Studies on hyper scaling dielectric loss of grading capacitors of 500kv breakers Abstract: To solve the problem of hyper scaling dielectric loss of grading capacitors for 500 kv high voltage substation breakers under the test voltage of 10kv, characteristics of the capacitors were analyzed and dielectric loss tests under a higher voltage more than 10 kv were conducted in a laboratory. The experimental data shows that the Garton effect existed in the capacitors is the cause for the problem. In addition to the high voltage and magnetic field interferences, a test with a normal voltage of 10 kv much lower than the rated voltage cannot reflect the real insulation condition of the capacitors. However, by raising test voltage or conducting the test under rated voltage, the influence of Garton effect can be eliminated, and the real insulation condition of capacitors in the normal working environment can be surveyed. This method may provide references for field tests and analyses of grading capacitors of 500kv breakers. Key words: grading capacitor; dielectric loss; Garton effect; insulation condition 0 Introduction Dielectric loss factor is an important indicator to reflect the insulation performance of grading capacitor between open contacts of circuit breaker. By measuring the dielectric loss value, to discover the defect of the insulation,which was damp overall, dirty or deterioration of oil or dipped material. Due to there is high voltage and magnetic field interferences in the testing site, the Garton effect exists in the import grading capacitors, the dielectric loss test in 10 kv voltage could not reflect the insulation condition of the equipment. Develop the dielectric loss test in high voltage, and with testing of tanδ U curve, can reflect the insulation condition of the equipment exactly. According to National Standards of People s Republic of China GB/T 4787 1996, about the dielectric loss angle tangent of grading capacitor of breaker, which regulates: at the rated voltage and rated frequency, the dielectric loss value should not be more than 0.002 under 20. 25 items Counter Measures requires that there should be high voltage dielectric loss test at site. Specific to the dielectric loss value under the close to the working voltage is less than the dielectric loss value under the 10 kv. In order to analyze and judge the insulation status of capacitor more exactly, carry out the dielectric loss test under the increased test voltage, and carry out the test of tanδ-u curve. 1 The principle and significance of testing of dielectric loss angle In order to analyze easily, we usually treat the insulation medium as a parallel circuit with equivalent resistance R and equivalent capacitance C. The current I flow through the insulation medium, consist of active current I R flow through R and reactive current I C flow through C. As shown in the Fig. 1.
The relationship between dielectric loss P and dielectric loss angle δ, see the formula (1), the tanδ of good insulation medium won t increase significantly as the increasing of voltage. If there is defect in the insulation medium, especially there exists air gap, the tanδ will increase significantly as the increasing of voltage, as shown in the Fig.2. In the formula: tanδ is the dielectric loss factor, P is the dielectric loss of the insulation medium, W; U is AC voltage RMS of the sample, V; w is frequency of power angle. 2 Test analysis of Garton effect 2.1 Garton effect In 1940, Mr. Garton found that in the insulation medium containing paper (or plastic & oil), the change of dielectric loss value under low voltage is more than the change of dielectric loss value under high voltage. Because under the effect of alternating electric field, the movement of oil type colloid charged particles was obstructed by paper fiber, and decreases with increasing electric field intensity. In low voltage, the micro contaminant of medium free in the medium, and polarization loss is larger, while in high voltage, the micro contaminant of medium concentrated on both ends of the
electrodes, and the dielectric loss is smaller, as shown in Fig.3. which is called Garton effect of Insulation Medium. Impurity distribution under low voltage Impurity distribution under high voltage 2.2 High voltage dielectric loss test of 280kv grading capacitor Choosing a 280kv grading capacitor with hyper scaling dielectric loss value under 10kv test voltage, to proceed the high voltage dielectric loss test, and there is no interference of high voltage and high magnetic fields, and the test uses dielectric loss bridge with external high voltage standard capacitor. The test voltage of grading capacitor was regulated by external voltage regulator, and rise the test voltage with boosting transformer (see Fig.4, Cn is rated value, Cx is measured value). With the increase of the test voltage, the dielectric loss value of capacitor is gradually lower, the capacitance is essentially constant. When the test voltage increase to a certain value, the dielectric loss value of the test sample is essentially constant, and with continued increasing of voltage, the dielectric loss is constant. From test data of table 1 and tanδ U curve of Fig.5, the dielectric loss value under the working voltage is much less than the value under 10kv test voltage, and after increasing the test voltage, the dielectric loss value meet the national standards. Experiment capacitor standard capacitor exciting transformer Fully shielded cable 0 400V voltage regulator 220V power supply Cn outlet Cx outlet Dielectric loss instrument
As there is no interference of high voltage and high magnetic fields, then the main reason for this result is insulation medium with conductive impurities cause ion conductance loss under the AC voltage. In the low voltage, the impurities of medium are weakly obstructed by paper fiber, and free in the medium, as a result of that, polarization loss is larger and the overall dielectric loss value is larger. While after increasing the test voltage, under the effect of high voltage, the impurities get rid of the obstruct of paper fiber, and concentrate on both ends of the electrodes, as a result of that, polarization loss is smaller, and the overall dielectric loss value is lower. When the test voltage increase to a certain value, most charge particles of the medium concentrate on both ends of the electrodes, and the active loss of the test sample is basically stable. About the above analysis, grading capacitor with Garton effect disturb the judgement of insulating property, and the tester mistake there are internal defects in test sample. Possibly make the qualified capacitor out of service by mistake, delay the time of power transmission, and reduce the stability and economy of power system. 3. Studies on high voltage dielectric loss of grading capacitors of 500kv breakers 3.1 routine dielectric loss test of grading capacitors of 500kv breakers
In Oct 2010, test for No.1 main transformer and switches of low, medium, and high voltage of a transformer substation. 10kv dielectric loss test for grading capacitor of No.5012 breaker & No.5013 breaker, adopt positive wiring, and wiring as Fig.6. This test, finding that the dielectric loss value of grading capacitor of No.5012 breaker s two sides of A phase, TA side of B phase, TA side of C phase, and No.5013 breaker s TA side of B phase, is obviously higher than over years test results( partial data see table 2). The grading capacitors are all paper oil insulation, under the 10kv test voltage, the tanδ value of grading capacitor between open contacts of circuit breaker go beyond to the standard, and the standard is the value should be less than 0.5% from Electric Power Equipment Handover and Preventive Test Procedures by NORTH CHINA GRID COMPANY LIMITED. Shield of high voltage Core wire of high voltage Shading ring C X core wire C X shield Ground Grading capacitor year No.5012 breaker TA side of A phase No.5013 breaker TA side of B phase 3.2 high voltage dielectric loss test of grading capacitor of 500kv breaker In the recent years, in the pilot test of Changzhi Power Supply Branch, during the tests of grading capacitor of 500kv breaker, there are several cases that the test results of dielectric loss value exceed the standard. Because there are interference of high voltage and high magnetic field, which will influence the judgement of insulation of capacitor. Based on the previous experience, build the dielectric loss test circuit of high voltage, wiring method as Fig.4, and use test voltage regulator with single frequency regulating mode 40/50/60Hz. Under the test voltage of 50HZ, the dielectric loss value is negative, because there are interference of high magnetic field and high electric field
of 50HZ power frequency, and test voltage is power frequency high voltage which could not anti interference, as a result of that, the test result is negative. When under the test voltage of 60HZ, the test results of increasing voltage obviously lower than the test results of 10kv (partial data see table 3). So suspect the hyper scaling dielectric loss of grading capacitor of 10kv test voltage is caused by Garton effect. No.5012 breaker TA side of A phase No.5013 breaker TA side of B phase So, grading capacitor of high voltage breaker, adopt high voltage dielectric loss bridge and use positive wiring method & pilot frequency 50±1Hz anti interference to proceed the dielectric loss method. As the limit of the test condition, choose the standard capacitor of 160kv, and exciting transformer with 100kv high side voltage, and test wiring as Fig.7. Test all dielectric loss value of each voltage grade after increasing and decreasing the voltage, here choose two grading capacitors separately hyper scaling dielectric loss and not hyper scaling dielectric loss (the dielectric loss value should be less than 0.5% under 10kv), and the test results (see table 4) and the data from tests develop tanδ U curves of grading capacitor (see Fig.8&9). After analyzing, when the test voltage increase, the dielectric loss value tanδ decrease, and when the test voltage increase to a certain value, the dielectric loss value basically stable. Standard capacitor Experiment capacitor Fully shielded cable Cn Outlet HV Dielectric loss instrument Cx Outlet Ground Null line
No.5013 HV breaker Grading capacitor TA side of B phase No.501 HV breaker Grading capacitor TA side of C phase 3.3 Studies on high voltage dielectric loss of grading capacitors of 500kv breakers From the above test results and tanδ U curves, adopt the pilot frequency 50±1Hz antiinterference, when increase the test voltage of two experimental article capacitors, the dielectric loss value decrease, and when the voltage increase to 50kv, the dielectric loss value is stable, and all the dielectric loss value won t exceed to 0.5% under the HV test voltage. The capacitance stay stable no matter the test voltage increase or decrease, which means the insulation is good. The reason for this phenomenon is oilpaper combination insulation capacitor occur Garton effect, and change the distribution of charged impurities of the insulation medium. As the grading capacitor of HV breaker running normally, the two poles at the same potential, and after running for a long time, the charged particles are obstructed by paper fiber and free in the
medium, so under the low test voltage the dielectric loss value increase. When the test voltage increase, the obstruction by paper fiber will be weaker, and concentrate on two poles of capacitor, and the acting of charged particles will decrease, so the dielectric loss value will decrease. When continue to increase the test voltage, the dissociative charged particles will mainly concentrate on two poles of capacitor, so the dielectric loss value will not change obviously. Analyzing from above tests, the dielectric loss test of capacitor under the 10kv, if the dielectric loss value exceeds the standard, we could consider to test it under high test voltage. Under the high test voltage, if the dielectric loss value decrease obviously and less than the specified value, and when the voltage increase to a certain value, the dielectric loss value keep stable basically, we could confirm that capacitor occur Garton effect, which means the insulation is good. Under 10kv test voltage to test the dielectric loss value, and the capacitor, the spare capacitor, have not been applied voltage for several months, and the dielectric loss value exceeds the standard, we could make the capacitor under the high test voltage for several hours, and then test the dielectric loss value under 10 kv, if the result is lower than the specified value, which means it occurs Garton effect. In order to make sure the equipment running safely, we should shorten the test period properly. 4. Conclusion Aiming at the phenomenon of the hyper scalling dielectric loss of grading capacitor under 10kv test voltage, this article analyze it intensively. Test the high voltage dielectric loss for the capacitor whose dielectric loss value exceed the standard, and make the tanδ U curves and analyze it, the result shows: due to the limit of test condition and the difference of material of experimental capacitor, it occurs Garton effect easily during the test, which influence the judgement of insulation seriously. While increasing the test voltage of dielectric loss test could identify the Garton effect exactly. This test method will be a great reference for field test and insulation analysis of grading capacitor of high voltage breaker.