Past Paper Questions - Dec 2017 (2)

Question 1

3-phase synchronous generator. Given the generator characteristics, what type of generator is being described? In what type of power generation scheme would such a generator be used?

Answer 1

A salient pole machine is being described, they are employed in low-speed power generation schemes such as hydro pumps and wind turbines. The hydraulic or wind turbine in the operates at low speeds compared to steam plants, therefore generators require a large number of field poles to produce the rated frequency.

Question 2

Briefly outline why it is advantageous to use very large synchronous generators in a centralised power system.

Answer 2

Size matters, a small synchronous generator will have an efficiency of around 50%, whereas a large machine will have near 90% efficiency as there are smaller losses.

Question 3

Describe a major factor which limits the size of these machines.

Answer 3

The upper limit on size is set by the cooling required with larger machines requiring more elaborate cooling.

Question 4

Explain why, for large consumers of electricity, it is desirable to operate an electrical load at close to unity power factor.

Answer 4

The phase difference between the voltage and current is zero and therefore removes the reactive power from the system. All the energy supplied by the source is consumed by the load, which results in a more efficient generation and transmission.

Question 5

The transformer which serves the factory is protected by a unit protection scheme, briefly outline the differences between a unit and non-unit protection scheme.

Answer 5

Unit type schemes protect a specific area of the system i.e. a transformer. It is possible to design protection systems that respond only to fault conditions occurring within a clearly defined zone. This type of protection system is known as unit protection.

The non-unit schemes, while also intended to protect specific areas, have no fixed boundaries. As well as protecting their own designated areas, the protective zones can overlap into other areas. While this can be very beneficial for backup purposes, there can be a tendency for too great an area to be isolated if a fault is detected by different non unit schemes.