Reactive or resistive

Ships often involve complex propulsion and other onboard systems which must operate reliably for long periods, often many hundreds of miles offshore. Ensuring the availability of power for these systems is a critical function, making reliable testing of power supplies of particular importance.

While some systems have a degree of interdependency, most incorporate various levels of redundancy, which in normal running conditions involves the efficient power management of several duplicate size generators. The correct and comprehensive testing of these systems at the commissioning stage is vital to ensure trouble-free, efficient power generation and the safety of personnel. The same principles apply to commissioning and testing of back-up emergency systems.

At the heart of any ship’s power system is one or more gensets. The total package is completed by various discrete systems and components such as alternators, regulators and switchgear. These additional components typically come from various manufacturers, and are usually designed to interface with a number of makes, models, and sizes of generators. As with any other mechanical or electrical components, all are potentially subject to failure, and have varying maintenance needs, at the very least requiring regular testing and servicing.

However, individually testing a series of components never answers the most important question of all: “How do you know that your system—and not merely its components—will work when it counts?” In an actual emergency, a vessel’s entire emergency power generation system will be stressed. Unlike in a series of short, component-by-component tests, the system must operate at full power, with all components working together to do their jobs. The stresses introduced by this mode of operation cannot be simulated by discrete tests of a system’s numerous individual components: automatic transfer switches, switchgear, load-sharing centres, voltage regulators, alternators, electrical cabling and connectors, ventilation, cooling systems, and fuel systems.

While the generators may have been tested at the factory, the installation variables of the interaction with other parallel-connected power generation units (UPS), load profile, ambient temperature, humidity, fuel, exhaust, and cooling systems can be significantly affected by the installation. Therefore, a system-wide test is the only way to ensure that the individual components of any power generation system will work together harmoniously, whether for continuous production demands, or in an emergency power outage situation.

Sometimes commissioning engineers may only consider testing generator engines rather than the whole system. The most common form of testing is using a resistive load bank to run the prime mover, connected at the generator’s bus. However, this fails to replicate the actual stresses produced during real-world generator operation.

A resistive-only loadbank provides an electrical load (at unity power factor) which when applied to a generator converts and dissipates the resultant generated power as heat. This electrical loading will highlight individual engine problems, but resistive loads are only part of a ship’s total power consumption. Often, the influence of a lagging power factor (pf) <0.8 due to reactive loads is underestimated or ignored.

Generally only incandescent lights and electric heaters operate on a purely resistive load. These draw a steady supply of electricity from a generator, but do not 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 will not identify how well it will actually perform when exposed to the real reactive load pattern.

A reactive load test of an installation’s power system can accurately simulate response to a changing load pattern, such as would be encountered during a real power failure.

Resistive/reactive combination load banks are used to test the genset at its rate pf. In most cases this is 0.8 pf. The reactive component of the load will have a current that ‘lags’ the voltage. The resulting power is described in two terms, the kW, or real power, and the kVA or apparent power. The combination of resistive and reactive current in the load will allow for the full kVA rating of the generator windings to be tested. Even though the genset is producing more kVA, it is actually not producing more kW. The ‘real’ power (kW) required from the engine is essentially the same.

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

It is clear that reactive as well as resistive testing is more comprehensive and thorough than a resistive-only test, closely replicating conditions in a real power outage, and more readily identifying potential problems. Carefully managed reactive/resistive testing is essential to guarantee the operation of emergency power systems.

Aside from commissioning performed onshore, loadbanks do have other uses for vessels. Permanently installed loadbanks allow comprehensive testing to be integrated into maintenance procedures of emergency systems. Oversized generators can be cleared of any carbon build-up with regular sessions of full load application, ensuring reliable performance when called upon for particular production cycles or an emergency.