High Voltage AC Transmission Interview Questions and Answers:
1. What are the transmission voltages that are used in India ?
Ans. 33, 66, 110, 132, 220 and 400 kV. 765/800 kV is also in used in India.
2. What are the advantages of adopting extra high voltage/ultra high voltage for transmission of ac electric power ?
Ans. Advantages of adopting of EHV/UHV for transmission of ac electric power are reduced line losses, high transmission efficiency, improved voltage regulation, reduced conductor material requirement, flexibility for future system growth, increase in transmission capacity of the line, increase of SIL and reduction in ROW.
3. Why does surge impedance loading (SIL) increase with the increase in voltage level ?
Ans. SIL varies as the square of the operating voltage, so SIL increases with the increase in voltage level.
4. What are the problems associated with an EHV transmission ?
Ans. The problems associated with EHV transmission are corona loss and radio interference, requirements of heavy supporting structures and erection difficulties and insulation requirements.
5. Why does the use of bundle conductors reduce corona loss in an EHV line ?
Ans. A bundled conductor is a conductor made of two or more sub-conductors and is used as one phase conductor. The inductive reactance of the bundle conductors is reduced because the self GMD of the conductor is increased.
By bundling the conductors the self GMD of the conductors is increased thereby, the critical disruptive voltage is increased and hence corona loss is reduced.
6. On what basis the conductor size is determined for transmission lines up to 220 kV ?
Ans. For overhead transmission lines operating at voltages up to 220 kV, the conductor size is determined on the basis of its current carrying capacity.
7. On what basis the conductor size of EHV line is determined ?
Ans. The conductor size of an EHV line is decided on the basis of corona.
8. What are the sources of audible noise generation in EHV transmission systems ?
Ans. The sources of noise generation in EHV transmission systems are corona, humming of transformers, cooling systems and mechanical and electrical auxiliaries.
9. What material is universally employed for overhead transmission lines ?
Ans. ACSR conductors.
10. What is economical value of span for 400 kV transmission line ?
Ans. 350-400 m.
11. What is line compensation ?
Ans. Neutralization or compensation of reactive power of line is called the line compensation.
12. What is series and shunt compensation ?
Ans. When the line compensating device is connected in series with the line, series compensation is achieved and when connected across the line, shunt compensation is achieved.
13. What for series and shunt compensation provided in EHV lines ?
Ans. Series compensation is provided in EHV lines to artificially reduce the series reactance of line so as to improve stability, voltage regulation and transmission efficiency where as shunt compensation is provided to artificially reduce the line susceptance so as to improve voltage regulation under light load conditions.
14. Why shunt compensation is preferred over series compensators ?
Ans. There are some major problems associated with series compensation as summarized below
- Series compensation introduces a subsynchronous frequency, proportional to the square root of the compensation in the system. In some cases, this frequency may interact with turbo-generator shafts and develop high torsional stresses.
- Switching’s in of an unloaded transformer through a series compensated line may produce ferroresonance.
- Series compensation may lead to maloperation of the protection system leading to unnecessary tripping.
- High recovery voltage may develop across the circuit breaker contacts due to series compensation.
15. Give advantages of series compensation.
Ans. Series compensation is provided in EHV transmission lines to artificially reduce the series reactance of the line, that results in improvement of stability, voltage regulation and transmission efficiency.