Control Of Three-phase Grid Connected Converter Under Typical Power Grid Conditions

With the sustained development of modern power system, the grid structure has become complicated. Accordingly, power quality problems such as grid frequency mutation, grid voltage fluctuations and flicker occur from time to time, and the distorted grid voltage is more common than before. The three-phase grid connected converters may operate under such complicated power grid conditions. In this case, the grid voltage may be ideal, non-ideal or even voltage-flicker-transient. The conventional control strategy of three-phase grid connected converters could not achieve unit power factor, sinusoidal AC current and stable DC bus under the non-ideal grid voltage conditions such as distorted or unbalanced grid voltage conditions. What’s more, the three-phase grid connected converters may be unstable when grid voltage dips. This paper mainly focuses on simplifying the control structure and parameters of the control strategy for three-phase grid connected converter under typical power grid conditions, improving the system ability of disturbance resistance, speeding up the transient response of the system during grid voltage dips and constraining the system states.In terms of the slow transient response of the conventional grid voltage synchronization, the low-order Kalman filters and the low-order FIR filters are used to realize the accurate and fast extraction of the fundamental positive and negative sequence components of the grid voltage in this paper. The experimental results show that the transient-response-speed of the proposed voltage synchronization method is much faster than the conventional method.The under-actuated characteristic, irreversible decoupling matrix and effect of non-ideal grid voltage of the typical three-phase grid connected converters are discussed in this paper. Based on the theory of multi-input and multi-output feedback linearization, this paper solutes the feedback law by overcoming the difficulty such as the under-actuated characteristic or irreversible decoupling matrix. According to farther research, the influence of the non-ideal grid voltage on the control of three-phase grid connected converters is reflected in two aspects of the model disturbance and the reference distortion. The model disturbance has mostly been concentrated in their instantaneous power models or high relative-degree three-phase grid-connected-converter models, in which there are the derivative of the grid voltage. Because of the difficulty of detecting the components of the non-ideal grid voltage, the derivative of the grid voltage is usually the uncertainty in the mathematical models. The reference distortion takes the form of distorted currents reference generated by the traditional reference strategy due to the non-ideal grid voltage. Through the analysis of power flow between the power grid and the three- phase grid connected converter, this paper puts forward an accurate reference signal calculation based on harmonic power injection ideal. The accurate reference signal calculation could produce undistorted grid side current reference through production or consumption specific harmonic power by three-phase grid connected converters. The accurate reference signal calculation makes restraining the grid side current harmonics from the control problem to a reference problem. This makes the design of the control system without the need to consider the problem of harmonic suppression under non-ideal grid voltage. Thus, it greatly simplifies the structure of the control system, and makes the application of tracking control strategy with excellent performance easily.Based on the theory of multi-input and multi-output feedback linearization and the proposed accurate reference signal calculation, this paper proposes a general design method for the control of three-phase grid connected converter under typical power grid conditions. Through the general design method, we can obtain the general feedback linear model of the three-phase grid connected converter system, and solve the problem of zero dynamic instability in three-phase grid connected converter. This makes the tracking controller and its parameter design can be applicable for a class of three-phase grid connected converters, so that the applicability of the tracking controller is improved. Based on the generalized linear model of the general design method, an improved tracking control is proposed by using the extended state observer. This method includes two forms of linear active disturbance rejection power control and improved internal model control. They can improve the tracking effect of the system and the system’s ability to resist disturbance. Linear active disturbance rejection power control is a linear active disturbance rejection PI control based on direct power control for three-phase grid connected converter. It has the advantages of simple structure and applicability to high relative-degree three-phase grid connected converter. Linear active disturbance rejection power control could make the three-phase grid connected converters operating nearly unity power factor under the ideal grid volt ages and low AC harmonics under the non-ideal voltages. The improved internal model control improves the disturbance resistance of internal model control by means of the observer for the disturbance. This method can realize the error free tracking of the periodic signal, and suppress the grid side current harmonics of the three-phase grid converter more effectively than linear active disturbance rejection power control under the non-ideal grid voltages. Further, because one relative-degree three-phase grid connected converters need only feedback of the output of the system, this paper designs a reduced order extended state observer for improved internal mode control, which reduces the difficulty of tracking controller.In this paper, a constrained adaptive robust control with integral feedback for for one relative-degree three-phase grid connected converters is proposed.The constrained adaptive robust control can effectively constrain the system state and keep grid side current from exceeding limits. Combined with direct power control and the an accurate reference signal calculation, the constrained adaptive robust control can reduce the THD of three-phase grid connected converter grid side current to 5% or less, also could make three-phase grid connected converter operate near unity power factor under the ideal grid voltages.Theoretical analysis and experiments show that the general design method proposed in this paper can ensure the zero dynamic stability of the three-phase grid connected converter system and simplify the design of the controller and its parameters. Although the proposed tracking control methods based on general design method focus on different performance such as the design complexity, tracking performance and state constraint, all of them have the ability of disturbance resistance, and can reduce the THD of grid side currents of three-phase grid connected converter to 5% or less, to ensure the three-phase grid connected converter operating near unity power factor under the ideal grid voltage. Comprehensive experiments show that combined with the proposed grid voltage synchronization method, the three-phase grid connected converter control system would have a fast response of voltage-flicker transient, so as to shorten the transition process and reduce the uncertainty of voltage flicker, which enhances fault ride-through capability of the system, and improves the stability of the system.