Insulation test - electrical performance test

Date：2021-10-29 17:24:00Views：1831

Measuring the insulation of electrical equipment is a work often carried out by power operators. Qualified insulation is an important index whether electrical equipment can be put into operation.

Insulation is a safety measure to isolate or wrap the charged body with non-conductive substances to protect against electric shock. Simply put, the insulation resistance of electrical equipment is the ability to isolate voltage. Insulation test is a test to test the dielectric strength of insulating materials or insulating structures of electrical equipment. The purpose is to check the safe operation of products and equipment and prevent dangerous accidents caused by short circuit.

Application scope of insulation test: electrical equipment, electrical components, insulating materials

Insulation test method:

In general, the insulation resistance test is to output a fixed DC voltage value, then detect the leakage current to the ground or other parts, and the degree obtained after pressurizing it for 1min is the insulation resistance value. In the insulation resistance test, the core link is the absorption ratio test, which can timely reflect the damage degree and moisture return of insulators in electrical equipment. Generally, the insulation of the task equipment can be qualified only when the insulation resistance of the equipment at room temperature is qualified and the absorption ratio is within 1.3.

Why do insulation tests?

Because the electrical equipment is always under the influence of various external factors during operation, its performance will constantly change. These external factors mainly include external electric field, environment, high voltage, corrosion degree and machinery. Under the influence of these adverse factors, the power equipment is more likely to have unpredictable faults and even lead to power operation interruption. Therefore, the insulation test of high-voltage electrical equipment in the power system should be carried out in advance before it is put into operation, so as to ensure the safe and stable operation of the power system. Therefore, in order to ensure the good insulation of high-voltage electrical equipment and the stable and safe operation of power system, it is very necessary to conduct insulation test on electrical equipment.

Procedure for measuring insulation resistance of electrical equipment with insulation megger

(1) According to the voltage level of the tested equipment, select the insulation megger corresponding to the voltage level.

(2) The measurement operation shall be carried out by two or more personnel.

(3) Before measurement, it must be verified that there is no voltage in the three phases of the tested equipment and there is no possibility of sudden electricity.

(3) The leads of insulated megger shall not be braided together.

(4) If necessary, wrap a metal shielding ring on the surface of the insulator and lead it to the shielding terminal through the conductor to eliminate the influence of leakage current.

(5) Before measurement, the insulation megger shall be tested to be in good condition; Short circuit the two leads, and then shake them slowly. The indication of the gauge needle is 0, indicating that the gauge is in good condition.

(6) Connect one lead of the insulation megger to a reliable grounding point and the other lead to the tested equipment (wear insulating gloves or other insulating tools).

(7) Be sure to keep the speed of the insulated megger fast and uniform.

(8) When the equipment under test has large capacitance or inductance, read the insulation value indicated by the insulation megger after a long enough time. ≥1.3。

(9) After the phase to ground insulation is measured respectively, the phase to phase connection shall be measured if necessary.

(10) When the tested equipment has large capacitance or inductance, discharge the tested equipment after measuring the insulation to prevent static electricity from hurting people.

(11) Keep records.

Insulation test precautions:

1. Check whether the DC cable of photovoltaic array and array grounding measures are effective

First of all, the reason for the abnormal insulation impedance of most photovoltaic arrays is the damage of DC cables. It is recommended to regularly inspect the non directly buried and visual cables during daily operation and maintenance inspection. Secondly, the insulation impedance will be low, including that the grounding hole of the module is not installed with a grounding wire, the protection pipe of the DC cable enters water, and the grounding failure of the photovoltaic array.

2. Rely on inverter to check series by series

If the DC side of the inverter is multi-channel access, the component string can be detected by troubleshooting one by one. Only one string is reserved at the DC side of the inverter. Check whether the inverter continues to report errors after startup. If it does not continue to report errors, it indicates that the insulation performance of the connected string is good. If it continues to report errors, it is likely that the insulation of the string does not meet the requirements.

3. Use megger or other professional equipment to test one by one

During on-site inspection, megger, insulation meter or other professional equipment shall be used to measure the insulation resistance of strings one by one, and the impedance shall be greater than the standard insulation impedance threshold.

Precautions for insulation resistance test:

1. The place where the megger is placed during telemetry shall be far away from the conductor with large current and permanent magnet objects, as well as near the line or device with high, medium and low frequency signal electromagnetic radiation. The interference of various electrical and magnetic factors on the measurement work shall be avoided to the greatest extent.

2. The test line connecting the end button of the megger to the tested object must be a single copper core flexible conductor with good insulation. Double stranded wire or parallel wire shall not be used. Even two single stranded wires cannot be contacted or twisted together. The length of the two test wires shall be appropriately selected according to needs. In principle, it should be short rather than long. The two wires shall be connected separately. The test line shall not be placed on the tested equipment, other metal objects or lines, nor dragged on the ground. During the whole test process, people and other parts of the body shall not contact the relevant parts of the tested object of the test line, because of the insulation defect of the test line, the equivalent parallel resistance formed when the distance between two conductors is very close or when they are coupled with each other through other conductive objects, And the coupling and leakage of the test line to the ground will have a certain impact on the measurement results.

3. In case of lightning at or near the measuring site, the megger and test line shall be removed immediately, and the test work shall be stopped to prevent lightning from endangering the safety of personnel and instruments through direct strike, induction and conduction.

4. In the process of shaking and reading, if it is found that the pointer has pointed to the zero scale, indicating that there is a short circuit in the measured object, stop rotating the shaking handle immediately to prevent the moving coil inside the megger from being damaged due to overheating.

5. Before the megger does not stop rotating or the tested object does not discharge, do not touch the tested part or its related parts by hand, nor the metal part of the terminal button of the megger, so as to avoid electric shock.

6. When measuring the insulation resistance of high-capacity electrical equipment and power lines and reading the value, do not stop the rotation of the megger handle immediately. First carefully remove the test line, and then stop the rotation of the handle, so as to prevent the megger from being damaged by the tested object discharging power to the measuring circuit. After the megger is removed, a certain amount of electricity is stored because the measured object is charged by the high voltage of the megger and shows a very high voltage, which may cause personal electric shock accident or damage other equipment. Therefore, appropriate methods must be taken to fully discharge the measured object.

7. When measuring the insulation resistance of high-voltage equipment with a high-voltage megger, two people must be responsible for the division of labor. One of them is responsible for lapping and separating the "L" end test line on the tested object with insulating gloves and insulating rod, and the other is responsible for mastering the megger and recording the measurement results. The operation of the megger and the "L" end test line shall follow the principle of "shaking before connection, dismantling before stopping".

8. For the measured results, the main factors affecting the insulation resistance measurement shall be taken into account for comprehensive analysis, such as the impact caused by the large difference in temperature and humidity in the measurement environment; Influence of residual charge during repeated test; Influence of current absorbed by insulating material on test time; The influence of the cleanliness of the key parts of the tested object; The performance of megger and the influence of wiring mode and operation method.