Major electrical equipment and overhead line insulators

Major electrical equipment and overhead line insulators

1. UNIT-2 Major electrical equipment and overhead line insulators

2. An insulator gives support to the overhead line conductors on the poles to prevent the current flow toward earth. In the  transmission lines , it plays an essential role in its operation. The designing of an insulator can be done using different materials like rubber, wood, plastic, mica, etc. The special materials used in the electrical system are glass, ceramic, PVC, steatite, polymer, etc. T he most common material used in the insulator is porcelain and also special composition, steatite, glass materials are also used. This article discusses an overview of different types of insulators and its working. I nsulator Prof. Tathe S. G. ICEEM

3. Prof. Tathe S G ICEEM These are classified into different types based on their rating which includes the following. These are used in transmission & distribution system where each insulator consists of several insulating discs. Pin Insulator Strain Insulator Suspension Insulator Shackle Insulator

4. Prof. Tathe S G ICEEM 1. Pin Insulator This kind of insulator is used in distribution systems. The voltage capacity of this insulator is 11kV . It is designed with a high mechanical strength material. These are connected in vertical as well as horizontal positions. The construction of this insulator is simple and needs less maintenance as compared with other types .

5. Prof. Tathe S G ICEEM 2. Suspension Insulator In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the insulator become more . Handling and replacing bigger size single unit insulator are quite difficult task. For overcoming these difficulties, suspension insulator was developed . In suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc like shape.

6. Advantages of Suspension Insulator Each suspension disc is designed for normal voltage rating 11KV (Higher voltage rating 15KV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level. If any one of the disc insulators in a suspension string is damaged, it can be replaced much easily. Mechanical stresses on the suspension insulator is less since the line hanged on a flexible suspension string. As the  current  carrying conductors are suspended from supporting structure by suspension string, the height of the conductor position is always less than the total height of the supporting structure. Therefore, the conductors may be safe from lightening. Prof. Tathe S. G. ICEEM

7. When suspension string is used to sustain extraordinary tensile load of conductor it is referred as string insulator . When there is a dead end or there is a sharp corner in transmission line , the line has to sustain a great tensile load of conductor or strain . A strain insulator must have considerable mechanical strength as well as the necessary electrical insulating properties . 3 . Strain Insulator

8. When there is a dead end of the line or the line has a sharp curve or corner, the line crosses the valley or river, or the line is subjected to greater tension, where strain-type insulators are used.  For higher voltage transmission line strain type insulators consisting assembly of suspension type insulators is used.   The strain insulators should have high dielectric properties and high mechanical strength. Where the tension is increased high, at river crossing spans, two to four strings of insulators in parallel are used.  The discs of strain insulators are employed in the vertical plane.  The strain insulator is also called a tension insulator. 

9. The shackle insulator (also known as a spool insulator ) is usually used in low voltage distribution network. It can be used in both the horizontal or vertical positions. The use of such insulator has decreased recently after increasing the using of underground cable for distribution purpose. Prof. Tathe S. G. ICEEM 4. Shackle Insulator

10. The tapered hole of the spool insulator distributes the load more evenly and minimizes the possibility of breakage when heavily loaded. The conductor in the groove of shackle insulator is fixed with the help of soft binding wire. Prof. Tathe S. G. ICEEM

11. The  suspension insulator  is the one which consists of a number of porcelain discs connected in series by metal links in the form of a string. The voltage applied across the string of the suspension insulators is not uniformly distributed across the various discs, i.e. the disc nearest to the line conductor has much higher potential than the other discs. This unequal potential distribution is undesirable and it is usually expressed in terms of string efficiency. Therefore , the string efficiency of the suspension type insulator is defined as follows − Prof. Tathe S. G. ICEEM S tring efficiency

12. “ The ratio of voltage across the whole string to the product of the voltage across the disc nearest to the conductor and the number of discs in the string is known as string efficiency.” Where, n is the number of discs in the string of suspension insulator. Hence, from the definition of the string efficiency, it is clear that the greater the string efficiency, the more uniform is the potential distribution across the string. In ideal case, the string efficiency is 100% for which the voltage across each disc will be exactly the same. However, it is impossible to achieve 100% string efficiency. Prof. Tathe S. G. ICEEM

13. As we know, the potential distribution in a string of suspension insulators is not uniform, i.e. the highest potential appears across the insulator disc nearest to the line conductor and decreases progressively as the cross-arm is approached. If the insulation of the disc nearest to the line conductor breaks down, the breakdown of the other discs will take place in succession. Therefore , it is required to equalize the voltage across the various discs of the string, i.e., needs to improve the string efficiency. The following methods are employed for improving the string efficiency of a suspension type insulator − Prof. Tathe S. G. ICEEM Methods of Improving String Efficiency

14. By using the longer cross-arms By grading the insulators By using a guard ring 1. By Using Longer Cross-Arms The value of string efficiency depends upon the ratio of shunt capacitance to the mutual capacitance (let  K ). That means, the lesser the value of  K , the greater is the string efficiency and hence, more uniform is the potential distribution Prof. Tathe S. G. ICEEM

15. The value of  K  can de decreased by decreasing the shunt capacitance. For reducing the shunt capacitance , the distance between the line conductor and the tower must be increased, i.e., longer cross-arms should be used. In practice,  K  = 0.1 is the limit that can be achieved by this method. It is because the limitations of cost and strength of the tower do not allow the use of very long cross-arms . 2. By Grading the Insulators: In this method of improving string efficiency, the insulators of different sizes are so chosen that each has a different capacitance. Hence, the insulators used are capacitance graded, i.e ., they are connected to form the string in such a way that the top disc has the minimum capacitance and it increases progressively as the disc nearest to the conductor is reached. Prof. Tathe S. G. ICEEM

16. Prof. Tathe S. G. ICEEM As the voltage is inversely proportional to the capacitance, hence the capacitance grading of the insulators tends to equalize the potential distribution across the various discs in the string. The main disadvantage of this method is that it requires a large number of different sized insulators . 3. By Using Guard Ring : The guard ring is a metal ring which is electrically connected to the conductor and surrounding the bottom insulator disc as shown in Figure . It equalizes the potential across each disc in the string.

17. Prof. Tathe S. G. ICEEM The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is designed in such a way that the shunt capacitance currents 𝑖1,𝑖2, etc., are equal to the metal fitting line capacitance currents 𝑖1′, 𝑖2′, etc. The result is that the same charging current (I) flows through each disc of the string. As a result, the potential distribution across the discs of the string will be uniform.

18. Prof. Tathe S. G. ICEEM

19. The time horizon for long-term and mid-term forecasting ranges between a few weeks to several years. Unfortunately, it is difficult to forecast load demand accurately over a planning period of this length . This fact is due to the uncertain nature of the forecasting process. A large number of influential factors characterize and directly or indirectly affect the underlying forecasting process; all of them are uncertain and uncontrollable . Therefore, any long-term forecast, by nature, is  inaccurate! Prof. Tathe S. G. ICEEM

20. As we know the string of suspension insulators consists of a number of porcelain discs connected in series through metal links. The porcelain portion of each disc is in between two metal links. Thus, the disc acts like a capacitor represented by C as shown in Figure-2. The capacitance C is called the mutual capacitance. If the string has only mutual capacitance, then the charging current would have been the same through all the discs and hence, the voltage across each disc would have been the same (say V/3) as shown in Figure. But in actual practice, a capacitance also exists between the metal fitting of each disc and the tower. This capacitance is called the shunt capacitance, represented by C1 in the Figure Potential Distribution over Suspension Insulator String Prof. Tathe S. G. ICEEM

21. Due to the presence of the shunt capacitance, the charging current is not the same through all the discs of the string, i.e. the voltage across each disc will be different . The disc nearest to the line conductor will have the maximum voltage. Therefore, the important points observed regarding the potential distribution over a string of suspension insulators are as follows − The voltage across a string of suspension insulators does not distributes itself uniformly across the individual discs due to the presence of shunt capacitance. The disc nearest to the line conductor has maximum voltage across it and it decreases as the cross-arm is approached. If the voltage applied across the string is DC, then the voltage across each disc would be the same because the insulator capacitances are ineffective for DC. Prof. Tathe S. G. ICEEM

22. Mathematical Analysis of Potential Distribution over Suspension Insulator String Prof. Tathe S. G. ICEEM

23. Refer Figure-2. Let the shunt capacitance  C 1  is some fraction  K  of self-inductance ( C ), i.e., 𝐶 1 =𝐾𝐶. Starting from the cross-arm, the voltage across each disc is  V 1 , V 2  and  V 3 , respectively. Prof. Tathe S. G. ICEEM

24. Prof. Tathe S. G. ICEEM

25. Hence, the following points can be observed from the above mathematical analysis − Let  K  = 0.3, then from equation (4), we have  V 2  = 1.3  V 1  and  V 3  = 1.99  V 1 . This shows that the disc nearest to the conductor has maximum voltage across it. The greater the value of  K , the more non-uniform is the voltage across the discs of the string and hence, lesser is string efficiency. The inequality in the potential distribution increases with the increase in the number of discs in the string of insulators. Prof. Tathe S. G. ICEEM

26. The Station Transformers are employed to supply auxillary loads to the power plant for starting the plant when generating unit is not in operation . There transformers are connected to the switchyard bus. LV side of the station transformer is connected to the auxiliary buses . Station transformers are provided with On Load Tap changer with ONAN/ONAF cooling . Star point of the transformer is grounded through resistor. Normal ratings are 20MVA and HV voltage 11kV or 13.8kV and LV voltage in the range of 3.6kV or 6.6kV. Prof. Tathe S. G. ICEEM Station Transformers

27. Prof. Tathe S. G. ICEEM Isolator is  a manually operated mechanical switch that isolates the faulty section of substation. It is used to separate faulty section for repair from a healthy section in order to avoid the occurrance of severe faults. It is also called disconnect or or disconnecting switch . It is also called disconnector or disconnecting switch. There are different types of isolators used for different applications. They are: single break, double break, bus isolator, and line isolator. The isolator will be a horizontal double break central rotating type with an earth switch. Isolators and earth switches can be hand operated. Earth switches and Isolators (in closed position) are designed to withstand thermal effects and other conditions due to short circuit current. Isolator

28. Prof. Tathe S. G. ICEEM Reactors are used in transmission substations to limit current flow between transmission lines and the ground. Shunt reactors and series reactors are the two main types of reactors used in transmission substations. S hunt Reactors: A shunt reactor is a reactor that is connected in parallel with a transmission line. In short, a  substation reactor  is essentially a coil that has several turns and fairly high resistance values. Electrical reactors are typically used on a transmission line when it’s necessary to limit currents that could damage a power transformer or other power system. This type of current-limiting reactor often relies on inductive reactance. Reactors

29. Prof. Tathe S. G. ICEEM The additional inductance and reactance are then added to the series which is key to the function of an electrical reactor or series reactor. Luckily, it’s easier to determine faults in a series reactor since it doesn’t require you to shut down your transformers or generators in the event of a short circuit or other electrical problem. It’s simpler to spot the fault current in your reactor and asses a neutral point for an inductor without having to dismantle an entire power system. This level of reliability makes reactors critical components of many substation electrical systems.

30. Prof. Tathe S. G. ICEEM Control panel Control panel is used to accommodate instruments for the purpose of measurement, monitoring, protection, detection, control and manage the processed The panels are located in a control room as well as a field termed as a local control panel (LCP) for operational convenience . The control panels are designed based on the type of energy and dynamics associated. Panels are named based on a functional media such as electrical, electronic, air, and oil . The panel is called PLC control panel with a PLC installed, associated wiring, power supplies. If the panel is Pneumatic controller/ Indicators which are operated by air is called pneumatic control panel with associated controllers, transducers gauges, etc .,

31. If a panel is called hydraulic, if it is operated by oil with associated pressure gauges, directional control valves, etc., Types of Control Panels There are three basic types of control panels. Flat Control Panels Breakfront Control Panels Console type Control Pane

32. In electric power distribution, a bus bar (also bus bar) is a metallic strip or bar, typically housed inside switchgear, panel boards, and bus way enclosures for local high current power distribution . A bus bar is a metallic bar in a switchgear panel used to carry electric power from incoming feeders and distributes to the outgoing feeders. In simple terms, bus bar is a electrical junction where incoming and outgoing currents exchange . Electrical Bus bar consists the number of lines electrically, which are operating at the same voltage and frequencies. Generally, copper or aluminum conducting material is used in the construction of bus bars. bus bar

33. A substation battery ensures all the essential electrical systems in a substation continue to operate in the event of a power outage. An absence of an electrical supply could result in damage to equipment and personnel. The DC system is the most important component of a high voltage industrial/utility substation. substation battery

34. Nowadays, the control house is one of the main parts of every modern HV switchyard and substation. Simply , because substations have grown in complexity and equipment such as circuit breakers, switch disconnectors , CT, VTs, and others must be controlled, monitored, and protected from some common point in the field. The necessity for supplemental equipment such as protection relays, controls, batteries, communications equipment, and LV distribution equipment also increases. And all that equipment must be placed somewhere in the field. control house

35. Substation regulators are one of the primary means, along with load-tap-changing power transformers, shunt capacitors, and distribution line regulators, for maintaining a proper level of voltage at a customer's service entrance. range of voltage swing between light and heavy load periods . Power distribution voltage regulators maintain power distribution system voltages within a defined range. Regulated voltages ensure that electrical products and equipment will operate optimally.  Electrical products and equipment are typically designed to operate within a limited voltage band. Poorly regulated voltage often results in adverse and unacceptable equipment performance. Problems that can occur include damage electronics, inefficient operation of motors, dim light bulbs and a myriad of other issues. voltage regulators

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