UHV AC transformers, switchgear, series compensation devices and lightning arresters are the main core equipments of UHV AC transmission projects. This time, we will focus on the latest technical developments of these four types of equipment.
Development of UHV gas insulated metal-enclosed switchgear
China's UHV substations use UHV GIS, which consists of circuit breakers, disconnectors, grounding switches, current transformers and busbars. In the early stage, domestic manufacturing enterprises and foreign companies cooperated in the research and development of UHV GIS. The rated short-circuit breaking current was 50kA. In 2009, the Jindongnan-Nanyang-Jingmen UHV AC test demonstration project was successfully put into operation. After 2011, the country began to work on the localization and technical upgrading of UHV GIS, and developed a product with a short-circuit breaking capacity of 63kA, and invested in the expansion project of the UHV AC test demonstration project. Subsequently, the localization of UHV circuit breaker operating mechanism, basin type and supporting insulator was realized, which was widely applied in the transmission of power transmission from East to East and Zhe Ford high-voltage AC transmission project, and the technical parameters reached the international leading level.
In the development of UHV GIS, the development of UHV circuit breakers, isolating switches, basin insulators, etc., as well as large short-circuit current interruption, ultra-fast transient over-voltage (VFTO) suppression and high insulation design are key challenges. Introduce separately.
1. UHV circuit breaker
The voltage of the UHV circuit breaker is high, the short-circuit breaking current is large, the DC component decay time constant is 120ms, and the breaking is difficult. It is necessary to establish a test circuit equivalent to the actual breaking condition, which puts forward higher requirements for the breaking test and needs to be carried out. Insulation, short-circuit breaking and test technology research. In-depth research on the insulation performance, breaking capacity and test technology of the circuit breaker has been carried out in China, and an ultra-high voltage circuit breaker with double-break/four-break and 63kA breaking capacity has been developed.
1) Insulation performance
According to the internal structure of the circuit breaker, combined with different dielectric constants of SF6, epoxy resin and ceramics, an electric field simulation model was established, and the insulation characteristics were analyzed and calculated, as shown in Fig. 6. The arc-break chamber fracture is calculated by applying a 2400kV lightning impulse voltage to one end and applying a power-frequency voltage with a reverse polarity peak of 635kV at the other end, that is, 900kV, which is the most demanding condition. The product structure is optimized according to the electric field requirements, and the test verification meets the insulation requirements.
2) breaking ability
On the basis of the breaking capacity 50kA circuit breaker, the structure of the arc extinguishing chamber is optimized. Considering the effects of the opening speed, the volume of the compressor chamber, the shape of the nozzle and the breaking current, the gas pressure in the process of breaking the 63kA short circuit current of the circuit breaker is simulated and analyzed. The pressure distribution is shown in Figure 7. The pressure chamber and the nozzle A higher, ideal air pressure is obtained. Through the performance verification test conducted by the circuit breaker prototype, the parameters such as opening speed, arcing time, arcing interval and pre-breakdown time were improved. The type test results show that the developed UHV circuit breaker has superior large short-circuit current breaking capability.
3) Operating mechanism UHV circuit breaker adopts high-power hydraulic operating mechanism, which puts high requirements on operating characteristics and stability. On the basis of the hydraulic mechanism of the UHV circuit breaker with a breaking capacity of 50kA, according to the modular design, the output power of the mechanism is increased, and the high-flow quick-acting valve, the integrated multi-stage buffer system, and the sealing structure in the form of grid ring and seal are used. By optimizing various structural parameters, excellent mechanical properties are obtained, and the operating speed is adjustable.
4) Short circuit breaking test technology
The short-circuit breaking test of the UHV circuit breaker in China adopts the current introduction synthesis test method. The test current is appropriately adjusted to introduce the current and frequency of the voltage source to meet the requirements of the zero zone assessment equivalence. When the rated voltage is higher than 800kV, the test equipment of the current introduction method reaches the limit and cannot meet the requirements of the integral test of the UHV circuit breaker. To this end, a "three-loop method" synthesis test circuit is proposed, and the test wiring is shown in Fig. 8. In the high current phase, the current through the sample is supplied by the current source, and the first set of voltage loops is input at the appropriate time before the zero-crossing break, and the current supplied by the current source and the first set of voltage loops flows through the circuit breaker under test; After the zero-crossing of the product, the first set of voltage loops provides the TRV starting part at one end of the circuit breaker. When the TRV rises to a certain stage, the second set of voltage loops is applied and simultaneously applied to the other end of the circuit breaker and the outer casing. The voltages of the voltage loops are superimposed to form the recovery voltage applied to both ends of the sample, and the full voltage evaluation of the casing is satisfied; in the circuit switching, the equivalence requirement of the current zero zone should be ensured. The "three-loop method" synthetic test circuit has good equivalence and high safety, and successfully completed the whole machine breaking test of T100s and T100a of 1100kV circuit breaker.
2. Isolation switch
UHV equipment has high insulation reliability requirements, and operation over-voltage suppression is more important. The extremely fast transient over-voltage (VFTO) caused by the isolation switch operation has the characteristics of high amplitude, short rise time and continuous multiple pulses. Larger, need to take reasonable measures to limit. UHV GIS isolating switches are mainly available in two configurations. The “vertical†isolating switch operates faster and produces a higher VFTO amplitude. The “vertical†isolating switch is equipped with a VFTO damping resistor on the static contact side. As shown in Figure 9, during the opening and closing of the isolating switch, the damping resistor is serially connected to the discharge circuit, effectively suppressing the high repetition of the contact gap. Frequency oscillations, thereby limiting the VFTO amplitude and frequency. The "horizontal" isolating switch operates at a slower speed, produces a lower VFTO amplitude, and does not have a damping resistor.
3. Basin insulator
The basin insulator provides insulation support for the high voltage conductor and divides the air chamber, which is a key insulation component of UHV GIS. In recent years, the localization and quality improvement of UHB basin insulators have been carried out. By analyzing the types and causes of defects generated by basin insulators, the structural optimization of the basin insulators has been carried out. The electric field distribution is shown in Fig. 10. The stress distribution is shown in Fig. 11. Show. The production process has been improved and adjusted, so that the mechanical and electrical performance of the basin insulator has reached a high level. From the two aspects of test and test methods (type test, material test, factory test) and process control (insert processing, casting, curing process, etc.), clear technical means of quality control are proposed. In particular, it is proposed to use five test samples to carry out several tests in sequence, covering various working conditions of production, transportation and operation, taking into account the assessment of product technology level, comprehensive performance and process stability. The manufacturing level and quality stability of UHV basin insulators have been greatly improved, and the products have been applied in the power transmission project of Yandong East and Zhejiang Ford.
At present, China has successfully developed an ultra-high voltage circuit breaker with a short-circuit breaking capacity of 63kA, which solves the VFTO suppression problem, realizes the quality improvement of the basin insulator and the localization of the UHV GIS. The technical performance has reached the international leading level and realized a large scale. Engineering Applications. In the future, the development of power grids puts forward higher requirements for UHV GIS. UHV GIS will develop in the direction of high performance parameters, environmental applicability and environmental protection, such as UHV circuit breakers with rated breaking current of 80kA, adapting to extreme environments. Materials such as insulating media, seals, greases and hydraulic oils, such as cold, high heat and sand, use new GIS such as SF6 instead of medium, and further improve the operational reliability of the equipment.
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