Examination Paper: Earthing, Protection and Switchgear for Electrical Power Systems, Exams of Materials science

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CORK INSTITUTE OF TECHNOLOGY

INSTITIÚID TEICHNEOLAÍOCHT CHORCAÍ

Semester 1 Examinations 2011/

Module title: Earthing, Protection and Switchgear

Module: ELEC 8007

School: Electrical & Electronic Engineering

Programme title: Bachelor of Engineering (Honours) in Electrical Power Systems – Award

Programme Code EELPS_8_Y

External Examiner(s): Ms M. Desmond, Mr C. Murray r. G. Beecher, Dr. M. Duffy Internal Examiner(s): Mr D Collins

Instructions: Answer any four questions. All questions carry equal marks. State any assumption made.

Duration: 2 Hours

Sitting: Winter 2011

Requirements for this examination:

Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt, please contact an Invigilator.

Q1.(a)(b)Figure 1(a) shows a line diagram for a radial distribution scheme and the fault levels at each stage. Figure 1(b) gives details of the electro-mechanical overcurrent relays deployed in the protection scheme. Propose suitable current and time settings for each overcurrent relay that will ensure proper discrimination during fault conditions. Use a margin of 0.4 s between stages to cater for security and circuit breaker clearing times. (25 marks)

Q2. (a) An industrial site that needs a high security of supply has two 20 kV substations. Power is transferred from one substation to the other by means of duplicate underground feeder cables. Explain why this configuration is considered appropriate. Sketch a line diagram for this system depicting the necessary switchgear and protection equipment. (15 marks) (b) Assume that a full short circuit fault occurs on one feeder cable. Describe how the protection system responds to the fault and how it fulfils the requirements outlined in part (a) of your answer. (10 marks)

Q3. The line diagram for a distribution network is shown in Figure 2. Estimate the maximum sustained short circuit current and the peak short circuit current at the fault location. Ignore the impedance of the grid, switchgear and fault arcs. (25 marks)

Q4. (a) A large industrial premises has an existing 10 kV distribution board of withdrawable design incorporating one spare section with busbars but no other equipment. The fault level at the busbars is 25 kA. An expansion of the facility is planned, the extra load being 2.5 MW at a power factor of 0.83 lagging. The new load is to be supplied from the spare section of the board. Specify the principal items of electrical equipment to be provided in the spare section of the distribution board in respect of the new load. (15 marks)

(b) The new load in (a) is to be supplied by means of a 2 km-long underground cable having a per-phase capacitance to earth of 0.2 μF/km. Estimate the charging current of the cable during normal operation. Estimate the fault current for an earth fault on one phase at the remote end of the cable, bearing in mind that the network is isolated (IT). (10 marks)

Q5. (a) Compare vacuum and SF 6 circuit breaker technologies for 20 kV applications. (10 marks)

(b) The supply to a high voltage cable is switched by a vacuum circuit breaker. The parameters of the system are as follows: capacitance to earth 0.5 μF, inductance 0.1 H, operating voltage 20 kV. Estimate the following… (i) the peak current on energising the unloaded cable; (ii) the worst-case arc voltage on interrupting a current of 150 A. (15 marks)

Useful information

Table 1: IEC current ratings are decimal multiples of…

1 A 1.25 A 1.6 A 2 A 2.5 A 3.15 A 4 A 5 A 6.3 A 8 A

Table 2: Current transformer ratios

100/5 150/5 200/5 250/5 300/ 400/5 500/5 600/5 700/5 800/ 1000/5 1200/5 1400/5 1600/5 2000/

Cold resistivity of copper = 0.018 -m/mm^2

Hot resistivity of copper = 0.022 -m/mm^2