Abstract: This paper analyzes the market and proposes research on gas-filled integrated high-voltage parallel capacitors, and introduces the characteristics of the developed products. Keywords: Gas-filled integrated high-voltage parallel capacitors, research and development 1 Introduction With the rapid development of China's economy and the continuous improvement of people's living standards, the demand for electricity is constantly increasing, and safety and environmental protection issues are receiving increasing attention. There is an urgent need for flame-retardant, explosion-proof, and environmentally friendly power transmission and distribution equipment. The clear guiding ideology for the construction and transformation of urban and rural power grids in China is to develop towards less oil and oil-free systems. Oil-free products are increasingly welcomed by the power sector, and the demand is constantly growing. With the continuous progress of power technology, new technologies are developing rapidly, and new products are emerging one after another. At present, oil-free dry-type insulated combined electrical appliances, transformers, transformers, reactors, and circuit breakers are increasingly widely used, but there is a lack of inexpensive and high-quality dry-type oil-free high-voltage capacitors. In high-voltage parallel compensation complete sets of equipment, other supporting equipment can be easily made oil-free, but high-voltage parallel capacitors, due to the special nature of their working medium, face great difficulties in achieving oil-free products. Currently, only a small number of dry-type oil-free high-voltage capacitors are in operation. The technology is not yet fully mature, product performance is not entirely stable, and the cost is high, hindering widespread adoption. Therefore, gas-filled, flame-retardant, and explosion-proof parallel capacitors with reduced oil content have emerged. Traditional oil-filled parallel high-voltage capacitors are popular due to their simple wiring, convenient installation, use, and maintenance, and small footprint. However, their large volume of insulating oil does not align with the trend towards oil-free power systems. The new gas-filled parallel high-voltage capacitor is developed based on the already mass-produced oil-filled capacitors with over a decade of manufacturing and operational experience. It replaces a large amount of insulating oil with a non-flammable insulating gas, retaining and leveraging the advantages of the original product while adhering to the concepts of safety, environmental protection, and reduced oil content. The successful development and widespread use of gas-filled parallel high-voltage capacitors have met market demands and are highly favored by users. Please log in to: Power Transmission and Distribution Equipment Network for more information 2 Research on Gas-Filled Parallel High-Voltage Capacitors Traditional oil-filled parallel capacitors suffer from drawbacks such as easy oil leakage, flammability and explosion under fault conditions, environmental pollution, and heavy weight, due to the large amount of flammable insulating oil (transformer oil or alkylbenzene oil) between the large casing and the unit capacitors, which fills hundreds to thousands of kilograms between the casing and the unit capacitors. Replacing the insulating oil with non-flammable insulating gas can overcome these disadvantages. However, compared with insulating oil, the insulating gas has poorer insulation and heat dissipation performance under normal conditions, and the gas is more difficult to seal. Therefore, how to select a suitable insulating gas, minimize the heat generation of the product, and solve the problems of heat dissipation, insulation, and sealing are the main research contents of this product development, among which heat dissipation is the focus and difficulty of the research. 2.1 Heat Dissipation SF6 gas is a poor conductor of heat, and its heat dissipation performance is much worse than that of insulating oil. If the oil in the oil-filled parallel capacitor is simply replaced with gas, it will inevitably cause the temperature rise of the capacitor unit to be too high, resulting in performance degradation and thermal breakdown damage. Therefore, solving the thermal stability and thermal breakdown problems of gas-filled capacitors, and minimizing the temperature rise of individual capacitor units to maintain their good operating condition, is the focus and challenge of this project. For all-film capacitors, the temperature of the hottest spot inside should not exceed 80℃, and to reduce the difficulty of other aspects of temperature rise reduction, the temperature rise between the hottest spot and the unit casing should be controlled below 6℃. To ensure that the internal operating temperature of the capacitor unit does not exceed the allowable value, the dielectric loss of the control unit must be strictly controlled. The factory standard is tanδ not greater than 3×10⁻⁴. Practice has proven that the company's current technical level can achieve this. Generally, the dielectric loss tangent tanδ of the unit does not exceed 2×10⁻⁴. In addition, a certain number of reinforcing ribs are added to the outer casing to maximize the heat dissipation area of ​​the large casing and reduce the temperature rise of the large casing to the ambient air. How to minimize the temperature rise between the capacitor unit casing and the large casing is the focus of our research. We have conducted a lot of research work on this. First, the structural arrangement of the capacitor units within the large enclosure was carefully designed to ensure simple and reliable wiring and smooth heat flow, preventing localized overheating. This allows the heat from the capacitor units to be effectively transferred to the large enclosure via conduction, convection, and radiation without requiring additional forced cooling measures. To ensure that the capacitor units do not degrade in performance due to poor heat dissipation, we conducted research and adopted more effective forced cooling measures, applying heat pipe cooling technology used in the aerospace field to this product. A heat pipe cooling device is installed between the capacitor unit shell and the large enclosure shell. This is an effective and practical result of our research project. The working principle of the heat pipe (see Figure 1) is as follows: the heat-absorbing section of the heat pipe absorbs heat energy and transfers it to the working fluid inside the heat pipe. Due to the special properties of the working fluid under certain environmental conditions, it absorbs heat and changes from a liquid phase to a gas phase. Under the action of pressure difference within the heat pipe, the gaseous working fluid reaches the cooling section through the transport section. Because the temperature in the cooling section is lower, the working fluid changes from a gaseous phase to a liquid phase, releasing heat energy. Heat pipes have low thermal resistance, high power transfer, and can achieve long-distance, variable-direction heating. They also have fast start-up, good transmission speed, and significant heat dissipation effect. Typically, heat pipes operate under high negative pressure. To ensure the lifespan of the heat pipes, we used heat pipes operating at atmospheric pressure (0.1 MPa). This means the working fluid vaporizes at its vaporization temperature and can condense back normally, dissipating heat. To verify the heat dissipation effect of the heat pipes, we conducted numerous comparative experiments on samples with and without heat pipes to determine the optimal structure. Table 1 shows the thermal stability test data of the samples. The test method followed standard JB7112-2000, and the test voltage was 1.2 times the rated voltage. A key aspect of developing gas-filled composite capacitors is solving the temperature rise problem, and we believe that using heat pipes for heat dissipation is a very effective solution. We conducted extensive, in-depth, and arduous research and experiments on various aspects, including the principle of heat pipes, various factors affecting heat dissipation and transfer, manufacturing processes, and the installation and use of heat pipe radiators. Overcoming various difficulties, we finally achieved a breakthrough and developed an internal heat pipe cooling device suitable for this product, with excellent performance and convenient installation, thus solving the long-standing problem of temperature rise. Type testing of the product proves that the temperature rise meets the requirements and has a certain margin, ensuring that the capacitor unit will not be damaged by heat due to poor heat dissipation, ensuring its safe operation, and thus meeting the requirements for explosion-proof and fire-proof performance. Only when the temperature rise problem is reasonably solved can the gas-filled composite capacitor fully exert its advantages. The successful use of the internal heat pipe cooling device provides a reliable guarantee for the safe operation of the product. 2.2 Insulation In gas-filled composite high-voltage parallel capacitors, the insulating gas undertakes the external insulation of the capacitor unit inside the large casing and the insulation and heat dissipation between the capacitor unit and the casing. Sulfur hexafluoride (SF6) gas is a gas with very stable physical and chemical properties and excellent insulation and arc-quenching properties. It was discovered in 1900 and began industrial production in the 1930s. Due to its excellent physical and chemical properties and insulation and arc-extinguishing performance, SF6 gas began to be used in power equipment in the 1940s. Currently, SF6 gas and its mixtures are widely used in power equipment such as circuit breakers, instrument transformers, and GIS. Undoubtedly, SF6 gas or its mixtures are the ideal choice for gas-filled parallel capacitors. The issue is that the pressure range of the gas and the ratio of SF6 gas to each mixture need to be carefully studied and determined. The insulation levels for 10kV and 35kV parallel capacitors are 75/42 (kV) and 95/200 (kV), respectively. Since the insulating gas in gas-filled parallel capacitors only provides external insulation for the high-voltage live parts within the large enclosure, and the structural arrangement allows for increasing the insulation distance with minimal cost increase, we have conducted thorough research and testing to select a suitable insulation structure and insulation distance to ensure reliable product operation. This allows the product to pass all electrical performance type tests under 0 gauge pressure air conditions. Taking into account factors such as ease of sealing and mechanical strength of the enclosure, we selected a gas pressure range of 0.001–0.06 MPa (all pressures mentioned here are gauge pressure) through experimental research. This range satisfactorily meets the requirements in all aspects, unlike SF6 circuit breakers and GIS which use higher gas pressures. Considering the manufacturing practices of gas-filled capacitor banks, high gas pressure offers little benefit to gas-filled capacitor banks but complicates product manufacturing, operation, and maintenance. Therefore, for ease of operation, this product does not consider equipping it with a complex automatic gas replenishment system. To prevent external moisture from seeping into the enclosure during operation, a positive pressure should be maintained inside the enclosure. Because our factory has many years of experience in manufacturing SF6 standard capacitors and, in recent years, gas-sealed products such as gas-filled CVTs, we can achieve an annual gas leakage rate of 0.5% for gas-filled capacitors at 0.065 MPa pressure. Therefore, the principle for determining the rated gas pressure (20℃) is based on an annual leakage rate of 0.5%. After 20 years, at the lower limit of the ambient temperature, the gauge pressure inside the casing when the product is not in operation should not be less than 0.001 MPa. Based on this, the rated gas pressure can be calculated to be 0.032 MPa. Similarly, based on the upper limit of the ambient temperature and the operating temperature of the product, the upper limit of the normal pressure inside the casing can be calculated to be 0.06 MPa. In reality, due to guaranteed process conditions, the annual gas leakage rate during the product's factory testing is below 0.03%. Therefore, under normal operating conditions, the product does not require gas replenishment during its 30-year lifespan. Although SF6 gas has excellent electrical properties, its price is relatively high, and it is quite sensitive to the non-uniformity of the electric field. While some measures have been taken for ease of manufacturing in multi-stage capacitors, the uniformity of the electric field is still not ideal. However, using a mixture of SF6 and inexpensive N2, provided the ratio is appropriate, can save costs and reduce the sensitivity of pure SF6 gas to electric field non-uniformity, ensuring good electrical strength without the need for significant effort to improve electric field uniformity. Currently, both domestically and internationally, SF6 mixtures are gradually replacing pure SF6 gas. Commonly used N2/SF6 ratios are 50%/50% or 60%/40%. Through research and experimentation, we have achieved good results using a suitable volume ratio of SF6 to N2. As mentioned earlier, for ease of sealing and increased reliability, the design basis for lead insulation strength is the "0" gauge pressure air state. The insulation of all conductors (excluding the internal insulation of the unit itself) inside the gas-filled integrated capacitor box is ensured by a reasonable insulation arrangement structure and special measures such as the use of insulating sheaths. This allows the product to meet type test requirements even under "0" gauge pressure air conditions. Therefore, filling with SF6 gas or a mixed gas provides a greater insulation margin, and due to the excellent sealing performance, no negative pressure will occur inside the box. Using SF6 gas instead of insulating oil, the insulation margin of the gas-filled integrated product is greater than that of oil insulation in terms of electrical insulation strength. Furthermore, the structural arrangement ensures that each phase is arranged independently, the phase leads do not cross, and the phase spacing is large, preventing phase-to-phase breakdown due to insufficient spacing. For the electrical strength inside the capacitor unit, we select a relatively low and reasonable dielectric working strength, and each small component is equipped with reliable internal fuse protection. These measures make the product a highly reliable and long-service-life electrical device. 2.3 Sealing Many people worry that gas sealing is more difficult than oil sealing. Even after decades, the sealing problem of oil has not been completely solved, let alone gas. We must pay close attention to this common problem and focus on researching and solving it. Gas-fired CVTs, PTs, and GIS products have been in use for decades. Our factory's standard SF6 gas capacitors have been in use for over thirty years. In recent years, we have made breakthrough progress in the research of gas-filled CVTs and GIS-grade CVTs, and our factory possesses rich and mature technology and manufacturing experience. Compared to other high-pressure electrical products, gas-filled integrated capacitors have lower gas pressure (0-0.065MPa) and lack dynamic seals for operating mechanisms. This product has fewer sealing surfaces; only the bushings require mechanical sealing using rubber gaskets, while other surfaces can be sealed by welding. Despite this, we still focused our research on this aspect. Firstly, we used specially shaped sealing materials with excellent sealing performance that can withstand SF6 gas and ultraviolet radiation. We employed a special sealing structure, strictly controlled the compression ratio, and used double-sided welding for the casing walls, applying advanced welding technology to ensure a strong and sealed weld. Furthermore, the process employed advanced and effective multiple leak testing procedures, and the leak detection instruments were advanced and reliable. Practice has proven that the methods and means used are feasible; the SF6 gas leak detector reading was close to 0 during the product's factory testing. 2.4 Special Protection Measures We conducted specialized protection research on gas-filled integrated capacitors. Specially ordered, highly reliable, and sensitive electrical contact pressure gauges are installed on the product. When the gas pressure inside the enclosure reaches the upper and lower limits, the contacts close and trip, ensuring the capacitor can operate normally while quickly withdrawing from operation in case of a fault to prevent the accident from escalating. 3 Product Performance 3.1 Main Performance Indicators 3.1.1 Capacitance Deviation The deviation between the measured capacitance and the rated value of the capacitor shall not exceed 0% to +5%. The ratio of the capacitance of any two phases shall not exceed 1.01. 3.1.2 Loss Tangent tanδ Under rated voltage and frequency, the loss tangent tanδ of the capacitor at 20℃ shall not exceed 3×10⁻⁴. 3.1.3 Dielectric Electrical Strength The dielectric between the two terminals of the capacitor can withstand one of the following test voltages for 10 seconds: a) Power frequency AC voltage: 2.15Un b) DC voltage: 4.30Un 3.1.4 Insulation Level The phase-to-phase and phase-to-case insulation levels of the capacitor are both 42/75kV. 3.1.5 Temperature Rise Capacitor: Within the allowable temperature range, under a power frequency AC voltage of 1.20Un, the temperature of the hottest spot inside the unit shall not exceed 75℃. 3.1.6 Gas Pressure Capacitor: The rated gas pressure at 20℃ is 0.032MPa, and the allowable gas pressure range is 0～0.06MPa. 3.1.7 Zero Gauge Pressure Performance: All electrical tests of the capacitor are completed under gauge pressure with an internal gas pressure of 0. 3.1.8 Annual Leakage Rate: Annual leakage rate ≤0.5%, no gas replenishment required for twenty years. 3.1.9 Other Performance Indicators: Other performance indicators comply with the requirements of GB/T11024.1～GB/T11024.4, JB7112, and DL/T628. 3.2 Product Testing: Based on the factory tests and type tests of the gas-filled integrated high-voltage parallel capacitors manufactured by our factory during this period, all mechanical and electrical performances are good, meeting the requirements of relevant standards, and operating normally. Table 2 shows the test results of the main items of our gas-filled parallel capacitor. Test results of the main items. 4. Product Features The gas-filled high-voltage parallel capacitor successfully developed by Guilin Power Capacitor Factory is based on the traditional oil-filled capacitor and developed through extensive testing and research. It has the advantages of traditional capacitors and overcomes the disadvantages of oil-filled capacitors. The product has the following features: 4.1 Flame-retardant, explosion-proof, and environmentally friendly. Non-flammable gas SF6 or a mixture of SF6 and N2 replaces the insulating oil in the enclosure, fundamentally eliminating oil leakage. More importantly, it eliminates the possibility of explosion and fire, ensuring the safety of the substation. Furthermore, because there is a gas chamber filled with non-flammable and flame-retardant gas as the oil storage chamber, even if the unit explodes, there is no risk of oil leakage and fire. Compared with transformer oil, SF6 gas is a green and environmentally friendly material that does not pollute the environment. 4.2 Low loss and low temperature rise. The internal unit generates little heat. Simultaneously, it adopts patent technology ZL01 212594.6, applying heat pipe cooling technology used in the aerospace field to the capacitor. The internal heat sink effectively conducts the heat generated by the unit to the product casing, thereby improving the unit's heat dissipation and reducing the internal temperature rise. 4.3 Lightweight: Due to the replacement of oil with gas, the product's weight is significantly reduced, making installation easier. 4.4 Long lifespan and wide applicability: Increased insulation margin in the design and the adoption of GE technology in the manufacturing process result in high partial discharge levels and a long lifespan. The capacitor has a wide range of applications and can be used indoors, outdoors, or in harsh environments, with a guaranteed lifespan of 20 years. 4.5 Good sealing: Ensures no gas replenishment is required for 20 years. 4.6 Excellent performance, mature manufacturing and operating experience, and high reliability: The capacitor performance meets or exceeds the requirements of GB/T11024, JB7112, and DL/T628. The integrated product's performance has been tested in operation for over ten years and has decades of experience using gas insulation, ensuring reliable product performance. 4.7 Low Product Cost: Compared to self-healing high-voltage capacitors, the product cost is significantly lower, offering a price advantage. 4.8 ​​Simple Operation and Maintenance: The product boasts a large insulation margin and excellent sealing. The electrical contact pressure gauge automatically alarms or trips at the lower and upper pressure limits. In contrast, oil-filled capacitors require oil sample testing, regular replacement of the desiccant in the desiccant, and cleaning of oil residue, while gas-filled capacitors do not. 4.9 Aesthetically Pleasing Appearance: Compared to traditional oil-filled capacitors, it lacks accessories such as oil conservators, desiccant, oil level indicators, drain valves, oil sample valves, and pressure relief valves, resulting in a clean and uncluttered appearance. Furthermore, being oil-free, it is free of oil stains, giving it a brighter and cleaner look. Our factory's sophisticated design and the use of a new sandblasting and zinc-spraying anti-corrosion process on the outer shell further enhance its corrosion resistance and aesthetic appeal.