Zhengxi Li
North China University of Technology
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Featured researches published by Zhengxi Li.
IEEE Transactions on Power Electronics | 2013
Yongchang Zhang; Wei Xie; Zhengxi Li; Yingchao Zhang
This paper proposes an improved model predictive direct power control (MPDPC) for a pulse width modulation (PWM) rectifier by using a duty cycle control. The conventional MPDPC achieves good steady-state performance and quick dynamic response by selecting the best voltage vector, which minimizes the errors between the reference power and the real power. However, due to the limited number of voltage vectors in a two-level converter, the sampling frequency has to be high to achieve satisfactory performance. This paper introduces the concept of a duty cycle control in the MPDPC by allocating a fraction of control period for a nonzero voltage vector and the rest time for a zero vector. The nonzero vector is selected by evaluating the effects of each nonzero vector and its duration is obtained based on the principle of power errors minimization. Simulation and experimental results prove that, compared to the conventional MPDPC, the proposed MPDPC with duty cycle achieves further steady-state performance improvement without affecting the dynamic response at a small cost of control complexity increase.
IEEE Transactions on Power Electronics | 2013
Yongchang Zhang; Zhengxi Li; Yingchao Zhang; Wei Xie; Zhengguo Piao; Changbin Hu
Conventional switching-table-based direct power control (STDPC) presents irregular power ripples and variable switching frequency, due to the use of predefined switching table and hysteresis comparators. This paper proposes an improved DPC for three-phase pulsewidth modulation (PWM) rectifier with simple calculation. The concept of duty cycle control is introduced in DPC to allocate only a fraction of control period to the voltage vector selected from conventional switching table in DPC, while a null vector is applied for the rest of time. The main advantages of the proposed duty cycle control are twofold. On one hand, it is independent of rectifier parameter such as line-side inductance, hence strong robustness against inductance variations is obtained. On the other hand, the principle to obtain the duty cycle is very simple and easy to implement. Compared to conventional DPC, significant power ripple reduction and more sinusoidal grid current can be observed in the improved DPC. The presented simulation results and laboratory tests validate the effectiveness of the proposed method.
IEEE Transactions on Industrial Electronics | 2014
Yongchang Zhang; Wei Xie; Zhengxi Li; Yingchao Zhang
Conventional model predictive power control (MPPC) achieves good steady-state performance and quick dynamic response by minimizing a cost function relating to the power errors. However, applying single voltage vector during the whole control period fails to reduce the power ripples to a minimal value; particularly in the two-level converter with limited switching states. Recently, the concept of duty cycle control has been introduced in MPPC to achieve further power ripple reduction. Although better steady-state performance is obtained, a lot of calculations are needed when deciding the best voltage vector and its corresponding duration. This paper proposes a low-complexity MPPC with quick voltage selection and fast duty cycle calculation. Different from prior MPPC, the negative conjugate of complex power in synchronous frame is selected as the control variable. As a result, only one prediction is required to select the best voltage vector, and its duration is determined base on the principle of error minimization of both active and reactive power. Further study reveals that the proposed low-complexity MPPC is equivalent to the recently reported MPPC with duty cycle control. Simulation and experimental results obtained from a two-level three-phase ac/dc converter are presented to confirm the theoretical study and the effectiveness of the proposed method.
IEEE Transactions on Power Electronics | 2016
Xiaoguang Zhang; Zhengxi Li
A mechanical parameter estimation algorithm for permanent magnet synchronous motor (PMSM) drive systems based on sliding-mode observer is proposed in this paper. First, an extended sliding-mode mechanical parameter observer (ESMMPO) is presented to track system disturbances in real time, which include the information of mechanical parameters. Based on this ESMMPO, the mechanical parameter can be extracted from the estimated system disturbance. Therefore, a simple algorithm for mechanical parameter estimation of the PMSM control system is presented. The parameter choice guidelines of ESMMPO are designed to guarantee the global stability of the observer. Then, the equivalent low-pass filter function of the ESMMPO is presented to suppress the sliding-mode chattering, which allows softening the output signal of observer while maintaining no phase lag incurring. Therefore, the smooth output of the ESMMPO can be directly used for the parameter estimation. Experimental results show the validity of the proposed parameter estimation approach.
international conference on electrical machines and systems | 2013
Yongchang Zhang; Changqi Qu; Zhengxi Li; Yingchao Zhang
PWM rectifier controlled by direct power control (DPC) is characterized by the merits of quick response, no need of inner current tuning and strong robustness, etc. The switching table is the key point of DPC and many researches have been done in this field. However, most papers fail to give a clear interpretation on the mechanism of switching table. This paper gives a detailed analysis on the mechanism of the switching table by using mathematical derivation and geometry illustration. It is found that there are six kinds of feasible switching tables, among which three tables have been reported in the literature while other three not. The performance of the six kinds of tables are compared and their respective pros and cons are summarized. Selected simulation and laboratory experimental results are presented to confirm the feasibility of the six switching tables.
european conference on cognitive ergonomics | 2012
Yongchang Zhang; Zhengxi Li; Zhengguo Piao; Wei Xie; Xianglong Wei; Wei Xu
Direct power control (DPC) has been proven to be an effective approach for the control of doubly fed induction generators (DFIG). However, conventional switching-table-based DPC presents large power ripples at steady state and exhibits a broad harmonic spectrum, which is difficult to be filtered. Some SVM based DPC methods have been proposed to tackle the problems above, but they are usually complicated and sacrifice the robustness and simplicity. This paper presents a simple but effective predictive DPC to achieve quick response and good steady state performance. Meanwhile the simplicity and robustness of the basic DPC are maintained as much as possible. Three vectors are applied during one control period and their durations are obtained in a very simple and straightforward way. The novel PDPC is compared with prior art and the results prove that the novel PDPC achieves reduced control complexity, better steady state performance and quicker dynamic response at similar switching frequency.
international conference on electrical machines and systems | 2013
Yongchang Zhang; Qin Zhang; Zhengxi Li; Yingchao Zhang
This paper presents the performance evaluation and comparisons of two different control strategies for three-phase pulse width modulation (PWM) rectifiers. The two control methods are voltage oriented control (VOC) and model based predictive current control (MPCC). VOC decomposes the grid currents into active and reactive power components and regulate them separately in synchronous frame using PI. Contrary to VOC, MPCC eliminate the use of PI for the current control. It uses a discrete-time model of the system to predict the future value of the load current for all possible voltage vectors and the one minimizing the current errors is selected. The theoretical background is described, and the advantages and disadvantages of each scheme are studied through analysis and a series of experiments. A comprehensive comparison and evaluation are implemented for the two methods from several aspects such as algorithm complexity, steady state and dynamic performances, current THD. Compared to the conventional control strategy VOC, model based predictive control (MPC) is intuitive in principle and easy to understand. Furthermore, it does not require any regulators such as PI in VOC. Simulation and experiment results prove that, MPPC can achieve similar steady state performance to VOC and the dynamic response is better.
international conference on electrical machines and systems | 2013
Yongchang Zhang; Haitao Yang; Zhengxi Li
Direct torque control (DTC) is a kind of powerful control scheme for high performance control of induction motor (IM) drives, which provides very quick dynamic response with simple structure. However, the use of hysteresis comparators and predefined switching table lead to high torque ripple and variable switching frequency. To overcome the drawbacks of conventional DTC, this paper proposes a simple space vector modulation (SVM) based deadbeat DTC to achieve low torque ripple and fixed switching frequency, which is composed of two steps of deadbeat control. In the first step, the reference stator flux vector is obtained based on the principle of deadbeat torque control, and in the second step, the reference stator voltage vector to achieve deadbeat control of stator flux is obtained based on the stator voltage equation. No tuning work is required in the two steps, which is very simple and intuitive to understand. The reference voltage vector is then synthesized by the SVM block. The proposed deadbeat SVM-DTC can achieve excellent steady state performance and quick dynamic response over a wide speed range. Both simulation and experimental results are presented to validate its effectiveness.
international conference on electrical machines and systems | 2011
Yongchang Zhang; Zhengxi Li; Jiefeng Hu; Wei Xu; Jianguo Zhu
This paper proposes a direct power control (DPC) strategy for cascaded brushless doubly fed induction generator (CBDFIG), which eliminates the slip rings and brushes in conventional DFIG wind turbine systems and features quick dynamic response and excellent steady state performance. Prior methods for controlling CBDFIG are based on vector control (VC), which is complicated and requires much tuning work and machine parameters. Appropriate decoupling and fine PI tuning are mandatory to obtain good performance over the entire operating range. Furthermore, there are few papers regarding the grid synchronization issue of CBDFIG. This paper firstly analyzes the influence of each voltage vector on active/reactive powers and then proposes a unified switching table for both grid-connected operation and grid synchronization process. The effectiveness of the developed DPC method for CBDFIG is confirmed by the presented simulation results.
international conference on electrical machines and systems | 2011
Yongchang Zhang; Zhengxi Li; Tianshi Wang; Wei Xu; Jianguo Zhu
Large torque ripple and variable switching frequency are the two most notable drawbacks of conventional switching table based direct torque control (DTC) for doubly fed induction generator (DFIG). By using one active vector and one null vector during one control cycle, the torque ripple can be significantly reduced while achieving almost constant switching frequency. This paper propose a very simple but effective method to obtain the duty ratio of the active vector, which is able to reduce the complexity and improve the system robustness while reducing both torque and flux ripples. Furthermore, this paper points that by appropriately arranging the sequence of active vector and null vector, the switching frequency can be further reduced and the performance is only slightly affected. This fact is useful for high power wind energy applications with restricted switching frequency. The developed method is compared with one of the prior analytical methods based on torque ripple RMS minimization and exhibits lower rotor flux ripple and better harmonic performance of stator and rotor currents. The presented simulation results obtained from a 15 kW DFIG validates its effectiveness.