DEPARTMENT OF ELECTRICAL ENGINEERING DIT UNIVERSITY HIGH VOLTAGE ENGINEERING

Transcription

1 1 DEPARTMENT OF ELECTRICAL ENGINEERING Introduction to High Voltage Testing It is essential to ensure that the electrical equipment is capable of withstanding the overvoltages that are met within the service. The overvoltages maybe either due to natural causes like lightning or system originated ones such as switching impulse or power frequency transient voltages. Hence testing for overvoltages is necessary. The overvoltage tests are classified into two groups: (i) Power frequency voltage test. (ii) Impulse voltage test. These tests together ensure the overvoltage withstand capability of an apparatus. Terms related with High Voltage Testing The following terms are most commonly associated with high voltage testing which have been explained briefly below: (i) Disruptive discharge voltage It is defined as the voltage which produces the loss of dielectric strength of insulation. It is that voltage at which the electrical stress in the insulation causes a failure which includes the collapse of voltage and passage of current. It causes a permanent loss of strength in solids whereas in liquids or gases only temporary loss maybe caused. (ii) Flashover When a discharge takes place between two electrodes (in a gas, liquid or solid) over the surface of the dielectric but not through the volume of the insulator then it is called flashover. It is mostly temporary failure of insulation of the insulator. The dielectric strength of the insulator is recovered completely once the adverse conditions which resulted in flashover are removed. (iii) Puncture If a discharge occurs in an insulator through the entire volume such that once the adverse conditions which caused discharge are removed the dielectric strength is not fully recovered. It is mostly permanent failure of insulation. (iv) Withstand Voltage The voltage which is to be applied to a test object under specified conditions without causing flashover or puncture of the insulator is known as withstand voltage. (v) Fifty Percent Flashover Voltage It is the voltage which has a probability of 50% flashover when applied to the test object. This is normally applied in impulse tests in which loss of insulation strength is temporary. (vi) Hundred Percent Flashover Voltage It is the voltage that causes a flashover at each of its applications under the specified conditions when applied to test object. (vii) Creepage Distance It is the shortest distance on the contour of external surface of the insulator unit or between two metal fittings on the insulator. (viii) Reference Atmospheric Conditions The electrical characteristics of the insulators and other apparatus are normally referred to the reference atmospheric conditions. Atmospheric Conditions Indian Standards British Standards Temperature 27 0 C 20 0 C Pressure 760 Torr 760 Torr Absolute Humidity 17 gm/m 3 11 gm/m 3 Tests on Insulators: The tests normally conducted on insulators are (i) type tests and (ii) routine tests. The type tests are intended to prove or check the design features and quality. The routine tests are intended to check the quality of the individual test piece. The type tests involves quality testing of equipment at the design and development level i.e. samples of the product are taken and are tested when a product is being developed and designed or an old product is to be redesigned and developed. The routine tests are meant to ensure the reliability of individual test objects, quality and consistency of materials used in their manufacture. The following tests are carried out on all insulators:

2 2 DEPARTMENT OF ELECTRICAL ENGINEERING (i) 50% dry impulse flashover test (ii) Impulse withstand test (iii) Dry flashover and dry one minute test (iv) Wet flashover and wet one minute test (v) Temperature cycle test (vi) Electromechanical test (vii) Mechanical test (viii) Porosity test (ix) Puncture test (x) Mechanical routine test (i) 50% Dry Impulse Flashover Test: This test is carried out on a clean insulator mounted as in normal working condition. An impulse voltage of 1.2/50 µs waveshape and of an amplitude which can cause 50% flashover of the insulator is applied i.e. out of the total number of impulses applied at least 50% of the impulses should cause flashover. The polarity of the impulse voltage is then reversed and the procedure is repeated. There must be at least 20 applications of the impulse in each case and the insulator must not be damaged. The magnitude of the impulse voltage should not be less than that specified in standard specifications. (ii) Impulse Withstand Test: In this test, the insulator is subjected to standard impulse 1.2/50 µs waveshape of specified value under dry conditions with both positive and negative polarities. If five consecutive applications do not cause any flashover and puncture, the insulator is deemed to have passed impulse withstand test. If out of five two applications cause flashover the insulator is deemed to have failed the test. (iii) Dry Flashover & Dry One Minute Test: The power frequency voltage is applied to the insulator and the voltage is increased to the specified value and maintained for one minute. The voltage is then increased gradually until flashover occurs. The insulator is then flashed over atleast four more times. The voltage is increased to reach flashover voltage in about 10 seconds. The mean of atleast five consecutive flashover voltages must not be less than the value specified in specifications. (iv) Wet Flashover & Wet One Minute Test: If the test is carried out under artificial rain it is called wet flashover test. The insulator is subjected to spray of water of following characteristics Precipitation rate 3 ± 10% mm/min. Direction 45 O to vertical Conductivity 100 µs ± 10% Temp. Of water ambient + 15 O C Initially a voltage which is 50% of one minute rain test is applied to the insulator and water of above specifications is sprayed for two minutes. The voltage is then raised to one minute test voltage in approximately 10 seconds and maintained there for one minute. The water is continuously sprayed on the insulator during the application of the voltage. The voltage is then further increased gradually till flashover occurs and the insulators is then flashed atleast four more times such that the time taken to reach flashover voltage in each case is 10 seconds. The flashover voltage must not be less than the value specified in specifications. (v) Temperature Cycle Test: The insulator is immersed in a hot water bath whose temperature is 70 o higher than normal water bath for T minutes. It is then taken out and immediately immersed in normal water bath for T minutes. After T minutes, the insulator is again

3 immersed in hot water bath for T minutes. The cycle is repeated three times and it is expected that the insulator should withstand the test without damage to the insulator or glaze. Here T = (15 + W/1.36) where W is the weight of the insulator in kg. (vi) Electromechanical Test: This test is carried out only on suspension or tension type of insulator. The insulator is subjected to a tension which is 2.5 times the specified maximum working tension maintained for one minute. Also simultaneously 75% of the dry flashover voltage is applied. The insulator should withstand this test without any damage. (vii) Mechanical Test: This test is a bending test applicable to pin type and line post insulators. The insulator is subjected to a load three times the specified maximum breaking load for 1 minute. There should be no damage to the insulator. (viii) Porosity Test: In this test the insulator is broken and immersed in a 0.5% alcohol solution of Fuchsin under a pressure of kn/m 2 for 24 hours. The broken insulator is taken out and further broken. It should not show any sign of impregnation. (ix) Puncture Test: In this test an impulse overvoltage is applied between the pin and the lead foil bound over the top and side grooves in case of pin insulator and between the metal fittings in case of suspension type insulators. The voltage is 1.2/50 µs with a magnitude twice the 50% impulse flashover voltage and negative polarity. Twenty such applications are applied. The procedure is then repeated for 2.5, 3, 3.5 times the 50% impulse flashover voltage and it is continued till the insulator is punctured. The insulator must not puncture at a voltage which is equal to or less than the voltage specified in the specifications. (x) Mechanical Routine Test: In this test the insulator is suspended vertically or horizontally and a tensile load 20% in excess of the maximum specified working load is applied for one minute. There should be no damage to the insulator string. Testing of Bushing: The bushing is an integral component of high voltage machines. It is used to bring high voltage conductors through the grounded tank or body of the electrical equipment without excessive potential gradients between the conductor and the edge of the hole in the body. In case of transformer the bushing extends into the surface of oil at one end and the other end is carried above the tank to a height sufficient to prevent breakdown due to surface leakage. The following tests are carried out on bushings: (i) Power factor test (ii) Impulse withstand test (iii) Chopped wave and switching surge test (iv) Partial discharge test (v) Momentary withstand test at power frequency (vi) One minute wet withstand test at power frequency (vii) Visible discharge test at power frequency (viii) Temperature rise and thermal stability test (i) Power Factor Test: The bushing is installed as in service or immersed in oil. The H.V. terminal of the bushing is connected to H.V. terminal of the Schering bridge and the tank or earth portion is connected to the detector of the bridge. The capacitance and power factor of the bushing is measured at different voltages as specified in the relevant specifications. The measured capacitance and power factor should be within the range specified. (ii) Impulse Withstand Test: In this test the bushing is subjected to impulse waves of both polarity and magnitude as specified in the standard specification. Five consecutive full waves of standard waveform (1.2/50 µs) are applied and if two of them cause flashover, the bushing is said to be 3

4 defective. If only one flashover occurs then ten additional applications are made. If no flashover occurs, bushing is said to have passed the test. (iii) Chopped Wave & Switching Surge Test: In this test chopped wave and switching surge of appropriate specifications are applied on H.V. bushings five times each. If two of them cause flashover then the bushing is said to be defective. If only one flashover occurs, ten additional applications are made. If no flashover occurs then the bushing is said to have passed the test (iv) Partial Discharge Test: This test is carried out in order to determine whether there is deterioration or not of the insulation used in bushing. This is done by using internal or partial discharge arrangement as shown in fig.1. The shape of the discharge is an indication of nature and severity of the defect in the bushing. C x r x R 3 C T D Band pass filter Amplifier Display Unit (CRO or Counter) r D C D C 4 R 4 Cx Test piece rx Internal resistance of test piece CD Dummy sample rd Internal resistance of dummy sample CT Filter capacitor D Power frequency detector Fig.1 Partial discharge arrangement for testing of bushing (v) Momentary Withstand Test at Power Frequency: This test is done as per the Indian standard specifications IS: 2099 applied to bushings. In this test the specified test voltage is applied for a minimum time of approximately 30 seconds. The bushing should not flashover or puncture. However at present this test is replaced by impulse withstand test. (vi) One Minute Wet Withstand Test at Power Frequency: It is the most common and routine test. In this test, the voltage specified is applied to the bushing mounted as in the service with rain arrangement. A properly designed bushing has to withstand the voltage without flashover for one minute. (vii) Visible Discharge Test at Power Frequency: This test is done to determine whether the bushing is likely to give radio interference in service when the voltage specified in IS: 2099 is applied. No discharge other than that from the arcing horns or grading rings should be visible to the observer in a dark room. The test arrangement is same as that of the withstand test but this test is conducted in a dark room (viii) Temperature Rise & Thermal Stability Test: The purpose of this test is to ensure that the bushing in service for long does not have an excessive temperature rise and also does not go into the thermal runaway condition of the insulation used. The temperature rise test is carried out in free air with an ambient temperature below 40 0 C at a rated power frequency a.c. current. The steady temperature rise above the ambient air temperature at any part of the bushing should not exceed 45 0 C. This test is carried out for such a long time till the temperature is substantially constant. 4

5 The thermal stability test is done for bushings rated for 132 kv and above. This test is carried out with the bushing immersed in oil at a maximum temperature as in service and the voltage applied is 80% of the nominal system voltage. The thermal stability test is a type test but in case of large sized H.V. bushings, it maybe necessary to make them routine tests. Testing of Surge Diverters: The lightning arrestor or surge diverter is the most reliable apparatus to protect the power system against lightning and switching surges. A surge diverter has to be a non conductor for operating power frequency voltages and should behave as short circuit for impulse voltages. The following tests are conducted on a lightning arrestor: (i) Power frequency sparkover test (ii) 100% standard impulse sparkover test (iii) Front of wave sparkover test (iv) Residual voltage test (v) Long duration impulse current test (vi) Operating duty cycle test (vii) Other test (mechanical test, pollution test, pressure relief test etc.) (i) Power Frequency Sparkover Test: It is a routine test. In this test a power frequency voltage which is 1.5 times the rated power frequency withstand voltage is applied across the lightning arrestor five times successively. If there is no flashover in all five applications then the arrestor is said to have passed the test. A series resistance is also inserted in the circuit to limit the current in case a flashover occurs. This test is generally done both under dry and wet conditions. (ii) 100% Standard Impulse Sparkover Test: This test is conducted to ensure that the arrestor operates positively when an overvoltage of impulse nature occurs. In this test an impulse voltage of standard specifications is applied to the lightning arrestor ten times successively. The arrestor should sparkover at each application. The test is conducted with both positive and negative polarity waveforms. (iii) Front of Wave Sparkover Test: This test is conducted in order to ensure that the surge diverter flashes over for very steep fronted waves of high peaks. This test is conducted using an overvoltage having a rate of rise of 100 kv/µs per 12 kv of the rating. The estimated maximum steepness of the waves is specified in standards and specifications. The test is done by conducting 100% sparkover voltage test for increasing magnitudes of standard impulse wave. The time to sparkover is measured. The volt time characteristics of diverter is plotted. The intersection of the line with slope of vertical steepness of front with volt time characteristics of the arrestor gives the front of a wave sparkover voltage. (iv) Residual Voltage Test: This test is conducted on a pro rated diverter of ratings in the range 3 to 12 kv only. The voltage developed across the non linear resistor units during the flow of surge currents through the arrestor is called the residual voltage. A pro rated arrestor is a complete, suitably housed section of an arrestor including series gaps and non linear resistors in same proportion as in the complete arrestor. A standard impulse current of rated magnitude is applied and the voltage developed across the diverter is recorded using a suitable voltage divider and CRO. The magnitudes of the currents are approximately 0.5, 1.0 and 2.0 times the rated currents. From the oscillogram, a graph is drawn between the current magnitudes and the voltage developed across the diverter pro rated unit. From the graph, the residual voltage corresponding to the exact rated current is obtained. The diverter is said to pass the test if V R < rv 2 2 5

6 Where V R is residual voltage of pro rated unit 2 V r is multiplying factor = RM where V RM is maximum permissible residual voltage V 1 V 1 is rating of complete unit V 2 is rating of pro rated unit tested (v) Long Duration Impulse Current Test: This test is also done on pro rated units of 3 to 12 kv. The circuit used to generate a rectangular impulse wave consists of an artificial transmission line with lumped inductances and capacitances. The duration of the current pulse t is given by 2( n 1) LC where n = number of stages of sections used and L, C are inductance & capacitance of each unit. The rectangular wave is generated if the surge impedance of the diverter is equal to LC at the test current. As per the specifications, 20 applications are made with specified current in five groups. The interval between the successive applications is about 1 minute. It is usual to record the waveforms in first two and last two applications of the current wave. The diverter is said to have passed the test if the following conditions are satisfied: (a) The power frequency sparkover voltage before and after the application of the current wave does not differ by more than 10%. (b) The voltage and current waveforms of the diverter do not differ significantly in two applications. (c) The non linear resistance elements in the diverter do not show any sign of puncture or external flashover. (d) The voltage across the diverter at first and last application does not differ by more than 8%. (vi) Operating Duty Cycle Test: This test is conducted on pro rated units of diverter and it gives better closeness to actual conditions. The diverter is kept energized at its rated power frequency supply voltage. The rated impulse current wave is applied first at a phase angle of about 30 0 from a.c. voltage zero. If the power frequency current is not established then the angle at which current impulse wave is applied is advanced in steps of 10 0 upto 90 0 or peak position of supply voltage wave till follow on current is established. During the follow on current period, the peak voltage across the diverter should be less than or equal to the rated peak voltage. Twenty applications of the impulse current at the selected points on the voltage wave are made in four groups. The time interval between each application is 1 minute and between successive groups it is about half an hour. The arrestor is said to have passed the test if the following conditions are met: (a) The average power frequency sparkover voltage before and after the test does not differ by more than 10%. (b) The residual voltage at the rated current does not vary by more than 10%. (c) The follow on power frequency current is interrupted each time. (d) There is no flashover or puncture to the pro rated unit. (vii) Other Test: The following tests are generally conducted on diverters used on EHV systems: (a) Mechanical test (b) Pressure relief test (c) Voltage withstand test on insulator housing of surge diverter (d) Switching surge flashover test (e) Pollution test 6

7 Testing of Isolators & Circuit Breakers: The tests conducted on circuit breakers and isolators give common characteristics for both. However these characteristics though directly relevant to the testing of circuit breaker are of not much relevance as far as testing of isolators are concerned because the isolators are not used for interrupting high currents. The following tests are conducted on circuit breakers and isolators: (i) Dielectric test (ii) Impulse test (iii) Short circuit test (iv) Thermal test (v) Asymmetrical test (i) Dielectric Test: This test consists of overvoltage withstand tests of power frequency, lightning and switching impulse voltages. These tests are done for both internal and external insulation with the switch or circuit breaker in both open and closed positions. In the open position, the test voltage levels are 15% higher than the test voltages used when the circuit breaker is in closed position. Due to this there is always a possibility of line to ground flashover which is avoided by mounting the circuit breaker on insulators above the ground. (ii) Impulse Test: In this test the lightning impulse wave of standard shape is applied to the circuit breaker similar to the insulators. In addition switching overvoltages are also applied to assess their performance under overvoltages due to switching operations. (iii) Short Circuit Tests: These are the most important tests carried out on circuit breaker since these tests assess the primary performance of these devices i.e. their ability to safely interrupt the fault current. These tests consist of determining the making and breaking capacities at various load current and rated voltages. In case of isolators, the short circuit tests are conducted only with the limited purpose to determine their capacity to carry the rated short circuit current for a given duration and no breaking, making current test is done. The different methods of conducting short circuit tests are as follows: (a) Direct Testing in Networks or in Fields The circuit breakers are sometimes tested for their ability to make or break the circuit under normal load conditions or under short circuit conditions in the network itself. This is done during period of limited energy consumption or when the electrical energy is diverted to other sections of the network which are not connected to the circuit under test. In this type of testing there is not much flexibility. (b) Direct Testing in Short Circuit Test Lab In order to test the circuit breaker at different voltages and at different short circuit currents, short circuit laboratories are provided. The fig.2 shows a schematic layout of a short circuit testing laboratory. It consists of a short circuit generator in association with a master circuit breaker, resistors, reactors and measuring devices. The rating of the master circuit breaker is always higher than the circuit breaker under test. A make switch is used to initiate the short circuit. The master circuit breaker isolates the test device from the source at the end of a predetermined time set on a test sequence controller. The master circuit breaker can also be tripped if the test device fails to operate properly. 7

8 G Fig.2 Schematic dig. of short ckt. testing lab for direct testing (c) Synthetic Testing of Circuit Breaker Due to very high interrupting capacities of circuit breakers, it is not economical to have a single source to provide the required short circuit current and rated voltage. Instead a combination of the effects of two sources, one of which supplies a.c. current and the other the high voltage is used. In the initial period of short circuit test, the a.c. current source supplies the heavy current at low voltage and then the recovery voltage is simulated by a source of comparatively high voltage of small current capacity. The fig.3 shows the schematic diagram of a synthetic testing station where Vc low voltage, high current generator Lc current controlling inductance MCB master circuit breaker Lv voltage waveform controlling choke Cv capacitor to give necessary recovery voltage Co capacitor to control the frequency of transient recovery voltage MCB Making Switch Aux CB LV LC VC CO CB under Test CV Fig.2 Schematic dig. of short ckt. testing lab for synthetic testing With the auxiliary circuit breaker and the test circuit breaker closed, the closing of the making switch causes the current to flow in the test circuit breaker. At some instant say to, the circuit breaker begins to operate. The trigger gap closes at the instant just before the generator current becomes zero and the higher frequency current from discharging capacitor CV flows through the arc. At the instant when generator current becomes zero, the master circuit breaker clears the circuit leaving only current from CV. At the zero of this current full test voltage will be available. (d) Composite Testing In this method, the circuit breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current. This method does not give a proper estimate of breaker performance and hence is not much preferred. 8

9 Testing Procedure for Short Circuit Test The circuit breakers are tested for their breaking capacity (B) and making capacity (M). The following procedure is to be adopted while conducting the short circuit tests on circuit breakers as per the specifications: B 3 B 3 B at 10% of rated symmetrical breaking capacity B 3 B 3 B at 30% of rated symmetrical breaking capacity B 3 B 3 B at 60% of rated symmetrical breaking capacity B 3 MB 3 MB at 100% of rated symmetrical breaking capacity B 3 MB 3 MB at 100% of rated symmetrical breaking capacity Here B and M represent breaking and making operations respectively. MB denotes making operation followed by breaking operation without any intentional time lag. 3 denotes time in minutes between successive operations of an operating duty. (iv) Thermal Test: These tests are made to check the thermal behaviour of the circuit breakers. In this test the rated current is passed continuously through all three phases of the switchgear for a long enough period to achieve steady state conditions. When the normal rated current is less than 800 A then the temperature rise above the ambient temperature should not exceed 40 0 C. In case of currents of magnitude 800 A and above the temperature rise above the ambient temperature should not exceed 50 0 C. (v) Asymmetrical Test: In this one test cycle is repeated for the asymmetrical breaking capacity in which the d.c component at the instant of contact separation is not less than 50% of the a.c. component. Testing of Cables: The high voltage power cables have proved quite useful especially in case of HVDC transmission. The following tests are done on a cable: (i) Bending test (ii) Loading cycle test (iii) Thermal stability test (iv) Dielectric thermal resistance test (v) Life expectancy test (vi) Dielectric power factor test (vii) Power frequency withstand voltage test (viii) Impulse withstand voltage test (ix) Partial discharge test However before performing the above mentioned tests, the cable sample has to be carefully prepared especially for electrical tests. This is essential to avoid any excessive leakage or end flashovers which otherwise may occur during testing and hence may give wrong information regarding the quality of cable. The length of the sample cable varies between 50 cm to 10 m. The terminations are usually made by shielding the ends of cable with stress shields so as to relieve the ends from excessive high electrical stresses. (i) Bending Test: In case of this test, the cable is bend round a cylinder of specified diameter to make one complete turn. It is then unwound or rewound in the opposite direction. This cycle is repeated three times. It is to be noted that a voltage test should be made before and after this test. The diameter of the cylinder depends on the diameter of the cable under bending test as follows: 9

10 Diameter of cylinder = 16 X diameter of cable (ii) Loading Cycle Test: In this test the cable in the form of loop alongwith its accessories is subjected to 20 load cycles with a minimum conductor temperature 5 0 C in excess of the design value and the cable is energized to 1.5 times the working voltage. The cable should not show any signs of damage. (iii) Thermal Stability Test: This test is conducted after the loading cycle test. The cable is energized with a voltage 1.5 times the working voltage for a cable of 132 kv rating (the multiplying factor i.e. 1.5 decreases with increase in operating voltage) and the loading current is so adjusted that the temperature of the core of the cable is 5 0 C higher than its specified permissible temperature. The current should be maintained at this value for six hours. (iv) Dielectric Thermal Resistance Test: The ratio of the temperature difference between the core and sheath of the cable and the heat flow from the cable gives the thermal resistance of the sample of the cable. It should be within the specified limits as per the specifications. (v) Life Expectancy Test: The life tests are carried out for reliability studies in service. In order to determine the expected life of the cable under normal stress, accelerated life tests using increased voltages are performed on actual cable lengths. It has been observed that the relation between the expected life of the cable in hours and voltage stress is given by g = Kt (Vn) where g = maximum electrical stress t = expected life of cable in hours K = constant depending on field conditions and materials n = life index depending on the material (vi) Dielectric Power Factor Test: This test is done using the high voltage Schering Bridge. The power factor is measured for different values of voltages e.g. 0.5, 1.0, 1.5 and 2.0 times the rated operating voltages. The maximum value of power factor at normal working voltage does not exceed a specified value. The fig.3 shows the arrangement used for dielectric power factor. A shielding shown by is used which is connected to ground to eliminate the effect of stray capacitances. r x C x C S D R 4 R 3 C 3 Fig.3 Schematic dig. of Schering bridge Test Voltages: The following tables give the test voltages required for different transmission voltages for high voltage testing. Table 1 Test voltages for a.c. equipments System nominal Power frequency Impulse withstand Switching surge 10

11 Voltage (rms) withstand voltage (rms) voltage withstand voltage Table 2 Test voltages for d.c. equipments Nominal Voltage D.C. withstand voltage ±400 ±600 ± Impulse withstand voltage Switching surge withstand voltage If the insulation requirement for a particular voltage are required to be studied in a research and development lab then table 3 and table 4 gives the voltage levels required in lab. Table 3 Test voltages for a.c. system voltage System nominal Power frequency Voltage (rms) withstand voltage (rms) Impulse withstand voltage Switching surge withstand voltage Table 4 Test voltages for d.c. systems Nominal Voltage D.C. withstand voltage ±400 ±600 ± Impulse withstand voltage Switching surge withstand voltage