TEXT 20.Distribution systems.Power System Loads.

The physical structure of most power systems consists of generation facilities feeding bulk power into a high-voltage bulk transmission network, that in turn serves any number of distribution substations. A typical distribution substation will serve from one to as many as ten feedercircuits. A          typical feeder circuit may serve numerous loads of all types. A light to medium industrial customer may take service from the distribution feeder circuit primary, while a large industrial load complex may take service directly from the bulk transmission system. All other customers, including residential and commercial, are typically served from the secondary of distribution transformers that are in turn connected to a distribution feeder circuit.

Load Classification

The most common classification of electrical loads follows the billing categories used by the utility companies. This classification includes residential, commercial, industrial, and other. Residential cus­tomers are domestic users, whereas commercial and industrial customers are obviously business and industrial users. Other customer classifications include municipalities, state and federal government agencies, electric cooperatives, educational institutions, etc.

A useful approach to classification of loads is by breaking down the broader classes into individual load components.The components of a particular load, be it residential, commercial, or industrial, are individually defined and modeled. These load components as a whole constitute the composite load and can be defined as a “load window.”

Modeling Applications

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.

Static applications: Models that incorporate only the voltage-dependent characteristic include the following.

• Powerflow (PF)

• Distributionpowerflow (DPF)

• Harmonicpowerflow (HPF)

• Transmissionpowerflow (TPF)

• Voltagestability (VS)

Dynamic applications: Models that incorporate both the voltage- and frequency-dependent charac­teristics include the following.

• Transientstability (TS)

• Dynamicstability (DS)

• Operatortrainingsimulators (OTS)

There are essentially two approaches to load modeling: component based and measurement based The component-based approach is a “bottom-up” approach in that the different load component types comprising load are identified. Each load component type is tested to determine the relationship between real and reactive power requirements versus applied voltage and frequency. A load model, typically in polynomial or exponential form, is then developed from the respective test data. The range of validity of each model is directly related to the range over which the component was tested. For convenience, the load model is expressed on a per-unit basis (i.e., normalized with respect to rated power, rated voltage, rated frequency, rated torque if applicable, and base temperature if applicable). A composite load is approximated by combining appropriate load model types in certain proportions based on load survey information.

The measurement approach is a “top-down” approach in that measurements are taken at either a substation level, feeder level, some load aggregation point along a feeder, or at some individual load point. Variation of frequency for this type of measurement is not usually performed unless special test arrangements can be made. Voltage is varied using a suitable means and the measured real and reactive power consumption recorded. Statistical methods are then used to determine load models. A load survey may be necessary to classify the models derived in this manner. The range of validity for this approach is directly related to the realistic range over which the tests can be conducted without damage to customers’ equipment.

Devices Contributing to Modeling Difficulties

Some load components have time-dependent characteristics that must be considered if a sequence of studies using static models is performed that represents load changing over time. Examples of such a study include Voltage Stability and Transient Stability.

The devices that affect load modeling by contrib­uting abrupt changes in load over periods of time are protective relays, thermostatically controlled loads, voltage regulation devices, discharge lamps (Mercury Vapor, Sodium Vapor, and Fluorescent Lamps).

1. Read and translate the text.

2. Complete the table below using a dictionary.

3. Read and decide if the following statements are true (T) or false (F).

1) Domestic users may take service directly from the bulk transmission system.

2) Static application involve both the voltage- and frequency-dependent charac­teristics.

3) A load model is developed from the respective test data.

4) According to a “top-down” approach measurements are taken at a feeder level only.

5) There are numerous devices that affect load modeling.

4. Writedownthedifferences:

5. Match the words from the text with their corresponding definitions.