Unit 1

Structure of Electrical Power Systems and Tariff

1. Explain the power system structure ?

Figure . Power System Structure.

Generating Substation

In generating station the fuel like coal, water, nuclear energy, etc. is converted into electrical energy. The electrical power is generated in the range of 11kV to 25kV, which is step-up for long distance transmission. The power plant of the generating substation is mainly classified into three types, that is thermal power plant, hydropower plant and nuclear power plant.

The generator and the transformer are the main components of the generating station.

Transmission Substation

The transmission substation carries the overhead lines which transfer the generated electrical energy from generation to the distribution substations. It only supplies the large bulk of power to bulk power substations or  big consumers.

The transmission lines mainly perform the two functions

1. It transports the energy from generating stations to bulk receiving stations.
2. It interconnects the two or more generating stations.The neighbouring substations are also interconnected through the transmission lines.

Sub-transmission Substation

The portion of the transmission system that connects the high voltage substations through the step-down transformer to the distribution substations is called the sub-transmission system.

The sub-transmission voltage level ranges from 90 to 138KV. The sub-transmission system directly serves some large industries.

Distribution Substation

The component of an electrical power system connecting all the consumers in an area to the bulk power sources is called a distribution system.The bulk power stations are connected to the generating substations by transmission lines. They feed some substations which are usually situated at convenient points near the load centres.

2.Define the following terms

(i) connected load

(ii)Maximum demand

(iii)Demand factor

(i)                Connected load. It is the sum of continuous ratings of all the equipment’s connected to supply system. A power station supplies load to thousands of consumers. Each consumer has certain equipment installed in his premises. The sum of the continuous ratings of all the equipment’s in the consumer’s premises is the “connected load” of the consumer.

(ii)              Maximum demand: It is the greatest demand of load on the power station during a given period. The load on the power station varies from time to time. The maximum of all the demands that have occurred during a given period (say a day) is the maximum demand.

(iii)           Demand factor. It is the ratio of maximum demand on the power station to its connected load

i.e., Demand factor = Maximum demand /Connected load.

The value of demand factor is usually less than 1.

3.     Define the following terms

(i)                Load factor

(ii)             Diversity factor

(iii)          Plant capacity factor.

Load factor. The ratio of average load to the maximum demand during a given period is known as load factor i.e.,

Load factor = Average load /Max. Demand

If the plant is in operation for T hours,

Load factor = Average load T /Max. Demand T

Diversity factor. The ratio of the sum of individual maximum demands to the maximum demand on power station is known as diversity factor i.e., Diversity factor = Sum of individual max. Demands /Max. Demand on power station

Plant capacity factor. It is the ratio of actual energy produced to the maximum possible energy that could have been produced during a given period i.e.,

Plant capacity factor = Actual energy produced /Max. Energy that could have been produced.

4.     Define the following terms

(i)Reserve Capacity

(ii) Plant use factor

(iii)Load curve

Reserve capacity :

The reserve capacity maintains the reliability of electricity system as it ensures that there is more supply available than the demand. If the system has the capacity which is exactly equal to the demand, there can be electricity shortage when just one power plant cannot operate as usual or there is a sudden increasing demand.

Plant use factor:

Plant use factor. It is ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation

i.e. Plant use factor = Station output in kWh/ Plant capacity Hours of use

Suppose a plant having installed capacity of 20 MW produces annual output of 7·35 × 106 kWh and remains in operation for 2190 hours in a year.

Then, Plant use factor = 7. 35 x 10 -6 / (20x 10 3) x 2190

= 16.7%

Load Duration Curve

When the load elements of a load curve are arranged in the order of descending magnitudes, the curve thus obtained is called a load duration curve.

Figure 2. Load duration curve

5.     Explain the concept of base load and peak load

(i) Base load. The unvarying load which occurs almost the whole day on the station is known as base load. Referring to the load curve of Fig. It is clear that 20 MW of load has to be supplied by the station at all times of day and night that is  throughout 24 hours. Therefore, 20 MW is the base load of the station. As base load on the station is almost of constant nature, therefore, it can be suitably supplied without facing the problems of variable load.

(ii) Peak load. The various peak demands of load over and above the base load of the station is known as peak load. Referring to the load curve of Fig. It is clear that there are peak demands of load excluding base load. These peak demands of the station generally form a small part of the total load and may occur throughout the day.

6.     What are the advantages of interconnected grid system?

The advantages of interconnected system

(i)                Exchange of peak loads : An important advantage of interconnected system is that the peak load of the power station can be exchanged. If the load curve of a power station shows a peak demand that is greater than the rated capacity of the plant, then the excess load can be shared by other stations interconnected with it.

(ii)             Use of older plants : The interconnected system makes it possible to use the older and less efficient plants to carry peak loads of short durations. Although such plants may be inadequate when used alone, yet they have sufficient capacity to carry short peaks of loads when interconnected with other modern plants. Therefore, interconnected system gives a direct key to the use of obsolete plants.

(iii)          Ensures economical operation : The interconnected system makes the operation of concerned power stations quite economical. It is because sharing of load among the stations is arranged in such a way that more efficient stations work continuously throughouts the year at a high load factor and the less efficient plants work for peak load hours only.

(iv)           Increases diversity factor : The load curves of different interconnected stations are generally different. The result is that the maximum demand on the system is much reduced as compared to the sum of individual maximum demands on different stations. In other words, the diversity factor of the system is improved, thereby increasing the effective capacity of the system.

(v)              Reduces plant reserve capacity : Every power station is required to have a standby unit for emergencies. However, when several power stations are connected in parallel, the reserve capacity of the system is much reduced. This increases the efficiency of the system. 64 Principles of Power System

(vi)           Increases reliability of supply : The interconnected system increases the reliability of supply. If a major breakdown occurs in one station, continuity of supply can be maintained by other healthy stations

7.     What is tariff?

The electrical energy produced by the power system is delivered to large number of customers. The tariff becomes the attention for the electric supply company. The company has to ensured that the tariff such that it not only recovers total cost of producing electrical energy but also earns profit on the capital investment.

8.     What are tariff setting principles?

The principal factors involved in fixing of a tariff are enu­merated below:

1. While fix­ing the tariff for different classes of consumers, it must be considered whether the tariff will result in a revenue meeting all the expenditure of the supply au­thority. In addition, the tariff should bring forth suffi­cient money to enable future expansion to meet an an­ticipated load requirement.

2. The consumers are encouraged to make more extended use of electricity.

3. The tariff should be simple and capable of easy expla­nation to the public.

4. The consumers are charged according to what the en­ergy costs. The tariff should be such as to satisfy the consumers of all categories.

5. The consumers are encouraged to use power during off-peak hours and penalised for high loads demanded at sys­tem peak by making provision for higher demand charges.

6. The consumers are penalised for poor power factor.

The type of service rendered by the supply authority also determines that tariff must be charged to a con­sumer. For example, consider Electric supply to a domestic consumer, having usually light and fan points, makes the life of an ordinary citizen conven­ient and pleasant.

9.     Explain the desirable characteristics of tariff?

The tariff must the following desirable characteristics

(i)Proper return - The tariff should be such that it ensures proper return from each consumer. In other words, the total receipts from the consumers must be equal to the cost of producing and supplying electrical energy plus a reasonable profit.This will enable the electric supply company to ensure continuous and reliable service to the consumers.

(ii) Fairness:  The tariff must be fair so that different types of consumers are satisfied with the rate of charge of electrical energy. Thus, a big consumer should be charged at a lower rate than a small consumer. It is because increased energy consumption spreads the fixed charges over a greater number of units, thus reducing the overall cost of producing electrical energy.
(iii) Simplicity:  The tariff should be simple so that an ordinary consumer can easily understand it. A complicated tariff may cause an opposition from the public which is generally distrustful of supply companies.

(iv) Reasonable profit: The profit element in the tariff should be reasonable. An electric supply company is a public utility company and generally enjoys the benefits of monopoly. Therefore, the investment is relatively safe due to non-competition in the market. This calls for the profit to be restricted to 8% or so per annum.

10.  Explain two part and three part tariff?

Two Part Tariff

In this tariff scheme, the total costs charged to the consumers consist of two components: fixed charges and running charges. It can be expressed as:

Total Cost = [A (kW) + B (kWh)] Rs.
Where, A = charge per kW of max demand ( A is a constant which when multiplied with max demand (kW) gives the total fixed costs.)
             B = charge per kWh of energy consumed ( B is a constant which when multiplied with units consumed (kWh), gives total running charges.)

The fixed charges depend on maximum demand of the consumer and the running charge will depend on the energy (units) consumed.

Three- part tariff

In this scheme, the total costs are divided into 3 sections: Fixed costs, semi-fixed costs and running costs.
Total Charges = [A + B (kW) + C (kWh)]
Where, A = fixed charges,
             B = charge per kW of max demand (i.e. B is a constant which when multiplied with max demand (kW) gives the total fixed costs.)
             C = charge per kWh of energy consumed (i.e. C is a constant which when multiplied with units consumed (kWh), gives total running charges.)
 

11. Explain availability of tariff and kVAH tariff?

Availability tariff comprises of three components:

Capacity charge: Towards reimbursement of the fixed cost of the plant to the plants declared capacity to supply MWs.

Energy Charge : To reimburse the fuel cost for scheduled generation.

Unscheduled Interchange: Payment for deviation from schedule at a rate dependent on system frequency conditions.

KVAH tariff

KVAh (kilo volt ampere hour) billing is more effective and is preferred by utilities. It takes care of both active power consumption as well as reactive power consumption. The measures taken by consumers to optimise electricity bill improve the efficiency of customer installation as well that of utility network besides helping utilities in better utilisation of their installed capacity.