On-load tap-changers for power transformers

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.

Over the decades both principles have been developed into reliable transformer components which are available in a broad range of current and voltage applications. These components cover the needs of today’s network and industrial process transformers and ensure optimal system and process control [1].

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 (fig. 1).

This paper mainly refers to OLTCs immersed in transformer mineral oil. The use of other insulating fluids or gas insulation requires the approval of the OLTC manufacturer and may lead to a different OLTC design, as shown in chapter 4.2.2.

The OLTC changes the ratio of a transformer by adding or subtracting 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.

Figure 2 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.

Simple changing of taps during an energized status is unacceptable due to momentary loss of system load during the switching oper ation (fig. 3). The “make (2) before break (1)" contact concept, shown in Figure 4, 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 ( IC ) 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.

The voltage between the taps mentioned above is the step voltage U_ir , 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 (details in chapter 3) and OLTC-designs, separate selector switches and change-over selectors (reversing or coarse type) are also used. 

The following basic arrangements of tap windings are used (fig. 5): Linear arrangement (fig. 5 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 (fig. 5 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 (for problems arising from this disconnection see chapter 6.2). The greatest copper losses occur, however, in the position with the min i mum 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 (fig. 5 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 (fig. 5 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 (special winding arrangements can cause the same disconnection problems as described above; in addition the series impedance of coarse winding/tap winding must be checked – see chapter 6.3). 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 (fig. 5 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.

Which of these basic winding arrangements is used in each individual case depends on the system and operating requirements. These arrangements are applicable to two winding transformers as well as to autotransformers and to phase-shifting transformers (PST). Where the tap winding and therefore the OLTC is inserted in the windings (high-voltage or low-voltage side) depends on the transformer design and customer specifications.

Two winding transformers with wye-connected windings have regulation applied to the neutral end, as shown in figure 6. This results in relatively simple and compact solutions for OLTCs and tap windings.

Regulation of delta-connected windings (fig. 7) requires a three-phase OLTC whose three phases are insulated according to the highest system voltage applied (fig. 7 a), or 3 single-phase OLTCs, or 1 single-phase and 1 two-phase OLTC (fig. 7 b). Today, the design limit for three-phase OLTCs with phase-to-phase insulation is the highest voltage for equipment of 145 kV (BIL 650 kV). To reduce the phase-to-phase stresses on the delta-OLTC the three pole mid-winding arrangement (fig. 7 c) can be used.

For regulated autotransformers, fig. 8 shows various circuits. The most appropriate scheme is chosen with regard to regulating range, system conditions and/or requirements, as well as weight and size restrictions during transportation. Autotransformers are always wye-connected. 

• Neutral end regulation (fig. 8 a) may be applied with a ratio above 1:2 and a moderate regulating range up to 15 %. This operates with variable flux.  
• A scheme shown in fig. 8 c is used for regulating high voltage U₁.
• For low voltage U₂ regulation, the circuits fig. 8 b, 8 d, 8 e and 8 f are applicable. The arrangements fig. 8 e and 8 f are two core solutions. Circuit fig. 8 f operates with variable flux in the series transformer, but it has the advantage that a neutral end OLTC can be used. In the case of arrangement according to fig. 8 e, the main and regulating transformers are often placed in separate tanks to reduce transport weight. At the same time, this solution allows some degree of phase shifting by changing the excitation connections within the intermediate circuit.

Over the last few years, the importance of phase-shifting transformers used to control the power flow on transmission lines in meshed networks has been steadily increasing [2]. The fact that IEEE provides a “Guide for the Application, Specification and Testing of Phase-Shifting Transformers“ [3] proves the demand for PSTs. These transformers often require regulating ranges which exceed those normally used. To achieve such regulating ranges, special circuit arrangements are necessary. Two examples are given in fig. 9 and fig. 10.

Fig. 9 shows a circuit with direct line-end regulation, fig. 10 an intermediate circuit arrangement. Fig. 9 illustrates very clearly how the phase-angle between the voltages of the source and load systems can be varied by the OLTC position. Various other circuit arrangements have been implemented.

The number of OLTC operations of PSTs is much higher than that of other regulating transformers in networks (10 to 15 times higher). In some cases, according to regulating ranges – especially for line-end arrangements (fig. 9) – the transient overvoltage stresses over tapping ranges have to be limited by applying non-linear resistors. In addition, the short-circuit current ability of the OLTC must be checked, as the short-circuit power of the network determines the current. The remaining features of OLTCs for such transformers can be selected according to the usual rules (see chapter 6).

Significant benefits resulting from the use of a PST are:

• Reduction of overall system losses by eliminating circulating currents
• Improvement of circuit capability by proper load management
• Improvement of circuit power factor
• Control of power flow to meet contractual requirements

4.1.1 Resistor oil-type OLTCs
The OLTC design that is normally used for higher ratings and higher voltages comprises a diverter switch (arcing switch) and a tap selector. For lower ratings, OLTC designs in which the functions of the diverter switch (arcing switch) and the tap selector are combined in a selector switch (arcing tap switch) are used.

With an OLTC comprising a diverter switch (arcing switch) and a tap selector (fig. 11), the tap-change operation takes place in two steps (fig. 12). The next tap is first preselected by the tap selector at no load (fig. 12 position a - c). The diverter switch then transfers the load current from the tap in operation to the preselected tap (fig. 12 position d - i). The OLTC is operated by means of a drive mechanism. The tap selector is operated by a gearing directly from the drive mechanism. At the same time, a spring energy accumulator is tensioned, which operates the diverter switch – after release at a very short time interval – independently of the motion of the drive mechanism. The gearing ensures that this diverter switch operation always takes place after the tap preselection operation has finished. The switching time of a diverter switch is between 40 and 60 ms with today’s designs. During diverter switch operation, transition resistors are inserted (fig. 12 position e - g) which are loaded for 20–30 ms, i. e. the resistors can be designed for short-term loading. The amount of resistor material required is therefore relatively small. The total operation time of an OLTC is between 3 and 10 seconds, depending on the respective design. 

A selector switch (arcing tap switch) as shown in fig. 13 carries out the tap-change in one step from the tap in service to the adjacent tap (fig. 14). The spring energy accumulator, wound up by the drive mechanism actuates the selector switch sharply after releasing. For switching time and resistor loading (fig. 14 position b - d), the above statements apply. The details of switching tasks including phasor diagrams are described in Annex A of [4], [5] and [6].
 

4.1.2 Reactor oil-type OLTCs
The following types of switching are used for reactor oil-type OLTCs:

• Selector switch (arcing tap switch)  
• Diverter switch (arcing switch) with tap selector

All reactor-type OLTCs are compartment types where the preventive autotransformer (reactor) is not part of the OLTC. The preventive autotransformer is designed by the transformer manufacturer and located in the transformer tank.

Today only selector switches (arcing tap switches) for voltage regulators are still in production whereas the reactor vacuum-type OLTCs are going to be the state-of-the-art in the field of power transformers. This oil technology is therefore not further discussed in this paper. For more detailed information about the switching tasks and phasor dia grams of reactor oil-type OLTCs, please see Annex B of [4], [5] and [6].

4.2.1 Basic principles of vacuum switching technology
Over the course of the last four decades, vacuum switching technology has become the predominant switching technology in the areas of medium-voltage substations and high-capacity power contactors, and has replaced oil- and SF6-technology. Today the demand for circuit breakers in the medium power voltage segment worldwide is mostly covered by vacuum-type circuit breakers.

Vacuum switching technology also best meets the new application requirements and increased performance demands for OLTCs by end users. Its superiority over competing switching technologies in the low and medium power ranges is based on a number of technical features  [7], [8]:

• The vacuum interrupter is a hermetically-sealed system  
  • There is no interaction with the surrounding medium, despite the arc
  • The switching characteristics do not depend on the surrounding medium
• The arc (drop) voltage in vacuum is considerably lower than in oil or SF6
  • Low energy consumption
  • Reduced contact wear
• Elimination of the insulating medium as the arc-quenching agent
  • Elimination of by-products e. g. carbon when using transformer oil
  • On-line filter is unnecessary
  • Easy disposal
• No aging of the quenching medium
  • Constant or even improving switching characteristics throughout the entire lifespan of the vacuum interrupters (getter effect)
• No interaction/oxidation during switching
  • High rate of recondensation of metal vapor on contacts extends contact life
  • Constantly low contact resistance
• Extraordinary fast dielectric recovery of up to 10 kV/µs
  • Ensures short arcing times (maximum one half-cycle) even in the case of large phase angles between current and voltage or high-voltage steepness dU/dt after the current zero (converter transformers) [9].

4.2.2 Application of the vacuum switching technology to on-load tap-changers
When developing a vacuum interrupter for use in an OLTC, the unique parameters are:

• Mechanical life in transformer oil (or any other given insulating medium) for the operating temperature range and expected life-span of the OLTC
• Switching performance
• Contact life
• Physical dimension

Since the early 1970s, vacuum interrupters that complying with the features required by reactor-type OLTCs have been developed. These OLTCs, which are in general external compartment-type designs, did not dictate any special requirements with regard to the physical size of the interrupter. This is not the case with resistor-type OLTCs, which usually have a very compact design. Today, after more than four decades of development, vacuum interrupters have reached an advanced technical performance level. The use of modern clean room and furnace soldering technologies during the production process, and new contact system and material designs are some of the milestones of this reliable product. This has enabled considerably smaller vacuum interrupters to be designed, opening the door for application in resistor-type OLTCs with overall dimensions equivalent to those of conventional resistor-type OLTC designs (see fig. 15 and 16).

In figure 17 the contact wear due to current breaking is shown for conventional copper-tungsten contacts under oil and for vacuum interrupters. The rate is more than one decade smaller for vacuum interrupters (e. g. rate: 1/30 at 1,000 A). Apart from the contact material, the contact geometry is the most important factor for this current range and OLTC applications. This results in contact life, where vacuum interrupters easily reach numbers of switching operations over 600,000 without changing the interrupters. 

Investigations in electric arc furnace applications (EAF) cleary demonstrate the superior performance of the special contact material design for use in OLTCs.

Fig. 18 shows the contact system of opened vacuum interrupters after 300,000 EAF-operations in a steel mill in Turkey. The vacuum interrupters were installed in VACUTAP^® VRF I 1,300 with a step voltage of 1,400 V and a maximum through-current of 1,200 A.

The contact surfaces were smooth and the total contact wear less than 1 mm. The findings exceeds all MR’s expectations for the contact lifespan and demonstrate that these vacuum interrupters are very far from the end of their lifespan.

As already described in the introduction to chapter 4, the future trend – in other words, the increasing demand for more fire safety, greater environmental compatibility and more freedom of maintenance in transformer technology – must be considered during the design stage of new generations of OLTCs.

Vacuum switching technology, which has no interaction with the surrounding medium and the use of state-of-the-art alternative liquids such as natural and synthetic esters meet all of these requirements.

All the liquid-immersed VACUTAP^®-OLTCs presented in chapter 4.2.2 have therefore been designed and tested for mineral oil as well as for selected natural/synthetic esters [10], [11].

During operation of the reversing or coarse change-over selector, the tap winding is disconnected momentarily from the main winding. At this point, it assumes a potential that is determined by the voltages of the adjacent windings as well as by the coupling capacities to these windings and to grounded parts. In general, this potential is different from the potential of the tap winding before the change-over selector operation. The differential voltages are the recovering voltages at the opening contacts of the changeover selector and, when reaching a critical level, they are liable to cause inadmissible discharges on the change-over selector. If these voltages exceed a certain limit value (for special product series, the said limit voltages are in the range of 15 kV to 35 kV), measures must be taken regarding the potential control of the tap winding.

Particularly in the case of phase-shifting transformers with regulation at the line end (e. g. fig. 9), high recovery voltages can occur due to the winding arrangement. Figure 33a illustrates a typical winding arrangement of PST according to fig. 9. Figure 33b shows the diagram of this arrangement without limiting measures. As can be seen, the recovery voltages appearing at the change-over selector contacts are in the range of the system voltages on the source and the load side. An OLTC certainly cannot be operated under such conditions. This fact must be taken into account during the planning stage of the PST design [2], [3], [4], [6], [12].

There are three methods of solving the above-mentioned problem:

• One way of decreasing the recovery voltages is to install screens between the windings. These screens must have the potential of the movable change-over selector contact 0 (fig. 9). See Figures 34 a and 34 b.  
• The second way is to connect the tap winding to a fixed potential by a fixed resistor (tie-in resistor) or by a resistor which is only inserted during change-over selector operation by means of a potential switch. This resistor is usually connected to the middle of the tap winding and to the current take-off terminal of the OLTC (fig. 35).
• The third possibility is to use an advance retard switch (ARS) as a change-over selector (fig. 36). This additional unit allows the change-over operation to be carried out in two steps without interruption. With this arrangement, the tap winding is connected to the desired potential during the entire change-over operation. As this method is relatively complicated, it is only used for high power PSTs.

The common method for the potential connection of tap windings is to use tie-in resistors. The following information is required to dimension tie-in resistors:

• All characteristic data of the transformer such as: power, high and low voltages with regulating range, winding connection, insulation levels
• Design of the winding, i. e. location of the tap winding in relation to the adjacent windings or winding parts (in the case of layer windings)
• Voltages across the windings and electrical position of the windings within the winding arrangement of the transformer which is adjacent to the tap winding
• Capacity between tap winding and adjacent windings or winding parts
• Capacity between tap winding and ground or, if present, grounded adjacent windings
• Surge stress across half of tap winding
• Service and test power frequency voltages across half of the tap winding

During the operation of the diverter switch (arcing switch) from the end of the tap winding to the end of the coarse winding and vice versa (passing mid-position, s. fig. 37 a), all turns of the whole tap winding and coarse winding are inserted in the circuit.

This results in a leakage impedance value which is substantially higher than during operation within the tap winding where only negligible leakage impedance of one step is relevant (fig. 37 b). The higher impedance value in series with the transition resistors has an effect on the circulating current which is flowing in the opposite direction through coarse winding and tap winding during diverter switch operation. Consequently a phase shift between switched current and recovery voltage takes place at the transition contacts of the diverter switch and may result in an extended arcing time.

In order to ensure optimal selection and adaptation of the OLTC to these operating conditions, it is necessary to specify the leakage impedance of coarse winding and tap winding connected in series.