Past Paper Questions - Jan 2015 (2)

Question 1

Explain why the use of power factor correction can help to reduce losses in a power system.

Answer 1

With power factor correction, we produce Q (reactive power) locally and so reduce the total apparent power
drawn from the network. As the supply voltage V stays the same, the current drawn from the network is reduced and so are network losses are proportional to i squared.

Question 2

Explain two key advantages of supplying AC power over three phases.

Answer 2

1. The amount of copper required in the system is less compared to a single phase system as there is a shared common neutral conductor, also if the system is designed properly and balanced, the current is neutral so the cross sectional area can be reduced.
2. The power delivered to a large three-phase load such as a motor is steady and does not pulsate as it does with single-phase AC, so torque and vibrations variations are reduced.

Question 3

The AC electricity network is sometimes described as a displacement system. Explain what this implies.

Answer 3

A displacement system is one in which what is withdrawn must be replaced, within the context of the electrical system this implies that demand must be matched by the supply. Failure to do this would result in a system collapse and failure of the power supply.

Question 4

Explain the difference between dispatchable and non-dispatchable generation and give an example of each.

Answer 4

Dispatchable electrical generation can be modulated or readily brought on or offline for the purposes of balancing supply and demand. One is hydro power, that can be brought on or taken offline in a matter of minutes and supply modulated by varying the water flow rate.

Non-despatchable generation is when the output cannot be readily controlled. An example is wind power, where the power supplied is proportional to the wind speed and for reasons of economic viability, power must be accepted into the network when available.

Question 5

In a future UK power system featuring significant quantities of nondispatchable generation, give up to four different mechanisms by which stability could be maintained, outlining some of the advantages and disadvantages of each.

Answer 5

Demand side management – varying the load to help match stochastic elements of the supply, this has the advantage that it shifts some of the responsibility for control of the power system to the demand side easing
the burden on dispatchable generation, however a disadvantages are that there are not that many loads that are suitable for control and response is uncertain.

Back-up generation – additional dispatchable generation can be provided in the power system. This has the advantage of allowing stability to be maintained, the disadvantage is that the generation would be idle for much of the time, and the back-up service would need to be paid for adding cost to the operation of the power system.

Energy Storage – additional storage could be added into the network to absorb surplus renewable output and supply load when renewable output is low. However there are relatively few large-scale storage technologies available e.g. pumped hydro and storage ads cost to the system.

Greater inter connection between countries – this greatly increases the size and diversity of the power system, reducing the impact that renewables have on the power supply and the need for backup or storage. A disadvantage is that interconnection has a high capital cost and also opens up the risk of faults affecting large areas.