In accordance with the PUE, PTEEP, SNiP and other regulatory and technical documents, all cable lines with a rated operating voltage of more than 1 kV are subjected to an increased voltage test. The frequency of such cable tests depends on the voltage class, type of insulation, service life, installation method, and the presence of visible defects. Be sure to test newly commissioned cable lines that have undergone major repairs, as well as periodically at certain intervals.
The test of cable lines with a rated voltage of up to 1 kV is carried out using a megohmmeter for a voltage of 2500 V for 1 minute. In this case, the insulation resistance value is measured between the current-carrying conductors and the grounded sheath, separately between each conductor. The method for testing the insulation of a cable line for a higher voltage involves the use of a megohmmeter as an auxiliary tool. With its help, obvious defects are determined in the form of a break in the integrity of the conductive core, breakdown and other types of damage to electrical insulation. The main type of control of such products is insulation testing with increased voltage. It is performed using a rectified voltage or power frequency voltage.
Why test the cable with increased voltage
During installation and subsequent operation, various negative environmental factors can affect the cable line:
- sharp fluctuations in temperature;
- cable bending over the permitted radius;
- soil shift and mechanical shocks;
- long-term operation in the mode of current overload.
High-voltage testing of the cable makes it possible to create an increased electric field strength in its insulation. This makes it possible to detect defects that cannot be diagnosed by other methods of control. To determine the dielectric strength of insulation, no more accurate and simple method has yet been invented than high-voltage tests. The characteristic of the electrical strength of the insulation depends on the duration of the application of increased voltage, the rate of its rise, the presence of thermal, mechanical effects on the cable during installation or operation. Thanks to such tests, it is possible to qualitatively control the condition of the product and carry out its repair in a timely manner. The result will be long and trouble-free operation throughout the lifetime declared by the manufacturer.
Cable line test standards
The normative values of the magnitude of the test voltage and the time of its application are set out in the Rules for the Installation of Electrical Installations (PUE) and the Rules for the Technical Operation of Consumer Electrical Installations (PTEEP).
For power cables laid through the air, which have only one current-carrying core with plastic insulation and cables with rubber insulation for voltages up to 1 kV, the tests are not carried out. The duration of the tests for products with plastic or oil-paper insulation with a rated voltage up to 35 kV inclusive is ten minutes. For rubber insulated cables with a rated operating voltage of 3 to 10 kV, the duration of the test is five minutes. Cables with a rated voltage of 110 - 500 kV are tested for fifteen minutes.
Testing cable lines with increased voltage is accompanied by control over the amount of leakage current through the insulation. Based on the data obtained, for each current-carrying core, the asymmetry coefficient is calculated, which is equal to the ratio of the maximum value of the leakage current to its minimum value. The test standards are detailed in the PUE:
Power cables must have a stable leakage current, which must decrease during the test. If this does not happen, the leakage current is unstable or increases, then the test is carried out until a defect is detected, but not more than 15 minutes.
Test preparation and safety
Testing a power cable with increased voltage is accompanied by a high potential hazard to personnel and the electrical equipment under test. For this reason, the methodology for testing the cable with increased voltage clearly regulates the sequence of all actions and activities related to labor protection and safety. The main requirements for such work are:
- Only adults who have passed a medical examination and periodic testing of knowledge of electrical safety are allowed to work.
- Registration of work with a work permit. All personnel who take part in high-voltage testing must be properly instructed.
- Workplace preparation. The cable is disconnected from the electrical network, and all its metal elements, which are not provided for the supply of increased voltage, must be grounded.
- Before connecting the test equipment, all metal parts of the cable are briefly grounded to remove residual charge.
- Before applying increased voltage, the cable line is inspected for the presence of visually detectable defects.
- Negative ambient temperature is a contraindication for high voltage testing. If the air temperature is 0 0 С or lower, then the cable tests with increased voltage are carried out only in emergency cases.
- Before applying high voltage, current-carrying conductors must be checked with a megger for the integrity of the conductors, the insulation resistance between the phases and the metal sheath.
- All used dielectric protective equipment must be marked with a test of their suitability and undergo a preliminary visual inspection before use.
How is the testing process
The test of the cable with increased voltage is carried out using direct current. This allows the use of lightweight mobile units with high power. Partial discharges in this case develop very weakly, active power losses are minimal and heat generation is practically absent. Immediately before and after high-voltage tests, it is imperative to check the resistance of electrical insulation using a megohmmeter with a voltage of 2.5 kV.
To increase the efficiency of personnel work and reduce the labor intensity of the operations performed, it is allowed to test cables without disconnecting them from the busbar system. Products laid in the ground are checked in the summer season. In the event of a breakdown, it will be much easier to excavate a place with a defect and carry out repair work. To supply increased voltage, special high-voltage rectifier installations of a stationary, portable or mobile type are used. The design of test facilities includes: transformer, rectifier, control panel. The leakage current is controlled through a milliammeter, which is connected to the secondary winding of the transformer.
Method of testing the cable with increased voltage
- Check the insulation of all current-carrying conductors one by one with respect to each other and with respect to the metal grounded sheath using a megohmmeter for a voltage of 2500 V.
- Connect the current-carrying core to the output of the test facility, ground the metal screen and the armor. If the cable has other current-carrying cores, they must also be grounded.
- On the opposite side of the cable, wire the bare ends to a distance of about 150 mm.
- The measurement technique assumes a smooth gradual increase in the test voltage at a rate of about 1 kV per second.
- After reaching the required value of the voltage value, the countdown begins and the value of the leakage current is fixed.
- After the completion of the measurements, the adjusting knob is set to the zero position, the power is turned off from the test facility, the blocking from accidental voltage supply to the cable line is turned on, the high-voltage output is necessarily grounded.
- After all preventive and protective measures have been taken, it is allowed to proceed with the disassembly of the test circuit.
If the insulation of the cable line is made of cross-linked polyethylene, then such a product is not tested with rectified current due to the possible accumulation of a large amount of space charges in the thickness of the insulation. Such products are tested using a low-frequency alternating voltage, which makes it possible to exclude the formation of a space charge.
The above diagram shows an example of connecting the AID-70 test facility, which provides the application of an alternating voltage of up to 50 kV or a direct current voltage of up to 70 kV. When using measuring equipment of other types, the power cable connection scheme will not fundamentally change.
Lightning impulse test
Testing electrical equipment using lightning impulses is strictly regulated by GOST 1516.3 - 96. Testing of a power cable is carried out with a standard lightning impulse with a front of 1.2 μs and a half-decay duration of 50 μs. This type of control of insulation parameters differs significantly from the standard voltage with an industrial frequency of 50 Hz. Lightning impulse tests are characterized by a steeper current-voltage characteristic.
Sheath test of XLPE cable
XLPE cable for voltage 110 - 500 kV is not tested with direct voltage due to a significant reduction in its service life during such tests. To solve this problem, control is used using an increased voltage of a sinusoidal form with a low frequency (0.01 - 0.1 Hz). In this case, the test time can be up to one hour. This test method is widely used in the USA, where it is called VLF (Very Low Frequency). The current regulatory legal acts of the Russian Federation in no way regulate high voltage tests for cable products for a voltage of 110 - 500 kV with XLPE insulation. The main types of checks in this case will be the recommendations of the manufacturer.
The sheath of cables with XLPE insulation can be damaged due to various mechanical influences. If this defect is not promptly and quickly eliminated, the main insulation of current-carrying conductors will quickly lose its protective properties. The test standards for the XLPE cable, namely its sheath, include the following requirements:
Cable line voltage, kV
DC test voltage, kV
Duration of test voltage application
Checking the sheath of cables with XLPE insulation is carried out in the following cases:
- Before commissioning the cable.
- After cable repair.
- Periodically during operation, the first time after 2.5 years, and then every 5 years.
- After excavation in the security zone of a particular cable.
To perform cable testing, PTS use specialized diagnostic equipment adapted to test such cable lines.
Conducting industrial frequency tests of switchgears
Testing the cable with increased voltage of industrial frequency is carried out for cable lines for a voltage of 110 kV - 500 kV. This type of test replaces the rectified current test. The value of the test voltage in this case is 145 kV for cables for a voltage of 110 - 220 kV, 288 kV for cable lines for a voltage of 220 - 500 kV. The duration of these tests is five minutes.
Testing Power Cables with Impregnated Paper Insulation
Testing a cable with impregnated paper insulation with increased voltage is characterized by its own characteristics:
- If there is a conductive screen, its galvanic connection with the shell is carried out.
- First, a test voltage of 40% of the required value is applied, after which it is gradually brought to the required value.
- If the increase in the test voltage is carried out smoothly, then the rate of its rise should be no more than 1 kV per second. If the regulation is carried out in steps, then the lifting speed at one time should be no more than 5% of the full value.
High-voltage tests must be carried out before putting the cable into operation, which ensures reliable and stable operation of electrical equipment. Systematic checks of cable lines with high voltage impregnated paper insulation are combined with visual inspection, checking the insulation resistance.
Frequency of inspection
High voltage cable test standards include the following frequency requirements for products for voltage 6 - 35 kV:
- Newly laid before switching on, before backfilling, after re-laying.
- Operating cables after repair, according to the schedule of preventive maintenance, unscheduled after emergencies.
Cable lines with impregnated paper insulation for a voltage of 6 - 35 kV are tested with increased voltage with the following frequency:
- Once a year for cable lines feeding especially critical facilities.
- Once every three years for cable lines supplying all other consumers.
- Once every five years for distribution lines.
It is allowed not to conduct tests for cable lines that are subject to decommissioning in the next five years or are conclusions from TP, RP to overhead power lines.
Products with XLPE insulation are tested with increased voltage after each repair and before commissioning. The protective sheaths of such cables are additionally checked after excavations in the protected zone of the cable lines and periodically once every 5 years (the first time after 2.5 years).
Registration of test results in the form of a protocol (example)
The cable test report is a technical report on the work done. It confirms the operability of the tested electrical equipment and the possibility of its further operation. Checking the existing cable lines makes it possible to exclude their breakdown during operation, which can cause a long interruption in the power supply to consumers. Information from the cable test report can be used to track the condition of the insulation and predict its future condition.
The protocol includes the following data:
- Technical information: brand, section, rated voltage.
- Description of the cable line: length, installation location.
- Measurement results for each phase.
- Information regarding tests of couplings and shells.
- Data on test equipment with serial number.
- The conclusion of the specialist who performed the measurements.
- Weather conditions: temperature and humidity level of the surrounding air.
- Links to official documents on the basis of which tests and measurements were carried out.
- Signature of the person responsible for carrying out the measurements, indicating his position and the electrical safety group.
- The license number of the company that conducts high voltage tests.