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A load behaviour is used to establish the voltage dependence of a load. A load behaviour is defined separately, and can be used for several (transformer) loads.

 

Adding a load behaviour is done with Insert | Trends | Load behaviour.

Editing a load behaviour is done with Start | Edit | Trends | Load behaviour.

Deleting a load behaviour that is not in use is done with Start | Edit | Delete | Load behaviour.

 

 

1)   The parameters Const R en Const X are always equal to respectively: 100 - Const P and 100 - Const Q.

 

Seven types of load behaviour have been predefined:

 

 

 

MODELLING

 

Load flow

Subdivision into constant power (P and Q) and constant impedance (R and X) is possible for both the actual and the imaginary part of the load. By then selecting a particular ratio between constant P and constant R, or constant Q and constant X, the voltage dependence of the load can vary between 0 and quadratic. The following applies to the load:

 

 Pload =  P * [ (const.P / 100) + (const.R / 100)( |U| / Unom)² ]

 Qload = Q * [ (const.Q / 100) + (const.X / 100)( |U| / Unom)² ]

where:

and

 const. P + const. R = 100 %

 const. Q + const. X = 100 %

 

Constant power

At constant power, the rated power will always remain constant, independently of the calculated node voltage |U|. The following then applies in a load flow calculation:

•if |U| increases : Iload decreases

•if |U| decreases: Iload increases

 

Constant impedance

At constant impedance, the Zload is determined at nominal node voltage using Pload and Qload. The following then applies in a load flow calculation:

•if |U| increases : Iload increases

•if |U| decreases: Iload decreases

 

Application

The modelling of loads is complicated because a typical load is composed of a large number of devices such as motors, fluorescent and incandescent lamps, air conditioners, household devices, electro-chemical devices, heaters, furnaces, etcetera. The exact composition of load is difficult to estimate. Also, the composition changes on time (season). The active and reactive power, consumed by the devices, depend on the actual voltage. Some examples:

•Asynchronous motors: the active power is constant at a voltage not exceeding 10% of the nominal value; the reactive power follows approximately the constant current model.

•Incandescent light bulbs: the active power changes with the deviation from the nominal voltage, to the power 1.5. This is achieved with 25% constant power and 75% constant admittance; virtually no reactive power is absorbed, so that the model does not matter.

•Resistive heating: the active power is quadratic depending on the deviation of the nominal voltage. This is achieved with 100% constant admittance; virtually no reactive power is absorbed.

 

In literature (Kundur, 1994) describes the load characteristics for a number of typical load classes. The following table has been derived from this:

 

 

In HV-network models it is custom to use the constant P and Q model for all general loads, because in fact these loads are connected at lower voltage levels to voltage regulated transformers.

In distribution networks the load behaviour tends to constant current or constant admittance, however with the increase in power electronic driven loads this is also shifting towards constant power.

 

Convergence

The increase in the load current at low node voltage, as occurs with a load which comprises constant power, can lead to divergence of a load flow solution. The increased load current causes a further lowering of the node voltage in this case. Increasing the proportion of constant impedance in the load behaviour will increase the chance of convergence. A load flow calculation almost always converges when the load behaviour for P and Q comprises 100 % constant impedance.

 

See also: Growth.