ENGI4487-WE01/ ENGI44I10-WE01 Smart Energy Networks 4 2023
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ENGI4487-WE01/ ENGI44I10-WE01
Smart Energy Networks 4
2023
Section A
Question 1
(a) Consider the power system shown in Figure Q1a. The reactance data is shown in per unit on a common base on the diagram.
For each of the following faults, compute the fault current in per unit. Assume that the fault has zero impedance.
(i) A single line to ground fault at bus 2. For this case, also calculate the phase voltages (Ⅴabc ) at the faulted bus. [20%]
(ii) A line to line fault at bus 3. [20%]
(iii) A double line to ground fault at bus 2. [20%]
(b) A 50-Hz synchronous generator has a transient reactance of 0.3 per unit and an inertia constant of 4 MJ/MVA. The generator is connected to an infinite bus through a transformer and a transmission line, as shown in Figure Q1b. Resistances are neglected and reactances are expressed on a common MVA base and are marked on the diagram. The generator is delivering a real power of 0.9 per unit and its internal voltage magnitude is |E′ | = 1.05 pu. The infinite bus
voltage is vbus = 1.0∠0° per unit.
Assume that a permanent three-phase bolted fault occurs on bus 2 and is cleared three cycles later. Draw the equal area curves. Determine the power angle δ after the three cycles, and also derive the equation for the maximum power angle. Determine the maximum value of δ. Is the
stability maintained?
[40%]
Section B
Question 2
(a) In your own words, explain the difference between adequacy of supply and security of operation concepts in power systems. Explain the challenges facing the Transmission System Operator for maintaining a required level of security of operation especially in systems with higher levels of Renewable Energy Sources (RES). [20%]
(b) Consider the circuit shown in Figure Q2. The 33/11 kV transformer has an on-load tap changer which maintains the load voltage at 11 kV. The load consists of a fixed load (Ld )
and a price-sensitive flexible load (Lf ) which is varying (both in units of MW). The fixed load
remains at 1 MW throughout the day.
(i) The flexible load Lf (unit MW) is a function of the electricity price p(t) (unit in £/MWh)
as given by the following expression:
In which p(t) is always positive and t denotes the time for any hour in the day. If the price of electricity shall remain constant at £40/MWh determine the total daily energy loss (in units of MWh) in the 33 kV line due to both the flexible load and the base load demands. [50%]
(ii) Propose a two-tariff Time-of-Use Demand Side Management (DSM) scheme to reduce the total energy losses in the line. The two-tariff structure should have the following characteristics:
- The average between off-peak and peak prices should equal the fixed rate of £40/MWh
- The difference between peak and off-peak demand throughout the day should be reduced by 40% from the same value in part (i). [30%]
2023-07-29