POWER TRANSFORMER AUTOMATIC TAP CHANGER CONTROLS BASIC INFORMATION

How Transformer Tap Changer Control Works?

The tap-changing mechanism is usually motor-driven and can be controlled manually and automatically. In the automatic mode, the output voltage of the transformer is compared to a reference voltage and a raise/lower signal is sent to the tap changer motor when the output voltage falls outside a specified band, called a dead band.

The dead band must not be smaller than the voltage between taps; otherwise, it will ‘‘hunt’’ endlessly and burn up the tap changer. The dead band must not be too wide, however, because the purpose of voltage regulation will be defeated.

Ordinarily, the dead band is set to a voltage between two and three tap increments. With the tap voltage typically around 1% of the nominal secondary voltage, this provides regulation within a 2–3% range.

If two or more transformers with load tap changers are connected in parallel, then it is important that all transformers operate at the same transformer turns ratio; otherwise, excessive circulating KVAR results. The way this is implemented is to have the tap changer on one of the parallel transformers control voltage as the lead tap changer, with the other tap changers as the followers.

Circuitry is installed on the followers to sense the direction of reactive power flow through each transformer. If too much reactive power is flowing from the primary to the secondary, then its secondary taps are lowered.

If too much reactive power is flowing from the secondary to the primary, then secondary taps are raised. Appropriate dead bands are established to prevent the LTCs from hunting and to limit interactions among the tap changer controls.

One such control scheme, called a load-balancing method, is depicted in Figure 6.16 for three transformers with voltage regulators supplying a common load bus.

  FIGURE 6.16 A load-balancing control scheme for three parallel transformers with load tap changers.

If the three transformer impedances equal and if the transformers are set on the proper tap positions, the transformer secondary currents will all be in phase with the load current and there will be no current unbalance.

If one or more transformer is set on the wrong tap, circulating currents will flow in all three transformers. The principle of operation of the load-balancing method is to separate each of the transformer secondary currents into a load current component and a circulating-current component. The transformer secondary currents flow through current transformers, labeled CT 1, CT 2, and CT 3 in Figure 6.16.

The currents at the CT secondaries split into two paths at each of the CT secondary windings. Path 1 (to the right) goes through a set of auxiliary transformers, labeled CT 4, CT 5, and CT 6. The secondaries of the auxiliary CTs are connected in series, forcing the currents in all three primary windings equal one another, each being one-third of the total load current. By default, the unbalance-current components must flow in path 2 to the left.

Each of the circulating-current components (also called unbalanced current components) flows through an inductive reactance element, called a paralleling reactor. The paralleling reactors are labeled jX in Figure 6.16. In general, the unbalance-current components of the three transformers are unequal.

The voltages developed across the paralleling reactors are added to the sensed voltages at the secondary windings of the main transformers, which are used to control the movement of the tap changers.

If transformer 1 is on a higher tap position than transformer 2 or transformer 3, the unbalanced currents flowing through the parallel reactors increase the sensed voltage at transformer 1 and reduce the sensed voltages at transformers 2 and 3.

This causes transformer 1 to lower its taps and transformers 2 and 3 to raise their taps. If transformer 1 is on a lower tap position than transformers 2 and 3, the unbalanced currents flowing through the parallel reactors decrease the sensed voltage at transformer 1 and increase the sensed voltages at transformers 2 and 3. This causes transformer 1 to raise its taps and transformers 2 and 3 to lower their taps.

RESISTANCE TYPE TRANSFORMER LOAD TAP CHANGER BASICS

The LTC design that is normally applied to larger powers and higher voltages comprises an arcing switch and a tap selector. For lower ratings, LTC designs are used where the functions of the arcing switch and the tap selector are combined in a so-called arcing tap switch.

With an LTC comprising an arcing switch and a tap selector (Figure 3.3.1), the tap change takes place in two steps (Figure 3.3.2). First, the next tap is preselected by the tap selector at no load (Figure 3.3.2, positions a–c). Then the arcing switch transfers the load current from the tap in operation to the preselected tap (Figure 3.3.2, positions c–g).

The LTC 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.

This operates the arcing switch — after releasing in a very short time — independently of the motion of the drive mechanism. The gearing ensures that this arcing switch operation always takes place after the tap preselection operation has been finished.

With today’s designs, the switching time of an arcing switch lies between 40 and 60 ms. During the arcing switch operation, transition resistors are inserted (Figure 3.3.2, positions d–f), which are loaded for 20 to 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 LTC is between 3 and 10 sec, depending on the respective design.

An arcing tap switch (Figure 3.3.3) carries out the tap change in one step from the tap in service to the adjacent tap (Figure 3.3.4). The spring energy accumulator, wound up by the drive mechanism actuates the arcing tap switch sharply after releasing. For switching time and resistor loading (Figure 3.3.4, positions b–d), the above statements are valid.

The details of switching duty, including phasor diagrams, are described by IEEE (Annex A [IEEE, 1995]) and IEC (Annex A [IEC, 2003]).

ON LOAD TAP CHANGER DESIGN TEST FOR MOTOR DRIVEN MECHANISM BASIC INFORMATION

Mechanical load test
If the LTC is operated by a separate motor-drive mechanism, the output shaft shall be loaded by the largest LTC for which it is designed or by an equivalent simulated load.

At such a load, 500 000 operations shall be performed at room temperature across the entire tap range. Additional cooling of the motor-drive is permissible during this test.

During this test, 10 000 operations shall be performed with the motor supply voltage at 85% of rated drive motor voltage. Also, 10 000 operations shall be performed at 110% of rated drive motor voltage. In addition, 100 operations shall be performed at a temperature of -25 °C.

The correct functioning of the tap position indicator, limit switches, restarting device, and operation counter shall be verified during this test. At the completion of this test, the LTC shall be operated manually, if applicable, through one cycle of operation.

The test shall be considered to be successful if there is no mechanical failure or any undue wear of the mechanical parts. Normal servicing according to the manufacturer's instruction book is permitted during the test. During this test, the heating system of the motor-drive mechanism shall be switched off.

Overrun test
It shall be demonstrated that, in the event of a failure of the electrical limit switches, the mechanical end stops will prevent operation beyond the end positions when a motorized tap-change is performed and that the motor-drive mechanism will not suffer either electrical or mechanical damage.

ON LOAD TAP CHANGER NAMEPLATE SAMPLE AND REQUIREMENTS

The LTC nameplate shall be in accordance with ANSI C57.12.10-1987 and shall include the items listed below:

a) Number and year of this standard
b) Manufacturer's name
c) Serial number
d) Manufacturer's type designation
e) Year of manufacture
f) Maximum rated through current
g) Basic lightning impulse insulation level to ground

The nameplate shall be permanently attached to the LTC compartment.

TRANSFORMERS TAPPING BASICS AND TUTORIALS

Transformers also provide the option of compensating for system regulation, as well as the regulation which they themselves introduce, by the use of tappings which may be varied either on-load, in the case of larger more important transformers, or off-circuit in the case of smaller distribution or auxiliary transformers.

Consider, for example, a transformer used to step down the 132 kV grid system voltage to 33 kV. At times of light system load when the 132 kV system might be operating at 132 kV plus 10%, to provide the nominal voltage of 33 kV on the low-voltage side would require the high-voltage winding to have a tapping for plus 10% volts.

At times of high system load when the 132 kV system voltage has fallen to nominal it might be desirable to provide a voltage higher than 33 kV on the low-voltage side to allow for the regulation which will take place on the 33 kV system as well as the regulation internal to the transformer.

In order to provide the facility to output a voltage of up to 10% above nominal with nominal voltage applied to the high-voltage winding and allow for up to 5% regulation occurring within the transformer would require that a tapping be provided on the high-voltage winding at about  13%.

Thus the volts per turn within the transformer will be: 100/87 D 1.15 approx. so that the 33 kV system voltage will be boosted overall by the required 15%. It is important to recognise the difference between the two operations described above.

In the former the transformer HV tapping has been varied to keep the volts per turn constant as the voltage applied to the transformer varies. In the latter the HV tapping has been varied to increase the volts per turn in order to boost the output voltage with nominal voltage applied to the transformer.

In the former case the transformer is described as having HV tappings for HV voltage variation, in the latter it could be described as having HV tappings for LV voltage variation. The essential difference is that the former implies operation at constant flux density whereas the latter implies variable flux density.

Except in very exceptional circumstances transformers are always designed as if they were intended for operation at constant flux density. In fixing this value of nominal flux density some allowance is made for the variations which may occur in practice.

TRANSFORMER TAP CHANGER DESIGN FOR MODERATE KVA AND CURRENT BASIC AND TUTORIALS

TRANSFORMER TAP CHANGER DESIGN FOR MODERATE KVA AND CURRENT BASIC INFORMATION
Notes On Transformer Tap Changer Design



Tap-Changer Designs for Moderate kVA and Current. In the smaller ratings, where both the voltage and the current are moderate, the energy to be ruptured in switching from tap to tap becomes relatively so small that light and simple equipments are feasible.

A variety of mechanical designs, together with special circuits, has been evolved with the purpose of providing simpler, smaller, and inherently less expensive equipments. The following may be noted:

1. Designing the tap changer so that it is capable of rupturing the current directly on the same switches which select the taps

2. Designing the circuit so that the tapped winding is reversed in going from maximum to minimum range, thereby securing a substantial reduction in the rating of core and coils for a given output

3. Using higher switching speed, by means of which the life of the arcing contacts is increased.


Tap Changers Designed to Interrupt Current. The contactors C (Fig. 10-27) operate to open the switching circuits so that there is no interrupting duty on the selector contacts which connect to the transformer taps.

When the rated current is moderate, it becomes possible to rupture the current directly on the tap-selector switches and thus obtain a major economy in the cost of the mechanical equipment.

POWER TRANSFORMER AUTOMATIC CONTROL FOR TAP CHANGERS BASIC AND TUTORIALS

POWER TRANSFORMER AUTOMATIC CONTROL FOR TAP CHANGERS BASIC INFORMATION
What Is Automatic Tap Changer Controls For Power Transformers?


Automatic Control for Tap Changers. It is usual practice to use some sort of voltage measuring device to control the operation of the motor which drives the tap changer.

Such devices may be mechanical, balancing the force of a solenoid actuated by the voltage against weights or springs, or they may be an electrical network, usually a bridge circuit which balances against the voltage of a Zener dioide.

With either type of device, a voltage higher than a desired upper limit will start the tapchanger driving motor to change to the next lower tap voltage; similarly, a voltage lower than the desired lower limit will cause a change to the next higher tap.

The circuit usually includes a time delay to prevent tap changes, which would occur unnecessarily during very short time variations in voltage. It also may include a line drop compensator to facilitate maintaining the voltage within a given band at a point (load center) some distance from the transformer.

The line-drop compensator introduces a signal into the voltage regulating relay circuitry. This represents the voltage drop due to line impedance between the transformer and the load center.

The voltage-regulating relay (or contact-making voltmeter) should be adjusted so that the voltage bandwidth, or spread between voltages at which the raising and lowering contacts close, will be not less than the percentage transformer tap plus an allowance for irregular voltage variations.

For example, a tap-changing transformer with 11/4% taps should have a minumum voltage bandwidth of approximately 11/4% 1/2% 13/4%.

In addition, the voltage-regulating relay may contain a component for use when load tap-changing transformers are operated in parallel. In this case, the tap changers must be controlled so that they are approximately on the same tap position.

The component, a paralleling reactor, is used with external circuitry to detect, and generate a signal to minimize, circulating current that results when the tap changers are not on like positions.

POWER TRANSFORMER TAP CHANGERS BASIC AND TUTORIALS

TAP CHANGERS OF POWER TRANSFORMERS  BASIC INFORMATION
What Are Power Transformer Tap Changers? How Tap Changers Work?

When a transformer carries load current there is a variation in output voltage which is known as regulation. In order to compensate for this, additional turns are often made available so that the voltage ratio can be changed using a switch mechanism known as a tapchanger.

An off-circuit tapchanger can only be adjusted to switch additional turns in or out of circuit when the transformer is de-energized; it usually has between two and five tapping positions. An on-load tapchanger (OLTC) is designed to increase or decrease the voltage ratio when the load current is flowing, and the OLTC should switch the transformer load current from the tapping in operation to the neighbouring tapping without interruption.

The voltage between tapping positions (the step voltage) is normally between 0.8 per cent and 2.5 per cent of the rated voltage of the transformer. The OLTC mechanisms are based either on a slow-motion reactor principle or a high-speed resistor principle.

The former is commonly used in North America on the low-voltage winding, and the latter is normally used in Europe on the high-voltage winding.

The usual design of an OLTC in Europe employs a selector mechanism to make connection to the winding tapping contacts and a diverter mechanism to control current flows while the tapchanging takes place. The selector and diverter mechanisms may be combined or separate, depending upon the power rating.

In an OLTC which comprises a diverter switch and a tap selector, the tapchange occurs in two operations. First, the next tap is selected by the tap switch but does not carry load current, then the diverter switches the load current from the tap in operation to the selected tap. The two operations are shown in seven stages in Fig. 6.15.

The tap selector operates by gearing directly from a motor drive, and at the same time a spring accumulator is tensioned. This spring operates the diverter switch in a very short time (40 – 60 ms in modern designs), independently of the motion of the motor drive.

The gearing ensures that the diverter switch operation always occurs after the tap selection has been completed. During the diverter switch operation shown in Fig. 6.15(d), (e) and (f), transition resistors are inserted; these are loaded for 20–30 ms and since they have only a short-time loading the amount of material required is very low.

The basic arrangement of tapping windings is shown in Fig. 6.16. The linear arrangement in Fig. 6.16(a) is generally used on power transformers with moderate regulating ranges up to 20 per cent. The reversing changeover selector shown in Fig. 6.16(b) enables the voltage of the tapped winding to be added or subtracted from the main winding so that the tapping range may be doubled or the number of taps reduced.

The greatest copper losses occur at the position with the minimum number of effective turns. This reversing operation is achieved with a changeover selector which is part of the tap selector of the OLTC. The two-part coarse–fine arrangement shown in Fig. 6.16(c) may also be used.

In this case the reversing changeover selector for the fine winding can be connected to the ‘plus’ or ‘minus’ tapping of the coarse winding, and the copper losses are lowest at the position of the lowest number of effective turns. The coarse changeover switch is part of the OLTC.


Regulation is mostly carried out at the neutral point in star windings, resulting in a simple, low-cost, compact OLTC and tapping windings with low insulation strength to earth. Regulation of delta windings requires a three-phase OLTC, in which the three phases are insulated for the highest system voltage which appears between them; alternatively three single-phase OLTCs may be used.