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Chapter – 4 Insulators Introduction •Overhead line conductors should be supported on the poles or towers in such a way that currents from conductors do not flow to earth through supports. This is achieved by securing line conductors to supports with the help of insulators. •Insulators provide necessary insulation between line conductors and supports and thus prevent any leakage current from conductors to earth. BSS 2 Desirable Properties of Insulators 1)High mechanical strength to withstand conductor load, wind load etc., 2)High electrical resistance to avoid leakage currents to earth. 3)High relative permittivity so that the dielectric strength is high. 4)Insulator material should be non-porous, free from impurities and cracks otherwise the permittivity will be lowered. 5)High ratio of puncture strength to flashover. BSS 3 •Most commonly used material is porcelain but glass, steatite and special composition materials are also used to a limited extent. •Porcelain is produced by firing at a high temperature a mixture of kaolin, feldspar and quartz. It is stronger mechanically than glass, gives less trouble from leakage and is less effected by changes of temperature. BSS 4 Types of Insulators •Successful operation of an overhead line depends to a considerable extent upon the proper selection of insulators. •There are several types of insulators but the most commonly used are pin type, suspension type, strain insulator and shackle insulator. BSS 5 1)Pin type insulators Part section of a pin type insulator is shown Pin type insulators... • Pin type insulator is secured to the cross-arm on the pole. • There is a groove on the upper end of the insulator for housing the conductor. • Conductor passes through this groove and is bound by the annealed wire of the same material as the conductor. Pin type insulators... These insulators are used for the transmission and distribution of electric power up to 33kV. Beyond 33kV, pin type insulators become too bulky & uneconomical. Causes of insulator failure • Insulators are required to withstand both mechanical & electrical stresses. • Electrical stress is primarily due to line voltage and may cause the breakdown of the insulator. • Electrical breakdown of the insulator can occur either by flash-over or puncture. Causes of insulator failure... In flashover, an arc occurs between the line conductor and insulator pin (i.e., earth) and discharge jumps across the air gaps, following shortest distance (a+b+c). Causes of insulator failure... • In case of flash-over, the insulator will continue to act in its proper capacity unless extreme heat produced by the arc destroys the insulator. • In case of puncture, discharge occurs from conductor to pin through the body of the insulator in which case, the insulator is permanently destroyed due to excessive heat. Causes of insulator failure... • In practice, sufficient thickness of porcelain is provided in the insulator to avoid puncture by the line voltage. • Ratio of puncture strength to flashover voltage is known as safety factor i.e., Puncture strength Safety factor of insulator Flash - over voltage 10 for pin type insulators 2) Suspension type insulators Usual practice to use suspension type insulators for high voltages (>33kV). Suspension type insulators... • Number of porcelain discs are connected in series by metal links in the form of a string. • Conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of tower. • Each disc is designed for low voltage, say 11kV. Number of discs in series will depend upon the working voltage – 6 discs for 66kV. Suspension type insulators... Advantages: 1) Cheaper than pin type insulators for voltages beyond 33 kV. 2) Each disc of suspension type insulator is designed for low voltage, usually 11 kV. Desired number of discs can be connected in series depending upon the working voltage. Suspension type insulators... 3) Whole string does not become useless because only the damaged disc can be replaced by the good one. 4) Suspension arrangement provides greater flexibility to the line. The connection at the cross arm is such that insulator string is free to swing in any direction and can take up the position where mechanical stresses are minimum. Suspension type insulators... 5) Additional insulation required for the raised voltage can be easily obtained in the suspension arrangement by adding the desired number of discs. 6) Suspension type insulators are generally used with steel towers. As the conductors run below the earthed cross-arm of the tower, therefore, this arrangement provides partial protection from lightning. 3) Strain insulators • When there is a dead end of the line or there is corner or sharp curve, the line is subjected to greater tension which can be relieved by using strain insulators. • For low voltage lines (<11kV), shackle insulators are used as strain insulators. • Strain insulator for HV transmission lines consists of an assembly of suspension insulators. Strain insulators... • The discs of strain insulators are used in the vertical plane. • When the tension in lines is exceedingly high, as at long river spans, two or more strings are used in parallel. 4) Shackle insulators • Shackle insulators were used as strain insulators earlier but now, they are frequently used for LV distribution lines. • These insulators can be used either in a horizontal position or in a vertical position. • These insulators can be directly fixed to the pole with a bolt or to the cross arm. Shackle insulators... • The fig. shows a shackle insulator fixed to the pole. • The conductor in the groove is fixed with a soft binding wire. Potential Distribution over Suspension Insulator String • A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links. • Fig. shows 3-disc string of suspension insulators. Potential Distribution over Suspension Insulator String... Porcelain portion of each disc is in between two metal links and hence forms a capacitor C as shown in Fig. This is known as mutual capacitance or selfcapacitance. Charging current I is same through all discs and hence voltage across each unit is same i.e., V/3 as shown. Potential Distribution over Suspension Insulator String... • Capacitance also exists between metal fitting of each disc and tower or earth in actual practice. This is known as shunt capacitance C1. • Due to this shunt capacitance, different current flows through all the discs of the string leading to different voltages. Observations regarding the potential distribution 1)Voltage impressed on a string of suspension insulators does not distribute itself uniformly across the individual discs due to the presence of shunt capacitance. 2)Disc nearest to the conductor has maximum voltage across it. As we move towards the cross-arm, the voltage across each disc goes on decreasing. Observations regarding the potential distribution... 3) Unit nearest to the conductor is under maximum electrical stress and is likely to be punctured. Thus, the potential across each unit need to be equalized. 4) If the voltage impressed across the string were DC, then voltage across each unit would be the same. This is because the insulator capacitances are ineffective for DC. String Efficiency • Voltage applied across the string of suspension insulators is not uniformly distributed across various units or discs. • The disc nearest to the conductor has much higher potential than the other discs. • This unequal potential distribution is undesirable and is usually expressed in terms of string efficiency. String Efficiency... • The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency i.e., Voltage across the string n Voltage across disc nearest to conductor n = number of discs in the string String efficiency where • The greater the string efficiency, the more uniform is the voltage distribution. Mathematical expression • Fig. shows the equivalent circuit for a 3-disc string. • Let the self capacitance of each disc be C. • Further, let the shunt capacitance C1 be some fraction K of self capacitance i.e., C1 = KC. Mathematical expression... • Starting from the cross-arm or tower, the voltage across each unit is V1, V2 and V3 respectively as shown. • Applying Kirchhoff’s current law to node A, we get, I2 I1 i1 or V2C V1C V1C1 or V2C V1C V1 KC V2 V1 1 K 1 Mathematical expression... • Applying Kirchhoff’s current law to node B, we get, I3 I2 i2 or V3C V2C V1 V2 C1 or V3C V2C V1 V2 KC or V3 V2 V1 V2 K V1K V2 1 K 2 V1 K 1 K V3 V1 1 3K K 2 V2 V1 1 K 2 Mathematical expression... • Voltage between conductor and earth (i.e., tower) is V V1 V2 V3 V1 V1 1 K V1 1 3K K V1 3 4K K 2 2 V V1 1 K 3 K 3 & hence, Voltage across top unit, V1 V 1 K 3 K Mathematical expression... • Voltage across second unit from top, V2 = V1 (1 + K) • Voltage across third unit from top, V3 = V1 (1 + 3K + K2) % String efficiency Voltage across the string 100 n Voltage across disc nearest to conductor 1 K 3 K V 100 3 V3 3 1 3K K 2 Methods of improving String Efficiency The potential distribution in a string of suspension insulators is not uniform The maximum voltage appears across the insulator nearest to the line conductor Decreases progressively as the cross arm is approached The insulation of the insulator nearest to the line conductor is stressed to the highest) This causes its break down or flash over The breakdown of other units will take place in succession This necessitates to equalize the potential across the various units of the string i.e. to improve the string efficiency Methods to improve the string efficiency: 1) By using longer cross-arms: The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution The value of K can be decreased by reducing the shunt capacitance In order to reduce shunt capacitance longer cross-arms should be used. Limitations of cost and strength of tower do not allow the use of very long cross-arms In practice, K = 0·1 is the limit that can be achieved by this method. 2) By grading the insulators: Insulators of different dimensions are so chosen that each has a different capacitance The insulators are capacitance graded The string is assembled in such a way that the top unit has the minimum capacitance, Increasing progressively as the bottom unit (i.e., nearest to conductor) is reached As voltage is inversely proportional to capacitance, this method tends to equalize the potential distribution across the units in the string This method has the disadvantage that a large number of different-sized insulators are required 3) By using a guard ring A guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator as shown The idea is to cancel exactly the pin to tower (shunt capacitance) charging currents The guard ring introduces capacitance between metal fittings and the line conductor The guard ring is designed in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i1’, i2’ etc. Consequently the same charging current I flows through each unit of the string There will be uniform potential distribution across the units. Testing of Insulators Three categories of tests: 1) Flash-over Tests: a) 50 per cent dry impulse flash-over test. b) Dry flash-over and dry one-minute test. c) Wet flash-over and one-minute rain test. a) 50 per cent dry impulse flash-over test The test is made on a clean insulator The impulse generator delivers at least 20 positive 1/50 microsecond impulse wave 50% of the positive impulses applied cause flash-over of the insulator Next, about 20 negative 1/50 impulse is applied 50% of the positive impulses applied cause flash-over of the insulator In each case the insulator must not be damaged b) Dry flash-over and dry one-minute test A voltage of power frequency is applied to a clean insulator The voltage is gradually increased up to the specified value This voltage is maintained for one minute. The voltage is then increased gradually until flashover occurs. The insulator is then flashed-over at least four more times The voltage is raised gradually to reach flash-over in about 10 seconds The insulator must not be damaged c) Wet flash-over and one-minute rain test The insulator is sprayed with water at an angle of 45 degrees The temp. of the water is within 10°C of the ambient temperature in the neighborhood The insulator must withstand the test-voltage specified for one minute The voltage is then gradually raised until flashover occurs The insulator is then flashed at least four more times The time taken to reach the flashover voltage being in about 10 seconds. 2) Sample Tests: a) Temperature-cycle test b) Mechanical test c) Electro-mechanical test d) Puncture test e) Porosity test a) Temperature-cycle test The insulator is subjected three times to the following temperature cycle Immersed for T minutes in a water-bath at 70° C The main water bath is at a lower temperature Next, taken out, and immersed as quickly in a main water bath and left in this bath for T minutes. The insulator must withstand this series of tests without damage to the porcelain or glaze. b) Mechanical Tests Applied to pin insulators and line post insulators. The test is a bending test, in which a load of three times the specified maximum working load (twice for a post insulator) is applied for one minute. There must be no damage to the insulator, In the case of the post insulator, the load is then raised to three times There must be no damage to the insulator or its pin (or pins) c) Electro-mechanical test Applied to suspension or tension units only the insulator is mechanically stressed to a tension of 2.5 times the specified maximum working load this being maintained for one minute. Simultaneously, 75 per cent. of the dry spark-over voltage is applied. Simultaneously 75% of the dry spark-over voltage is applied The insulator should not be damaged d) Puncture test The insulator is completely immersed in insulating oil at room temperature The voltage raised as rapidly as is consistent with correct measurement The insulator must not be punctured