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

used on transformers with

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.

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.

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.

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)

Tap point is placed In star connected winding, near the star point.

In delta connected winding, at the center of the winding.

In autotransformer, between the series and common windings

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.

No-Load Tap Changer (NLTC or

DETC)

On Load Tap Changer (OLTC

Mechanical tap changers

Thyristor-assisted tap changers

Solid state (thyristor) 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 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

changersA 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

temporarily place a large

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.

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.

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

The OLTC changes the ratio of a transformer by adding or

subtracting to and turns from either the primary or the secondary

winding.

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.

Examples of commonly used winding

schemesIn 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 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.

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.

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

combined in a selector switch (arcing tap switch) are used.

1. With a diverter switch & a tap selector operation takes place in two steps :

a. The next is preselected by the tap selector at no load.

b. The diverter switch then transfers the load current from the tap in

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 step from the

tap in service to the adjacent tap

Reactor oil-type OLTCsThe 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

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

large phase angles

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

Rated step voltage: Ui

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

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

specify the leakage impedance of coarse winding and tap

Reduction of power losses

Voltage profile enhancement

Voltage stability

The tap changing transformer is connected at the load terminal, its off

tap ratio is ‘t’. Transformer reactance at unity off-nominal tap ratio is X.

The approximate voltage drop formula is

System voltages and impedance referred to the system load side are

respectively

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.

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