Voltage and Power Flow Control

Module 4 : Voltage and Power Flow Control

Lecture 19 : Power Flow Control

Phase Shifting Transformers

Another method of changing power flow through a transmission line is by use of a phase shifter.

A phase shifter introduces a small phase shift in the voltage at its two terminals by injection of a series voltage. The series injected voltage of one phase is in phase with the line voltage across the other 2 phases. This implies that the voltage injected in one phase is in quadrature to the bus voltage at one terminal of that phase. This small quadrature voltage injection mainly affects the bus voltage angle at the other terminal (which has superscript 1). The magnitude is not affected much if the series injected voltage is small. This is illustrated in the figures below. In the phasor diagram, the mechanism of obtaining a phase shift of q is shown. Similarly, a phase shift of -q can be obtained by reversing the injected voltage.

A phase shifter can alter the power flows in a transmission system.

In the figures shown on the left, for the given phasor angles at both ends, the power flow in line 2 can be altered. Thus the sharing of the total power flow :

between the two lines can be altered. The expression for power flow in line 2 is given by,

A phase shifter can alter the power versus (d1-d2) curve in a way that benefits stability.

In particular, by changing q when the rotor angular separation increases due to a disturbance, it can ensure that the "restoring torque" on the machines at the two ends is positive for larger values of (d1-d2).

In the figures shown above, if only line 2 is present, then the electrical power flow P versus (d1-d2) with and without a phase shift is as shown in the figure on the right. Note that a phase shifter does not change the maximum value of electrical power.

A device such as a capacitor connected in series with a line, on the other hand, increases the maximum value of power (and therefore restoring torque) but does not cause much shift in the point until which restoring torques are positive.

Both series connected capacitor and phase shifter, therefore enhance stability (the mechanisms are however different). Therefore they permit increase in actual line loadability.