F Fei Wang

Pliant Active and Reactive Power Control for Grid-Interactive Converters Under Unbalanced Voltage Dips

During voltage dips continuous power delivery from distributed generation systems to the grid is desirable for the purpose of grid support. In order to facilitate the control of inverter-based distributed power generation adapted to the expected change of grid requirements, generalized power control strategies based on symmetric-sequence components are proposed in this paper, aiming to manipulate the delivered instantaneous power under unbalanced voltage dips. It is shown that active and reactive power can be independently controlled with two individually adaptable parameters. By changing these parameters, the relative amplitudes of oscillating power can be smoothly regulated, as well as the peak values of three-phase grid currents. As a result, the power control of grid-interactive inverters becomes quite flexible and adaptable to various grid requirements or design constraints. Furthermore, two strategies for simultaneous active and reactive power control are proposed that preserve flexible controllability; an application example is given to illustrate the simplicity and adaptability of the proposed strategies for online optimization control. Finally, experimental results are provided that verify the proposed power control.

Modeling and Analysis of Grid Harmonic Distortion Impact of Aggregated DG Inverters

This paper proposes an impedance-based analytical method for modeling and analysis of harmonic interactions between the grid and aggregated distributed generation (DG) inverters. The root cause of harmonic interaction/resonance problems is the impedance-network quasi-resonance between the effective output impedance of the inverter and the equivalent grid impedance at the connection point. Starting with the output impedance modeling of an inverter, a Norton model of the inverter is derived. Comparing with the switching model and the average model of the inverter, simulation results show the effectiveness of the model. This paper proposes that impedance limits should be specified and used as an extra design constraint for DG inverters in order to minimize the harmonic distortion impact on the grid. Assuming the impedance models of individual inverters and local loads within a distribution grid are known, especially in the case of new grids under construction, harmonic interactions between the grid and a certain number of DG inverters can be preliminarily estimated.

Control of grid-interfacing inverters with integrated voltage unbalance correction

This paper presents the control of a grid-interfacing inverter with integrated voltage unbalance correction. It is proposed to add an additional function to the inverter to decrease the negative-sequence voltage at the point of connection with the utility grid. Based on a symmetric sequence voltage decomposition and using an improved multi-variable filter, the grid-interfacing inverter intentionally absorbs a small amount of negative-sequence current from the grid, thereby helping to correct the negative-sequence voltage. Although the amplitude reduction contributed by each individual inverter system is small compared to the total negative-sequence component, grid-interfacing inverter modules can collectively achieve substantial results in the grid. The integrated function and proposed control has been verified in simulations and by experiments on a laboratory prototype.

High Performance Stationary Frame Filters for Symmetrical Sequences or Harmonics Separation Under a Variety of Grid Conditions

This paper proposes a group of high performance filters for fundamental positive / negative sequences and harmonics detection under varied grid conditions based on a basic filter cell. The filter cell is demonstrated to be equivalent to a band-pass filter in the stationary frame, and is easily implemented using a multi-state-variable structure. To achieve high performance in different grid conditions, cascaded filters are developed for distorted and unbalanced grids. This paper also analyzes the robustness of the filter for small frequency variations, and improves its frequency-adaptive ability for large frequency changes. Furthermore, it is proved that this filter can also be applied for the synchronization in a single-phase system. Considering the digital implementation of the filter, four discretization methods and the resulting limitations are investigated. The effectiveness of the presented filters is verified by experiments.

Active and reactive power control schemes for distributed generation systems under voltage dips

During voltage dips continuous power delivery from distributed generation systems to the grid is desirable for the purpose of grid support. In order to facilitate the control of distributed generation systems adapted to the expected change of grid requirements, generalized power control schemes based on symmetric-sequence components are proposed in this paper for inverter-based distributed generation, aiming at manipulating the delivered instantaneous power under voltage dips. It is shown that active power and reactive power can be independently controlled with two individually adaptable parameters. By changing these parameters, the relative amplitudes of oscillating power can be smoothly regulated, as well as the peak values of three-phase grid currents. As a result, the power control of grid-side inverters becomes quite flexible. Furthermore, two strategies for simultaneous active and reactive power control are proposed that preserves adaptive controllability. Finally, the proposed schemes are verified experimentally.

Sequence-Decoupled Resonant Controller for Three-phase Grid-connected Inverters

This paper proposes a set of sequence-decoupled resonant controllers constructed with an easy-to-implement structure. It shows that either positive-sequence or negative-sequence components of three-phase unbalanced AC signals can be regulated individually in the stationary frame, while conventional and widely used resonant controllers cannot separately cope with these two sequences. Consequently, the extraction of the corresponding sequence component is not necessary for an individual sequences control. Moreover, controllers designed for positive and negative sequences can be assigned with different gains to meet practical applications. The principle of new sequence-decoupled resonant controllers is described based on a multi-state-variable structure. In addition, the relation with conventional resonant controllers is investigated. The controllers are found to be equivalent when dealing with both sequences together. Furthermore, two examples are presented to show the application of the proposed controller. The effectiveness of the proposed new controller and its performance in the two system applications are verified by experiments.