Transmission and distribution Typical outline of an ac transmission and distribution supply system

The bulk transmission of electric power from the generating station to a substation in the surrounding area of cities or load centers is called the primary transmission system.

1. GENERAL LAYOUT

2. Typical outline of an ac transmission and distribution supply system

3. Primary transmission: The bulk transmission of electric power from the generating station to a substation in the surrounding area of cities or load centers is called the primary transmission system. In Fig. 1.1, the section shown between points A and B is called the primary transmission system. The power is transferred by a 3- ф, three-wire overhead transmission system. The voltage levels are stepped down at the receiving substations for secondary transmission.

4. Secondary transmission: The secondary transmission system includes power transmission via 3-ф three-wire overhead transmission lines from the receiving substation located in the surrounding area of the city or load centers to the substations located at various points near the consumers in the city. The section marked between points C and D in Fig. 1.1 shows the secondary transmission system. The voltage levels are further stepped down to 33 kV or 11 3-ф three-wire at these substations.

5. Primary distribution: From the substation located in the city, power is supplied to large H.T. consumers via 3-ф three-wire overhead or underground system. The power is further stepped down by these large consumers at their own premises to 415/220 V, 3-ф, four-wire for further uses by the large consumers in their premises. The section of the supply system between substations and large consumers supplied at 11, 33 kV via 3-ф three-wire overhead is called the primary distribution system.

6. Secondary Distribution: The electric power from the primary distribution lines is delivered to pole-mounted transformer located near the residential localities. The distributed line voltage is stepped down to 415/220 V, 3-ф four-wire for secondary distribution. Residential consumers are supplied with single phase supply whereas motor or three phase loads are supplied with 3-ф, 4 wire supplies. The secondary distribution system consists of feeder, distributor and service mains. The section between the distribution substation and consumer terminal is called Secondary distribution

7. Classification of Transmission Lines The classification of the transmission lines depends on its voltage and the length of the conductor. AC Transmission Line Short-Transmission Line Medium Transmission Line Pi Model of a Medium Transmission Line T Model of a Medium Transmission Line Long Transmission Line DC Transmission Line

8. 1. AC Transmission Line The transmission line has  resistance  R,  inductance  L,  capacitance  C and the shunt or leakage conductance G. These parameters along with the load and the transmission line determine the performance of the line. The term performance means the sending end voltage, sending end currents, sending end  power factor,  power loss in the line, efficiency of the transmission line, regulate and limit of power flow during efficiency and transmission, regulation and limits of power during steady state and transient condition.

9. i ) Short Transmission Line If the line is not more than 80 Km or if the voltage is not over than 20 KV then the line is known as the  short transmission line . The capacitance of the line is governed by their length. The effect of capacitance on the short transmission line is negligible, but for cable where the distance between the conductor is small, the effect of capacitance cannot be ignored. While studying the performance of the short transmission line only resistance and the inductance of the line is calculated.

10. ii) Medium Transmission Line The line which is ranging from 80 to 240 km is termed as a medium transmission line. The capacitance of the medium transmission line cannot be ignored. The capacitance of the medium transmission line is considered to be lumped at one or more point of the lines . The effect of the line is more at high frequency, and their leakages inductance and capacitance is considered to be neglected. The medium transmission line is sub-divided into Pi – model and T – model. Pi Model of a Medium Transmission Line T – Model of a Medium Transmission Line

11. iii) Long Transmission Line The line having a length more than 240 km is considered a long transmission line. All the four parameters (resistance, inductance, capacitance, and leakage conductance) are found to be equally distributed along the entire length of the line.

12. Standard voltage level used in India As per the (erstwhile) Indian Electricity Rules, 1956, vide Rule 2(av), the following were the limits: -Low Voltage: Not exceeding 250 V. -Medium Voltage: Not exceeding 650 V. -High Voltage: Not exceeding 33000 V. -Extra High Voltage: Exceeding 33000 V However, it may be noted that the IE Rules have now been superseded by the Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations, 2010 and the above definition is removed in the CEA Regulations, 2010. As such, as per the latest Regulations in vogue in India, there is no such classifications as LV, MV, HV & EHV. The National Electric Code (of India) 2011 (Reaffirmed in 2016): Part 1 – Section 2 Low Voltage:  The voltage which does not normally exceed 250 V (Cl. 3.3.37) Medium Voltage:  The voltage which normally exceeds 250 V but does not exceed 650 V (Cl. 3.3.38) High Voltage:  The voltage which normally exceeds 650 V (but less than 33 kV ) (Cl. 3.3.39) Extra High Voltage:  The voltage exceeding 33 kV under normal conditions (Cl. 3.3.40)

13. HIGHEST VOLTAGE INDIA The highest transmission voltage in India is 765 kV Anpara -Unnao S/C Line (UPPCL): This line spans approximately 409 circuit kilometers1 . Kishenpur-Moga L-1 (W) S/C Line (PGCIL): It covers around 275 circuit kilometers1 . Tehri-Meerut Line-1 S/C Line (PGCIL): This line has a length of approximately 186 circuit kilometers WORLD The highest operational transmission line voltage in the world is found in Kazakhstan. The  Ekibastuz-Kokshetau power transmission line, built during the Soviet Union era as power line 1101, runs at an impressive  1,150 kilovolts (kV).  1150 KV

14. High voltage AC (Alternating Current) transmission has several advantages: Reduced Conductor Material: High voltage AC transmission requires a thin conductor, reducing the volume of conductor material needed12. Long Distance Transmission : It enables the transmission of bulk power over long distances1. Improved Voltage Regulation : High voltage AC improves voltage regulation and reduces voltage drop12. Increased Power Efficiency : Power efficiency tends to increase at high voltage levels1. Reduced Power Losses : High voltage AC reduces power losses, especially line losses1. Increased Transmission Efficiency : The transmission efficiency increases with the increase in transmission voltage2. Decreased Line Drop : The percentage line drop decreases when the transmission voltage increases2. Convenience of Transformers : AC transmission has established itself as the preferred global platform over the past century, due to the convenience of transformers in stepping voltage up or down as needed

15. LINE CAPACITANCE

16. Necessity of Extra High Voltage (EHV) Transmission 1. Reduction of Electrical Losses: Line losses are reduced since line losses are inversely proportional to the transmission voltage, 2.   Transmission efficiency increases because of reduction in line losses 3. Voltage regulation is improved because of reduction of percentage line drop 4. Lesser conductor material is required being inversely proportional to the square of transmission voltage. 5. Transmission of bulk power: Economic considerations have led to the construction of power stations of large capacity (Peat head Steam Plants & Remote Hydro Power Plants) and so need of transfer of bulk power over long distances arose. Transmission of bulk power from generating stations to the load centers is technically and economically feasible only at voltages in the EHV/UHV range 6. Increase in Transmission Capacity of the Line: Power transferred is expressed as:

17. 7. Possibility of Interconnections of Power Systems: It is practically not possible to have interconnections of two or more power systems, which is necessary to achieve sharing of installed reserves and for development of integrated systems and grids, without EHV transmission. 8.  Increase of Surge Impedance Loading: The load carrying capability of a power transmission line is known as Surge impedance loading. It is the line that carries power when each phase is terminated by a load equal to the surge impedance of the power transmission line. 9. Reduction in Right of way (ROW): Individuals or property managers are paid for giving the space or away or land by the organization installing the power transmission lines.

18. SUBSTATION LAYOUT AND COMPONENTS

19. NOTE:

20. FACTS: Flexible AC Transmission System (FACTS) is defined as ‘ Alternating current transmission systems incorporating power electronic-based and other static controllers to enhance controllability and increase power transfer capability’ The FACTS controller is defined as ‘a power electronic based system and other static equipment that provide control of one or more AC transmission system parameters’.

21. Why do we need FACTS? The increase in the loading of the transmission lines sometimes can lead to voltage collapse due to the shortage of reactive power delivered at the load centers. This is due to the increased consumption of the reactive power in the transmission network and the characteristics of the load (such as induction motors supplying constant torque). FEATURES OF FACTS: Helps in maintaining economic and secure operation of large interconnected systems. Maintains sufficient margins (in power transfer) with ease down the impact of voltage collapse Required safe operating margin can be substantially reduced by the introduction of fast dynamic control over reactive and active power by high power electronic controllers. FACTS can make the AC transmission network ‘flexible’ to adapt to the changing conditions caused by contingencies and load variations

22. Series Compensator: Thyristor Controlled Series Capacitor (TCSC) Thyristor Controlled Series Reactor (TCSR) Thyristor Switched Series Capacitor (TSSC) Static Synchronous Series Compensator (SSSC) Shunt Compensator: Static VAR Compensator (SVC) Thyristor Controlled Reactor (TCR) Thyristor Switched Capacitor (TSC) Thyristor Switched Reactor (TSR) Static Synchronous Compensator (STATCOM) Series-shunt Compensator: Unified Power Flow Controller (UPFC) Series-series Compensator: Interline Power Flow Controller (IPFC)

23. Flexible AC Transmission System (FACTS) is a power electronic-based system that enhances and increases the power transfer capability and controllability in conventional AC transmission networks. Here are some key features of FACTS: Fast Voltage Regulation: FACTS devices allow rapid adjustments to voltage levels, maintaining stability and reliability in the power grid. Increased Power Transfer over Long AC Lines: FACTS controllers enhance the power transfer capacity of transmission lines, enabling efficient energy transmission over extended distances1. Damping of Active Power Oscillations: FACTS systems mitigate oscillations in active power, improving grid stability and performance1. Load Flow Control in Meshed Systems : FACTS devices optimize load distribution and flow control within complex meshed power systems. These features contribute to better voltage stability, transient stability, voltage regulation, and thermal limits in the transmission network

24. LAYOUT & SINGLE LINE DIAGRAM OF A POLE MOUNTED SUBSTATION

25. Components of Transmission and Distribution Line Overhead Conductors: Properties of material Types of conductor with trade names, Significance of sag

26. List out the P roperties of conductors used for the overhead power transmission: When it comes to selecting conductors for overhead power transmission, several properties are crucial. Let’s explore them: High Electrical Conductivity : A good conductor should have high electrical conductivity. This property allows efficient flow of electric current through the conductor. High Tensile Strength : Conductors must withstand mechanical stresses due to factors like wind, ice, and temperature changes. High tensile strength ensures that the conductor remains intact under these stresses. Relatively Lower Cost : While maintaining other properties, the cost of the conductor should be reasonable. Cost-effectiveness is essential for long-distance power transmission. Lower Weight per Unit Volume : Lightweight conductors are preferred because they reduce the overall weight of the transmission line.  This property makes installation and maintenance easier Easy availability Should not be brittle.

27. Types of conductor with trade names Stranded Hard drawn Copper Aluminum Galvanized Steel Cadmium copper Copper Clad Steel Phosphor bronze

28. SAG Purpose of Sag: The inclusion of appropriate sag is crucial for several reasons: Tension Control: Sag prevents the conductor from being overstretched and experiencing unsafe tension levels. Without sag, wind pressure could lead to conductor breakage or detachment from its end support. Structural Integrity: Proper sag calculation ensures the structural integrity and operational reliability of transmission lines. Adverse Conditions: Under adverse conditions (such as wind or ice), sag helps maintain stability by adjusting the effective weight of the conductor.

29. PIN TYPE INSULATOR Conductor inserted into the top semi-circular groove and tied along its neck with the aid of a separate wire made of the same material as the conductor The rain sheds (projection facing downward) are designed in a way that even when insulators get wet, a sufficiently dry space is provided by those inner sheds to avoid flashover and leakage current These insulators can be used for up to 33 kV.

30. SUSPENSION TYPE INSULATOR Beyond 33kV transmission voltage, pin type insulators become expensive. As a result, it is common practice to use suspension type insulators for voltages greater than 33 kV. These consist of a series of porcelain discs connected by metal links in the shape of string The conductor is suspended at one end of this string, while the other end is secured to the tower's cross arm Each unit or disc is rated for 11 kV, and the number of discs connected in series is determined by the operating voltage. For example, if the line's operating voltage is 66 kV, the string will have six discs connected in series .

31. STRAIN TYPE INSULATOR The strain insulators are used either at the line's dead end, at steep curves, at river crossings, or corners. When the pull applied on the string of suspension insulator is high, as, in the case of long spans across the river, two or more strings are employed in parallel.

32. SHACKLE TYPE INSULATOR Shackle insulators were previously used as strain-type insulators; however, they are now frequently employed for low-voltage lines. A conductor is passed through leftover space between the clamp & insulator & is tied along the groove with the use of flexible binding wire made of the same material as the conductor.

33. Selection of Insulating material T he choice of insulating material for EHV overhead transmission lines involves a delicate balance between electrical, mechanical, and environmental factors. Engineers carefully evaluate these criteria to ensure the safe and efficient operation of the power grid: Voltage Level : Insulators must be designed to withstand the  voltage level  of the transmission line. EHV lines operate at significantly higher voltages than standard transmission lines, so the insulating material must have superior  dielectric strength  to prevent flashovers and breakdowns 1 . Mechanical Strength : Insulators should possess  adequate mechanical strength  to support conductors and resist environmental stresses. EHV lines often span long distances and are exposed to wind, ice, and other external forces.  Robust insulators are essential to maintain structural integrity 1 . Insulation Properties : Insulators must provide  sufficient electrical insulation  to prevent flashovers and punctures.

34. Causes of Failure : 3. Porosity in Insulation Materials : Porcelain insulators manufactured at  low temperatures  may become porous. 4. Improper Glazing on Insulator Surface : If the surface of a porcelain insulator is  not properly glazed , moisture can adhere to it. 5. Flash Over Across Insulator : Flashovers can cause insulators to  overheat , leading to their failure. 6. Mechanical Stresses on Insulator : Weak portions due to manufacturing defects may break when  mechanical stress  (applied by the conductor) exceeds their limits 1 . Cracking of Insulator : Porcelain insulators consist of three main materials: the porcelain body, steel fittings, and cement used to fix the steel part with porcelain 2. Defective Insulation Material : If the insulation material used for the insulator is  defective , it increases the risk of punctures. Any weak points in the material can lead to  breakdowns  under electrical stress

35. String efficiency is defined as the ratio of the voltage across the entire string to the product of the string's number of discs and the voltage across the disc closest to the conductor