Load Bank Sizing Calculations – Part Two

In Article “Load Bank Sizing Calculations – Part One”, we explain the following points:

• What is a Load Bank?,
• Why we don’t use the actual facility loads to test the power source?,
• Wet Stacking Problem,
• Load Bank Applications,
• Applicable standards for Using load banks with emergency power generating systems.

Today, we will explain the different types of Load Banks.

Types of Load Banks

There are many types of load banks which can be categorized to the following categories: According To the Load Element Type, According To Portability, According To Cooling Method, According To Method of Control, According To Operating Mode, According To Application, According to no. of Load Steps, According to Load Bank Voltage and Frequency.

First: According to the Load Element Type

Load elements is the basic component in any load bank, they vary in construction and function to develop different types of loads used for testing generators. These load elements can be one of the following types: Resistive Load Element, Reactive Load Element, Combined (resistive/reactive) Load Element.

1- Resistive Load Element Resistive load banks are the most common. They allow you to test a prime mover and a generator at 100% capacity at unity or 1 power factor (PF).  But actually only 80% of the generator set nameplate kVA rating can be achieved when utilizing resistive units because they do not test the reactive power produced by the generator. Resistive load banks prove equivalent loading of both generator and prime mover. 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. Design: Resistive load elements  Resistive load elements use conductors manufactured of high resistance metals such as nickel-chromium alloy that can quickly transfer heat to surrounding air. When these are wound in a helical pattern, they provide surface area for transferring heat to air. Helical Pattern Other resistive load element designs use conductors that are sheathed and sealed. These elements position conductors between inner and outer sheathing and are bedded in magnesium oxide powder to impart certain thermal and insulating properties. The outer sheath is stainless steel to resist corrosion. Sheathed elements typically do not operate at the high temperatures reached by other designs, and thus provide long service life under continuous operation. Load banks with sheathed elements are well-suited for heavy-duty applications in locations with varied climatic conditions. Principle of Operation: The “load” of a resistive load bank is created by the conversion of electrical energy to heat by power resistors. Large amounts of heat are generated by these power resistors, approximately 3,412 British Thermal Units per hour for every kilowatt of load. This heat must be quickly dissipated from a load bank to prevent overheating either by normal convection to air or using water, or using forced air cooling, which is provided by a dedicated power circuit and one or more blowers. Cooling Fan and Motor Advantages: Resistive load elements simulates real-life loads, such as lighting and heating loads as well as the resistive or unity power factor component of magnetic (motors, transformers) loads. Resistive load testing verifies that the engine and generator system will produce and maintain full load without overheating and shutting down. Resistive load banks test the prime mover and generator at 100% capacity at unity or 1 power factor (PF) Resistive load elements can be used with AC or DC power sources. Resistive load elements Test the fuel delivery system operation at maximum rating and fuel consumption Resistive load elements Demonstrate the cooling system operation at the generator set’s full operating capacity Resistive load elements Allow the exhaust and after treatment system to reach normal operating temperatures Resistive load elements Eliminates exhaust wet-stacking by burning off built-up fuel and oil carbon deposits and reseating the rings when part load or low load conditions are encountered during periodic testing Resistive load elements Evaporates moisture from the engine oil that reduces wear causing acid formation Resistive load elements Identifies deficiencies than can be corrected with proper maintenance and repair before failure, avoiding downtime and additional expenses. Disadvantages: Resistive load banks have limited testing capabilities of alternator capacity (kVAR), alternator controls, load-sharing controls (kW only), distribution bus, and transient response (voltage). They actually test only 80% of the generator set nameplate kVA rating because they do not test the reactive power produced by the generator. Applications: Resistive load banks may best be used for: Generators with a capacity less than 200 kVA, Portable generators, Small generators, UPS systems.

2- Reactive Load Element  A reactive load bank has either inductive load elements or capacitive load elements.

2.A- Inductive load Element Inductive load bank is the more common type of reactive load banks and they are rated in kilovolt-amperes reactive (kVAR). Using an inductive load bank in conjunction with a resistive unit provides the capability to test the generator set fully at 100% nameplate kVA rating. Inductive load banks add inductance to the load and therefore reduce the PF to less than unity (lagging) to provide a lagging power factor load test. 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 100KW of resistive load, 75KVAR of inductive load is provided. Other ratios are possible to obtain other power factor ratings. Design: Inductive load Element Inductive elements are constructed of copper wire wound on molded nylon bobbins that provide maximum coil-to-ground insulation. The wire is gauged to provide an inductive power factor of 0.05 or less. High-temperature reinforced insulation is positioned between each layer of the winding to maintain coil-to-coil insulation. The bobbins are mounted on a ferrous core constructed of iron laminations. When compared to resistive elements, inductive load elements generate very little heat. Consequently, convective cooling or a small fan is sufficient for most inductive load bank designs. Principle of operation: The inductive load bank is converting electrical energy into a magnetic field. The power used to create and maintain these magnetic fields places corresponding load onto the power source to test it. Also, a very little heat is generated. Consequently, convective cooling or a small fan is sufficient for most reactive load bank designs. Advantages: Using inductive load elements in conjunction with resistive ones provides the capability to test the generator set fully at 100% nameplate kVA rating. When compared to resistive elements, inductive load elements generate very little heat. Consequently, convective cooling or a small fan is sufficient for most inductive load bank designs Inductive load elements allow for proper calibration of load sharing and voltage regulating systems in parallel operation installations. Installations with critical large motor loads might warrant this type of load banks. Disadvantages: It has a higher cost therefore it is only used for new installation start-up. Applications: Inductive loads are used to simulate real-life mixed commercial loads consisting of lighting, heating, motors, transformers, etc.

2.B- Capacitive Load Element A capacitive load bank is similar to a reactive load bank in rating and purpose, except that leading power factor loads are created. Also, they are rated in kilovolt-amperes reactive (kVAR). Design: Capacitive Load Element Industrial load capacitors utilize an oil-filed, welded metal case with large diameter, low profile, input bushings. Typical load bank capacitive elements absorb and store electrical charge to create capacitive load. The bushings provide minimum inductance for fast discharge and high frequency response. Oil-filed capacitors offer a typical 3 to 100 kilojoules energy storage per unit. Principle of operation: Capacitive load elements use industrial grade capacitors that absorb and store electrical charge to create capacitive load to test the power source and provide a lead power factor load test. Advantages: Capacitive load elements simulate certain electronic or non-linear loads typical of telecommunications, computer or UPS industries. Capacitive load banks are often used at electrical substations to increase power factors. In facility power systems, capacitive load banks can be used to increase the power factor to offset inductive load. Capacitive load elements allow for proper calibration of load sharing and voltage regulating systems in parallel operation installations. Disadvantages: It has a higher cost therefore it is only used for new installation start-up.

3- Combined (Resistive/Reactive) Design: Combined load banks can consist of resistive, inductive, and capacitive (RLC). Combined load banks incorporate resistors and inductors all in one construction which can be independently switched to allow resistive only, inductive only or varying lagging power factor testing. Combined load banks are rated in kilovolt-amperes (kVA). Advantages: Tests the alternator and voltage regulator at its full rated (KVA/KVAR) capacity, Simulates the actual load (KW, KVA, KVAR) that the systems are specified and designed for, Simulates transient loads to provide voltage and frequency response characteristics Tests are used to simulate and verify synchronizing, load sharing and voltage regulation on multiple unit paralleled systems under actual load conditions, Allows thermographic/infrared inspection of the electrical systems and identification of potential hot spots and the condition of cables, terminations and buss work, Combined load banks can test: Prime mover capacity (kW), Prime mover controls, Alternator capacity (kVAR), Alternator controls, Load-sharing controls (kW and kVAR), Distribution bus, Transient response (Hz), Transient response (voltage). Applications: Combined load banks are useful for testing turbines, switchgear, rotary UPS, generators greater than 200kVA and battery UPS systems. They can also be used to test utility substation protection systems, particularly those that use complex relay arrangements to control or mitigate directional power phenomena.

Comparison between Resistive and Combined Load Banks

The differences between testing with a resistive-only load bank and a combined load bank are compared in Table 1, 2 &3. Table 1: Resistive-Only and combined Load Bank Testing Comparison Resistive load bank testing kW = kVA at unity power factor Combined load bank testing Reactive power component Characteristics Tests the prime mover (engine) at 100% load. Tests the alternator and voltage regulator at its fully rated (KVA/KVAR) capacity. Tests the fuel delivery system operation at maximum rating and fuel consumption. Simulates the actual load (kW, kVA and KVAR) for which the systems are specified and designed. Demonstrates the cooling system operation at the generators full operating capacity. Simulates transient loads to provide voltage and frequency response characteristics. Allows the exhaust and after-treatment system to reach normal operating temperatures Simulates and verifies synchronizing, load sharing and voltage regulation on multiple-unit paralleled systems under actual load conditions. Eliminates exhaust wet-stacking by burning off built-up carbon deposits from unburned fuel and oil, and reseals the rings when partial- or low-load conditions are encountered during periodic testing . Allows thermographic/infrared inspection of the electrical systems; identification of potential hot spots: and the condition of cables, terminations, and buss work. Evaporates moisture from the engine S which reduces wear-causing acid formation. Identifies deficiencies that can be corrected with proper maintenance and repair before failure, avoiding down-time and additions expenses. Generator engine governors respond to loads by reducing engine speed. Figure-1 compares the transient response for a large diesel standby generator when applying a block load using restive-only and resistive/reactive load banks. The resulting initial synchronous voltage dip (Vdip1) using the 75% load at 0.80 power factor results in a voltage dip that is approximately 25% greater when compared to the equivalent resistive-only load applications. The engine speed related voltage dip (Vdip2) is similar, in both cases, due to the manufacturer’s standard V/Hz-type voltage regulator.  Figure-1 Table-2 : Resistive vs. combined Load Bank Characteristic Resistive Load Bank combined load Bank Prime Mover Capacity KW Yes Yes Prime Mover Controls Yes Yes Alternator Capacity KVAR Limited Full Load Alternator Controls Limited Yes Load Sharing Controls KW Only KW And KVAR Distribution Buss Limited Full Load Transient Response HZ Yes Yes Transient Response VOLT Limited Yes A detailed explanation for Table-2 is indicated in Table-3 Table-3 Characteristic Resistive Load Bank Combined load Bank prime mover testing, alternator testing and load sharing A resistive-only load bank can provide adequate testing of the individual prime mover and load sharing (including load add/load shed) controls of a multiple unit facility A combined load bank allows testing of the alternator, load sharing, and transient responses because it can apply loads that approach those experienced during normal generator operation.  transient voltage dips During testing with a resistive-only load bank, a system that is sensitive to transient voltage dips would not necessarily provide an indication of a power supply or system condition that would lead to a potential problem during operation. Solid-state controls and power supplies are particularly sensitive to transients and can shut down unexpectedly during load changes unless specifically backed up with a dedicated power source capable of riding through the voltage and frequency transients associated with block loading of the generators.  Load sharing for multiple unit generator systems Note: the ability to share reactive loads (kVAR) equally is critical to achieving the maximum-rated power system output. When load sharing controls are not properly configured (i.e., droop settings, cross-current compensation, and measurement and control device polarities), resistive-only testing can fail to determine how the reactive load is accepted by an individual generator. In addition, the paralleling switchgear and protective relays may perform adequately under resistive load applications The reactive load bank testing will provide load acceptance and rejection that simulates real-world conditions more closely.

How to choose the right load bank Resistive or Combined?

When selecting the type of the proper load bank some key features must be considered like: The generator capacity, Number of generator units, Ease of operation, Onboard diagnostics, Metering, The ability for an operator to control multiple units from a single controller, Data download capabilities since Load banks offering automatic step loading and duration, and data collection and reporting capabilities are beneficial in providing the necessary records to demonstrate compliance with the facility and regulatory requirements.  Other features for selection are: Type of application, AC or DC Voltage, Applied voltage and frequency, Required load step, Method of control, Method of cooling, Indoor or outdoor service, Portability. The correct size and type of load bank based on the generator capacity are shown in Table-4. Table-4: Types of Resistive and Resistive/Reactive Load Banks Generator Capacity Load Bank Type Application < 100 kVA Resistive Only Portable, Small Generators and UPS System (120/208, 240 VAC, 60 Hz) < 200 kVA Resistive Only Small Generators and UPS Systems (480VAC, 60Hz) >200 kVA combined Single Units (480VAC, 60 Hz) 1MVA – 6MVA combined Single Units (480/5kV/15kV, 60 Hz) >6 MVA combined Multiple Units Combined (480/5kV/15kV, 60 Hz)

In the next article, we will explain other types of Load Banks. So, please keep following.

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