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# Tap Changer

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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. On-load tap-changers (OLTCs) are indispensable in regulating power transformers used in electrical energy networks and industrial applications
2. 2.  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.
3. 3.  By the transformation ratio V1/V2= N1/N2 V2=N2/N1×V1 From 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= 400Volt Transformation ratio V1/V2= 11000/400=27.5
4. 4.  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.
5. 5.  An on load tap changer has 3 major steps a. Tap selector- it selects the appropriate tap b. Fixed contact- These contacts hold a fixed position c. Moving contact- This contact moves to open/ close the appropriate taps.
6. 6.  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)
7. 7.  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.
8. 8. Tap selector Diverter switch / Oil compartment MDU
9. 9.  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.
10. 10.  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.
11. 11.  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.
12. 12. Vacuum OLTC Vacuum interrupter
13. 13.  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.
14. 14.  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.
15. 15.  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.
16. 16.  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.