级联型电力电子变压器并联运行的改进下垂控制策略

Improved Droop Control Strategy for Parallel Operation of Cascaded Power Electronic Transformers

为了提升级联型电力电子变压器并联运行时的稳态和动态性能,在传统下垂控制的基础上提出了一种改进下垂控制策略。首先,建立传统下垂控制的并联系统等效电路,研究其存在电压控制偏差较大、均流控制精度不高且动态特性较差等问题的原因;然后,通过闭环控制实现对输出电压控制偏差的补偿,推导负载电流与移相角的函数后对其泰勒级数展开式进行线性化处理,并将处理结果作为电流前馈的系数,提升稳态时电压控制的精准度、电流控制的均衡度以及动态时的快速调节能力;最后利用公共直流母线电压取代各自采样的输出电压作为控制反馈值,降低了线路阻抗对电流均衡度的影响。通过一套3台电力电子变压器组成的并联系统进行验证,实验结果表明:该改进下垂控制策略可将并联系统的功率离散度降低至0.18%,直流电压控制偏差降低至0.2 V,验证了工程应用的有效性、可行性和实用性。

In order to improve the static and dynamic performance of cascaded power electronic transformers(PETs) in parallel operation, an improved droop control strategy is proposed on the basis of the traditional droop control. Firstly, the equivalent circuit of the traditional droop control for parallel system is established, and the reasons for the problems such as large voltage control deviation, low current sharing control accuracy and poor dynamic characteristics are studied. Then, the closed-loop control realizes the compensation of the output voltage control deviation, and the Taylor-expansion of the function of the load current and the phase shift angle is derived and linearized to use as the current feedforward coefficient, thus the accuracy of voltage control and the balance of current control in static performance, as well as the rapid adjustment ability in dynamic performance, can be improved. Finally, the common DC bus voltage is used to replace the output voltage of each PET as the control feedback, which reduces the influence of the line impedance on the current sharing. The experimental results of a set of 3 PETs in a parallel system show that the improved droop control strategy can reduce the power dispersion of the parallel system to 0.18% and the DC voltage control deviation to 0.2 V, which verifies the effectiveness, feasibility and practicability of the engineering application.