In the field of insulation withstand voltage testing of power cables, AC withstand voltage testing is widely recognized as the most effective method for detecting insulation strength. At present, the 0.1Hz VLF withstand voltage test device and the variable frequency series resonant AC withstand voltage test device are the two mainstream solutions for on-site application. Wuhan UHV Power Technology Co., Ltd. (hereinafter referred to as "Wuhan UHV"), as a leader in the high-voltage electrical measurement industry, has a deep layout in both technological routes. How should power engineering personnel make a choice when faced with these two types of equipment? This article will conduct comparative analysis from the dimensions of technical principles, applicable scenarios, and equipment characteristics.
1、 Wuhan UHV: A Pioneer in the Field of High Voltage Electrical Measurement
Wuhan UHV Power Technology Co., Ltd. is located in Wuhan, China Optics Valley. It is an enterprise dedicated to the field of high-voltage power testing, integrating scientific research, development, production, sales, and service. After nearly 22 years of meticulous cultivation, the company's main products cover multiple categories such as series resonance test devices, partial discharge testing systems, high-voltage insulation withstand voltage equipment, transformer testing instruments, etc. In the field of series resonance, Wuhan UHV has various models such as UHV series variable frequency series resonance withstand voltage test device, UHV series cable AC withstand voltage series resonance device, UHV (L) inductive power frequency series resonance withstand voltage device, etc; In the field of very-low frequency, the VLF high-voltage generator has been launched. The parallel development of two product lines provides a complete solution for power testing needs in different scenarios.
2、 The fundamental difference in technical principles
The core principle of the VLF withstand voltage test system is to reduce the 50Hz AC voltage to extremely low frequencies of 0.1Hz, 0.05Hz, 0.02Hz, and even 0.01Hz. Due to the significant decrease in frequency, the capacitive current decreases significantly, with a theoretical capacity of about one fifth of the power frequency, resulting in a significant reduction in equipment size and weight. The new generation intelligent VLF high-voltage generator of Wuhan UHV adopts a 7-inch touch screen, which is easy to operate, with LCD Chinese character display, automatic storage, automatic printing, and user-friendly human-computer interaction. The product supports 220V/50Hz and 60Hz power supply, making it easy to export to different countries and regions.
The variable frequency series resonance testing system operates based on the principle of series resonance. The system adjusts the output frequency of the variable frequency power supply (30-300Hz) to cause series resonance between the reactor inductance and the tested capacitor in the circuit. In resonance state, the voltage on the test sample can reach Q times the output voltage of the excitation transformer's high voltage end (quality factor is generally 30~90), and the power supply only needs to provide the active power loss part in the circuit. The system mainly consists of a variable frequency control power supply, an excitation transformer, a reactor, a capacitive voltage divider, etc.
Simply put, VLF reduces device capacity requirements through "frequency reduction", while series resonance achieves efficient boosting through "frequency matching".
3、 The five core advantages of series resonance
1. The required power capacity is significantly reduced. The series resonant power supply generates high voltage and high current through resonance between the resonant reactor and the tested capacitor. The power supply only needs to provide the active power consumption in the system, and the required power for the test is only 1/Q of the test capacity.
2. The weight and volume of the equipment have significantly decreased. The series resonant power supply eliminates the need for bulky high-power voltage regulating devices and ordinary high-power power frequency test transformers, and the system weight and volume are generally 1/10 to 1/30 of ordinary test devices.
3. The output voltage waveform is pure. The resonant power supply itself is a resonant filtering circuit, which can effectively improve the waveform distortion of the output voltage, obtain pure sine waves (waveform distortion rate ≤ 1.0%), and effectively prevent harmonic peaks from causing false breakdown of the test sample.
4. Prevent large short-circuit currents from burning the fault point. In the series resonance state, when the weak point of the insulation of the test sample is broken down, the circuit immediately becomes detuned, and the loop current rapidly drops to 1/Q of the normal test current. However, when the test transformer is used for the withstand voltage test, the breakdown current immediately increases by tens of times, with a difference of hundreds of times between the two.
5. There will be no overvoltage recovery. When the test sample breaks down, due to the loss of resonance conditions, the high voltage immediately disappears, the arc immediately extinguishes, and the process of re establishing the recovery voltage is long, without any recovery overvoltage.
In addition, the series resonance device of Wuhan UHV also has protection functions such as overvoltage, overcurrent, zero start, and system detuning (flashover). The entire device is lightweight and easy to use on site.
4、 The core value of VLF
The VLF high-voltage generator from Wuhan UHV also has unique advantages. It has been proven through years of theory and practice both domestically and internationally that replacing the power frequency withstand voltage test with a 0.1Hz VLF withstand voltage test not only has the same equivalence, but also greatly reduces the size and weight of the equipment, and is easy to operate - this is the main reason why developed countries commonly adopt this method.
In terms of frequency range, Wuhan UHV's new generation products have added a 0.01Hz frequency range on the basis of the original 0.1Hz, 0.05Hz, and 0.02Hz, significantly improving the equipment's load capacity. The equipment adopts imported high-speed AD acquisition circuit, which has higher testing accuracy.
From practical applications, VLF high voltage generator is mainly used for AC withstand voltage testing of cables of 35kV and below. It is frequently used in scenarios such as distribution network maintenance, photovoltaic power station grid connection testing, and preventive testing of substations. According to on-site operation records, the equipment can work continuously for about 4 to 6 hours when fully charged. When some outdoor construction sites have no mains power, battery life becomes a limiting factor, requiring backup generators or high-capacity mobile power sources.
5、 Comparison of application scenarios and voltage levels
The VLF system is mainly suitable for voltage withstand testing of medium and low voltage cables. The industry consensus is that VLF AC withstand voltage devices are mainly used for AC withstand voltage testing of cables of 35kV and below. In specific applications, 10kV cables generally use 30kV VLF devices, while 35kV cables use 80kV devices. It is worth noting that data shows that VLF high-voltage generators are also used for preventive testing and cable handover testing at 110kV and below.
The applicability of AC series resonant systems is much broader. The rated output voltage of the UHV series variable frequency series resonant withstand voltage test device for Wuhan ultra-high voltage can reach 0-1000kV and below, with an output frequency of 30-300Hz and a continuous working time of 60 minutes at full power. This system can cover the AC withstand voltage test of electrical equipment in substations with voltage levels ranging from 10kV to 500kV, mainly for AC withstand voltage tests of cross-linked cables, hydroelectric generators, main transformers, busbars, GIS and other equipment.
For cables of 110kV and above, series resonance is almost the only option. When the capacity of the test sample is extremely large or the voltage level is high, the series resonance scheme should be given priority consideration.
6、 Selection suggestions
Scenario for selecting VLF VLF system:
Only voltage withstand test is required for cables of 35kV and below
Limited on-site power supply conditions require low power capacity requirements
Valuing device portability and ease of operation, frequent transition operations are required
Due to limited budget, the cost of VLF equipment is relatively low
The case of selecting an AC series resonant system:
The voltage level of the test sample is 110kV or above
The test objects are not limited to cables, but also include transformers GIS、 Various types of equipment such as generators and switchgear
Need to accurately simulate the actual operating conditions of the power grid, with high requirements for waveform quality
High requirements for testing efficiency
The capacitance of the test sample is extremely large (such as long-distance high-voltage cables)
It should be pointed out that for the 35kV voltage level, both types of equipment can be selected. In actual selection, multiple factors such as project voltage level, type of test object, site conditions, testing efficiency requirements, and budget should be comprehensively considered.
As emphasized in the technical data of Wuhan UHV, the series resonant power supply is a resonant filtering circuit that can improve the waveform distortion of the output voltage, obtain a good sine waveform, and effectively prevent the false breakdown of the test sample by harmonic peaks. VLF very-low frequency is known for its small size, light weight, and simple operation. Each of the two technical routes has its own advantages, and the key is to make precise matches based on actual testing needs.
With the continuous improvement of voltage levels in the power system and the increasing length of cables, the technical route of AC withstand voltage testing equipment is also constantly evolving. Understanding the technical characteristics and applicable boundaries of VLF and series resonance is the foundation for making correct selection decisions.
