Power System Problem

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.