Power system analysis software: Sizing study; Fault analysis

Sizing study

The sizing study selects the conductor size based on the minimum conductor material and cross-sectional area necessary to meet defined feeder-current-carrying capability, and associated voltage drop criteria. Transformers are sized based on their full load kVA rating.

Feeder sizing:

Feeder sizes are based on the design load value from the demand load study. The demand load study calculates the total connected demand and design load in each branch of the value is larger than its demand load value. The NEC (USA) requires branch circuits that serve continuous loads to be rated so that not more than 80% of the feeder current rating is used. Selecting a design load value of 125% (1/80) of the demand load value meets the NEC standard. The feeder current rating is defined as the current in amperes that a conductor may carry continuously under the conditions of use without exceeding its temperature rating.

The sizing study bases its calculations on two separate criteria: the minimum conductor cross-sectional area to meet feeder rating values and a user-defined voltage drop value. If you consider parallel feeder combinations for a specified conductor type, the sizing study can select multiple feeders in parallel for that conductor. Cable sizes are usually specified in a cable library in order to be available to the sizing study. Derating factors are determined based on the temperature derating factor and duct bank design detail criterion.

The sizing study selects the cable that best meets defined current rating values and has the smallest cross-sectional area. Once the cable is selected, the voltage drop for the cable or cable pair is calculated. If the voltage drop criterion is exceeded, the sizing study selects the next larger cable size and begins the comparison of cross-sectional area, rated current and voltage drop.

The sizing study algorithm determines the feeder branch design load value in amperes, then determines the selected feeder design current rating. The feeder design rating is the product of the rated current, the temperature derating factor and the number of parallel cables. The design load value is the rated size of the load multiplied by specified demand factors and the long continuous load factor (or design factor). Once the current rating conditions are met, the sizing study then checks the calculated voltage drop on the cable, based on the branch design load current and power factor, cable impedance, and length.

The voltage drop is calculated using the following formula:

3cosXsin

% Voltage drop = 100

If the voltage drop exceeds the specified level, the sizing study selects the next largest feeder and restarts the sizing study in order to select a cable or combination of parallel cables which meets the rating criteria of minimum cross-sectional area and acceptable voltage drop.

Transformer sizing:

Transformer sizing is normally based on either the calculated branch demand or design load value, depending on user selection. The sizing algorithm compares the demand or design load value to the transformer's full load size. The transformer's full load size is:

Full load size = Nominal kVA rating × Transformer capacity factor

Typical capacity factors are:

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Transformer Cooling Characteristic Capacity Factor Dry type (DT) 1.00 Oil/air cooled (OA) 1.15 Oil/air/forced air (OAFA) 1.25 power system. Some loads are defined as continuous loads and, as such, the design load

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Transformer feeders:

Feeders for the primary and secondary of transformers are based on a factor (usually 125%) of the transformer's full load rating and an allowable voltage drop criteria. This factor is usually defined in the demand load study set-up dialog box.

Fault analysis

A fault analysis program derives from the need to adequately rate switchgear and other busbar equipment for the maximum possible fault current that could flow through them. Fault analysis programs were also developed in the 1950s alongside load flow programs.

Initially only three-phase faults were considered and it was assumed that all busbars were operating at nominal voltage prior to the fault occurring. The load current flowing prior to the fault was also neglected.

By using the results of a load flow prior to performing the fault analysis, the load currents can be added to the fault currents allowing a more accurate determination of the total currents flowing in the system.

Unbalanced faults can be included by using symmetrical components, mathematically dividing fault currents in negative sequence, positive sequence and zero sequence currents.

When the fault levels in an industrial plant are calculated the contributions of motors and generators should also be taken into account. Their contribution to the fault current can change the final value significantly. Modern programs also take the DC effect into account.