Transformer – FAQ | Interview Questions -Part 3

What is the purpose of transformer oil?

Transformer oil functions as a coolant as well as insulating material. It also enables transformer protection during internal faults. It is chemically stable and has high thermal conductivity and dielectric strength. On the event of faults it decays rapidly and forms gases which operate Buchholz relay.

What are the different cooling methods adapted in a transformer?

Based on the coolant used the cooling methods can be classified into:

  1. Air cooling
  2. Oil and Air cooling
  3. Oil and Water cooling

1. Air cooling (Dry-type transformers)

  • Air Natural(AN)
  • Air Blast (AB)

2. Oil cooling (Oil-immersed transformers)

  • Oil Natural Air Natural (ONAN)
  • Oil Natural Air Forced (ONAF)
  • Oil Forced Air Natural (OFAN)
  • Oil Forced Air Forced (OFAF)

3. Oil and Water cooling (For capacity more than 30MVA)

  • Oil Natural Water Forced (ONWF)
  • Oil Forced Water Forced (OFWF)

What is the function of a Buchholz relay?

Buchholz relay is a protective device used to detect faults occurring inside a transformer (short circuits, inter-turn faults etc). It is an oil actuated relay. It is operated by the avalanche of gases formed by the decomposition of transformer oil during faults.

What are the protection features in a transformer?

Faults occurring inside the transformer: Inter turn faults, Insulation degradation, short circuits

External faults: External surges and spikes may affect the transformers, Overloads and external short circuits

What is a current transformer?

While measuring high AC currents measuring instruments cannot be connected in series to the circuit. Doing so will damage the instrument or even it is not practical to manufacture instruments for various magnitudes of currents. A current transformer is a device that gives us output in the range of 0-1A or 0-5A proportional to the actual current flowing through the circuit. This makes the measurement easy and size of measuring device small and economical.

What is a potential transformer?

As like current transformers, potential transformers are also instrument transformer. It facilitates the measurement of high voltages by stepping down them. It is connected in parallel to circuit. A voltmeter is connected to the secondary side of the transformer.

Why is the transformer used in the power system?

Transformers are the most important part of power transmission and distribution network. It is used to step up and step down voltage and for the measurement of current (Current transformer) and voltage (potential transformer). Transformer helps in reducing the power transmission losses, cost of transmission and helps in delivering the required voltage to the consumers.

What is an isolation transformer?

An isolation transformer is a safety device, used to electrically isolate two circuits, still allowing power transfer between them. It has a turns ratio of 1:1. It blocks transmission of DC signals from one circuit to the other. Also, it is used to reduce noise in audio and radio frequency amplifiers.

What is the difference between iron loss and copper loss in a transformer?

In transformers, Iron losses are the one that occurs in the transformer core and are constant in nature. Eddy current loss and hysteresis loss are the two major types of iron losses.

Copper loss occurs in the transformer winding due to the resistance of the conductor used in it. It varies according to the load current.

What is a dry transformer?

Dry-type transformers are natural air-cooled transformers those are not oil-immersed. It has better overload capacity than the oil-immersed transformers. These types of transformers are available up to 25kVA.

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