SUBSTATION ENGINEERING – 2023/2024 PAST QUESTIONS AND ANSWERS

This Article shows Past Questions and Answer workings in Substation Engineering (also known as Power System 4) for Higher National Diploma (HND) Students of Electrical/ Electronics Engineering Auchi Polytechnic, Auchi.

This Past Questions and Answers is helpful to all students in the Department of Electrical/ Electronics Engineering offering this course; Substation Engineering (EEP 436). For those currently writing their on going exams in Electrical Engineering HND 2 Power Option. This is the past questions and answers for you to study.

N.B – These questions and answers are uploaded only as a guide and ease for your studies.

DEPARTMENT OF ELECTRICAL /ELECTRONIC ENGINEERING,  SCHOOL OF ENGINEERING TECHNOLOGY

AUCHI POLYTECHNIC, AUCHI

FIRST SEMESTER EXAMINATION 2023/2024 ACADEMIC SESSION

CLASS: HND2. COURSE TITLE: SUB-STATION ENGINEERING

COURSE CODE: EEP436 TIME ALLOWED 3 HRS. EXAMINER: ENGR. A.B OSHOMAH

INSTRUCTION: ATTEMPT ANY FIVE (S) QUESTIONS

Question 1

(a) Define a sub station

(b) Explain with aid of a diagram distribution substation

(c) With a diagram show a secondary distribution consumer substation with underground feeder arrangement

Question 2

(a) What is a feeder?

(b) With a neat diagram, explain the meaning of a switching sub-station

(c) Find the value in ohms of the reactance per phase external to a 30MVA, 33KV, 50Hz, 3-phase generator such that the steady state current on short-circuit shalt not exceed 4 times the full-load current. The internal reactance of the generator is 7.5%

Question 3

(a) What are the primary functions of current limiting reactors?

(b) With the aid of a diagram only explain a tie bar system

(c) The figure below shows a system having four alternators each rated at 11kv, 80 MVA and having at sub transient reactance of 15%

Find: (i) Fault level for a fault on one of the feeders (near the bus) with zero value of X

(ii) The reactance of the current limiting reactor X to limit the fault level to 1200 MVA for a fault on one of the feeders (near bus).

Question 4

(a) Design and explain a single line busbar system for a large generating station were several units are to be installed. 

(b) A generating station has five section bus-bars connected with a tie-bar through 12.5% reactors rated at 4000KVA with 15% reactance are connected to each section bus- bar. Find the total steady input to a dead short circuit between the lines on one of the section bus- bars.

I. With reactors

II. Without reactors

Question 5

(a) Explain with the aid of waveform diagram the principles of voltage recovering in arc extinction.

(b) in a short circuit test with earth neutral on a 132kv, 3-phase circuit breaker the power factor of the fault was 0.3, the recovering voltage was 0.95 of full line value, the breaking current was symmetrical and the restriking transient had a neutral frequency of 16000 Hz. Estimate the rate of the restriking voltage.

Question 6

(a) Briefly explain the effect of the following on voltage recovering: (i) resistance and reactance (ii) power factor (iii) armature reaction (iv) circuit condition

(b) An iDMT type over-current relay is used to protect a feeder through 600/1 CT. The relay was a PS of 1.26% and TMS = 0.3 the tie of operation of the said a…. if a fault current of 6,000 A flows through the feeder. Make use of the following characteristics

PSM	2	3	6	8	10	15
Time Operations	10	6	45	32	3	25

Question 7

(a) What is a static relay? State its limitations

(b) It is given at Auchi transmission station that the fault current level on 33 KVside is 2,700 A: CT ratio at 33 V side is 200:1 and it is given

that the fault current level at 33 kV side is 2.700A; CT ratio at 33 kV side is 200:1 and 132KV side is 100:1 (refer fig below). if both the relays and R2 are set for 100% plug setting. determine the operating time for both the relays when time grading margin of 0.6 second is en and TMS for relay R, is 0.18.

ANSWERS TO PAST QUESTIONS

Question 1

(a) Defining a sub station – a substation is an assemblage on equipment for purposes honest and generation or utilization, through which electrical power in bulk is pass on the objectives or modifying its characteristics.

(b) Explain with aid of a diagram distribution substation – in distribution substation is the Sub station close to low center which delivers electric energy from substations mission feeder to primary distribution Fields or directly into a low volt network.

(c) With a diagram show a secondary distributiorn consumer substation with underground feeder arrangement

Question 2

(a) What is a feeder? – feeders Anders power lines through which electricity is transmitted on power system. A feeder transmit power from generating station or substation to the distribution point.

(b) With a neat diagram, explain the meaning of a switching sub-station

A switching sub station through which power delivered without transformation, and which is meant for the purpose of making different connections between various transmission lines by switching operations.

(c) Find the value in ohms of the reactance per phase external to a 30MVA, 33KV, 50Hz, 3-phase generator such that the steady state current on short-circuit shalt not exceed 4 times the full-load current. The intarnal reactance of the generator is 7.5%

Solution

Reactance required limiting the short-circuit current to 6 times of full-load current

= Full current / short- circuit current

= 1/4 = 0.25p.u

Internal synchronous reactance per phase 7.5% = 0.075p.u

External reactance required per phase (Xpu)

= 0.25 – 0.075 = 0.175p.u

External reactance required per phase in ohms

Xpu x ( KVB/MVB) = 0.175 x (33/30)

= 0.1925Ω ≈ 0.193Ω

Question 3

(a) The primary functions of current limiting reactors

1. Reduce short-circuit currents to levels that can be handled by existing distribution equipment

2. Limit starting current or as part of a speed control system for electric motors

3. Protect other system components from high current levels

4. Limit the inrush current when starting a large motor

5. Reduce short-circuit currents to prevent damage to equipment and ensure safe operation

6. Provide a cost-effective alternative to upgrading circuit breakers or other equipment

7. Allow for the use of smaller conductors and other equipment

8. Improve system stability and reliability by reducing the risk of cascading faults.

(b) diagram of a Tie bus bar system

(C)

4a. Diagram of a single bus bar system

firstly, if a fault occurs on any section of the bus bar, that section can be isolated without affecting the supply of other sections. Secondly, if a fault occurs on any feeder, the fault current is much lower than with unsectionalize bus bar. This permit the use of current breakers of low T capacity in the feeders. Thirdly, repairs and maintenance of any section of the bus bar can be carried out by e- energizing the section only eliminating the possibility f complete shutdown.

it is worthwhile to keep in mind that a cordite breaker should be used as the sectionalizing switch so fast uncoupling of the bus-bars may be carried out safely during low transfer. Moreover, recycle breaker itself should be provided with isolators on both sides so that it’s maintenance can be done while the bus bars are alive.

(b)