Question 1: State two [2] advantages and [1] one disadvantages of alternating current compared to direct current.

Answer:Advantage of alternating current compared to direct current:

After generation transformers are used to step up the value of alternating current for transmission.

Transformers can be use again to step down the alternating current values for distribution A.C can be generated at higher voltages with fewer problems of heating and arcing A.C is produced by varying magnetic fields and as a result raising and lowering of

voltages is possible, cheaper also higher voltage means lower current and thus smaller I2R losses.

Disadvantages of alternating current compared to direct current: It causes radio frequency interference, when compared to direct current

Question 2: [a] Explain with the aid of diagrams:[1] Radial Distribution systems.[2] Closed ring distribution systems.

[b] State one advantage and one disadvantage of each distribution system in [a]

Answer: A) (i) Radial systems: This is the most commonly used system which comprises of separate

feeders or circuits “radiating” out of the sub-station or source. The feeder may consider as consisting of a trunk/main portion from which there radiate spurs or laterals to which distribution transformers are connected.

(ii) Closed Ring Systems: The feeders may be operated as a closed loop. Disconnecting devices are usually more expensive circuit breakers which are activated by relays, which

operate to open only the circuit breakers on each end of the faulted section, leaving the remaining portion of the feeder energized. Pilot wires are used to run from circuit breaker to circuit breaker in many instances and are very costly to maintain.

B) One advantage of radial systems is that it should a fault occur on the spur or lateral, the fuse through it is connected to the primary main.One disadvantage: loss of cable, primary supply or transformer will cut off service.

One advantage of close ring distribution system is that, a close ring main distribution system is more reliable than a radial distribution system.One disadvantage is that it is cheaper than the ring/loop.

Question 3: Explain with the aid of diagram how single phase and three phase voltages can be obtained from a three phase four wire system.

Answer: The diagram above shows how single phase and three phase voltages are obtained from a three phase four wire system. With the presence of the neutral wire connected to the system single phase can be obtained using any one of the line and the neutral. Three phase can be obtained without the use of the neutral.

Question 4: Describe with the aid of diagrams the following types of earthing of supply systems:(A) TT

(B) TN-C(C) TN-S(D) TN-C-S

(E)IT

Answer:

The first letter indicates the connection between earth and the power-supply equipment (generator or transformer):T: Direct connection of a point with earth (Latin: terra);I: No point is connected with earth (isolation), except perhaps via a high impedance.

The second letter indicates the connection between earth and the electrical device being supplied:T: Direct connection of a point with earthN: Direct connection to neutral at the origin of installation, which is connected to the earth

(A)TT

In a TT earthing system, the protective earth connection of the consumer is provided by a local connection to earth, independent of any earth connection at the generator.The big advantage of the TT earthing system is the fact that it is clear of high and low frequency noises that come through the neutral wire from various electrical equipment connected to it. This is why TT has always been preferable for special applications like telecommunication sites that benefit from the interference-free earthing. Also, TT does not have the risk of a broken neutral.In locations where power is distributed overhead and TT is used, installation earth conductors are not at risk should any overhead distribution conductor be fractured by, say, a fallen tree or branch.

In pre-RCD era, the TT earthing system was unattractive for general use because of its worse capability of accepting high currents in case of a live-to-PE short circuit (in comparison with TN systems). But as residual current devices mitigate this disadvantage, the TT earthing system becomes attractive for premises where all AC power circuits are RCD-protected.

(B) TN-C

TN-C: combined PE and N conductor all the way from the transformer to the consuming device. Rarely used.

(C) TN-S

TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.

(D)TN-C-S

TN-C-S earthing system: Part of the system uses a combined PEN conductor, which is at

some point split up into separate PE and N lines. The combined PEN conductor typically

occurs between the substation and the entry point into the building, and separated in the

service head. In the UK, this system is also known as protective multiple earthing (PME),

because of the practice of connecting the combined neutral-and-earth conductor to real

earth at many locations, to reduce the risk of broken neutrals - with a similar system in

Australia being designated as multiple earthed neutral (MEN).

E) IT

In an IT network, the distribution system has no connection to earth at all, or it has only a high impedance connection. In such systems, an insulation monitoring device is used to monitor the impedance.

Question 5: Sketch a single line diagram of a small commercial distribution systems, include the followings:

[1] A workshop with a 400V motor.[2] Lighting circuits with 220\110V.[3] Lathe 480V

Answer:

Question 6: Explain [4] four installation and environmental factors influencing the choice of cable for a particular application.

Answer:

Four installation and environmental factors influencing the choice of cable for a particular application are :

The cross sectional area of the cables chosen should be of the optimum size to carry the rated current load and short circuit current without overheating and should be within the required voltage drop.

The installation applied to the cable must be adequate for continuous operations at the specified working voltage with a high degree of thermal stability, safety and reliability.

Lathe 480v

All materials used in the construction must be carefully selected in order to ensure a high level of chemical and physical stability throughout the life of the cable in the selected environment.

The cable must be mechanically strong and sufficiently flexible to withstand the drumming operations in the manufacturer’s works, handling during transport or when the cable is installed by the direct burial, in trenches, pulled into ducts or laid on cable racks.

Adequate external mechanical and or chemical protection must be applied to the insulation and metal or outer sheathing to enable it to withstand the required environmental service conditions.

Question 7: State two advantages and two disadvantages of the following types of materials commonly used for manufacturing cables:

[1] Copper[2] Aluminum[3] PVC[4] Mineral insulation

Answer:

1) Copper: Advantages:

High conductivity Greater flexibility

Disadvantages: High cost of primary metal Corrosion

2) Aluminum: Advantages:

Favorable conductivity to weight ratio (light weight) Lower cost of primary metal Readily available

Disadvantages: Aluminum requires a larger conductor than copper for the current Formation of a film of Aluminum Oxide(increased resistance at terminals)

3) Polyvinyl Chloride: Advantages:

Doesn’t deteriorate under moist condition

Practically impervious to chemical reactions

Disadvantages: Softens at high temperatures Cracks at low temperatures.

4) Mineral Insulation: Advantages:

Can withstand temperatures as high as 150o

Can be used successfully is high corrosive environment Disadvantages:

Specially designed joints and termination are required Cannot be used where there are vibrations, as the cladding/cores will begin

to crack which will eventually lead to failure.

Question 8: Explain the difference between dielectric stress and dielectric strength with respect to electrical cables.

Answer: Dielectric stress in the electrostatic force acting on the C.S.A of the cable. It results in dielectric breakdown of the cable. Whereas dielectric strength is the maximum voltage that the cable can withstand before the insulation starts conducting electricity.

Question 9: Explain one method of stress control in H.V cables.

Answer:One method of stress control in H.V cables is to reduce the operating voltage, since external over voltages are associated with lightning discharges are not dependent, the voltage of the system. As a result, the importance of stresses produced by lightning decreases as the operating voltage increases. Internal over voltages are generated by changes in the operating conditions of the systems such as switching operations, a fault on the system fluctuation in the load or generators.

Question 10: Explain one method of arc control in circuit breakers.

Answer:One method of arc control in circuit breakers is using high pressure force. When a gas blast is directed along a discharge, efficient cooling is obtained. If the blast is axially directed, the arc shrinks in the diameter and the temperature of its core rises. Because the gases are generated at high pressures, there will be turbulence near the surface of the arc and this effect may be used for deionization in circuit breakers where gas blast is used for extinction of the arc.

Question 11: State the function of the arc chute in a circuit breaker.

Answer:Functions of the arc chute in a circuit breaker are:

It moves the arc away from the contacts, hence giving the breaker a longer life span. It is responsible for stretching the arc and increasing the resistance providing a wider path

to quench the arc.

Question 12: State two disadvantages of using a fuse in a distribution system.

Answer:Disadvantages:

Time response is not ideal( could react contrary to fusing factor) Can be only be used for low current circuit Fuses tend to suffer from oxidation depending on the type of material used.

Question 13: A HRC fuse has a fusing factor of 1.1, determine its minimum fusing current if the fuse rating is 30A.

Answer: Fusing Factor = 1.1 ; Fuse rating = 30A

Fusing Factor=minimum fusingcurrentfuse rating

MinimumFusingCurrent=30×1.1= 33A

Question 14: State one advantage and one disadvantage for using a relay for system protection.

Answer: Advantage:

Reliable system since service can be maintained under all conditions, faulty components are removed quickly, injurious heating due to short circuits or heavy over currents are prevented.

Disadvantage: It requires regular maintenance since the device is idle most of the time.

Question 15: Sketch and explain a circuit of a relay operated circuit breaker, the circuit should include a current transformer with a ratio of 400:1. The relay should be dc operated.

Answer: The diagram shows a relay with its trip coil (current operated) connected to the secondary of a current transformer, C.T, a trip contact, T.C, with a fuse positive supply on one side and a wire connected to the trip coil of the High Voltage, H.V, breaker on the other side. At the full load primary current, 400A, a current of 1A will flow in the relay coil, causing the relay after a set time to operate. The relay coil would close the trip contact there by putting a positive supply to the breaker trip coil causing the breaker to operate.

Question 16: Explain the principle of operations of a IDMT relay .

Answer:The IDMT relay is of the electromagnetic type. A metal disc iss pivoted so as to be free to rotate between the poles of two electromagntics, each energized by the current being monitored. The tordue produced by the interaction of flux and eddy current induced in the disc is a function of current. The disc speed is proportional to the torque, as operating time is inversely proportional to speed, operating time is inverse;y proportional to a function of current. The disc is free to rotate against the restraining or resetting torque of a control spring. Contacts are attached to the disc spindle and under preset current levels operate to trip, via the the appropriate circuitry, rhe required circuit breaker.This relay operatres on the severity of the fault i.e the higher the overcurrent, the faster the relay would operate the circuit breaker. This is shown in the following characteristic graph.

Question 17: Explain a single line diagram of supply voltage from generation- transmission-distribution-utilization include voltages on the diagram.

Answer:

over current

Operating Time(ms)

Question 18: Explain the features\operations of oil circuit breakers.

Answer:

In an oil circuit breaker, over current results in temperature rise which results in a decrease in viscosity which causes a trip. Fire is prevented since oil is dielectric.

Question19: Explain the features\operations of air circuit breakers.

Answer:In an air circuit breaker, over current results in a temperature rise which results in a pressure build up which causes a trip. Fire is prevented by lack of oxygen in the air breaker.

Question 20: Explain why transformers are used in transmission and distribution systems.

Answer:Transformers are used in transmission in order to step up the voltage after generation. In distribution, transformers are used to step down the voltage after transmission.