Understanding Variable Load on Power Stations in Electrical Systems

1. Electrical Power Systems (1) Prof. Dr. Magdi M. El-Saadawi www.saadawi1.net E-mail : saadawi1@gmail.com www.facebook.com/magdi.saadawi Course Code: EE 2224 Prof. Dr. Magdi El-Saadawi

2. Contents Chapter 1 Variable Load on Power Stations Chapter 2 Power Factor Improvement Chapter 3 Corona Effects Chapter 4 Components of Over Head Lines Chapter 5 Analysis of Sag and Tension Chapter 6 Underground Cables

3. Chapter 1Variable Load on Power Stations 1.1. Power Systems: an Overview 1.2. Structure of Electric Power System 1.3. Variable Load on Power Stations 1.4. Load Curves 1.5. Basic Definitions of Commonly Used Terms 1.6. Units Generated per Annum 1.7. Load duration Curve 1.8. Types of Loads 1.9. Base Load and Peak Load on Power Station 1.10. Method of Meeting the Load 1.11. Interconnected Grid System

4. Variable Load on Power Stations Chapter 1 Prof. Dr. Magdi El-Saadawi

5. 1.1. Power Systems: an Overview • The power demands of different consumers vary in accordance with their activities. The result of this variation in demand is that load on a power station is never constant; rather it varies from time to time. • Most of the complexities of modern power plant operation arise from the inherent variability of the load demanded by the users. • The power station must produce power as and when demanded to meet the requirements of the consumers. • On one hand, the power engineer would like that the alternators in the power station should run at their rated capacity for maximum efficiency and on the other hand, the demands of the consumers have wide variations. Prof. Dr. Magdi El-Saadawi

6. 1.2 Structure of Electric Power System • Generation, Transmission and Distribution systems are the main components of an electric power system. • Electric power is generated in the range of 11 kV to 25 kV, which is increased by stepped up transformers to the main transmission voltage. • Transmission level voltages are in the range of 66 kV to 400 kV (or higher). Large amounts of power are transmitted from the generating stations to the load centers at 220 kV or higher. • In USA it is at 345 kV, 500 kV and 765 kV, in Britain, it is at 275 kV and 400 kV and in Egypt it is at 500 kV and 750 kV. Prof. Dr. Magdi El-Saadawi

7. In Egypt, networks operate at 220, 132 kV, 66 kV, 33 kV, 11 kV or 6.6 kV and supply the final consumer feeders at 380 volt three phase, giving 220 volt per phase. • Figure 1.1 shows the schematic diagram of a power supply network. Prof. Dr. Magdi El-Saadawi

8. Electrical Power System Prof. Dr. Magdi El-Saadawi

9. 1.3 Variable Load on Power Stations • A power station is designed to meet the load requirements of the consumers. An ideal load on the station, from stand point of equipment needed and operating routine, would be one of constant magnitude and steady duration. However, such a steady load on the station is never realized in actual practice. • Effects of variable load: • Need of additional equipment. • Increase in production cost. Prof. Dr. Magdi El-Saadawi

10. Spinning Reserve • To meet sudden increases in load, a certain amount of generating capacity (in each area) known as the "spinning reserve" is required. • This consists of generators running at normal speed and ready to supply power instantaneously. • It is always better to keep gas turbines and hydro generators as "spinning reserve". • Gas turbines can be started and loaded in 3 minutes or less. Hydro units can be even quicker. Prof. Dr. Magdi El-Saadawi

11. 1.8 Types of Load • General nature of load is characterized by the load factor, demand factor, diversity factor, power factor and utilization factor. • The types of load can be divided into following categories: (1) Domestic منزليةأحمال (2) Commercial (3) Industrial (4) Agriculture Prof. Dr. Magdi El-Saadawi

12. 1.8 Types of Load (cont.) Domestic Load: lights, fans, refrigerators, air-conditioners, mixer, grinders, heaters, ovens, small pumping motors etc. Commercial Load: lighting for shops, offices, advertisements etc., and other electrical appliances used in commercial establishments such as market places, restaurants etc. Industrial Loads: small-scale industries, medium-scale industries, large-scale industries, and heavy industries. Agriculture Loads: motor pump-sets load for irrigation purposes. Prof. Dr. Magdi El-Saadawi

13. 1.4 Load Curves Definition: They are curves showing the variation of load on the power station with reference to time. The load on a power station varies from time to time. These load variations during the whole day (i.e. 24 hours) are recorded half-hourly or hourly and are plotted against time on the graph. There are: daily load curve, monthly load curve, and yearly (annual) load curve. Prof. Dr. Magdi El-Saadawi

14. Fig. 1.4 daily load curve Prof. Dr. Magdi El-Saadawi

15. Important Notes associated to Load Curves The area under the load curve gives the number of units generated in the day (in kWh). The highest point on the load curve represents the maximum demand on the station on that day. The area under the load curve gives the number of units generated in the day. Daily Pave= . Prof. Dr. Magdi El-Saadawi

16. Important Notes associated to Load Curves The ratio of the area under the load curve to the total area of rectangle in which it is contained gives the load factor. The load curve helps in selecting the size number of generating units. The load curve helps in preparing the operation schedule of the station. Prof. Dr. Magdi El-Saadawi

17. Prof. Dr. Magdi El-Saadawi

18. Difference between load curves of summer and winter Prof. Dr. Magdi El-Saadawi

19. Yearly Load Curve Prof. Dr. Magdi El-Saadawi

20. 1.7 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.. • The area under the load duration curve and the area under the load curve are equal. Prof. Dr. Magdi El-Saadawi

21. Prof. Dr. Magdi El-Saadawi

22. Load Curve and Load Duration Curve Prof. Dr. Magdi El-Saadawi

23. Points about load duration curve • The load duration curve gives the data in a more presentable form. • Area under daily load duration curve (in kWh) will give the units generated on that day. (same as load curve) • The duration curve can be extended to include any period of time • Daily load duration curve • Monthly load duration curve • Annual load duration curve Prof. Dr. Magdi El-Saadawi

24. 1.9. Base Load and Peak Load on Power Station • The load curve shows that load on the power station can be considered in two parts, namely; (i) Base load (ii) Peak load. Prof. Dr. Magdi El-Saadawi

25. 1.9. Base Load and Peak Load on Power Station Base load: The unvarying load which occurs almost the whole day on the station. Peak load:The various peak demands of load over and above the base load of the station. Prof. Dr. Magdi El-Saadawi

26. Prof. Dr. Magdi El-Saadawi

27. Prof. Dr. Magdi El-Saadawi

28. Load Duration Curve Prof. Dr. Magdi El-Saadawi

29. 1.7. Method of Meeting the Load In order to achieve overall economy, the best method to meet load is to interconnect different power stations. The more efficient plants are used to supply the base load and are known as base load power stations. The less efficient plants are used to supply the peak loads and is known as peak load power stations. Prof. Dr. Magdi El-Saadawi

30. 1.10. Method of Meeting the Load When water is available in sufficient quantity as in summer and rainy season, the hydroelectric plant is used to carry the base load and the steam plant supplies the peak load as shown in Fig 1.5 (i). However, when the water is not available in sufficient quantity as in winter, the steam plant carries the base load, whereas the hydro-electric plant carries the peak load as shown in Fig. 1.5 (ii) Prof. Dr. Magdi El-Saadawi

31. Prof. Dr. Magdi El-Saadawi

32. 1.9 Basic Definitions of Commonly Used Terms • Connected Load: Each electrical device has its rated capacity. The sum of the continuous ratings of all the electrical devices connected to the supply system is known as connected load. • Example If a consumer has connections of: five 100-watt lamps and a power point of 500 watts, then connected load of the consumer is: 5 x 100 + 500 = 1000 watts. Prof. Dr. Magdi El-Saadawi

33. 1.9 Basic Definitions of Commonly Used Terms Demand: the load at the receiving terminals averaged over a specified interval of time. It is given in kW, kVA, kilo-amperes, or amperes. Demand Interval: the time period over which the average load is computed. It may be 30 minute, 60 minute or even longer. Maximum Demand: The maximum demand of an installation or system is the greatest demands of load on the power station during a given period. It must express the demand interval used to measure it. So it may be such as daily, weekly, monthly or annual.

34. 1.9 Basic Definitions of Commonly Used Terms • Maximum Demand<connected load because all the consumers do not switch on their connected load to the system at a time. • The knowledge of maximum demand is very important as it helps in determining the installed capacity of the station. • The station must be capable of meeting the maximum demand..

35. 1.5 Basic Definitions of Commonly Used Terms • Demand factor: It is the ratio of maximum demand on the power station to its connected load < 1 • The knowledge of demand factor is vital in determining the capacity of the plant equipment. Prof. Dr. Magdi El-Saadawi

36. 1.5 Basic Definitions of Commonly Used Terms • Average load (Average demand): The average of loads occurring on the power station in a given period (day or month or year) given in kW or MW Prof. Dr. Magdi El-Saadawi

37. 1.5 Basic Definitions of Commonly Used Terms • Load factor:The ratio of average load to the maximum demand during a given period • If the plant is in operation for T hours, Prof. Dr. Magdi El-Saadawi

38. 1.5 Basic Definitions of Commonly Used Terms The load factor plays key role in determining the overall cost per unit generated. Higher the load factor of the power station, lesser will be the cost per unit generated (It is because higher load factor means lesser maximum demand. The station capacity is so selected that it must meet the maximum demand. Now, lower maximum demand means lower capacity of the plant which, therefore, reduces the cost of the plant).. Prof. Dr. Magdi El-Saadawi

39. Prof. Dr. Magdi El-Saadawi

40. 1.5 Basic Definitions of Commonly Used Terms • Diversity of Load يعنى أن الأحمال المختلفة لا تعمل فى فترة زمنية واحدة أو أن شركات الكهرباء تتبع سياسات تدفع إلى ذلك • Diversity Factor The ratio of the sum of individual maximum demands to the maximum demand on power station Prof. Dr. Magdi El-Saadawi

41. Practical Example on Diversity Factor Solution 1:Transformer Size (Assuming Diversity Factor of 1.0) 2,000 + 1,500 + 1,000 = 4,500 kVA Choose 4,500 kVA Solution 2:Transformer Size (Assume Diversity Factor of 1.2) (4,500 kVA divided by 1.2 = 3,750 kVA) Choose 3,750 kVA Prof. Dr. Magdi El-Saadawi

42. 1.5 Basic Definitions of Commonly Used Terms • Diversity Factor Diversity factor will always be greater than 1. The greater the diversity factor, the lesser is the cost of generation of power (Greater diversity factor means lesser maximum demand. This in turn means that lesser plant capacity is required. Thus, the capital investment on the plant is reduced). Prof. Dr. Magdi El-Saadawi

43. 1.5 Basic Definitions of Commonly Used Terms • Capacity Factor The ratio of actual energy produced to the maximum possible energy that could have been produced during a given period. النسبة بين الطاقة التى أنتجتها المحطة إلى مايمكن أن تنتجه المحطة Prof. Dr. Magdi El-Saadawi

44. 1.5 Basic Definitions of Commonly Used Terms • The plant capacity factor is an indication of the reserve capacity of the plant. • A power station is so designed that it has some reserve capacity for meeting the increased load demand in future. • Therefore, the installed capacity of the plant is always somewhat greater than the maximum demand on the plant. • Reserve capacity = Plant capacity - Max. demand. Prof. Dr. Magdi El-Saadawi

45. ? What is the difference between load factor and capacity factor Can they have the same value? Prof. Dr. Magdi El-Saadawi

46. 1.5 Basic Definitions of Commonly Used Terms The difference between load factor and capacity factor is an indication of reserve capacity. If the maximum demand on the plant is equal to the plant capacity, then load factor and plant capacity factor will have the same value. In such a case, the plant will have no reserve capacity. Prof. Dr. Magdi El-Saadawi

47. 1.5 Basic Definitions of Commonly Used Terms • Plant use Factor It is the ratio of kWh generated to the product of plant capacity and the number of hours for which the plant was in operation لو كانت المحطة مكونة من عدة وحدات It Indicates how best the plant capacity has been utilized Prof. Dr. Magdi El-Saadawi

48. ? What is the difference between Plant use Factor and Capacity factor Can they have the same value? Prof. Dr. Magdi El-Saadawi

49. 1.5 Basic Definitions of Commonly Used Terms الفرق بينهما: أن هذا المعامل يتحدث عن الاستخدام الحقيقى للمحطة والسابق يتحدث عن امكانيات تشغيل المحطة • Capacity Factor The ratio of actual energy produced to themaximum possible energy that could have been produced during a given period. Plant use Factor The ratio of actual energy produced to the product of plant capacity and the number of hours for which the plant was in operation Prof. Dr. Magdi El-Saadawi

50. 1.11. Interconnected Grid System pp. 29 • Although interconnection of station involves extra cost, yet considering the benefits derived from such an arrangement, it is gaining much favor these days. • Exchange of peak loads • Ensures economical operation: • Increases diversity factor: • Reduces plant reserve capacity: • Increases reliability of supply • Use of older plants: Prof. Dr. Magdi El-Saadawi