The Grid Optimization Blog

Load Coefficients (Load Varying with Voltages)

It is crucial to comprehend how the electric load reacts to changes in the supply voltage, especially when voltage reduction is now being utilized more often by utilities as means to reduce electric stress and demand response. After voltage reduction was implemented at a specific station, various engineering departments review the load change and compare the impact to their models and expectations. Often, the results don’t concur with their analysis, mainly due to the lack of modeling proper load coefficients. In the next paper, I can discuss the impact of voltage reduction on the substation transformers, sub-transmission feeders and the transmission system.

There are three types of loads, also known as load coefficients:

  • Constant power load
  • Constant current load
  • Constant admittance (inverse of the impedance) load

Even though several load flow programs models the electric load as constant power only, the other two types play a key role as well. Most modern equipment is of a constant power load type. The main difference between these types is in their response to changes in voltage....(Read rest of "Load Coefficients"...)

Discuss this Blog Entry 2

lilantha (not verified)
on Jul 15, 2014

Could you please give some examples for 3 types of loads.

Nicholas Pratley (not verified)
on Jul 15, 2014

Some examples of different types of load:

1. constant Power load: induction motors (if voltage is slightly depressed, torque decreases as the square of voltage, slip increases and current draw increases to compensate). The bulk of Industrial load consists of this type of load.

2. constant Current load: power electronic rectifiers. Perhaps the power supplies for computers and consumer electronics and ballasts for fluorescent, compact fluorescent and even LED lamps?

3. constant Impedance load: resistance heaters, such as in dishwashers and electric clothes dryers, as well as certain industrial heating/drying processes, and incandescent electric lighting.

In the past, the change in active power load on a distribution circuit has been about 1% for each 1% change in voltage, meaning that the sum of different types of loads behaves approximately as a constant current load. There is no substitute in modeling for having actual measurements! The voltage dependence characteristic will be unique to each feeder circuit, and will also evolve as the load changes over time and as the utility installs equipment like voltage regulators and capacitors.

At the higher-voltage (69 kV and above, let us say) Transmission level, the details of the individual load characteristics may be masked by the action of voltage control devices such as under-load tap changers (TCUL) on transformers, as well as voltage regulators on underlying distribution circuits. The action of these adjustable devices makes the voltage applied to the load more uniform even as the transmission level voltage is depressed, making the load appear to behave more like constant power type load, until the limit of the voltage control device's operating range is reached.

This explains why so much load is modeled as constant power load in transmission planning studies. The details of the circuits below about 69 kV are modeled by an "equivalent" load.

Users of power flow software should remain aware that the assumed characteristic of load in their data may be applicable only over the normal narrow range of steady-state voltages, and that transient recovery from contingencies (sudden equipment failures) especially may involve the voltage passing through some very low values for some time. Some loads may trip off during this time. For example, one use of CVR (Conservation Voltage Reduction) is to force the tripping-off of customers' air conditioning by deliberately decreasing the distribution supply voltage. An air conditioner uses an induction motor to drive a compressor. Under prolonged low voltage conditions, the motor will draw high current, largely reactive, to try to develop enough torque to keep the compressor turning. The high reactive current will cause even more voltage drop to the load, raising current still more until, after some delay, the high current will cause protective devices like circuit breakers to trip the load off. When a good deal of load trips off, the voltage rebounds.

Note that the common induction motor is a counter-example to the assertion that reactive power consumption decreases as voltage is depressed.

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Matthew C. Cordaro, PhD

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