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Monday, 8 September 2014

Phase Shifting Transformers: A crucial component for Transmission Lines

Phase Shifting Transformers (PST), also called Phase Angle Regulator (PAR), are crucial components in the ongoing endeavor for improved transmission capacity, reliability and security. They are an effective and economical tool to ensure reliable and efficient power flow control in overloaded transmission lines.  

PST help to control the power flow in the transmission line enabling the existing line to be loaded up to the thermal limits, deferring or avoiding the investment in new transmission lines. The electrical losses in the system and hence the operating cost are also minimized by balancing the power flow in the network and optimizing the power flow. System reliability is also increased by employing PSTs which help in mitigating post-contingency overloads and undesired load flows. The impedance of two or more lines, which are in parallel, determine the load sharing between them. In case of different impedance and hence unbalanced loading, PSTs help to balance the load sharing. 

Concept of Phase Angle Regulation:

The basic concept of phase angle regulation is the addition of an appropriate quadrature voltage component to the prevailing bus voltage in order to increase or decrease its magnitude or angle to the desired value. This injected voltage will change the existing phase angle of the system voltage. For comparatively small angular modifications, the resultant angular change is nearly proportional to the injected voltage. But with large angular compensations, the magnitude of the system voltage changes significantly.


Application of Phase Shifting Transformers:

PSTs have more windings and tap changer than the traditional power transformers and hence are more complex. Two 1630 MVA, 400 kV, + 18o PSTs are installed in Italy to increase the power transfer from France to Italy and to increase reliability. Similarly, to optimize power flow and to increase the system reliability three 1400 MVA, 400 kV, ± 25o units are installed in Belgium. A 600 MVA phase shifting transformer was manufactured by Siemens for a Slowenian sub-station. This PST had a No-load phase angle of ± 40o  with ± 32 steps. Thus, PSTs are crucial components in any power transmission line and are used through out the world. 


Phase Shifting Transformers by BHEL: 

India’s first PST, 315 MVA, 400/220/55/33 kV class, 3-phase, ± 15%, was developed by BHEL Bhopal for APGENCO. This PST is a quadrature type, dual core, dual tank design which consisted of two transformers, called the shunt and the series unit. The complexity of the transformer can be judged by the factor that it has 6 numbers of 220 kV bushing on the series transformer, whereas the shunt unit consisted of 15 number bushings.  
    
Modern phase angle regulators with fast power electronic control using thyristors etc can also be used to mitigate dynamic disturbances, for transient stability enhancement, damping of power oscillation and minimization of post-contingency overloads and corresponding voltage dips.

Ref:
·         S Sachdeva, R K Singh, S.K Gupta, J.S. Kuntia, and R.K.Tiwari, “Development of India’s first phase shifting transformer”, W& E International (Energy Section).

·         Hingorani & Gyugyi, “Understanding FACTS”, John Wiley India.