National Conference on High Voltage Engineering ...

National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017

Quality Analysis of Insulators Subjected to HVDC Applications- A Review B. Mallikarjuna Jain University Bangalore, India [email protected]

V. Muralidhara Jain University Bangalore, India

Abstract—The thermal runaway and ion-migration are the two type tests suggested by IEC 61325-1995 on ceramic and glass insulators to be used for DC voltages. The ion-migration test verifies whether the quality of material is suitable for DC voltages or not. The electrical body resistance measurement test, thermal run away test, ion-migration test, and SF6 puncture withstand test are the important tests and to verify that whether the parameters given in the standards are adequate or not. This paper presents the work carried out in order to find the quality of DC insulators by Ion-migration, Thermal runaway test and the measurement of the body resistance of the insulators. I.

INTRODUCTION

1.1 In order to achieve India’s ambitious mission of POWER FOR ALL, the entire country expansion of the regional power transmission network is essential. Also inter regional capacity to transmit power would be essential because resources are unevenly distributed in the country and power needs to be carried over longer distances to areas where load centers exist. 800 kV DC seems likely to account for a significant part of world growth in power transmission capacity. China and India are the main users of the new technology as they seek to secure reliable power supplies and exploit sources, particularly hydro, that are located far from the load centers. In North -East region and Himalayan region of India, work is under progress for transmitting bulk power over long distances using 800kV Bi-polar UHV DC lines. For UHV-DC line transmissions, it is important that the reliability of these lines shall have to be given utmost priority, and outages of these lines shall be of bare minimum. One of the major points in the outages of these lines is pollution performance of line insulators and failure rate of these insulators, which should be as low as possible. The most important factor for HVDC system is the pollution performance of insulators and failure rate of insulators in DC system is higher compared to AC system. The contamination problem is more serious for DC line than for AC lines. The negative DC pollution flashover voltage of an insulator string is lower than that of AC lines. The arc shortening the neighboring under rib tips reduces the effectiveness of the insulator leakage length. The flashover

N. Vasudev HV Division, CPRI Bangalore, India

K.N. Ravi Sapthagiri College of Engineering Bangalore, India

voltage is highly dependent upon arc propagation route, which in turn is a direct function of insulator profile. Both the flashover performance and the failure of DC insulators under dry conditions are very important factors to be considered in the design of DC insulators. The various factors like thermal runaway, ion migration inside the insulator volume, anode growth, surface erosion, cement growth etc, are the causes for failure of insulators. However, the rate of failure is higher in polluted zones where surface erosion and puncturing of insulators occur. Therefore, insulators have to be subjected to accelerated aging tests to simulate observed on site and to check their capability to withstand the stresses. Porcelain insulators puncture and glass insulators shatter into small pieces. Breakage of porcelain insulators occur due to electrolytic corrosion at the pin resulting in reduction of weight of the electrode and lesser mechanical strength. The resulting increase in the size of the pin due to corrosion may cause breakage of the porcelain shell. Environmental conditions also affect influence rate The accumulation of pollution in DC system is more due to existence of continuous electrostatic force on the pollutant and the absence of current zero, which results in longer duration of scintillation are the factors that the pollution performance of insulator poses a serious threat to HVDC transmission systems than in the case of AC systems. Because of the above reasons the rate of contamination is more and scintillation duration is longer as a consequence of which the flashover and aging tests are severe in DC. The main factors that determines the reliability of a transmission system is the design of external insulation and becomes increasingly important as the system voltage level is increased. The problem is more acute in HVDC systems as compared to AC systems. The thermal runaway and ion-migration are the two type tests suggested by IEC 61325-1995 on ceramic and glass insulators to be used for DC voltages. The ion-migration test verifies whether the quality of material is suitable for DC voltages or not. The total charge, which would have been passed through the volume of insulator in its estimated life of 50 years (Q50), is allowed to pass through an insulator in an accelerated manner, within a few thousands of hours. The voltage during the test is kept at a voltage between 70kV to 95kV and the temperature between 75oC to 130oC.

Quality Analysis of Insulators Subjected to HVDC Applications- A Review

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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 The body resistance measurement test, thermal run away test, ion-migration test, and SF6 puncture withstand test are the important tests and to verify that whether the parameters given in the standards are adequate or not. This paper presents the work carried out in order to find the quality of DC insulators by Ion-migration, Thermal runaway test and the measurement of the body resistance of the insulators. The voltage and temperature were varied during these tests in order to identify the good insulators. The result will be useful for the design of insulators for the HVDC transmission system considering effect of pollution. All tests were carried out as per standards, on insulators and observed that the tests prescribed are inadequate for testing the quality of the DC insulators. The thermal runaway tests are carried at higher temperatures above 800C (though the standard specifies 800C), with the increased level of voltages resulted in the failure of few insulators. The test procedures given in IEC-61325,1995 to check the quality of insulators are not sufficient. It was observed in many tests of ion-migration and thermal runaway test that even though the volume resistance is lower, the insulator passes all the tests. Weak insulators i.e., with low volume resistance, have to be tested for lesser number of days, and hence the possibility of withstand is more. Where as If the volume resistance values are higher, which indicates that it is a good DC insulator, then the number of days required for ion-migration test is very high. The tests were conducted in order to study the above effects. The tests given in the standards, namely electrical body resistance measurement, ion-migration test, thermal runaway test, and SF6 puncture withstand test have been carried out on batches of both AC and DC insulators. The insulators withstood all the above tests. Therefore, it can be inferred that few of the insulators, which are not having good resistance values at higher temperature, also withstand the tests as per the standard. Hence, it is envisaged that the variation in the parameters of the tests namely temperature and voltage may detect the inferior quality of insulators for DC applications. Accordingly, the temperature of thermal runaway test was increased to 120oC and 150oC. It was thought that inferior insulators might fail in this thermal runaway test. The results of the tests were found to be encouraging. This paper presents the details of the experimental setup, test procedure, results, discussions and conclusions.

1.2: FAILURE MECHANISMS IN DC INSULATORS The behavior of ceramics under DC current has been investigated by taking three different points of view viz. 1. Study of the puncture mechanisms of ceramics: Scientists have generally concluded that the intrinsic dielectric strength of the ceramics is extremely high, but thermal breakdown could occur under certain conditions. The dielectric breakdown usually obtained in the laboratory can be explained by some kind of defect mechanism, very often starting by a mechanical failure possibly enhanced due to

electrical forces or thermal stresses or by electrical discharges occurring in a void. 2. Study of the long term behavior of materials under DC current, considering the possible modifications in the material due to migration of the charge carriers, such as Na +, which could give rise to various phenomena such as surface depletion layer, space charges near the electrodes, ion accumulation, and their possible neutralization near heterogeneity or surfaces. Finally they give rise to the formation of trees, extending throughout the thickness of the of the material. 1.2.1: Mechanism Involving Fractures: The observation of insulators, which have failed within the cap when energized by a DC stress, shows that the failure process ends up in propagation of fractures. The science of fracture mechanics has been well developed which could explain that micro-cracks existed on the surfaces of glass and could propagate under the influence of an external stress. This concept has been extremely useful in explaining the mechanical strength of brittle materials like glass. In the case of porcelain, numerous studies have been published, and the effects of stress, environment, defects, inclusions and stress corrosion have been investigated. When considering the case of insulators, the local distribution of mechanical stress resulting from the combination of the residual internal stresses and of the service external stresses, has to be taken into account. Scientists have reviewed the effect due to defects on the strength of porcelain insulator shells and have found that shells containing microscopic defects, such as micro-cracks, tiny voids and inclusions, have a strength reduced to 62% of normal strength during the mechanical test. However, in the case of glass and porcelain insulators, this effect due to defects is well known and the manufacturing processes and testing have been designed so that most of harmful defects are eliminated. The good performance of the insulators used on AC lines shows that the defects, which are harmful on a purely mechanical point of view, are effectively eliminated. Thus, we have to conclude that the failures appearing in DC involve some other mechanisms, which are not be purely mechanical. The effect of the DC electrical stress on the defects makes them more harmful than under an AC stress. 1.2.2: Thermal Runaway: Another phenomenon which explains the consequence of the voltage stress on the insulator units, but not strictly related to the presence of defects in the insulator body that could explain the insulator failure is the thermal runaway. For a single insulator unit subjected to DC stresses this phenomenon may be considered a possible origin of the thermal stresses causing failure of the insulator. Fig 1.2 summarizes the results of the theoretical and experimental works, and gives the insulator failure conditions as a function of voltage and temperature. 1.2.3: Ion Migration and Accumulation: Two research organizations, one in England and one in Russia have studied the modifications of composition,


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 structure and properties brought by a prolonged DC stress. The main summary of the works is given below. The manufacturers of HVDC Mercury Valves were concerned about the use of porcelain housing working at temperatures up to 1500C. They found the formation of alkali deposits and conducting dentritic growths (trees) at the negative electrode and the formation of ionic depletion layer, in which most of the charge carrier especially sodium, have migrated out, at the positive electrode, leading to a non uniform voltage distribution inside the insulator. This effect was not observed at 200C in the lifetime of the equipment, became significant at 800C and serious at 1500C. In such a case, it is possible to see clearly the Na+ depletion layer with a microprobe. Then it is established that the cause of breakage of DC insulators was electrolysis connected with a technical deficiency: the presence in the insulators of foreign inclusions in the form of iron oxides with dimensions from 0.01mm to 0.1mm. The mechanism proposed to explain the phenomenon involves displacement of sodium ions to the inclusions of iron oxides where the sodium becomes neutral and the ion goes from Fe+2 to Fe+3. The diameter of the sodium atom being almost twice as much as the diameter of the ion, tension stresses are induced in the material around the inclusion which might allow propagation of a previously sub-critical microfacture. Further, it was inferred from research works that the ion accumulation in the volume may increase due to surface erosion under polluted conditions. These ions may migrate towards cathode, inside the volume of insulator and may lead failure of insulators. 1.2.4: Heterogeneity Mechanisms: In addition to the defect mechanism due to the accumulation and neutralization of sodium ions at heterogeneity, various other mechanisms connected to heterogeneity could be proposed. 1.2.5: Pore Mechanism: Discharges in voids, which would gradually damage the material, are well known as partial discharges. Their voltage inception level depends on the diameter of the pore and on the pressure and type of gas inside the pore. In DC, conductivity of the walls of the void is certainly the prime parameter. The discharges could increase the local temperature and lead to local thermal breakdown. 1.2.6: Local Thermal Breakdown and Stresses Mechanism: Local disturbances of the current flow due to the differences in conductivity of the heterogeneity and the material, could, in near critical conditions of thermal runaway, cause a local thermal breakdown initiating the failure. And increased locally the temperature and given raise to a thermal stress field. Such a thermal stress field could also be due to the difference between the coefficients of thermal expansion of the material and of the heterogeneity. These stresses could help to start the propagation of a sub-critical micro-fracture. 1.2.7: Local Electrical Breakdown Mechanism:

Local electrical breakdown near a heterogeneity, possibly enhanced by a space charge, due to the lower strength of the heterogeneity or of its interface with the material (then this mechanism would be close to the pore mechanism). 1.2.8: Surface Erosion: It has been noticed that leakage currents in DC conditions produce more severe surface roughening in the areas of the insulator disc just surrounding the metal parts than in AC. conditions, possibly because of the longer duration of the arcs, which do not extinguish twice per period. In many case radial tracks of more or less deep erosions are also superimposed to the roughening. Experience has shown that contaminant sticks to these damaged areas, gets into deeply and can no longer be fully removed by washings, even if accurate. In, polluted sites the areas near to the cap and the pin of DC insulators would become fairly soon both eroded and permanently polluted (cleaning action of wind and rainfall being rather ineffective) so acting as extension of the electrodes themselves, able to increase the current in the insulator body near the cap (zone 2 of Fig. 1.1).

Figure 1.1: Definition of different zones in the dielectric of a cap and pin suspension insulator for the classification of the origination of failures. Zone-1 - most of the failures were observed in zone-1 when the insulators were in clean conditions. Zone-2 - most of the failures were observed in zone-2 when the insulators were in polluted conditions. Zone-3 - failures in this zone are very few


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 1.Electrical body resistance measurements were carried out on number of insulators. 2.Ion migration test and Thermal-runaway tests were carried out as per the standards. Almost all the batches of the insulators have withstood the tests. These experiments suggest that tests as per IEC may not be adequate to assess the quality, since the batches of insulators which are having a steep fall in Electrical Body resistance characteristic also pass the ion migration test and thermal runaway test.

Fig. 1.2 Conditions for thermal runaway of a single insulator unit as a function of voltage, temperature and time II. SCOPE OF EXPERIMENTAL WORK Based on the detailed study by the researchers on Ion-migration and Thermal-runaway, IEC has stipulated the standard for assessing the quality of DC insulator. The standard was published in the year 1995. After the formulation of the standard number of Ionmigration and Thermal-runaway tests were carried out on batches of insulators. Electrical body resistance measurements were also carried out on number of batches of insulators. These tests given in the standards, namely electrical body resistance measurement, ion-migration test, thermal runaway test, and SF6 puncture withstand test have been carried out on batches of both AC and DC insulators. The insulators withstood all the above tests. Therefore, it can be inferred that few of the insulators, which are not having good resistance values at higher temperature, also withstand the tests as per the standard. Hence, it is envisaged that the variation in the parameters of the tests namely temperature and voltage may detect the inferior quality of insulators for DC applications. Accordingly, the temperature of thermal runaway test was increased to 120oC and 150oC. It was thought that inferior insulators might fail in this thermal runaway test. The results of the tests were found to be encouraging. IEC 61325-1995 on ceramic and glass insulators to be used for DC voltages suggest type tests like thermal runaway and ion-migration tests. The ion-migration test, which has been suggested, verifies whether the quality of material is suitable for DC voltage or not. The tests given in IEC-61325, namely electrical body resistance measurement, ion-migration test, thermal runaway test, and SF 6 puncture withstand test have been carried out on batches of both AC and DC insulators. The insulators withstood all the above tests. Therefore, it can be inferred that few of the insulators, which are not having good resistance values at higher temperature, also withstand the tests given in IEC-61325. Therefore, it was envisaged to carry out systematic study which may lead to the modification in the procedure of standard tests. The scheme of experimental work is given below.

1.Therefore it was envisaged that thermal runaway tests if conducted at higher temperature may detect the lower quality insulator. 2.Thermal run away tests are envisaged to be carried out at higher temperatures and volume currents were measured where ever it was possible. 3.Based on Electrical Body Resistance tests, an easy approach for calculating Q50 was derived. Based on detailed experiments thermal runaway tests it was possible to get failures of insulator or the insulators were drawing higher currents whose Electrical Body Resistance characteristics were not good. II.

EXPERIMENTAL SETUP AND TEST PROCEDURES In order to assess the quality of DC insulators many tests have been suggested in IEC 61325-1995 on “Insulators for overhead lines with a nominal voltage above 1000 volts”. The important tests are Thermal runaway test and Ion migration test. The experimental set up and test procedure for conducting Ion-migration, Electrical body resistance measurement and Thermal runaway tests were as per IEC 61325-1995. 3.1: ELECTRICAL BODY RESISTANCE MEASUREMENT: Electrical body resistance measurements were carried out as per IEC 61325-1995. In order to prevent surface leakage currents from introducing errors in the body resistance measurement, the arrangement used is as shown in figure 3.1. All the insulators are kept in an oven of dimensions 9×9×8 ft, as shown in figure 3.1. A 280 kV, 10 mA, two-stage DC source is used as the voltage source for the body resistance measurement. A positive DC voltage of 70 kV was applied to all the insulators. The measurements were made after a preheating of two hours at the temperature of measurement. The current was recorded fifteen minutes after the voltage was applied. 3.2: ION-MIGRATION TEST Ion-migration test given in IEC 61325-1995, takes into consideration that the total charges that would have passed through the volume of insulator in its estimated life of fifty years. This is allowed to pass through the insulator in an accelerated manner, within a few thousands of hours in the laboratory. Ion-migration test is one of the important tests for checking the quality of DC insulator. The ionic impurity like Na+, K +, which may be present inside the volume of insulator,


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 affects the life of the insulator by their migration towards negative electrode. This is significant at higher temperatures and causes non-linear voltage distribution inside the insulator. If the impurity is higher, then the life of the insulator is shorter. The insulators are subjected to a temperature between 90ºC to 130ºC and the voltage applied was between 65 kV to 75 kV.

Hence, it is envisaged that the variation in the parameters of the tests namely temperature and voltage may detect the inferior quality of insulators for DC applications. Accordingly, the temperature of thermal runaway test was increased to 120oC and 150oC. It was thought that inferior insulators might fail in this thermal runaway test.

The insulator should not flashover or puncture during the test. Test voltage is maintained on each insulator until the total of the mean daily charges reaches the expected Q50. When Q50 has been reached on all insulators, they were allowed to cool and then were subjected to a dry DC withstand test under both polarities. The value of the voltage in both polarities was equal to the specified dry DC Positive withstand voltage corrected for atmospheric conditions. Ageing test was also carried out on a batch of insulators under AC voltages.

IV : RESULTS AND DISCUSSIONS Series of experiments were carried out on batches of insulators for Electrical body resistance measurements, Ionmigration tests and thermal runaway tests. The tests were carried out on DC and AC insulators. Tests were also carried out on porcelain, glass and Polymeric insulators. Tests were carried out with AC voltages. Ageing tests were carried out with AC voltages similar to that of Ion-migration on few batches insulators. Thermal runaway tests were carried out as per the standards and also at higher temperatures of 120°C and 150°C. Detailed results of these tests are presented in the following sections. The results of Electrical body resistance test, thermal run away test and Ion migration test are given in the following section. 4.1 STUDIES ON THERMAL COEFFICIENT 4.1.1 ELECTRICAL BODY RESISTANCE MEASUREMENT Electrical body resistance measurements were carried out prior to the Ion-migration tests in order to calculate the Q 50. The positive polarity DC was applied to the pins of all the insulators and the currents of all the insulators were measured during the Ion-migration tests. The resistance of all the insulators was measured at temperatures 90°C, 120°C and 150°C with a tolerance of ± 5°C. The electrical body resistance measurements were conducted on bath of insulators. The procedure for the measurement electrical body resistance gave in the chapter 3, section 3.1. The electrical body resistance measurement was conducted on porcelain insulators of 160kN – AC insulators, 210kN – DC insulators, glass insulators and also on polymeric insulators. Electrical body resistance measurements were carried out at three different temperature viz. 90 oC, 120oC and 150oC. It was observed from the tests that the values of resistance decrease with increase in applied voltage and temperature. The electrical body resistance of DC insulators generally decreases when the temperature is increased. The values vary from about 1000GΩ at 90oC to few tens of GΩ at 150oC. This is confirmed from graphs 4.1 and 4.2. Few of AC insulators gave very low resistance values, see figs. 4.3 and 4.4 and it can be inferred from the results that the variation of resistance with respect to the temperature is wide . In case of glass insulators, the value of body resistance varies from few hundreds of MΩ at 90oC, to few tens of MΩ at 120oC and shattered at 120oC when the voltage was high.

Figure 3.1: Arrangement for measurements and Ion migration test

body,

resistance

. 3.3 THERMAL RUNAWAY TEST This test is performed on insulators to verify the thermal characteristics of insulators. The insulators were placed in a forced air oven, which are heated to a temperature of 80°C. This temperature was maintained for at least 8 hrs for preheating. After 8 hrs of preheating period, a test voltage of 110kV was applied for 8 hrs continuously with the oven at 80°C temperature. After the 8 hour of voltage application period, the applied voltage and oven heaters were switched off. The oven door was kept closed. The insulators kept in position for a period of 30 minutes. After the waiting period, the test voltage was applied to the insulators for 1 minute in order to verify that whether any puncture has occurred. The test procedures given in IEC-61325, 1995 are not sufficient to check the quality of the DC insulators. It was observed in many tests of ion-migration and thermal runaway test that even though the volume resistance is lower, the insulator passes all the tests. If the volume resistance values are higher, which indicates that it is a good DC insulator, then the number of days required for ion-migration test is very high. Therefore, possibilities of failure are very high. Where as weak insulators i.e., with low volume resistance, have to be tested for lesser number of days, and hence the possibility of withstand is more. Many tests were conducted in order to study the above effects.


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 thereafter. In order to verify the consistency, the tests were carried out on more number of batches of insulators. The results are shown in figs. from 4.5 to 4.7.

ELECTRICAL BODY RESISTANCE - DC INSULATORS WITH RESPECT TO TEMPERATURE.

VARIATION OF BODY RESISTANCE WITH RESPECT TO TIME AT CONSTANT TEMPERATURE 150o C.

DC-1 DC-2 DC-3 DC-4 DC-5 DC-6 DC-7 DC-8 DC-9 DC-10

100

100 Resistance (Giga ohm)

Resistance (G ohm)

1000

10 80

100

120

140

160


10

1

Temperature (oC)

0

50

Fig. 4.1.

100

150

200

250

Time (minutes)

Fig. 4.5

ELECTRICAL BODY RESISTANCE - DC INSULATORS WITH RESPECT TO TEMPERATURE.


100

10

BODY RESISTANCE WITH RESPECT TO TIME AT o

CONSTANT TEMPETURE 90 C

10000

RESISTANCE (Giga ohm)

Resistance (Giga Ohm)

1000

1000 DC-1 AC-1 DC-2 DC-3 DC-4 AC-2 AC-3 Poly-1 AC-4

100 10 1

80

100

120

140

160

0

100

200

300

400

500

TIME (minutes)

Tempeture (oC)

Fig. 4.6

Fig. 4.2:

10000 1000 100 10 1

AC-1 AC-2 110

130

Resistance (Giga ohm)


AC Insulatores

90

BODY RESISTANCE WITH RESPECT TO TIM E AT CONSTANT TEM PERATURE 120 OC.

1000

100 DC-1 AC-1 DC-2 DC-3 DC-4 AC-2 AC-3 Poly-1 Poly-2 AC-4

10

150 1

Temperature (oC)

0

100

200

300

400


TIME (MINUTES)

Fig. 4.3

Fig. 4.7

AC Insulatores

4.1.2 ION MIGRATION TEST Ion migration tests were carried out on few batches of HVDC insulators. The procedure for the test is described in section 3.2. The expected charge Q50 and number of days for the test was evaluated as per IEC-1325, 1995. The number of days for ion migration test of all the insulators is given in Tables 4.8 to 4.10 along with the results of body resistance measurements. Table– 4.8 showing the calculation of Q50 and number of days. Voltage = 70 kV, temperature = 1100 C.

60 40 AC-3 AC4

20 0 90

110

130

150

Temperature (oC)

Fig. 4.4 Tests were carried out at 90oC, 120oC and 150oC for about four to five hours on all the insulators and volume currents of the insulators were measured. Generally the value of the resistance decreases and settles after about two hours. In few cases, resistance decreases initially and slightly increases

6

132.9 156.7 130.4 122.8


R120 (G Ohms) 12.68 14.4 13.14 12.46

Slope (A) Between R90 to R120 11173 11351 10913 10880

Q50Total Calculated (Coulombs) 2.02 1.57 2.33 2.51

No. of days 8.89 7.96 10.454 10.671

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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 168.0 189.2 173.6 173.6 179.5 161.5

17.07 20.59 20.96 21.6 17.99 16.68

10873 10547 10053 9910 10939 10796

1.84 1.91 2.65 2.85 1.67 1.99

10.715 13.146 17.95 19.653 10.287 11.249

Average R90 = 1.588*1011 Ω Average slope (A) = 10743.72 Average R120 = 1.675*1010 Ω Average Q cal = 2.076 C.

Calculated R120 = 1.658*1010 Ω Average No. of days = 11.62

803.85 865.05 977.83 702.90 608.27 726.04 803.85 726.04 833.33 807.32 702.90 833.33

103.20 127.83 122.28 109.22 70.53 79.78 103.20 100.90 117.79 96.98 71.20 102.74

9761.17 9092.41 9886.04 8853.78 10245.35 10500.85 9761.17 9384.40 9303.69 10077.16 10888.03 9953.95

Average R90 = 8.05*1011 Ω Average R120 = 1.04*1011 Ω Average Q cal = 0.632 C.

Table – 4.9 Table showing the calculation of Resistance, slope, Q50 and number of days. Voltage + 75 kV, temperature = 1000 C. R90 (G Ohms) 526.32 448.72 583.33 489.50 619.50 466.67 466.67 538.50 552.50 700.00 583.33 636.36 777.78 677.64 912.65

R120 (G Ohms) 53.03 49.41 75.30 98.60 100.97 86.10 85.70 78.90 87.50 124.30 112.30 106.60 137.30 138.15 123.50

Slope (A) 10913.66 10491.36 9735.39 7619.48 8626.54 8036.99 8059.14 9133.06 8763.13 8218.97 7834.73 8496.20 8247.07 7562.24 9511.13

Average R90 = 5.986*1012 Ω Average R120 = 9.717*1012 Ω Average Q cal = 1.47 C.

Q50Total Calculated (Coulombs) 0.5772 0.8301 0.9239 3.1962 1.5117 2.7065 2.6761 1.3490 1.5824 1.6446 2.4013 1.5718 1.4592 2.3779 0.6596

No. of days 20.94 26.48 40.52 137.53 76.43 107.66 106.28 57.10 70.63 96.82 121.19 82.41 95.25 142.25 46.02

Average slope (A) = 8749.93 Calculated R120 = 9.506*1010 Ω Average No. of days = 71.17

Table – 4.10 Table showing the calculation of Resistance, slope, Q50 and number of days. Voltage + 75 kV, temperature = 1000 C. R90 G Ohms

R120 G Ohms

Slope (A)

Q50Total Calculated (Coulombs)

No. of days

833.33 1023.19 977.83 775.59 661.95 803.85 803.85 833.33

6.27 131.58 184.41 76.01 61.81 120.32 101.80 122.95

11370.42 9753.48 7932.66 11045.38 11275.10 9031.30 9826.16 9099.97

0.293 0.522 1.362 0.367 0.386 0.950 0.641 0.886

16.27 40.11 114.43 19.46 17.15 60.54 38.49 58.20

0.662 0.857 0.511 1.188 0.690 0510 0.662 0.883 0.800 0.564 0.437 0.580

39.93 58.45 37.21 67.04 30.40 26.34 39.93 49.46 51.80 33.41 21.24 35.81

Average slope (A) = 9852.12 Calculated R120 = 0.014*1011 Ω Average No. of days = 37.93

The tests was conducted at a test voltage of 70 kV or at 75kV and the corresponding test temperature was maintained constant at 110°C or at 100°C respectively with a tolerance of ± 5°C during the test and currents were recorded at every four hours. The daily charge passing through each insulator was evaluated. Depending upon the average charge per day, which were actually measured, the number of days the voltage shall have to be applied to all the insulators were evaluated. During this period there was no failure either by way of flashover or by puncturing of any insulators, though more charges would have been passed through all the insulators than the charge that would have passed during their life Q50. Based on the results of number of experiments a graph of the product of Q50 and R90 versus Temperature coefficient A was evaluated. By measuring the resistance at 90°C and 120°C and evaluating Temperature coefficient A, expected charge Q50 can be immediately evaluated. The above results suggest that this ready reckoner can be used for evaluating Q50. Ion-migration tests were conducted on number of batches of DC insulators. On analyzing the results it is clear that the number of days of the test depend on the slope of resistance of the insulator with respect to temperature. If the slope is of high value then the number of days is less. It is clear from the results of the tests that the insulators which are having lower resistance at higher temperature has higher slopes. It can be seen from the results that these insulators also pass the test with lower number of test days. It can be inferred from the results that though the insulators have higher values of resistance and better slopes, the test duration is higher. The insulators having lower values of resistance and higher values of slopes have less number of test days. Therefore, it is suggested that after ascertaining standard values of resistance and values of slope, standard value of Q50 can be recommended. 4.2 CONCLUSIONS Ion migration tests were carried out on batches of porcelain insulators and there were no flashover or puncturing of insulators during the tests. Charges, more than that would have flown in the life of insulators have been passed through all the insulators and there were no failures during the test. The ready reckoner of product of Q50 and R90 v/s temperature coefficient A was evaluated which can be used to arrive at


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017

Insulators

R90oCG 

R120oCG 

Q50 Col

No. of days

DC-1

132.9

12.68

2.02

8.89

DC-2

156.7

14.4

1.57

7.96

DC-3

130.4

13.14

2.33

10.45

DC-4

122.8

12.46

2.51

10.67

DC-5

168

17.07

1.84

10.71

DC-6

189.2

20.59

1.92

13.14

DC-7

173.6

20.96

2.65

17.94

DC-8

173.6

21.6

2.85

19.65

DC-9

179.5

17.99

1.67

10.29

DC-10

161.5

16.6

1.99

11.25

value of Q50 expeditiously. Even though the insulators have higher values of resistance and better slopes, the test duration is higher. The insulators having lower values of resistance and higher values of slopes have less number of test days. Therefore, it is suggested that after ascertaining standard values of resistance and values of slope, standard value Q50 Insulators R95oCG 

R120oCG 

Q50 Col

No. of days

67.094

11.67

16.322

97.96

AC 2

67.094

12.13

17.921

109.03

AC 3

100.634

12.13

4.564

36.12

AC 4

100.634

10.67

3.393

25.67

AC 5

67.094

10.67

13.161

76.55

AC 6

67.094

10.76

13.429

78.33

AC 7

100.634

12.72

5.098

41.03

AC 8

100.634

11.67

4.172

32.57

AC 9

100.634

12.72

5.098

41.03

AC 10

67.094

14

25.427

162.69

AC 1

can be recommended. 4.3 ION-MIGRATION TEST: Ion migration test were conducted further on porcelain AC and DC, and also on polymeric insulators. The procedure for the ion migration test is as described in the section 3.2 Ion-migration test conducted further on batches of AC and DC insulators. The electrical body resistance measurements were carried out as per procedure given in section 3.2. The batch of insulators includes 160 kN–AC insulators, 210 kN–DC insulators, and also polymeric insulators. The arrangement is shown in figures 3.1 and 3.2. It was observed from the test that the values of resistance decreases with increase in applied voltage and temperature. It was observed that all the AC insulators have passed the test even though the charge passed is much higher than that of calculated ones. The test temperature was maintained at 120oC. In order to estimate the number of hours for ionmigration test, electrical body resistance measurements were carried out for the batch of insulators and volume currents were measured for AC and DC insulators and total charge that would through the insulators, were estimated. The number of days for which the voltage has to be applied was calculated.

No failure occurred during this test. DC ion-migration test was also carried out with AC insulators and the results are given in table 4.11 similarly ion-migration test was carried out for DC insulators with DC voltage and the results are given in table 4.12. Table 4.11 Table showing the number of days as per the average charge per day

Table 4.12 Table showing the number of days as per the average charge per day

4.4 THERMAL RUNAWAY TEST: The thermal runaway tests were carried out on batches (each batch consisting of 10 numbers) of insulators using the procedure described in section 3.3. Thermal runaway test, were conducted on both ceramic and non-ceramic insulators. The total current (both surface and volume current) was monitored for all the insulators. Tests were carried out at temperatures of 90o C, 120o C and 150o C. The values of the volume current of all the insulators were monitored, and the electrical body resistance values are evaluated. The values of the voltage during the measurement of currents were 70 kV and not 110 kV. This is because of the fact that flashover were occurring at higher voltages. During the measurement of volume currents in the thermal runaway test, it was evident that AC insulators were drawing higher currents at 120o C. Two of the AC insulators were drawing currents more than 2 mA at 70 kV. In order to assess the quality of insulators, for DC applications, thermal runaway tests were carried out at 120o C and 150o C instead of 80o C, which was stipulated in the standards. Thermal runaway test were carried out, with 4 hours of preheating and 4 hours of voltage applications at constant temperature of 120 o C and 150o C on glass, ceramic and non-ceramic insulators. During this period glass insulators have shattered This failure of glass insulators suggests that, inferior insulators fail at thermal runaway test, if carried out at higher temperature i.e. at 120o C and 150o C. Later thermal runaway test were carried out at 120o C and 150o C for 8 hours of preheating and 8 hours of voltage application. The tests were carried out after removing the failed insulators. It was not possible to carry out the test at higher voltages, especially when the temperature was about 150o C, therefore input voltage was reduced significantly from 85 kV to 30 kV or even to 10 kV in some cases. One of the AC insulators failed during thermal runaway test at 150o C. Thermal runaway test was carried out at a temperature of 80oC and as per IEC-1325, 1995 on batch of insulators.


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National Conference on High Voltage Engineering & Technology (NCHVET2017), 27-28 January 2017 Three of the AC insulators were drawing higher currents during this test. The above results are very significant for assessing good quality for DC applications. The insulators gave varied body resistance values at 90oC and 120oC. Many insulators failed at thermal runaway test at 120 oC and 150oC. These failed insulators were drawing higher currents at 90oC and at 120oC. Though the tests specified in standards check quality of DC insulators, it seems that almost all the insulators including the tough AC pass the specific test. From the above study it is possible that Thermal runaway test at higher temperature may detect faulty insulator for DC applications. Therefore it is recommended that the test procedure has to be modified and Thermal runaway test have to be carried at higher temperature than specified temperature in the standards.

REFERENCES [1] Peixoto, G.Marrone, L.Paragamin and C.Carra, “ Failure of transmission line cap and pin insulator under DC stresses”, IEEETrans. P.D. Vol PWRD-2, No.-1 PP777782, 1988. [2] K.Naito, S.Mastu, and Y.Suzuki,” Special aspects of insulators for HVDC Transmission lines and stations”, CIGRE SC 33-93(Col 1) IWD, PP2.12. [3] R.Mailfert, L.Paragamin and D.Rivere, “Electrical reliability of DC line insulators”,- IEEE Trans. EI-16, No.3,PP276, 1981. [4] D.Durmora, L.Paragamin and R.Parrand, “ State of art concepts of insulators strings for HVDC lines”, CIGRE 1- 106-114, 1991. [5] T. Kawamura, K. Nagai, T. Seta and K. Naito,“DC Pollution performance of insulators”, CIGRE, , Paris, France, Report 33-10, 1984. [6] M. Fazelian, C. Y. Wu, T. C. Cheng, H. I. Nour, and L. J. Wang, “A study on the profile of HVDC insulators—DC flashover performance,” IEEE Trans. Elect. Insul., vol. 24, no. 6, pp. 119–125, Feb. 1989. [7] Ravi. K.N. “ Pollution ageing studies of insulators under DC Voltage”, Ph.D Thesis 1995. [8] N.Vasudev, A.K.Mujumdar, K.N.Ravi, N.S.Mohan Rao, Channakeshava. “Study of the results of ion-migration in insulators subjected to DC voltages”. CPRI., Bangalore [9] V.Muralidhara, B.Ramachandra, N.Vasudev, P.V.Vasudevan Nambudiri, K.N.Ravi and Sriramulu “ Study of Thermal-runaway tests on Insulators subjected to DC voltages” International Confernce on High Voltage Engineering and Applications. Chon gqing, China. November 9-13, 2008

BIOGRAPHIES B. Mallikarjuna obtained B.E Degree from Gulbarga University and M.E Degree from Bangalore University during the year 1992 and 1997 respectively. He is working as the Assistant Professor at RNSIT, Bangalore with teaching experience of 22 years. His area of interest is High voltage Engineering. V.Muralidhara obtained B.E., M.E. Degree from Mysore University, Mysore during the year 1975 and 1978 respectively. He worked as a lecturer at PESCE, Mandya from 1976 to 1981 and Joined Bangalore Institute of Technology (BIT) Bangalore during 1981 and at present Associate Director, School of Engineering and Technology, Jain University, Bangalore. He published papers in International and National Journals / conferences and having 40 years of teaching experience. His area of interest is HV Engineering and Quality analysis of DC insulators.

N.Vasudev obtained B.E , M.E, PhD Degree from the University of Bangalore, Mysore and Bangalore during the year 1982, 1986 and 1999 respectively . He worked as lecturer at RV College Of Engineering during 1982 to 1984. Joined CPRI High voltage Division during the year 1987 as an Engineering Officer. At present he is the Head of HV Division. His areas of interest are design of external insulation from the point of view of pollution. Ageing studies on polymeric AC and DC insulators under polluted conditions. He has more than 50 publications at National and International forums. He is an IEEE member. K.N.Ravi born in Salem, India during the year 1956. Received BE degree in Electrical Engineering from Bangalore University during 1978. Received ME degree in Electrical Engineering with specialization in High Voltage from Indian institute of Science during 1981. Obtained Phd degree from Indian Institute of Science during the year 1995 for the thesis “Pollution ageing studies of Insulators under DC voltages”. Received Badkas medal during 1996 for best thesis. Joined CPRI during 1982 and was working in High Voltage Division till July 2007. His areas of interest are Design of external insulation from the point of view pollution for AC and DC voltages, Pollution Performances of DC insulators, lightning arrester and polymeric insulators. Presently working as Prof and Head of Electrical and Electronics Engineering of Sapthagiri College of Engineering, Bangalore. He has published more than 50 papers in national and international conferences.


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