Past Paper Questions - Dec 2016 (2) Flashcards
Explain why salient pole synchronous generators are used in hydro power generation and sometimes in wind turbines.
These are commonly used in these applications as they have a slow rotational speed. These low speed rotors have a large diameter to accommodate the large number of poles required.
The poles are mounted on a large steel frame fixed to a revolving vertical shaft.
In the electricity system supply and demand must balance at any point in time. With the aid of a diagram, explain how this balance can be achieved using a mix of generation.
Draw MegaWatt Magnitude on Y axis, Time on X axis. Block chart of various generation mechanisms such as wind, nuclear etc over the peak times in a day.
During a renewable deficit period, interconnection with the rest of the UK will be required and energy supplies such as nuclear and gas will be required to back up the demand. During renewable surplus power will need exported to the rest of the UK.
What two basic components could be used in an equivalent circuit to model a single phase AC machine (phase angle of the current is measured at -40 degrees relative to the voltage) and briefly explain why.
A resistor and an inductor.
A resistor would be used, the loss manifested as a voltage drop across the component.
An inductor would be used as the phase angle indicates the current lags the voltage by 40 degrees.
Explain why it is beneficial to supply large rotating machines with three phase power.
The power delivered to a large three-phase load is steady and does not pulsate as it does with single phase AC, so torque and vibration variations are reduced.
Also the amount of copper required in the system is less compared to a single phase system as there is a shared common neutral conductor. If properly designed and balanced, the current is neutral so the cross-sectional area can be reduced.
Explain the how real and reactive power output from a synchronous generator can be controlled physically and how this differs from an induction generator.
In a synchronous generator it can be operated to export and absorb reactive power, whereas an induction generator always absorbs reactive power. The real power output P, can be controlled by varying angle (X) in a synchronous generator. The reactive power Q, can be controlled by varying the Eo.
