Mechanism And Countermeasures Of High-impedance Transformer Inrush-current And Its Impact On Protection

With the rapid development of the economy and society,the problem of decreasing system impedance and severely exceeding short-circuit currents in the main power networks interconnected by large regional grids and urban power networks with heavy loads has generally arising.The cost of circuit breakers will rise sharply with the increasing demand for interrupting capacity.The traditional solution to limit the short-circuit current on the low-voltage side of transformer is to connect inductances in series outside the low-voltage winding,which add additional power devices.With the ability of increasing the short-circuit impedance to reduce the short-circuit current without adding additional power devices,the high-impedance transformers have been used more and more widely.However,in recent years,there have been many cases in which high-impedance transformer have caused the the zero-sequence overcurrent protections of the bus-tie circuit breaker or even its upper level line misoperated when it is energized.The power supply reliability of the power grid is severely affected,and the safety of the system faces huge challenges.At present,there are two commonly used high-impedance transformers,one is the high-impedance transformer with built-in high-voltage windings(T-Hin for short),and the other is the high-impedance transformer with inductance connected in series in low-voltage winding(T-Lse for short).The two types of transformers increase the short-circuit impedance through increasing the magnetic leakage area and connecting inductances in series in the low-voltage delta windings of the traditional transformer(T-Tra for short)respectively.Multiple accidents waveforms show that the inrush current of T-Hin has two obvious and common characteristics.(1)The initial value of the zero-sequence current is large and after attenuating for the delay time of zero-sequence overcurrent protection,its value is still greater than the setting value;(2)The three-phase inrush current of the primary winding shows a large imbalance and there is always one-phase inrush current significantly smaller than the other two phases.Y₀YD connected T-Hin has frequent misoperation accidents.However,the misoperation accidents of T-Lse and T-Tra with the same iron core and connection mode are rarely reported.According to the analysis of the classical transformer zero-sequence equivalent circuit,the high short-circuit impedance should make the zero-sequence current smaller,which is contrary to the facts.The short-circuit parameters of the two high-impedance transformers is same,the zero-sequence current performance should also be same,which is not consistent with the facts.Therefore,the classic transformer zero-sequence equivalent circuit of is no longer applicable.It is urgent to propose a transformer inrush theory or equivalent circuit that can be used to analyze the impact of short-circuit impedance and winding arrangement on the zero-sequence current during energization.It is also necessary to propose a transformer simulation model that reflects the differences in impedance and structure,as well as parameter calculation method.This paper carried out research works by theoretical analysis,modeling and simulation,dynamic model tests,and on-site recorded waveforms to ensure the safe,stable and reliable running of the power grid.Research works include:1)Circulating current coupling considered inrush current analytical method that is suitable for transformers with different winding structures,2)Parameter demonstration and parameter calculation method of three-winding high-impedance transformer during energization,3)Characteristics comparison of high-impedance transformers inrush current and their impact on protections,4)Countermeasures in differnet scenariosAiming at difficulties in analyzing zero-mode inrush current of Y₀D transformers with different structures,the relationship between the winding magnetic flux distribution and the equivalent circuit parameter is figured out and it is pointed out that the hollow inductance is the key parameter reflecting the difference in winding structures.An analytical expression of single-phase inrush current reflecting the difference in winding structures was proposed.It is figured out that the circulating current is the unbalanced current due to the breaking of symmetrical relationship among three-phase secondary demagnetization current when secondary demagnetization current of the saturated-phase becomes smaller.The circulating current decreases as the hollow inductance and the secondary leakage inductance increase.Based on above,a zero-mode inrush current expression considering circulating current coupling and suitable for transformers with different winding arrangements is proposed.For the first time,the zero-mode equivalent circuit for transformer that can analyze changes in short-circuit impedance and winding structure has been constructed.It lays a theoretical foundation for analyzing the inrush current characteristics of high impedance transformers.Aiming at the parameter calculation method for zero-mode inrush current of three-winding high-impedance transformer,analysis on the differential equations based on flux linkage between windings showed that the T-equivalent circuit of three-winding transformer has application limitations due to ignoring the excitation branch and decoupling transformation.It is revealed that the zero value of the medium-voltage winding equivalent leakage inductance of the three-winding transformer derived from the mathematical equivalent transformation without physical meaning.It is demonstrated for the first time that the actual leakage inductances when the Y₀YD three-winding transformer work as a Y₀D two-winding transformer are different from the traditional T-equivalent leakage inductances calculated based on the short-circuit impedance.There have differences in physical concepts and theoretical values.The primary T-equivalent leakage inductance of traditional transformer is smaller than the actual value while that of T-Hin is larger than the actual value,and the deviation of the T-Hin is larger.It will cause the zero-sequence current analytical value of the traditional transformer to be larger,and that of T-Hin to be too smaller.A loop equation based calculation method for the actual leakage inductance of a two-winding transformer is proposed,solving the difficulties of prior risk assessment and accurate accident analysis of zero sequence overcurrent protection misoperation.According to the analysis of the zero-mode equivalent circuit and the different parameters,the amplitude,attenuation and unbalance of high-impedance transformer inrush current are studied.It is pointed out that the zero-mode inrush current of T-Hin is the largest.Although the attenuation rate of the T-Hin is slightly faster,its attenuation time is long,which may easily cause the zero-sequence protection to misoperate.The opening and closing angle of the misoperation waveforms is relatively close to the feature angles.The two phases of the feature angles are deeply saturated and its primary inrush current is large while one phase is not saturated and its primary inrush current is small for it is the coupling current of the circulating current.Therefore,the primary inrush current of T-Hin shows a large imbalance.It provides a theoretical basis for the suppression method of zero-mode inrush current.Based on the principle of“consistent structure and equivalent parameters”,physical models of T-Hin,T-Lse and T-Tra were designed and manufactured for the first time.A dynamic simulation test system was constructed.The effects of remanence,closing angle,voltage,winding connection,and transformer type on the zero-mode inrush current were carried out.The theoretical and simulation analysis were verified by dynamic physical simulation tests.In order to resist the misoperation risk of the zero sequence protection caused by the zero-mode inrush current of the high-impedance transformer,countermeasures considering the application constraints of the actual scenario are studied based on the above researches.In terms of adjusting the coordination status for system running,operations such as energizing when there are much more incoming lines,energizing in large gears,energizing with load,and temporarily improving the sensitivity of the transformer zero-sequence overcurrent protection have been proposed.In terms of optimizing control of the opening and closing angle,a zero-mode inrush current suppression method based on consistent matching of the opening and closing angles is proposed.In terms of improving the protection,an improved zero-sequence overcurrent protection method based on the zero-mode inrush current harmonic content and waveform inertia restraint is proposed.Simulations and recorded waformers verify the effectiveness of the above methods.At the end of this dissertation,the research work and results are summarized and the directions of further research are pointed out.