Research On Differential Protection Of Converter Transformer In LCC-HVDC System

The line commutated converter-high voltage direct current（LCC-HVDC）system based on grid commutation has many advantages,such as low line cost,rapid transmission adjustment,and strong line fault recovery capability.Engineering applications such as power transmission are very extensive.One of the more prone failures in this system is commutation failure.At present,most of the converter transformers adopt the differential protection based on the second harmonic braking principle.However,because the converter transformer is in an important position in the coupling of the AC and DC systems,it is affected by the combination of them,and after the commutation failure occurs,a large number of harmonics are injected,which has a greater impact on traditional protection.The safe and reliable operation of the converter transformer is crucial to the operation of the entire system.Therefore,the research on commutation failure in HVDC transmission system and its differential protection of converter transformer is of great significance.First,the HVDC transmission system model is built on the PSCAD/EMTDC simulation platform,the corresponding control strategies are analyzed and introduced.The commutation process and the causes of commutation failure are deduced theoretically,and the commutation failure is classified and discussed from the valve level.The influencing factors of commutation failure are theoretically analyzed,and the influence of converter transformer connection mode,fault occurrence time,fault type and transition resistance on commutation failure is simulated and verified.Secondly,the principle of the ratio differential protection based on the second harmonic braking is analyzed,and the harmonic content of the equivalent current on the AC side of the inverter is analyzed by the superposition switching function method,and it is obtained that there may be a large number of The DC component is injected into the converter transformer,which causes the change of the harmonic characteristics of the converter transformer.The valve-side fault of the converter transformer with YNy connection is divided into two cases:the common anode valve of the fault phase is turned on and the common anode valve of the fault phase is not turned on.wave content,the ground fault on the valve side is accompanied by commutation failure,and the differential protection delays action or even refuses to act.The impact on differential protection is small;for external faults accompanied by commutation failure,it has no impact on differential protectionFinally,in view of the problem of delayed action or even refusal to act in the second harmonic braking differential protection of the converter transformer in the HVDC transmission system,an improved differential protection scheme is proposed.Analysis of the zero-sequence current harmonics on the neutral point side when the converter transformer has internal faults under different working conditions accompanied by commutation failure and excitation inrush current,the internal faults are divided into two categories: the first type of internal faults is the fundamental wave and the third The faults with large harmonics,the second type of internal faults are faults with small third harmonics;the third harmonics are large and the fundamental wave is smaller than the first type of internal faults during inrush current.A new scheme for differential protection of converter transformers based on the amplitude of the zero-sequence current fundamental and third harmonic components on the neutral point side.The effectiveness of the proposed scheme is verified by a large number of simulations.The simulation results show that the protection is not affected by commutation failure,and is suitable for both YNy connection and YNd connection converter transformers;when internal faults occur,the protection can operate correctly;excitation When inrush current,the protection can reliably not operate.