Inverter without AC voltage sensor control strategy under voltage imbalance

Researchers Hu Shuju, Meng Yanfeng, Li Fenglin, Song Bin, and Deng Ya from the Institute of Electrical Engineering, Chinese Academy of Sciences, in the 24th issue of Journal of Electrical Engineering, in the study of a non-ideal grid voltage condition, to study a voltage imbalance in the grid. The grid-connected inverter has no AC voltage sensor control strategy.

Firstly, based on the second-order generalized integrator, the orthogonal filter and the three-phase grid-connected inverter voltage observer suitable for the grid voltage unbalance condition are constructed, and the grid voltage is observed in the two-phase stationary coordinate system; then based on the orthogonal filter The output is positively and negatively sequenced, so that the positive and negative sequence separations are synchronized with the grid voltage observation. Finally, the inverter is combined with the PR control in the two-phase stationary coordinate system, and the negative sequence compensation algorithm is used to realize the inverter. No AC voltage sensor control under grid voltage imbalance conditions.

This strategy can avoid the problems of integral saturation, initial value sensitivity and static error in traditional virtual flux linkage observation, and overcome the problem that the existing orthogonal filter-based inverter without AC voltage sensor can not adapt to the grid voltage imbalance. . The effectiveness of the proposed strategy is verified by simulation and experiment.

When a renewable energy power generation system is connected to a power distribution network such as an inverter or a distribution network with a weak grid structure, some non-ideal conditions that may exist in the grid voltage may adversely affect the stable operation of the grid-connected system. [1]. In view of the grid-connected inverters, in recent years, some researchers at home and abroad have begun to study the AC-free voltage sensor control that does not rely on the grid voltage signal to improve the adaptability of the inverter under non-ideal grid conditions [2, 3].

Similar to voltage sensor control, in a voltage-free sensor control algorithm, the reconstructed voltage/virtual flux linkage signal can be explicit and vector controlled; it can also be implicit and direct power control. The existing grid voltage/virtual flux linkage reconstruction methods can be roughly divided into two categories: one is the grid voltage/virtual flux reconstruction method based on complex power estimation, which belongs to the open loop estimation method, and the accuracy is not high, and The current differential term is easy to cause interference; the second is the grid voltage/virtual flux reconstruction method based on the grid side current deviation adjustment, which belongs to the closed loop estimation method and has high accuracy.

In order to reduce interference and improve the accuracy of observation, it is generally necessary to use a low-pass filter, but the low-pass filter itself has problems such as zero drift, integral saturation, steady-state error, and initial value sensitivity [4, 5]; When the voltage is unbalanced, the cascade algorithm of flux linkage observation and positive and negative sequence separation makes the control structure more complicated, increases the delay time and reduces the dynamic response speed of the system [6].

In [7], an inverter-free AC voltage sensor control method based on quadrature filter is proposed, which can realize the AC-free voltage sensor control of the inverter under the condition of grid voltage balance. Based on the virtual synchronous machine technology, the literature [8,9] introduces a virtual current to synchronize the voltage control signal with the grid voltage, which can well track the amplitude, frequency and phase of the fundamental component, but not for the non-ideal grid voltage. Conditions are specifically studied.

In [10], based on the instantaneous power theory and virtual flux linkage technology, the literature [10] proposes two improved AC voltage-free sensors that adapt traditional direct power control to grid disturbances and operate well. The situation requires further experimental verification. Therefore, for the inverter-free voltage sensor control of grid voltage imbalance conditions, both the construction of the filter and the control strategy of the inverter need further research, verification and improvement.

Aiming at the above problems, this paper studies the inverter-free AC voltage sensor control strategy under the condition of grid voltage imbalance. Based on the second-order generalized integrator, a variable frequency orthogonal filter is constructed to further establish a voltage observer. The method of separating the positive and negative sequence components based on the orthogonal filter makes the voltage observation and voltage and current positive and negative sequence separation can be completed simultaneously, and the negative sequence compensation current control realizes the inverter without AC voltage sensor control, thereby simplifying The control structure of the system and get better dynamic performance. Finally, the simulation and experimental verification results and conclusions are given.

Figure 1 Structure diagram of variable frequency orthogonal filter

Figure 2 Experimental system structure

Figure 3 experimental system device physical map

in conclusion

In this paper, we propose to establish a variable frequency orthogonal filter based on the second-order generalized integrator, and further construct a voltage observer that adapts to the grid voltage imbalance condition, and perform positive and negative sequence separation based on the output of the orthogonal filter to make positive and negative The sequence separation is synchronized with the grid voltage observation, combined with the negative sequence current compensation control, to achieve the inverter-free AC voltage sensor control under the grid voltage imbalance condition.

The effectiveness of the proposed control strategy is verified by simulation and experiment. The proposed AC-free voltage sensor control strategy can adapt to the grid voltage imbalance condition and reduce the inverter hardware cost, which can effectively improve the grid adaptability of the grid-connected inverter. The next step will be to further improve the method proposed in the current inverter without voltage sensor control, and compare it with other methods.