It is helpful to understand the applications of load modeling before discussing particular load characteristics.

The applications are divided into two broad categories: static (‘‘snap-shot’’ with respect to time) and dynamic (time varying). Static models are based on the steady-state method of representation in power flow networks. Thus, static load models represent load as a function of voltage magnitude.

Dynamic models, on the other hand, involve an alternating solution sequence between a time-domain solution of the differential equations describing electromechanical behavior and a steady-state power flow solution based on the method of phasors.

One of the important outcomes from the solution of dynamic models is the time variation of frequency. Therefore, it is altogether appropriate to include a component in the static load model that represents variation of load with frequency.

The lists below include applications outside of Distribution Systems but are included because load modeling at the distribution level is the fundamental starting point.

Static applications: Models that incorporate only the voltage-dependent characteristic include the following.
* Power flow (PF)
* Distribution power flow (DPF)
* Harmonic power flow (HPF)
* Transmission power flow (TPF)
*Voltage stability (VS)

Dynamic applications: Models that incorporate both the voltage- and frequency-dependent characteristics include the following.
* Transient stability (TS)
* Dynamic stability (DS)
* Operator training simulators (OTS)

Strictly power-flow based solutions utilize load models that include only voltage dependency characteristics.

Both voltage and frequency dependency characteristics can be incorporated in load modeling for those hybrid methods that alternate between a time-domain solution and a power flow solution,such as found in Transient Stability and Dynamic Stability Analysis Programs, and Operator Training Simulators.

Load modeling in this section is confined to static representation of voltage and frequency dependencies. Static models are justified on the basis that the transient time response of most composite loads to voltage and frequency changes is fast enough so that a steady-state response is reached very quickly.

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