Phase-Shifting Transformer tests

A phase-shifting transformer is a specific form of electrical transformer used to manage the flow of active power on 3Ø electric transmission lines. It is often called a phase angle regulating (PAR) in the US and a quadrature booster in the UK. In this application note, we will use the phase-shifting (PST) term.

Physically, phase-shifting transformers are static machines whose general construction characteristics are in some respects similar to conventional transformers with two separate transformers: A Shunt unit or Exciting unit (EU) and a series unit (SU). Due to their internal connections, they make it possible to obtain a phase shift between the input and output voltages. The phase shift between the input and output voltages is obtained by the addition in series with the line of a voltage taken or generated by another phase. Using the on-load tap changer, a variable angle can be obtained by changing tap positions.

When two transmission lines are in parallel (Fig.1.A), direct manipulation of angle allows adjusting the distribution of the current between the two lines and optimizing the capacity of transmission while avoiding the overloads [1]. Thus, phase-shifting transformers provide a means of relieving overloads on heavily loaded circuits and rerouting power through more convenient paths.

WHY WE SHOULD PERFORM TESTS FOR PHASE-SHIFTING TRANSFORMER?

Due to the predominant inductive nature of the power system, an active power flow between source and load must be achieved with a phase lag between terminals. Phase-Shifting Transformers are a preferred tool to achieve this goal.

The WACC cost of a PST can be expensive: as much as (4-6 million GBP) about (6–9 million USD) for a unit rated over 2 GVA. However, the gain in flexibility, in the speed of operation, also in the loss in line (a better balance between the lines limits the losses by Joule effect) justifies the investment and thus can quickly recover the cost of PSTs as long as they are placed under constant supervision and maintenance.

PST is usually used in critical nodes in the network, in case of any malfunction, maintenance time is very important, so both the transport system operators (TSOs) and the distribution system operators (DSOs) should test and maintain the PSTs permanently using advanced and sophisticated devices under the requirements of the international standard (IEC 60076) [2], in order to control power flow as much as possible.

HOW DO WE TEST PHASE-SHIFTING TRANSFORMER?

The terminals of the Exciting unit of the PST installed in the field are usually not accessible. Therefore, specific tests of the EU and the SU are usually performed only in the factory. the list below shows some of the PSTs tests, which are part of standard acceptance testing of the series and exciting units.

• Turns Ratio
• Exciting Current
• Phase shift
• DC Winding Resistance: (see Application Note: ‘’How Do We Measure Transformer Winding Resistance?’’)
• Dynamic Winding Resistance (OTCL) (see Application Note: ‘’Tests on the on-load tap changer (OLTC)’’)

The tests mentioned above are suitable to show the characteristic behaviour of the PST and are generally part of any factory acceptance test procedure.

• To perform a DC Winding Resistance and Dynamic Winding Resistance (OTCL) measurement a winding resistance test set is used (WinRes-20).
• Performing Turns Ratio, Exciting Current and Phase shift tests with the required accuracy, under the international standards and without making any complicated calculations by hand, will be so easy even for less trained staff by using the HighTest Turns Ratio Tester (TURA-X).

HighTest Technology Ltd. has developed a wonderful true three-phase test device to perform a simultaneous measurement of PST’s tests (Turns Ratio, Exciting Current and Phase shift) mentioned above.

HIGHTEST DEVICE USED TO MEASURE PHASE ANGLE OF PHASE-SHIFTING TRANSFORMER.

The fully automatic and true three-phase turns ratio tester TURA-X  based on a state-of-the-art technology using the most advanced technique available today, enables users to perform easily the two types of turns ratio tests: simultaneous 3-phase test and sequential 3-phase test, In addition, it is capable to perform the test for any transformer type without a need to know its winding configuration and its phase displacement.

Reference:

[1] Guile, A. Paterson, W. (1977). Electrical Power Systems vol 1. Pergamon. ISBN 978-0-08-021729-1.                                                                                                                                                      [2] IEEE Guide for the Application, Specification, and Testing of Phase-Shifting Transformers,” in IEEE Std C57.135-2001, vol., no., pp.1-52, 24 May 2002.