Tariffs and Power Factor Improvement Interview Questions and Answers:
1. What is meant by tariff?
Ans. Tariff means the schedule of rates or charges. Tariff, in case of electric supply, means the schedule or rates framed for supply of electrical energy to different classes of consumers.
2. Name the different types of tariffs.
Ans. (i) Flat demand tariff (ii) Simple tariff (iii) Flat rate tariff (iv) Step rate tariff (v) Block rate tariff (vi) Hopkinson demand or two part tariff (vii) Maximum demand tariff or Wright demand rate (viii) Power factor tariffs (ix) Three part tariff or Doherty rate tariff (x) Off peak tariff.
3. What is tariff? What is general form of tariff?
Ans. Tariff means the schedule of rates or charges. Tariff, in case of electric supply, means the schedule or rates framed for supply of electrical energy to different classes of consumers. General form of tariff is
Total energy charges, e = Rs a + b kW + c kWh
- a is a constant charge made each billing period.
- b = Unit charge in Rs per kW of metered maximum demand in kW during billing period. In some cases it is also charged in Rs per kVA so that the consumers are penalized for poor power factor.
- c = Unit charge for energy in Rs per kWh of energy consumed.
4. Why is tariff less for power load than lighting load?
Ans. Power load spreads the fixed charges over greater number of units, thus reduces the overall cost of generation of electrical energy. Lighting load has low load factor and therefore cost of generation of electrical energy is higher. These are the reasons that tariff is less for power load than lighting load.
5. What type of tariff is employed for domestic consumers ?
Ans. Block rate tariff.
6. What is two part tariff ?
Ans. In the two part tariff, the total charge to be made to the consumer is divided into two components namely fixed charge (proportional to connected load or maximum demand) and the running or operating charges (proportional to the units consumed).
7. For which category of consumers two part tariff is used ?
Ans. Two part tariff is mostly applicable to medium industrial consumers.
8. How do demand factor, load factor and diversity factor in a power system affect the fixation of tariffs?
Ans. The tariff must cover the following items :
- Recovery of cost of capital investment in generating, transmitting and distributing equipment.
- Recovery of cost of operation, supplies and maintenance of equipment.
- Recovery of cost of metering equipment, billing, collection costs and miscellaneous services and
- A satisfactory return on the total capital investment.
The unit cost depends quite substantially on the load factor. The fixed cost remains constant irrespective of the load factor. At low load factor the fixed cost is shared by a smaller number of units of energy resulting in a relatively higher energy cost. At high load factor, the same fixed cost is shared by a large number of units of energy thereby reducing the unit energy cost.
The demand factor, being the ratio of maximum demand to connected load, determines the capacity of the system and hence the cost of power equipment required to serve a given load. Thus demand factor affects the fixtation of tariff.
Diversity between the loads of different consumers and different areas leads to reduction in peak demand resulting in reduction in generation, transmission and distribution costs. So the fixation of tariff is affected.
9. Explain reasons, why power factor tariff is imposed ?
Ans. Since the efficiency of plant and equipment depends upon the power factor, therefore, in order to increase the utility of plant and equipment to the maximum, the plant must be operated at the most economical power factor. That is why, sometimes consumers are penalized for poor power factor by imposing the power factor tariff.
10. Which type of tariff encourages the consumers to keep the load factor and power factor high ?
Ans. KVA maximum demand tariff encourages the consumers to keep the load factor and power factor high.
11. Can a power factor be included in a tariff ?
Ans. Yes, power factor is included in power factor tariffs such as kVA maximum demand tariff, kWh and kVARh tariff and sliding scale or average power factor tariff.
12. What type of tariff is usually applied to bulk consumers.
Ans. Three part tariff.
13. What are the advantages of using industrial/agricultural loads during night hours ?
Ans. During night hours a large proportion of the generating and distribution equipment remains idle due to extremely low demand. In case industrial and agricultural consumers are encouraged to use electricity during night and other off-peak hours by giving a special discount, the energy can be supplied without incurring any additional capital cost.
14. What are the different types of loads ?
Ans. The main types of loads on a power system are domestic, commercial, agricultural, industrial, municipal, traction etc.
15. Define power factor.
Ans. The cosine of the angle between voltage and current in an ac circuit is known as power factor.
16. Define active, reactive and apparent power.
The product of voltage V, current I and power factor cos Φ i.e. VI cos Φ is known as active power.
The product of voltage V, current I and sine of phase angle Φ i.e., VI sin Φ is known as reactive power.
The product of voltage V and current I i.e., VI is known as apparent power.
These three powers are related as follows :
17. Why is the power factor not more than unity?
Ans. Power factor being the cosine of the phase angle between voltage V and current I can never exceed unity.
18. What is the effect of power factor on the cost of generation?
Ans. Lower power factor means higher kVA for the given load, therefore, large sized generators, transformers, switchgear, bus-bars, more energy loss and poor voltage regulation thereby increasing the cost of generation.
19. What is the need of improving power factor?
Ans. Low power factor causes the ratings of generators and transformers, cross-sectional area of the bus-bars and the contact surface of the switchgear, the size of the feeders and distributors, energy losses, voltage drops in generators, transformers, transmission lines and distributors to increase. So power factor needs to be improved to reduce the generation, transmission and distribution costs.
20. What are the advantages of power factor improvement?
Ans. The advantages of good (or improved) power factor are : (i) reduction in load current ; (ii) increase in voltage level across the load ; (iii) reduction in energy losses in the system (generators, transformers, transmission lines and distributors) due to reduction in load current ; (iv) reduction in kVA loading of the generators and transformers which may relieve an overloaded system or release capacity for additional growth of load and (v) reduction in kVA demand charge for large consumers.
21. What is the importance of power factor?
Ans. Low power factor leads to high capital cost for generators, transformers, switchgears, transmission lines, distributors and cables. So the power factor is very important.
22. Why it is not economical to raise the power factor to unity?
Ans. As the power factor approaches unity, the capacity of the power factor improving equipment increases more rapidly i.e. the power factor of an installation can be improved from 0.8 (lagging) to 0.9 (lagging) by a much small capacity kVAR than which will be required to raise the power factor from 0.9 to unity. So it is not economical to raise the power factor to unity.
23. What factors determine the economical limit of power factor correction ?
Ans. Economical limit of power factor correction is governed by the relative costs of the supply and power factor correcting equipment.