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Transformer interview Questions & Answers paper-11

191. what is called marked ratio of an instrument transformer ?

Ans : The making ratio of a current or a potential transformer is the ratio of the primary current or voltage, as the case may be, to the secondary current or voltage as given on the rating plate.

192. What is called ratio correction factor ?

Ans : The ratio correction factor is that factor by which the marked ratio of a current or a potential transformer must be multiplied to obtain the true ratio. So R.C.F = (True ratio)/(Marked ratio.)

193. What are the types of current transformers ?

Ans : The two types are (1) Air-cooled current transformers and (2) Oil immersed current transformers.

194. What are the types of current transformers according to the magnetic circuit ?

Ans : There are three types of current transformers in common use according to the magnetic circuit, namely, “Ring type”, “Core type” and “Shell type”.

195. How are the terminals marked in a current transformer ?

Ans : The primary terminals are marked by the letters ‘M’ and ‘L’ for main & load ends respectively and the secondary terminals by the same letters enclosed in a circle i.e. (M) and (L).

196. What are the errors in C.Ts. ?

Ans : There are two of errors, e.g. ratio error and phase difference error. The ratio error is the amount by which the secondary current differs from exact proportionality to the primary current and is expressed as a percentage of the rated secondary current. The phase difference error is the angle by which the secondary current differs in phase form the primary current and is expressed in minutes of arc.

197. At what load the phase angle error of a c.t. will be negative ?

Ans : At the load of very low power factor the phase angle error will be negative.

198. why are the errors of a C.T. generally greater with relatively small loads than at rated full load?

Ans: The errors are generally increased with relatively small loads than at rated full load due to greater proportion of exciting current to the reversed secondary current .

199. If the core of a C.T. is permanently magnetized how will you demagnetize it ?

Ans: To demagnetize the core of a C.T. a variable resistance of high value is connected across the secondary terminals and with full rated current flowing through the primary the resistance is gradually reduced to zero.

200. What is meant by “turn compensation” and why is it done ?

Ans : Turn compensation means to keep the number of secondary turns one or two less than that obtained by normal ratio and it is done to reduce the current ratio error.

201. How are the ratio error and the phase error of a C.T. improved ?

Ans : The errors of a C.T. can be improved by the following methods – By using high permeability and low loss magnetic material of the core. By reducing the length of the flux path in the core and increasing the area of the path with all joints reduced to a minimum or avoided altogether. By increasing the primary ampere-turns. By using one or two turns less in the secondary winding. By reducing the internal secondary burden as far as possible. By keeping the connected burden on the secondary as small as possible. By specifying the rated burden as near to the actual burden as possible.

202. What are the categories of current transformers and their applications ?

Ans : There are two categories of C.Ts, e.g, Measuring current transformers are used with ammeters, wattmeters, KVA meters, KWH meters, power factor meters etc. Protective current transformers are used with over current relays, earth fault relays, differential protection, impedance protections etc.

203. Which instruments transformer is used in a Tong tester ?

Ans : Current transformer.

204.why is only one turn of very thick conductor used in the primary of a current transformer ?

Ans : In the primary of a current transformer only one turn of very thick conductor having negligible resistance is used not only for reducing the induced voltage but also for reducing the voltage drop to nearly zero. As large current flows through the primary and small current as per current ratio flows through the secondary if one turn is selected for primary the secondary turns will be smaller.

205. What materials are used for the core of a C.T. & why ?

Ans : In order to reduce the ampere turns required the core of a C.T. must have high permeability, small iron loss, a low flux density and low phase difference error. Two suitable materials are Silicon steel having the maximum permeability of 4500 at flux densities in the neighborhood of 5000 lines/cm2 with phase difference error of 1.5 to 2⁰ . Mumetal, a nickel-iron alloy containing a small amount of copper having the maximum permeability of about 80000 at flux densities of about 3500 lines/cm2 with phase difference error of below 0.5⁰. As the nickel-iron alloy reduces the magnetizing current and core loss to about 10% of that of the corresponding silicon steel values, Mumetal is best suited for the core of a C.T. where a high degree of accuracy is desired.

206. What is the effect of saturation on the performance of C.T. ?

Ans : On reaching saturation the transformer ratio is affected resulting in an increase of exciting current due to reduction in magnetizing inductance and introducing a phase error in the C.T. deviating primary and secondary current from their desired phase opposition i.e. 180⁰. The increase in exciting current causes the reduced secondary output and hence reduced the speed of over current relays. The saturation of C.T. disturbs the balance in differential relays thus affecting the stability of protection. Therefore the effect of saturation makes the operation of the C.T. unsatisfactory.

207. What are the basic differences between current transformers and potential transformers ?

Ans : (1) A C.T . is connected in series with the line whereas a P.T. is connected across the supply lines. (2) In a C.T the number of turns is inversely proportional to the current but in a P.T. the number of turns is directly proportional to the voltage. (3) In a C.T. the primary has one or more turns of heavy conductor and the secondary has a large number of turns of thin wires whereas in a P.T. the primary has a large number of turns of thin wires and the secondary has a few number of turns of comparatively thick wires.

208. What are the types of potential transformers ? State the applications of potential transformers ?

Ans : There are two types of potential transformers. E.g. Electromagnetic type potential transformer, in which primary and secondary windings are used on the magnetic core like usual transformers. Capacitor type potential transformer, in which a series capacitor group is used with an auxiliary voltage transformer. The primary voltage is applied to the series capacitor group and the voltage across one of the capacitor is taken to the primary of the auxiliary voltage transformer. Then the secondary of the auxiliary voltage transformer is finally taken to the metering and/ or control panel. Potential transformers are used with voltmeter, KWh meter, power factor meter, frequency meter, protective relays for the purpose of measurement and protection. Electromagnetic type potential transformers are used upto 66KV lines while capacitor type potential transformers are used for the voltages 66KV and above.

209. How are the terminals of a potential transformer marked ?

Ans : In single phase potential transformer the primary terminals are marked with letters V1,V2 and the secondary terminals bearing similar markings are encircled. In the case of tapped windings the primary terminals are marked with the letters V1, V2,V3,V4 and the secondary terminals bearing similar markings are encircled. In three phase star/star potential transformer the primary terminals are marked with the letters A B C N and the secondary terminals are marked with similar letters but encircled. The terminal marked with the letter ‘N’ is the neutral.

210. What do you mean by the “ratio error” and “phase difference error” of potential transformer ?

Ans : The amount by which the secondary voltage of a potential transformer differs in magnitude from exact proportionality to the primary terminal voltage is called the ratio error of a potential transformer. The angle by which the secondary voltage of a potential transformer differs in phase from the primary terminal voltage is called the phase difference error of a potential transformer.

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