Power flow on transmission lines needs to be controlled in order to achieve the optimum utilization of transmission lines capacity.

The distribution of the power flow between two parallel lines is dictated by their impedances.  The line with smaller impedance carries more power and vice versa.

The active power and reactive power over a transmission line is given by,

Where, VS  is sending end voltage
.   Vis receiving end voltage
.   XL is line impedance
.   δ is power angle (angle between sending end and receiving end voltages)

As observed from the above formulae, active power transmitted can be controlled by
.  (i)   Altering sending/receiving end voltage(s)
.  (ii)  Lowering line impedance
.  (iii) By controlling power angle

Altering sending/receiving end voltage
Active power flow on transmission lines can be controlled either by altering sending end voltage or receiving end voltage.  But, whenever voltages are altered for controlling active power, it has a bigger impact on reactive power component.  Hence, this method is not effective.

Lowering line impedance
Total line reactance can be lowered by placing series capacitor to compensate line inductance. This increases not only active but also reactive power flow.  Hence, this method is also not effective.

Controlling power angle
By altering the power angle, active power can be increased with decrease in reactive power and vice versa as per above formulae.

Phase Shift Transformers (PSTs) are the best option to control power angle……
*  A phase shift transformers (PST) can be employed for power control on transmission
.   lines by altering power angle.
*  These are special transformers used to create a phase shift between the primary
.   side and secondary side voltages.
*  Both magnitude and direction of power flow can be controlled by varying the phase shift.

Types of PSTs
.  i)    Direct PSTs
.  ii)   Indirect PSTs
.  iii)  Symmetrical PSTs
.  iv)  Asymmetrical PSTs

Direct PSTs
The phase is obtained by connecting the windings in an appropriate manner.

Indirect PSTs
Connections are based on two separate transformers; one variable tap exciter to regulate the amplitude of the quadrature voltage and one series transformer to inject the quadrature voltage in the right direction.

Symmetrical PSTs
Creates an output voltage with an altered phase angle compared to the input voltage, but with same amplitude.

Asymmetrical PSTs
Creates an output voltage with an altered phase angle and amplitude compared to the input voltage.

Lets us consider direct, asymmetrical PST (as follows)…

Direct, Asymmetrical PSTs
The following is the configuration of the direct type asymmetrical PST.

The tap controls angle between sending end voltage and receiving end voltages by inducing a quadrature voltage. The direction of the phase shift can be controlled by switches.

From the above phasor diagram,

From above equation the output voltage is always greater than the input voltage.

Substitute α in the following equation for real power transmitted over the transmission line.

By Er. B S Jaya Prakash Nayak
Asst. Divisional Engineer (Elec) / Technical to CMD and SAP