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TAP CHANGER:

A transformer’s rated voltage may not match the system voltage exactly, or it may necessary to raise or lower the output voltage to supply a certain load. For this purpose, we need to change transformer turn ratio by adding or removing a portion of winding. A tap changer is doing the same. It changes the taps on the primary winding (H.V winding) and follows the principle “break before make”. As the name indicates such a device cannot operate when transformer is loaded or energized, because it would break load current and magnetizing current. So, these types of tap changers are known as no load tap changer or off load tap changer. It has five tap positions; two tap position above the nominal voltage rating, one at nominal voltage, and other two positions below the nominal voltage. These voltages between two taps are usually +2.5% of nominal voltage. So, total tap changes voltage are +5%.
Generally rotary switch changes the tap by a wheel, assembled outside the transformer. With the movement of wheel, tap position changes and indicates the tap position. An insulating rotating shaft aligns a spring loaded shorting wedge between pairs of hollow cylindrical electrodes. Winding taps are connected by soldering with this electrode.
Decades ago power transformer equipped with this type of off load tap changer, and now a days it is available in

    LOAD TAP CHANGERS:

The LTC introduces for voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. From the beginning of LTC development, two switching principles have been used for the load-transfer operation, the high-speed-resistance type and the reactance type.

Design Principle:

The LTC changes the ratio of a transformer by adding turns to or subtracting turns from either the primary or the secondary winding in load condition. So, a load tap changer (LTC) must be a make-before-break switching device, requiring bridging over two adjacent taps before moving on to the next tap. An impedance must be inserted between the taps in order to limit the short-circuit current that flowing in the bridging position. The transition impedance in the form of a resistor or reactor consists of one or more units that are bridging adjacent taps for the purpose of transferring load from one tap to the other without interruption. The majority of resistance-type LTCs are installed inside the transformer tank (in-tank LTCs), whereas the reactance-type LTCs are in a separate compartment that is normally welded to the transformer tank.

Resistance-Type Load Tap Changer:

The LTC design for larger powers and higher voltages comprises an arcing switch and a tap selector. For lower ratings, LTC designs are used where arcing switch and the tap selector are combined in a arcing tap switch.
The complete resistance-type tap-changing mechanism, complete with moving contacts, arcing contacts, transfer switch, and resistor can be housed in a single unit. As per figure, the tap change takes place in two steps. First, the next tap is preselected by the tap selector at no load. Then the arcing switch transfers the load current from the tap in operation to the preselected tap. The tap selector is operated by a gearing directly from a 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 pre-selection operation has been finished. The switching time of an arcing switch lies between 40 and 60 ms. During the arcing switch operation, transition resistors are inserted, which are loaded for 20 to 30 ms, i.e., the resistors are 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.
The transfer switch is open only when the tap changer is in the bridged position, with the resistor limiting the circulating current that flows between the taps. Unfortunately, the resistor is also in the load current path, so I2R heating in the resistor is of concern. In this type, the bridging position only long enough to let the second moving contact to meet up with the first, thus limiting the I2R heating in the resistor.
The tap-changing unit is usually located in the main transformer tank with the parts insulated by the same transformer oil that bathes the core and coils, with the exception of the arcing contacts which are in a sealed self-contained oil compartment to prevent contaminating the main transformer with arcing products. Resistance-type load tap changers have a very long life and can perform hundreds of thousands of operations between overhauls.

Reactance-Type Load Tap Changer:

For reactance-type LTCs, preventative auto-transformer method is used. The preventative auto-transformer has a 1:1 turn’s ratio and with the tap change in the bridged position and the ampere-turns in each half of the preventative auto-transformer must cancel each other. Therefore, the load current flows virtually unimpeded through both halves of the auto-transformer winding. Any circulating current that flows in the closed loop between the taps encounters the magnetizing impedance of the preventative autotransformer, which effectively blocks the circulating current. The voltage at the output of the preventative autotransformer is the average of the voltages of the bridged taps.
When the tap changer in the un-bridged, full-tap position, all of the load current is coming from a single tap through both moving contacts and the output voltage is equal to the voltage at that tap. Again, the ampere-turns in the two halves of the 1: 1 preventative autotransformer cancel, so the load current flows virtually unimpeded.
When, the tap changer begins to move to the next tap position, the upper moving contact moves while the lower moving contact remains on the tap. All of the load current in the upper moving contact is then suddenly transferred to the lower moving contact. Since all of the load current now flows through one-half of the autotransformer winding, it encounters the magnetizing impedance of the autotransformer, which would normally create a large voltage drop. By designing the preventative autotransformer with relatively low magnetizing impedance by building in air gaps in the core and by allowing the core to saturate at a relatively low winding voltage this problem may overcome. A set of stationary arcing contacts are necessary in order to prevent burning and pitting of the moving contacts when breaking load current.


When both moving contacts are on the same tap, all of the arcing contacts are closed. Before the upper moving contact moves, the upper transfer contact opens. The middle arcing contact and the lower transfer contact are closed; the upper transfer contact is merely breaking a parallel current, so minimal arcing occurs there. Next, the middle arcing contact opens, interrupting the load current through the upper moving contact. Finally, with the upper moving contact isolated, it can move on to the next tap position and all of the arcing and transfer contacts are closed to establish the bridging position.
The movement of the lower moving contact can proceed in a similar manner. The main advantage of the preventative auto-transformer scheme is that the tap changer can stay on the bridging position indefinitely.
LTCs require periodic maintenance. The motor-driven mechanism needs to be cleaned and lubricated and the controls inspected and tested. The main moving contacts should be periodically inspected and replaced if worn or pitted. The arcing contacts are the components require frequent overhaul. If the arcing contacts operate in oil, the oil must be changed periodically.

BUSHINGS:

A transformer designed for outdoor use has its core and coils completely enclosed by a steel enclosure. In order to connect the winding to the electrical system, the leads are brought out of the tank through bushings. Since the leads are energized at line voltages, the bushings must insulate and isolate the leads from each other and from the tank.
Bushings are rated by the maximum voltage and current for which they are designed.
According to Construction:
Construction-ally, there are two types of bushing, the solid or bulk type and the capacitance-graded or Condenser type.

Solid Bushing:

The solid-type bushing is made with a central conductor and porcelain or epoxy insulators at either end and is used primarily at the lower voltages through 25 kV. Solid bushings are commonly used in applications ranging from small distribution transformers and circuit switchers to large generator step-up transformers and hydrogen-cooled power generators. The central conductor lead connected directly to the transformer winding. The space between the lead and the insulator may consist of only air on lower-voltage solid-type bushings, or this space may be filled with electric-grade mineral oil or some other special compound on higher-voltage bushings. Oil and compounds are used for better cooling, higher dielectric constants, higher breakdown strengths than air.
The primary limitation of the solid bushing is its ability to withstand voltages above 90 kV and limited for 25 KV equipment rating.

Capacitance-Graded Bushings:

A condenser bushing gets its name from the grading capacitor that relieves the electrical stresses internal to the bushing. Currently, this construction is used for virtually all voltage ratings above 25-kV system voltage and has been used for bushings through 1500-kV system voltage. This construction uses conducting layers at predetermined radial intervals within oil-impregnated paper or some other insulation material that is located in the space between the central conductor and the insulator for making capacitance-graded bushings; conductive foils, typically aluminum or copper, in oil-impregnated Kraft paper are used. The principal elements are the central circular conductor, onto which the capacitance-graded core is wound. A capacitance tap is electrically connected to one of the plates of the grading capacitor for measuring the power factor of the insulation when the bushing is tested. This tap is normally grounded when the bushing is in operation. Oil is used to impregnate the paper insulation.
There are various types of insulating media are used inside the bushing. Air and oil insulated bushing are used for solid type bushing. Whereas, oil impregnated paper and resin impregnated paper insulation are used for capacitance graded bushing for its superior dielectric-withstand characteristics. Gas insulated bushing, where pressurized SF6 gas are used as insulating media, uses in circuit breaker bushing.
According to Insulating Media, An air-to-oil bushing has air insulation at one end of the bushing and oil insulation at the other whereas an air-to-air bushing has air insulation on both ends.

Design Parameters:

Conductor size and material: This is determined primarily by the current rating.
Insulators: That must have sufficient length to withstand the steady-state and transient voltages that the bushing will experience.
Flange: Material can be cast aluminum for high-activity bushings which mount the bushing to the apparatus on which it is utilized, and, to contain the gaskets.
Oil Reservoir: It required on larger bushings with self-contained oil for, mineral oil expands and contracts with temperature, and oil impregnated insulating paper must be totally submerged in oil in order to retain its insulating qualities. For the purpose of checking the oil level in the bushing, an oil-level gauge is often incorporated into the reservoir.
Clamping system: It used on bushings, provides the mechanical integrity of the bushing.
Temperature limits: Within bushings depend on the type of bushing and the materials used in them. Solid type bushings are limited to the maximum allowable temperatures of the sealing gaskets and possibly the epoxy insulators, if used. The Kraft-paper insulation the maximum temperature that this paper can endure without accelerated loss of life is 105˚C.
Bushing Current Transformer Pockets: The bushing flange is extended on its inner end, and the BCTs, having 500 to 5000 turns in the windings, are placed around the flange. This location is called the BCT pocket .In this case, the bushing central conductor forms the single-turn primary of the BCT, and the turns in the windings form the secondary.
Corona ring: EHV (extra high voltage) bushings have corona rings attached to the top of the bushing to prevent corona from the high voltage stresses at sharp edges and corners. Corona rings are generally used at voltages above 230 kV.

Maintenance of bushing:

Bushing needed some periodic inspections and maintenance.

Oil Level:

Proper oil level is very important in the operation of a bushing, and abnormal change in oil level can indicate problems within the bushing. Loss of oil can indicate that the bushing has developed a leak, possibly through a gasket, a soldered or welded seal, or an insulator that has been cracked or broken. A leak on the air-end side may be an indication that water has entered the bushing.

Power-Factor/Capacitance Measurements:

Two methods are used to make power (dissipation) factor and capacitance measurements. The first is the grounded specimen test (GST), where current, watts, and capacitance of all leakage paths between the energized central conductor and all grounded parts are measured. The second method is the ungrounded specimen test (UST), where the above quantities are measured between the energized center conductor and a designated ungrounded test electrode. This test can be made by tan- delta test machine. It is recommend that power factor and capacitance measurements be made at the time of installation, a year after installation, and every three to five years thereafter. A significant increase in a bushing’s power factor indicates deterioration of some part of the insulating system. It may also indicate deterioration within the bushing.
if power-factor/capacitance measurements indicate that something is wrong with the bushing, dissolve gas analysis of oil is required.
Bushings used in highly polluted environments should be washed periodically. Improper installation of either the top or bottom terminals can cause thermal problems.

Keep in pocket:→

✓For necessary output voltage of transformer, tap changer is required.
✓There are two types of tap changer; off load and on load tap changer.
✓Off load tap changer works at no load condition and follows break before make principle.
✓On load tap changer works at loaded condition and follows make before break principle.
✓Resistance type load tap changer and reactance type load tap changer are two types of on load tap changer.
✓Solid bushing or Capacitance Graded bushing is used for transformer bushing.
✓Oil level and power factor measurement test are common test of bushing.

Short questions related to this topic:

Q. What does tap changer changes?

A. Output voltage.

Q. How tap changer changes output voltage?

A. By changing the transformer turn ratio.

Q. Does resistance type tap changer need extra chamber?

A. No, reactance type need it.

Q. Can resistance type tap changer loss free?

A. No reactance type is loss free.

Q. Which type can stay in bridge condition?

A. Reactance type.

Q. Why bushing require?

A. To brought lead from transformer tank.

Q. Which type of bushing used in high voltage?

A. Condenser type.

Q. Why capacitance measurement is require?

A. For checking bushing deterioration.

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