DC high voltage generator power cable withstand voltage test

1. Measuring insulation resistance

It should be conducted on each phase separately, and the other two phase conductors, metal shields or metal sheaths and armor layers at both ends of the cable are connected. For this test, it is only necessary to note that the cable is a capacitive device, and the conditions that should be noted when doing insulation resistance and absorption ratio tests on capacitive devices. For example: full discharge before and after the test, fire first and then overlap, disconnection first and then power off and shake meter, etc.

The insulation resistance changes with temperature and the ambient temperature. The buried cable needs to record the soil temperature. The temperature correction coefficient of the viscous impregnated paper insulated cable is shown in Table 1.

Table 1 Temperature correction coefficients for viscous impregnated paper insulated cables

Conversion formula

R20=KRL

Where R20 is the insulation resistance at 20℃;

K——Insulation material temperature correction coefficient.

2. DC withstand voltage and leakage current test

Oil-paper insulated cables are only tested for DC withstand voltage, not AC withstand voltage. This is because an increase in AC Ig may lead to thermal breakdown; when hot, the electric field is unevenly distributed, which can easily damage the cable. Attention should be paid to the following: The cable core is connected to negative polarity: When the cable is damp, the moisture carries a positive charge. If the core is connected to negative polarity, the moisture will concentrate on the core, the moisture in the insulation will increase, the leakage current will increase, and defects will be easy to find. If the core is positive polarity, the moisture will penetrate into the lead sheath, the moisture in the insulation will decrease, the leakage current will decrease, and defects will not be easy to find.

3. Rubber and plastic cable test

Rubber-plastic cable refers to polyvinyl chloride, cross-linked polyethylene, and ethylene-propylene rubber insulated cables. Its characteristics are large capacity, high voltage level, light structure, and easy bending. It has gradually replaced oil-paper insulated cables. The difference between cross-linked polyethylene cable and the familiar oil-impregnated paper turnkey cable is that in addition to the main phase insulation being cross-linked polyethylene plastic, there are also two layers of semiconductor glue coating. A layer of semiconductor glue is coated on the outer surface of the core wire to overcome corona and free discharge, so that there is a good transition between the core wire and the insulation layer. On the outer surface of the phase insulation, a second layer of semiconductor glue is coated inside the copper tape shielding layer. The copper tape shielding layer is just a thin copper tape with a thickness of 0.1mm, which constitutes the phase shielding layer.

1. How to determine whether the inner and outer sheaths of rubber-plastic cables have been flooded

Use an insulation resistance meter to measure the insulation resistance. Use a 500V insulation resistance meter. When the insulation resistance per kilometer is lower than 0.5MΩ, the following method should be used to determine whether water has entered the outer sheath.

The basis of this method of measuring insulation resistance with a multimeter is that different metals form a galvanic cell in an electrolyte. The electrode potentials of some metals are shown in Table 2.

Table 2 Electrode potentials of different metals forming galvanic cells in electrolytes

When the outer sheath of the cross-linked cable is damaged and water enters, since it is not an electrolyte, a potential of (-0.76) V is generated on the galvanized belt of the armor layer to the ground. If the inner lining layer is also damaged and water enters, there will be a potential of +0.334 V on the copper shielding layer. Use the "positive" and "negative" test leads of the multimeter to measure the resistance between the armor layer and the ground, and between the armor layer and the copper shielding layer. If the difference between the positive and negative values ​​is large, it means that a primary battery has been formed and the sheath is damaged. At this time, in the measurement circuit, since the formed primary battery is connected in series with the dry battery of the multimeter, when the polarity combination makes the voltage added, the measured resistance is smaller, otherwise, the measured resistance value is larger. If there is no damage, the resistance values ​​measured by positive and reverse connection should be the same.

Before the cable is put into operation, after the terminal or joint is redone, and after the inner lining is damaged and water enters: use a double-arm bridge to measure the DC resistance of the copper shield and conductor at the same temperature. When the ratio of the former to the latter increases compared with that before commissioning, it indicates that the DC resistance of the shield increases and the copper shield may be corroded. When the ratio decreases compared with that before commissioning, it indicates that the contact resistance of the alternating connection in the accessory may increase.

2. Pressure test

The withstand voltage test is divided into DC withstand voltage test and AC withstand voltage test. At present, rubber-plastic cables, especially cross-linked polyethylene cables, have been rapidly developed. Since high-voltage cross-linked polyethylene cables use DC withstand voltage, there are obvious disadvantages. It is not suitable to use DC voltage test. There are two reasons: ① During the DC test voltage process, space charges are formed in and around the cross-linked polyethylene cable, which has a cumulative effect on the insulation, accelerates insulation aging, and shortens the service life. ② The insulation electric field distribution under DC voltage is different from that under actual operating voltage. The former is distributed according to resistivity while the latter is distributed according to dielectric constant.

Due to the insulation characteristics of rubber-plastic cables, DC withstand voltage cannot simulate operating conditions. In addition, because rubber-plastic cables have a memory effect on DC voltage, DC tests have a cumulative superposition effect, causing the cable to be subjected to overvoltage after operation, resulting in insulation breakdown. Another feature is that water dendrites are easily generated in the insulation of rubber-plastic cables, and the charging current, capacity and weight are small when tested in DC test equipment.

In addition, most test voltage values ​​of DC tests are below 4.0U0. For rubber-plastic cables of 110KV and above, even if there are defective joints or serious air gaps, their tolerance to DC is greater than 4U0. Therefore, DC is almost ineffective in discovering defects in high-voltage rubber-plastic cables.

(1) AC withstand voltage test equipment.

The resonance test device uses the capacitance of the cable as the resonance parameter and the current is provided by the resonance circuit. Therefore, the test equipment is relatively small in size and light in weight and can be implemented on site.

Resonance test devices can be divided into: frequency modulation resonance, inductance modulation series resonance. For cross-linked cables, the detection effect of 20HZ~300HZ alternating current is the same.

Generally, a frequency modulation resonance device can be selected, which is the preferred equipment for high-voltage cable withstand voltage test. It has the advantages of high quality factor, small test current, small input power, small input power, and short-circuit current during breakdown.

(2) Common accidents and diagnosis of cross-linked polyethylene cables;

1) Water tree deterioration is the main cause of cross-linked polyethylene cable accidents, accounting for about 70%. Pay special attention to cables with harsh operating environments, such as those with poor heat dissipation.

2) Shielding copper tape breaks: In a cable with one end of the shielding copper tape grounded, when the shielding copper tape breaks, a high voltage will be induced on the copper tape at the non-grounded end, causing discharge at the broken part, often destroying the insulation. The accident is characterized by more cores than three cores, and fire and smoke at the broken part.

3) Copper shield grounding fault: mostly occurs at the joint. Due to poor sealing, the cable head is damp, which reduces the insulation between the copper shield and the steel armor. Cable sheath fault - judged by the principle of primary battery, using the rubber and plastic cable sheath damage detector to locate, the core of the electric dry cable is prone to sharp burrs during the production process. The electric field distortion causes the main insulation to deteriorate and discharge. Therefore, cross-linked polyethylene cables above 3KV have two layers of semiconductor materials, the core shield and the insulation shield. If the shielding layer is not thick enough, the thickness is uneven, which directly affects the safe operation and life of the cable.