Voltage Source Converter System Research Articles

Interval analysis of the small‐signal stability of grid‐connected voltage‐source converter system considering multiparameter uncertainty

AbstractGrid‐connected voltage source converters (VSCs) have been broadly applied in modern power system. However, instability issues may be triggered by the integration of grid‐connected VSCs, jeopardising the operation of the power grid. Conventional stability analysis methods can be utilised to derive system stability margins under nominal conditions. Whereas grid‐connected VSCs inevitably operate under multiparameter uncertainty, which may result in overly optimistic or even incorrect estimations of stability margins, thereby posing potential risks to system operation. To address this issue, an interval small‐signal stability analysis approach is proposed to investigate the system stability under multiparameter uncertainty. First, the interval state‐space model of the grid‐connected VSC system is constructed based on interval symbolic linearisation. Furthermore, the interval eigenvalue decomposition is introduced to calculate the interval eigenvalue distribution of the interval state‐space model. Eventually, the upper bounds of the real part of the dominant interval eigenvalues are utilised for deriving interval stable parameter regions. Results of Monte Carlo analysis and time‐domain simulations of the grid‐connected VSC system are utilised to verify the effectiveness of the proposed interval stability analysis method.

Coordinated DC voltage control strategy for the receiving end hybrid LCC-VSC system

The receiving end hybrid line commutated converter (LCC) — voltage source converter (VSC) system has the obstacle that when an AC fault occurs at the sending LCC side, the DC voltage of the rectifier side will fall, resulting in the power transmission reduction of the whole system. Particularly, when the rectifier side DC voltage falls below the inverter side DC voltage, the power transmission is interrupted, posing a huge threat to the stability of the power grid at both the sending and receiving ends. To tackle these problems, a coordinated DC voltage control strategy for the receiving end is proposed. By controlling the DC voltage of the receiving VSC, the proposed method can improve the DC system power transmission capability when voltage drops at the sending side. The proposed control strategy is simple but practical, which is good for the stable and secure operation of both the sending and the receiving power system.

Study on the topology of a system for delivery of electricity from deep sea wind through voltage source converter

It is a difficult problem to maintain high reliability while designing the topology of the voltage source converter (VSC) for deep-sea wind power in a compact manner. In this paper, the topology of the converter, the main wiring type and grounding method, and the DC system wiring scheme are studied. The advantages and disadvantages of each scheme are compared in offshore wind power application scenarios. Finally, the half-bridge converter topology and the true bipolar with metal return line topology are recommended for the topology design of the VSC system for deep and remote offshore wind power. The power operating range is also presented. The scheme proposed in this paper paves the way for promoting the lightweight and highly reliable system for the delivery of electricity from deep sea wind through VSC.

Transient synchronization stability of photovoltaics integration by singular perturbation analysis

The integration of large-scale photovoltaics (PVs) into the power grid has significantly altered the transient synchronization dynamics of traditional power systems dominated by synchronous generators (SGs) and posed great challenges to modeling and analysis of PVs integration. In this paper, the transient synchronization stability of the PV-SG system is studied using the singular perturbation technique. Firstly, a nonlinear model of a PV-SG system is established to reveal the multiscale transient synchronization characteristics. Further, the full system is decomposed into a slow subsystem and a fast subsystem by the singular perturbation technique. The fast subsystem containing the dynamics of the DC voltage control, terminal voltage control, and phase-locked loop, and the slow subsystem containing the dynamics of rotor motion can perfectly reflect the dynamics of the full system within the electromagnetic and electromechanical timescales, respectively. The proposed model provides a clearer physical picture of dynamics in the PV-SG system within the electromagnetic and electromechanical timescales. Subsequently, the stability of the slow and fast subsystems is investigated using the energy function and eigenvalue analysis methods, respectively. Meanwhile, the impacts of various operating, control, and structural parameters on the transient synchronization stability are uncovered. Different from the most existing research endeavors on the wide simulations of the PVs integration, the impact of PVs on the synchronization dynamics of SGs without considering the dynamical characteristics of the PV system, and the transient synchronization stability analyses of the PLL-based voltage source converter systems, it is the key contribution to study the transient synchronization dynamical characteristics of the PV system and its interaction with the SG under different timescales. All these are helpful and easy to extend to more complicated PV-SG systems. Finally, the analysis results are validated by extensive simulations.

Transient Stability of Multiple Phase-Locked-Loop-Type Voltage Source Converters System Based on Amplitude Mapping Model

Compared to the traditional power grids, the multiple converters integration system are more vulnerable to the disturbances from transient events. However, the high-order characteristics of the system model is still a barrier not ignored. Inspired by averaging method of nonlinear vibration, the amplitude mapping model (AMM) is firstly proposed to evaluate the transient stability for multiple phase-locked-loop-type (PLL-type) voltage source converters (VSCs) system in this paper. Specifically, the origin and meaning of amplitude mapping model is introduced. Next, the mathematical model of the PLL-type VSCs system is derived. Based on AMM, the stability evaluation strategy of the PLL-type VSCs system is obtained. After that, the influence of key parameters, including the system gSCR, d-axis current reference, and PLL control parameters, on the system stability and the coupling effect between the VSCs are analyzed. Finally, a 6-VSCs 14-nodes system is applied to verify the correctness of the method proposed in this paper.

Transient Stability Analysis and Improvement for the Grid-Connected VSC System With Multi-Limiters

As a widely configured controller, the limiter is essential to ensure the security of the voltage source converter (VSC), which affects the stability characteristics of the grid-connected VSC system under severe faults. However, due to limiters' hard algebraic constraints, the existing methods have yet to reveal the stability mechanism related to the multi-limiters thoroughly. In this paper, the stability mechanism of the grid-connected VSC system with multi-limiters is investigated, and an improved Filippov Theorem is proposed to study the dynamic characteristics. Two kinds of points are essential to the stability, namely, the fixed switching point(FSP) and the fixed ending point(FEP). Expressly, transient stability can be guaranteed by the condition that FSP is in the region of attraction(ROA), even if other parts of the trajectory are outside the ROA. Moreover, the existence of FEP contributes to some control loop failure, which does not induce instability but may cause the power into the grid to be inconsistent with the reference value. This paper presents the analytical methods for these crucial points, which avoid time-domain simulation. What follows illustrates how to enhance the stability characteristics by configuring the FSP and FEP. Another advantage of the proposed method is easily extended to the VSC with multi-limiters and different control structures.

Unified AC/DC hybrid admittance modelling and oscillation stability analysis for interlinking voltage source converter system

AbstractThe voltage source converter (VSC) interlinking AC and DC power networks is a core equipment in AC/DC hybrid power systems. To analyze the stability of this system based on the impedance method, it is necessary to establish the hybrid AC/DC admittance model of interlinking VSC. This paper presents a unified and systematic AC/DC hybrid admittance modelling framework which is widely applicable to different control methods of interlinking VSCs. The hybrid admittance models for interlinking VSCs under PQ control, VdcIq control, and Vf control are explicitly formulated, with their elements individually reflecting the dynamics of the AC side, DC side, and the interaction between them. Subsequently, the impedance network model of interlinking VSC systems is formulated based on the hybrid admittance, and furthermore the frequency‐domain modal analysis is performed to analyze the system oscillation stability. Finally, frequency scanning and time‐domain simulation results in MATLAB/Simulink are provided to validate the effectiveness of the modelling and oscillation stability analysis method.

Analytical Dual-Sequence Current Injections Feasible Region of Weak-Grid Connected VSC Under Asymmetric Grid Faults

For phase-locked-loop synchronization (PLL-Syn) based Weak-Grid connected Voltage Source Converter (WG-VSC) system under asymmetric grid faults, published works mainly focus on existing of equilibrium point and small signal stability, which cannot address PLL-Syn transient stability problems. For the first time, we have proposed an analytical solution to the question of “ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Whether the WG-VSC system can reach to a new available equilibrium point (EP) of fault-on state from original EP of pre-fault state without loss of PLL-Syn transient stability under asymmetric faults?</i> ”. Dual-sequence current injections feasible region (CIFR) of WG-VSCs in positive and negative sequence current injection space is defined firstly. Then, by considering <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PLL-Syn transient stability constraint, Maximum available current constraint, and PCC Voltage constraint</i> , an analytical expression of CIFR has been obtained. Finally, experiments results based on detailed switching model in RT-BOX hardware-in-loop platform have been provided to verify the CIFR constructed in this paper. It should be noted that the work of this paper can not only address PLL-Syn transient stability problems of WG-VSC system under asymmetric faults, but also can pave the way for asymmetric fault ride-through control considering transient stability constraints.

An Intelligent Method for Fault Location Estimation in HVDC Cable Systems Connected to Offshore Wind Farms

Large and remote offshore wind farms (OWFs) usually use voltage source converter (VSC) systems to transmit electrical power to the main network. Submarine high-voltage direct current (HVDC) cables are commonly used as transmission links. As they are liable to insulation breakdown, fault location in the HVDC cables is a major issue in these systems. Exact fault location can significantly reduce the high cost of submarine HVDC cable repair in multi-terminal networks. In this paper, a novel method is presented to find the exact location of the DC faults. The fault location is calculated using extraction of new features from voltage signals of cables’ sheaths and a trained artificial neural network (ANN). The results obtained from a simulation of a three-terminal HVDC system in power systems computer-aided design (PSCAD) environment show that the maximum percentage error of the proposed method is less than 1%.

Transient synchronous stability analysis and enhancement control strategy of a PLL-based VSC system during asymmetric grid faults

The stability of a voltage source converters (VSC) system based on phase-locked loop (PLL) is very important issue during asymmetric grid faults. This paper establishes a transient synchronous stability model of a dual-sequence PLL-based VSC system during low voltage ride-through by referring to the equivalent rotor swing equation of synchronous generators. Based on the model, the synchronization characteristics of the VSC system under asymmetric grid faults are described, and the interaction mechanisms, as well as the transient instability phenomena of positive and negative sequence PLL during asymmetric faults are explained. Using the equal area criterion, the influences of sequence control switching action, detection delay, and interaction between the positive and negative sequence PLL on the transient synchronous stability of the VSC system are analyzed, respectively. In addition, a transient stability assessment criterion based on the critical fault clearance angle and time and an enhancement control strategy based on the improved positive and negative sequence PLL are proposed. Finally, the analytical results are validated through simulation and experiments.

Evaluation of VSC Impact on Power System Using Adequate P-Q Capability Curve

Renewable energy sources, which are becoming increasingly popular, often use a voltage source converter (VSC) for connection to the power system. Assessing the effects of connecting such a source to the power system is essential to ensure the proper operation of the power system and the connected source. For this purpose, it is necessary to know the range of active and reactive power generation by the converter. The authors indicate that the interaction between the power system and the converter affects its range of available active and reactive power. Therefore, a strictly defined range of the converter’s generating capability should not be assumed as invariant, but its capability for a given operating condition of the power system should be determined iteratively. In order to confirm this thesis, the authors analyzed the operation of the VSC-based energy source in an example power system using the PowerFactory software. Extending the calculation procedure to include iterative determination of the converter’s available power range showed a significant influence of the system’s operating state on the converter’s generating capabilities. The results obtained in this work extend the knowledge, and thanks to them, the operation of VSC systems can be modelled more accurately.

A protection method for LCC-VSC hybrid HVDC system based on boundary transient power direction

In a line-commuted converter–voltage source converter (LCC-VSC) system, in the hybrid multi-terminal high-voltage direct-current (HVDC) sending end, a line-commuted converter (LCC) converter is used, while the receiving end uses a full-half hybrid modular multi-level converter (MMC), with a fault self-clearing capability. The DC line faults can be self-cleared within 1–2 ms. Therefore, there is an inevitable problem of short availability of faulty information since only 1-ms complete fault data are available to identify faults. Thus, it is necessary to develop a fast and reliable protection scheme for an LCC-VSC system. To address this need, this paper proposes a DC line protection method based on the boundary transient power direction. First, the characteristics of boundary transient power direction of the internal and external faults are analyzed. For an internal fault, the boundary transient power direction is positive, whereas for an external fault, this direction is negative. Therefore, this paper uses the sum of the boundary transient power directions to structure internal- and external-fault identification elements and a pole detection element. The proposed method is validated by real-time digital simulations using Kun-Liu-Long LCC-VSC three-terminal hybrid HVDC–RTDS test system. Numerous tests have demonstrated that the proposed protection method is effective in detecting different fault types under various fault distances, fault transition resistance values, and signal-to-noise ratios in the period of 1 ms. Compared with the existing methods, the proposed protection principle has a better performance in withstanding fault transition resistance (600 Ω) and noise interference (10 dB).

Modeling and Analysis of the Torque-Frequency Dynamics for Multi-VSC Parallel System Based on the Equivalent Admittance

The increasing use of voltage-source converters (VSCs) in modern power systems has led to extensive studies on their frequency response and stability, which are characterized by power-frequency oscillations and overshoots following a disturbance. However, the existing small-signal analysis models do not adequately describe the impact of the types of disturbance on the torque-frequency dynamics of the multi-VSC parallel system. In this paper, based on the principle of electromechanical analogy, a unified equivalent admittance modeling method is proposed by analogizing the rotor dynamic model of a synchronous generator (SG) to the circuit network, which has the advantages of simple modeling, clear physical meaning and easy expansion. Firstly, the equivalent admittance model of a single SG/VSC is established, and then easily extended to the multi-VSC system. The proposed modelling method not only applicable to a variety of VSCs, but also can handle different types of disturbance. More importantly, the circuit theory can be applied to analyze the torque-frequency dynamic characteristics. Subsequently, the equivalence between the proposed model and the state-space model under different damping terms is compared. In addition, the response characteristics of two different damping terms are compared. Next, the dynamic response characteristics of single VSC and multi-VSC parallel systems under different disturbances are analyzed. Finally, related simulations and experiments are executed to verify the accuracy and effectiveness of the proposed method.

A Nonlinear Adaptive Stabilizing Control Strategy to Enhance Dynamic Stability of Weak Grid-Tied VSC System

Large scale renewable energy systems interfaced with voltage source converters (VSCs) experience several challenging issues when integrated to weak grids. One of the specific issue i.e. control loop interaction of vector-controlled VSC under high grid impedance leads to several instability mechanism ultimately resulting in unwanted tripping of the VSC unit. As a countermeasure, a unique nonlinear adaptive stabilizing control (NASC) technique has been proposed in this paper to enhance the dynamic stability of the VSC during such scenarios. A small-signal model is developed first to examine the instability mechanism of VSC connected to weak grids under abnormal operating conditions and varying set points of the power controller. The dominant modes are identified from the analysis and a stabilizing controller is developed based on adaptive dynamic surface control philosophy for enhancing its stability. The proposed controller has been proved to be uniformly ultimately bounded using Lyapunov analysis. Besides, the stabilizing mechanism of VSC is analyzed with the proposed controller by performing small-signal stability analysis of the entire system. The efficacy of the proposed control scheme is demonstrated by performing numerical simulations in MATLAB 2017a and conducting experimental validations in controller-hardware-in-the Loop (CHIL) platform.

Generalized Swing Equation and Transient Synchronous Stability With PLL-Based VSC

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems.

Comparative evaluation of converter-based compensation schemes for VSC systems to achieve full-range active power transfer in very weak grids

Voltage source converter (VSC) is the expected core technology that supports power system de-carbonization by allowing renewable energy development. However, with the increasing penetration of renewables and continuing decommissioning of thermal generators, challenges of integrating VSC-based systems into weak and even very weak ac grids become apparent. Therefore, this paper presents theoretical analysis describing the relationship between active power transfer range and weak grid factors of the generic VSC-grid system, and aims to identify the most effective way to allow the VSC to exchange the rated active power in both directions (±1 pu) with the weak grid. Thus, converter-based compensation is presented to extend the operation boundary and avoid voltage collapsing. Nevertheless, the effects of reactive current provision and series voltage compensations should be recognized; therefore, operational characteristics of two arrangements, namely, shunt and series VSC-based compensation schemes, are comparatively evaluated. In extremely weak grid cases, shunt compensation converter cannot ensure a full active power transfer range of the targeted VSC due to the inherent voltage limitation, whilst series compensation converter can assist the targeted VSC to achieve full-range active power transfer. Effectiveness and performance of the presented compensation methods during power reversal and ac fault are demonstrated with a typical extremely weak grid, and system boundaries with different schemes are given.

Transient Synchronization Stability Improvement Control Strategy for Grid-Connected VSC Under Symmetrical Grid Fault

Under grid fault, the grid-connected voltage source converters (VSCs) have the risk of transient synchronization instability during low voltage ride through. Based on the equivalent rotor swing equation of VSC, this letter proposes an improved transient synchronization stability control strategy, which improves the existence of equilibrium points and transient synchronization behaviors of the VSC system. Thus, the transient synchronization stability of grid-connected VSC can be significantly enhanced. The simulation and experimental results demonstrate the validity of the proposed strategy.

A Three-Phase Active-Front-End Converter System Enabled by 10-kV SiC MOSFETs Aimed at a Solid-State Transformer Application

The use of high-voltage silicon carbide (SiC) devices can eliminate multilevel and cascaded converters and their complicated control strategies, making converter systems simple and reliable. A three-phase two-level voltage-source converter system serves as a simple converter system for interfacing any dc source to a three-phase grid. However, when the high-voltage devices are used in two-level converters, they are exposed to a high-voltage peak stress and a high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$dv/dt$</tex-math></inline-formula> (up to 100 kV/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula> s). Operating these semiconductor devices at these stress levels requires careful design not only of the semiconductor die and the module, but also of the gate drivers, busbars, and passive filters. This article demonstrates the operation of 10-kV SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s and discusses the design considerations, advantages, and challenges associated with the operation of the three-phase two-level medium-voltage converter system used as the active-front-end converter system. Reliable operation of the medium-voltage converter system requires the development of reliable high-voltage modules and auxiliary parts, such as gate drivers, busbars, inductors, voltage and current sensors, and proper design of the controller system. Successful tests demonstrating continuous field operation of the medium-voltage active-front-end converter at a nominal rating of 7.2-kV dc-link voltage is demonstrated for the first time in the literature. The results indicate that these devices can accelerate the growth and deployment of medium-voltage SiC devices for field operation, as demonstrated by the operation inside the mobile container.

A Multiple Open-Circuit Fault Diagnosis Method for Two-Level Three-Phase Voltage Source Converters Based on Average-Phase Voltage Model

This article presents a fault diagnosis method in the two-level three-phase voltage source converter (VSC) systems based on the average-phase voltage (APV) model without additional sensors or hardware circuits, which can diagnose and locate multiple open-circuit (OC) faults correctly and quickly. First, based on the idea of APV within one switching period and the changing rules of basic voltage vector after OC faults, the APV model consisting of grid-side APV (GAPV) model, prefault converter-side APV (CAPV) model, and postfault CAPV model is proposed. The APV model can be used to estimate the phase voltage under different OC faults. And then, the residual feature between GAPV and CAPV is analyzed under different single or multiple OC faults. With the help of the proposed APV model, both single and multiple OC faults can be diagnosed by deviation polarity, which avoids the complex deviation polarity sequences judgment. Meanwhile, the fault diagnosis method has the ability of correctness verification, which can strengthen the robustness and prevent false alarms. Finally, the effectiveness of the proposed model and method is validated with experiments; the multiple OC faults can be located within 2.5 ms.

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.