Power System Problem

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Questions and Answers

What is the primary purpose of a breather in a transformer?

To provide cold air in the transformer

To filter the transformer oil

To increase the transformer's KVA rating

To absorb moisture of air during breathing (correct)

What is the necessary condition for the parallel operation of two single-phase transformers?

They must have the same KVA rating

They must have the same copper loss (correct)

They must have the same polarity

They must have the same turn ratio

What is the primary purpose of performing a short circuit test on a transformer?

To measure its insulation loss

To measure its iron loss

To measure its copper loss (correct)

To measure its core loss

What is the minimum flashpoint of transformer oil?

140 degree Celsius Signup and view all the answers

Which test determines the iron loss of a transformer?

Both a and b Signup and view all the answers

What type of winding is used in a three-phase shell type transformer?

Sandwich type Signup and view all the answers

Which type of transformer has its secondary winding always kept closed?

Current transformer Signup and view all the answers

Which loss in a transformer changes with the load?

Copper loss Signup and view all the answers

What is the correct relationship between the turns ratio of a transformer and its primary and secondary impedances?

The primary impedance is equal to the secondary impedance divided by the square of the turns ratio Signup and view all the answers

What is the power transformed in an ideal transformer with a turns ratio of a?

The primary power Signup and view all the answers

What is the purpose of a no-load test on a transformer?

To determine the magnetizing current and loss Signup and view all the answers

What is the typical phase angle between the no-load current and voltage in a transformer?

Lagging by about 15° Signup and view all the answers

What is the characteristic of the no-load current in a transformer?

Small magnitude and low power factor Signup and view all the answers

What is the condition of the secondary terminals during a no-load test?

Open Signup and view all the answers

What is the relationship between the turns ratio and voltage ratio of a transformer?

The turns ratio is equal to the voltage ratio Signup and view all the answers

What is the effect of increasing the turns ratio of a transformer on the secondary voltage?

The secondary voltage increases Signup and view all the answers

What is the main purpose of laminating transformer cores?

To reduce both eddy current and hysteresis loss Signup and view all the answers

What is the unit of measurement for transformer ratings?

Kilo-Volt-Ampere (KVA) Signup and view all the answers

What is the purpose of the back-to-back test in transformers?

To compare the efficiency of two identical transformers under load conditions Signup and view all the answers

What is the primary reason for performing a short circuit test on a transformer?

To test the transformer's impedance Signup and view all the answers

What is the relationship between the primary and secondary voltage in a transformer?

Primary voltage is inversely proportional to secondary voltage Signup and view all the answers

What is the effect of laminating transformer cores on eddy current loss?

It minimizes eddy current loss Signup and view all the answers

What is the purpose of the open circuit test in transformers?

To determine the transformer's core loss Signup and view all the answers

What is the relationship between the number of windings in the primary and secondary coils of a transformer?

The number of windings in the primary coil is inversely proportional to the number of windings in the secondary coil Signup and view all the answers

Study Notes

Synchronous Generators

A power system has 3 synchronous generators with turbine-control characteristics equivalent to X1 = 60(60 - f), X2 = 120(70 - f), and X3 = 140(80 - f), where f is the system frequency in Hz.
The system frequency for a load of 800 MW is 60 Hz.

Silicon Steel

Silicon steel is a type of steel.

Transformer Core

Transformer cores are laminated to reduce eddy current and hysteresis loss.

Transformer Performance

A transformer has a primary coil with 1600 loops and a secondary coil with 1200 loops. If the current in the primary coil is 6 Ampere, the current in the secondary coil is 8 Ampere.
The short circuit test in a transformer is performed on the low voltage side.
The transformer ratings are expressed in terms of KVA (Kilo-Volt-Ampere).
The secondary voltage is 440 Volt, and primary voltage is 220 Volt, so the comparison of the secondary coil and primary coil is 2/1.

Efficiency of Transformers

The back-to-back test determines the efficiency of two identical transformers under load conditions.

Transformer Characteristics

The transformer has 6 windings in its primary core and 3 in its secondary coil. If the primary voltage is 440 V, the secondary voltage is 800 V.
The impedance of primary with respect to secondary is secondary impedance/a2.
The power transformed in the ideal transformer with turns ratio a is equal to primary power.
For a transformer with primary turns 100, secondary turns 400, if 200 V is applied at primary, we will get 800 V at secondary.
For a transformer with primary turns 400, secondary turns 100, if 20A current is flowing through primary, we will get 80A at secondary.
The no-load current drawn by transformer is 2 to 5 per cent.
The purpose of no-load test on a transformer is to measure magnetising current and loss.
The no-load current in a transformer lags behind the voltage by about 75°.
In no-load test, we keep secondary terminals open.

Transformer Design

The primary purpose of performing a short circuit test in a transformer is to measure its copper loss.
The flashpoint of transformer oil must be greater than 140 degree.
The open circuit test determines the iron loss of the transformer.
The frictional loss in a transformer is 0%.
The three-phase shell type transformer uses a sandwich type winding.
The secondary winding of a power transformer is always kept closed.

Power Transmission

The highest voltage for transmitting electrical power in India is 400 kV.

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Description

A power system problem involving synchronous generators, turbine-control characteristics, and system frequency. Find the system frequency for a load of 800 MW.