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Knec Electrical Power Systems And Electromagnetic Fields Theory October/November 2015 Question Paper

Knec Electrical Power Systems And Electromagnetic Fields Theory October/November 2015

Course:Diploma In Electrical And Electronic Engineering Module Iii

Exam Year:2015

ELECTRICAL POWER SYSTEMS
AND ELECTROMAGNETIC FIELD THEORY
Oct./Nov.2015 Date............................
Time: 3 hours
THE KENYA NATIONAL EXAMINATIONS COUNCIL
DIPLOMA IN ELECTRICAL AND ELECTRONIC ENGINEERING
(POWER OPTION)
(MODULE III)
ELECTRICAL POWER SYSTEMS AND ELECTROMAGNETIC FIELDS THEORY
3 Hours
INSTRUCTION TO CANDIDATES
Write your name and index number in the space provided above.
Sign and write the date of the examination in the space provided above.
You should have a non – programmable scientific calculator and drawing instruments for this examination.
This paper consists of two sections A and B
Answer any THREE questions from section A and TWO question from section B in the space provided in this question paper.
All the question carry equal marks.
Maximum marks for each part of questions are as shown.

SECTION A:PHYSICAL SCIENCE
Answer any THREE question from this section.
1. (a) State three assumptions made in unsymmetrical fault calculations in power system.
(3marks)
(b) Outline the procedure for calculating short – circuit voltamperes and short –circuit current for a balanced three- phase fault in a power system.
(5marks)
(c)A 25 MVA,11 kV generator has sequence impedances Z₁=Z₂=j0.22 p.u. and Z_o=j0.04 p.u.If a double line to ground fault occurs on the terminals of the generator when operating at no load and at rated voltage, determine the;
i. Line currents;
ii. Fault current.
( Assume that the generator is solidly grounded)
(12 marks)

2. (a)State three reasons why power system stability studies are carried out.
(3 marks)
(b)With reference to power system stability,define the following:
(i) steady state limit;
(ii) transient limit.
(4 marks)
(c) A generator is delivering 0.7 of its maximum power to an infinite bus through a transmission line. A fault occurs such that the reactance between the generator and the infinite bus is increased to three times its pre fault value. When the fault is cleared, the maximum power that can be delivered is 0.85 of the original maximum value.With the aid of power angle curves , determine the critical clearing angle.
(13marks)

3. (a) (i) Stat three limitations of horn gap arresters
(ii) Differentiate between surge arrester and surge absorber.
(5marks)
(b) A concentric cable has an external diameter of D metres and internal diameter of r metres .S how that the velocity of propagation of a surge through the cable is given by

$V= \frac {3 \times 10^8}{\sqrt r}m/s$

Where r is the permittivity (relative) of the cable dielectric.
(4marks)
(c) An underground cable having an inductance of 0.3mH per km and a capacitance of 0.4F per km is connected in series with an overhead line having an inductance of 2.om H per km and a capacitance of 0.014 F per km. If a voltage surge of 100kV originates from the cable side towards the cable – line junction, determine the;
i. Transmitted voltage and current;
ii. Reflected voltage and current.
(11marks)

4. (a) State two disadvantages of corona formation.
(2marks)
(b) Explain two methods of reducing corona effects.
(4marks)
(c) Explain how sag of an overhead line conductor is affected by the following factors;
i. Conductor weight per unit length;
ii. Temperature.
(d)A trasmission line has a span of 280 metres between level supports.The conductor has a diameter of 20mm, weighs 8.5N/M and has an ultimate breaking strength of 80 kN.
Each conductor has a radial covering of ice 9.6mm thick and is subjected to a horizontal wind pressure of 400N/m² of the ice covered projected to area. If the factor of safety is 2 and one cubic metre of ice weughs 9.14kN,calculate the;
i. Deflected sag;
ii. Vertical sag.
(10marks)

5. (a)W ith the aid of a diagram, describe the operation of induction type directional over current relay
(9marks)
(b) With the aid of a circuit diagram, explain the operation of merz –price circulating current
Protection scheme for a delta/delta three phase power transformer.
(7marks)

(c)A star connected, 3 –phase,20MVA ,11kV alternator is protected by merz –price circulating current principle using 1000/5 amperes current transformers.The star-point of the alternator is earthed though a resistance of 10 $\Omega$. If the minimum operating current for the relay is 0.5 A,determine the percentage of each phase of the stator winding which is unprotected against earth faults when the machine is operating at normal voltage.
(4marks)

SECTION B: ELECTROMAGNETIC FIELD THEORY
Answer any TWO questions from this section.
6. (a)Describe the force between two electric charges Q₁ and Q₂ separated by distance d in space.
(b) A charge Q₁ =4 c is located at point A whose cartesian co- ordinates(x,y,z) is (3,5,6) metres,while another charge Q₂=-c is at B(5,1,10)metres.Determine the:
i. Force experienced by Q₁ due to Q₂;
ii. Electric field intensity at the origin due to charge Q₁.
(10marks)
(c) A solid conductor of circular cross section is made of a homogenous non –magnetic material. If the radius of the conductor is 4mm and the current flowing is 30 along the z-axis,determine the:
I. H at r = 2mm;
II. Total magnetic flux outside the conductor.
(7marks)

7. (a) Define each of the following with respect to electromagnetic waves:
i. TE wave;
ii. Wavelength
iii. Intrinsic impedance.
(3marks)
(b) A 12 GHz uniform plane wave propagates in a lossless medium for which r=5 and r=2.5.Determine the:
i. Velocity of propagation;
ii. Propagation constant;
iii. Wavelength;
iv. Intrinsic impedance.
(8marks)
(c) (i) Explain the poynting theorem.
(4marks)
(ii) The electric field intensity of a wave is given by
$E(z,t)=120 sin(wt-z)\hat a_ xVm^-1$
Determine the total power passing through a rectangular area of sides 45mm x10mm in the z=0 plane.
(5marks)
8. (a) Explain farady’s law of electromagnetic induction.
(3marks)

(b) A 50cm wire aligned with x –axis moves in a plane z = 4 m with a velocity =$2\hat a_x + 5\hat ay\ m/s$ in magnetic field with flux density
$ \underline {B}=4x^2z \hat a_x + 5 \hat a_y - 2xz^2\hat a_z\ Wb/m^2 $
Derive the expression for the e.m.f induced.
(4marks)
(c) A material with =5 S/m and r =1 is placed in an electric field whose electric field density ,E= 250 Sin (10⁹t) v/m.
Determine the expressions for:
(i) Conduction current density;
(ii) Displacement current density;
(iii) Frequency at which the conduction and displacement current densities are equal.
(7marks)
(d) The electric field intensity ,E, in free space is given by the expression:
.Determine the expresions for:
(i) Magnetic flux;
(ii) Magnetic field intensity,H
(iii) Electric flux density ,D.
(6 marks)

Knec Electrical Power Systems And Electromagnetic Fields Theory October/November 2015 Question Paper

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