IEC60502-1997 (applicable to medium and low voltage cables 1~30kV), IEC60840-1999 (for high voltage cables 45~150kV), and IEC62067-2001 (for extra-high voltage cables 220~550kV) standards have been promulgated, with varying technical requirements for cross-linked cable products and more stringent specifications.
1.1 Partial Discharge Performance Indicator
The following are the test indicators for partial discharge of various cables:
Medium and Low Voltage Cables
Factory tests now require the partial discharge quantity to be no more than 10 pC at 1.73 U0 voltage, down from the previous limit of 20 pC at 1.5 U0. Type tests now require the partial discharge quantity to be no more than 5 pC at 1.73 U0 voltage, also down from the previous limit of 20 pC at 1.5 U0.
b) High-voltage Cables
Factory tests have confirmed that the partial discharge is not greater than 10 pC under 1.5 U0 voltage; type tests show the partial discharge is not greater than 5 pC under 1.5 U0 voltage.
c) Ultra-High Voltage Cable
The factory test specifications state that partial discharge quantity is 10 pC or less at 1.5 U0 voltage, or there is no discernible partial discharge at a lower sensitivity level of background noise.
The type test specification states that partial discharge quantities are 5 pC or less at 1.5 U0 voltage or unresolvable under the sensitivity of lower background noise.
1.2. Tensile Test
The following are the specifications for different cable withstand voltage tests:
Low and medium voltage cables
The withstand voltage for factory tests has been increased from the previous 2.5 U0 to 3.5 U0; for type tests, it has been raised from the previous 3 U0 to 4 U0, with the withstand time remaining unchanged at 4 hours.
b) Extra-high voltage cables
Factory testing includes both power frequency withstand voltage tests to extend the voltage endurance time and an additional pre-qualification test project beyond the traditional test items; the high-field strength thermal cycling test aims to evaluate the long-term safe operational performance of the cable system.
1. Post-installation laying test
The post-installation testing emphasizes the AC withstand voltage test. In recent years, both domestically and internationally, there is a consensus that for the withstand voltage test of cross-linked cables, the AC testing method is preferred over the DC withstand voltage test, which is adapted from the oil-paper insulated cable testing method.
Testing of extra-high voltage cables is permitted only using AC testing methods, conducting AC withstand voltage tests within the range of 20 to 300 Hz, and selecting voltages between 1.1 U0 and 1.7 U0.
High-voltage cable testing now prioritizes the AC method over the DC method, reversing the previous order of selection from DC to AC.
Considering the current实际情况 and operational convenience, DC withstand voltage testing is still retained for new installations of medium and low voltage cross-linked cables.
1. Focus on the overall quality level of cross-linked cable products
The cable core is just a part of the cable line, and cable accessories are also one of the components of the cable line. Only when both the cable core and its accessories function properly can the transmission cable line operate safely. To this end, the new standard has specifically added a pre-qualification test item — the high-field strength thermal cycle test. The specific implementation method is: for one year, apply a continuous voltage of 1.7 times the rated operating voltage, while heating the cable for at least 8 hours and cooling it for at least 16 hours in each cycle, with a total of at least 180 cycles. After the test, the cable undergoes lightning impulse withstand voltage testing, followed by inspection of the sample, ensuring there is no moisture intrusion, leakage, or corrosion. Only after passing all the tests is the pre-qualification test considered to be合格, and the cable can then be safely put into commercial operation.
Method for AC Cable Dielectric Withstand Test
Power cables typically have a significant capacitance, presenting a capacitive load under AC test voltages. Long power cables require large AC test equipment and power sources. To achieve testing purposes with smaller capacity test power sources, the following resonance methods are generally employed.
2.1 Power Frequency Resonance Test Transformer with Fixed Reactor Compensation
Most of the capacitor current is supplied by the compensating reactor, with a small portion of the capacitor current resonating with the resonance test transformer to achieve the testing objectives. This method requires a minimal power supply capacity, only necessitating the active power capacity of the entire circuit. To employ this method, a resonance transformer is required, whose output voltage must reach the test voltage. Additionally, the output current range of the resonance transformer must be compatible with the reactor to meet the needs of cables of varying lengths.
2.2 Tuned Series Resonant Reactor
By utilizing the capacitive characteristics of the cable lines, a tunable reactor is used to achieve a resonant series connection with it. The test power capacity only needs to meet the active power loss of the circuit, with relatively low requirements for the capacity of the step-up transformer and the output voltage. However, due to the impracticality of making the tunable reactor too large (a reactor with 250 kV and a rated current of 2 A is already quite large, and an 8-ton car can only carry two of these reactors), a set (multiple) of fixed high-current reactors must be used in conjunction with it to test cables of varying lengths.
2.3 Tuned Radio Frequency Series Resonant Voltage
Selecting the appropriate reactor and cable in series, using a tunable frequency power supply, and stepping up the voltage through a transformer, adjust the power supply frequency to induce resonance in the circuit and generate high voltage at the cable ends, thereby achieving the testing objectives.
As of now, the domestic testing level status.
Historically, the cable AC withstand voltage tests conducted within the province were generally entrusted to the Guangdong Provincial Electric Power Testing Research Institute, as other entities had yet to possess the capability to initiate such projects. The original Guangzhou Power Supply Bureau's testing facility featured a tuning inductor that could be used for cable AC withstand voltage tests (power frequency). However, this tuning inductor was only capable of testing cables with a voltage of 220 kV, a cross-sectional area of 1,200 mm², and a length of 382 meters.
Guangdong Provincial Electric Power Research Institute uses a series-frequency tuned resonant voltage withstand device for cable testing, which is at the forefront domestically. The Zhejiang Province and East China Research Institute both utilize this equipment. The power source for this device is an imported Siemens frequency tuning power supply, which is easy to adjust, reliable, and requires minimal maintenance. Following the frequency tuning power supply is an intermediate step-up transformer, which boosts the voltage from hundreds of volts to thousands of volts and then applies it to the series circuit composed of the reactor and cable. By changing the frequency of the power supply, the circuit is brought into resonance, and the output voltage of the frequency tuning power supply is increased to reach the required withstand test voltage at the cable ends.
This unit is not only used for cable testing but also allows for the stacking of reactors in series to conduct GIS withstand tests from 110 to 500 kV. Additionally, equipped with a medium-voltage (approximately 10kV) reactor with high current (dozens of amperes) and appropriate inductance parameters, it can also perform tests such as induction withstand voltage, partial discharge measurement, and neutral point withstand voltage for 110kV and 220kV transformers.
As of now, the cable testing status at Shenzhen Power Supply Branch.
Shenzhen Power Supply Branch has predominantly used cables produced by Mitsubishi Corporation of Japan for its 110 kV cables, with nominal cross-sectional areas of 630 mm² and 800 mm², and lengths up to approximately 5 km. Similarly, for its 220 kV cables, the majority have been from Mitsubishi Corporation, with a nominal cross-sectional area of 1,200 mm², and the longest installed cable extends about 2.7 km.
With the improvement of domestic cable manufacturing, since 1998, Shenzhen Power Supply Branch has started using domestically produced cross-linked cables in urban and rural power grid construction projects. From 2000, it has independently undertaken the AC withstand voltage tests for 110 kV cross-linked cables. Equipped with a sufficient capacity of frequency-modulated power sources and four reactors with a rated voltage of 250 kV and a rated current of 20 A, the electrical testing department is capable of conducting AC withstand voltage tests for cross-linked cables up to 220 kV and even 500 kV. It can test cables up to approximately 5.9 km in length. However, for 220 kV cables, it is not yet possible to monitor local discharge during the withstand voltage test process.
