Importance of specifying capacitor placement in distribution system.

A distribution feeder with fixed and switched capacitors is typically designed with consideration for the location and size of capacitors. Their locations are determined optimally to minimize feeder losses and comply with voltage standards at different loading levels. Banks are usually placed near a load or in areas where low voltage results from loading to provide reactive power locally so the current for the reactive load does not have to be sent through the distribution system. The reduced amount of reactive power flowing on the distribution lines between the source and capacitor bank allows a lower current flow and improves the power factor. Line losses are smaller and thus the voltage becomes higher at the load. Here are some of the benefits of capacitor use:

• Released capacity in the substation and/or feeders
• Improved transfer capabilities
• Higher voltages
• Reduces losses/energy savings
• Additional generation and transmission capacity

Placing capacitors near the load creates the risk of high voltage if the load value declines or is tripped off. As the load cycles off, capacitors can overcompensate and bring about problems with high voltage, motor damage, excessive switching transients, and even flashover. Capacitor compensation is useful for fixed loads. Switched capacitors can be used to meet the demand of time-varying loads. The switching of banks may cause considerable system transients. The level and duration of these transients is dependent on the time constant associated with the line and capacitor. It should also be noted that an operating capacitor bank makes a power system more sensitive to transients since a large di/dt through the capacitor causes a large voltage spike.

Capacitor kV Rating
Capacitors are designed for 10% overvoltage and 30% over current. Oftentimes, capacitors are installed with ratings higher or even lower that the expected voltage at their connection point. The output kvar of a capacitor bank varies by the square of the voltage.

Sactual = (V squared actual/ V squared rated) x Srated

If a voltage rating is not specified for a capacitor then the nominal voltage is assumed. If the rated voltage of a capacitor is known then that value should be entered into the capacitor editor.

Substation Control
Capacitor switched modules can be tripped or closed based on substation or feeder kvar or power factor levels. This type of operation simulates radio based capacitor controls that operate capacitors remotely from the substation. There are two types of substation controls in the SynerGEE model:

• Substation kvar
• Substation pf

When either of these control options is selected, the capacitor will operate based on the load flow INTO the substation serving the capacitor. If a substation is not a part of the model then the feeder values will be used.
Capacitor Placement
Capacitor placement is a comprehensive tool for aiding you in finding locations for new capacitor installations. The application is designed to operate under a variety of placement objectives and to provide results on a feeder-by-feeder basis in a concise manner. The analysis is an interactive application that allows the engineer to act as the optimizer as opposed to the standard report-based application.

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