Abstract:With the rapid development of flexible AC/DC distribution systems, the large-scale access of nonlinear elements such as power electronic devices and constant power loads make the transient stability problem of the system increasingly serious. Firstly, a mathematical model of AC/DC distribution system with constant power loads based on AC/DC and DC/DC converters connected in parallel under double-loop control is established, and a nonlinear Taylor expansion of the system is carried out. Secondly, the original high-order system is decoupled into a series of low-order subsystems based on the nonlinear decoupling theory. The dominant decoupled subsystem is identified and analyzed in terms of transient stability by using the phase-plane method and the inverse trajectory method, and the validity of the theoretical analysis is verified by physical simulation. Then, the concept of the nonlinear participation factor is introduced based on the principle of nonlinear decoupling, and the participation factor of the state variables is calculated, so as to put forward the analytical method of the control parameters affecting the stability under the constant power load perturbation, and further give the feasible domain of the control parameters. Finally, an AC/DC distribution system with double-loop control under constant power load is constructed by using PLECS software, and simulation and experiment verify the effectiveness of the proposed method.

Abstract:Two-terminal DC distribution networks are a type of network structure powered by two-terminal power sources, providing a more stable and reliable supply voltage for DC loads. However, power fluctuations and unbalanced loads increase the voltage drop on the line, which may lead to the voltage deviation and imbalance index of DC loads exceeding the limit, obstructing the normal operation of DC loads. Based on the outer-loop voltage control of the voltage source converter (VSC), and considering the neutral line voltage compensation, this paper proposes a joint suppression strategy for the voltage deviation and imbalance of DC distribution network based on the voltage compensation equivalent model, to realize the local coordinated voltage control of DC distribution network. First, the paper establishes a power flow model of the two-terminal DC distribution network, introduces the VSC voltage droop control into the model, and analyzes the characteristics of voltage deviation and imbalance under different control strategies. Then, on this basis, a simplified equivalent model of voltage drop in the two-terminal power supply circuit is obtained by taking the lowest voltage point as the dividing point, and the least square method is used to identify the equivalent impedance parameters, constructing the voltage compensation equivalent model of two-terminal DC distribution network. Taking the parameter identification results as the voltage outer-loop control input, a local coordinated voltage control strategy for two-terminal DC distribution networks considering the neutral line voltage compensation is proposed. Finally, a simulation model is built in the MATLAB/Simulink to verify the correctness of the power flow models of the two-terminal DC distribution network and the effectiveness of the control strategy.

Abstract:Under the condition of high power disturbance, the limiter is prone to the saturation phenomenon of duty cycle, and the DC power distribution system will be degraded from closed-loop control to open-loop response and transient instability. Therefore, this paper carries out the transient stability analysis of DC power distribution system considering the duty cycle limiter. Firstly, this paper uses Sigmoid function to simulate the nonlinear characteristics of duty cycle limiter, and establishes a large-signal model of DC distribution system which is closer to the engineering reality. Secondly, the estimated attraction domain taking into account the influence of the duty cycle limiter is characterized by the Takagi-Sugeno (TS) fuzzy modeling method. Then, the variation of the estimated attraction domain with different control parameters reveals the transient instability phenomenon of the DC distribution system induced by duty cycle saturation. Finally, for the limitations of the existing transient stability improvement methods due to the duty cycle limiter, an optimization design method of control parameters is proposed to provide a scheme for the control parameter selection to improve the transient stability of the system. The effectiveness of the theoretical analysis is verified by the RT-Box hardware-in-the-loop experiment.

Abstract:In the flexible DC distribution system, the DC pole-to-pole short-circuit fault makes the converters of the whole network quickly locked, which is faster than the action time of the DC circuit breaker, leading to the system outage and greatly reducing the reliability of the system. Therefore, this paper proposes a line protection method based on incipient fault section location for DC cable incipient short-circuit faults. Through the active injection of the local converter, the attenuation of the line impedance to the injected signal is weakened; the characteristics of incipient faults are displayed; the accurate identification and section location of incipient short-circuit faults are realized; and the problem of system power loss caused by short-circuit faults is solved. To avoid frequent injection, an injection starting criterion based on differential current measurement is proposed. The simulation results with PSCAD/EMTDC show that this method can accurately identify and locate the incipient fault section and promptly isolate the fault branch, and has a small impact on the system operation. It has a certain anti-noise ability, and guarantees the continuous and safe operation of the flexible DC distribution system.

Abstract:When the bipolar short-circuit fault occurs at the low-voltage DC side of the multi-branch flexible DC distribution network which takes the power electronic transformer with cascaded H-bridge (CHB-PET) as the main station, the fault current rises rapidly and the peak value is high. However, power electronic devices lock up soon after the fault, resulting in the loss of fault information, which brings challenges to the reliable operation of DC distribution network line protection. To solve this problem, a block-free fault ride-through scheme is proposed based on the cooperation between the input stage and the isolation stage of CHB-PET. By setting the inward shift ratio of the dual active bridge high-voltage switching tube in the isolation stage, the DC capacitor discharge of the input stage is controlled, so that it does not overcharge or overdischarge. And the overcurrent of the power electronic devices in the CHB-PET is eliminated, so as to ensure the sustainable stable current output of the CHB-PET after the fault. On this basis, the Fourier transform is used to extract the high frequency component of the low-voltage DC-side current of CHB-PET with the fault ride-through strategy, and the protection criterion is constructed by the ratio of the integral value of the high-frequency current amplitude to the integral value of the full current. Then a single terminal protection scheme is designed. A multi-branch DC distribution network model including CHB-PET is built on the PSCAD/EMTDC simulation platform. The simulation analysis shows that the proposed method can identify and remove faulty lines quickly and reliably.

Abstract:Aiming at the problem of low coverage of real-time measurements and uncertainty of probability distribution of measurement errors, an interval state estimation method based on a pseudo measurement modeling method using deep neural networks (DNN) is proposed for regional AC/DC distribution network. Firstly, DNN is trained offline in this method. Then the real-time measurement data and the variable values controlled by VSC are used as the input features of DNN to establish a pseudo-measurement model. Secondly, the trained DNN is used to generate the pseudo measurements quickly when the real-time measurements are updated. Finally, the uncertainty of the pseudo-measurement and the real-time measurement is modeled in the interval form and the interval state estimation is carried out in order to accurately monitor the states of the AC/DC distribution system. The simulation results of the calculation example show that the proposed method can avoid assumptions about the probability distribution of the measurement errors, and it can obtain the accurate upper and lower bounds of the state variables in the case of low real-time measurement redundancy or insufficient configuration.

Abstract:With the rapid development of DC distribution technology, the DC distribution systems have gradually shown the remarkable characteristics of multiple voltage levels, multiple DC buses and multiple power electronic converter access, which leads to more complex impedance coupling relationship among converters and the increasingly prominent problem of system stability. In order to evaluate the small-signal stability of the DC distribution systems with multiple voltage levels, the small-signal equivalent model of the system is established, and then the unified transfer function from all system input disturbances to any bus voltage is derived according to the different control modes of the DC transformers between adjacent buses. On this basis, the system stability assessment method is proposed. Furthermore, a distribution system with two-voltage level DC buses is taken as an example to analyze the impedance factors affecting the system stability. At the same time, the stability criterion based on the system equivalent loop gain is proposed. Finally, two system cases are designed, and the correctness of the proposed stability assessment method of the DC distribution system with multiple voltage levels is verified based on MATLAB/Simulink simulation.

Abstract:The power supply of the DC distribution system has multiple output levels. The load operation mode is flexible, and the system operation state increases sharply, which leads to the reduction of reliability assessment efficiency. This paper proposes a reliability assessment method for the DC distribution system based on state merging. First, this method establishes a system state space model based on the multiple operating states of the DC power distribution system with source-load coupling. The conditions of state merging are studied according to the intrinsic connection among different states. Then, the fault tree analysis method is combined with the Markov process to propose the state merging method based on the truncation factor. The method realizes state merging in the case of differential transfer ratios. Finally, the IEEE-RBTS BUS 6 DC distribution system is used as an example to verify the effectiveness of the proposed method.