I. (a) What is meant by 'discrimination' when applied to protective relays on an electrical supply system and why is it necessary?
(b) Which relay on the radial feeder shown in the diagram will have the shortest operating time and operate first in the event of the fault shown?
(c) Which types of relay would be found on a ring feeder and why?
2. (a) State briefly the main advantages and disadvantages of radial and ring feeders.
(b) Explain the relationship between power and energy and hence determine the number of megajoules in one unit of electricity.
3. (i) A medium-sized clothing factory takes its supply at low voltage and has the following meter readings for the month of September.
MD (day) 428 kW
MD (night) 87 kW
kWh (day) 72180
kWh (night) 10060
If the average power factor is 0.843, find the bill for the month, using the supplier's tariff shown in lesson ESD - I - 2.
(ii) A large continuous process foundry works a night shift and takes its supply at high voltage.
For the month of December the following meter readings are recorded.
| MD |
(day) |
1792 kW |
(night) |
1607 kW |
| Units |
(day) |
224368kWh |
(night) |
289560kWh |
| Reactive |
(day) |
131140kVArh |
(night) |
195510 |
| kVarh Chargeable Capacity |
2123 kVA |
|
|
|
Find the average power factor for the month and calculate the electricity bill.
4. The diagram below shows the single phase equivalent circuit of a 275 kV 3-phase source connected to a line and a circuit breaker, and a fault on the system is shown.
Calculate:
(a) the frequency of the recovery voltage transient
(b) the maximum voltage to appear across the CB contacts
(c) the average rate of rise of recovery voltage transient.