The symmetrical bipolar configuration shows a great advantage in modular multilevel converter (MMC) based high-voltage dc-current (HVDC) transmission. In a bipolar MMC-HVDC system, station internal ac grounding faults cause sub-module (SM) capacitors to discharge violently. These faults do great harm to MMCs and require special protection schemes. In this paper, a single-phase-to-ground fault is taken as an example to investigate transient characteristics of station internal ac grounding faults in a bipolar MMC-HVDC system. A calculation theory of fault current based on the improving RLC model and second-order differential equation with variable coefficients is introduced. A novel transient zero-mode current based protection criterion with better reliability, quickness, and ability to endure fault resistances is proposed to detect internal ac grounding faults. The fault clearing strategy applicable to three types of grounding faults is also designed. The correctness of the proposed calculation theory and protection schemes is verified through simulations performed in PSCAD/EMTDC. INDEX TERMS MMC-HVDC, station internal ac grounding fault, second-order differential equation with variable coefficients, transient zero-mode current, protection scheme.
Accurate and reliable fault location method for alternating current (AC) transmission lines is essential to the fault recovery. MMC-based converter brings exclusive non-linear characteristics to AC networks under single-phase-to-ground faults, thus influencing the performance of the fault location method. Fault characteristics are related to the control strategies of the converter. However, the existing fault location methods do not take the control strategies into account, with further study being required to solve this problem. The influence of the control strategies to the fault compound sequence network is analyzed in this paper first. Then, a unique boundary condition that the fault voltage and negative-sequence fault current merely meet the direct proportion linear relationship at the fault point, is derived. Based on these, a unary linear regression analysis is performed, and the fault can be located according to the minimum residual sum function principle. The effectiveness of the proposed method is verified by PSCAD/EMTDC simulation platform. A large number of simulation results are used to verify the advantages on sampling frequency, fault resistance, and fault distance. More importantly, it provides a higher ranging precision and has extensive applicability.
Power flow controller (PFC), fault current limiter (FCL) and DC circuit breaker (DCCB) are important equipment in the DC grid. There are a large number of power electronic components in these equipment, resulting in high cost. In order to reduce the use of additional components, a current‐limiting DC circuit breaker with power flow control capability is proposed here, which integrates the PFC, FCL, and DCCB into one equipment. Further, the main breaker in the proposed circuit breaker is shared by all lines connected to a DC bus to reduce cost. The topology and operating principle of the circuit breaker are introduced. Theoretical analysis and parameter design are carried out. Besides, the proposed circuit breaker is verified in PSCAD/EMTDC. Finally, the performance and cost are compared with existing schemes. The simulation results show that the proposed circuit breaker can effectively realize power flow control, current limiting and fault isolation at a lower cost.
Non-member Junting Zhang, Non-member Multi-port hybrid DC circuit breakers (MP-HCBs) are rapidly developing due to their obvious advantages in economy compared to traditional two-port hybrid circuit breakers (HCBs). However, most of the existing MP-HCB topologies do not have the currentlimiting capability, and a large number of insulated-gate bipolar transistors (IGBTs) are still needed in their main breakers. In response to the problems, a multi-port current-limiting hybrid DC circuit breaker (MP-CLHCB) based on thyristors is proposed in this paper. MP-CLHCB uses low-cost thyristors instead of IGBTs, and suppresses the rise of fault current through currentlimiting branch. In an attempt to prevent the MP-CLHCB from reclosing in a permanent fault and causing damage to the DC grid again, an adaptive reclosing scheme is also proposed. In this paper, the topology, operation stages, theoretical derivation, and parameter design of the MP-CLHCB are provided. Finally, the performance of the MP-CLHCB is verified and compared through simulations in PSCAD/EMTDC.
The development of flexible HVDC transmission technology requires the improvement of the theory of relay protections. The protection method based on the time domain has poor resistance to transition resistance and is easily affected by interference factors. The protection based on frequency domain analysis can better reflect the essential characteristics of the fault, and it is also an urgent problem for relay protections. The blocking effect of boundary elements on both sides of the line on high-frequency components is an important entry point for current protection research, but high-frequency protections are very sensitive to noise signals. Therefore, this paper analyses the frequency characteristics of the voltage at the measuring point from the perspective of the traveling wave transmission process, and uses the S transform to design a protection method based on high-frequency energy to realize the identification of internal and external faults. Finally, the ability to withstand transition resistance and anti-noise of the protection proposed here is verified by using PSCAD/EMTDC. The protection can complete fault identification within 2 ms, which means that it has both reliability and quick action.
Many communities have their own renewable energy generators to reduce longdistance power transmission losses and to decrease greenhouse gas emissions. However, in high power demand regions, power transmission congestion is often occurred when abundant renewable energy is generated and injected into the grid, which affects the security of transmission lines and power grids. In this paper, the congestion was caused by redundance power produced by community renewable energy generators. Double-layer power allocation strategy was presented to optimize the issue. In scheduling layer, linear optimization was used to allocate power flow, and fairness was also considered. In community layer, inverters were launched or stopped in turn by operation time to minimize the number of working units and to reduce costs. 6 communities with different power production were chosen for the simulation to justify the strategy. The results show the Double-layer strategy can alleviate congestion and increase the average utilization rate of inverters.
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