Development of the medium and low voltage DC distribution system is of great significance to a regional transmission of electric energy,increasing a penetration rate of new energy,and enhancing a safety of the operati...Development of the medium and low voltage DC distribution system is of great significance to a regional transmission of electric energy,increasing a penetration rate of new energy,and enhancing a safety of the operation of the AC/DC interconnected grid.This paper first summarizes the medium and low voltage DC distribution system schemes and plans put forward by many countries,and then elaborate status of under-construction medium and low voltage DC distribution system project cases in China.Based on these project cases,this paper analyzes key issues involved in the medium and low voltage DC distribution system topologies,equipment,operation control technologies and DC fault protections,in order to provide theoretical and technical reference for future medium and low voltage DC distribution system-related projects.Finally,this paper combines a current China research status to summarize and give a prediction about the future research direction of medium and low voltage DC distribution system,which can provide reference for the research of medium and low voltage DC distribution system.展开更多
An HVDC link with line-commutated converter(LCC)as a rectifier at the power sending end and with a serial-connected LCC and voltage source converter(VSC)hybrid inverter(SLVHI)at the power receiving end,has been adopte...An HVDC link with line-commutated converter(LCC)as a rectifier at the power sending end and with a serial-connected LCC and voltage source converter(VSC)hybrid inverter(SLVHI)at the power receiving end,has been adopted for the forthcoming Baihetan HVDC engineering project in China.To realize the AC fault ride-through(FRT)of SLVHI,a new strategy based on DC chopper(DCC)is proposed in this paper.Firstly,the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure(CF)and related factors.Secondly,a modified DCC topology of SLVHI is designed to adjust the unbalanced power dissipation based on voltage drop depth.Thirdly,an enhanced voltage-dependent current order limiter(VDCOL)is proposed to deal with the unbalanced power after DCC switch-off.With the cooperation between modified DCC and enhanced VDCOL,the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously.Finally,the inverter side AC FRT performances of the HVDC link with SLVHI were studied using PSCAD software/EMTDC algorithm.The simulation results validate the effectiveness and superiority of the proposed strategy,with better CF mitigation and VSC DC overvoltage suppression abilities than offered by other existing FRT strategies under different fault scenarios.展开更多
In DC distributed power systems(DPSs),the complex impedance interactions possibly lead to DC bus voltage oscillation or collapse.In previous research,the stability analysis of DPSs is implemented based on mathematical...In DC distributed power systems(DPSs),the complex impedance interactions possibly lead to DC bus voltage oscillation or collapse.In previous research,the stability analysis of DPSs is implemented based on mathematical analysis in control theory.The specific mechanisms of the instability of the cascade system have not been intuitively clarified.In this paper,the stability analysis of DPSs based on the traditional Nyquist criterion is simplified to the resonance analysis of the seriesconnected port impedance(Z=R+jX)of source and load converters.It reveals that the essential reason for impedance instability of a DC cascade system is that the negative damping characteristic(R<0)of the port the overall impedance amplifies the internal resonance source at reactance zero-crossing frequency.The simplified stability criterion for DC cascade systems can be concluded as:in the negative damping frequency ranges(R<0),there exists no zero-crossing point of the reactance component(i.e.,X=0).According to the proposed stability criterion,the oscillation modes of cascade systems are classified.A typical one is the internal impedance instability excited by the negative damping,and the other one is that the external disturbance amplified by negativity in a low stability margin.Thus,the impedance reshaping method for stability improvement of the system can be further specified.The validity of the simplified criterion is verified theoretically and experimentally by a positive damping reshaping method.展开更多
基金supported by the National Key Rese arch and Development Program of China(2018YFB0904100)Science and Technology Project of State Grid(SGHB0000KXJS1800685)
文摘Development of the medium and low voltage DC distribution system is of great significance to a regional transmission of electric energy,increasing a penetration rate of new energy,and enhancing a safety of the operation of the AC/DC interconnected grid.This paper first summarizes the medium and low voltage DC distribution system schemes and plans put forward by many countries,and then elaborate status of under-construction medium and low voltage DC distribution system project cases in China.Based on these project cases,this paper analyzes key issues involved in the medium and low voltage DC distribution system topologies,equipment,operation control technologies and DC fault protections,in order to provide theoretical and technical reference for future medium and low voltage DC distribution system-related projects.Finally,this paper combines a current China research status to summarize and give a prediction about the future research direction of medium and low voltage DC distribution system,which can provide reference for the research of medium and low voltage DC distribution system.
基金This work was supported by the State Grid Science&Technology Foundation“NY71-19-037:Research on power transmission of large-scale renewable power bases by VSC-LCC hybrid HVDC”.
文摘An HVDC link with line-commutated converter(LCC)as a rectifier at the power sending end and with a serial-connected LCC and voltage source converter(VSC)hybrid inverter(SLVHI)at the power receiving end,has been adopted for the forthcoming Baihetan HVDC engineering project in China.To realize the AC fault ride-through(FRT)of SLVHI,a new strategy based on DC chopper(DCC)is proposed in this paper.Firstly,the mathematical model is built to investigate the VSC DC overvoltage mechanism after SLVHI commutation failure(CF)and related factors.Secondly,a modified DCC topology of SLVHI is designed to adjust the unbalanced power dissipation based on voltage drop depth.Thirdly,an enhanced voltage-dependent current order limiter(VDCOL)is proposed to deal with the unbalanced power after DCC switch-off.With the cooperation between modified DCC and enhanced VDCOL,the proposed FRT strategy can realize CF mitigation and VSC DC overvoltage suppression simultaneously.Finally,the inverter side AC FRT performances of the HVDC link with SLVHI were studied using PSCAD software/EMTDC algorithm.The simulation results validate the effectiveness and superiority of the proposed strategy,with better CF mitigation and VSC DC overvoltage suppression abilities than offered by other existing FRT strategies under different fault scenarios.
基金supported by National Key Research and Development Program of China(2018YFB0904100)Science and Technology Project of SGCC(SGHB0000KXJS1800685).
文摘In DC distributed power systems(DPSs),the complex impedance interactions possibly lead to DC bus voltage oscillation or collapse.In previous research,the stability analysis of DPSs is implemented based on mathematical analysis in control theory.The specific mechanisms of the instability of the cascade system have not been intuitively clarified.In this paper,the stability analysis of DPSs based on the traditional Nyquist criterion is simplified to the resonance analysis of the seriesconnected port impedance(Z=R+jX)of source and load converters.It reveals that the essential reason for impedance instability of a DC cascade system is that the negative damping characteristic(R<0)of the port the overall impedance amplifies the internal resonance source at reactance zero-crossing frequency.The simplified stability criterion for DC cascade systems can be concluded as:in the negative damping frequency ranges(R<0),there exists no zero-crossing point of the reactance component(i.e.,X=0).According to the proposed stability criterion,the oscillation modes of cascade systems are classified.A typical one is the internal impedance instability excited by the negative damping,and the other one is that the external disturbance amplified by negativity in a low stability margin.Thus,the impedance reshaping method for stability improvement of the system can be further specified.The validity of the simplified criterion is verified theoretically and experimentally by a positive damping reshaping method.
