Important Multiple choice Question Of Transformer 2022

Answer.4. both cooling and insulation

Explanation: There are two main functions of the transformer oil

Functions of Transformer Oil

Oil is an equally important part of a transformer’s overall insulation.

(i) Electrical Insulation:- The main function of insulating oil in a transformer is to provide electrical insulation between the various energized parts; it also acts as a protective coating layer to prevent oxidation of the metal surfaces.

(ii) Heat Dissipation/Coolant:- Another important function of the oil is to enhance heat dissipation. Transformer cores and windings get heated up during operation due to various power losses. Oil takes heat away from the core and windings by the process of conduction and carries heat to the surrounding tank, which is then radiated out to the atmosphere. In order that the mineral oil can dissipate the heat away effectively, certain specifications — including viscosity, pour point, and flash point — need to be maintained.

(iii) Diagnostic Purposes:- The third (very useful) function of insulating oil in a transformer is that it acts as a health indicator for the device. Both the chemical and electrical conditions of the transformer can be monitored by examining the oil periodically. Oil samples are collected from designated sampling points of the tank and taken to laboratories for several tests to be performed.

When a fault develops within the transformer, the energy is dissipated through the oil, which causes chemical degradation of the liquid. Testing oil samples for degradation products can provide useful information about the nature and severity of possible faults inside a transformer.

Answer.1. Open Circuit test

Explanation: 

The purpose of the open-circuit test’ is to determine the excitation admittance of the transformer- equivalent circuit, the no-load loss, the no-load excitation current, and the no-load power factor.

• In the open-circuit test, the transformer load terminal is kept open. Open circuit test is also known as the no-load test.
• The current drawn by shunt parameters is a no-load current a very small current. Therefore the current that will flow in the circuit in the open circuit test is very low so the measurement of the quantities voltage, current, and power must be on the low voltage side so that the corresponding value will be readable in the instruments. And therefore, the open circuit test must be performed on the low voltage side. This means the high voltage side must be kept open and for the measurement of power, voltage, and current on the low voltage side the wattmeter, voltmeter and ammeter must be connected
• We know that as the output terminal is open the parameters that are in the shunt can be found out by this test. Since the shunt circuit has the core parameter so we can say that the open circuit test gives the core parameter.
• The open-circuit test on the transformer is performed to determine magnetizing reactance and equivalent resistance due to iron loss.
• As the normal rated voltage is applied to the primary, therefore, normal iron losses will occur in the transformer core. Hence, a wattmeter will record the iron losses and small copper loss in the primary. Since the no-load current is very small (2 to 5% of rated current), copper losses in the primary under no-load conditions are negligible as compared with iron losses. Hence, wattmeter reading practically gives the iron losses in the transformer.

2. Low reluctance

Explanation:

• Magnetic reluctance, or magnetic resistance, is a concept used in the analysis of magnetic circuits. It is analogous to resistance in an electrical circuit, but rather than dissipating electric energy it stores magnetic energy.
• Low the reluctance, less the opposition to flux therefore more flux can pass through the transformer core.

Also from the equation

Flux = MMF/R_e

Where Re = Reluctance

So lower the reluctance higher will be the flux flow

Answer.4. Transformer

Explanation

The Buchholz relay is a gas-operated relay used for the protection of oil-immersed transformers against all types of internal faults. The slow-developing faults called incipient faults in the transformer tank below oil level operate Buchholz relay which gives an alarm. If the faults are severe it disconnects the transformer from the supply.

It uses the principle that due to the faults, oil in the tank decomposes, generating the gases. The 70% component of such gases is hydrogen which is light and hence rises upwards towards the conservator through the pipe. The Buchholz relay is connected to the pipe, as shown in Fig. Due to the gas collected in the upper portion of the Buchholz relay, the relay operates and gives an alarm.

There are many types of internal faults such as insulation fault, core heating, bad switch contacts, faulty joints etc. which can occur. When the fault occurs the decomposition of oil in the main tank starts due to which the gases are generated. As mentioned earlier, the major component of such gases is hydrogen. The hydrogen tries to rise up towards the conservator but in its path, it gets accumulated in the upper part of the Buchholz relay.

When gas gets accumulated in the upper part of the housing, the oil level inside the housing falls. Due to which the hollow float tilts and closes the contacts of the mercury switch attached to it. This completes the alarm circuit to sound an alarm. Due to this operator knows that there is some incipient fault in the transformer. The transformer is disconnected and the gas sample is tested. The alarm circuit does not immediately disconnect the transformer but gives the only indication to the operator. This is because sometimes bubbles in the oil circulating system may operate the alarm circuit though there is no fault.

However, if a serious fault such as an internal short circuit between phases, earth fault inside the tank etc. occurs then a considerable amount of gas gets generated. Thus due to fast reduce the level of oil, the pressure in the tank increases.  This energizes the trip circuit which opens the circuit breaker. Thus transformer is totally disconnected from the supply.

The connecting pipe between the tank and the conservator should be as straight as possible and should slope upwards conservator at a small angle from the horizontal This angle should be between 10 to 11°

For the economic considerations, Buchholz relays are not provided for the transformers having a rating below 500 kVA.

Answer.2. Magnetostriction in iron core

Explanation:

Noise in transformer

The transformers using ferromagnetic core produces noise. Such a noise exists in the form of an electric hum around transformers. Such noise may be annoying to the nearby residential area. We can think of a transformer core, therefore, as behaving like a giant loudspeaker producing a continuous humming or buzzing noise, which is well within a human’s audible frequency range, and extremely irritating.

The noise is produced due to the vibration of the enclosure and accessories. The vibrations are produced mainly due to stray magnetic fields and due to magnetostriction.

Magnetostriction is a phenomenon due to which the length of the ferromagnetic core increases when magnetized and gets back to the original when demagnetized. Due to this, there is an increase and decrease in the cross-section of the core. As the laminations change their dimensions, the core vibrates to produce the noise. The core sound is dominant in no-load conditions.

In load conditions, the vibrations in the tank and winding cause noise. The load noise is caused due to electromagnetic forces resulting from leakage fields produced by load currents. These forces are proportional to the square of the load currents. Similarly, there is noise due to sound generated by cooling equipment such as pumps and cooling fans.

Thus the various factors responsible for producing transformer noise are,

1. Magnetic forces producing vibrations due to stray magnetic fields.
2. The vibrations are produced due to magnetostriction.
3. The vibrations are due to electromagnetic forces generated due to the load currents.
4. Mechanical vibrations due to the degree of tightness of clamping the core by nuts and bolts.
5. The joints in the core.
6. The sound is produced by cooling equipment like fans and pumps.
7. The damping

The noise productions in transformers can be reduced by

1. The no-load sound generated due to the magnetostriction can be reduced by lowering the flux density in the core.
2. By using proper core material with low loss, high permeability, and low noise generation.
3. By properly tightening the core by clamps and bolts and properly fixing laminations and frames.
4. By providing sound insulation to the tank from the ground and surrounding air.
5. By using cushion padding and oil barriers which help in insulating the sound of vibration.
6. By using proper innovative designs and materials like stiffeners while designing the tank walls.

The noise cannot be completely eliminated but taking the frequency of vibration of the transformer outside the audio frequency range is most helpful.

Answer.4. Equal to

Explanation:

• The transformer is said to be a constant main flux device. It is due to the high permeability and greater mutual flux which maintain a constant value.
• When a load resistance is connected to the secondary winding the voltage induced into the secondary causes a flow of secondary current.
• This current produces a secondary flux field which is in opposition to the primary field flux (Lenz’s law). Thus, the secondary flux cancels some of the primary flux.
• With less flux surrounding the primary, the primary EMF is reduced and more current is drawn from the source. The additional primary current generates more lines of flux nearly reestablishing the original number of total flux lines.
• Similarly,  for reducing the load, the secondary current decreases, and hence the primary current also reduces.

To summarize, as current is drawn from the secondary, total flux momentarily decreases, and primary current increases restoring the flux lines to almost their original number. Thus, over the normal operating range of load § current, the total core flux does not change more than two or three percent.

3.Damage the transformer

Explanation:

• • A transformer works on the principle of mutual induction, in which you need a varying magnetic field in a winding to induce an EMF in the secondary winding.
  • In DC generally change in frequency with respect to time is zero. If the primary of a transformer is connected to a d.c. supply, the primary will draw a steady current and hence produce constant flux. Consequently, no back e.m.f. will be produced.
  • So a dc source cannot provide varying magnetic fields, hence mutual induction is not possible.
  • The primary winding will draw excessive current due to the low resistance of the primary. The result is that the primary will overheat and burn out or the fuses will blow. Care must be taken not to connect the primary of a transformer across the d.c. supply.

3. Both step up and step down

Explanation:

An autotransformer is a type of transformer that uses a single tapped winding rather than the two separate and electrically isolated windings used by mutual transformers.

Because autotransformers don’t have separate windings, unlike mutual transformers there is no electrical isolation between the primary and secondary circuits.

• For example, if 230 V is applied between points A and B which involves 115 turns of the autotransformer winding, then the volts per turn (230/115) will be 2.
• By a suitable selection of taps, one may select the number of turns to supply the necessary voltage to the other components.
• A selection of 55 turns will provide a voltage of 110 V while a selection of 160 turns can provide 320 V.
• Thus, an autotransformer can function as a step-up or step-down transformer.
• In the step-up version, the voltage induced in the additional winding is added to the supply voltage.
• In the step-down version, the induced voltage reduces the supply voltage.
• It is possible to design an autotransformer to a required secondary output, which is variable in fractions or multiples of applied voltage.

2. 0%
Explanation:

The transformer is a static device that is used to transfer electric power from one circuit to another without changing its frequency. The main function of a transformer is to raise as lower the voltage in a circuit with a corresponding decrease or increase in current at the same frequency. It works on the principle of Faraday’s law of Electromagnetic induction. Transformers have no moving parts, rugged and durable in construction.

Since operation does not involve rotation of any armature, field system, or commutator, rotational, friction loss. and windage losses do not occur and its efficiency is thus high.

For electrical ‘power’ purposes, i.e. transformers operating at 50 or 60 Hz, iron cores are essential and iron losses will occur. Winding copper losses are also present when current is supplied, nonetheless, the transformer is the most efficient of electrical machines and has a full-load efficiency of 95.5% for units of 5 kVA and 97.5% for units up to 1 MVA may be achieved.

3.Both electrically & magnetically

Explanation:

An autotransformer is a type of transformer that uses a single tapped winding rather than the two separate and electrically isolated windings used by mutual transformers. Because autotransformers don’t have separate windings, unlike mutual transformers there is no electrical isolation between the primary and secondary circuits.

• Therefore the primary is electrically connected to the secondary, as well as magnetically coupled to it.
• The alternating current applied between the input points will induce a flow of magnetic flux around the core.
• This magnetic flux will link with all the turns forming the coil, inducing a voltage into each turn of the winding.
• Since the volts-per-turn is the same in both windings, each develops a voltage in proportion to its number of turns.
• In an autotransformer, part of the current flows directly from the input to the output, and only that part is transferred.

Where electrical isolation between the primary and secondary windings is unimportant, the use of an autotransformer has a number of advantages over a mutual transformer.

• With only one winding, the volume of copper required in its manufacture can be lower than for a mutual transformer.
• There are no secondary copper losses, so autotransformers are more efficient than a corresponding mutual transformer.