Line Protection Scheme Of AC/DC Systems With Large-Scale Renewable Energy Resources

For renewable energy resources（RES）integration in a centralized approach,DC grid is an effective scheme for large-scale RES integration over long-distance.For RESs access to power grids in a distributed approach,some RES based distributed generators are connected to distribution networks through inverters.When large-scale power electronics are integrated into power grids,fault characteristics of power grids have changed substantially.When a DC line fault occurs,the DC fault current rises to a large amplitude within several milliseconds,which threatens the safe operation of inverters.When an AC line fault happens,the output current amplitude of the inverter is limited and the phase is adjusted based on the requirements of fault-ride through controls,which decreases the sensitivity of existing distance protections.In addition,distributed generators（DGs）bring about the bidirectional power flow of distribution networks,which imposes challenges for the coordination among overcurrent protections.To overcome the above challenges and guarantee the safe operation of AC and DC systems,dc grid line protection schemes suitable for different fault resistances and DC boundaries,a novel distance protection scheme for AC transmission networks considering impacts of inverter based resources（IBRs）,and a novel current-only directional element for AC distribution network considering impacts of inverter based DGs are proposed.The effectiveness of the proposed schemes is verified by simulation cases or experiment cases.The main contributions of this paper are as follows:1)To cope with low-impedance and medium-impedance DC faults,an ultra-fast boundary protection scheme based on current-limiting reactor voltage in the modal domain is proposed for DC grid.Firstly,phase-mode transformation is used to decouple the dependency between positive and negative poles of transmission lines,and the fault equivalent network in the mode domain is established.Then,fault characteristics of line-mode and zero-mode current-limiting reactor voltages under solid faults are analyzed quantitatively.Finally,line-mode current-limiting reactor voltage(∫V_Lmn1)is adopted for internal and external fault detection while zero-mode current-limiting reactor voltage(∫V_Lmn0)is adopted for faulted pole selection.The maximum fault resistance that the proposed method can endure is 200Ω.Besides,the detection speed is pretty fast,no more than 1.1ms.Compared with some time-domain travelling-wave protections,the proposed scheme is robust to noise disturbance（10d B）.Compared with Wavelet Transform based methods,the proposed scheme has a lower computing burden and sampling frequency.In addition,fault type and operations of DC circuit breakers have no impact on the proposed method.2)To cope with high-impedance DC faults,an improved boundary protection scheme using the polarity and propagation time characteristic of backward voltage traveling-waves is proposed for DC grid.Firstly,to achieve close-in fault and remote fault detection,the transmission line is divided into four zones.Then,according to different fault zones,the reflection and refraction of travelling-waves under high-impedance faults are quantitatively analyzed.Finally,Math Morphological Gradient（MMG）is adopted to detect the polarity and arrival of backward voltage traveling-waves,and a fault protection criterion is designed.The proposed method can work well under high-impedance faults（500Ω）.Compared with some time-domain travelling-wave protections,this scheme can work well under close-in faults and remote faults.In addition,the scheme is not affected by fault distance,and the detection time meets the requirement of speediness.3)To improve the applicability of DC line protection schemes to different DC boundaries,a novel fault protection scheme independent of DC boundary is proposed for MMC based DC grid.Firstly,the propagation characteristics of line-mode backward voltage traveling-wave and the polarity of zero-mode backward voltage traveling wave are quantitatively analyzed.Then,the distortion factor of line-mode backward voltage travelling-wave is proposed to detect internal faults while the polarity of zero-mode backward voltage travelling-wave is proposed to select faulted poles.Besides,pilot protection using directional overcurrent criterion is adopted to detect remote faults so that the proposed method can protect the whole line.The proposed method is robust to fault impedance（300Ω）and the detection speed is fast,no more than 1.5ms.In addition,the proposed method is not affected by fault distance and fault type.4)For AC systems with IBRs,an improved distance protection benefiting from negative-sequence fault current behavior is proposed.Firstly,the output current phase of IBR is quantitatively analyzed to reveal the limitation of existing distance protections.Then,negative-sequence fault current characteristics of IBR under asymmetric faults are quantitatively analyzed.Finally,an enhanced protection criterion(φ_OP-f)benefiting from negative-sequence fault current behaviors is designed.Simulation and experimental cases show that the proposed method is robust to fault impedance（300Ω）,fault distance,and asymmetric fault type.5)For AC distribution networks with IBDGs,an improved current-only directional element is proposed to identify forward and backward faults.Firstly,the impacts of IBDGs on existing directional elements are evaluated.Then,line current behaviors under low-impedance faults and high-impedance faults are quantitatively analyzed to extract effective phase and amplitude information to design the criterion.Simulation and experimental cases demonstrate that this method can correctly identify backward and forward faults.In addition,the method is robust to fault impedance and fault position.