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A tap changer is a device fitted
to power transformers for
regulation of the output voltage
to required levels. This is
no...
Voltage regulation is normally achieved by changing
the ratios of the transformers on the system by altering
the number of...
 To supply a desired voltage to the load.
 To counter the voltage drops due to loads.
 To counter the input supply volt...
 Some form of impedance is present to
prevent short circuiting of the tapped
section.
 A duplicate circuit is provided s...
 Nominal Voltage set point.
 Bandwidth (the amount of variation allowed before a
tap change
occurs).
 Time delay (The a...
Tap point is placed
 In star connected winding, near the star point.
 In delta connected winding, at the center of the w...
Tap changers connected to the primary or
secondary side windings of the transformer
depending on:
 Current rating of the ...
No-Load Tap Changer (NLTC or
DETC)
On Load Tap Changer (OLTC
 Mechanical tap changers
 Thyristor-assisted tap changers...
No-Load Tap Changer (NLTC or
DETC)In low power, low voltage transformers, the tap point can
take the form of a connection ...
Solid state (thyristor) tap changer
Recently developed which uses thyristors both to
switch the load current and to pass t...
On Load Tap Changer (OLTC)
OLTCs enable voltage regulation and/or phase shifting by
varying the transformer ratio under lo...
The resistor-type OLTCs are installed inside the transformer
tank (in-tank OLTCs)
The reactor-type OLTCs are in a separate...
The OLTC changes the ratio of a transformer by adding or
subtracting to and turns from either the primary or the secondary...
The “make before break contact concept”, is used. The transition
impedance in the form of a resistor or reactor consists o...
Examples of commonly used winding
schemes
In star/wye connection, windings have
regulation applied to the neutral end.
Regulation of delta-connected windings requires a three-
phase OLTC whose three phases are insulated according
to the high...
For regulated autotransformers, the most appropriate
scheme is chosen with
regard to regulating range, system conditions a...
The switching capacity itself is primarily a function of the
contact design, contact speed and arc-quenching agent.
Based ...
1. With a diverter switch & a tap selector operation takes place in t
a. The next is preselected by the tap selector at no...
Reactor oil-type OLTCs
The following types of switching are used for reactor oil-
type OLTCs:
1. Selector switch (arcing t...
Technical features
 The vacuum interrupter is a hermetically-sealed system.
 There is no interaction with the surroundin...
To select the appropriate OLTC, the following key data
of the corresponding transformer windings should be
known:
 MVA ra...
The appropriate tap-changer can be
determined:
 OLTC type
 Number of poles
 Nominal voltage level of OLTC
 Tap selecto...
During the operation of the diverter switch (arcing switch)
from the end of the tap winding to the end of the coarse
windi...
 Reduction of power losses
 Voltage profile enhancement
 Voltage stability
The tap changing transformer is connected at...
Voltage value of sec. terminal of transformer can be
regulated using tap changer. This regulation also affects
the calcula...
In the radial distribution system, each radial feeder is
divided into load sections with a tap changing transformer at
the...
Tap changer
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Tap changer
Tap changer
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Tap changer

general info about the tap changer used in power transformer

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Tap changer

  1. 1. A tap changer is a device fitted to power transformers for regulation of the output voltage to required levels. This is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s. Tap changers offer variable control to keep the supply voltage within the limits. The 2 ½% step can be
  2. 2. Voltage regulation is normally achieved by changing the ratios of the transformers on the system by altering the number of turns in one winding of the appropriate transformer/s. Tap changers offer variable control to keep the supply voltage within these limits. Tap changers can be on load or off load. On load tap changers generally consist of a diverter switch and a selector switch operating as a unit to effect transfer current from one voltage tap to the next. Tap changers can be adjusted to fit the application needs.
  3. 3.  To supply a desired voltage to the load.  To counter the voltage drops due to loads.  To counter the input supply voltage changes on load.  Additionally required to perform the task of regulation of active and reactive power flows.
  4. 4.  Some form of impedance is present to prevent short circuiting of the tapped section.  A duplicate circuit is provided so that the load current can be carried by one circuit whilst switching is being carried out on the other.
  5. 5.  Nominal Voltage set point.  Bandwidth (the amount of variation allowed before a tap change occurs).  Time delay (The amount of time the voltage must be outside the bandwidth before a tap change occurs) .  Line drop compensation ( a way vary the set point voltage to compensate for heavy loads)
  6. 6. Tap point is placed  In star connected winding, near the star point.  In delta connected winding, at the center of the windin  In autotransformer, between the series and common
  7. 7. Tap changers connected to the primary or secondary side windings of the transformer depending on:  Current rating of the transformer.  Insulation levels present.  Type of winding within the transformer (eg. Star, delta or autotransformer).  Position of tap changer in the winding.  Losses associated with different tap changer configurations eg. Coarse tap or reverse winding.  Step voltage and circulating currents.  Cost.  Physical size.
  8. 8. No-Load Tap Changer (NLTC or DETC) On Load Tap Changer (OLTC  Mechanical tap changers  Thyristor-assisted tap changers  Solid state (thyristor) tap changers
  9. 9. No-Load Tap Changer (NLTC or DETC)In low power, low voltage transformers, the tap point can take the form of a connection terminal, requiring a power lead to be disconnected by hand and connected to the new terminal. Since the different tap points are at different voltages, the two connections can not be made simultaneously, as this would short-circuit a number of turns in the winding and produce excessive circulating current. Mechanical tap changers A mechanical tap changer physically makes the new connection before releasing the old using multiple tap selector switches, but avoids creating high circulating currents by using a diverter switch to
  10. 10. Solid state (thyristor) tap changer Recently developed which uses thyristors both to switch the load current and 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. Thyristor-assisted tap changers Thyristor-assisted tap changers use thyristors to take the on-load current while the main contacts change over from one tap to the previous. This prevents arcing on the main contacts and can lead to a longer service life.
  11. 11. On Load Tap Changer (OLTC) OLTCs enable voltage regulation and/or phase shifting by varying the transformer ratio under load without interruption. On load tap changers generally consist of a diverter switch and a selector switch operating as a unit to effect transfer current from one voltage tap to the next. The selector selects the taps and is operating in the transformer oil. The diverter is the actual switch with high current contacts that balances the load from one tap to the other. The divertor is inside a separate compartment inside the transformer tank. The diverter and selector are positionned above each-other and driven by the same axe. The voltage between the taps is known as the step voltage, which normally lies between 0.8 % and 2.5 % of the rated voltage of the transformer. Two switching principles have been used for load transfer operation : 1.the high-speed resistor-type OLTCs 2.the reactor-type OLTCs.
  12. 12. The resistor-type OLTCs are installed inside the transformer tank (in-tank OLTCs) The reactor-type OLTCs are in a separate compartment which is normally welded to the transformer tank
  13. 13. The OLTC changes the ratio of a transformer by adding or subtracting to and turns from either the primary or the secondary winding.
  14. 14. The “make before break contact concept”, is used. 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.
  15. 15. Examples of commonly used winding schemes In star/wye connection, windings have regulation applied to the neutral end.
  16. 16. Regulation of delta-connected windings requires a three- phase OLTC whose three phases are insulated according to the highest system voltage applied. Today, the design limit for three-phase OLTCs with phase- to-phase insulation is the highest voltage for equipment of 145 kV. To reduce the phase-to-phase stresses on the delta- OLTC the three pole mid-winding arrangement (fig. 7 c) can be used.
  17. 17. For regulated autotransformers, 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.
  18. 18. The switching capacity itself is primarily a function of the contact design, contact speed and arc-quenching agent. Based on that OLTC are of two type: 1. Oil-type OLTCs 2. Vacuum-type OLTCs Resistor oil-type OLTCs In an oil-type OLTC, the OLTC is immersed in transformer oil and switching contacts make and break current under oil. 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
  19. 19. 1. With a diverter switch & a tap selector operation takes place in t a. The next is preselected by the tap selector at no load. b. The diverter switch then transfers the load current from the tap operation to the preselected tap.  The OLTC is operated by means of a drive mechanism.  Switching time of a divertor switch is b/w 40 &60 ms.  Transition resistor are inserted which are loaded for 20-30 ms.  Total operation time 3-1o sec. 2. A selector switch(arcing tap switch) carries out the tap in one ste tap in service to the adjacent tap
  20. 20. Reactor oil-type OLTCs The following types of switching are used for reactor oil- type OLTCs: 1. Selector switch (arcing tap switch) 2. Diverter switch (arcing switch) with tap selector
  21. 21. Technical features  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.  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 vapour 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 halfcycle) even in the case of
  22. 22. To select the appropriate OLTC, the following key data of the corresponding transformer windings should be known:  MVA rating.  Connection of tap winding (for wye, delta or singlephase connection).  Rated voltage and regulating range.  Number of service tap positions.  Insulation level to ground.  Lightning impulse and power frequency voltage of internal insulation. The following OLTC operating data may be derived from this information:  Rated through-current: Iu
  23. 23. The appropriate tap-changer can be determined:  OLTC type  Number of poles  Nominal voltage level of OLTC  Tap selector size/insulation level  Basic connection diagram
  24. 24. 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, 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. 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, it is necessary to
  25. 25.  Reduction of power losses  Voltage profile enhancement  Voltage stability The tap changing transformer is connected at the load terminal, its tap ratio is ‘t’. Transformer reactance at unity off-nominal tap ratio The approximate voltage drop formula is System voltages and impedance referred to the system load side a respectively
  26. 26. Voltage value of sec. terminal of transformer can be regulated using tap changer. This regulation also affects the calculation of the thevenin equivalent parameters. Changes of equivalent parameters cause a change of voltage stability conditions.
  27. 27. In the radial distribution system, each radial feeder is divided into load sections with a tap changing transformer at the beginning of the distribution network. However, there is the need to find the tap setting of the substation transformer that would give minimum distribution loss while satisfying the operating constraints under a certain load pattern. These operating constraints are voltage drop, current capacity and radial operating structure of the system. The mathematical formulation for the minimization of power loss tap changer problems is

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