For PV system, Earth issue of PV system is very common on solar power plant. The reason is that most of time, PV cables have been damaged during the engineering . Some are severe and easy to be found, while some of them just have small defetcts. These potential earth issues will affect or damage inverter,even combiner box or inverter may be burned in severe situation. In the article, we will introduce how to check earth issue fast via multimeter.
Testing Principle Introduction
Photovoltaic effect, a phenomenon in which an electromotive force is generated when a semiconductor is irradiated with light
Built-in electric field inside semiconductor silicon will drive electronic and hole into positive or negative side, then form PV voltage.
So the voltage between positive and earth, negative and earth should be same , and equal half of PV voltage in the instaneous time. Meanwhile, the voltage is not real , and they are virtual voltage. then it will decline until 0. As earth is not connected to positive or negative side directly, when you put multimeter on them, it will have the process like discharging. That’s why it will drop unitl 0 voltage.
1 Detecting earth issue via multimeter firstly
a. Turn off Inverter or Make inverter stop running
b. Disconnect PV system from inverter
c. Put red tong on Positive cable of PV wire, black tong on earth . wait for a while, then record the voltage
d. Put black tong on negative cable of PV wire, read tong on earth, wait for a while , then record the voltage
Based on Photovoltaic effect, if normal PV string, DC voltage between positive and earth, negative and earth will be half of PV voltage in the instaneous time, once you put multimeter on them instaneously, you can get the reading from multimeter, then it will decline until 0.
If abnormal PV string, you can see inbalance on the two voltages, including the voltage between postive and earth, voltage between negative and earth. The most severe situation, you will see that one side has PV voltage, while the other side is 0. In the situation, definitely , earth issue exsists. if voltage between positive and earth is 0, it means positive cable has earth issue, if voltage between negative and earth is 0. it means negative cable has earth issue.
You can test every PV string unitl you find abnormal one.
2 Confirm the earth issue further via megger or insulation resistance testers
How to choose megger
The meggers are commonly used in three types: 500V, 1000V, and 2500V according to cable withstanding voltage. If high-voltage megger is used for low voltage device or cable, the equipment or cable under test may be damaged. If the working voltage of the equipment is above 500V and below 3000V, 1000V megger can be used. 2500V megger can be used to measure 3000V and above
a. Before use, check whether the insulation layer of the connecting wire of the megger is in good condition and whether there is any damage.
b. Check whether there is any sliding wire on the fixed terminal of the megger
c. Open circuit test: Before the megger is connected to the measured cable, shake the handle softly to make the generator reach the rated speed of 120r/min, and observe whether the pointer points to the “∞” position of the scale
d. Short circuit test: make read and black tong be interconnected , then shake the handle softly and observe if the pointer points to “0”
How to use megger
a. Observe whether the device under test or the line will be measured is on the state of power failure
b. Connect “E” of the megger to the ground reliably, and “L” to the positive or negative cable under test
c. When shaking the handle, the speed should increased from slow to fast until the rated speed of 120r/min
d. Read the insulation resistance value on megger
Digital insulation resistance testers