Marine Transformer questions and answers for ETO

Introduction

The transformer is a static device used for transferring electrical power from one circuit to another circuit without changing frequency

Transformer works on the principle of electromagnetic induction (Mutual induction) which states that any change in flux linking with the coil, EMF is induced in the coil.

The transformer consists of two coils or windings which are electrically separated but linked through a common magnetic circuit.

The two coils have high mutual inductance. If the current in one coil is changed then an EMF gets induced in the other coil.

The winding which is connected to the supply side is called primary winding and the one which is connected to load side is called secondary winding.

The primary winding is connected to alternating voltage and the alternating current which produces alternating flux and this flux links both the primary and secondary winding. Hence according to faraday law of EMI, mutual induced EMF is developed in the secondary winding. This EMF drives the current if the secondary winding is connected to the load thus the electrical power is transferred from one circuit to another circuit

Types of transformer

Based on the construction of the core

Core type
Shell type

Core type transformer

Windings are placed on either limb
Windings surround core
Natural cooling is possible
Winding cross section can’t be thick
The core is rectangular and is made up of silicon steel, windings of copper

Shell type transformer

Core surrounds winding
No natural cooling
Cross-section of winding can be thicker
Both primary and secondary winding is placed on the central limb

Losses in Transformer

Iron losses – hysteresis, eddy current
Copper losses

What is Hysteresis Loss?

It is due to magnetisation and demagnetisation of the core which is produced by alternating flux. During this process, some amount of power is lost which is hysteresis loss

For silicon steel, hysteresis coefficient is very less, so it is used for core construction the minimise the loss.

What is Eddy current loss?

Eddy current losses are because of circulating currents or eddy currents in the metal block. Eddy currents are set up by induced EMF. To reduce the losses, laminated construction is done to the core thickness of lamination.

Copper loss

These are due to power wasted in the form of I2R losses due to the resistance of the primary and secondary windings

It can be minimized by reducing the resistance of windings i.e. by increasing the cross-sectional area of windings.

Ideal transformer

It has no losses
Winding resistance is zero
Leakage flux is zero

Voltage regulation of transformer

Voltage regulation of the transformer is defined as a change in the magnitude of terminal voltage from no load to full load expressed as a % of the no-load voltage

Voltage regulation = [(no-load voltage – full load voltage) / (no-load voltage)] x 100

All-day efficiency of transformer

The ratio of output power to input power is defined as the efficiency which is the power efficiency. But for special types of the transformer, this may not measures the correct performance

The special types of transformer used for industrial and commercial loads are distribution transformer

The load on such transformer varies considerably during the period of the day. But the primary of the transformer is energised at it is rated voltage for 24 hours to provide continuous supply to the consumers.

The core loss depends on the voltage which takes place continuously whether the load is tnt or not whereas copper loss depends on the load condition at no load copper loss is negligibly small while on the full load it is at rated value. So there will be no measures of true efficiency. In such a transformer, the energy output is calculated in kWh.

Energy spent in supplying to various losses is also calculated in kWh. Output to total energy thus the ratio of total energy input is called as energy efficiency or all-day efficiency.

All-day efficiency = energy o/p during a day kWh / energy i/p during a day kWh

All-day efficiency = energy o/p during a day (kWh) / energy o/p in kWht (energy spent in supplying the losses)

Construction of Three phase transformer

The three-phase transformers can be core or shell type. The three core type single-phase transformers can be combined to get a three-phase core type transformers and similarly shell-type also

Advantages of single three-phase transformer

A 3 phase transformer of occupies less space for the same rating compared to a bank of 3 single-phase transformers
Its weight is less
Cost is less
Only one unit is required to be handled which make it easy for the operator
The core will be of smaller size and the material required for the core is less
The overall structure and installation of a single three-phase unit is simple

Bank of three single phase transformers

Three single-phase transformer are connected together to have a three-phase transformer. The 3 single-phase transformers from a bank have identical ratings.

In a bank of 3 single-phase transformer, the phases are electrically connected but the three magnetic circuit circuits are independent.

Advantages of Bank of three single-phase transformers

When one of the transformers is out of service then also the system separations are possible with remaining two transformers using Y-Y connections at induced capacity.
It is possible to have one transformer in a bank with a higher KVA rating than the other so that unbalanced load can be supplied.
In the bank of three single-phase transformers, only one single-phase transformer is to be kept spare rather than to keep a where 3 phase unit which is expensive. This requirement of standby is less in case of bank of 3-single phase transformers.

Three-phase transformer connections

The primary and secondary of three-phase transformers can be connected in different ways such as star and delta connection.

The following types of connections are used

Y-Y connection
Δ-Δ connection
Δ-Y connection
Y- Δ connection
V-V connection or open delta
Scott connection or T-T

Advantages of Delta – Delta [Δ-Δ] connection in transformer

In order to get the secondary voltage is sinusoidal, the magnetising current of transformer must contain 3^rd harmonic content.
The delta connection provides a closed path for the circulation of 3^rd harmonic component of current.
The flux remains sinusoidal which results in sinusoidal voltage.
This type of connection can be used for supplying unbalanced load also.
Due to Δ connection, V_ph is same as V_L but I_ph = 1/ root 3 I_L
Therefore the cross-section of winding is very less, this makes the condition economical for a low voltage transformer.
If there is a bank of single-phase transformers connected in Δ-Δ and if one of the transformers is disabled, then the supply can be continued with remaining two transformers with reduced capacity.

Disadvantages

It is not suitable for 3 phase 4 wire system.

Labelling of transformer terminals

For high voltage winding, upper case letters are used and LV windings by lower case letters.

Feature of silicon steel

Silica + pure steel = silicon steel

Silicon steel belongs to Ferromagnetic materials among all magnetic materials Ferro is a superior magnetic material.

Silicon steel allows low reluctance and high permeability.

Permeability-conductivity-analogies

Silicon steel has low hysteresis coefficient.

If the full load current is 100amps, what would be the starting current when connected in DOL, Star Delta and Auto Transformer?

In DOL starter it is 5 times full load current i.e. 500 A
In Star Delta one by third of DOL starting current 500/3 = 166.66 A
In autotransformer, x^2 × DOL starting current
X is tapping percentage
50% tapping means x is .5
I.e. (0.5) ^ 2 × 500 is 125 A
Autotransformer is depended upon tapping. 25 to 100amps.

Why secondary of CT should not be kept open?

Since we know that CT is a step-up transformer. So if the secondary side is open then it will create a very large potential difference between the open ends and which will cause medium to ionise and produce arc and can lead to a fire. So it's shorted and not left open.

It also becomes a hazard to personnel.

Transformer safeties?

Ventilations for Air Cooling
Mechanical interlock for avoiding paralleling two transformers.
Earthed System.
Breaker protection
Fuses.
Isolation arrangements.

Why can't we use step-up transformer instead of a transistor for voltage amplification?

Because if the Transformer steps up the voltage and at the same time reduce current because transformer power is always the same in primary and secondary. So there is no power gain but in a transistor, we amplify signal same time we can achieve power gain also.

What is Isolation Transformer and what is its function?

An isolation transformer is used to isolate the two circuits electrically by providing galvanic isolation between them.

Isolation transformers block transmission of the DC component in signals from one circuit to the other, but allow AC components in signals to pass.

Isolation transformers are used for impedance matching to get the most efficient power transfer between stages and to keep different stages electrically isolated to prevent ground loops.

Isolation transformers prevent harmonics from transferring from one side to the other side.

Applications:

Telecommunication equipment
Remote control equipment
Computers & peripherals
Analytical instruments etc.

Reference: https://www.quora.com/

How do you identify the primary and secondary winding of a transformer? When there is no marking

We can identify by observing the connecting cable dia. 440 side cable having less dia as compared to 220 sides.
For step-up transformer low voltage side is primary and high voltage side is secondary.
For a step down high voltage side is primary and low voltage side is secondary,
The technical terminology is HT and LT instead of the primary and secondary side. The HT/LT may be primary or secondary.
In case of a step-down transformer, primary windings must have more length than the secondary windings. So, obviously, the winding having more length will have more resistance than the other.
In short, check the resistance of both windings and the windings having more resistance will be the primary one.

Why transformer should not be connected star - star?

We can connect is Y-Y configuration also. But every configuration has its own advantage and disadvantage. In Y if the fault occurs in any phase we have to isolate whole winding but in Delta, if the fault occurs in one phase we can run the transformer in V (open delta) connection having 57.7% efficiency. In star connection, harmonic generation is more as compared to delta

Also, in Y-Y connection the 3rd harmonics present in supply, to eliminate 3rd harmonics there is a requirement of an additional 3rd winding which connected in delta called tertiary winding in Y-Y connection transformers.

What happens when we use a copper core instead of iron steel core in the transformer?

As copper is a paramagnetic material it has low permeability as compared to iron. Hence to maintain the same flux in core transformer draw more current and windings damage.

What will happen if we connect dc voltage to the input of a transformer?

When a DC voltage is applied to the primary winding of a transformer, due to low resistance, the winding acts as a short circuit across the terminals of the DC source that lead to the flow of heavy current through the winding resulting in overheating of the winding. Eventually, the source or the winding will be damaged. The effect of current on winding depends on the applied DC voltage, rating of the transformer, winding resistance and how powerful the DC source is. “Nothing will happen if 5Vdc is applied to a 50KVA transformer!

Reference: https://www.electricalclassroom.com/

In order to replace ammeter why should we open CT?

The CT converts current in the range of 1A or 5A from a higher value. That is high ampere current to lower ampere rating by using step up transformer

As the voltage gets stepped up this can lead to arcing at the secondary if left open while working.

To avoid this, we short the secondary of CT.

How do you carry out the Transformer changeover on ship?

The changeover should be carried out in anchorage or port
Note down the parameters of working transformer
Carry out IR test for standby transformer
Make sure everything's perfect
Inform to bridge and must shout down which are not get supply from ESB and UPS
After having to carry out the changeover operation
Note down parameters and compare to previous work
transformer. If everything perfect and we have to switch ON back to which are shout down.

What are the types of transformer using in the ship?

Power distribution transformer (High Rating) – for step down, lighting, and 220VAC consumers
Potential transformer (Low Rating) – indication lamp and instrumentation
Current transformer – instrumentations circuits and metering
Isolation transformer – PCB and control circuits
Ignition transformer – boiler and incinerator
Low voltage to high voltage with Low current spark
Autotransformer – Starter
Welding transformer – Welding machine