Testing the Limits

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There is a problem with having backup gene­rators. The problem is that the operator may not know if the generator is capable of meeting necessary power demands in an emergency situation. What is needed is a device that allows for the testing of a simulated electrical load. Load banks are just the right devices.

A load bank is a self-contained testing unit that includes both electrical load elements and operational controls and auxiliary devices necessary for operation. By simulating the real demand load from a neighborhood, commercial enterprise, residential unit, industrial center, or critical emergency facility (such as a hospital, fire station, or police department), the load bank serves the electrical power source by testing, supporting, and protecting it.

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In particular, a load bank allows for the independent testing of generator sets (either turbines or diesel reciprocating engines), the emergency backup power sources that are used when the main electrical supply grid goes down. The types of testing performed by load banks are diverse and extensive, including, but not limited to:

  • The quality control testing of turbines and diesel engine generators performed in the factory prior to shipping the unit to a customer
  • The analysis and reduction of “wet stacking” problems (the oily exhaust from the diesel engine generator’s exhaust pipe that results from not having all of its fuel efficiently burned) that can occur in diesel engine generators running at light loads
  • The standard testing at regular intervals of emergency backup electrical power sources to ensure that they still function properly
  • Ground power testing to measure the ground resistance for an earth electrode system to ensure that resistance does not increase over time
  • Adjusting the output of a power source by optimizing the load in prime power applications
  • Running the generator to allow for cleaning and removal of carbon that may have built up in the diesel engine generator’s piston rings
  • Ensuring the proper functioning of critical support systems such as air conditioning for data center and server facilities during emergency blackouts
  • Performing load rejection tests to ensure that a system can withstand the shock of a sudden loss of load and still return to normal functions using its internal governor
  • Ensuring that load optimization in primary power system applications by simulating the load that the system will have to supply

Load Bank Design and Installation
So how does a load bank simulate the demands of an entire industrial plant, commercial operation, emergency facility, or residential development? This is a question of vital importance, since emergency backup gensets will be mothballed for extended periods of time. Because of this, the system operator can never be completely sure that they will be ready when needed. It is not practical that an entire system be shut down even for a short period of time to allow for the testing of a backup power supply, and it is extremely risky for an operator to test the genset’s capabilities, exactly as it is required to operate. This is where load banks prove their worth.

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By definition, as an auxiliary device to the main power supply system, a load bank is a device that simulates and applies an electrical load to a power source. In practice, this means a load bank is configured to simulate the electrical load demand of the facility in danger by a general power outage. During a test, the backup genset is used to supply power to the simulated demand load of the load bank to evaluate its performance. By closely mimicking the actual system load, the load bank allows for a near real-world evaluation of the emergency genset’s operational capabilities.

There are several types of loads that can be evaluated. The simplest and smallest are those that test batteries. These battery discharge testers will evaluate battery capacity under a variety of simulated load conditions. Laboratories also use load banks to test critical equipment, especially high-tech aerospace power generation applications. At the large end of the scale are those that can test both alternating current (AC) and direct current (DC) power source for large-scale uses. These include the simulation of major electrical components such as breakers and relay stations.

A load bank is internally configured to simulate real electrical demand loads, receive power from the generator being tested, and then convert and dissipate this power in the same manner and with the same levels of resistance and power consumption that would result from use by the actual facility. The generator can then be operated as it would in actual use to see if it can meet the demand and overcome the resistance to the system being modeled by the load bank—and can do so in a controlled fashion.

In the real world, the actual electrical power demand will fluctuate (often unpredictably) with time of day and variations in operational demand. A load bank can be fully controlled by the operator to simulate a wide variety of extreme electrical demand scenarios. By testing the genset’s operational characteristics, a load bank not only ensures that the genset can meet demand at a critical moment, it also operationally protects the genset—which triggers the question of how to best configure the pairing of load banks and gensets.

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Credit: Eagle Eye Power Systems
Load bank testing in action

Gensets and Load Banks
What is the relationship between a genset and its load bank? A genset is an electrical power generator. Though primarily used for emergency backup power, they can be used as primary electrical power sources in off-grid applications. They are often small enough to be portable, but can also be large, fixed facilities. They can derive their power from either turbines or reciprocating diesel engines.

In any configuration, size, or application, gensets consist of three main components. The first is electrical output provided by the actual generator. This is a dynamo (a bundled coil of electrical wire set spinning relative to a permanent magnet) that converts mechanical energy into electrical energy. Though it actually comprises only one of three components, the generator usually provides the name of the entire unit. The second component is a reciprocating engine or spinning gas turbine that provides the source of the mechanical energy. The engine or turbine is mounted directly on the generator by means of a drive shaft that delivers the mechanical motion to the spinning dynamo. Lastly, there is the fuel that provides the mechanical energy that drives the engine. A generator is a device that converts mechanical energy into electrical energy, and an engine or turbine converts chemical energy into heat energy and then into mechanical energy. The fuel is the source of the initial chemical energy. The fuel used is usually diesel for reciprocating engines and natural gas for turbines, though other fuels (propane, biodiesel, hydrogen, etc.) can also be used.

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Credit: Eagle Eye Power Systems
Load banks come in a variety of sizes.

Typically, smaller, mobile gensets are single-phase generators, while the larger, fixed facilities are three-phase. Single-phase is normally for residential or small commercial use. Mixed commercial loads include motors, transformers, and equipment—in addition to basic heating and lighting. Three-phase is primarily used for industrial applications. In addition to its three main components, a genset includes a governor (to regulate its mechanical speed), a coolant system (with coolant liquid and radiator to vent off excess heat), exhaust vents for burned fuel, a lubrication system (with pumps and lubricant reservoir), automatic starts (electrical jump-starting for smaller gensets and compressed air activator for larger models), and voltage regulator (to prevent voltage from rising to a potentially harmful level). Other regulators control and limit the amount of fuel and air being fed into the engine intake. Prior to hooking up the emergency backup genset, the main service load needs to be disconnected. This can be done manually for smaller mobile systems, while larger fixed systems utilize automatic switches that simultaneously disconnect the main service load at startup.

So, in this arrangement, a load bank’s primary function is to serve as a testing apparatus for the genset to ensure that it will function properly when the time comes to put it to use. This is a function that is usually required by local electrical code. A whole range of operating loads needs to be tested because a genset may function properly under a light demand load but fail when required to meet a heavier demand. This heavier demand will most likely coincide with a critical application, such as a safety system or medical application. This ability to vary loads will also allow a load bank to test the full operational range of the load system, while accurately simulating its operations.

By developing an electric load, a load bank receives, converts, and dissipates as heat the electrical power generated by the genset. Instead of using the electrical power to perform work, a load bank utilizes the electrical power to perform tests. The testers apply variable and discrete electrical loads, and then measure the endurance of the generator’s responses. The resultant feedback allows the testers to adjust the generator and calibrate its controls. To perform these tests, a load bank needs to consist of both resisting load elements and the controls needed to adjust the test load and accurately simulate the actual system load. And load banks are not just used to test AC power created by the genset. They can be designed to test multiple types of power sources—hydroelectric dams, wind power farms, solar energy PV arrays, internal aircraft power generators, DC power generated by batteries and fuel cells, inverters that convert DC to AC current, etc.

The kinds of testing performed by load banks can be as varied as the number of power sources they can evaluate. Manufacturers can use load banks to test backup power generators by mimicking the variable electrical demand loads of entire factories. The generators themselves can be tested at the factory to allow for fine-tuning and adjustments prior to shipping. The same factory quality control process can be applied to gensets consisting of reciprocating diesel engines with adjustments made to the engine performance. This testing can also be performed to prevent excessive exhaust from wet stacking. Outside the factory, field testing is performed on a regular basis for emergency backup gensets, allowing for adjustment prior to critical uses. Testing can also be performed in accordance with National Fire Protection Association standards for generator operations. Lastly, there are non-critical and
non-performance demonstration tests conducted as part of genset sales efforts.

Types of Load Banks and Their Applications
But how exactly does a load bank perform these tasks? What are they made of? How do they work? The three basic types of load banks are resistive, inductive, and capacitive.

The first type, resistive load banks, are the most commonly used. As suggested by the name, the load applied by a resistive load bank consists of resistors that transform the electrical power into radiant heat. This type of load bank is serviced by a coolant system that carries away and radiates this heat via coolant systems that utilize either water or air as a working fluid. The individual resistors are arranged to provide a level of resistance equal to that of the primary service load to which the genset must supply power. Their relatively simple configuration lends them to testing fixed loads such as continuous DC discharge from electrical storage batteries or steady power production from a genset. They match resistance loads to the amount of power supply. If a battery or generator produces 100 kW, then the resistive load bank supplies a load demand of 100 kW, allowing it to accept the entire energy produced by the power source (less the amount of energy spent on waste head carried away by the generators cooling system, the energy required for controls and instrumentation, and the energy lost through venting exhaust).

The second type, the inductive load bank, is used to augment a resistive load bank. Inductive loads are lagging (current lags voltage). This type of load bank is configured to a lagging factor load. The lagging power factor indicates that a load is inductive with a negative power factor. This inductive load is created by iron core reactive elements that create magnetic inductive loads, and is typically configured to be three-quarters of the resistive load (75 kilo volt amps resistive [kVAR] of apparent reactive load compared to 100 kW of applied power from the genset). This allows the load bank to simulate actual mixed and variable commercial loads. With this inherent variability, an inductive load bank can test full power systems.

Capacitive load banks are also used in conjunction with reactive load banks and can be added to inductive load banks. Capacitive loads are leading (current leads voltage). This type of load bank is configured to a leading factor load. The leading power factor indicates that a load is capacitive with a negative power factor. Capacitive load banks are used to simulate loads from electronics, telecommunications, computer systems, or other systems with non-linear loads.

Load Bank Testing Procedures
How is load bank testing performed? Testers begin with the need to ensure an uninterrupted power supply (UPS) that is the primary source of power for a demand load, but needs to be backed up by an emergency power source if the system experiences a critical power outage. So, the goal of testing is to ensure that these backup systems continue to provide UPS. These tests are typically carried out concurrent with regularly scheduled preventative maintenance. If multiple batteries or gensets are being kept in reserve, load bank testing can be used sequentially to test individual power supply units. Individual batteries that are approaching the end of their operational lives can be replaced, and defective gensets maintained and repaired.

Depending on the service contract that comes with the purchase agreement, load bank testing is often a service provided by the supplier or manufacturer to its customers. Timing of the initial testing is critical, however. The initial test should take place at least a week after UPS is initially established to the demand load to ensure that the batteries are fully charged and that the voltage across the supplying batteries has been equalized. Until this balance is achieved, load bank testing will not produce accurate results. Most load bank testing provided by suppliers is performed with small, portable load banks (100 kw or smaller) owned by the supplier. Larger load bank testing is often performed by larger units rented from other suppliers for the purpose. Most testing is done with standard resistive load banks that don’t evidence a power surge when switched on. Instead, the current immediately assumes a steady state to allow for accurate testing.

In the field, load banks are provided with appropriately rated power cables that are long enough to allow them to be set back at least 60 feet from the UPS terminals, and still be connected to the power supply unit. Testers should be prepared to manage heat dissipation and noise abatement during testing (so they should avoid testing in active work zones or areas with fire control sprinklers that could be triggered). For these and other reasons, load bank testing is typically performed outside of normal working hours.

When scheduling the testing, diesel engine gensets should be tested monthly to a minimum of 30% of their standby nameplate rating. In general, load testing should meet the requirements of NFPA 110: “Diesel-powered EPS installations…shall be exercised monthly with the available EPSS load and shall be exercised annually with supplemental loads [e.g., a load bank] at not less than 50% of the EPS nameplate kW rating for 30 continuous minutes and at not less than 75% of the EPS nameplate kW rating for one continuous hour for a total test duration of not less than 1.5 continuous hours.”Cables and breakers should never be disconnected during a load test. Load bank testing should be performed in parallel with the demand load with a dedicated downstream bus with an over-current protection, collocated with a switch board in order to avoid the need for reconnection.

A typical step-by-step procedure for load bank testing is a series of embellished steps. First, the generator is started and should run until temperature stabilizes. Then, the manual or automatic switches are transferred to the backup power source. Following this, step load with the load bank until the power level required by the test is achieved. Measure and evaluate the results. After recording the test results, the load bank load should be removed from the circuit. All transfer switches can then be switched back to normal operating positions and allow the genset to cool down and shut down in accordance with manufacturers specifications and guidelines. In accordance with NFPA 110, all reciprocating diesel engine gensets need to be tested monthly for a minimum of 30 minutes at standard operating temperatures, and at least once every three years at 30% of nameplate load for at least four hours of continuous operation.

Major Manufacturers
Eagle Eye Power Systems 
offer constant current DC and AC load banks for both general and specialized load testing requirements. The DC load banks allow for users to predefine test parameters allowing technicians to safely continue other duties while the test takes place— improving work safety and efficiency. During the test, the units have the capability to automatically stop current load if any of the several safety parameters are reached. Adjustable time, low system, cell voltage, capacity, and load steps provide users wider control over their unique testing requirements.

Eagle Eye’s LB-AC Series offer classic analog banks, as well as cutting edge digital banks that are each designed for a wide variety of applications, including UPS and generator testing. The company offers a digital LB-kW line to accommodate high-power testing up to 1,000 kW, with the flexibility for custom applications beyond this. Units can be operated manually or with included software controls that allow the tester to create automatic load profiles that both display and record test data.

Load Banks of America (Sunbelt Rental Division) has an extensive fleet of load banks for rent, including 3-Phase AC-resistive/reactive solutions, portable 50/60 Hz transformers with multi-taps, and specialty medium voltage solutions, all of which are supported by their experienced technical staff. In addition to renting load banks, they provide the following technical and consulting services: rental equipment selection and system design, project management support, freight coordination, delivery and installation, operations support, round-the-clock technical support, and onsite operator service.
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