Table of Contents
A load bank is a device that develops an electrical load, via resistance, inductance or capacitance and applies said load to an electrical power source. The load bank converts said load into an electric or magnetic field, or dissipates the resultant power output of the source as heat, and is cooled by an electric motor and fan providing the appropriate cubic-foot-minute (CFM) of air.
Constructed using load elements with protection, control, metering, and accessory devices required for operation, they are most typically used for testing power sources, such as standby generators and batteries.
A load bank is a calibrated device, which converts the electric power from a power-generation source, into one of the three aforementioned electrical loads, and can be incrementally controlled to test the operation of critical systems under varying conditions, to determine the system response to changes.
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.
The main goal of load bank testing is to uncover problems in a controlled situation rather than during an actual power failure. Load bank testing should be conducted during initial system installations, but also as part of a regular maintenance program.
Load bank testing is the only way to verify a backup power system will operate during an outage. Backup systems such as generators and UPS equipment may perform adequately under light loads. However, without regular testing, they may fail to deliver a full-power load.
Testing ensures your backup systems are up to the task. When operating under light loads for extended periods, diesel generators may experience a condition called “wet stacking.” Wet stacking happens when carbon or unburned fuel oil accumulates in the exhaust system. This accumulation reduces output and may permanently damage the generator. Wet stacking can be corrected simply by running the generator engine under a sufficient load for a few hours.
Standards compliance may dictate load bank testing. For example, the National Fire Protection Association (NFPA) “Standard for Emergency and Standby Power Systems” provides requirements for initial commissioning of an emergency power supply, as well as ongoing testing. Other standards organizations imposing mandates for load testing include the ANSI/NETA Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems and the Joint Commission on Accreditation of Healthcare Organizations.
During data center commissioning, load banks can be used to test mechanical cooling systems by simulating the heat load of critical electrical systems. Water-cooled load banks also test chiller plants to ensure they meet their design ratings.
Chiller plant testing is a part of an integrated system test where all systems are tested as a whole – as they are installed. This testing will sometimes uncover problems that do not get detected in component level testing and factory testing.
Load bank testing is a way of validating the correct operational performance and battery autonomy of the UPS system. It tests the UPS and generator under load conditions. It is most often carried out during preventative maintenance. As a UPS battery set is only as strong as its weakest battery cell, load bank testing can also be used to ascertain the condition of UPS batteries and battery sets (or ‘strings’ as they are also known) to indicate if any individual cells are approaching the end of their working life and not holding a charge or about to fail. This enables them to be replaced in advance of critical usage.
Testing is offered as a service by suppliers of power protection equipment. Some suppliers offer load bank testing as part of the UPS commissioning process but caution must be exercised: ideally it should be carried out at least one week after the UPS has been commissioned to permit the voltage across the battery blocks to equalize and batteries to be fully charged. Load bank testing before this will not yield accurate results, or give a true picture of how the system is running, and thus be a waste of time and money.
A generator load bank test involves an examination, assessment and verifies that all primary components of the generator set are in proper working condition. The equipment used to conduct a load bank test produces artificial loads on the generator by bringing the engine to an appropriate operating temperature and pressure level. This is especially important for standby and emergency generator sets that do not run very often and/or may not be exposed to carrying heavy loads frequently. The general rule is - if your generator is not exposed to higher than 30% of its rated kW load then you should be considering a load test.
A load bank test ensures that your generator will run properly when it’s needed so that you can fully depend on it during an emergency. The key to a proper load bank test is that it tests your generator at its full kilowatt (kW) output rating. Because many generators do not regularly operate at their full kW rating, you must verify your generator can produce the highest possible horsepower that may be required - while at the same time maintaining adequate temperature and pressure levels that will allow it to run as long as necessary.
There are multiple reasons should perform a load bank test on an annual basis, including the following:
| Alternative | Battery Systems | Fuel Cells | Diesel Generators | UPS's | Healthcare | Maritime | Power Electronics | DOD |
|---|---|---|---|---|---|---|---|---|
| Wind Farms | Switchgear | Sales / Service | Sales / Service | Sales / Service | Hospitals | Shipyards | Testing Labs | Submarines |
| Solar | Valve Regulated | Manufacturing | Manufacturing | Manufacturing | Pharmaceuticals | Drydock | Universities | Port Power |
| Geothermal | Gel | Controls | Controls | Controls | Surgery Centers | Cruise Ships | Automation | Base Ports |
| Tidal | Flooded | Proton Exchange | Blackstart | Batteries | Nursing Homes | Workboats | Rail and Transit | R&D |
| Hydro | Chargers | Solid Oxide | Hydroelectric | IST | Treatment Facilities | Power Barge | Automobile | DOE |
| Wave | Sales / Service | Alkaline Fuel Cell | Supermarkets | Urgent Care | Cold Ironing | Power Supplies | GSA | |
| Biomass | Manufacturing | Direct Methanol | Water Treatment | Blood Banks | Shorepower | |||
| Aerospace | Oil & Gas | Utilities | Mining | Telecom | Commercial | Turbines | Data Centers | Nuclear |
| NASA | FPSO | CHP | Generators | Cell Towers | Electrical Contractor | Sales / Service | Building Owner | UPS |
| Aircraft | Rigs | Island Grid | Blackstart | Standby Power | General Contractor | Manufacturing | Container | Batteries |
| Military | Refineries | Switchgear | UPS | Switchgear | Commissioning Firms | Controls | Colocation | Generators |
| Airport | LNG Plant | Blackstart | Switchgear | Batteries | Engineering Firms | Blackstart | Blackstart | Controls |
| Substation | Defense Contractors | Hydroelectric | Facilities | Blackstart | ||||
| Excitation | IST | IST |
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 elements opposition to an alternating current, caused by the buildup 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.
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 the coolant, exhaust and generator losses, and energy consumed by accessory devices. A resistive load bank impacts all aspects of a generating system.
A load from a resistive load bank is created by the conversion of electrical energy to heat via high-power resistors such as wire-wound resistors. This heat must be dissipated from the load bank, either by air or by water, by forced means or convection.
Industrial electrical loads are comprised of both resistive and reactive type loads. Reactive, or reactance as it is commonly known, 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. Both fields act to produce counter EMF that is proportional to either the rate of change (time derivative) or accumulation (time integral) of the current. In vector analysis, reactance is the imaginary part of electrical impedance, used to compute amplitude and phase changes of sinusoidal alternating current going through the circuit element.
Reactance, in itself, can mean either inductance (Positive Reactance) or capacitance (Negative Reactance).
An inductive load produces lagging power factor loads and consists of an iron-core or an air-core wound reactive element which, when used in conjunction with a resistive load bank, creating 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 inductance load is provided. Other ratios are possible to obtain other power factor ratings. An inductive load is used to simulate real-life mixed commercial loads consisting of lighting, heating, motors, and transformers.
A capacitive load produces leading power factor loads and consists of multiple metal plates separated by a dielectric or an air-core wound reactive element which, when used in conjunction with a resistive load bank, creating a leading power factor load.
These loads simulate certain electronic or non-linear loads typical of telecommunications, computer or UPS industries. Harmonic currents generated by nonlinear loads are becoming a pervasive problem, as "nonlinear" electrical load is a load on the electrical utility that requires current from the utility, whose electrical waveform is non-sinusoidal. Computers, fax machines, printers, electronic lighting ballasts, and variable-speed drives are prime examples of nonlinear loads that are capable of generating harmonic currents
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. Furthermore, the following functions can be performed.
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.
Namely there are two types, AC and DC and commonly used to test power supplies, batteries and fuel cells.
Also, any active or passive current-carrying devices such as switches, circuit breakers, fuses, connectors and power semiconductors can be tested. Traditionally, many of these products are tested using resistive load banks. This approach does not simulate real-world conditions such as switching DC/AC converters found in many AC powered products. This type of conventional testing does not fully exercise the equipment under test (EUT) under worst-case operating conditions. High peak currents and low power factor loads can significantly impact the operating characteristics of a UPS or AC power product.
Typically five modes of operation:
Direct current load banks are commonly used to test UPS and telecom battery strings. The acceptance load test is a capacity test performed to determine if the battery is capable of supplying the manufacturer’s rated discharge current for a given duration under a specific set of conditions. The battery is discharged at a constant current or power to a predefined end of discharge voltage. The actual discharge time is then compared to the manufacturer’s rated discharge time to determine actual capacity.
A medium-voltage load bank is just like any other load bank commonly used, in that it contains resistors, reactors or capacitors. The voltage rating typically is 15kV being the maximum impressed potential allowed at the present time. This negates the need for transformers and allows for longer runs of cables and smaller conductors.
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.
Generally, there are three kinds of medium-voltage load banks in use today:
The use of an integrated transformer allows for finer load-resolution load-steps. The direct connection method, being much lighter and less labor-intensive, just requires medium-voltage cable form the source.
Load banks built to mimic real-world scenarios, confirming air-flow computational fluid dynamics modeling and maximizing power utilization effectiveness (PUE). Constructed from 19" wide rack-mountable chassis and are used to model true hot-aisle/cold-aisle containment systems, heat, and air flow in data centers.
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.
Variable controls provide for granular airflow, Delta-T and deferential pressure compensation at various altitudes. This allows the operator to fine-tune the load bank, as to mimic the server it is modeling and match the environmental conditions.
Data-centers are built to maintain a 2N (A/B) utility grid redundancy and the majority of server simulating loads are designed to facilitate that network design. Either channel A or B can be intentionally disabled via a master load switch and in doing so, the fans and electrical load on that channel can be shut off to inhibit load application for that channel.
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 nonsinusoidal 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 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 Switchmode 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 to the extra harmonic content.
Power frequencies as high as 400 Hz are used in aircraft, spacecraft, submarines and military equipment. 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.
Induction motors turn at a speed proportional to frequency, so a high-frequency power supply allows more power to be obtained for the same motor volume and mass. Transformers and motors for 400 Hz are much smaller and lighter than at 50 or 60 Hz, which is an advantage in aircraft and ships.
Resistive load banks rated for 60Hz can function at 400Hz without a problem, only the reactive loads must be built specifically for the frequency.
Regenerative braking is the process of opposing inertial load by converting kinetic energy to electricity than heat using resistors, which is essential for generator-powered moving machinery that carries and moves vast weights such as elevators, cranes, and hoists.
All-electric cars built today, use regen to capture a large portion of the load during braking, and stores it back into the battery. Regen usually loses around 10-20% of the energy being captured, and then the car loses another 10-20% or so when converting that energy back into acceleration.
These are seen primarily used to capture and dissipate regenerative power from large motors and are found in the following settings:
Often, 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.
In these situations, very long runs of cables may be used, but voltage-drop due to Ohms law may pose and add to the existing challenge. So this leaves us with an open-loop, water-cycle load cell, whereby chilled water is supplied to a device with heating elements, think of it as an expensive hot-water heater.
These can also be used to test data center chillers, a UPS system that is underground, located in a parking garage or located on top of a rooftop and may be used to provide heated water to simulate water-based server and or chillers.
These are typically built into existing ISO containers of various sizes and are used extensively in severe weather environments, off-shore platforms and international shipping. Crestchic and ASCO are two suppliers that exclusively deal with this model load bank.
This is a load bank bolted to a trailer and towed to various job sites. Far easier than having to constantly load and off-load a trailer with a forklift.
If the trailer is low enough, these can be used to test underground generation using oversized loads, without having to run very long runs of cables to the outside.
A radiator mounted load bank is specifically designed to mount in front of the radiator on a diesel generating set. The engine fans of the diesel gen-set provide suitable airflow to cool the resistive elements. Radiator mounted load banks are intended to supply supplemental load between 50-70% of nameplate kW rating of the gen-set to alleviate the effects associated with light loading.
These typically weigh less than 75lbs. as OSHA suggests a two-man lift being anything over 50lbs. Built for inside use, and have a higher watt density.
When the load bank is shipped by carrier, it is best to keep the weight under 150lbs. when conbined with shipping case and cables.
Portable load banks are built to transport safely to job sites, including remote locations. Once onsite, portable load banks are designed to be easily moved from one testing location to another.
These are a special class of load banks combining an exact balance of resistive, inductive and capacitive loads into one package, in a 1:1:1, or 1:2.5:2.5 ratios to provide a specific quality – factor. They function by building a resonant electrical network in either a series or parallel arrangement for quality factor testing of inverters.
Resonant loads are becoming increasingly prevalent due to solar inverter anti-islanding testing and commissioning per UL1741 and IEEE 1547 on photovoltaic systems.
The Q, quality factor, of a resonant circuit, is a measure of the “goodness” or quality of a resonant circuit. A higher value for this figure of merit corresponds to a more narrow bandwidth, which is desirable in many applications. More formally, Q is the ratio of power stored to the power dissipated in the circuit reactance and resistance.
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 the automatic reconnection of devices. For that reason, distributed generators must detect islanding and immediately stop producing power; this is referred to as anti-islanding.
As the majority of load banks are built in a delta configuration, wye configured loads have an added neutral connection. If properly configured, this can be used to test a source with an unblanced load.
Built with very fine load resolution load-steps and can be comprised of all three linear elements mention previously. Typically used in research and development and university settings for product design.
Recent years have seen expanded interest in the development and deployment of microgrids. A microgrid distribution model uses a local source or group of sources to supply electrical power to facilities and communities. Microgrids may use both renewable and fuel-based energy sources, and are increasingly used as sources of primary and supplemental power. Like other power systems, they must be tested to ensure correct commissioning, operation, and maintenance.
Typically a microgrid load bank test consists of both AC and DC load bank arrangements.
Load bank testing will indicate:
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Resistive
Inductive
Capacitive
Resistive Reactive
Medium Voltage
Direct Current
Electronic
Server