Voltage Source Converter System Research Articles

Leaky-momentum control algorithm for voltage and frequency control of three-phase SEIG feeding isolated load

This paper dealt the implementation of a Leaky-Momentum Control Algorithm (LMA) for controlling a voltage source converter (VSC) to enhance the power quality of a three-phase self-excited induction generator (SEIG) used in a distributed generating system. This LMA technique operates the VSC to regulate voltage and frequency of SEIG within a permissible limit. The LMA control is implemented to reduce the higher demand of reactive power, harmonics distortions and balancing of loads under different operating conditions. During the electrical and mechanical dynamical conditions, the LMA technique is maintaining a constant voltage and frequency at point of common coupling (PCC). The proposed technique is a modified control technique of basic Leaky and Momentum Algorithms. This control has removed the drawbacks of Leaky and momentum algorithms. Moreover, it is observed that LMA performs better when there are uncertainties in input conditions. The whole system comprising SEIG, nonlinear load, voltage source converter and battery storage system is made in MATLAB /SIMULINK. It has shown promising performance under both dynamical state and steady state of the system.

Small-Signal Stability Analysis and Enhanced Control Strategy for VSC System During Weak-Grid Asymmetric Faults

Small-signal stability issues of the voltage source converters (VSC) connected weak-network during asymmetric faults is currently rarely studied. This paper develops mathematic model of VSC system during weak-grid asymmetric faults, which reveals dynamic coupling between the phase-locked loop (PLL) and both positive-sequence (PS) control system and negative-sequence (NS) control system. Based on the deduced model, the generalized Nyquist criterion (GNC) method and modal analysis are adopted to study the small-signal stability of the VSC system during asymmetric fault steady-state. The analysis demonstrates that the small-signal stability of the system during asymmetric faults is dominated by the PLL mode, and the dynamic coupling between PLL and current control loops (CCLs) will produce negative damping, which will introduce the small-signal instability risk to the system. Moreover, an improved control strategy is designed for enhancing the small-signal stability of the VSC system during weak-grid asymmetric faults, by compensating the disturbance terms related to PLL dynamic to suppress the negative damping introduced by coupling between the PLL and both the PS and NS CCLs. Finally, this paper verifies the effectiveness of the proposed control strategy through simulation and experiment.

An Advanced Wideband Model and a Novel Multitype Insulation Monitoring Strategy for VSC-Connected Transformers Based on Common-Mode Impedance Response

The insulation monitoring technique for converter transformers has been explored using characteristic harmonics in the voltage source converter (VSC) system by the authors. The previous work enables the monitoring of aged groundwall insulation but cannot cover other insulation components. In order to explore the possibility of multitype insulation aging monitoring, in this article, we first derive an advanced model considering unevenly distributed circuit parameters along transformer windings and their frequency-dependent feature. Model improvements provide an accurate tool to describe multitype insulation in a wide frequency range. Based on the advanced model, a novel wideband common-mode (CM) impedance-response-based monitoring strategy for the VSC-connected transformer is proposed to achieve the evaluation of the aging in different insulation components. An illustrative case is studied on a 15 kVA transformer driven by an industrial inverter. Offline and online experiments are conducted to prove the effectiveness of the advanced model and the proposed strategy. Compared with the existing methods, the CM impedance-response-based strategy enables the online monitoring of multitype insulation components in an effective and noninvasive manner. In addition, the aging severities can be estimated reliably, even at incipient stages.

Quasi-Z-Source Inverter-Based Photovoltaic Power System Modeling for Grid Stability Studies

Quasi-Z-source inverters (qZSIs) are becoming a powerful power conversion technology in photovoltaic (PV) power systems because they allow energy power conversion in a single stage operation. However, they can cause system resonances and reduce system damping, which may lead to instabilities. These stability problems are well known in grid-connected voltage source converter systems but not in quasi-Z-source inverter (qZSI)-based PV power systems. This paper contributes with Matlab/Simulink and PSCAD/EMTDC models of qZSI-based PV power systems to analyze transient interactions and stability problems. These models consider all power circuits and control blocks of qZSI-based PV power systems and can be used in sensitivity studies on the influence of system parameters on stability. PV power system stability is assessed from the proposed models. The causes of instabilities are analyzed from numerical simulations and possible solutions are proposed.

Nonlinear Modeling of Multi-Converter Systems Within DC-Link Timescale

The emergence of various power electronic devices, including voltage source converters (VSCs), has greatly changed the dynamics of power systems. It is a great challenging to put forward new theories for modeling, analysis, and understanding of power-electronic-based power systems. In this paper, we first establish a detailed nonlinear model of multiple VSC systems, and then, by using the singular perturbation technique, obtain a reduced-order nonlinear model within the DC-link timescale. The latter model consists of differential algebraic equations (DAEs) and keeps slow dynamics in transient processes, showing a balance between computational accuracy and system complexity. Furthermore, the model is compared with the traditional electromechanical dynamical model of power systems, which are dominated by synchronous generators (SGs). Our model of multi-converter systems can be put into a similar diagram to the traditional power system dynamics, with which most of power electrical engineers are familiar. Finally, wide simulation results verify the efficiency of the proposed model.

Modeling and control of an electromechanical system with a permanent magnet generator and a voltage source converter

In the paper simulation dynamic models for the analysis of characteristics and transients of electromechanical system using a permanent magnet electric generator (PMG) connected to a variable speed fixed pitch wind turbine (WT) and a voltage source converter (VSC) mathematical models are developed. The system supplies a direct current (dc) resistive load through a controlled switch. The objective of the controlled resistive load connected to the VSC dc circuit of the system is to perform the dc output voltage stabilization for simulating the mode of generating the active power into a grid via a grid invertor. Two algorithm structures of realization of tracking the optimal WT motion trajectory in the coordinates “WT aerodynamical power – rotary speed” have been considered. The tracking algorithms function is to provide the maximization of power extraction from wind flow by WT. Electromechanical processes in the PMG – VSC system for both tracking algorithms have been investigated using developed dynamic simulation models and vector control technique of electromagnetic torque of the generator at variable WT mechanical power. The results of numerical investigations of the electromagnetic processes in the system have been analized. The efficiencies of the proposed control algorithms applied to the VSC in the considered electromechanical system have been compared and discussed.

Voltage Quality Enhancement of Grid-Integrated PV System Using Battery-Based Dynamic Voltage Restorer

The advancement of power electronic-based sensitive loads drives the power utilities’ devotion to power quality issues. The voltage disturbance could be happening due to fault conditions, switching of loads, energizing of transformers, or integration of highly intermittent energy sources such as PV systems. This research work attempts to enhance the voltage fluctuation of a sensitive load connected to a grid-integrated PV system using a battery-based dynamic voltage restorer (DVR). The proposed battery energy storage-based DVR has two separate controlling stages that are implemented at the DC–DC buck/boost converter of the battery and voltage source converter (VSC) system. Charging and discharging of the battery is operated based on the state-of-charge (SOC) value of the battery and the measured root mean square (RMS) voltage at the point of common coupling (PCC). The VSC of the DVR detection and reference generation control is done appropriately. In the detection control of the VSC, a combination of RMS and dq0 measurement techniques is used, whereas in the reference generation control, pre-fault strategy is implemented to restore both phase jump and magnitude distortions. Symmetrical and asymmetrical voltage sags scenarios are considered and the compensation demonstration is carried out using power system computer-aided design (PSCAD/EMTDC) software.

Influence of Active Power Output and Control Parameters of Full-Converter Wind Farms on Sub-Synchronous Oscillation Characteristics in Weak Grids

Active power outputs of a wind farm connected to a weak power grid greatly affect the stability of grid-connected voltage source converter (VSC) systems. This paper studies the impact of active power outputs and control parameters on the subsynchronous oscillation characteristics of full-converter wind farms connected weak power grids. Eigenvalue and participation factor analysis was performed to identify the dominant oscillation modes of the system under consideration. The impact of active power output and control parameters on the damping characteristics of subsynchronous oscillation is analysed with the eigenvalue method. The analysis shows that when the phase-locked loop (PLL) proportional gain is high, the subsynchronous oscillation damping characteristics are worsened as the active power output increases. On the contrary, when the PLL proportional gain is small, the subsynchronous oscillation damping characteristics are improved as the active power output increases. By adjusting the control parameters in the PLL and DC link voltage controllers, system stability can be improved. Time-domain results verify the analysis and the findings.

Damping control of high‐frequency resonance based on voltage feedforward for voltage source converter under a parallel compensated grid

When a voltage source converter (VSC) connects to a parallel compensated grid, high-frequency resonance (HFR) is a subsistent issue that exists in the interconnected system. HFR will introduce a significantly high-frequency voltage component at the point of common coupling, thereby the performance of the VSC system can be deteriorated seriously. For improving the operation of the VSC system under the parallel compensated grid, this study proposes an HFR damping strategy based on a voltage feedforward control to reshape the impedance of grid connected VSC. The VSC system with the proposed control strategy can damp HFR in a wide high-frequency range without employing the frequency detection and introducing the differential elements so that the negative influence caused by the high-frequency noise can be avoided. Theoretical analysis and experimental results are given to verify the effectiveness and availability of the proposed damping control strategy.

Small-Signal Stability Enhancement Approach for VSC-HVDC System Under Weak AC Grid Conditions Based on Single-Input Single-Output Transfer Function Model

The voltage source converter (VSC) system has stability issue under weak AC grid conditions. In this paper, the multi-input multi-output (MIMO) transfer function model of VSC system in frequency-domain is derived. Then, the individual channel analysis and design (ICAD) approach is employed to develop the equivalent single-input single-output (SISO) feedback control model for stability analysis. On that basis, the stability of VSC system and the coupling interaction intensity among the multiple control loops, including the active power control (APC), reactive power control (RPC) and phase-locked-loop (PLL) are investigated. The results show that larger PLL bandwidth increases the coupling degree of active power control (APC) and reactive power control (RPC) closed loops, resulting in the instability of the system under weak AC grid conditions. Finally, a derivative feedback compensation (DFC) approach is proposed to enhance the stability margin for VSC system connected to weak AC grid. The results based on MATLAB-based theoretical analysis and electromagnetic transient (EMT) simulations show that the proposed DFC approach can effectively suppress the instability of VSC system due to the close coupling of APC and RPC closed loops caused by the large PLL bandwidth, thus effectively improve the stability margin of the system.

Generalised average modelling of grid‐connected three‐phase VSC with closed‐loop vector control and regular‐sampled modulation

Based on the generalised averaging method, a comprehensive mathematical model for a three-phase grid-connected voltage source converter (VSC) with closed-loop vector control and symmetrical regular-sampled modulation is proposed here. The double Fourier series of switching functions is presented to construct the state average vectors in this model. To involve the conventional vector current control strategy, the coordinate transformation method of the state average vector is derived. The multiplication properties are also illustrated to calculate the product average vector for state variables. The proposed model is validated in both MATLAB simulations and hardware experiments. The results show that both the fundamental and the switching behaviours of the converter as well as the closed-loop dynamics of the controller can be accurately demonstrated by the derived model with fast simulation speed. The steady-state harmonic distribution can also be directly achieved by computing the equilibrium point of the system. The proposed model is appropriate for system-level studies of power electronic-based power systems and can provide a beneficial reference for the practical design and the performance assessment of the closed-loop VSC system, which indicates the broad prospect of the application of generalised average models in power system and power electronics fields.

High Frequency Resonance Damping Method for Voltage Source Converter Based on Voltage Feedforward Control

High frequency resonance (HFR) is a subsistent problem which affects the operation of the voltage source converter (VSC) connected to the parallel compensated grid. The appearance of HFR introduces a significant high frequency component in the grid voltage, thereby the operation of VSC system will be seriously affected. For enhancing the operation capability of VSC system, an HFR damping method based on the voltage feedforward control is proposed in this paper, which can reshape the VSC system impedance effectively in a wideband range. Besides, different with the existing HFR damping methods, the proposed method introduces a correction factor instead of the series virtual impedance with fixed value, so that the effect of impedance reshaping is irrelevant to the parameters of controlled object. In addition, this paper analyzes the fundamental control performance of VSC system after equipping the proposed method, for verifying that the proposed method will not worsen the fundamental control. Experimental results are given to validate the effectiveness of the proposed damping method.

A Novel Leakage-Current-Based Online Insulation Monitoring Strategy for Converter Transformers Using Common-Mode and Differential-Mode Harmonics in VSC System

Online insulation monitoring techniques for power transformers have been widely studied. However, challenges rise when they are applied to the converter-connected transformers which are operated under high electrical stress conditions. This article proposes a nonintrusive leakage-current-based insulation monitoring strategy for converter transformers, using inherent characteristic harmonics in voltage source converter system. Analytic relation between common-mode and differential-mode harmonics and insulation capacitances is first presented. Then, a combined strategy is proposed based on the differential measurement of leakage current, which enables online prognosis of not only overall ageing degree but also ageing localization of the phase-to-ground insulation. Experiments are conducted on a 15-kVA transformer driven by an industrial inverter. The proposed combined strategy shows robust performance over effective characteristic frequency spectrums. Several considerations for field implementation are further discussed. The major features of the proposed strategy are as follows: 1) easy implementation and nonintrusive measurement; 2) little impact on system operation and immunity to system interferences; and 3) lower requirements of data processing.

Active damping technique based on H ∞ controller for VSC under parallel compensation grid

Aimed at damping the high-frequency resonance (HFR) which exists in the voltage-source converter (VSC) system connected to the parallel compensation grid, an active damping method based on the H ∞ theory is proposed. Compared to the existing HFR damping methods, the H ∞ damping controller has the advantage of strong robustness, which means that the damping effect can be maintained when parameters of VSC system change. For obtaining the H ∞ damping controller, the design procedure of weight functions for the synthesis of H ∞ controller based on the requirements of HFR damping is investigated. Theoretical analysis and experimental results are given to verify the effectiveness and availability of the proposed active damping technique.

Nonlinear Modeling and Analysis of Grid-Connected Voltage-Source Converters Under Voltage Dips

In power systems, voltage-source converters (VSCs) are gradually replacing traditional devices and dramatically changing power system dynamic behavior, as the VSC's intrinsic high-order nonlinearity and multiple time-scale characteristics. As a simple case of transient stability for power-electronic-based power systems, in this article, we study a VSC system with a fifth-order nonlinear model, with the aid of nonlinear dynamics analysis. For the transient behavior, we find that both the amplitude and the phase of the terminal voltage of VSC show discontinuity when the fault occurs or is cleared. The behavior of time-scale separation is also generally observable, namely, the fast time-scale dynamics damps and disappears quickly, and the system bulk behavior is dominated by the slow time-scale dynamics. In addition, we find that the system transient stability is uniquely determined by whether the fault-clearing state is within or out of the basin of attraction of the post-fault equilibrium point. Finally, simulations and hardware-in-the-loop experiments verify the analytical results. We expect that all these interesting findings could provide an improved physical insight on our understanding of transient stability of power-electronic-based power systems.

RETRACTED: Application of Machine Learning and Big Data in Doubly Fed Induction Generator based Stability Analysis of Multi Machine System using Substantial Transformative Optimization Algorithm

With the increase in the amount of data captured during the manufacturing process, surveillance systems are the most important decision making decisions. Current technologies such as Internet of Things (IoT) can be considered a solution to provide efficient monitoring of productivity. In this study, it has suggested a real-time monitoring system that uses an IoT, big data processing and an Offshore Wind Farm (OWF) model is proposed. The Offshore Wind Farm (OWF) is an extended level invasion in modern power electronics systems, in this proposed work Doubly Fed Induction Generator (DFIG) based multi machined OWF was designed, and power stability was analyzed using Substantial Transformative Optimization Algorithm (STOA). The Voltage Source Converter (VSC) and High Voltage Direct Current (HVDC) system was combined with onshore network. The terminal voltage of onshore network was controlled through Onshore Side Converter (OSC), active and reactive power was regulated separately using VSC. The performance of the onshore network was evaluated under renewable network errors (Total Harmonics distortion and steady state error) beside with OWF. The OWF - DFIG active and reactive power was controlled smoothly with in the limit of HVDC, and the power framework security can be updated by controlling the active power of the OSC to help its terminal voltage using STOA methodology. From the voltage control mode, the electrical faults are recovered rapidly with minimum fluctuation. The dynamic simulation comes about additionally demonstrate that onshore network fault can't impact OWF behind HVDC transmission system. Because of the specialized favorable circumstance, VSC-HVDC innovation, the constancy in OWF is very much ensured against the onshore grid faults. The proposed STOA based system has validated through simulation in Matlab Simulink environment. General, 97% effectiveness, accomplished at full load condition in light of the proposed system. The results showed that the IoT system and the proposed large data processing system were sufficiently competent to monitor the manufacturing process.

A Non-Invasive Method for Estimating Circuit and Control Parameters of Voltage Source Converters

Voltage source converters (VSCs) have been widely used in modern power systems. Consequently, reliable operations of VSCs are essential for the power system to operate properly. A parameter estimator (PE) for a VSC system acts as a useful tool for problem diagnosis and preventive maintenance. Generally speaking, depending on how the measurement data are attained, a parameter estimation method can be classified into invasive and non-invasive. Non-invasive methods, which do not require any external excitation, are generally preferred for parameter estimation. However, most of the PEs can only estimate some parameters of the converters while treating others constant. This paper proposes a non-invasive parameter estimation method, which is able to predict simultaneously the circuit and control parameters of a VSC. To achieve this, the accurate open-loop and closed-loop steady-state models of the VSC are first derived. These models will be used to generate the training data sets to obtain the artificial neural network models to account for various uncertainties in a practical circuit. All the training data are generated by the derived VSC models, which are constructed in MATLAB and run off-line. The proposed method has been verified via PSCAD/EMTDC and validated with the experiment.

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.

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.

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.