Abstract: This paper analyzes the shortcomings and problems of the DC voltage withstand test on the XLPE cable. The field AC voltage withstand test is carried out by comparing the frequency conversion resonant device.
Key words: high voltage cable, DC withstand voltage test, AC withstand voltage test
Preface:
Power cables are commonly used as power lines for power plants, substations and industrial and mining enterprises, as well as for crossing rivers and railways.
Power cable used as urban transmission and distribution lines and industrial and mining enterprises inside the main line can occupy less land, beautify the environment.
The development of electric power construction directly led to the development of the country, power cable in the power construction play an important role, it to the affected by external climate, concealment, durable, high insulation, waterproof and acid can good, strong tensile, compressive, and an electric power users love, but in use process is easy to appear some fault, such as mechanical damage, lead corrosion, heat aging, etc.
So the power cable must check its hidden fault by its routine preventive test to ensure the normal operation of the power system.
According to IEC840 or CIGREWG21.03 recommended procedures, the purpose of the field test is not to test the cable manufacturing quality or cable accessories manufacturing quality, which has been confirmed in the type test and factory test.
The purpose of the field completion acceptance test is to check whether the cable laying and accessories are installed correctly.
Accidental damage may occur to cables in the process of transportation, handling, storage, laying and backfilling.
The inspection method is according to IEC229, for the cable whose outer sheath thickness is greater than or equal to 2.5mm, 10kV DC is applied between the cable shield and the ground and the voltage is withstanding for 1 minute.
IEC recommends two methods for voltage resistance test of cable main insulation:
DC withstand voltage: 3U015 minutes;
AC withstand voltage: U05 minutes.
The traditional DC voltage resistance method has the advantages of light weight, good mobility and low capacity of the test equipment, and it has a good effect on the application of oil-paper insulated cable. However, for XLPE cable, it has been proved that it is not suitable to adopt the DC voltage resistance method in both theory and practice.
GB Article 18.0.1 stipulates the high voltage cable test items:
1. Measure insulation resistance;
DC withstand voltage test and leakage current measurement;
AC pressure test; 3.
4. Measure the resistance of metal shielding layer and the resistance ratio of conductor;
5. Check the phase at both ends of the cable circuit;
6. Insulation oil test for oil-filled cables;
7. Cross connection system test.
There is no test item required to detect the inlet water of cable lining and outer sheath in the national standard. Now the test and judgment are discussed as follows:
1. Because the provisions of the national standard cannot detect whether the lining layer of the cable outer sheath is flooded, the test items added by the provinces include:
1.1. Use the resistance of copper coating and the resistance ratio of conductor to judge.
The procedure is to measure the DC resistance of the copper shield and conductor at the same temperature using a double-wall bridge.
When the ratio of the current layer to the latter increases, it indicates that the DC resistance of the copper shield increases, and the copper shield may be corroded.
When this ratio is reduced compared to before commissioning, it indicates that the contact resistance at the conductor connection point in the attachment is likely to increase.
Generally in the field experiment, the resistance value of steel armor and shielding insulation is measured, and the resistance ratio is used to judge whether the outer sheath and lining of the cable are flooded.
1.2. Use megohmmeter to measure insulation resistance value to judge.
Its steps to use 500 V megohmmeter measured lining rubber and plastic cable outer sheath layer of insulation resistance, when the insulation resistance of less than 0.5 ohms per kilometer, then use the following method to further judgment, a multimeter is used to measure the insulation resistance, using the galvanic battery principle, as a result of the rubber and plastic cable metal layer, armoured layer and the coating material is copper, lead, iron, zinc and aluminum, etc., when the outer sheath of the cable inner layer in water, the metal electrode, potential + respectively 0.334, 0.122, 0.44, 0.76 V and 1.33 V, the principle is that,
When the outer sheath of the rubber and plastic cable is damaged and water is taken into the cable, the groundwater is an electrolyte, and the galvanized steel strip of the armored layer will produce a potential of -0.76V to the ground.
When the outer sheath or inner lining is damaged and the water is taken into the water, when the insulation resistance per kilometer is lower than 0.5 megaohms, the positive and negative meter pen of the multimeter is used to measure the insulation resistance of the armor to the ground or the armor to the copper shielding layer in rotation. At this time, the galvanic cell formed in the measurement loop is connected in series with the dry cell in the multimeter.
When the polarity combination makes the voltage add, the measured resistance value is small.
On the contrary, the measured resistance value is larger.
Therefore, when the insulation resistance values measured by the two above are large, it indicates that the galvanic cell has been formed, and it can be judged that the outer sheath and lining layer have been damaged and flooded.
For example, a rubber and plastic cable sheath damage damp, measured resistance of 7 thousand ohms and 55 thousand ohms respectively.
2, cable voltage test, the national standard for DC voltage, AC voltage test, but the local provinces according to their own actual situation to choose one of them, now the advantages and disadvantages of the two are compared as follows: XLPE cable should not do DC voltage test, but should do AC voltage test.
2.1 DC voltage withstand test:
As a general principle of high voltage testing, the test voltage field applied to the test item should simulate the operation of the high voltage appliance.
While DC withstand voltage test is very effective for finding defects in paper insulated cables, it is not necessarily effective for XLPE insulated cables, and may have negative effects, mainly in the following aspects:
2.1.1 The electric field distribution of the XLPE cable is different under AC and DC voltage. The XLPE insulation layer is made of polyethylene by chemical cross-linking. It is a monolithic insulation structure, and its dielectric constant is 2.1--2.3, which is less affected by temperature changes.
Under AC voltage, the electric field distribution in the insulation layer of XLPE cable is determined by the dielectric constant of each medium, that is, the electric field intensity is inversely proportional to the dielectric constant, and this distribution is relatively stable.
Under the direct current voltage, the electric field distribution in the insulation layer is determined by the volume resistivity of the material and distributed in direct proportion, while the insulation resistance distribution coefficient is not uniform.
Especially in cable terminal head, connector box and other cable accessories, the distribution of AC electric field intensity and DC electric field intensity distribution is completely different, and the aging mechanism of insulation under AC voltage and DC voltage is different.
Therefore, the DC withstand voltage test can not simulate the running condition of the XLPE cable.
2.1.2 XLPE cable will produce "accumulation" effect under DC voltage to store accumulated unipolar residual charge.
It takes a long time for this residual charge to be released from the charge accumulation caused by the DC withstand voltage test.
If the cable is put into operation before the DC residual charge is fully released, the DC residual voltage will be superimposed on the peak power frequency voltage, making the voltage value on the cable exceed the rated voltage under operating conditions, which will accelerate insulation aging and shorten the service life of the cable, or even insulation breakdown.
2.1.3 One of the fatal weaknesses of XLPE cable is that it is easy to generate water branches in the insulation. The water branches will quickly turn into electric branches under DC voltage and form discharge, which accelerates insulation deterioration and leads to breakdown under power frequency voltage after operation.
However, the water branch can maintain a considerable voltage resistance for a period of time under AC working voltage.
2.1.4 Flashdown or breakdown during the field DC HV test may cause harm to the normal insulation of cables and joints.
In addition, the DC withstand voltage test can not effectively find some defects under the action of AC voltage, such as in the cable accessories, insulation if there is mechanical damage or stress cone misplaced defects.
Where insulation is most likely to break down under AC voltage, it is often unable to break down under DC voltage.
The insulation breakdown under DC voltage usually occurs in the place where the insulation breakdown does not occur under AC working conditions.
2.2 AC pressure test:
Since the DC withstand voltage test cannot simulate the operating field strength of the XLPE insulated cable and cannot achieve the desired test effect, we consider to use the AC high voltage test.
Because the capacitance value of the cable is different, we should first measure the capacitance value of the power cable before the test, calculate the capacitance current under the test voltage according to the capacitance value, so as to select the appropriate test instrument.
2.2.1 It is understood that the rated voltage of most power plant cables is 6kV and the length of most cables is less than 1.5km, so we can adopt the conventional AC voltage withstand test method.
If a test transformer of 50kV and 20kVA is used, its maximum output current is 1000mA. According to I=2π Fuc, taking 6kV cable as an example, the maximum capacitance value of the cable tested by this test transformer is 265NF (F =50Hz,U= 12kV).
2.2.2 For some large-capacity cables, such as the conventional AC voltage withstand test method, large-capacity test transformers are required, and the capacity of voltage regulator and power supply is also particularly large.
The site is often difficult to do, the transport of test instruments, often need to use large cars, cranes, etc., both time-consuming and laborious.
Therefore, according to the specific situation, we use frequency conversion test, series or series and parallel resonance method to carry out the cable voltage test.
2.2.3 Ultra-low frequency 0.1Hz voltage test:
According to the test capacity (formula S= WCUS2 =2∏ FUS2KVA, C-cable capacity under the formula, US - is the test voltage, F - power frequency, 50Hz in China), it can be seen that 0.1Hz AC voltage and 50Hz voltage, the former needs power equivalent to 1/500 of the latter, therefore, it can be used in the field without any problem to produce portable equipment.
At present, this method is mainly used in the test of medium and low voltage cables.
According to field practice, when the voltage withstand test of XLPE cable is carried out, the test voltage can be 1.5-1.8 times of that of 50Hz when the ultra-low frequency voltage of 0.1Hz is used. It is easier to find the cable insulation defects than the DC voltage, and it is easier to expose and break down the insulation defects than the 50Hz AC voltage.
2.2.4 Frequency conversion resonant voltage test:
The frequency conversion resonant test system can not only meet the voltage resistance requirements of high voltage XLPE cable, but also has the advantages of light weight and good mobility, so it is suitable for field test.
The device uses a fixed reactor as a resonant reactor to achieve resonance in the way of frequency modulation. The frequency adjustment range is 30-300Hz, which is in line with the AC voltage of power frequency and approximate power frequency (30 ~ 300Hz) recommended in CIGREWG21.09 "Guidelines for the Completion Test of High Voltage Exsqueezed Insulated Cables".
The AC voltage can reproduce the same field intensity as the operating condition, with good equivalence, high efficiency, portable equipment, and almost unlimited length of the sample.
To sum up, in view of the small capacity and volume of cable field power frequency test equipment, easy to carry and operate, and the cable defects are more effective than the conventional DC voltage resistance, so the power frequency or frequency conversion resonance test method should be adopted to carry out the cable field completion acceptance test.
Moreover, frequency conversion resonant device can meet the requirements of XLPE cable handover test of L10kV and 220kV and above, so it is suggested that frequency conversion resonant voltage withstand should be the first choice.