Standard system and applicable boundaries
The standards involved in the cable series resonance test can be divided into three levels.
The acceptance and commissioning level is based on GB 50150-2016 “Electrical Installation Engineering – Electrical Equipment Acceptance Test Standard”, which is used for the pre-commissioning assessment of newly installed cables. The 17th chapter “Power Cable Lines” of this standard specifies the test voltage value, the duration of voltage application, and the criteria for qualification.
The condition-based maintenance level is based on DL/T 596-2021 “Power Equipment Preventive Test Code”, which is applicable to the periodic preventive tests of operating cables. When this code was revised in 2021, it added requirements for 750kV equipment and incorporated new detection items such as partial discharge detection, reflecting the penetration of the state maintenance concept into the test practice.
The test method level is undertaken by GB/T 3048.8-2007 “Electrical Performance Test Methods for Wire and Cable – Part 8: AC Voltage Test”. This standard corresponds to IEC 60060-1:1989 Chapter 5, but has made multiple non-equivalent modifications. The specific provisions for the series resonance circuit make it the most direct methodological basis for resonant test.
The relationship among the three can be summarized as: GB/T 3048.8 addresses “how to test”, GB 50150 and DL/T 596 address “what level of qualification is considered acceptable”.
Technical compliance of the resonant circuit
Quantitative boundaries of waveform requirements
GB/T 3048.8-2007 provides clear quantitative constraints for the test voltage waveform: the waveform should be an approximately sinusoidal wave with the same half-wave, and the ratio of peak value to effective value should be √2 ± 0.07; the effective value of the resonant wave should not exceed 5% of the fundamental wave effective value. This requirement transforms “approximately sinusoidal wave” from a qualitative description into a measurable engineering criterion.
The quality of the voltage waveform output by the series resonance device depends on the harmonic content of the variable frequency power supply and the linearity of the reactor. The THD of the variable frequency power supply is usually required to be controlled within 1%, and the non-linearity of the inductance near the resonant point of the reactor will introduce waveform distortion. During on-site tests, if it is found that the voltage waveform deviates significantly from a sine wave or the peak ratio exceeds the tolerance, the first thing to check is whether the reactor core is saturated or the output of the variable frequency power supply is normal.
Applicability boundaries of variable frequency and variable reactance
Appendix A of GB/T 3048.8-2007 specifically stipulates “Variable Reactance Series Resonance Test Circuit and Parameter Selection”, indicating that the variable reactance scheme was still an important technical path when the standard was formulated. Variable reactance adjusts the inductance by changing the air gap of the reactor core or the number of turns of the coil, making the circuit resonate near the power frequency; variable frequency adjusts the frequency of the power supply to achieve resonance.
There is no superiority or inferiority in terms of standard compliance between the two methods, but the applicable scenarios are different. Variable frequency resonance can be adjusted within the range of 30-300Hz, which is more convenient for matching different capacitance test samples; variable reactance is limited by mechanical adjustment accuracy and has a lower search efficiency for the resonant point, but the frequency is closer to the power frequency, making it more suitable for equipment that is sensitive to frequency (such as certain oil-paper insulation structures). In cable test practice, variable frequency methods have dominated due to their operational convenience, but attention should be paid to the influence of test frequency on the starting voltage of partial discharge.
Quality factor and power supply capacity
The core advantage of series resonance lies in the quality factor Q. During the test, the voltage on the test sample is Q times the output voltage of the excitation transformer, and the power supply only needs to provide the active power loss of the circuit, requiring a power of approximately 1/Q of the test capacity. In engineering practice, the Q value of equipment below 35kV is usually required to be no less than 40, and for equipment above 110kV, it is no less than 50.
The Q value is not necessarily the higher the better. An excessively high Q value indicates a large stored energy in the circuit. Once the test specimen breaks down, the released energy will be even greater, which may cause additional damage to the fault point. The standard’s response to this is reflected in the circuit design concept: when the test specimen breaks down under resonance, the short-circuit current automatically drops to approximately 1/Q of the test current. This “self-limiting current” characteristic is an inherent safety advantage of the series resonance method, but it is conditional on the presence of an adequate linear range and heat capacity of the reactor.
The standard’s operational guidelines are based on:
Rate control of the voltage rise procedure
GB/T 3048.8-2007 specifies the specific voltage rise procedure: starting from a value not exceeding 40% of the test voltage, slowly increase the voltage; when exceeding 75% of the test voltage, the voltage rise rate should be controlled at approximately 2% of the test voltage per second. After maintaining the specified time, the voltage should be reduced to below 40% of the test voltage before cutting off the power supply to avoid transient processes causing faults or incorrect readings.
The engineering significance of this procedure lies in: in the low-voltage initial stage, it eliminates operational transient interference; when above 75% of the test voltage, using a constant rate of voltage rise ensures sufficient time to observe and record near the breakdown voltage. For series resonance devices, the output regulation accuracy of the variable-frequency power supply should meet this rate requirement. Rapid voltage rise may trigger transient oscillations in the resonant circuit, while slow voltage rise will prolong the time that the cable is subjected to voltages close to the test voltage.
Selection of test frequency
The typical frequency range for cable series resonance tests is 30-300Hz, and the actual commonly used frequency is around 35Hz. GB/T 3048.8-2007 defines the power frequency test voltage as 49-61Hz, but it also recognizes that the frequency of the series resonance circuit can be outside this range. This seemingly contradictory regulation actually reflects the standard’s pragmatic compromise for the special characteristics of cable loads: due to the large capacitance of the cable, if forced to resonate at 50Hz, the required reactor inductance would be too large, and the equipment would not be practical.
The selection of the standard is: allowing deviation from the power frequency, but ensuring the equivalence of the test through waveform requirements and voltage measurement accuracy. During on-site operations, the actual resonant frequency should be recorded and confirmed to be within the design range of the device; if there is abnormal drift in the frequency (such as due to the temperature rise of the reactor causing a change in inductance), the test should be suspended until it stabilizes before re-tuning.
Determination of test voltage
GB 50150-2016 Chapter 17 stipulates the AC withstand voltage test voltage for power cable lines, and Appendix D provides the AC withstand voltage test voltage values for power transformers and reactors. For cables, the test voltage is usually expressed as a multiple of U₀ (the rated voltage between conductors and ground). For a 10kV cable, the common test voltage is approximately 22kV (at the level of 2.5U₀); for a 35kV cable, it is approximately 52kV; for a 110kV cable, it reaches above 128kV.
When selecting the device, the rated output voltage should be higher than the test voltage and leave a margin. Industry practice is to configure at 1.1-1.25 times, covering the offset of the resonant point caused by the capacitance deviation at the site and avoiding the transformer being in the limit voltage range for a long time.
Judgment of breakdown and abnormal handling
Distinguishing between breakdown and “false breakdown”
GB/T 3048.8-2007 Chapter 7 “Test Results and Evaluation” stipulates: if the test sample shows no breakdown phenomenon within the specified test voltage and duration, it is judged as passing. However, the standard also introduces the concept of “false breakdown” – when the test sample has excessive capacitance, low insulation resistance, excessive leakage current, or the setting value of the equipment’s overcurrent protection is too low, the device may trip, presenting a false appearance similar to breakdown.
False breakdown is particularly common in series resonance tests. The reason is that the cable has a large capacitance, and the capacitive component dominates the resonant current. If the overcurrent protection threshold is set improperly, the excitation current or stray current may trigger the protection during the voltage increase process. The handling principle is: after an abnormal trip occurs, it should not be directly scrapped. Instead, the protection setting values should be checked, the insulation resistance should be remeasured with an ohmmeter, and if necessary, the voltage should be reduced for retesting. After eliminating the false breakdown, a judgment can be made.
Handling after breakdown
If it is confirmed that a real breakdown has occurred (accompanied by abnormal sounds, sudden increase in current, and sudden drop in voltage), the voltage should be reduced and the power supply should be cut off immediately. The self-limiting current characteristic of the series resonant circuit ensures that the breakdown point will not withstand a continuous large current, but the location of the fault point still requires subsequent handling. GB/T 3048.8-2007 stipulates that if the test terminal of the sample undergoes surface flashover or internal breakdown, another test terminal can be made and the test can be repeated.
Safety grounding and discharge
Grounding and discharging before and after the test are mandatory safety requirements. Chapter 8 of GB/T 3048.8-2007 clearly stipulates: after the test, grounding and discharging should be performed, the grounding resistance should be less than 4Ω, and the grounding end of the test device and the grounding end of the sample should be reliably connected. The reactor and capacitor divider of the series resonant device may store charges, and insufficient discharge is one of the main causes of on-site electric shock accidents.
The technical advantages of the series resonant test device have been fully verified in the field of cable withstand voltage tests, but “easy to use” does not mean “random use”. The regulations of GB/T 3048.8-2007 regarding waveforms, voltage rise procedures, and measurement accuracy, as well as the requirements of GB 50150-2016 and DL/T 596-2021 for test voltage and cycle, together form a standard defense line. What the on-site test personnel need to understand is: the reason why the resonant method is recognized by the standard is not because it is “convenient”, but because its waveform characteristics and breakdown self-limiting current characteristics can be equivalent to traditional power frequency tests under controllable conditions. Deviating from the standard operation undermines the basis of equivalence.
With the introduction of new standards such as DL/T 2557-2022 “Field Test Guidelines for AC External Applied Voltage and Partial Discharge on Valve Side of Converter Transformers”, the application scope of the series resonant method is expanding from cables to more equipment types. The evolution of the standard reminds us: technical compliance is not a one-time choice, but a continuous practice calibration.
Post time: Oct-09-2026