Robust Predictive Current Control for Grid Synchronization and Power Generation of Cascaded Brushless Doubly-Fed Generators Based on Extended State Observer
Yang Changshan, Zhang Yongchang, Jiang Tao
School of Electrical and Electronic Engineering North China Electric Power University Beijing 102206 China
The cascaded brushless doubly-fed generator (CBDFG) is composed of two coaxial wound rotor induction motors, whose rotor windings are directly connected to eliminate the need for brushes and slip rings. Therefore, it has the advantages of long service life, high reliability and low maintenance cost. CBDFG can be used for both independent generation and grid-connected generation, and has broad prospects in wind power generation and hydroelectric power generation. At present, vector control and deadbeat control are commonly used to control the CBDFG. Because the above two methods both use a large number of motor parameters, when the machine parameters used in the controllers are different from the actual values due to temperature, saturation and other reasons, the control effects have varying degrees of deterioration.
To overcome this problem, this paper proposes an extended state observer (ESO) based robust predictive current control (RPCC) method for CBDFG.
Firstly, the proposed RPCC can be applied to both grid synchronization and power generation process. The only difference between the two processes is the calculation of the reference value of the control winding current. In the process of grid synchronization, because the power side does not emit power, the virtual flux is needed to calculate the reference value of the control winding current. During power generation process, the reference value of control winding current is calculated directly according to the power reference value.
Secondly, the proposed RPCC is based on ultra-local model and uses an ESO to rapidly estimate the total disturbance caused by model uncertainty. The ESO can extend the total disturbance to a new disturbance and feed this total disturbance back to the controlled system. Based on the state space equation of ESO, the closed loop transfer function of the ESO for CBDFG is obtained. According to the closed loop transfer function obtained, the two parameters of ESO are simplified to one. Then, the control block diagram of the proposed RPCC in z domain is given, and the parameters of ESO are designed according to the characteristic equation of the proposed RPCC. In addition, the parameter stability of the proposed ESO is analyzed.
Finally, according to the designed ESO parameter and the obtained disturbance value, one-step delay compensation is carried out for the current value of the control winding, and deadbeat control is further incorporated to achieve fast and accurate control of the control winding current.
Compared with the traditional field-oriented control and deadbeat control, the experimental results show that this method not only has good steady-state and dynamic waveforms when the motor parameters are accurate, but also maintains a good control effect when the CBDFG parameters change. It shows that this method has good parameter robustness. In order to reduce the switching loss in actual wind power system, the switching frequency is usually set at several kHz. The simulation results of the proposed RPCC at low switching frequency show that although the decrease of switching frequency will increase the current THD, it is still far less than 5%, which meets the requirements of grid-connection.
杨长山, 张永昌, 蒋涛. 基于扩张状态观测器的级联无刷双馈电机并网同步和发电鲁棒预测电流控制[J]. 电工技术学报, 0, (): 8935-.
Yang Changshan, Zhang Yongchang, Jiang Tao. Robust Predictive Current Control for Grid Synchronization and Power Generation of Cascaded Brushless Doubly-Fed Generators Based on Extended State Observer. Transactions of China Electrotechnical Society, 0, (): 8935-.
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