Capacitive Voltage Regulation in Motor Starting

When running a motor starting calculation, EasyPower performs a regular power flow calculation to model the source as a Thevenin equivalent. Utility information that is entered in EasyPower, by default, maintains a power flow solution based on the utility voltage controlled at 1.0 pu at an angle of 0.

To maintain the control voltage at the utility bus, the internal Thevenin voltage of the source depends on the loading conditions. When there are no capacitors, larger internal voltage is required to maintain the utility bus voltage at 1.0 pu. Larger internal (Thevenin) voltage is used when the motor starts. When the capacitors are added to a system, less internal voltage is required.

Proposed Modeling Solution

Model the utility as an equivalent system, using an infinite source utility behind a cable that limits the point of connection to the same fault level. This is the Thevenin equivalent for the source. In this case, a control bus is maintained for power flow and is identified as the bus in which the infinite source exists. During regular power flow analysis, you will have some voltage drop at the point of connection. During motor start calculations, you will achieve the expected results. Turning on capacitors will improve the voltage at the point of connection.

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