2020, 44(5):154-160. DOI: 10.7500/AEPS20190225001
Abstract:Large-capacity renewable energy is transmitted through bipolar voltage source converter based high voltage direct current (VSC-HVDC) system in the island mode, which has broad application prospects. This paper analyzes the power surplus characteristics under the non-fault-pole overload and DC overvoltage conditions. Then the scheme of grouped AC energy dissipation resistor is proposed. The power surplus control strategies are designed in the cases of converter fault in sending end and DC overvoltage. By accurately switching the grouped AC energy dissipation resistor, the outage of bipolar VSC-HVDC system caused by power surplus is avoided. The proposed two power surplus control strategies are verified in real-time digital simulator (RTDS) and EMTDC simulation system of four-terminal VSC-HVDC power grid. Simulation results show that the proposed strategies can realize fault ride-through under the condition of power surplus, and the expansion of fault coverage can be avoided.
2020, 44(5):146-153. DOI: 10.7500/AEPS20190425008
Abstract:For MMC-based flexible DC grid with symmetric monopolar topology, this paper firstly studies the evolution law of fault currents of single-pole grounding fault and bipolar short-circuit fault, and thus reveals that non-faulted line charging process and fault waves are respectively the main reasons for the overvoltage of single-pole grounding fault and bipolar short-circuit fault. Secondly, a three-terminal flexible DC grid is used for simulation and verification, and the influences of different factors on overvoltage are analyzed. The results show that the fault resistance has a great effect on the peak value of the overvoltage, while the converter blocking, grounding scheme and line protection have few effects. Finally, the overvoltage of single-pole grounding fault and bipolar short-circuit fault are summarized and compared.
2020, 44(5):122-129. DOI: 10.7500/AEPS20190529006
Abstract:At present, the protection schemes of DC microgrid mostly depend on the fast breaking ability of DC circuit breakers at both ends of the line and the reliability of communication equipment. However, at this stage, the cost of DC circuit breakers is high, and the communication between the two ends of the line will greatly increase the construction and operation cost of DC microgrid. Based on the above background, this paper proposes a single-end ranging protection technology based on coordinated control and protection for the four-terminal ring DC microgrid. This method can be divided into two stages: fault control and protection implementation. In the stage of fault control, the fault current of DC line is controlled to be zero by changing the active control strategy of voltage source converter (VSC) itself and the external controllable elements. In the stage of protection implementation, based on the periodicity (20 ms) of the output voltage at the DC side of VSC after active control and the controllability of power electronic components, the unique loop between VSC and fault point is constructed. Then the single-end fault location without error can be realized based on the traditional RL algorithm. Differential fault location will lead to continuous zero-crossing of line current. On this basis, fault isolation can be achieved by fast disconnector. Based on the idea of coordinated control and protection, this method eliminates the interference of terminal current in single-end fault ranging of ring network, and there is no need to configure DC circuit breakers at both ends of the line. Fault isolation can be achieved only by using fast disconnector and fault control strategy to cooperate logically in time sequence. Finally, a model of four-terminal ring DC microgrid is built on the PSCAD/EMTDC simulation platform, which verifies the effectiveness of the control and protection scheme.
2020, 44(5):91-100. DOI: 10.7500/AEPS20190601001
Abstract:The three-wire bipolar structure based high voltage direct current (TWBS-HVDC) transmission system can greatly enhance the transmission capacity of the DC lines, which has become an effective method for converting AC line into DC line and improving line capacity. Aiming at the difference between TWBS-HVDC and bipolar DC system in connection ways, a current calculation method of short-circuit fault at DC side for two-terminal TWBS-HVDC system is proposed. Firstly, the transient equivalent model of TWBS-HVDC system is established based on modular multilevel converter (MMC) based transient equivalent circuit in the stage from different faults at DC side to the operation of DC breakers of lines. The number of independent circuits and the order of dynamic components in the transient equivalent circuit are taken as the standard, all DC faults are classified into three categories, and the state equations of each kind of fault are analyzed and deduced. The analytical expression of fault current is obtained by solving the eigenvalue and eigenvector of coefficient matrix in the state equation. Finally, a two-terminal TWBS-HVDC system simulation model is established in the MATLAB/Simulink digital simulation platform, and the simulation results validate the effectiveness and accuracy of the state equation solution method of short-circuit fault at the DC side of the proposed TWBS-HVDC system, which can provide a scientific basis for the selection of DC breakers and parameter setting of current limiting reactors.
2020, 44(5):23-29. DOI: 10.7500/AEPS20190607003
Abstract:Fault current limitation in DC power grid has become one of the important problems that must be faced and urgently solved in related fields. Most of the existing fault limitation methods adopt single component. In order to fully limit the rising rate and peak value of fault current, this paper proposes a fault limitation and optimal configuration method combining the characteristics of DC reactor and capacitive current limiter. Firstly, based on the fault equivalent circuit of modular multilevel converter, the necessity of installing fault current limiter is analyzed from two aspects of limitation principle and action sequence. Secondly, the limitation characteristics of inductance and capacitance to fault current are calculated and analyzed, and the equivalent circuit and solution method are extended to DC ring grid. The optimal configuration model is constructed with the objective of fault current and DC reactor. Finally, the optimization results are applied to PSCAD/EMTDC simulation model. Compared with the scheme using DC reactor only, the configuration results can further reduce 40% of the fault current without prolonging the fault clearance time. It is verified that the combination of DC reactor and capacitive current limiter can significantly reduce the fault current and the breaking capacity of DC circuit breaker.
2020, 44(5):60-67. DOI: 10.7500/AEPS20190128005
Abstract:With the increase of voltage and capacity of high voltage direct current (HVDC) system, the requirement for rapid clearance and isolation of DC faults is getting higher and higher. As the most effective solution, hybrid DC circuit breaker is not mature enough, which restricts its engineering application. Accordingly, this paper proposes a low-cost DC fault clearance scheme suitable for clearing fault in DC grid. In this scheme, the traditional half-bridge modular multilevel converter (MMC) is partially innovated to have the ability of auxiliary breaking operation. With the effect of the MMC auxiliary breaking operation at both ends of the fault line, the DC fault current can be quickly interrupted and isolated by low-cost circuit breaker unit installed in the DC transmission line. The equivalent circuit in the fault isolation process is modelled and analyzed, and the fault isolation sequence is designed. Furthermore, the comparative analysis of devices usage among typical schemes is performed. Finally, a simulation model with the proposed scheme is built, and an MMC prototype is developed. The simulation and experimental results validate the effectiveness of the proposed scheme.
2020, 44(5):3-13. DOI: 10.7500/AEPS20190626003
Abstract:The structure and operation modes of flexible DC grid are diverse, and there are strong coupling relationships among various power electronic devices. The fault characteristics are complex, coordination control is difficult, and active control of fault current still faces great challenges. Based on the development mechanism of fault current in flexible DC grid, this paper compares and summarizes the fault characteristics of AC and DC grids, and illustrates the necessity and possibility of active control of fault current in flexible DC grid. The basic scheme of active control scheme for fault current of flexible DC grid is determined. Domestic and foreign research results are summarized, and active control schemes are classified based on selectivity principle and suppression principle. Taking several typical schemes as examples, the characteristics of different types of active control schemes are analyzed and simulated in PSCAD/EMTDC, and the characteristics of different active control schemes are compared and summarized. The results show that the active control of fault current coordinated by the equipments on both source side and grid side can isolate fault quickly, save the cost of line investment and recover from fault rapidly.
2020, 44(5):138-145. DOI: 10.7500/AEPS20190715001
Abstract:The highly efficient electromagnetic transient (EMT) simulation of hybrid modular multilevel converter (MMC) is an important basis for the related research about hybrid MMC. However, because there are various kinds of sub-modules (SMs) can be employed, and a lot of power electronic switches are included in each kind of SMs, the detailed EMT model of hybrid MMC will reduce the simulation efficiency seriously. In view of this background, the unified dynamic averaging equivalent model of the series structure of various SMs is proposed based on the switching function and the dynamic characteristic of capacitor. In addition, a unified terminal highly efficiency EMT model of hybrid MMC based on the proposed dynamic model of the series structure is also presented and analyzed. The proposed unified model not only is convenient but also has the great simulation accuracy and efficiency, which has good portability and is especially important for a research tool to satisfy the convenience demand of modification. Finally, the simulation accuracy and efficiency of the proposed model are validated by the comparison with the detailed model based on the simulation components in ADPSS.
2020, 44(5):77-83. DOI: 10.7500/AEPS20190715003
Abstract:In view of AC/DC outlet grounding faults of modular multilevel converter (MMC) in bipolar flexible high-voltage direct current (HVDC) transmission systems, the transient characteristics of voltage and current after converter fault blocking are studied. The mathematical analytical formula of the fault component is derived. The research results show that a single-phase grounding fault at the AC outlet will lead to the overvoltage and the overcurrent of the upper arm and the lower arm of blocked converter on the non-fault phase, respectively. And the DC bias on the AC-side current causes the fault phase short-circuit current to have no zero-crossing. When a pole-to-ground fault occurs at the DC outlet, the short-circuit current of the bridge arm of the blocked converter is mainly composed of the steady-state current injected by the AC system and the circulating current between the upper and lower arms. A phase-selective trip protection strategy is proposed for this special fault characteristic of single-phase grounding fault at AC outlet, which can solve the problem that the AC circuit breaker cannot be normally opened when the zero-crossing point of the fault current does not exist. A simulation model of the Zhangbei flexible DC grid of China is built. The simulation results show that the analysis of the fault characteristics at the converter outlet is accurate and the proposed phase-selective trip protection strategy is effective and feasible.
2020, 44(5):68-76. DOI: 10.7500/AEPS20190729015
Abstract:As an effective means to support the adoption of high proportion of renewable energy, modular multilevel converter based high-voltage direct current (MMC-HVDC) has become an important development direction of power grid. The bipolar fault is the most serious fault in transmission lines. At present, state equations of DC system are usually written in s-domain, and then fault currents are solved based on the inverse Laplace transformation. A practical engineering calculation method for fault current is needed urgently. Zhangbei MMC-HVDC grid of China is taken as the research object, the fault characteristics and coupling mechanism for bipolar fault of transmission line are analyzed firstly. On this basis, the two ends of the fault lines near the valve side are regarded as two ports respectively, and the relationship between the fault currents and the voltages of the two ports are analyzed. Secondly, based on the idea that the voltage of the two ports of the positive and negative lines does not change too much, the annular MMC-HVDC grid is simplified to a two-terminal network or an open network. The practical calculation method of fault line current is obtained to calculate fault current directly, while it is no longer necessary to solve the high-order inverse Laplace transformation. Finally, the feasibility and efficiency of the practical calculation method are verified by comparing with the electromagnetic transient simulation results.
2020, 44(5):53-59. DOI: 10.7500/AEPS20190730002
Abstract:As the key equipment for the future DC power grid, the high-voltage and high-capacity DC/DC converter has become the hot research topic in recent years. This paper proposes a hybrid DC/DC converter topology combing thyristors with half-bridge sub-modules. The hybrid DC/DC converter can block the short-circuit faults on the high-voltage and low-voltage sides by latching sub-modules and thyristors, and has the advantages of low cost and high efficiency. The working principle, fault protection mechanism, control strategy, parameter design and economy of the topology are analyzed and demonstrated. Finally, the feasibility of the proposed topology and control strategy are verified by simulation results based on MATLAB/Simulink.
2020, 44(5):38-46. DOI: 10.7500/AEPS20190730005
Abstract:To solve the problem of lack of freedom of power flow control in multi-terminal DC transmission system, a modular DC power flow controller (DCPFC) is proposed, which adopts modular structure to facilitate the expansion of multiple lines and has the ability of DC fault current limiting. Firstly, the topology of DCPFC is introduced, and its equivalent circuit model is established, while the principles of power flow control and fault current limiting are described. Then the power transfer characteristics of bridge arm of DCPFC in the power flow control mode are analyzed, and the power balance mechanism of bridge arm based on AC circulation is studied. On this basis, the power distribution control and power balance control methods of DCPFC are proposed, and their control strategies in the fault current limiting mode are illustrated. Finally, a three-terminal DC transmission system is built in PLECS simulation software to verify the effectiveness of DCPFC in the conditions of power flow distribution, power flow reversal, power step and fault current limiting.
2020, 44(5):130-137. DOI: 10.7500/AEPS20190805005
Abstract:In order to accurately simulate the dynamics of large-scale AC/DC systems and interactions between individual components, a simulation method with high precision and efficiency is highly required. A multi-domain simulation method for large-scale AC/DC systems based on time-frequency coordination transform is proposed. In this method, DC systems are partitioned into the phasor-shift sub-systems, which are presented by their phasor-shift models based on the time-frequency coordination transform. This method can adopt large time-step to improve the efficiency while the accuracy is guaranteed. Further, a multi-domain interface model between phasor-shift sub-systems and electromagnetic sub-systems，which is also called interface transmission line model， is optimally designed. The interface model can reflect both instantaneous and wide-band phasor waveforms simultaneously. Finally, an actual AC/DC system integrating four-terminal flexible DC system in China has validated the effectiveness of the proposed method.
2020, 44(5):47-52. DOI: 10.7500/AEPS20191029007
Abstract:A large-step-ratio DC/DC converter with the abilities of automatic current-limiting and bidirectional power flow is proposed. It is suitable to be used as an interface circuit between high voltage direct current (HVDC) and low voltage direct current (LVDC) transmission lines. The DC/DC converter works with three-level voltage. By adjusting its duty ratio and phase shift, the transmission power of the converter can be changed, while the bidirectional power flow can be realized. The circuit operates with a trapezoidal current wave, and realizes partial soft switching with a high efficiency. When a short-circuit fault occurs the DC/DC converter can automatically limit the current, whose peak current value through the switch is same as that in normal operation. Based on these features, a converter is designed and validated by simulation, whose input voltage is 12 kV, output voltage is 1 kV, and transmission power is 100 kW.
2020, 44(5):114-121. DOI: 10.7500/AEPS20191114003
Abstract:DC faults in flexible DC distribution network could easily lead to overcurrent, which seriously threatens the safe operation of power grid. Modular multilevel converter (MMC) blocking is mostly used to cut off the fault current in FBSM-MMC based distribution network, but the blocking will cause power outage of the whole network for a moment, which reduces the reliability of the power supply. To solve the problem, a protection and fault isolation scheme based on active current-limiting control of FBSM-MMC for flexible DC distribution network is proposed, which consists of three stages. After a short-circuit fault occurs, the output DC current of the inverter will be limited to 1.2 times of the rated current by the control of MMC (stage 1). The faulty line is identified according to the synchronous zero-crossing characteristic at both ends of each line (stage 2). A fault isolation scheme which contains the cooperation of DC circuit breakers and high-speed switches is proposed (stage 3). By disconnecting the DC breaker associated with the fault and controlling the output DC current of the corresponding converter station to be reduced to 0, the mechanical switch on the fault line can also be quickly turned off, thereby achieving fault isolation. Finally, a four-terminal flexible DC distribution network model is built in PSCAD/EMTDC, and the feasibility of the proposed protection and fault isolation scheme are verified through a large number of simulations.
2020, 44(5):84-90. DOI: 10.7500/AEPS20190408013
Abstract:In order to prevent the trip of modular multilevel converter (MMC) due to block, the rising speed of DC short-circuit current should be limited. The active current-limiting control of MMC is a new current-limiting method, which can limit the DC short-circuit current by reducing the discharging time of capacitor without extra cost. The existing calculation method of DC short-circuit current cannot reflect the change of circuit structures and parameters caused by active current-limiting control. This paper proposes a calculation method of DC short-circuit current considering active current-limiting control of MMC. A duty circle index for discharging state of capacitor is introduced to represent the effect of active current-limiting control on DC short-circuit current. The state equation of DC short-circuit current is built based on state space averaging method, and the time-domain analytical expression of DC short-circuit current is given. The effectiveness of the proposed calculation method is validated by the simulation results based on PSCAD/EMTDC.
2020, 44(5):30-37. DOI: 10.7500/AEPS20190418012
Abstract:In order to eliminate the adverse effects of conventional current-limiting reactance on the operation stability of DC system and the breaking speed of DC circuit breaker, the bridge-type current-limiting solid-state circuit breaker has achieved excellent performance of both self-adaptive fault current limiting and breaking. However, the DC bias power supply in the bridge circuit has the shortcomings of no overcurrent protection, relatively high power capacity and high investment cost. For the bridge-type current-limiting solid-state circuit breaker, this paper designs a DC bias power supply based on three-phase half-wave rectifier circuit. The method of selecting the parameters of bias power supply and setting the voltage is put forward, which effectively reduces the number of power electronic devices, realizes the over-current protection circuit of bias power supply, and reduces the designed capacity and cost of bias power supply. The prototype experiment and simulation examples verify the advantages of the proposed bias power supply of self-adaptive current-limiting solid-state circuit breaker.
2020, 44(5):14-22. DOI: 10.7500/AEPS20190626004
Abstract:Fault current limiter (FCL) could retrain fault current after identifying the short-circuit fault of the flexible DC grid to reduce the DC circuit breaker （DCCB） requirements. By analyzing the working principle of FCL, the equivalent circuit of flexible DC converter station including FCL with a fault is obtained. For the calculation of fault current and voltage stress of FCL with different impedance types, the characteristics of fault current limiting impedance parameters are analyzed, and the ideal current limiting impedance is proposed and its characteristics are analyzed. To optimize the DCCB breaking current and FCL cost, an optimization method for current limiting impedance parameter is proposed. Based on simulation in PSCAD/EMTDC, the correctness of the voltage stress calculation and optimization model is verified.
2020, 44(5):101-113. DOI: 10.7500/AEPS20190418004
Abstract:The isolation and recovery of faults are the key issues to guarantee the supply reliability in the DC distribution network. Using converters with fault clearance capability and isolating switches to achieve fault current blocking and isolation has become an important development direction of DC distribution network. From the three aspects of protection technology and fault location, fault isolation and fault recovery, this paper summarizes the research status of fault processing technology based on blocking converters in DC distribution network. Finally, the difficulties of fault processing technology and the development trend of key technologies based on blocking converters in the DC distribution network are sorted out.