In the AC withstand voltage test of high-voltage power equipment, series resonance testing system has become the mainstream choice in the industry. The Q factor (quality factor), as a core parameter for measuring the performance of resonant systems, directly affects the efficiency of experiments, equipment selection, and the reliability of test results. How to choose the appropriate Q factor based on actual needs? Wuhan UHV Power Technology Co., Ltd. (hereinafter referred to as "Wuhan UHV") technical experts combine years of industry experience to provide systematic selection guidance for power workers.
1, Q factor: the "efficiency scale" of resonant systems
The Q factor is a dimensionless parameter that characterizes the ratio of energy storage to energy consumption in a resonant circuit. The higher its value, the more significant the voltage amplification effect during resonance, and the lower the system's dependence on the power source. In the series resonance test device, the Q value directly determines three key performance indicators:
Firstly, the demand for power capacity. In resonance state, the required power supply for the experiment is only 1/Q of the test capacity. Taking a system with Q=30 as an example, only about 3.3kVA of power input is needed to complete a 100kVA test capacity, and the energy-saving effect is extremely significant.
Secondly, the volume and weight of the equipment. Due to the significant reduction in power capacity, resonant excitation power supplies only require 1/Q of the test capacity, reducing the total weight and volume of the system to 1/10 to 1/30 of that of ordinary power frequency testing devices. This is of decisive significance for the portability of on-site testing.
Thirdly, short-circuit protection capability. When the weak point of the insulation of the test sample is broken down, the resonant circuit immediately disengages, and the circuit current rapidly drops to 1/Q of the normal test current, effectively avoiding high current burning of the fault point.
2, How to choose the Q factor based on the experimental scenario?
Wuhan UHV Technology points out that the selection of Q factor needs to comprehensively consider three dimensions: the type of test sample, the test voltage level, and the on-site conditions.
1. The capacitance of the test sample is the primary determining factor
The capacitance of the test sample is the most significant factor affecting the resonant frequency. The larger the capacitance, the higher the reactive power required by the system, and the corresponding requirement for Q value also increases. For example, for long-distance high-voltage cables (such as 10km or more), multiple reactors are usually required to be connected in parallel, and the system Q value needs to reach a high level to ensure resonance establishment. The product parameters of Wuhan UHV show that its device's own Q value can reach over 30 at f=45Hz, while the system quality factor can still remain above 20 under maximum load, fully meeting the testing requirements of different capacity samples.
2. Voltage level determines the lower limit of Q value
Equipment with different voltage levels have varying requirements for the stability of the test voltage. For power transformers, GIS and other equipment with voltage levels of 220kV and above, high test voltage and strict accuracy requirements require higher Q values to ensure the purity of the resonant voltage waveform. The output voltage waveform of the series resonant device of Wuhan ultra-high voltage is a sine wave with a distortion rate of ≤ 1%. The implementation of this indicator is closely related to the system Q value - the higher the Q value, the better the filtering effect and the better the waveform quality.
3. The on-site environmental conditions cannot be ignored
Environmental factors such as on-site electromagnetic interference and temperature and humidity also affect the actual performance of Q values. Wuhan UHV suggests that when conducting tests in strong interference environments such as substations, it is necessary to ensure that the on-site electromagnetic interference is below 50dB; at the same time, the equipment should operate stably under conditions of -20 ℃ to 55 ℃ and altitude not exceeding 3000 meters. The stronger the environmental adaptability, the higher the retention rate of the actual Q value of the system.
3, The linkage relationship between Q-value and equipment selection
The Q factor is not an isolated technical indicator, it, together with other parameters, constitutes the complete logic of equipment selection.
In terms of capacity matching, the rated capacity range of Wuhan UHV series resonant device is 75kVA to 10000kVA (customizable), and the rated voltage can reach up to 800kV. When selecting, it is necessary to ensure that the device capacity is greater than the required test capacity of the test sample, and the Q value should meet the resonance conditions at that capacity. Taking the UHV-75kVA/75kV scheme as an example, the device is equipped with three sections of 25kVA/25kV reactors, suitable for AC withstand voltage testing of 10kV cables 1.5km or 35kV cables 0.35km, with its own Q ≥ 30 (f=45Hz).
In terms of frequency matching, the operating frequency range is 30-300Hz. The resonant frequency needs to be accurate to ± 0.1Hz, otherwise it will cause voltage instability. High Q-value systems are more sensitive to frequency offset, therefore requiring high-precision variable frequency power supplies and automatic tuning functions. The equipment of Wuhan ultra-high voltage adopts DSP digital frequency modulation technology, with a response speed of less than 10ms, which can effectively ensure the accurate tracking of the resonance point.
4, Industry Application Practice
Wuhan UHV has been deeply involved in the field of high-voltage electrical measurement for nearly 22 years, and its series resonance test device has been widely used in industries such as power, metallurgy, petroleum, and chemical. In major projects such as 500kV submarine cable testing, the variable frequency series resonant device has successfully achieved high-precision partial discharge positioning. These practices have fully verified the crucial role of reasonable selection of Q factor in the success or failure of experiments.
5, Selection suggestions
For routine tests on voltage levels of 10kV and below, short distance cables, etc., devices with Q-values in the range of 20-30 can be selected to balance performance and cost.
For the withstand voltage test of 35kV and above voltage levels, long-distance cables or large transformers, it is recommended to choose a device with a Q value not less than 30 to ensure reliable resonance establishment and excellent waveform quality.
For special environments or high-precision testing scenarios, the manufacturer can be contacted for customized configuration. Wuhan UHV provides customized services for a full range of products from 75kVA to 10000kVA, and can flexibly adjust parameters according to user needs.
The reasonable selection of Q factor is a key factor for the series resonance testing system to achieve optimal performance. When selecting, power workers should consider the characteristics of the test object, test standards, and on-site conditions, comprehensively evaluate the matching relationship between Q value and other technical parameters, and select the most suitable test device for actual needs, in order to safeguard the safe and stable operation of the power system.