Load Bank Sizing Calculations – Part Five



Today, we will explain the Load Bank Power Connections and an example for Installation details, Power and Control Wiring Diagrams for A Load Bank.



Load Bank power Connections




1- Is a Local Isolator required for disconnecting the load bank?



When planning the installation of the load bank consider if a local isolator switch is required for disconnecting the load bank or not. If the output circuit breaker of the Supply-on-Test (generator or other) is easily accessible, then this can perform the isolating function.

  


2- Load bank power terminals configurations

A- Three Phase Connection:

The power terminal compartment of the load bank contains terminals for each of the three supply phases, earth and neutral. Depending on the load bank specification these power terminals may be configured to provide a star or delta connection.

Typical arrangement of the Supply-on-Test terminal bus bars for a Delta

Typical arrangement of the Supply-on-Test terminal bus bars for a delta configured

For connection to a three phase generator, the star or Delta configuration of the load bank makes little difference to the wiring. Irrespective of how the load bank is configured, if the Supply-on-Test (generator or other) has a neutral terminal it must be connected to the load bank neutral terminal to achieve maximum accuracy of the instrumentation.

Connection for a three phase supply with a neutral connection


Note:

  • Some manufacturers prefer that for Four wire wye power sources can be easily connected to the standard load bank by connecting phases A, B, and C to their respective input terminals. The standard load bank is a balanced 3 phase load, so the generator's neutral wire is not required.
 balanced 3 phase load bank , the generator's neutral wire is not required



B- Neutral connection:

  • Star connected load banks have a power neutral terminal which is connected to the load elements during testing and is capable of carrying a load.
  • Delta configured load banks are provided with an instrumentation neutral only. The terminal is not connected to any of the load elements and is provided for connection to instrumentation only.


C- Protective earth connection:

An earth conductor must always be bonded to the frame of the Supply-on-Test and connected to the grounding terminal of the load bank (marked PE).





3- Making connections for single-phase operation

3-phase load banks can be used for testing single-phase power sources. The method of connection (and the load available) will vary depending on the rating of the load bank, the supply voltage and frequency, and whether the load bank is configured to provide star or delta connections.

A- Single phase wiring for star configured load banks

Star configured load banks have a power neutral that is connected to the load elements, and this can be used to carry current during single phase testing. The load available depends on the connection method.

A.1- Normal Connection

Normally the load bank can be operated at 33% (one third) of full load capacity by connecting between one phase and neutral. The Supply-on-Test should be connected as shown in below Figure.

Normal star configured single phase connection


The maximum single-phase voltage that can be applied in this connection is 58% of the 3-phase load bank rated voltage.
For example, a 230V single-phase supply can be connected to a 400V 3-phase star configured load bank, giving 33% loading capacity.

A.2- Full Load Connection

To obtain full load capacity the load bank neutral terminal must be rated at the full single-phase current. Confirm the rating before connecting. The Supply on-Test should be connected as shown in below Figure.

Full load star configured connection

The maximum single-phase voltage that can be applied in this connection is 58% of the 3-phase load bank rated voltage.
For example, a 230V single-phase supply can be connected to a 400V 3-phase star configured load bank, giving 100% loading capacity.


B- Single phase wiring for delta configured load banks

Delta configured load banks have only an instrumentation neutral (marked N in the terminal compartment). This is not connected to any of the load elements and is not capable of carrying any load current. Because of this, single-phase operation is achieved by connection between two phase terminals, one of which is used as neutral. The neutral terminal is connected only for instrumentation purposes.

B.1- Basic Connection

The basic connection shown in below Figure will give approximately 50%
loading capacity when the nominal load bank supply voltage is connected or 17% loading capacity when a single-phase (√3) equivalent supply is used.

basic connection for a delta configured single phase supply


B.2- Alternative Connection for Maximum Loading

The loading capacity can be increased by linking together L1 (U) and L2 (V) as shown in below Figure.

Alternative Delta configured connection for maximum loading

In this case the connection will provide approximately 67% loading capacity when a 400V single-phase supply is connected or 22% loading capacity when a 230V single-phase supply is used.




4- Making connection for Using Multiple Load Banks




connection for Using Multiple Load Banks
  • Some load banks’ control system allows up to fourteen load banks to be interconnected and controlled from a single terminal as if they were a single unit.
  • This means that multiple load banks can be combined to match particularly large generating sets, or that a combination of resistive, capacitive or inductive loads can be mixed for special purpose or one-of tests.
  • One example of the use of multiple load banks might be where a purely resistive load bank is to be permanently installed for ongoing routine maintenance engine tests. A load bank with inductive elements could be added for a short period so that commissioning and acceptance tests can be carried out.



Notes:
  • When multiple load banks of different capacities are used the load applied is shared proportionally depending on the ratio of the load banks’ capacity.
  • The cable sizes for the Supply-on-Test must take this into account.





5- General Recommendations for Load Bank power Connections

  • It is good practice to route the three phase conductors in a close trefoil layout, held together with cable-ties. This minimizes stray magnetic fields from the cable array, and reduces inductive losses in the cables. In the event of a high fault current flowing this arrangement minimizes the risk of sudden and violent cable movements.
  • To eliminate induced magnetic fields and their associated eddy currents and heating effects all three phase conductors must pass through the same opening into the load bank terminal area.
  • If the connections are made using more than one conductor for each phase connection all the cables on any one phase should be of exactly the same length, and laid along a similar route. 
  • Ensure that the three phase conductors are equally shared between multiple cable entry openings, to minimize eddy current losses.
  • The load bank supply cables must be protected by the short circuit protective device (SCPD) which is suitably rated to the capacity of the supply cables.






An Example of Specification, Installation details, Power and Control Wiring Diagrams for A Load Bank




Here are the installation details, power and control wiring diagrams of a stationary resistive load bank for your study and review which will give you a complete picture about the load bank construction and connections.

1- The Selected Load Bank Description:


Resistive load bank

front and side view

This Load Bank is a vertical airflow unit designed to provide a balanced resistive load of unity power factor, at a specified 3-phase voltage.

2- Capacity and Steps:

The total load capability of the this Load Bank is 750 kW at 400 VAC, 3-phase, 50 Hz. Load steps are 50, 50, 50, 100, 100, 100, 100, and 200 kW.

3- Control:

  • Using the toggle switches on the control panel, any combination of the available load steps may be selected to achieve a desired load.
  • All load step switches are the toggle type with metal levers. The control panel also contains a FUSE for protection of the control circuit.
  • This load bank has an automated Load Shed Controller circuit. The automated Load Shed Controller will automatically maintain a minimum on the unit under test when AUTO operation is selected.

Functions of automated Load Shed Controller:


Automated Load Shed Controller



  • It is designed to add or remove load steps to maintain an approximate target load level on the generator to which the load bank is connected.
  • It is intended for use with standby or emergency backup power systems. The controller is employed during a utility service power outage when the standby generator is supplying power to the building load.
  • maintaining optimum loading of the generator and to prevent “wet stacking” on the generator which might occur when the building load alone is low in comparison to generator capacity.



Theory of operation for automated Load Shed Controller:



Typical Automatic Load Shed controller toggle switch and response graph


  • Operation When Automatic Load Controller (Load Shed) controls are initiated in the Automatic Mode, the controller will start the load bank by turning on the blower (free standing outdoor load banks only). After a time delay (typically two to five seconds), the controller begins adding load steps, with a time delay between the addition of each step. The load steps are added in sequence, usually from smallest to largest and removed in reverse sequence; first on/last off, last on/first off. 
  • The building load is continually monitored by means of a current transformer placed between the load bank and the building load. Correct placement of the current transformer is critical to proper operation of the automatic load controller (load shed). The current transformer does NOT monitor the load bank itself, and so does not monitor the total load on the generator. Only the building load is monitored, downstream from the generator and load bank. 
  • With current transformer input from actual building load, the controller adds or sheds load steps to supplement the building load and maintain optimum loading on the generator. The current transformer provides input to a series of current sensing relays. There is one adjustable current sensing relay for each load step. These settings are set at the factory and are based on generator KW capacity. 
  • The current sensors enable a load step if the building load is less than the relay set point. The load step is disabled if the building load equals or exceeds the relay set point. 
  • The Automatic Load Controller (Load Shed) load steps are determined by the load steps available in the load bank. Load step resolution with the controller is different from the load step resolution on the load bank. The controller window is determined by the largest step in the controller. The window is the difference between the target maximum load at which load steps are shed, and the minimum load on the generator after the controller has dropped its largest step.
Note:

A mode selector switch is added to the load bank control panel and is used to select load bank operation in either Manual or Automatic modes as follows:
  • MANUAL MODE: In the manual mode the automatic load controller (load shed) is disabled and the load bank can be used manually to test and exercise the generator. 
  • AUTOMATIC MODE: In the automatic mode, the load bank will sit idle until an external normally open contact closes initiate load controller (load shed) operations.

Control panel


The control panel contains:

  • POWER ON-OFF switch with a CONTROL POWER light,
  • BLOWER POWER light,
  • Blower power START and STOP pushbutton switches,
  • AIR FAILURE light,
  • Load Bank OVER TEMP light,
  • AUTO/MANUAL switch,
  • AUTO ON light,
  • MASTER LOAD ON-OFF switch,
  • Individual KW LOAD STEP switches.


4- Cooling:

The Load Bank contains a fan with a 3-phase, 400 VAC, 50 Hz, 10 hp, 1500 rpm fan motor which provides the necessary cooling air for the load elements. The fan motor is controlled by a motor starter contactor.

5- Safety:

  • Overcurrent protection is provided for the fan motor by three fuses and an overload relay.
  • An airflow switch is provided to monitor the flow of cooling air. This differential pressure switch is electrically interlocked with the load application controls to prevent load application if the fan is not working properly (AIR FAILURE light comes on).
  • An over-temperature switch is provided to monitor exhaust air temperature. This switch is electrically interlocked with the load application controls to prevent load application if the resistor stack exhaust temperature exceeds 375°F.


6- Dimensions:

This load bank is sized for mounting in a 19-inch rack enclosure, the control panel has a vertical height of 14.00 inches, and requires a minimum of 8 inches of clearance behind the panel.

7- Enclosure:

The Load Bank enclosure is fabricated using heavy-gauge aluminized sheet steel, making a rigid structure.

8- Components:

  • The load element resistor assemblies, load contactors, and the cooling fans/blowers are Mounted within the structure.  
  • The resistive elements are porcupine type, folly supported along their length.
  • The motor starter, motor overload relay, fuses, and customer connection terminals are mounted on separate panels.
  • The cooling air is drawn in from the two sides and the back, forced across the resistor elements, and exhausted out the top.
  • The panel includes a temperature controlled heater, which may be used to prevent condensation from hindering operation of the Load Bank. A 100 watt strip heater is located inside the Load Bank enclosure on the control/blower panel. It is controlled by a temperature switch. The heater is used to control condensation problems. The switch is factory calibrated to 50°F.
  • The Load Bank is equipped with screens mounted at the inlet openings and exhaust hoods.
  • The Load Bank uses a welded base to facilitate handling by a forklift truck. Bolt holes are also supplied in the base to permit permanent mounting to a concrete pad or metal foundation.

  
9- Location and Installation Requirements:

  • The Load Bank must be used in a cool, well-ventilated area. It must be installed where cool air is continually available and where hot exhaust air will not be recirculated through the Load Bank. The Load Bank must not be operated in a closed space. Exhaust temperature may exceed 400°F over ambient, under lull load conditions.
  • The Load Bank should be positioned so that there is a minimum of 36 inches of clearance on all sides to provide room for intake air flow and maintenance. A minimum of twelve feet of clearance from the exhaust is required for proper airflow. (Refer to below Outline Drawing)
Outline Drawing

  • The exhaust air may exceed 400°F above ambient under full load conditions. The unit must not be installed near any equipment, wiring, or plumbing which may be damaged by high air temperatures or which may constitute a fire hazard.


10- Airflow Considerations:

Even with an ample supply of cooling air, the Load Bank may overheat if it is not properly installed. There are two types of airflow problems that should be avoided:

A- Recirculating Airflow

  • If the hot, exhausted air is permitted to recirculate through the Load Bank, it will reach such a high temperature and low density that it will no longer cool the elements. A Load Bank should not be installed so close to any surface as to reflect the exhausted air back to the air intake.
  • A Load Bank should not be installed so close to any surface as to reflect the exhausted air back to the air intake.


B- Restriction of Cooling Air
  • Any obstruction located within five (5) feet of the inlet and twelve (12) feet of the exhaust hood (duct) will restrict the Load Bank's airflow. Airflow is also restricted when two or more Load Banks have air inlets positioned close to each other.
  • This competition for cooling air causes a low pressure area, restricting adequate airflow. It is recommended that the factory exhaust hoods be used.
  • If the exhaust hoods are not used, the Load Banks are designed to tolerate up to 0.25" water gage additional system airflow resistance. This includes air intake resistance (building make up air pressure) and resistance due to exhaust duct/louvers/screen. The exhaust must be screened to keep debris from entering unit. The screen must be a minimum 75% open area and/or exceed Load Bank duct area by a minimum of 50%.
  • If exhaust duct exits through motorized louvers, the design must be interlocked to assure full open prior to operation. Consideration must also be given for louver design and actuator to prevent heat problems.
  • The fan/blower is designed to move 20,000 CFM per resistor stack. Therefore, if installed indoors, the building air intake system must be upgraded to provide this continuous additional 20,000 CFM of free air.


11- Power and Control Connections:

A- Load Bank Power Terminals Connections

  • Load connections are made to the 3-phase bus bars located in the Load Bank. (Refer to above Outline Drawing) The connections are marked A, B, and C. Cables to the Load Bank should be of adequate size to handle maximum rated load according to the National Electrical Code and any local codes.
  • The ampacities of these load connections are shown on the Load Bank/Control Panel Schematic Diagram figure.


the Load Bank/Control Panel Schematic Diagram 


B- Fan/Blower Motor Connection

  • The fan/blower circuit consists of fuses, a motor starter contactor, an overload relay, and the fan motor.
  • The blower input power connections are wired to TB1 (19), (20), and (21) located on the control/blower assembly and are then wired directly to the main load bus.
  • Required power for the blower motors is 400V, 3-phase, 50 Hz, 14 amps/phase, and is derived directly from the load bus.
  • Make sure that the correct phase rotation is wired to the fan motor. Improper phase rotation will cause the fan to run in the reverse direction. The cooling air should be pushed from the fan, across the resistor elements, and out the top exhaust hood/duct. This phase rotation check is mandatory each time the source or fan connections are changed.
  • Safe practice dictates that the fan power be wired through a safety disconnect switch that can be locked out.


C- Grounding

  • A permanent ground conductor must be connected to the Load Bank enclosure by an individual ground wire to prevent a potential above ground on the enclosure. There is a ground nut in the base of each Load Bank frame for this connection. 
  • This ground conductor should be run with the load power conductors to provide the lowest impedance fault path. The ground nut must be connected to both the power source frame and to a good earth ground. The ground conductor should be sized per the National Electrical Code Table 250.122, if not superseded by local codes.


12- Control Connections:

A- Load Shed Controller

Wire the current transformer into the control circuitry as shown on Interconnection Diagram in below.

Interconnection Diagram


B- Control Panel Connection

  • The control panel is made to be mounted in a 19-inch rack-type enclosure. Connect terminals of the control panel to terminals of the Load Bank as shown on Interconnection Diagram above.
  • Control power is obtained internally from a 400:120 volt, 50 Hz control transformer mounted in the Load Bank and wired to the blower motor circuit. see Schematic Diagram-1 in below.
Schematic Diagram-1
Schematic Diagram-2


Schematic Diagram-3

13- Theory of Operation:

  • The automatic operation circuitry of the Load Bank allows the generator to maintain a minimum percentage of its rated output for efficient generator operation. The control range is generally between 60% and 80% of the output rating of the generator.
  • When the Auto/Transfer Contact is closed, the Auto Load Shedder will be active. The Auto Load Shedder will automatically start the blower and apply load steps.
  •  The load steps are added in sequence, usually from smallest to largest and removed in reverse sequence; first on/last off, last on/first off.
  • As the load on the generator lessens, the Load Shed Controller will automatically apply load steps until the overall generator load exceeds the minimum set point.
  • The Load Shed Controller automatically removes load steps when the generator load exceeds the maximum set point.
  • When the Auto/Transfer Contact is opened, the Auto Load Shedder will be inactive and the blower will be stopped automatically.


Notes:

  • The current set points for the Load Shedder option have been preset by
    the factory.
  • Before the current set point(s) can be adjusted, the operator must determine the maximum load the Controller should maintain (generally 60% to 80% of the total generator kVA).





Example for Load Shedder Set Point Calculation

If we have the following data:

  • Generator Resistive Rating = 2000 kW @ 480 VAC, 3 PH.
  • Load Bank Shedder Capacity: 1000 kW @ 480 VAC, 3 PH.
  • Load Steps: 50, 50, 100, 100, 200, and 500 kW

What are the Load Shedder Set Points?

Solution:

Actual Generator Output = Load Bank Load + Building Load.


  • As the actual generator output reaches 1600 kW (80% of its rated), the controller “sheds” a load step; The current flowing to the building load at this point will be called the current trip point.
  • At 0 kW building load, the Load Bank will provide 1000 kW worth of load. Actual generator output will be 1200 kW (60%), after a preset time-delayed ramp up period.
  • As the building load increases and the actual generator output approaches 1600 kW (80%), the controller disables one load step; in this example, the last 500 kW load step “sheds”.
  • This load shed control continues as the building load continues to increase. If the building load decreases, causing the actual generator load to fall below 60%, the controller will add the appropriate load step(s) to maintain the desired range of control.

So, the Load Shedder Set Points (Trip points) will be as follows:

Trip Point
Building Load
Load Bank Load
Actual Generator Output
% Of Generator
>,= 0 to <600 kW
1000 kW
1200-1600 kW
(60%-80%)
TP.1
>,=600 to <1100 kW
500 kW
1200-1600 kW
(60%-80%)
TP.2
>,=1100 to <1300 kW
300 kW
1200-1600 kW
(60%-80%)
TP.3
>,=1300 to <1400 kW
200 kW
1200-1600 kW
(60%-80%)
TP.4
>,=1400 to <1500 kW
100 kW
1200-1600 kW
(60%-80%)
TP.5
>,=1500 to <1550 kW
50 kW
1200-1600 kW
(60%-80%)
TP.6
>,=1550 kW
0 kW
Building Load
(>,=73%)



In the next article, we will explain how to calculate the proper rating of Load Banks and how to perform the load bank test. So, please keep following.

The previous and related articles are listed in below table:

Subject Of Pervious Article
Article
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



Types of Load Banks:
First: According to the Load Element Type

Second: According To Portability,
Third: According To Cooling Method,
Fourth: According To Method of Control,
Fifth: According To Operating Mode,
Sixth: According To Application,
Seventh: According to no. of Load Steps,
Eighth: According to Load Bank Voltage and Frequency.


Load Bank Basic Components:
Enclosure,
Load elements,
Controls & Instrumentation,
Cooling system,
System Protections,
Load and supply Connectors.

The load bank essential circuits:
Control Circuit,
Cooling Circuit,
Load Element Circuit.
The Relationship of the Control, Cooling, and Load Element Circuits.


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