Grid-connected Voltage Source Converters Research Articles

Analysis of Harmonic Oscillations Caused by Grid-Connected VSCs

In this paper, a method of harmonic modal analysis to examine the harmonic oscillations caused by grid-connected voltage source converters (VSCs) is proposed. The linearized harmonic model of grid-connected VSCs is derived. Based on the model, analysis was conducted, which indicated that the damping of harmonic oscillation modes is mainly determined by parameter settings of the VSCs' control systems. Interactions between the VSCs' control systems affect the damping and frequency of the harmonic oscillation modes. High-order harmonic interactions are superimposed on the low-frequency variations of variables associated with the VSCs' control systems and consequently pass through the output filters of the VSCs. Subsequently, strong harmonic interactions between the VSCs occur over the AC network. In addition, harmonic resonance occurs when one of the harmonic oscillation modes of one VSC is closed to another harmonic oscillation mode of the other VSCs on the complex plane. When harmonic resonance occurs, damping of one resonant harmonic oscillation mode degrades, and growing harmonic oscillations may occur. Hence, the parameters of the VSCs' control systems must be carefully tuned to avoid harmonic resonance. An example power system with grid-connected VSCs is presented to demonstrate and evaluate the correctness of the harmonic modal analysis.

Asymmetric Complex-Vector Models With Application to VSC–Grid Interaction

The properties of complex space-vector models for asymmetric three-phase systems are investigated in this article. Most importantly, three alternative methods for the stability analysis of the asymmetric closed-loop systems are presented. The end results avoid the usage of matrix manipulations. It is shown how the theory can be applied to modeling and stability analysis of a grid-connected voltage-source converter (VSC). The methods are compared using numerical examples.

Design of Multifrequency Proportional–Resonant Current Controllers for Voltage-Source Converters

In recent years, the applicability of multifrequency proportional-resonant (PR) current controllers has been extensively explored. The analysis and controller design are typically performed by means of frequency response methods. In this context, the open-loop system's gain crossover frequency and the associated phase margin are most commonly used as design parameters. However, those parameters are not well defined in applications with multifrequency PR controllers, where multiple 0-dB crossings of the open-loop gain emerge. In contrast, it is shown that the system's gain margin can mostly be utilized as a reliable stability measure. Furthermore, since each resonator introduces an unwanted phase lag to its higher neighbored resonator, the desired stability margin might not be achieved by “conventional” controller designs or the system could even be destabilized. Hence, this article reviews common analytic design guidelines for PR current controllers and extends them to controllers that implement multiple resonators in either a stationary frame or a synchronous reference frame. To achieve a good compromise between the stability margin and a small steady-state control error, a recursive adjustment procedure for the controller's integral gains is proposed. The “plug-and-play” controller design is applied to a grid-connected voltage-source converter and the theoretical findings are validated by computer simulations and experiments.

Improving the transient performance of DFIG wind turbine using pitch angle controller low pass filter timing and network side connected damper circuitry

In wind turbine systems, pitch angle controllers are employed to reduce the aerodynamic power gained during high wind speeds. On the other hand, passive filters are usually used to mitigate disturbances in grid-connected voltage source converters (VSCs). To avoid the risk of instability in the grid-connected VSC, as a result of resonance in the capacitive and inductive components, it is necessary to consider damping. In this study, the low pass filter timing constant of the pitch angle controller of the doubly-fed induction generator (DFIG) wind turbine was varied considering different time constants. The best timing response of the low pass filter was further used to analyse different filter topologies, in addition to a new control strategy that uses two trap passive filters with shunt resistor–capacitor as an active damper, in augmenting the DFIG wind turbine during grid fault. Simulation studies in PSCAD/EMTDC were carried out to compare the performance of the proposed scheme with some other conventional filter solutions for the DFIG wind generator, during a severe bolted three-phase to ground fault. The simulation results demonstrate the improved performance and faster recovery of the wind generator variables after the fault considering the proposed filter scheme.

Current Compensation in Grid-Connected VSCs using Advanced Fuzzy Logic-based Fluffy-Built SVPWM Switching

A main focus in microgrids is the power quality issue. The used renewable sources fluctuate and this fluctuation has to be suppressed by designing a control variable to nullify the circulating current caused by voltage fluctuations and deviations. The switching losses across power electronic switches, harmonics, and circulating current are the issues that we discuss in this article. The proposed intelligent controller is an interface between a voltage-sourced converter and a utility grid that affords default switching patterns with less switching loss, less current harmonic content, and overcurrent protection, and is capable of handling the nonlinearities and uncertainties in the grid system. The interfaced controller needs to be synchronized to a utility grid to ensure that the grid–lattice network can be fine-tuned in order to inject/absorb the prominent complex reactive energy to/from the utility grid so as to maintain the variable power factor at unity, which, in turn, will improve the system’s overall efficiency for all connected nonlinear loads. The intelligent controller for stabilizing a smart grid is developed by implementing a fuzzy-built advance control configuration to achieve a faster dynamic response and a more suitable direct current link performance. The innovation in this study is the design of fuzzy-based space vector pulse width modulation controller that exploits the hysteresis current control and current compensation in a grid-connected voltage source converter. By using the proposed scheme, a current compensation strategy is proposed along with an advanced modulation controller to utilize the DC link voltage of a voltage source converter. To demonstrate the effectiveness of the proposed control scheme, offline digital time-domain simulations were carried out in MATLAB/Simulink, and the simulated results were verified using the experimental setup to prove the effectiveness, authenticity, and accuracy of the proposed method.

Unified Modular State-Space Modeling of Grid-Connected Voltage-Source Converters

This article proposes a modular state-space modeling framework for grid-connected voltage-source converters, where the different control loops, including the ac current control, the phase-locked loop, the dc-link voltage control, and the ac voltage magnitude control, can be modeled separately as building blocks. Moreover, the mathematical relationship between state-space models in the rotating ( dq -) frame and the stationary ( αβ -) frame is explicitly established, and, thus, the modal analysis can be performed directly in the αβ -frame, which allows intuitive interpretation of voltage and current oscillation modes in the αβ -frame. Experimental tests of a 3-kW back-to-back converter system validate the effectiveness of the unified modular state-space modeling and analysis.

Current Control of Single-Phase VSC Systems With Inductor Saturation Using Inverse Dynamic Model-Based Compensation

In voltage-source converter (VSC) systems, utilization of filter inductors with deep saturation characteristics is often advantageous due to the improved size, cost, and efficiency. However, with the use of conventional current regulation (CCR) methods, the inductor saturation results in significant dynamic performance loss and poor steady-state current waveform quality. This paper proposes the inverse dynamic model (IDM)-based compensation method to overcome these performance issues. The method converts the nonlinear inductor of the plant to a fictitious linear inductor such that linear current regulators can perform satisfactorily. In conjunction, identification of L–R parameters is performed to obtain IDM of the system within an acceptable range of sensitivity yielding satisfactory performance. The performance issues of CCR methods with saturable inductor are demonstrated by simulations and experiments. Then, the proposed method is experimentally verified on a single-phase 1.5-kW grid-connected VSC. The performance improvement over the CCR methods is verified by a detailed comparison of experimental waveforms.

A Robust Power Regulation Controller to Enhance Dynamic Performance of Voltage Source Converters

This paper presents a novel power regulation controller for grid-connected voltage source converter (VSC). The proposed controller is usually applied to achieve smooth active and reactive power dynamics against the variation of parameters. The dynamic performance of grid-connected VSC is sensitive to parameter variation, threatening the safe operation especially when the relative stability is low. Moreover, there is no possibility to modify the dynamic response without the loss of power regulation precision or output voltage and frequency accuracy. In this paper, an improvement in the dynamic response is achieved by introducing feedforward branches into the VSC power regulation controller. Owing to the model reduction mechanism, the VSC provides smooth dynamic response and robustness against the variation of parameters. Simulation and experiment results are presented to prove the effectiveness of this approach, which shows excellent performance as opposed to the conventional controller paradigm.

Modeling and Analyzing the Effect of Frequency Variation on Weak Grid-Connected VSC System Stability in DC Voltage Control Timescale

The effect of frequency variation on system stability becomes crucial when a voltage source converter (VSC) is connected to a weak grid. However, previous studies lack enough mechanism cognitions of this effect, especially on the stability issues in DC voltage control (DVC) timescale (around 100 ms). Hence, this paper presented a thorough analysis of the effect mechanism of frequency variation on the weak grid-connected VSC system stability in a DVC timescale. Firstly, based on instantaneous power theory, a novel method in which the active/reactive powers are calculated with the time-varying frequency of voltage vectors was proposed. This method could intuitively reflect the effect of frequency variation on the active/reactive powers and could also help reduce the system order to a certain extent. Then, a small-signal model was established based on the motion equation concept, to depict the effect of frequency variation on the weak grid-connected VSC system dynamics. Furthermore, an analytical method was utilized to quantify the effect of frequency variation on the system’s small-signal stability. The quantitative analysis considered the interactions between the DC voltage control, the terminal voltage control, phase-locked loop, and the power network. Finally, case studies were conducted, and simulation results supported the analytical analyses.

Modeling and Analysis of DC Terminal Impedance of Voltage-Source Converters With Different Control Modes

A dc terminal impedance model of voltage-source converter (VSC) is required to analyze the stability of multiterminal high-voltage direct current system. This article proposes a comprehensive dc terminal impedance model for conventional three-phase two-level VSC under different control modes. The control strategy includes inner decoupled current control and outer control, the latter contains dc voltage control, active/reactive power control, and droop control for grid-connected VSC and ac voltage control for VSC connected to a load. When a VSC is connected to the ac grid, the phase-locked loop (PLL) is also taken into account. The influences of PLL dynamics, control time delay and dead time on the dc-side impedance are explained. Also, how the parameters of the inner current control and outer control affect the characteristics of dc terminal impedance is analyzed in detail. Finally, experimental measurements verify the proposed impedance models and analysis.

Evaluation of supraharmonic emission levels of multiple grid‐connected VSCs

Supraharmonic is an inevitable power quality issue, with the rapid development of power electronic technologies, receiving more and more attention. Firstly, this study analyses the supraharmonic emission mechanism of voltage source converter (VSC) based on sinusoidal pulse-width modulation. The mathematical analysis shows that the supraharmonics emitted by grid-connected VSC mainly distribute near the integer multiple of its switching frequency, and grid-connected VSCs are essentially supraharmonic voltage sources. Then, this study proposes the supraharmonic equivalent circuit model of multiple VSCs grid-connected system, by modelling grid-connected VSCs as supraharmonic voltage sources and connecting them in parallel with the LCL filters. Based on the proposed model, considering the primary and secondary emissions, a method is proposed to evaluate the supraharmonic current emission of one VSC and the total supraharmonic current emission flowing into the grid. Finally, simulation and field measurements show that the proposed method can effectively evaluate the supraharmonic emission levels of multiple VSCs grid-connected systems.

A Novel Orthogonal Current Decomposition Control for Grid-Connected Voltage Source Converter

Grid-connected voltage source converter is used to control the flow of active power from renewable energy sources with several functionalities, such as harmonic mitigation, reactive power compensation, and power factor improvement. The reference current calculation and its accuracy decide the performance of the control algorithms during harmonic disturbances. Fundamental signal extraction techniques are utilized to overcome the disturbances present in the system voltage and current. This paper presents a novel orthogonal current decomposition control for load current decomposition into active and reactive components. These current components are obtained through character of orthogonal vector in the inner product space. The block diagram of the proposed technique is implemented with the help of adder, multiplier, and windowed integrator. Phase voltage and load current for each phase are used as inputs to this functional diagram for getting desired fundamental active current signal. Subsequently, adjustable dc-link voltage control is also presented to diminish the switching losses in the converter and to avoid the false tripping of the grid-connected system during susceptible grid conditions. The proposed control technique is simulated in MATLAB/Simulink and verified through developed hardware prototype using ARM Cortex M4-based microcontroller (STM32F407VGT6) using Waijung environment.

Transient Stability of Voltage-Source Converters With Grid-Forming Control: A Design-Oriented Study

Driven by the large-scale integration of distributed power resources, grid-connected voltage-source converters (VSCs) are increasingly required to operate as grid-forming units to regulate the system voltage/frequency and emulate the inertia. While various grid-forming control schemes have been reported, their transient behaviors under large-signal disturbances are still not fully explored. This article addresses this issue by presenting a design-oriented transient stability analysis of the grid-forming VSCs. First, four typical grid-forming control schemes, namely, the power-synchronization control (PSC), the basic droop control, the droop control with low-pass filters (LPFs), and the virtual synchronous generator (VSG) control, are systematically reviewed, whose dynamics are characterized by a general large-signal model. Based on this model, a comparative analysis on the transient stabilities of different control schemes is then carried out. It reveals that the PSC and the basic droop control can retain a stable operation as long as there are equilibrium points, due to their noninertial transient responses, while the droop control with LPFs and the VSG control can be destabilized even if the equilibrium points exist, due to the lack of damping on their inertial transient responses. With the phase portrait, the underlying stability mechanism is explicitly elaborated, and the quantitative impacts of the controller gains and the virtual inertia are clearly identified. Subsequently, controller design guidelines are proposed to enhance the system damping as well as the transient stability. Finally, experimental results are provided to verify the theoretical analysis.

Combined Control of Grid Connected Converters Based on a Flexible Switching Table for Fast Dynamic and Reduced Harmonics

Grid connected voltage source converters are inevitable means now for expanding and supporting power systems. Nevertheless, their harmonic injection and high frequency switching may cause degradation of power quality and detraction of the equipment reliability. Therefore, the converters injected harmonics must observe the corresponding standards, while lower switching frequencies and rapid transient responses are desirable. In this paper, a new combined control method with a flexible switching table is presented for the converters to enhance the power quality and transient response. The method employs a virtual flux in connection with a combined control for the first time. In comparison with the conventional virtual flux direct power control, the proposed method provides better power quality, reduced THD and faster transient response, in addition to less grid parameters dependency. Compared to the conventional combined control method, it determines more accurately the converter voltage vectors, while avoiding grid voltage measurement and cumbersome controller tuning.

Symmetric Admittance Modeling for Stability Analysis of Grid-Connected Converters

This paper proposes a novel admittance model for the stability analysis of grid-connected voltage-source converters (VSCs) which features a symmetric structure. On this basis, the grid-connected VSC system can be represented by a reciprocal two-port circuit to explore the oscillation mechanism of the system. Furthermore, the grid-connected VSC system can be modelled as a single-input single-output (SISO) system rather than a multi-input multi-output (MIMO) system. We analytically evaluate the influences of the VSC control parameters and the power factor on the stability of the grid-connected VSC by employing the Nyquist criterion and the stability margin. In addition, we demonstrate that the VSC's current output is in inverse proportion to the system's gain margin. Further, an analytical expression of the current limit constrained by small-signal stability is derived in closed form so as to calculate the admissible current output. All of analysis results are verified by simulations based on Simulink and hardware-in-the-loop (HIL) platform.

Frequency coupling characteristics of electromagnetic oscillation in grid‐connected converters

Under certain conditions, grid-connected voltage source converter (VSC) may result in electromagnetic oscillations. Many coupling components with different frequencies could be observed in converter outputs when oscillation happens. The characteristics and mechanism of frequency coupling phenomenon in oscillations of VSC are studied. The magnitude relationship between first-order components in the three-phase domain is drawn by impedance model. The two first-order components will result in higher-order components due to the non-linearity of VSC controller. There are mainly three coupling paths in a controller: phase-locked loop, DC voltage fluctuation and overmodulation. The coupling mechanism and frequency distribution characteristics are investigated in this study. Theoretical results are validated by simulations with PSCAD/EMTDC.

Analysis and Damping Control of Small-Signal Oscillations for VSC Connected to Weak AC Grid During LVRT

The instability issues of grid-connected voltage source converters (VSC) may easily occur during low voltage ride-through (LVRT), especially when connected to a weak ac grid. In this study, a small-signal model of the grid-connected VSC system was developed to deal with the stability problems during deep voltage sags. Based on the model, the interaction between the phase-locked loop (PLL) and current control has been illustrated. In addition, the eigenvalue and modal analysis method was employed to investigate the influencing factors of the VSC system stability, which include the bandwidths of the PLL and current control loop, grid strength, and voltage sags. Furthermore, on the basis of the interaction between PLL and current control loop, a novel additional damping controller that is placed in the active current control loop was proposed and designed. Finally, experiments were conducted to verify the theoretical analysis and proposed control strategy for enhancing VSC system stability during LVRT with a high impedance grid connection.

Admittance study of grid‐connected VSCs for harmonic oscillatory instabilities

Voltage source converters (VSCs) can reduce damping of grid-connected VSC systems at harmonic resonances, leading to system instability. The conductance of the VSC admittance can be shaped selecting appropriate VSC parameter values to increase VSC damping at poorly damped harmonic resonances and ensure system stability. This study extends existing findings and makes contributions in this direction. Analytical expressions of the positive- and negative-sequence VSC conductances are presented to study exhaustively the impact of VSC parameters on harmonic oscillatory instabilities. Two applications, i.e. a traction system and an offshore wind power plant, are used to show the contribution of this study. The analytical study is validated from PSCAD/EMTDC time-domain simulations.

Impedance-Based Prediction of Distortions Generated by Resonance in Grid-Connected Converters

Small-signal impedance-based analysis can effectively predict the frequency and damping of resonance modes in power electronic systems. However, it cannot predict the magnitude of resonance or explain resonance-generated distortions in the absence of an external disturbance at the resonance frequency. This paper presents an impedance-based theory for the prediction of the magnitude of resonance or resonance-generated distortions in grid-connected converters. It is discovered that the impedance response of a converter starts changing with the magnitude of resonance at its terminals. The changing converter impedance response may stabilize an unstable growing resonance mode beyond a certain magnitude, at which point the converter enters a limit cycle mode of sustained oscillations. The proposed theory uses large-signal impedance for the prediction of resonance-generated distortions; the large-signal impedance of an electrical equipment represents its impedance response for different magnitudes of perturbation injected at its terminals. Large-signal impedance-based prediction of resonance-generated distortions is demonstrated for a three-phase grid-connected voltage-source converter.

Large-Signal Impedance-Based Modeling and Mitigation of Resonance of Converter-Grid Systems

Large-signal impedance of grid-connected converters can be used to predict resonance-generated distortions in converter-grid systems. Note that the large-signal impedance of a network represents its impedance response for different magnitudes of perturbation injected at its terminals. This paper presents large-signal impedance-based modeling and mitigation of resonance of grid-connected voltage source converters. Challenges of large-signal modeling because of the inapplicability of the small-signal approximation are addressed by leveraging the dominating influence of hard nonlinearities (such as pulsewidth modulation saturation and limiters) over soft nonlinearities (such as Park's transformations and phase-locked loop (PLL)) in shaping the large-signal behavior of the converter. The paper develops large-signal gains of hard nonlinearities using different types of describing functions. The paper shows that the large-signal impedance of a voltage source converter (VSC) can be shaped to reduce resonance-generated distortions by inserting limiters in the control system of the VSC. Developed large-signal impedance models are validated using numerical simulations of a VSC with dq current control and PLL. Large-signal impedance measurements of a commercial 1 MW VSC-based inverter and a medium-voltage doubly-fed induction generator with approximate 4 MW rating are presented to experimentally demonstrate the influence of the injected perturbation magnitude on the impedance response.