1. Difference between Voltage and Current.

Answer: Voltage is the Pressure, Current is the Load


2. What is an Injection Substation?

Answer: An injection substation is a station where a higher voltage is stepped down to a lower voltage, especially for distribution in a densely populated area. In Nigeria, the most common injection substations are 15MVA or 7.5MVA 33/11kv injection substations.

Also, a substation is an indoor or outdoor location in which there are located transformers, switchgear, busbar systems, and protection equipment.


3. What is the main difference between Substation and Injection Substation?

Answer: Substation is associated with Transmission (it does not supply consumers), while Injection Substation is associated with Distribution (it supplies consumers). – Injecting consumers with electricity. Power Station is associated with Generation.


4. Mention and explain briefly some equipment found in the Substation.

Answer:
a. Power Transformer: To stepping down 33KV to 11KV for distribution and to be further stepped down by, and it is mostly 15MVA and 7.5MVA capacities.

b. Bus Bar: Busbar is used when two or more transformers are to be connected together provided the transformers operate at the same voltage, frequency for synchronization. Busbar can be termed as electrical common point for all incoming and outgoing lines/feeders. It receives supply from incoming circuits and sends supply to outgoing circuits.

c. Lightning/Surge Arrestor: This is the first electrical device which is usually seen from the incoming overhead line and also near transformer's terminal phase to the ground. It works as shunt resistors to divert the lightning and high voltage surges to earth and protect other equipment from H.V surges.

d. Isolators: It is also called disconnecting switch or “on no-load” switch. In substation/switchyard, it is located before the circuit breaker.

e. Earthing switch: It discharges trapped charges within the circuit to earth for safety after with the circuit breaker and the isolator have been opened. They are mounted on the frame of isolators.

f. Current Transformer (CT): An instrument transformer for stepping down the current for measurement, protection and control. It is always connected in series to the circuit.

g. Voltage (potential) transformer (PT): An instrument transformer is for stepping down the voltage for measurement, protection and control. Its location is at the feeder side of the circuit breaker.

h. Circuit Breaker (C.B): Circuit breaker is an electromechanical appliance for switching during normal and abnormal operating conditions, and also interrupting over current and earth fault. Also called “on-load” switch.

i. Marshalling Kiosk: Also known as marshalling box can be used in either indoor or outdoor applications, but mostly in outdoor switchyard. Marshalling kiosk provide convenient points for the various control, protection and instrumentation wires which connect to and from different areas located within the substation, including power and instrument transformers.


5. Define Switchyard, and explain its types

Answer: As the name implies – switching substation, it is a place/yard where electrical switching occurs. We have Indoor switchyard/Control Room and Outdoor switchyard/Control Room.


6. Why is Transformer rated in VA?

Answer: [/b]There are 2 losses in transformer. One is copper loss which depends on current and the other is iron loss which depends on voltage.

[b]7. What is the relationship between Real Power and Apparent Power?

Answer: Apparent Power (VA) = Real Power (W)/Power Factor
Example: The real load a 15MVA transformer can comfortably carry is 15 x power factor = 15MVA x 0.8 = 12MW

8. Mention 2 faults you know.

Answer: Over-current, and Earth Fault.


9. Difference between Ohmic and non-Ohmic conductors

Answer: In an Ohmic conductor the current flowing is proportional to the voltage applied. In a non-Ohmic conductor there is no linear relationship.

Conductors that obey Ohm’s Law are called Ohmic conductors while those that do not obey Ohm’s Law are called non Ohmic conductors.


10. Why is AC not used instead of DC in batteries?

Answer: When we connect a battery with AC Supply, then it will charge during positive half cycle and discharge during negative half cycle, because the Positive (+ve) half cycle cancel the negative (-Ve) half cycle, so the average voltage or current in a complete cycle is Zero. So there is no chance to store AC in the Batteries.


11. State the difference between Alternator and Generator

Answer: In an alternator, electricity is produced when a magnetic field spins inside the stator winding. On the other hand, the armature winding of wire in a generator spins inside a fixed magnetic field to generate electricity.


12. Full meaning of SMS,ICT, WwW,GSM,ISP

Answer:
SMS – Short Message Service
ICT - Information and Communications Technology (ies)
WWW – World Wide Web
GSM - Global System for Mobile Communications
ISP – Internet Service Provider


13. How can we improve power factor?

Answer:
- Minimize operation of idling or lightly-loaded motors.
- Avoid operation of equipment above its rated voltage.
- Replace standard motors as they burn out with energy-efficient motors. Power factor is also significantly affected by variations in load. A motor must be operated near its rated capacity to get the highest power factor.
- Install capacitors in your AC circuit.

14. What is the full meaning of SCADA?

Answer: Supervisory Control And Data Acquisition

15. Explain tripping unit.

Answer: The tripping unit consists of a charger which converts 220 volts AC to 110 volts DC, and a set of wet cell batteries connected in series. The charger charges the battery with 110 DC volts and the batteries in turn supply the transformer panels with 110 DC volts. This is done to ensure continuous supply of power to the relays when there is an outage.

16. Explain Ring Main Unit (RMU) and Feeder Pillar.

Answer: Ring Main Unit is Distribution Feeder Load Switches used in maintaining supply network in a Ring System (mesh) or (loop) in looping of 2 or more transformers in Ring system. It is used in underground system .It is mainly used in 11KV grid (HV), while Feeder Pillars are low voltage (LV) distribution boards with fuses incorporated. They help to improve voltage distribution by distributing load evenly and in a tidy manner. They are mostly used at the 0.415KV grid. Also incorporated in the feeder pillar are an ammeter and voltmeter.


17. Give reasons why electrical installation be earthed? State two types of earthing you know

Answer: It is used to provide a low resistance earthing for doing the following tasks:

- discharge currents from surge arrestors, overhead shielding and earthing switches
- for equipment body earthing
- for safe touch potential and step potential in substation
- for providing path for the neutral to ground currents for the earth fault protection

Two types of earthing are Protective (Equipment) Earthing and Neutral Point Earthing.




18. Why are cables on the Primary side always smaller than the ones on the Secondary side?

Answer: The cable at the primary side of the xformer is always smaller in width than the cable on the secondary side becuase the current at the secondary side is always far higher than the primary's side as a resulting of stepping down. Primary side(Low Current and High Voltage), Secondary side (High Current and Low Voltage)


19. List protection relays in transformer.

Answer: Differential relay, Over-current relay, Bulcholz relay, Earth fault relay.


20. List tests carried on transformer you know.

Answer: Capacity test, Ratio test, Insulation resistance test, pressure test, oil test, Excitation test, Earth resistance test, physical inspection test as well.



Source: http://www.johnomolaolu.com/2015/09/likebales-and-fqas-for-those-seeking.html

more pictures

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