Several methods and presents of AC withstand voltage test of power cables

The first is the ultra-low frequency approach to cable testing. In the ordinary power frequency withstand voltage test, the cable capacity is large, the test transformer capacity is large, and a considerable test power supply is required on site to provide reactive power for the cable. Therefore, the power frequency withstand voltage test is not suitable for the field.

Therefore, it is necessary to use ultra-low frequency as a test power supply, which can both reduce the capacity of the test transformer and make the field test easier to operate. However, since this method is not very effective in detecting insulation defects, the term method is generally used in the testing of low-voltage cables.

Then there is the cable test oscillation voltage method. The principle of the test is to charge the cable with direct current. After the voltage reaches a certain value, the resistance and inductance are discharged through the gap to obtain the damping oscillation voltage to check the insulation defects of the main insulation and accessories of the cable.

This method is more effective than the DC withstand voltage test, but there is an attenuation of the oscillation voltage, which cannot meet the needs of long cables, and the damage of high-frequency voltage to the cable is great, which is a problem of this method.

Finally, there is the resonant withstand voltage test method. This method can meet the requirements of high-voltage, high-current testing. The resonant withstand voltage method is divided into inductance modulation and frequency modulation according to the adjustment mode; According to the resonance mode, it is divided into series resonance and parallel resonance.

The cable adjustable inductor resonance withstand voltage is to adjust the inductance of the loop reactor, so that the capacitance of the reactor and the cable resonate at the power frequency (50Hz) to meet the test requirements. The cable frequency modulation resonance withstand voltage is to change the output frequency of the test power supply, so that the reactor with a fixed inductance in the circuit resonates with the product under test to meet the test requirements.

The cable series resonance method is to test the reactor and the tested product in series when the current of the test transformer meets the test requirements but the voltage does not reach the test voltage. When the circuit is in a resonant state, the test product can generate the output voltage of the Q times (Q) transformer, which is the figure of merit of the circuit. The energy provided by the power supply is only the active power consumed in the circuit.

The cable parallel resonance method is to connect the reactor in parallel with the tested product when the voltage of the test transformer meets the test requirements but the current does not meet the requirements, so that the loop parameters meet the resonance requirements of the test. The induced current of the reactor compensates for the capacitive current of the test piece.

1.Power frequency withstand voltage: select the appropriate equipment according to the length, cross-sectional area and voltage level of the cable, and last for 1 minute at the specified test voltage, and pass without flashover breakdown.

In order to ensure the safe and smooth progress of the test work, some key points in the test operation need to be paid attention to

Before the test, the test voltage of the tested cable should be understood, and other test items of the tested cable and the previous test results should be understood. If it is found that the tested cable is defective or abnormal, the AC withstand voltage test should be carried out after elimination. According to the capacitance of the cable.

and test voltage to estimate the size of the test current, judge whether the capacity of the test transformer is sufficient, and consider the setting value of overcurrent protection (generally should be set to the capacitance current of the tested cable.

times). The test site should be fenced or fenced, hung up signs, and sent a special person to supervise it. The test product should be disconnected from other equipment, and keep a sufficient safety distance, and when the distance is insufficient, an insulating baffle or other protective measures should be considered.

Before the test, the surface of the tested cable should be wiped clean.

After the test wiring is connected, it should be checked by an experienced person and confirmed that it is correct before it can be prepared for boosting.

Adjust the variable frequency series resonant withstand voltage test device to protect the ball gap so that the discharge voltage is 110% 120% of the test, and there should be no obvious difference in the continuous test for three times, and check the reliability of the overcurrent protection action.

Before pressurizing, first check whether the regulator is in the zero position. The voltage regulator can only be boosted at the zero position, and each other should be reminded to pay attention to safety when boosting.

The variable frequency series resonant withstand voltage test device should not only monitor the voltmeter during the step-up process.

The ammeter should also be monitored.

and the current of the test piece. When boosting, it should be boosted at a constant speed, not too fast. When the specified test voltage is raised, the time is calculated, and when the time is up, the voltage is slowly and evenly lowered.

It is not allowed to jump the power switch first without stepping down. Because the power switch jumps without stepping down is equivalent to doing an operation wave test for the tested product, it is very likely to damage the equipment insulation.

If it is found that the voltage fluctuates greatly or the tested cable has abnormal sound, smoke, fire, etc., the voltage should be lowered immediately, the power supply should be pulled out, and the grounding wire should be hung on the high-voltage side.

After that, the cause will be identified.

The insulation resistance of the tested cable should be measured before and after the AC withstand voltage test.

When possible, a partial discharge test is also carried out.

The figure shows the schematic diagram of the AC withstand voltage test. The test detects the defects and problems in the insulation performance of the sample under AC high voltage. The state stipulates that if the tested cable can withstand the power frequency test voltage for one minute without breakdown, insulation flashover or other abnormal problems, the cable can be judged to be well insulated.

The function of R1 in the figure is to limit the short-circuit current of the transformer when the tested cable is discharged, so that it is lower than the allowable value, and the voltage gradient of the high-voltage winding needs to be lower than the dangerous value; The function of R2 is to limit the current of the spherical gap discharge; The function of the voltage regulating transformer is to adjust the amplitude of the test voltage and the speed of voltage rise and fall; In terms of the selection of test transformers, single or cascade test transformers can be used, depending on the specific conditions of the test.

The AC withstand voltage test should be selected.

The purpose of the DC withstand voltage test is to test the highest voltage peak that the operation of electrical equipment can withstand;

The purpose of the AC withstand voltage test is to test the dielectric strength of power equipment;

For the cable, it is just a power transmission device, the resistance is extremely low, and it is pointless to test how much voltage it can withstand without heating up; The cable needs to be insulated from the surrounding environment to avoid loss of power resources or danger, so the AC withstand voltage test should be selected.

Cable DC withstand voltage test.

What is the difference between the AC withstand voltage test and the AC withstand voltage test?

1.Insulation preventive test can be divided into two categories: one is the non-destructive test or insulation characteristic test, which is a variety of characteristic parameters measured at a lower voltage or with other methods that will not damage the insulation, mainly including measuring insulation resistance and leakage current.

dielectric loss tangent value, etc., so as to judge whether there are defects inside the insulation. Experiments have proved that this method is effective, but it cannot be relied on alone to reliably judge the electrical strength of insulation. The other type is the destructive test or withstand voltage test, the voltage applied by the test is higher than the working voltage of the equipment, and the insulation test is very strict, especially to expose those dangerous concentrated defects, and can ensure that the insulation has a certain electrical strength, mainly including DC withstand voltage, AC withstand voltage, etc.

The disadvantage of the withstand voltage test is that it will cause certain damage to the insulation.

2.DC withstand voltage test, DC withstand voltage test voltage is high, has a special role in finding some local defects of insulation, and can be carried out at the same time as leakage current test. Compared with the AC withstand voltage test, the DC withstand voltage test has the advantages of light test equipment, small damage to insulation and easy to find local defects of the equipment.

Compared with the AC withstand voltage test, the main disadvantage of the DC withstand voltage test is that due to the different voltage distribution inside the insulation under AC and DC, the test of the DC withstand voltage test is not as close to the reality as the AC.

3.Effect of DC withstand voltage test on cross-linked power cable: cross-linked polyethylene.

The insulating material is a cross-linked polyethylene plastic generated by the cross-linking process, which is an integral insulating material with a dielectric constant.

and are generally not affected by temperature changes. The strength of the electric field in the insulating layer at DC voltage.

is in accordance with the insulation resistivity.

The distribution of insulation resistivity is uneven (in the production process of cross-linked polyethylene plastics, due to process reasons, there are inevitably impurities in the main material, they have small insulation resistivity, and radially distributed along the insulation layer, the distribution is uneven), so the electric field distribution of cross-linked polyethylene insulation under AC and DC voltages is different, resulting in inconsistent breakdown characteristics. The DC withstand voltage test not only can not effectively find the insulation defects such as water branches in the cross-linked polyethylene insulation material, but also due to the effect of space charge, the original internal weaknesses of the insulation are further developed and expanded, and the insulation performance is gradually attenuated to form the accumulation effect of the deterioration of the insulation, which is easy to cause the cable to be put into operation soon after the cable is put into operation under the action of AC voltage. In addition, some parts of the cable, such as the cable head and the intermediate head, have some defects under the alternating voltage, but they will not break down during the DC withstand voltage test.

First, the nature is different.

1. AC withstand voltage: the most effective and direct method to identify the dielectric strength of power equipment.

2. DC withstand voltage: the maximum voltage peak value of the detection equipment under the high voltage test.

Second, the destructiveness is different.

1. DC withstand voltage: Since the insulation under DC voltage basically does not produce dielectric loss, the damage to the insulation caused by DC withstand voltage is small. In addition, since only a small leakage current is required for the DC withstand voltage, the required test equipment has a small capacity and is easy to carry.

2. AC withstand voltage: AC withstand voltage is more damaging to insulation than DC withstand voltage, because the test current is capacitive current, large-capacity test equipment is required.

The cable series resonance test device adopts the method of adjusting the frequency of the power supply, so that the reactor and the tested capacitor can achieve resonance, and obtain high voltage and high current on the tested product, which is a new method and trend of the current high voltage test, and has been widely used at home and abroad. The cable series resonance test device adopts a special SPWM digital waveform generation chip, with a frequency resolution of 16 bits, and the frequency fineness can be reached at 20 300Hz. The orthogonal asynchronous fixed carrier modulation mode is adopted to ensure that the output waveform is good in the whole frequency range. The power part adopts the IPM module to ensure the stability and safety of the instrument at the minimum weight. It has been shown that a considerable number of cable failures are caused by the negative effects of regular DC withstand voltage tests.

Therefore, many experts at home and abroad have suggested that the AC withstand voltage test of the cable should be carried out on site instead of the DC voltage test, so as to improve the distribution quality of the power cable, and the appropriate use and safety. Prepare the required equipment and installation materials sufficiently, and conduct self-inspection of test equipment such as series resonance withstand voltage device, megohmmeter, and nuclear phase instrument in advance.

The staff and relevant persons in charge involved in the construction of the cable withstand voltage test should make preparations in advance, and evacuate other personnel to a safe range to ensure the safety of the personnel in the station. The inspection of the cable phase is carried out on the two ends of the cable line, and the inspection results indicate that the phase at both ends of the cable is consistent with the phase of the power grid. Use a megohmmeter to measure the insulation resistance of each core and sheath of a multi-core cable.

After comparing the measurement results with the original results, it is determined that the cable has good insulation performance and can be tested for AC withstand voltage. Connect the components of the frequency conversion series resonance and the tested cable, and ensure that the grounding wire is correct. The series resonance is operated to raise the voltage to 52 kV, and then the hipot test is started, which is set to 60 minutes.

After the withstand voltage test is completed, the power supply is cut off, and the discharge is completed with a discharge rod, and the insulation resistance of the cable is measured again. Within 60min of the withstand voltage of the cable, the cable is regarded as qualified if there is no flashover and discharge. The results of the re-conducted insulation resistance test should not be significantly different from the previous results, otherwise the insulation performance will be regarded as unqualified.

AC withstand voltage test is the most effective and direct method to identify the dielectric strength of power equipment, and it is an important part of preventive test. In addition, because the AC withstand voltage test voltage is generally higher than the operating voltage, so after passing the test, the equipment has a large safety margin, so the AC withstand voltage test is an important means to ensure the safe operation of power equipment. With the development of the national economy and the improvement of the voltage level of urban power supply, XLPE insulated power cable (XLPE) has been more and more widely used at home and abroad due to its reasonable process and structure, excellent electrical performance and safe and reliable operation characteristics.

In particular, great progress has been made in the field of high-voltage power transmission. Compared with oil-filled cables, cross-linked cables are easy to lay and install, simple to operate and maintain, and there is no problem of oil flow. However, the operation and research in recent years have shown that the insulation of XLPE cable is prone to dendritic discharge during operation, resulting in insulation aging and damage, which seriously affects the service life of XLPE insulated power cable.

Therefore, it is of great significance to fully understand the insulation characteristics of cross-linked cables and find and prevent some defects in insulation in a timely and effective manner to ensure the safe operation of equipment and even systems. The main factors affecting the insulation of cross-linked cables and the principle of handover test of cables are expounded, and it is considered necessary and feasible to carry out AC withstand voltage test on cross-linked cables in the field.

In recent years, the test and operation experience at home and abroad have proved that the DC withstand voltage test can not effectively find the insulation defects in the cross-linked cable, and even cause the hidden danger of cable insulation. In 1978 to 1980, the German company Sesechiswag failed 87 times in XLPE cables of the 10 kV voltage class with 41 circuits; Sweden's 3 kV KV voltage class XLPE cable has been put into operation for more than 9 000 km, with 107 failures, and there have been many cable accidents in China, and a considerable number of cable failures are caused by the negative effects of regular DC withstand voltage tests.

Therefore, the relevant departments at home and abroad widely recommend the use of AC withstand voltage instead of the traditional DC withstand voltage.