Off-Circuit and On-Load Tap Changers For Transformers

Tap changer is an essential element of every transformer, be it a power transformer or a distribution transformer, It is used for changing the tapping connection of the transformer winding while it is de energized or on load, This paper discusses in detail the basic function, connection circuits, design concepts, construction and main parts of off-circuit and on load tap changers used in distribution and power transformers Various types of on-load tap changers and melor drive units are discussed. The significance of preventive and condlilion-based maintenance philesophy is highlighted. Annual maintenance and major inspeclion activities are also discussed.

Voltage Regulation

Voltage variation in electrical systems is a normal phenomenon because power demand changes constantly. It is essential to maintain the system voltage within the specified limits and power utilities are under obligation to their customers to maintain the supply voltage between such limits. By changing the turn ratio of transformer, the voltage ratio and secondary voltage is changed and voltage regulation is achieved. The device used for adding or subtracting the turns of primary or secondary winding of transformer is called a tap changer. The output voltage of transformer can be kept constant by adding or subtracting number of turns in the primary or secondary winding depending upon location of the tap changer. At lower voltages, the tap changer may be located at either the low voltage or high voltage winding, however, at medium, high and extra high voltage levels, tap changers are installed on high voltage winding. Tap changers can be divided into two categories: off-circuit and on-load. The off-circuit tap changers are installed on the distribution as well as power transformers.

On the distribution transformers, they are used for control of seasonal voltage variations and operate only when the transformer is de-energized. The on-load lap changers are installed on power transformers and used for control of daily voltage variations due to changing load, short period voltage variations, and the reactive and active power variations.

Without On-Load Tap Changer (OLTC) transformers, these parameters can only be controlled at the generating plant or by supply interruptions and de-energized tap changing. OLTC offers variable control to keep the supply voltage within the specified limits generally ±10%. The introduction of OLTC improved the operating efficiency of electrical systems considerably.

The Off-Circuit Tap Changer

The most economical method of changing the turn ratio of a transformer is the use of Off-Circuit Tap Changer (OCTC), also known as De-energized Tap Changer (DTC) as the transformation ratio can be adjusted only when transformer is de-energized. Usually the turn ratio of the high voltage (HV) winding is regulated to ±2.5% or ±5% to suit the local condition. The distribution transformers are normally equipped with 5 or 7 positions OCTC. Position 1 of the tap-changer (+ position) corresponds to the highest ratio, which gives the lowest voltage on the low voltage (LV) side. Position 5 or 7 of the tap-changer (- position) corresponds to the lowest ratio, which consequently gives the highest voltage on the LV side. The control shaft of the OCTC is brought through the cover or the tank wall. The shaft end is provided with a handle, position indicator and locking device. When the tap position is changed, the locking device must be secured to ensure that the OCTC has been set to the desired operating position. The OCTC shall be turned from one extreme position to another for contact wipe approximately at least once a year. This is necessary when the tap changer is used infrequently. For dry-type transformers, the de-energized tap changing is generally done by means of bolted links.

An off-circuit tap changer consists principally of three parts: operating handle projecting outside the transformer, fixed contact with connecting terminals and insulating shaft with moving contact as shown in Figure 1.

The basic transformer winding circuit arrangements using OCTC are shown in Figure 2.

They are: linear, single-bridging, double-bridging, series-parallel and star-delta. Depending upon the requirement, any one arrangement of the winding can be used to get desired voltage regulation.

The On-load Tap Changer

The on-load tap changer is a device for changing the tapping connection of a winding while the transformer is on load.

On-load regulation allows continuous adjustment of the transformer ratio. When a transformer's load increases, the system impedance causes the voltage to drop.

When the load decreases, the voltage rises.

The OLTC senses the change in voltage and adjusts regulating winding to keep the voltage within the acceptable limits. The passage from one tap to another without interrupting the principal circuit requires two taps to be connected together for a very short period. To avoid a complete short circuit, this connection is made through a resistance or less commonly through an inductance. On-load regulation is required for each phase, therefore, one or more windings for regulation, a tap selector, a resistor and some switches are needed to enable the change to be made. The OLTC contacts are connected to the taps of a regulating winding. About 96% of all power transformers today above 10MVA incorporate on-load tap changers as a means of voltage regulation. OLTs are used in applications where variable voltage is required for manufacturing processes.

Design Concepts of OLTC

To effect the change in turn ratio of a transformer under load, two design concepts are used.

a) The OLTC comprising either of a tap selector with or without reversing switch or coarse tap selector switch and a diverter switch, known as diverter switch type.

b) The OLTC having a selector switch combining the function of tap selector and diverter switch in one device, known as selector type.

Principle Circuits of OLTC Transformers

The tapped portion of the winding may be located at one of the following locations, depending upon the type of winding:

a) At the line end of the winding;

b) In the middle of the winding;

c) At the star point.

For two-winding power transformers, the most popular arrangement is the regulation at the neutral end of the winding. This solution provides the most economical tap winding arrangement with generally graded insulation combined with a compact three-phase neutral point OLTC.

Constructional Features of OLTC

The main parts of an on-load tap-changer are: diverter Switch, tap Selector and motor drive unit. The reversing or course-fine switch is part of tap selector. The motor drive, diverter and tap selector operate as a unit to effect transfer current from one voltage tap to the next. Most substation type regulating transformers have a 10% tap winding. The polarity of the tap winding can be reversed under load. This gives the transformer the ability to lower or raise the voltage ratio by 10% above or below its nominal voltage rating. Tap changers possess two fundamental features; some form of impedance is present to prevent short circuiting of the tapped section, and a duplicate circuit is provided so that one circuit can carry the load current while switching is being carried out on the other.

The impedance can either be a resistor or a center-tapped reactor. Transition resistor type tap changers which carry out selection and switching on the same contacts are single compartment design and those which have tap selectors and separate diverter switch are double compartment design.

The switching arrangements may be linear, reversing or coarse-fine. Figure 3 shows typical switching arrangements. The linear switching is not very common; it is used in around 20% cases. The design of transformer and OLIC is simple, regulating range is smallest and losses are smallest. The reversing (plus/minus) switching is most common; used in around 70% cases. The tap winding is either additive (+) or subtractive (-) depending upon position of reversing switch and it has change over selector. The coarse/fine switching is least common switching arrangement used in around 10% cases. The coarse winding is inserted in the main winding, having change over selector and also losses are small.

Single Compartment Design

In the single compartment design, same contacts are employed for selection and transfer of current. All the contacts are housed in the same compartment. The insulating oil of this compartment is kept isolated from the main transformer oil. Figure 4 shows electrical principle for selector type OLTC.

Double Compartment Design

Large transformers of high voltage class employ double compartment type OLTCs. A compartment houses make-break contacts and transition resistors and another compartment houses the tap selector. The electrical principle for the diverter type OLTC is shown in Figure 5.

As the tap selector is housed in the same tank along with transformer, the transformer buchholz relay takes care of tap selector also. However, for diverter switch a separate oil surge relay is provided because the oil in diverter switch compartment is not in contact with the transformer tank oil. The oil surge relay trips the transformer for any electrical fault that takes place in the diverter switch compartment.

The Resistor Type OLTC

The Resistor type OLTC may consist of diverter switch or diverter/selector combined switch, oil compartment, tap selector, change-over Selector, tie-in resistor, drive mechanism, conservator tank and accessories. The major components of OLTC are further discussed below.

Diverter Switch

A diverter switch is a major OLTC component. It is used to transfer the load from the selected to the pre-selected tap without interruption of the transformer load current. Diverter switches are installed in the tap changer oil compartment, which separates the tap-changer oil from the oil of the transformer main tank.

Transition Resistors

In the mid-position of a tap change operation, both the selected tap as well as the pre-selected tap are connected with the output terminal of the tap changer. In order to avoid a short circuit between these two taps, a transition resistor is necessary for limitation of the circulating current.

Spark Gaps

Spark gaps are a standard protective device against surges between selected and pre-selected tap for diverter switches. The spark gaps are arranged at the diverter switch contact shell. In some tap changer applications varistors are used instead of spark gaps.

Varistors

Varistors are non-linear resistors like the ones that are used in surge arresters.

Varistors can be used to limit surges between selected and pre-selected tap inside the tap changer compartment.

Tap Selector

The purpose of a tap selector is to transfer connection from a common terminal to a multiplicity of leads connected to the tap winding of a transformer. Tap selector contacts do not switch currents but are always operated under no-load conditions.

Tap selectors are usually arranged beneath the tap-changer oil compartment and are immersed in the insulating oil of the transformer main tank.

Change-over Selector

A tap changer with changeover selector allows the tap selector to move through a second revolution and thus increases the tapping range. It can be used either as reversing switch for the buck-and-boost connection or as coarse tap selector to connect the beginning or end of a coarse tap winding to the main winding. In any case the resulting range will be doubled.

Tie-in Resistor

The tap winding is briefly galvanically isolated from the main winding during the change-over operation of the reversing switch or coarse tap selector. It assumes a potential resulting from the voltages of the adjacent windings and coupling capacitances to these windings or to grounded parts. This potential shift of the tap winding generates corresponding voltages between the breaking change-over selector contacts as one contact is alwavs connected with the tap winding and the other contact is always connected with the main winding. This voltage is called recovery voltage. When the change-over selector contacts open, a capacitive current is interrupted, called breaking current. The recovery voltage and braking current may cause excessive discharges on the change-over selector and damage the transformer. Therefore, tie-in resistor shall be used when recovery voltage exceed maximum permissible value given by the tap changer manufacturer.

Motor Drive Unit

The motor drive unit is used for operating on-load and off-circuit tap changers in regulating transformers. The tap change operation can be activated by starting the motor drive from local or remote raise/lower push buttons or a control pulse triggered by voltage regulator relay. The tap change operation is always completed regardless of any other control pulses emitted during the operating time.

OLTC Conservator Tank

An oil filled tank, mounted above the OLTC and piped into it is usually filled half way and is used to keep the level of the oil in the diverter switch compartment always full. The conservator tanks are available in two designs, without and with bellows. The former has the advantage that there is no direct contact of oil with air thereby prevents oxidation and moisture contents in oil.

Accessories

The accessories for on-load tap changer are; OLTC protective relay, pressure relief device, oil level indicator, dehydrating breather, oil filter plant, OLTC on-line monitor and their application depends on individual requirement.

Vacuum Switching OLTC

The vacuum switching technology is used in reactor-type tap changers. This has revolutionized resistor-type tap-changers.

The advantages of vacuum switching are service requirements after 150,000 to 500,000 switching operations which drastically reduce running costs for the whole life and increases availability of the transformer. Also, there is no oil carbonization, no replacement of contacts and no oil filter systems required.

Thyristor-assisted Tap Changer

Thyristor-assisted tap changer uses thyristors to take the on-load current while the main contacts change over from one tap to the next. This prevents arcing on the main contacts and can lead to a longer service life between maintenance activities. They are more complex and require a low voltage power supply for the thyristor circuitry. They also can be more costly.

Solid state (Thyristor) Tap Changer

They use thyristors to switch the load current as well as to pass the load current in the steady state. Their disadvantage is that all of the non-conducting thyristors connected to the unselected taps still dissipate power due to their leakage current and they have smaller short circuit withstand capacity. This power can add up to a few kilowatts which has to be removed as heat and leads to a reduction in the overall efficiency of the transformer, in exchange for a compact design that reduces the size and weight of the tap changer device. Solid state tap changers are typically employed only on smaller power transformers.

Maintenance of Tap Changers

Malfunctions in tap changers may result in spectacular and costly failures. An international survey on failures in large power transformers showed that tap changers were the source of some 40% of transformer faults. Maintenance strategies such as time-based or Preventive Maintenance (PM) and predictive or Condition-Based Maintenance (CBM) could be adopted in order to keep the operational reliability of tap changer and transformer on a high level. Tap changer PM activities include periodic checks, annual inspection and major overhauls. The CBM tests are: on-line Dissolved Gas in oil Analysis (DGA) of diverter switch compartment, acoustic emission and vibration technique.

Annual Maintenance

The annual maintenance activities of on-load tap changer include the following:

1. Visual checks of OLTC head, protective relay and motor drive unit.

2. Visual checks for oil tightness at the sealing locations of the OLTC head. the protective relay and the pipe connections.

3. Visual checks of the gaskets at the protective housing of the motor drive unit.

4. Check for proper function of the electric heater in the protective housing of the motor drive unit.

5. Check of state of the drying medium (silica gel) and the oil level in the dehydrating breather.

6. Operational checks.

7. Inspection of position switches.

8. Check of upper and lower electrical limits, mechanical stops, safety switch and bevel gears.

9. Operation of tap changer from end to end positions 5 times. It is necessary for contact wipe.

10. General inspection and completion of inspection sheets.

Major Inspection

Depending on the type, the first inspection may be after two years and then four to seven years thereafter. In countries with tropical and subtropical climate, the humidity should be taken into account.

Maintenance work may be started only if the relative humidity is less than 75%. The major inspection activities of OLTC include the following:

1. Perform ratio test at minimum, mid, mid-1, mid+1 and maximum tap positions e.g. if there are 27 tap positions then tap positions will be 1, 13, 14, 15 and 27.

2. Drain oil from diverter switch compartment.

3. Dismantle and remove diverter switch for inspection and clean diverter housing.

4. Inspect and/or change contacts (wear dependent).

5. Confirm integrity of transition resistors (ohmmeter).

6. Re-install diverter switch and fill the insulating oil.

7. Confirm dielectric strength of oil and filter (OILTAP).

8. Vacuum check of vacuum interrupters (VACUTAP).

9. Review and lubricate motor drive.

10. Check tap position indicators in motor drive and tap changer head to indicate the same position.

11. Perform ratio test to ensure that driving mechanism and tap changer are in their proper position.

12. Perform functional test of the protective relay and de-air it.

13. Check manual cranking balancing.

14. Check rotation lag balancing.

15. Complete inspection check-sheets.

Contact Replacement

Contact life estimate is stated in equipment manual or on name plate. Average power transformer experiences approximately 20 tap changing operations a day which equates to 219,000 operations over 30 years. Since contacts have a long life expectancy, contact replacement will most likely not be necessary over useful life of transformer.

Tap Selector Inspections

Tap selector should be inspected after 1 to 1.5 million operations, depending on the type of tap changer and on the selector design.

Summary

Tap changers are one of the indispensable components for the regulation of power transformers used in electrical energy networks and industrial applications. Taps are used in the winding to change the turn ratio of transformer and maintain output voltage. An off-circuit tap changer allows the voltage ratio of the transformer to be adjusted while it is de-energized and an on-load tap changer changes the turn ratio of the transformer winding while it is energized. The OLTC active component construction can be either of the diverter or selector type switching principle. There are three basic tapping arrangements; linear, reversing switch or coarse-fine switch and each have their own advantages and disadvantages depending upon the application. The OLTs are classified as oil type, vacuum type and electronic/thyristor type. Switching from one position to another has to be performed through impedance; either a resistor or a reactor to avoid a short circuit between two steps of the regulating winding. The resistor type OLTCs may be single compartment design or double compartment design.

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. In new OLTC designs, the contacts for make-and-break currents are replaced by vacuum interrupters. Since the year 2000, high speed resistor type OLTC are commercially available for in-tank installation on oil filled transformers. As tap changers are major source of transformer failures, maintenance strategies have been described for the regulating transformer's high level of reliability.