Monday, 6 January 2020

Power Transformer: Specifications, Selection criteria, Types of transformer

Power Transformer

Power transformer
Power Transformer

1) kVA rating of transformer
2) Voltage ratings for the primary and secondary voltages
3) HV and LV currents
4) Operating frequency of the transformer
5) % impedance of transformer
6) Allowable temperature rise.
7) Wiring instructions for HV and LV windings/terminal diagram
8) Model number and serial number of the transformer
9) Weight of the transformer
10) Information related to the tap changer
11) Transformer vector group
12) Winding connection diagrams
13) Type of cooling
14) Insulation class
15) Name of the manufacturer
16) Weight of core
17) Weight of winding
18) Volume of oil in liters.

Criteria of Selection of Power Transformer:
i) Ratings:
  • The no load secondary voltage should be specified 5% above the nominal voltage in order to partially compensate for regulation of transformer.
  • If the transformers need to operate in parallel with each other, then their voltage ratios should be adjusted as per guidelines given in IS:10028(Part-II)-1981
  • On load tap changers on HV side should be specified. For On load tap changer should be 16taps of 1.25percent.
ii) Taps:
  • If the oil circuit taps are to be specified, they should be in the range of +2.5% and +5% and should be provided on the HV side of transformer.
  • If the oil circuit taps are to be specified, they should be in the range of +2.5% and +5% and should be provided on the HV side of transformer.
iii)Connection Symbol:
  • For power transformers upto 66kV high voltage side(HV) rating, the most preferred connections are delta/star(Dyn) and star/star(YNyn)
  • If the voltage rating is higher, then star/star(YNyn) and star/delta(YNd) connections should be preferred.
  • The section of group is made on the basis of requirement of parallel operation with other transformers
  • The value of transformer impedance is decided by considering the secondary fault levels and the associated voltage dips. For deciding the precise value of transformer impedance by referring IS:2026(Part-I)-1977.
  • If the transformer is to be operated in parallel then the impedance be selected as per the guidelines given IS:10028(Part-II)-1981.
v)Termination Arrangement:
  • The HV & LV terminals may be one of following three types depending upon on the method of installation:              
  • Rating                        Upto 10 MVA       12.5 to 40 MVA
  • Voltage Class             Upto 66kv             Upto 132Kv      
  • Type of cooling          ONAN                   ONAN/ ONAF
Types of Transformer
  • Isolation Transformer
  • Auto-transformer
  • Current Transformer
  • Potential Transformer
  • Welding Transformer           
Isolation transformer
  • In isolation transformer, the primary and secondary are physically isolated (no electrical connection)
  • Voltage spikes that might occur on the primary are greatly reduced or eliminated in the secondary
  • If the primary is shorted somehow, any load connected to the secondary is not damaged
  • Example: In TV monitors to protect the picture tube from voltage spikes in main power lines.
  • An autotransformer uses only one coil for the primary and secondary. 
  • It uses taps on the coil to produce the different ratios and voltages. 
  • Sometimes, it is desirable to change the voltage by a small amount (for instance, when the consumer
  • is far away from the generator and it is needed to raise the voltage to compensate for voltage drops). 
  • In such situations, it would be expensive to wind a transformer with two windings of approximately
  • equal number of turns. An autotransformer is used instead. 

Current Transformer
  • C.T. has bar type conductor, which behaves as primary winding.
  • The primary of C.T. carries large current Ip which is to be measured, so the bar is of large cross
  • sectional area.
  • The secondary of C.T.is made up of large number of turns. It is wound on core. The secondary
  • winding is a low current winding. Hence its cross sectional area is small.
  • An ammeter of small range is connected across the secondary as shown in fig.

Potential Transformer
  • Higher voltage ‘33kV’ is the voltage to be measured
  • Primary of PT is connected across this voltage
  • PT is step down transformer
  • Due to PT, voltage across voltmeter gets reduced by   a factor equal to the turns ratio of PT.
  • Hence low range voltmeter is used to measure voltage.
Welding Transformer 

  • It is a step down transformer that reduces the voltage from the source voltage to a voltage desired
  • according to the demands of the welding process.
  • Have large number of primary turns and low secondary turns.
  • The secondary currents are quite high.
  • The secondary has several taps for adjusting the secondary voltage to control the welding current.
  • The transformer is normally large in size compared to other step down transformers as the windings
  • are of a much larger gauge.
  • Common ratings:
–Primary voltage – 230 V, 415 V–Secondary voltage – 40 to 60 V–Secondary current – 200 to 600 A


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