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How to Check earth issue on PV side

Testing earth issue on pV side

For PV system, earth issue is very common on solar power plant. Most of time, PV cables are damaged in the engieering. Potential earth issues will affect or damage inverter,even combiner box will be burned in severe situation.

Testing principle:

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 equals half of PV voltage. Earth is like zerio potential. As earth is not connected to positive or negative side directly, so they are virtual voltage. In the instaneous time, you can detect half of PV voltage via multimeter,then it will decline until 0.

Testing Tool
  • Traditional megger

it is pretty good, not only cheap , but also convenient. You can prepare both 500V and 1000V megger. Voltage level depends on PV cable.

  • Digital insulation resistance testers,

More expensive and accurate, wider voltage range

  • Multimeter
Testing Method
  • Turn to DC voltage position on multimeter
  • Disconnect PV system from inverter
  • Put red tong on Positive cable of PV wire, black tong on earth. wait for a while, then record the voltage
  • 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. 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 voltage of postive with earth, voltage of negative with 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. It is easy to understand. At this time, zero potential, only when same positve or same negative can cause this.

  • You can test every PV string unitl you find abnormal one.

When abnormal PV string has been found. You can use megger to test and confirm it finally.

How to use traditional megger

The hand-cranked insulated megohmmeter is also called a shaker. It is scaled in megaohms (MΩ). The megohmmeter consists of medium and large scale integrated circuits. Its output power is large, the short-circuit current value is high, and the output voltage level is many. Megohmmeters are commonly used and indispensable instruments for electric power, post and telecommunications, communications, electromechanical installation and maintenance, and industrial enterprises that use electric power as industrial power or energy.

how to choose megger

The megohmmeters used by electricians are commonly used in three types: 500V, 1000V, and 2500V according to the voltage measured. Without special timing, use a 500V megohmmeter to measure the working voltage of the equipment at 500V and below. If a high-voltage megohmmeter is used, the insulation of the equipment under test may be damaged. If the working voltage of the equipment is above 500V and below 3000V, use a 1000V megohmmeter. Meter measurement; use a 2500V megohmmeter to measure at 3000V and above.

Safe Check
  • Before use, check whether the insulation layer of the connecting wire of the shaker is in good condition and whether there is any damage.
  • Check whether there is any sliding wire on the fixed terminal of the shaker
  • Check whether there is any sliding wire on the fixed terminal of the shaker
  • Open circuit test: Before the shaker is connected to the measured resistance, shake the handle to make the generator reach the rated speed of 120r/min, and observe whether the pointer points to the “∞” position of the scale.
  • Short-circuit test: short-circuit the terminal buttons L and E, slowly shake the handle, and observe whether the pointer points to the “0” position of the scale
Use of the megger
  • Observe whether the device under test and the line are measured in the state of power failure. And the connecting wire between the shaker and the device under test cannot use double-stranded insulated wire or stranded wire, and should be connected with single-stranded wire separately.
  • Correctly wire the device under test to the shaker. When shaking the handle, it should increase from slow to fast to the rated speed of 120r/min
  • Correctly read the measured insulation resistance value.
  • Measuring the insulation resistance of PV cable to ground

Connect (E) of the shaker terminal to the ground reliably, and (L) to the positive or negative cable under test. Shake the generator handle of the shaker clockwise, the speed changes from slow to fast, and the speed reaches 120/min and shakes at a constant speed. After about one minute, when the generator speed is stable, the hands of the watch will also stabilize. At this time, the value indicated by the hands is the Measured insulation resistance value. The measurement method is shown in the figure

Normal PV cable has at least 500M resistance.

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