In the Know

The Case for Reactive Load Bank Testing for Stationary Engines | Part 2

Facilities managers and business owners are occasionally hesitant to conduct a test of their facility’s emergency power generation systems (as opposed to just testing their system’s engine) because this sort of comprehensive testing is substantially more involved (and expensive) than a simple test of the engine. Power systems must be interrupted. A service provider must complete arduous and complex attachments to various points in the facility’s power system to test it, both as a collection of several discrete units and as a whole.

However, a system-wide test is the only way to insure that the individual components of any emergency power generation system will work together. And unlike an actual power emergency, facilities managers can schedule a test of their complete backup power generation system at their convenience. Despite the inconveniences it represents, only a complete system-wide test will provide an accurate picture of how a facility’s emergency power generation equipment will perform during an interruption in utility power, particularly in the actual conditions under which the system will operate.

While the generator may have been tested at the factory, the installation variables of the interaction with other parallel-connected power generation units, consumer’s load profile, altitude, ambient temperature, fuel, exhaust, and cooling systems can be significantly affected by the installation. The on-site acceptance test, typically done on a new installation, is a valuable tool for the engineer, building owner, and can be done periodically by maintenance personnel to determine the operating capabilities of the generator in its installed location and many years after installation, to verify that the systems continue to perform reliably as designed.

Resistive Testing: Only Part of the Story

Facilities managers typically only test their generator’s engine(s). The most common form of testing is to use a resistive load bank to run the engine, but this fails to account for the actual stresses produced during real-world emergency generator operation. Facilities managers seldom attempt to simulate real-world conditions by conducting periodic system-wide tests of their generation equipment primarily because they don’t always know this service is available.

In a typical test of a facility’s emergency power generation equipment using a resistive load bank , the device develops an electrical load (at unity power factor) and applies it to a standby generator set, which converts or dissipates the resultant power output as heat. To perform the test, a service provider hooks up the loadbank at the generator’s buss. The service provider who performs the test will diagnose engine problems (and identify potential problems) with the engine tested individually. But this resistive testing does not provide a complete picture of a facility’s preparedness in an emergency because resistive loads are usually only a small part of any facility’s total power consumption. Quite often, the influence of a lagging power factor <0.8 due to reactive loads is underestimated.

Generally, in any facility, the only equipment operating on a resistive load are incandescent lights and electric heaters. These units draw a steady supply of electricity from a generator but they don’t produce the large block loads that truly test a generator’s performance. A resistive load test will verify that a generator’s prime mover is working, but it won’t identify how well it will perform when stressed in an actual emergency.

Imagine a doctor attempting to ascertain a patient’s health using a treadmill: verifying that the patient can walk provides little information to the doctor—to be considered healthy, the patient must be able to run at a steady pace (e.g. full load simulation) and climb (reactive load simulation) simultaneously.

Brownouts, blackouts, hurricane storm surges, strong winds, ice storms, tornados, and grid interruptions can all disrupt utility power supply to a facility, and resistive-only testing cannot simulate the conditions in which the facility’s equipment will operate.

The same can be said of emergency power generation equipment: in order to provide adequate “insurance” against lost power, it must be able to keep a facility fully operational during an interruption in utility service, which involves substantially more than supplying heat and light.

To continue our analogy, by only testing a generation system’s engine, one has only verified that the patient’s legs work, not that he can use them properly. In order to properly insure that an entire system will operate when it matters, the entire system must be tested as it would operate under normal circumstances: resistive load testing is insufficient and only part of the simulation of the real load situation.

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