includes comprehensive, full-system resistive/reactive testing in addition to the tests and maintenance required for the individual components in your system. You might be surprised at how inexpensive these services can be when combined, and how expensive they can be when ignored!

In addition to helping to ensure that your emergency power generation system meets your facility’s current power needs during an emergency, your service provider should be able to help you plan equipment purchases as your business—and power consumption needs—grow. Partnering with a knowledgeable and competent provider is an investment in your company’s future that will surely pay off when the lights go out... Or when they don’t.

Questions for Your Service Provider

  • Where will you place the load bank?
  • Is there ample room in the switchgear to connect the load test cables?
  • How will the test cables be routed? How long should the load test be?
  • When is the most convenient time for the test? 
  • Is there a contingency plan for EPG during the test?
  • What are the parameters for the test?
  • Do I want a resistive-only or a resistive/reactive test?


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Chris Giannaris
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