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  • Volume 44,Issue 5,2020 Table of Contents
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    • >Special Topic on Active Control of Fault in Flexible DC Grid
    • Guest Chief Editor's Note

      2020, 44(5):1-2. DOI: 10.7500/AEPS20200110011

      Abstract (205) HTML (228) PDF 414.90 K (959) Comment (0) Favorites

      Abstract:

    • Overview on Typical Schemes for Active Control of Fault Current in Flexible DC Grid

      2020, 44(5):3-13. DOI: 10.7500/AEPS20190626003

      Abstract (173) HTML (86) PDF 883.47 K (1204) Comment (0) Favorites

      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.

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    • Parameter Optimization for Fault Current Limiting Impedance of Flexible DC Converter Station Based on Voltage Stress Analysis

      2020, 44(5):14-22. DOI: 10.7500/AEPS20190626004

      Abstract (81) HTML (30) PDF 2.59 M (780) Comment (0) Favorites

      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.

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    • Characteristics and Optimal Configuration of Capacitive Current Limiter Considering DC Reactor

      2020, 44(5):23-29. DOI: 10.7500/AEPS20190607003

      Abstract (116) HTML (48) PDF 716.74 K (1403) Comment (1) Favorites

      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.

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    • Design of DC Power Supply for Self-adaptive Current-limiting Solid-state Circuit Breaker

      2020, 44(5):30-37. DOI: 10.7500/AEPS20190418012

      Abstract (88) HTML (37) PDF 1.62 M (1023) Comment (0) Favorites

      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.

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    • Modular DC Power Flow Controller with Current Limiting Function and Its Control

      2020, 44(5):38-46. DOI: 10.7500/AEPS20190730005

      Abstract (94) HTML (28) PDF 966.34 K (1021) Comment (0) Favorites

      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.

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    • Large-step-ratio DC/DC Converter with Capability of Short-circuit Current Limiting

      2020, 44(5):47-52. DOI: 10.7500/AEPS20191029007

      Abstract (179) HTML (50) PDF 803.55 K (849) Comment (0) Favorites

      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.

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    • DC/DC Converter with Fault Blocking Capability for HVDC Transmission System

      2020, 44(5):53-59. DOI: 10.7500/AEPS20190730002

      Abstract (146) HTML (51) PDF 810.16 K (993) Comment (0) Favorites

      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.

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    • An Augmented Modular Multilevel Converter for Clearing Fault in DC Grid

      2020, 44(5):60-67. DOI: 10.7500/AEPS20190128005

      Abstract (126) HTML (47) PDF 707.20 K (1093) Comment (0) Favorites

      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.

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    • Practical Calculation for Bipolar Short-circuit Fault Current of Transmission Line in MMC-HVDC Grid

      2020, 44(5):68-76. DOI: 10.7500/AEPS20190729015

      Abstract (201) HTML (78) PDF 819.70 K (1135) Comment (0) Favorites

      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.

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    • Converter Grounding Fault Characteristics and Protection Strategy in Bipolar MMC-HVDC System

      2020, 44(5):77-83. DOI: 10.7500/AEPS20190715003

      Abstract (156) HTML (33) PDF 713.46 K (1326) Comment (0) Favorites

      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.

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    • Calculation Method of DC Short-circuit Current Considering Active Current-limiting Control of Modular Multilevel Converter

      2020, 44(5):84-90. DOI: 10.7500/AEPS20190408013

      Abstract (147) HTML (104) PDF 732.17 K (1095) Comment (0) Favorites

      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.

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    • Current Calculation Method of Short-circuit Fault at DC Side for MMC Based Two-terminal TWBS-HVDC

      2020, 44(5):91-100. DOI: 10.7500/AEPS20190601001

      Abstract (60) HTML (26) PDF 829.75 K (962) Comment (0) Favorites

      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.

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    • Overview on Fault Processing Technology for DC Distribution Network Based on Fault Blocking Converter

      2020, 44(5):101-113. DOI: 10.7500/AEPS20190418004

      Abstract (91) HTML (66) PDF 746.29 K (864) Comment (0) Favorites

      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.

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    • Protection and Fault Isolation Scheme Based on Active Current-limiting Control for DC Distribution Network

      2020, 44(5):114-121. DOI: 10.7500/AEPS20191114003

      Abstract (113) HTML (39) PDF 677.53 K (770) Comment (0) Favorites

      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.

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    • Single-end Ranging Protection Technology for Ring DC Microgrid Based on Coordinated Control and Protection

      2020, 44(5):122-129. DOI: 10.7500/AEPS20190529006

      Abstract (135) HTML (51) PDF 738.15 K (1229) Comment (0) Favorites

      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.

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    • Multi-domain Simulation Method for Large-scale AC/DC Systems Based on Time-Frequency Coordination Transform

      2020, 44(5):130-137. DOI: 10.7500/AEPS20190805005

      Abstract (81) HTML (53) PDF 965.77 K (879) Comment (0) Favorites

      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.

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    • Unified Terminal and Highly Efficient Electromagnetic Transient Model of Hybrid Modular Multilevel Converter with Various Sub-modules

      2020, 44(5):138-145. DOI: 10.7500/AEPS20190715001

      Abstract (99) HTML (102) PDF 947.44 K (1158) Comment (0) Favorites

      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.

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    • Analysis on Overvoltage Mechanism of DC Line Fault in Flexible DC Grid

      2020, 44(5):146-153. DOI: 10.7500/AEPS20190425008

      Abstract (90) HTML (23) PDF 1.27 M (1152) Comment (0) Favorites

      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.

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    • Surplus Power Dissipation Strategy for Bipolar VSC-HVDC System with Integration of Islanded Renewable Energy Generation System

      2020, 44(5):154-160. DOI: 10.7500/AEPS20190225001

      Abstract (98) HTML (40) PDF 799.64 K (846) Comment (0) Favorites

      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.

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    • >Basic Research
    • System and Framework Design of Risk Coordination Control for Whole Operation Process of Power System

      2020, 44(5):161-170. DOI: 10.7500/AEPS20190429011

      Abstract (139) HTML (31) PDF 854.41 K (542) Comment (0) Favorites

      Abstract:For the multiple uncertain risk factors faced by the operation of power grid, this paper analyzes the necessity of risk coordination control from the perspective of the sequential characteristic of risk development and the dispatch mode of power grid. Based on the view point of risk coordination control, a system of risk coordination control for the whole operation process of power grid including medium and short-term, day-ahead, real-time and post-contingency is proposed. The risk optimization models, control measures and coordination strategies at each stage are elaborated in detail. The solution based on a five-tier software framework is proposed and key technologies supporting the system application are discussed. Finally, the preliminary implementation of the whole process risk coordination control system in the Zhejiang Power Grid of China is introduced. The results of theoretical analysis and engineering practice show that the proposed system and framework can adapt to the “open, fair and just” power dispatch mode, coordinate the risk control decisions at different stages and realize the step-by-step tracking and rolling control for the operation risk of power grid.

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    • Operation Risk Analysis of Electric Vehicle Integrated to Distribution Network Based on Weighted Distribution Entropy

      2020, 44(5):171-179. DOI: 10.7500/AEPS20190303007

      Abstract (65) HTML (22) PDF 771.51 K (377) Comment (0) Favorites

      Abstract:Aiming at analysis on the operation risk of the electric vehicle integrated to the distribution network, the index of loss severity with amplification factor is proposed, and the distribution entropy is used to measure the risk caused by the distribution uncertainty of the state variables of the distribution network. By weighting the distribution entropy with loss severity, the weighted distribution entropy (WDE) is established, based on which the voltage risk index and the load-flow risk index are established. The weighted sum of WDE is calculated according to the component importance, so as to evaluate the voltage risk and the load-flow risk of the network. Taking into account both types of risks, a comprehensive set of indices is constructed to evaluate the operation risk of distribution networks. Finally, simulation studies are conducted on an improved IEEE 33-node distribution system with wind power and photovoltaic accessed, and the superiority of WDE over traditional risk indices and the rationality of the comprehensive index are analyzed by the dynamic probabilistic load flow simulation. In addition, the variation of the operation risk with the access location and capacity of electric vehicle load are studied. The effectiveness of the proposed indices and models is verified through the simulation studies.

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    • Method of Determining Cascading Number of π-Shaped Equivalent Circuit Chain Network Oriented to Distributed Network

      2020, 44(5):180-186. DOI: 10.7500/AEPS20190127009

      Abstract (68) HTML (13) PDF 863.96 K (327) Comment (0) Favorites

      Abstract:Increasing attention has been paid to the physical simulation of the distribution network. The transmission line often adopts the π-shaped equivalent circuit chain network in simulation. It is very important how to construct the transmission line model economically and effectively. Firstly, this paper gives the input impedance expressions of the lossless π-shaped circuit chain network with different cascading numbers, and clarifies the difference in phase-frequency characteristics between the chain network and the uniform transmission lines without losses. Then, the amplitude-frequency error of the characteristic impedance between the π-shaped circuit chain network and the uniform transmission line without losses is analyzed. It is found that when the operation frequency approaches or exceeds the upper limit frequency, with which the π-shaped circuit chain network simulate the uniform transmission line without losses, the two are essentially different. The ratio of any frequency to the upper limit frequency is only related to the characteristic impedance error. Each π-shaped circuit in the chain network is premised on working within its first resonant frequency, so that the reasonable error range of the characteristic impedance can be determined. Furthermore, the reasonable cascading number which is suitable for π-shaped circuit chain network of the distribution network can also be obtained. Finally, through the calculation and simulation of typical transmission line parameters and comparing the experimental results of physical simulation system, the method is verified to be reasonable and effective.

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    • Model Predictive Control Based Optimal Operation of Distribution Network with Charging-Swapping-Storage Integrated Station

      2020, 44(5):187-197. DOI: 10.7500/AEPS20190509008

      Abstract (76) HTML (65) PDF 1.29 M (443) Comment (0) Favorites

      Abstract:An optimal scheduling model is proposed, which comprehensively considers electric vehicle (EV) charging-swapping-storage integrated station and active distribution network. Based on driving behavior characteristics of fast-charging users and speed-flow practical model of city roads, the fast charging station model and battery swapping station model are established respectively, which are combined with the cascaded energy storage system to form the integrated station model. The optimal scheduling model is established in the active distribution network, which includes wind turbines, photovoltaics, micro-turbines and the integrated station, and the model is transformed into a mixed integer second-order cone model to solve the problem. The multi-time scale optimal scheduling strategy based on model predictive control is used to realize day-ahead scheduling, intra-day rolling scheduling and real-time feedback correction of distribution network, which reduces the impact of distributed generator and load prediction errors on the distribution network operation. Taking the actual road conditions of bus lines in a city as an example, it is verified that the proposed optimal scheduling strategy has advantages of meeting the charging load demand of EVs, suppressing power fluctuation and reducing operation maintenance cost of distribution network.

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    • Reinforcement Learning Based Distributed Secondary Optimal Control for Multiple Microgrids

      2020, 44(5):198-206. DOI: 10.7500/AEPS20190521007

      Abstract (133) HTML (60) PDF 832.95 K (493) Comment (0) Favorites

      Abstract:Aiming at the static deviation problems of system frequency and voltage caused by primary control of distributed generators in microgrid, a distributed secondary optimal control based on reinforcement learning local feedback method is proposed, which addresses the need of frequency recovery and voltage adjustment by using the local information. Firstly, according to the demand of microgrid economy, frequency and voltage control and the comprehensive performance of distributed generators (environmental benefit, economic benefit and technical benefit), a local reward is defined to coordinate the frequency recovery and voltage regulation of multiple microgrids. Secondly, according to the actual operation of the power grid, the multi-agent reinforcement learning algorithm is used to optimize and modify the global reward feedback, while satisfying the balance between supply and demand, so that the frequency deviation can be eliminated asymptotically and the stable operation of the microgrid can be guaranteed. Finally, the effectiveness and adaptability of the proposed control are verified by the analysis of case study.

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    • Voltage Dynamic Compensation Control of DC Microgrid Based on Robust Disturbance Observer

      2020, 44(5):207-214. DOI: 10.7500/AEPS20190805002

      Abstract (85) HTML (47) PDF 768.35 K (386) Comment (0) Favorites

      Abstract:Aiming at the problem of bus voltage control in DC microgrid, a dynamic compensation control strategy based on robust disturbance observer is designed to complete voltage compensation of DC-DC converter. Firstly, based on the architecture of DC microgrid, the theoretical analysis of the bus voltage fluctuation is carried out. Secondly, the state space mathematical model of the DC-DC converter of the DC microgrid system is established, and the input and output relationship of the control system is obtained. According to the robust dual-mass decomposition and the theory of Youla’s parameterized stability controller, the control architecture based on robust disturbance observer is obtained. The model matching theory is applied to compensate the output value generated by the current disturbance. The voltage loop compensation controller is solved by the linear matrix inequality (LMI) method. The current loop compensation controller is designed according to the dynamic structure diagram of the DC-DC converter. The results of semi-physical experiments show that the architecture can improve the dynamic performance of DC-DC converter without changing the structural parameters of the original system, and suppress DC bus voltage fluctuation caused by load switching, power fluctuation and AC side load imbalance, and enhances the robustness of the system.

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    • Non-intrusive Load Disaggregation Method Considering Time-phased State Behavior

      2020, 44(5):215-222. DOI: 10.7500/AEPS20190225006

      Abstract (92) HTML (61) PDF 689.45 K (358) Comment (0) Favorites

      Abstract:Load monitoring is an important part of intelligent electricity consumption. A non-intrusive load disaggregation method considering time-phased state behavior is proposed to solve the problem that existing low frequency non-intrusive load disaggregation methods require more priori information and have lower accuracy for load with similar or lower power. Firstly, power data of the load device is clustered to construct a load state template.An intelligent optimization method for the typical behavior time period that does not require a specified number of time periods is proposed. Load state behavior law is extracted by time-phase to construct a load behavior template. Then, on the basis of the traditional power characteristics, considering the two dimensions of probability and time, the time-phased state probability factor (TSPF) is introduced into the objective function as a new load characteristic, and the load disaggregation is realized by multi-feature genetic optimization iteration. Finally, the validity and accuracy of the method are verified on the public data set.

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