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9210-209
Level 7 Post graduate Diploma in Electrical Engineering
Power system economics and planning
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, drawing
instruments
A Reference booklet is
attached
General instructions
• This paper consists of eight questions.
• Answer any five questions.
• A non-programmable electronic calculator may be used but candidates must show
sufficient steps to justify their answers.
• Drawings should be clear, in good proportion and in pencil. Do not use red ink.
• All questions carry equal marks. The maximum marks for each section within a
question are shown.
© The City and Guilds of London Institute 2014
ST00042736 H1 PO4500102241 10175598
1
a)
b)
c)
2
a)
b)
3
a)
b)
Explain why long term load forecasting is essential in the generation
planning exercise.
Describe three different methods available for load forecasting giving merits and
demerits of each of the methods.
The peak electricity demand of a province for five consecutive years are given as
1893, 1841, 1922, 1868 and 1955 MW respectively in chronological order.
i)
Obtain the 5 year simple compound growth rate for the given data.
ii) Fit a linear trend profile matching for the above data. Also obtain the
goodness of fit (R2) and comment on the acceptability of the fit.
(Attached equations may be used for this calculation).
iii) What is your recommendation on the period of applicability of the forecast?
Give clear reasons for your answer.
Discuss the following aspects related to transmission network planning.
i)
Developments that could lead to transmission bottlenecks in future.
ii) Factors that should be taken into account in deciding the operating voltage,
number of circuits and the route of a new transmission line.
iii) The (n-1) criterion is used in transmission planning. What more advanced
criteria could be used to improve the transmission reliability?
iv) The main factors determining the voltage regulation and its significance in
the supply of transient and fluctuating loads.
What are the advantages and disadvantages of an HVDC link over an ac link?
Why is it desirable to operate grid connected generators at the same marginal cost?
Two generators A and B are connected to two grid substations X and Y respectively.
A transmission line T connects the substations X and Y. The cost characteristics of
the generators A and B are given below, where the generated quantities PA and
PB are in MW and the total costs of generation C(PA) and C(PB) are in Currency
Units (CU) per hour :
(3 marks)
(6 marks)
(3 marks)
(6 marks)
(2 marks)
(3 marks)
(6 marks)
(4 marks)
(3 marks)
(4 marks)
(4 marks)
C(PA) = 75 + 6PA + 0.08PA2
C(PB) = 100 + 7PB + 0.09PB2
A power flow of PT (in MW) along the transmission line causes a power loss
of Ploss (in kW) given by Ploss = 0.94PT 2. The marginal cost at the bus bar Y is
16 CU/MWh,
i)
Find the optimal loading of the generators, assuming that the transmission
line is dedicated for transmission of power generated from generator A.
State other assumptions made in the calculation.
ii) Calculate the marginal cost at bus bar X based on the marginal cost at the
bus bar Y.
iii) How does the answer to part i) and ii) above change if a wheeling charge is
to be incorporated?
2
(11 marks)
(3 marks)
(2 marks)
4
a)
b)
c)
Briefly describe the term ‘unserved energy’.
Explain, giving examples, why the cost of unserved energy depends on the type
of customer and customer’s activities.
The daily load profile of a province is estimated as given in the Table Q4a. This
load is to be supplied by the three generators with details given in Table Q4b.
Time from (hrs)
Time to (hrs)
Load (MW)
00.00
06.00
100
06.00
09.00
150
09.00
18.00
250
18.00
21.00
400
21.00
24.00
150
(2 marks)
(4 marks)
Table Q4a: Load profile
Generator
Capacity (MW)
Average cost
(Units/MWh)
Availability (%)
1
200
1000
90
2
100
2000
80
3
200
1500
90
Table Q4b: Generator details
Using an appropriate loading order for the generators, determine the following.
i)
Energy served by each generator.
ii) Total production cost.
iii) Loss of load probability.
iv) System unserved energy.
5
a)
b)
c)
Give four main factors that are of importance in selecting a site for a substation.
Using appropriate diagrams explain a ring bus bar system giving its advantages
over a one and half breaker system.
Answer the following with reference to applications in substations.
i)
Why different voltage/current transformers are used for metering and
protection applications?
ii) Explain the purpose of auto-reclosing option of circuit breakers.
iii) Describe different safety precautions taken at the design stage of substations.
3
(8 marks)
(2 marks)
(2 marks)
(2 marks)
(4 marks)
(6 marks)
(3 marks)
(3 marks)
(4 marks)
See next page
6
a)
b)
c)
Give four reasons to justify the statement ‘Rural electrification projects are
financially less attractive compared to urban electrification projects’.
A large number of small loads are closely spaced along a distribution line of 5 km
length. Some of the loads are single phase but they are well-balanced among the
three phases. A three-phase four wire Y-grounded 13.2 kV line is planned along
the total length, fed through a distribution substation transformer from a
high-voltage line. The connected load is linearly distributed at the rate 1.35 kVA/m
at an average power factor 0.9 lagging. Assuming an annual demand factor of
0.6 and a diversity factor of 1.20 among all loads calculate the peak kVA load of
the substation transformer. Assuming a conductor resistance of 0.443 Ω/km
calculate the percentage voltage drop for optimum positioning of the
distribution transformer.
Consider the simple radial distribution system shown in Figure Q6. The assumed
failure rates and repair times of lines A, B, and C are 0.20 f/yr, 0.10 f/yr & 0.15 f/yr
and 6.0 hr, 5.0 hr and 8.0 hr respectively.
i)
Calculate the failure rates and mean restoration times at load points L2 and
L3, assuming that the primary supply is 100% reliable.
ii) If the load points L1, L2 and L3 supply 1000, 800 and 500 customers
respectively, calculate the reliability indices SAIFI and SAIDI for the set of
customers supplied by this radial system.
(4 marks)
(8 marks)
(4 marks)
(4 marks)
Figure Q6: Radial distribution system
7
a)
b)
c)
Explain briefly, why electricity suppliers give high priority to promote Demand
Side Management (DSM) measures.
What purpose is served by each of the following tariff components/ mechanisms?
i)
Fixed charge.
ii) Maximum Demand Charge.
iii) Time of use tariff.
A utility faces a power shortage of 150 MW in meeting its evening peak which is of
3 hour duration every day. As load shedding is against the government policy and
the next nearest solution is to hire short-term emergency power plants costing
£0.15 million every day. On the other hand, it has been estimated that about
4 million household customers use two or more incandescent lamps with a total
wattage of at least 100 W during the peak period which can be replaced by energy
efficient, compact fluorescent lamps. 100 W comprises of usually two lamps
(1×40 W + 1×60 W) and can be replaced by two CFL lamps with total wattage of
approximately 30 W, at a cost of £20 per household. The average price of
electricity to this segment of customers is 7p per kWh.
i)
Establish an analysis to determine whether there is a financially viable
investment on DSM to resolve the capacity shortage of the generating
system through energy efficient lamps provided free of charge to customers.
Assume that the transmission and distribution losses make 10% of the
generation. State any other assumptions made.
ii) In a different proposal customers are requested to pay £2 for the CFL
lamps per household. Estimate the financial benefit to each of the
households if the average lifetime of the lamps is taken as one year.
4
(4 marks)
(2 marks)
(2 marks)
(2 marks)
(6 marks)
(4 marks)
8
a)
b)
What is meant by ‘cost reflective pricing’?
Write short notes on the following covering all important aspects under each of
the topics.
i)
Driving forces for restructuring of the electricity industry.
ii) Vertical and horizontal unbundling.
iii) Single buyer model.
iv) Open access to transmission assets.
5
(4 marks)
(4 marks)
(4 marks)
(4 marks)
(4 marks)