Tap ChangerA tap changer is a device fitted to power transformers for regulation of the output voltage to required levels.

On-load tap-changers (OLTCs) are indispensable in regulating power transformers used in electrical energynetworks and industrial applications

Tap changer alters the turn ratios of the transformer on the system by altering the number of turns in one winding of the appropriate transformer.

Tap changers offer variable control to keep the supply voltage within these limits.

Tap changers are of two types ◦ Off load◦ On load However since the offload tap

changer causes interruption in the supply on load tap changers are more preferred in in today power system.

By the transformation ratioV1/V2= N1/N2V2=N2/N1×V1From the above equation by altering the turn ratio to suitable values the secondary voltage can be adjusted desired level.

Lets – N1=2750, N2=100, turn ratio= N1/N2=2750/100=27.5

why we choose this turn ratio - V1=11 KV, V2= 400VoltTransformation ratio V1/V2= 11000/400=27.5

Each tap corresponds to different transformation ratio. For Initial installation we need to choose a suitable tap as per our voltage levels.

Tap changers provides 5 to 20 %

voltage range for voltage regulation.

Transformer tap changer position and related voltage level in the primary & transformation ratios

On Load Tap Changer An on load tap changer

has 3 major steps

a. Tap selector- it selects the appropriate tapb. Fixed contact- These contacts hold a fixed positionc. Moving contact- This contact moves to open/ close the appropriate taps.

Switching sequence of tap selector & diverter switch

Based on the AVR feedback the suitable tap is selected fig (a) to (c)

After the tap is selected the moving contact moves and it closes both taps for a while. It moves further and finally completely opens to earlier tap and closes to the next tap fig (d) to (i)

The moving contact closes two taps for a while during the tap changing, due to which a circulating current starts to flow. This current needs to be minimized. For limiting this current resistor or reactors are used.

The moving contact is operated by charged spring tension and it completes the process within 30 to 70ms. The spring is charged by motor drives.

While opening and closing the contacts sparks are produced due to arising recovery voltage. The contacts are kept in oil or SF6 gas , which work as insulation medium as well as arc quenching.

Physical construction of Tap changer

Tap selector

Diverter switch / Oil compartment

MDU

Motor drive unit (MDU)- This unit has got a motor diver and other necessary controls. Motor charges the spring to rapidly perform the switching. Power is transferred via shaft & gear mechanism.

Diverter switch /Oil compartment- This has the diverter switch, which has moving and fixed contacts. It is filled with oil or SF6 gas to avoid the sparking & quenching during opening and closing of the contacts.

Tap selector- It has different tap combinations as specified by the customer requirements and different standards.

Diverter unit & tap selector

Due to opening closing of contacts the sparks produce and the oil degrades over the time. The oil test & analysis needs to be done in lab at regular interval

The contacts wear over the time after number of operations.

The oil seals damage over the time and leakage happens. Maintaining the oil level is must. For condition monitoring its must to check for oil leak and level regularly.

Oil dripping is dangerous for environment as well. OLTC includes a lot of gear, spring and mechanical

parts which have a usual wear and tear, that need to be taken care as an when required.

Condition monitoring and maintenance for OLTC

Vacuum type OLTC use the vacuum interrupters which has some advantages over oil / SF6.

It is hermetically sealed and has no interaction with surrounding medium despite the arc.

Contacts wear is lower than OIL/SF6. No aging or quenching medium is required. Its more environment friendly. Vacuum interrupters perform up to 600000

operation with out replacement. Has low maintenance cost and greater reliability.

Vacuum type OLTC

Vacuum OLTC

Vacuum OLTC Vacuum interrupter

A dual voltage transformer can be defined as the one which is capable of providing two types of voltage.

A dual voltage transformer has two windings in primary and two other windings in secondary.

By connecting the both windings either of primary or secondary in series or parallel two different voltages are produced.

Dual Voltage Transformer

From the figure two primary windings and two secondary windings are connected in series resulting in 240 volt input and 24 volt out put voltage.

From the adjacent figure the primary and secondary windings are connected in parallel, resulting 120 volt input and 12 volt out put voltage.

There can be either type of arrangements as per requirement.

The details in the adjacent figure shows the winding arrangement of a dual transformer. It produces 33/7.1 and 33/11 kV voltages according to arrangement of winding.

The unit produces different outputs, based on the connections as advised in the name plate details.

Upon simplifying the connection diagrams they turn out to be series connected winding for 33/11 kv output and combination of series and parallel for 33/7.1 kv output.

Winding connections for dual transformer

Thank you