Switching Principle of On Load Tap Changers for Power Transformers

Posted on September 21, 2017 in Transformers by With 0 Comment
Switching Principle of On Load Tap Changers for Power Transformers

Switching Principle of On Load Tap Changers for Power Transformers

Power transformers equipped with on load tap changers (OLTCs) have been the main components of electrical networks and industrial applications. On load tap changers (OLTCs) enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption.

From the start of tap-changer development, two switching principles have been used for load transfer operation – the high-speed resistor-type OLTCs and the reactor-type OLTCs.

The majority of resistor-type OLTCs are installed inside the transformer tank (in-tank OLTCs) whereas the reactor-type OLTCs are in a separate compartment which is normally welded to the transformer tank.

The on load tap changers (OLTCs) consists of a high speed resistor transition diverter switch, tap selector, driving mechanism and external driving shaft.

In an application it may be needed:

  1. To supply a desired voltage to the load.
  2. To counter the voltage drops due to loads.
  3. To counter the input supply voltage changes on load.

Switching Principle of On Load Tap Changers for Power Transformers

The OLTC changes the ratio of a transformer by adding or subtracting to and turns from either the primary or the secondary winding. The transformer is therefore equipped with a regulating or tap winding which is connected to the OLTC.

Principle winding arrangement of a regulating transformer in wye delta connection

Figure 1 – Principle winding arrangement of a regulating transformer in wye delta connection

Figure 1 shows the principle winding arrangement of a 3-phase regulating transformer, with the OLTC located at the Wye-delta-connection in the high voltage winding.

Loss of system load with single contact switching

Figure 2 – Loss of system load with single contact switching

Simple changing of taps during an energized status is unacceptable due to momentary loss of system load during the switching operation (Figure 2). The “make (2) before break (1) contact concept”, shown in Figure 3, is therefore the basic design for all OLTCs. The transition impedance in the form of a resistor or reactor consists of one or more units that bridge adjacent taps for the purpose of transferring load from one tap to the other without interruption or appreciable change in the load current. At the same time they limit the circulating current (I C ) for the period when both taps are used. Normally, reactor-type OLTCs use the bridging position as a service position and the reactor is therefore designed for continuous loading.

Basic switching principle “make (2) before break (1)” using transition

Figure 3 – Basic switching principle “make (2) before break (1)” using transition

The voltage between the taps mentioned above is the step voltage, which normally lies between 0.8 % and 2.5 % of the rated voltage of the transformer.

The main components of an OLTC are contact systems for make and break currents as well as carrying currents, transition impedances, gearings, spring energy accumulators and a drive mechanism. Depending on the various winding arrangements and OLTC-designs, separate selector switches and change-over selectors (reversing or coarse type) are also used.

Basic arrangements of Regulating Windings

The following basic arrangements of tap windings are used (Figure 4):

Basic connections of tap windings

Figure 4 – Basic connections of tap windings

Linear arrangement (Figure 4 a), is generally used on power transformers with moderate regulating ranges up to a maximum of 20 %. The tapped turns are added in series with the main winding and changes the transformer ratio. The rated position can be any one of the tap positions.

With a reversing change-over selector (Figure 4 b) the tap winding is added to or subtracted from the main winding so that the regulating range can be doubled or the number of taps reduced. During this operation, the tap winding is disconnected from the main winding. The greatest copper losses occur, however, in the position with the minimum number of effective turns. This reversing operation is realized using a change-over selector which is part of the tap selector or of the selector switch (arcing tap switch). The rated position is normally the mid position or neutral position.

The double reversing change-over selector (Figure 4 c) avoids the disconnection of tap winding during the change-over operation. In phase-shifting transformers (PST) this apparatus is called the advance-retard switch (ARS ).

Using a coarse change-over selector (Figure 4 d) the tap winding is connected either to the plus or minus tapping of the coarse winding. During coarse selector operation, the tap winding is disconnected from the main winding. In this case, the copper losses are lowest in the position of the lowest effective number of turns. This advantage, however, places higher demands on insulation material and requires a larger number of windings.

The multiple coarse change-over selector (Figure 4 e) enables multiplication of the regulating range. It is mainly used for industrial process transformers (rectifier/furnace transformers). The coarse change-over selector is also part of the OLTC.

References:

  1. On Load Tap Changers for Power Transformers by Dr. Dieter Dohnal

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Hello, I'm Kalpesh, An Electrical Engineer and the founder of Substation System.

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