Section 1: Review Questions
1 List the basic principles or factors thatneed to be considered during design of a power distribution system.
2 Describe how a shunt capacitor can be used to improve voltage regulation on a distribution network
3 List thecategories of equipment that areused in power networks.
4 List the different electrical faults that can occur in a power system and use a diagram to illustrate these.
5 Calculate the magnitude of the short-circuit current in a single-phase circuit with a nominal open circuit voltage of 1000V if the equivalent source impedance is 5 milli (5 x 10-3) Ohms (NB show all calculations)
6 What assumptions are usually madewhen performing simplified fault current calculations?
7 Describe three main functions of switchgear.
8 Briefly describe the characteristics of the following MV switchgear:
(a) Switches
(b) Isolators/disconnectors
(c) Circuit breakers
9 Compare the advantages and disadvantages of the following types of switchgear insulation:
(a) Oil
(b) Air break
(c) Sulphur-Hexaflouride (SF6)
(d) Vacuum
10 What factors should be considered when selecting switchgear for a specific application.
11 List the functions of a typical modern protection relays.
12 In what applications would the following types of cable be used.
(a) XLPE
(b) PILC
(c) PVC
(d) Elastomeric
13 What factors need to be considered when selecting a cable for a specific application?
14 Describe what methods can be used to calculate the voltage drop in a length of cable.
15 Describe how a cable is selected to ensure it can safely carry a fault current.
16 How do environmental conditions influence the current rating of cables?
17 Describe the precautions should be taken when installing cables to ensure the maximum service life?
18 With the use of a diagram illustrate how a delta/star transformer can be connected to give standard polarity and standard phase sequence.
19 List three methods how transformer neutrals can be earthed and comment on their prospective fault current.
20 Compare the advantages and disadvantages of forced and natural transformer cooling.
21 Whattypes of faults transformer faults can occur in power transformers?
22 What is differential protection? Describe how it is used to protect a power transformer.
23 Use an illustration to explain the concept of power factor.
24 List the effects of sustained low power factor on equipment.
25 What methods can be used to improve power factor?
26 Are there any economic benefits of power factor correction? Explain.
27 Discuss the fundamental reasons why equipment and structures are grounded/earthed.
28 Describe how equipment grounding can reduce the shock hazards to personnel.
29 Describe the zoned protection approach to mitigation of lightning and switching surges.
30 Illustrate the principles of core balance protection using a diagram.
31 Compare the advantages and disadvantages of solid earthingwithresistance earthing systems
Section 2: Practical Exercise
1. Determine the symmetrical fault MVA on the secondary side of a 10MVA 11kV/3.3kV three phase power transformer with an impedance of 4.7% if the primary to the transformer is considered to be supplied from an infinite bus (ie a source with zero source impedance). (NB show all calculations)
2 What would be the reactance per phase in Ohms of the primary winding of a 11kV, 1MVA three-phase transformer with a percentage reactance of 6%? (NB show all calculations)
3 Calculate the size of three core PVC insulated copper cable required to supply a 3 phase, 400 volt, 100 kW motor over a distance of 50m buried direct in the ground. The motor is known to have a power factor of 0.9 lagging and will be started with a star-delta starter. The voltage drop should not exceed 5%. (You can use the data from the cable tables in the readings or any other manufacturers reference - please supply these with your answer if you are using another source) (NB show all calculations)