The implementation of solar power systems globally has continued to increase in the past decade, presenting new opportunities for measuring the insulation resistance of those systems in the course of their maintenance and management.
By Masao Higuchi, Application Engineer, Field Design Department, Hioki E.E. Corporation.
The implementation of solar power systems globally has continued to increase in the past decade, presenting new opportunities for measuring the insulation resistance of those systems in the course of their maintenance and management. This situation has led to the ongoing development of maintenance inspection guidelines that govern how insulation resistance testing is carried out.
IEC62446-1 is the international standard for PV generation systems, and is divided into three categories: “Category 1”, required for all systems, and “Category 2” and “Additional Tests”, required for larger or more complex systems. Despite the additional time and costs required, however, in order to obtain optimal performance, “Category 2” and “Additional Tests” are recommended even for small or simple PV systems.
Within Category 1 testing, insulation resistance testing is required assure electrical safety by showing sufficient insulation between electricity-conducting components and the PV panel’s frame, or between the PV module and the external environment. Improper insulation can lead to electric shocks and other hazards.
There are two methods to test insulation resistance, one of which is to short circuit the positive and negative electrodes of the PV string before measuring the insulation resistance between the shorting point and earth. The other is to measure the insulation resistance between the positive electrode and earth, and then between the negative electrode and earth separately without short-circuiting. Both methods are described in IEC62446-1 standard, so either can be used. However, for each method, there are several issues to consider.
A PV module is considered a constant current source, so electrically it is not a problem to short-circuit the positive and negative electrodes. Once the electrodes are shorted, the insulation resistance can be measured correctly using general standard insulation resistance testers.
However, careful attention must be paid on how to short-circuit the electrodes. To short the positive and negative electrodes, a large switch with sufficient integrity to shut down the discharge voltage and short-circuit current must be used. If a short is performed without using such a switch, a DC arc may be generated, which may lead to fire and/or electrical shock. As such, be sure to arrange an appropriate switch to short the two electrodes. This method presents an accurate, but still unsafe way to test for insulation resistance, with the only alternative when using the short-circuit method is to measure during the night when the PV panels are offline and not generating any energy.
The second method of testing insulation resistance is to test without shorting the electrodes. With this method, risk of electrical shock will be minimised, but in turn, you risk the possibility of obtaining incorrect measurement values due to the testing method used by general insulation resistance testers. This is caused by the electric potential existing in the PV module. General insulation resistance testers are designed to measure an object that has no electric potential. When electric potential is present, the measured value can be higher or lower than the actual value depending on the measurement conditions.
Figure 1 illustrates an insulation resistance test without short-circuiting the electrodes using a tester that can output negative test voltage. The insulation resistance is tested between the positive electrode and earth while the negative electrode of the PV module is connected to the earth fault. The diode circled on the left is the PV module, while the diode not circled is the bypass diode. Because the negative electrode is connected to the earth fault, when the insulation resistance tester is connected between the positive electrode and the earth, this in turn connects the earth fault to the tester.
Subsequently, a closed loop is created to flow the current generated by the PV module, and this current flows to the insulation tester. Since the insulation resistance tester outputs a negative test voltage, the direction of the measured current and PV generated current will be the same. The tester will detect a higher current due to the additional current generated by the PV module and calculate a lower insulation resistance than the actual value, resulting in a false negative.
Figure 2 shows a situation opposite of the previous example, where there is an attempt to measure the insulation resistance of the negative electrode while the positive electrode is connected to the earth fault. A closed loop to flow the current generated by the PV panel is once again created by the meter and the earth fault resistance, causing the PV-generated current to flow to the insulation resistance meter. However, in this case, the directions of the measured current and PV-generated current are opposite one another, causing some of the measured current to cancel out. As a result, the calculated insulation resistance is higher than the actual value through the detection of the lower current, resulting in a false positive. In the worst case, the insulation resistance can be incorrectly displayed as “infinity”, or maximum resistance, despite the earth fault.
Figures 3 and 4 illustrate ways to measure insulation resistance accurately using a general insulation resistance tester. As you can see, in both examples, there is no closed loop to allow the flow of current generated by the PV panel. Since the current has no path to flow to the tester, the insulation resistance can be accurately measured even when there is an earth fault. Naturally, good insulation resistance can also be tested accurately when there is no earth fault.
These differences and problems exist because general insulation resistance testers are not designed to measure objects containing electric potential. However, PV modules by design are always charged with electric potential as long as there is sunlight, so this problem has always been evident; yet, it not addressed in the IEC 62446-1 standard. To resolve those issues, Hioki has developed an insulation resistance tester that features a special function for when performing PV maintenance.
Models IR4053 Insulation Resistance Meter features a PV resistance mode in addition to the standard insulation resistance measurement function. By using PV resistance mode, the insulation resistance can be measured without the effect of the PV-generated current, resulting in accurate measurement under any system conditions. The IR4053 achieves this by measuring the voltage and current produced by the PV module as it generates electricity, and then subtracting those values from the measured current before dividing it by the applied voltage V, minus the PV-generated voltage, to calculate the resistance.
Measuring the insulation resistance of photovoltaic systems while they are continuing to generate electricity from the sun pose potential hazards if the short-circuit method is used. On the other hand, using the non-short-circuit method stipulated by the IEC 62446-1 standard leads to inaccurate results due to the additional energy introduced into the measurement system by the PV panels themselves. The Hioki IR4053 Insulation Resistance Meter provides a solution by taking into consideration that very energy into the calculation of resistance to deliver the correct insulation resistance value of PV panels, ensuring safety both during measurement and handling of the PV panels upon proper testing.
READ MORE INSTRUMENTATION & MEASUREMENT
WANT MORE INSIDER NEWS? SUBSCRIBE TO IAA NOW!
CHECK OUT IAA’S CURRENT AND PAST ISSUES: DIGITAL MAGAZINE