Past Paper Questions - Dec 2017 Flashcards

1
Q

Briefly describe the three main processes that contribute to the energy losses in a transformer.

A

Eddy losses in transformers: Some of the flux gets linked with other conducting parts, which will result in induced emf in those parts, causing small circulating current.

Hysteresis losses: Due to reversal of magnetisation in the transformer core. Depends upon the volume and grade of the iron, frequency of magnetic reversals and flux density.

Copper wire losses: Occur due to ohmic resistance of the transformer windings.

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2
Q

Briefly, explain what the term dispatchable means when applied to electricity generating plant and give an example of a dispatchable generator.

A

A generators output can be increased or decreased according to the level of demand of which can be brought on or offline.

An example of a dispatchable generator is a hydro pump, which can be given more flow rate to increase the output to match the level of demand.

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3
Q

Summarise why dispatchable plant is important in the operation of electrical power systems, particularly those with a significant amount of electricity derived from stochastic renewable sources like wind power.

A

Adding wind power to the generation reduces the ability of the power system to match the demand and causes increased pressure on dispatchable sources. Dispatchable generation is therefore important to modulate for the purposes of balancing supply and demand.

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4
Q

Briefly describe how power factor correction actually works.

A

Power Factor is a measure of how effectively incoming power is used in your electrical system. A poor power factor can contribute to equipment instability and failure, as well as significantly higher than necessary energy costs since it means that more current is required to perform the same amount of work. By optimising and improving the power factor, power quality is improved, reducing the load on the electricity distribution system.

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5
Q

Explain why two parallel current carrying conductors exert a physical force on each other.

A

When a conductor carrying a current is placed in a magnetic field, the conductor experiences a magnetic force. Two parallel electric current carrying conductors creates a magnetic field and the fields attract or repel depending on their orientation.

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6
Q

If current is flowing in the same direction in each conductor, describe what will happen to the wires if they are unrestrained.

A

When the currents flow in the same direction the magnetic field will be opposite and the wires will attract.

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7
Q

Briefly outline three main differences between a synchronous generator and an induction generator.

A
  1. Rotor speed lags synchronous speed in induction generators whereas synchronous generators have a rotor and synchronous speed which matches.
  2. Induction generators always absorb the reactive power, whereas the synchronous generators can be operated to export or absorb reactive power.
  3. Construction of induction generator is less complicated as it does not require brushes and slip ring arrangement. Brushes are required in synchronous generator to supply DC voltage to the rotor for excitation.
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