Influence of different test methods on the accuracy of high current busbar insulation resistance

Compared with the traditional cable, the high current busbar fully embodies its superiority in the process of current transmission, and thus its insulation performance test is of great significance to the safe operation of the line. In this paper, the insulation resistance of the high current busbar is measured by the basic test principle and the voltage injection test method respectively, and then the accuracy and precision of the two test methods are compared. The test results show that the relative error of insulation resistance of positive and negative busbar fluctuates in the range of 0.35%-4.20% and 0.10%-3.33%, respectively, while that of voltage injection test fluctuates in the range of 0.60%-2.80% and 1.10%-1.80%, respectively. Regardless of the basic test principle or voltage injection test, the relative error of insulation resistance in the high current busbar is less than 5%, and the voltage injection test method is more accurate. Compared with the basic test principle, the maximum relative error of Rn and Rp is reduced by 0.87% and 1.00% respectively.


Introduction
With the emergence of modern engineering facilities and equipment, the electricity consumption of all walks of life has increased rapidly [1][2].High high-current busbar is the main device for distributing electric energy.Compared with the traditional cable, it fully reflects its superiority in high current transmission and plays a very important role in the whole power transmission and distribution [3][4].When the busbar fails, it directly affects the safe and reliable operation of all devices connected to the busbar, which results in a large area of accidental power outage or serious damage to equipment, and then it is of great harm to the entire power system [5].Therefore, the insulation performance test of the high current busbar is of great significance to the safe operation of transmission lines [6][7].
It is generally believed that insulation resistance is the standard for determining the insulation performance of electrical systems [8].In this paper, the insulation resistance of the high current busbar is measured by the basic test principle and the voltage injection test method, and the accuracy of the two test methods is compared.Then the advantages and disadvantages of the basic test principle and the voltage injection test method are analyzed.This lays a foundation for the wide application of the insulation test technology of the following high current busbar.doi:10.1088/1742-6596/2728/1/012062 2

The testing principle of insulation resistance
It is assumed that the DC system voltage (the total battery voltage) is U, the insulation resistance between the positive and negative busbar and the ground is Rp and Rn, respectively, and the voltage between the positive and negative busbar and the ground is Up and Un, respectively.The equivalent model of the DC system is shown in the dotted box in Figure 1   In Figure 1, Rc1 and Rc2 are standard resistances during the measurement.The basic principle of insulation resistance measurement is as follows: When S1 and S2 are all disconnected, voltages between positive and negative busbar and ground are Up0 and Un0, respectively, which can be obtained from circuit law [11].
When S1 is closed and S2 is disconnected, the standard bias resistance Rc1 is added between the positive busbar and the ground, and voltages between the positive busbar and negative busbar and the ground are Upp and Unp, which can also be obtained [11].

The test results analysis of insulation resistance
In the insulation resistance test system, the measurement parameters and calculation results are sent to the monitoring computer by CAN interface to record data.After testing, RP0 and Rn0 are measured by the test system, and the results show that both resistance values are 6.5 MΩ because the measurement limit of the system is 6.5 MΩ.When the measured resistance exceeds the upper limit, the system will also display the upper limit, so that the initial insulation condition is good.
When the incorporated resistance RnI between the negative busbar and ground remains unchanged at 2, 000 kΩ, the incorporated resistance RpI between the positive busbar and ground gradually increases from 25 kΩ to 1, 000 kΩ.The insulation resistance RpM and RnM of positive and negative busbars are obtained by testing.The insulation resistance RpC and RnC of positive and negative busbars are calculated theoretically.The test resistance incorporated between positive and negative busbar and ground are RpI and RnI, respectively.The comparison results are shown in Figure 2. As can be seen from Figure 2, when RnI remains unchanged at 2, 000 kΩ, there is little difference between the test value RnM of the negative busbar and the theoretical calculation result RnC, and between the test value RpM of the positive busbar and the theoretical calculation result RpC.The maximum relative error is 0.41%.It shows that the basic test principle of busbar insulation resistance is accurate, and this method can be used for follow-up tests.In addition, by comparing the maximum relative errors of RnM and RnI, as well as RpM and RpI, it is found that the maximum relative errors are 4.35% and 1.80%, respectively, which are both within the measurement accuracy range of 5%.To further verify the accuracy of test results, RpI is kept unchanged at 2, 000 kΩ, and then RnI is gradually reduced from 1, 000 kΩ to 25 kΩ.RpM, RpC, and RpI, as well as RnM, RnC, and RnI, are compared respectively, as shown in Figure 3.As can be seen from Figure 3, when RpI remains unchanged at 2, 000 kΩ, there is little difference between the test value RpM of the positive busbar and the theoretical calculation result RpC, and between the test value RnM of the negative busbar and the theoretical calculation result RnC.The maximum relative error is 0.41%.The accuracy of the basic test principle of busbar insulation resistance is explained again.In addition, by comparing the maximum relative errors of RpM and RpI, as well as RnM and RnI, it is found that the maximum relative errors are 4.65% and 2.80%, respectively, which are both within the measurement accuracy range of 5%.This is one of the most used insulation resistance detection methods of high-current busbars, which can directly calculate the busbar insulation resistance.However, the method has some disadvantages, such as reducing the insulation performance of high current busbar and no solution in the theoretical calculation in special cases, which has some influence on the test and theoretical calculation results of insulation resistance.

The principle of voltage injection insulation test
Figure 4 shows the schematic diagram of voltage injection insulation detection [12][13][14].The specific measurement process is as follows: 1) When S1 and S2 are closed, and then when S3 and S4 are disconnected, the insulation resistance ratio of positive and negative busbar to the ground is shown in Formula (5) [15]: 2) When S2 is disconnected and S1 is closed, the insulation resistance between the positive and negative busbar and the ground is obtained, as shown in Formula ( 6) and Formula ( 7) [15]: where U1' and U2' are the voltages on R3 and R4, respectively.

Result analysis of voltage injection insulation detection
To test the effectiveness of the voltage injection insulation detection, the test platform was built, as shown in Figure 5 [15].The NiMH 144 V/6 A battery pack is selected to simulate the access resistance test board between the positive and negative busbar and the ground.The insulation resistance Rn and Rp are compared with the actual value Rs, as shown in Figure 6.It can be seen from Figure 6 that during the process of Rs gradually rising from 8.20 kΩ to 300 kΩ, the maximum absolute error of Rn is 10 kΩ and its corresponding relative error is 3.33%, while the maximum absolute error of Rp is 4.20 kΩ and its corresponding relative error is 4.20%.

Accuracy analysis of high current busbar during insulation resistance testing
Based on the results obtained from the basic test principle and the voltage injection test method, the relative error between Rn and Rp is shown in Figure 7.As can be seen from Figure 7, for Rn, the relative error obtained by using the basic test principle fluctuates in the range of 0.60%-4.35%,while the relative error obtained by using the voltage injection test fluctuates in the range of 0.10%-3.33%.At the same time, for Rp, the relative error obtained by using the basic test principle fluctuates in the range of 1.1%-4.65%,while the relative error obtained by the voltage injection test fluctuates in the range of 0.35%-4.20%.
To sum up, for the basic test principle and the voltage injection detection method, the relative error of insulation resistance in the high current busbar is less than 5%.However, compared with the two test methods, it is found that the voltage injection insulation detection method is more accurate.Compared with the basic test principle, the maximum relative error of Rn and Rp is reduced by 1.02% and 0.45%, respectively.

Conclusion
In the paper, the insulation resistance of a high-current busbar is tested by the basic principle and the voltage injection detection method, and then the accuracy of the two test methods is compared.The conclusions are as follows: IOP Publishing doi:10.1088/1742-6596/2728/1/0120626 1) Based on the basic principle of insulation resistance, the maximum relative error of a high current busbar can reach 4.65%, and the test accuracy is less than 5%; 2) The maximum relative error of insulation resistance obtained by the voltage injection detection method can reach 4.20%, and the test accuracy is also less than 5%; 3) The maximum relative error difference of the high-current busbar is 1.02%, which shows that the voltage injection insulation detection method is more accurate in the high-current busbar insulation performance test. [9][10].

Figure 1 .
Figure 1.Basic schematic diagram of insulation resistance measurement.

Figure 2 .
Figure 2. Test results of actual operation when RnI is fixed.

Figure 3 .
Figure 3. Test results of actual operation when RpI is fixed.

Figure 4 .
Figure 4. Schematic diagram of voltage injection insulation detection.

Figure 5 .
Figure 5. Schematic diagram of voltage injection insulation detection.

Figure 6 .
Figure 6.Comparison between insulation test and actual value.

Figure 7 .
The relative error changes of Rn and Rp under different test methods.