Load Bank Testing
The purpose of a load bank test is to simulate real-world scenarios that can be used to confirm proper functionality. Load banks are provided to simulate electrical load, as it is important to test various electrical apparatus in its installed location. While the equipment under test (EUT) may have been tested at the factory, the installation variables (Eg. altitude, ambient temperature, air intake, fuel, exhaust, cooling systems and contractor/sub-contractor workmanship quality) can be significantly affected by the installation. The proper use of a load bank can be used to effectively test the operating capability of the complete system, as delivered from the manufacturer.
A load bank is a device which develops an electrical load, applies the load to an electrical power source and converts or dissipates the resultant power output of the source.
The purpose of a load bank is to accurately mimic the operational or “real” load that a power source will see in actual application. However, unlike the “real” load, which is likely to be dispersed, unpredictable and random in value, a load bank provides a contained, organized and fully controllable load.
Consequently, a load bank can be further defined as a self-contained, unitized, systematic device that includes load elements with control and accessory devices required for operation.
Whereas the “real” load is served by the power source and uses the energy output of the source for some productive purpose, the load bank serves the power source, using its energy output to test, support or protect the power source.
- 1 Applications
- 2 Load bank types
- 3 Alternative load banks
- 4 Load Bank Manufacturers
Load banks are used in a variety of applications, including:
- Factory testing of turbines and engine diesel generator sets
- Reduction of wet stacking problems
- Periodic exercising of stand-by engine generator sets
- Battery and UPS system testing
- Ground power testing
- Load optimization in prime power applications
- Removal of carbon build-up on piston rings
- Load rejection test
Load bank types
The three most common types of load banks are resistive, inductive, and capacitive. Both inductive and capacitive loads create what is known as reactance in an AC circuit. Reactance is a circuit element's opposition to an alternating current, caused by the build up of electric or magnetic fields in the element due to the current and is the "imaginary" component of impedance, or the resistance to AC signals at a certain frequency. Capacitive reactance is equal to 1/(2⋅π⋅f⋅C), and inductive reactance is equal to 2⋅π⋅f⋅L. The unit of reactance is the ohm. Inductive reactance resists the change to current, causing the circuit current to lag voltage. Capacitive reactance resists the change to voltage, causing the circuit current to lead voltage.
Resistive load bank
A resistive load bank, the most common type, provides equivalent loading for both generators and prime movers. That is, for each kilowatt (or horsepower) of load applied to the generator by the load bank, an equal amount of load is applied to the prime mover by the generator. A resistive load bank, therefore, removes energy from the complete system: load bank from generator—generator from prime mover—prime mover from fuel. Additional energy is removed as a consequence of resistive load bank operation: waste heat from coolant, exhaust and generator losses and energy consumed by accessory devices. A resistive load bank impacts upon all aspects of a generating system.
The load of a resistive load bank is created by the conversion of electrical energy to heat via high-power resistors such as grid resistors. This heat must be dissipated from the load bank, either by air or by water, by forced means or convection.
In a testing system, a resistive load simulates real-life resistive loads, such as incandescent lighting and heating loads as well as the resistive or unity power factor component of magnetic (motors, transformers) loads.
The most common type uses wire resistance, usually with fan cooling, and this type is often portable and moved from generator to generator for test purposes. Sometimes a load of this type is built into a building, but this is unusual.
Rarely a salt water rheostat is used. It can be readily improvised, which makes it useful in remote locations.
Reactive load banks
Inductive load banks
An inductive load includes inductive (lagging power factor) loads.
An inductive load consists of an iron-core reactive element which, when used in conjunction with a resistive load bank, creates a lagging power factor load. Typically, the inductive load will be rated at a numeric value 75% that of the corresponding resistive load such that when applied together a resultant 0.8 power factor load is provided. That is to say, for each 100 kW of resistive load, 75 kVAr of inductive load is provided. Other ratios are possible to obtain other power factor ratings. An inductive load is used to simulate a real-life mixed commercial loads consisting of lighting, heating, motors, transformers, etc. With a resistive-inductive load bank, full power system testing is possible, because the provided impedance supplies currents out of phase with voltage and allows for performance evaluation of generators, voltage regulators, load tap changers, conductors, switchgear and other equipment.
Capacitive load banks
An capacitive load includes capacitive (leading power factor) loads.
A capacitive load bank or capacitor bank is similar to an inductive load bank in rating and purpose, except leading power factor loads are created, so reactive power is supplied from these loads to the system, hence improves the power factor. These loads simulate certain electronic or non-linear loads typical of telecommunications, computer or UPS industries.
Resistive/Reactive load banks
These are a combination of primarily resistive and inductive load elements suitable for performing power factor testing. Resistive/reactive combination load banks are used to test the engine generator set at its rated power factor and in the majority of cases this will be 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, or real power and the KVA or 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 to be 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.
Alternative load banks
Electronic load banks
An electronic load bank tends to be a fully programmable, air- or water-cooled design used to simulate a solid state load and to provide constant power and current loading on circuits for precision testing.
These units tend to be somewhat rare, but are great for testing labs to perform constant power, voltage and current load tests.
Water-Cooled load banks
Often times, situations require customers to test power supplies, namely generators and UPS systems, in locations that are not conducive to normal forced – air cooled load banks, either due to environment or physical constraints. For example, a UPS system that is underground, located in a parking garage or located on top of a rooftop.
In these situations, very long runs of cable sometimes more then 300'+ may be used, but due increased resistance and subsequent voltage – drop, may pose and add to the existing challenge.
In addition to the above scenario, water – cooled load banks can be used to provide heated water to simulate water – based server and or chillers
So this leaves us with an open – loop, water – cycle load cell, whereby chilled water is supplied to a device with heating elements. An easy way to envision this is to think of it as an expensive hot – water heater.
Medium-Voltage load banks
The notion of using medium – voltage load bank for portable testing purpose, came about from thinking outside the box. The extra logistics costs for transporting, craning into position and connecting the low – voltage cable, became rather tedious.
A medium – voltage load bank is just like any other load bank in that it contains resistors, reactors or capacitors. However, the voltage ratings are greater than 600 VAC and below 69,000 VAC.
Medium – voltage load banks are often used for commissioning power plants, maritime electrical systems, standby generator systems and substations, essentially the same as low – voltage load banks, the only difference being the voltage.
The alternate method of testing without medium voltage load banks is to use low – voltage load banks with step down transformers used to serve the low voltage load banks.
Typically it is always advantageous to test with medium – voltage load banks versus just using low – voltage load banks, as the added logistics and procurement costs for low – voltage load banks are far more expensive.
Server Simulating load banks
These are typically rack – mountable and used to simulate true hot – aisle/cold – aisle containment systems, heat and air flow. These simulate physical servers as installed at a data – center by way of heat – discharge, electrical – resistance and air – flow. The resultant heat output of the load banks are of a benefit in that they are used to load computer room air – conditioners, whilst creating an electrical load to the primary and standby systems in the data center.
Non-linear load banks
A load is considered non – linear if its impedance changes with the applied voltage. The changing impedance means that the current drawn by the non – linear load will not be sinusoidal even when it is connected to a sinusoidal voltage. These non – sinusoidal currents contain harmonic currents that interact with the impedance of the power distribution system to create voltage distortion that can affect both the distribution system equipment and the loads connected to it.
In the past, non – linear loads were primarily found in heavy industrial applications such as arc furnaces, large variable frequency drives (VFD), heavy rectifiers for electrolytic refining, etc. The harmonics they generated were typically localized and often addressed by knowledgeable experts.
Times have changed and harmonic problems are now common in not only industrial applications but in commercial buildings as well. This is due primarily to new power conversion technologies, such as the Switch – mode Power Supply (SMPS), which can be found in virtually every power electronic and power conversion device. Namely wind – farms are notorious for this and require special considerations due tot eh extra harmonic content.
400Hz load banks
These are normally used on aircraft and ship based systems that require commissioning or testing before being placed in service. 400 Hz systems are used where weight is important because the magnetic cores can be smaller/lighter. This applies to transformers as well since a lower inductance (smaller core/number of turns) can be used than required by lower frequencies.
It is worthy to note that resistors rated for 60Hz can operate at the higher value of 400Hz without a problem, but inductors and capacitors cannot.
Regenerative load banks
Reverse power/regenerative power protection of generator by sensing power direction, magnitude and automatic addition to act as a power sink.
These are seen primarily used to capture and dissipate regenerative power from large motors and are found in the following settings:
- Cranes and rigs
- Cable laying vessels
Resonant load banks
Resonant loads are becoming increasingly prevalent due to solar inverter anti – islanding testing and commissioning per IEEE on photovoltaic systems. These are comprised of an exact balance of resistive, capacitive and inductive loads in a 1:1:1, or 1:2.5:2.5 ratios so as to provide a specific quality – factor.
Many solar inverters are designed to be connected to a utility grid, and will not operate when they do not detect the presence of the grid. They contain special circuitry to precisely match the voltage and frequency of the grid.
Traditional utility electric power systems were designed to support a one way power flow from the point of generation through a transmission system to distribution level loads. These systems were not originally intended to accommodate the back – feed of power from distributed generation systems at the distribution level.
Islanding refers to the condition in which a distributed generator continues to power a location even though electrical grid power when the electric utility is no longer present. Islanding can be dangerous to utility workers, who may not realize that a circuit is still powered, and it may prevent automatic re – connection of devices. For that reason, distributed generators must detect islanding and immediately stop producing power; this is referred to as anti – islanding.
Load Bank Manufacturers
Here is a brief listing of commonly known load bank manufacturers:
- Crestchic UK
Note: This listing is by no way complete or absolute.