Study On Parallel Protection Of The Air-water Gap And The Arrester For Neutral Points Of Power Transformers

110kV and220kV systems are effectively grounded systems in my country. To limit short circuit current of single phase grounding fault and meet the requirements of relay protection, part of neutral points of power transformers are ungrounded according to relevant industry standards. When faults occur in the system transmission line (the typical faults are the single phase grounding fault, open phase operation and lightning incoming surge), there is high overvoltage on ungrounded neutral points of power transformer. Graded insulation is used at the neutral points of110kV and220kV power transformers in my country, so the insulation level of neutral points is much lower than the insulation level of phase line head. When the overvoltage on the neutral points of power transformers rises to a high value, the power transformer insulation and arrester will be threatened, so the ungrounded neutral points of power transformers in system must be protected by taking effective measures.To protect the insulation of ungrounded neutral points of power transformers in110kV and220kV systems, generally three protection modes (such as arrester, air gap and parallel connection of an air gap and a arrester) is used. The protection mode of parallel connection of an air gap and a arrester is most universal, but the traditional protection mode of parallel connection of a air gap and a arrester also has some disadvantages. Distance of air gaps is hard to master and dispersion of breakdown charaterastics of air gaps is large, so maloperation or miss trip are initiated easily; maloperation of air gaps results from that the breakdown of air gap occurs due to high transient overvoltage, then maloperation of relay protection occurs; parameters of arresters and air gaps are hard to cooperate, so it is hard to meet protection principle completely.To overcome disadvantages of traditional protection mode, the new protection mode of the parallel connection of the protection air-water gap and the arrester for neutral points of power transformers is presented in the paper. The control system is established and the control device meeting the system requirement is made. Simulation test research of the protection system is conducted successfully according to the overvoltage on ungrounded neutral points of110kV power transformers. The background and purpose of research, MOA arresters, breakdown theory of fluid dielectric and transformer neutral point protection and so on are summarized in the exordium part of the paper first, then the new protection mode and related content are studied as completely as possible from five aspects as follows.At first, the power frequency overvoltage and transient overvoltage containing lightning overvoltage probably appearing on the neutral points of power transformers is discussed to provide basis for configuration of neutral point protection. Study results show that when the single phase grounding fault occurs in the110kV effective ground system, the power frequency overvoltage on neutral points of power transformers will be lower than43.8kV. When the single phase grounding fault occurs in the220kV effective ground system, the power frequency overvoltage on neutral points of power transformers will be lower than87.6kV; when the open phase operation occurs or the single phase grounding fault occurs and the system is ungrounded, the power frequency overvoltage on the neutral points of power transformers will rise to phase voltage of the system or higher voltage and so on.Secondly, because the MOA arrester is the important component of the new protection mode presented in the paper, related test and calculation of volt-ampere characteristic of the MOA arrester are conducted. In test aspect, the test of the Y1.5W-55/140protection arrester used at neutral points of110kV transformers is conducted to measure the DC and AC volt-ampere characteristic and the equivalent capacitance. In calculation aspect, according to two aspects mainly involved in MOA arrester calculation which are fit of MOA volt-ampere characteristic and solution of corresponding nonlinear equations, the study is conducted. The new mathematical model of the DC volt-ampere characteristic of MOA which is mixed model of exponential functions and power functions is established by fitting test data, and then algorithm of steady-state responses of the nonlinear discrete systems to multiple input frequencies based on Picard iteration principle is presented and used to solve corresponding nonlinear equations. Comparative analysis is conducted to the test results and calculation results showing that the algorithm has advantages of high precision and fast convergence and the new mathematical model of the arrester is correct and effective.Thirdly, the new gap concept of "the air-water gap" is presented. To study and master discharge characteristic and discharge mechanism of the air-water gap, the special test devices are established in this paper and the power frequency discharge test of the air-water gap is conducted. Some laws are obtained from test results that breakdown of the air-water gap of the same length can occur more easily than air gap and water gap; there is a V shape curve relation between the power frequency breakdown and the air segment length when the total length is constant. Air and water are two kinds of common fluid, but they have different electrical characteristics. In general Air is insulative and water is conductive which has large resistance. When air and water are combined to conduct discharge test, its own phenomenon and law will be shown. Based on the bubble breakdown theory and different electrical characteristic of air and water, the kinds of discharge phenomenon and laws obtained from test are analyzed and the breakdown mechanism of the air-water gap is studied.Fourthly, to understand and master the more common laws in the discharge of the air-water gap further and for the convenience of engineering application, discharge laws of the air-water gap are analyzed and discussed further based on the above test study and mechanism analysis and the mathematical analytic formulas of power frequency discharge characteristic of the air-water gap are established by fitting methods. Calculation and test show that the calculation results of the established mathematical analytic formulas agree with test data well.Fifth, based on the above study, the new protection mode of the parallel connection of the protection air-water gap and the arrester for neutral points of power transformers is presented in this paper. The control system is established and the control device meeting the system requirement is made. The operating principle and operating conditions and operating process of the protection device are introduced and the appropriate range of gap distance of the air-water gap for neutral point protection of110kV power transformer is presented. According to the overvoltage on ungrounded neutral points of110kV transformers in typical faults, the simulation test study of the whole protection system in laboratory is conducted. The test shows that the new gap and the new protection mode presented in this paper can overcome the disadvantages of traditional protection mode well to protect the insulation of ungrounded neutral points of power transformers.