Design, Testing and Commissioning of First Phase Shifting Transformer in Indian Network

Interconnection of various generating resources is a common practice among utilities to increase the reliability of the system. With the interconnections the system becomes complex and it is a well known fact that the power flows in the interconnected network are mostly uncontrolled through certain segments of the network. Un controlled power flows in the networks are attributed to various reasons including impedances of parallel transmission lines, addition of new lines, addition of new generating stations, variation of loads and their phase angles. With the de regularization of power industry the grid operator requires tools to ensure control on the power flows in the network. Moreover efficient utilization of the existing transmission assets is required for economical operation of the grid. Phase Shifting Transformer (PST), which is a flow control device, will address the above problems in the operation of the network. PST also known as Quadrature Booster (QB) is extensively used worldwide to control or regulate power flows in power system networks. It alters or controls the power flow in the network by creating a specified phase shift the magnitude of the same is altered through an on load tap changer.

For the first time in Indian power system a Phase Shifting Transformer is designed and manufactured to address the power flow problems. Kothagudem Thermal Power Station (KTPS) in Andhra Pradesh of Indian Power grid has a generating capacity of 1220 MW. There are number of parallel 220 kV lines emanating from KTPS complex to various load centres for evacuation of the power from this complex. With the addition of a new 500 MW generation unit the expansion of network has become a necessity for evacuation of power. A double circuit 400 kV transmission line is planned for this purpose and the new lines are expected to carry their full capacity by reducing the present burden on the existing 220 kV network in the generating station. For this purpose it is proposed to interconnect the 400 and 220 kV buses through an interconnecting transformer. The initial load flow analysis with the proposed interconnection indicated the 400kV lines remain under utilized and the 220 kV lines are further over loaded.

The solution to the above problem is addressed by designing a Phase Shifting cum interconnecting transformer between the existing 220 kV bus and new 400 kV bus. This paper presents the experience of design, manufacture and commissioning of 315 MVA ±15° Phase Shifting transformer for power flow control application. PST proposed for this application is a combination of series and shunt transformers. Shunt transformer is a four winding 400/220/55/33 kV transformer of 315 MVA capacity and the Series transformer is of 35/55 kV 105 MVA. Vacuum tap changer is considered for better arc quenching properties and reduced maintenance requirements. The paper discusses load flow and simulation studies for the conceptual design. The detailed design, testing of the first PST for Indian Power grid will be presented.