Transformer interview Questions

1) Where is a Buchholz relay placed in the transformer ?

Ans: Relay is a gas actuated protection relay which is generally used in large oil immersed transformers of rating more than 500 kVA. It is used for the protection of a Transformer from the faults occurring inside the transformer.

Buchholz relay `can prevent the development of conditions leading to a fault in the transformer, such as the falling of the oil level due to leaks, or the penetration of air as a result of defects in the oil circulating system.

The adoption of other forms of protection does not therefore exclude the use of the gas-actuated Buchholz relay, as this device is the only means of detecting incipient faults, which if unnoticed, can cause heavy failures.

2)What is the use of breather in transformer?

Ans: As far as breathers are concerned ,silica gel breathers are used in transformers (power ,distribution transformers)!!

Oil immersed transformers find silica gel applications.In a transformer along with main tank there is another tank typically cylindrical in physical appearance(conservator tank) ,it contains a balloon like structure (used for understanding purpose only here).

Now ,

Transformer in loaded condition-temperature increases inside main tank,oil inside starts heating and expands the excessive oil starts moving to the conservator tank and thus the balloon structure inside compreses and with the help of breather air is sent out

Transformer in the opposite condition ,or say in extreme condition (lower temperature ),oil inside contracts and then air is taken inside to main the balloon like structure through silica gel breather .

You might end up why only silica gel ,the atmospheric air consists of moisture content (say 1%only),this may end up in detoriration of insulation (paper+water),and Hence to avoid this and breather acts as medium and provide dry air (moisture free). after all moisture is harmful why not dust ,yes dust in air is even to be avoided ,there is a oil in sealed cup and this helps in avoiding dust.

3) What are the Relays in transformer?

Ans: There are many types of protection relays used in transformers depend upon its construction, size, use etc. 
They are 
1. Buchholz relay for incipient faults like short ciruit, 
winding temparature protection, oil temparature protection, 
oil level protection, oil pressure protection, Pressure 
relief valve (for oil filled transformers)
2. Differential protection
3. Thermal Protection
4. Earth fault protection
5. Over current protection
6. Over excitation protection(where OLTC is used)  etc.

4) Why is the transformer placed in an oil tank?

Ans: Many people know that for the oil type transformer, it must be full of transformer oil. Transforme r oil is a kind of insulation oil, it has many functions and to protect the transformer.

Transformer oil serves following purpose in a transformer.

I. Primary: Electrical Insulation

The main purpose of oil in transformers is to provide an effective electrical insulation between (a) the winding and tank and (b) between the different winding. Insulation function.

II. Secondary: Cooling of transformer

The secondary purpose of oil is to mobilize heat from core and winding to cooling surfaces of the transformer tank. Thermal isolation

The basic qualities of transformer oil are required:

I. The oil shall be free from dirt, moisture, metal ions fibers etc.

II. It should be stable against oxidation at operating distinct temperatures

III. It should not be corrosive in nature

IV. It should not be soluble with respect to transformer material of construction

For the professional company, they can test the transformer oil after some time using transformer.

For the old transformer, firstly, we will choose the oil purifier for purify it, and then use it again. For considering the cost of the oil purifier is expensive.

5) Is Buchholz relay used for External Fault also?

Ans : Buccholz relay is a Mechanical relay used to Give two Commands: Alarm & Trip to avoi catastrophic breakdown of a transformer. Given below, is an image of how the bucholz relay assembly looks. On one end of the pipe is the conservator tank and on other end is the main tank. So Bucholz relay is connected between conservator tank and main tank.

Below is a photo of the internal assembly of the bucholz relay.

So how does Bucholz relay operate? See the video below for an interactive example.

Bucholz relay operates when Oil inside transformer breakdowns and creates various gases or pressure buildup due to faults. It is mechanically actuated. Whenever there will be a minor internal fault in the transformer such as an insulation faults between turns, break down of core of transformer, core heating, the transformer insulating oil will be decomposed in different hydrocarbon gases, CO2 and CO. The gases produced due to decomposition of transformer insulating oil will accumulate in the upper part the Buchholz container which causes fall of oil level in it. More severe types of faults, such as short circuit between phases or to earth and faults in the tap changing equipment, are accompanied by a surge of oil which strikes the baffle plate and causes the mercury switch of the lower element to close. This switch energized the trip circuit of the circuit breakers associated with the transformer and immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the circuit breakers associated with both LV and HV sides of the transformer. This is how Buchholz relay functions.

So, you will realize that the Bucholz relay protects only against internal faults that take place inside the transformer tank.

6) Why do transformer make noise while working ?

Ans : Transformers make noise while working is due to Magnetostriction. This noise is known as Humming.

Inside every element dipoles are there, so as inside the iron core of the transformer. When we apply 50hz AC to it, the dipoles vibrate at a frequency double of it's input frequency, that is at 100hz, which is audible to human ear. This phenomena is known as Magnetostriction. Due to this we hear a noise.

7) Why do you mean by KVA of Transformer ? 

Ans :  A volt ampere is used to measure apparent power (the vector sum of true power and reactive power. Transformers are rated in volt amperes, rather than in watts because to rate transformers in watts, it's necessary to know the power factor of the load, and transformer manufacturers have no way of knowing that.

A kilovolt ampere (kV.A) is one-thousand volt amperes. Distribution transformers are typically rated in kilovolt amperes. For example, pole-mounted distribution transformers are typically 5 kV.A, 50 kV.A, etc., and are used to reduce high-voltages to residential low voltages.

Power transformers, used on transmission systems, are usually rated in megavolt amperes.

8) Why is the transformer impedance given in percentage ?

Ans : Percentage impedance is a significant parameter of a transformer. Percentage impedance means -

1. The voltage drop occurring in the secondary winding of a transformer when the transformer is loaded fully. In your case, there will be a voltage drop of 12.4% from no-load secondary winding terminal voltage when the transformer supplies 55MVA. In this case, the percentage impedance is specified along with winding temperature since the resistance of the winding increases with winding temperature. As the load current increases, transformer winding temperature also increases and hence its impedance and thus the voltage drop.
2. The percentage of primary voltage when applied to the primary winding of the transformer will result in full load secondary winding current with the secondary winding terminals short-circuited.
3. The short circuit capacity of the transformer. The percentage impedance specifies the maximum short circuit current that can be fed by the transformer when its secondary winding is short circuited and rated voltage is applied to its primary winding. In this case, the percentage impedance is used to determine the available short circuit capacity or the fault level at the point in the electrical system where the transformer secondary is connected. Percentage impedance is also used to calculate the voltage drop that will occur in the system when a large motor connected to the system to which the transformer feeds power is started.

9) Why do we conduct an OC test on the LV side of Transformer ?

Ans : We can perform test in either sides.

As OC tests are done in rated voltage , and very high rating measuring instruments are not available ( or comes at very high cost) we prefer to do it at LV side.

Second point availability of voltage source ( i.e. rated voltage to be given for test ) low voltage is readily available but high voltages are not. ( not even college labs .. consider 33kv or 66kv etc).

10) Why CT not used as a Power Transformer?

Ans : The main purpose of Power transformer is to transfer power from one circuit to other by maintaining electrical isolation between the two circuits though the voltage level and current value in primary and secondary winding may different depending on application (step up or step down).

So in power transformer the whole supply voltage appears in the primary winding (neglecting cable losses) , hence it's windings need to be properly insulated depending upon the voltage level.

Whereas the main purpose of CT is to measure the current hence full current passes through its primary but a very small voltage drop across primary winding, because CTs are connected in series with the load hence by connecting CT voltage across load must not change noticeably that's why insulation level is not of that level as required in power transformer.

THANK YOU...