Unbalanced Grid Conditions Research Articles

Dynamic Performance Analysis of VSC-HVDC Based Modular Multilevel Converter under Fault

The use of high voltage direct current based voltage source converter (VSC-HVDC) in power transmission systems knows a great progress in recent years. Above all, with the new generation of power electronics converters such as the modular multi-level converter (MMC), with his scalable structure it can theoretically meet any voltage level requirement, which allows to increase the size of the power transferred compared to conventional converters. In this sense, this paper presents a study of a VSC-HVDC system based on a modular multi-level converter (MMC). The main objective of this work is to analyze the performance of the VSC-HVDC system based MMC without the AC filters and its control in the event of a fault, during set point changes and unbalanced grid conditions. After realization a mathematical model of the system studied and its control, simulations are done over in Simpower System/Matlab. The results obtained confirm the robustness of the system control and the system gives a good energy quality, that manifests by a good output currant and voltage curves with no need to use a voluminous AC filter.

Control of transformerless T-type DVR using multiple delayed signal cancellation PLL under unbalanced and distorted grid condition

In the near future, the power system network may experience severe voltage distortions due to association of renewable energy sources (RES), and the large penetration of power electronic based loads into the microgrid. To protect sensitive loads from these distorted and unbalanced grid conditions, the commonly used dynamic voltage restorer (DVR) requires effective control algorithms. The effective control algorithm comprises a controller, and accurate estimation of grid phase angle/frequency, and magnitude, thereby the reference DVR voltage is accurately estimated. The synchronous reference frame based phased locked loop (SRF-PLL) is commonly used for phase angle estimation. However, during unbalanced, distorted, and dc-offset conditions, the performance of SRF-PLL is unsatisfactory. To improve its performance, it is cascaded with prefilters such as dual second order generalized integrator (DSOGI), cascaded delayed signal cancellation (CDSC), and multiple delayed signal cancellation (MDSC). Among these prefilters, MDSC has superior performance under all grid conditions with less delay time and memory requirement. In addition to an effective control algorithm, the DVR system requires high power density converters. The power density of the system is increased using transformerless T-type converter. In this paper, the effectiveness of MDSC-PLL for the transformerless T-type converter based DVR is evaluated in MATLAB/Simulink for various grid conditions such as voltage sag/swell, unbalance voltage sag/swell, harmonics, and dc-offset.

Resistance-Emulating Control Strategy for Three-Phase Voltage Source Rectifiers Under Unbalanced Grids

In this article, a resistance-emulating control strategy without grid voltage sensors is proposed for three-phase voltage source rectifiers under unbalanced grid conditions. It consists of angle estimator, coordinate correction, resistance-emulating, common-mode resistance (CMR) and the differential-mode resistance (DMR) regulation, extended state observer, and other critical steps. In this scheme, the CMR regulates the average power, and the DMR controls the ripple power caused by the unbalanced supplies. Three different control objectives including constant active power, symmetrical input currents, and constant reactive power are achieved by regulating the DMR. Since the grid voltage sensors and the information of the grid-side circuit parameters are not required, the cost is reduced and the robustness and reliability of the scheme is improved. Experiments verify the effectiveness of the proposed method.

Zero-Sequence Voltage Injection Method for DC Capacitor Voltage Balancing of Wye-Connected CHB Converter under Unbalanced Grid and Load Conditions

Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.

DC-link voltage compensation with fuzzy-based MMC to mitigate the low-order circulating current

ABSTRACT In this paper, a novel fuzzy controller-based technique is used to control the harmonics of the circulating currents in Pulse width modulation (PWM) based modular multilevel converters (MMC). This technique is based on average model circuit of MMC, which recognises the cross-relation in between the sub-module and system-level elements using dependent sources. In addition to improving the damping of the system, the main superiority of this technique is that it maintains the natural balancing property of DC bus system in a PWM-based MMC by combining upper and lower arms together, instead of considering separately like in existing schemes. Fuzzy controller is most suitable controller for the decision-making. Fuzzy gives more accuracy, for the nonlinear loads or heavy industrial applications. Fuzzy controller allows the reduction of the harmonics of the circulating current in the dc-link system and works effectively during the unbalanced AC grid conditions also. Therefore, it is suitable for multi-frequency AC systems. The accuracy of the proposed method is verified using simulation results which include the arm currents, common mode (circulating) currents and DC-link voltages along with AC grid voltage and currents.

Modeling and Mitigation Control of the Submodule-Capacitor Voltage Ripple of a Modular Multilevel Converter under Unbalanced Grid Conditions

A modular multilevel converter’s (MMC’s) submodule (SM)-capacitor voltage will increase under unbalanced grid conditions. Depending on the imbalance level, the voltage ripple can be considerably high, and it can exceed the pre-defined safe limits. If this occurs, the converter will trip, which can lead to serious stability problems for the grid. This paper first proposes an analytical solution for deriving the three-phase imbalanced SM ripple of an MMC under an unbalanced grid. With this analytical tool, the imbalance mechanism of the SM voltage ripple can be easily understood. What is more, the symmetrical component method is first applied to analyze the three-phase SM capacitor ripple, and the positive-/negative-/zero-sequence components of the three-phase SM voltage ripple are easily identified by the proposed analytical method. Then, based on this powerful analytical tool, the proper circulating-current profile to be injected can be obtained, allowing for the right compensation of the voltage ripple. Based on this approach, two new voltage ripple compensation methods are proposed in this paper. Simulations were carried out to validate the analytical description of the submodule-capacitor voltage ripple proposed in this paper. Moreover, simulation and experimental results are provided to validate the new compensation techniques introduced in this paper.

Observer-Based Sliding-Mode Control for Grid-Connected Power Converters Under Unbalanced Grid Conditions

This article proposes a novel robust control strategy for three-phase power converters operated under unbalanced grid conditions. A consolidated control objective is obtained in the stationary <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\alpha \beta$</tex-math></inline-formula> frame, which can be flexibly adjusted according to the degree of oscillation in the active and reactive powers and the balance of the three-phase current. Based on the dynamics of the converter and control objective, a control scheme in a cascaded framework is presented, in which an adaptive observer is applied to estimate the positive and negative sequences of the grid voltage. In the current tracking loop, a super-twisting algorithm current controller coupled with a super-twisting differentiator is implemented to track the current references, featuring rapid dynamics, and improved robustness. Additionally, in the voltage regulation loop, an effective composite controller is developed to regulate the dc-link voltage, where a super-twisting observer is used to estimate the load disturbance, thereby improving the performance of the converter. The experimental results are provided to confirm the effectiveness and superiority of the proposed control strategy.

Parallel Hybrid Converter-Based STATCOM and Capacitor Voltage Control Technique

The parallel-hybrid converter (PHC) is a promising high-power topology. Smaller capacitor size, lower capacitor and switch counts, and low-energy storage requirement are some of its important features. However, it requires a large dc-filter inductor to filter the dc-side harmonics. Moreover, since its dc and ac sides are strongly coupled, only a single stable operating point in terms of a fixed modulation-index (MI) value is possible. Even with an additional dedicated control effort, the obtained MI variation remains still limited. This article proposes the PHC-static compensator (STATCOM) configuration that eliminates the dc-filter inductor requirement and can also operate at any MI value without necessitating additional control efforts. A detailed power analysis of the PHC-STATCOM for balanced and unbalanced grid conditions (UGCs) is performed in this article. A new control technique for PHC-STATCOM is proposed, which effectively balances the capacitor voltages of its chain-links even during UGCs. The energy storage requirement of PHC-STATCOM is evaluated to obtain the capability curve and determine its capacitor size. The PHC-STATCOM is compared with the popular STATCOM configurations in terms of the capacitor and switch counts. The proposed principles of PHC-STATCOM for several three-phase balanced and UGCs are verified using simulation and experimental studies.

An Improved Three-Stages Cascading Passivity-Based Control of Grid-Connected LCL Converter in Unbalanced Weak Grid Condition

This paper proposes an improved three-stages cascading passivity-based control (PBC) for a grid -connected LCL converter in unbalanced weak grid condition. In general, the traditional double-loop control based on positive and negative sequence transformations is used in grid-connected converter control in unbalanced weak grid condition. However, it is time consuming for second harmonic filtering, and positive and negative sequence currents need to be controlled separately. The PBC has strong robustness to interference, and the line voltage based PBC can deal with the voltage unbalance effectively and easily without negative sequence transformation. But the traditional PBC needs six variables and three damping coefficients for the grid-connected LCL converter in unbalanced grid condition, and it has the disadvantage of difficult implementation. The improved PBC can realize the same control effect with three-stages cascading PBCs of two variables and one damping coefficient, and it has the advantages of easy implementation, good performance and high stability. First, the modeling and controller design are detailed described. Then the SIMULINK simulation results demonstrate the benefits of the improved control strategy. Finally, a grid-connected LCL converter prototype of 5kW is built and the experimental results verify the correctness and effectivity of the improved three-stages PBC strategy.

Novel Converter Topology With Reduced Cost, Size and Weight for High-Power Medium-Voltage Machine Drives: 3x3 Modular Multilevel Series Converter

The modular multilevel converter is considered the state-of-the-art power electronic solution for high-power high-voltage applications that require a DC stage and that operate with fixed frequency at their AC terminals. However, in high-power AC-AC variable-frequency applications, it has been proved that this converter topology presents an unnecessarily large number of components and an intolerably high submodule-capacitor voltage ripple at low frequencies. The modular multilevel matrix converter was proposed as a promising alternative for high-power medium-voltage electrical-machine drives since it presented improved performance at low frequencies and a reduced number of components in comparison to the modular multilevel converter resulting in reduced costs and improved efficiency. Even though, the modular multilevel matrix converter is one of the converter solutions that stands out the most for high-power machine drives, it presents some operational drawbacks such the unstable behavior under unbalanced grid conditions and the extremely complex internal control and modulation. Recently, a new converter topology named modular multilevel series converter was proposed presenting attractive operational characteristics such as the high performance at low frequencies, high performance under unbalanced grid conditions, and straightforward control and modulation, which are important advantages compared to the modular multilevel converter and the modular multilevel matrix converter. Nonetheless, as demonstrated in this paper, the modular multilevel series converter presents large component count and poor efficiency. Thus, in this paper, a new converter solution is proposed, which is an enhanced version of the modular multilevel series converter that preserves the previously mentioned advantageous operational characteristics and that presents an improved efficiency and a considerably reduced number of components in comparison to the other three converter topologies, resulting in reduced cost, size and weight.

Model Predictive Direct Power Control of Grid-connected Converters Considering Unbalanced Filter Inductance and Grid Conditions

The grid-connected converter (GCC) is widely used as the interface between various distributed generations and the utility grid. To achieve precise power control for GCC, this paper presents a model predictive direct power control (MPDPC) with consideration of the unbalanced filter inductance and grid conditions. First, the characteristics of GCC with unbalanced filter inductance are analyzed and a modified voltage control function is derived. On this basis, to compensate for the power oscillation caused by unbalanced filter inductance, a novel power compensation method is proposed for MPDPC to eliminate the DC-side current ripple while maintaining sinusoidal grid current. Besides, to improve the control robustness against mismatched filter inductance, a filter inductance identification scheme is proposed. Through this scheme, the estimated value of filter inductance is updated in each control period and applied in the proposed MPDPC. Finally, simulation results in PSCAD/EMTDC confirm the validity of the proposed MPDPC and the filter inductance identification scheme.

A Comprehensive Motivation of Multilayer Control Levels for Microgrids: Synchronization, Voltage and Frequency Restoration Perspective

The current paradigm in integrating intermittent renewable energy sources into microgrids presents various technical challenges in terms of reliable operation and control. This paper performs a comprehensive justification of microgrid trends in dominant control strategies. It covers multilayer hierarchical control schemes, which are able to integrate seamlessly with coordinated control strategies. A general overview of the hierarchical control family that includes primary, secondary, tertiary controls is presented. For power sharing accuracy and capability, droop and non-droop-based controllers are comprehensively studied to address further development. The voltage and frequency restoration techniques are discussed thoroughly based on centralized and decentralized method in order to highlights the differences for better comprehend. The comprehensive studies of grid synchronization strategies also overviewed and analyzed under balanced and unbalanced grid conditions. The details studies for each control level are displayed to highlight the benefits and shortcomings of each control method. A future prediction from the authors’ point of view is also provided to acknowledge which control is adequate to be adopted in proportion to their products applications and a possibility technique for self-synchronization is given in this paper.

Small-Signal Model and Dynamics of MMC-HVDC Grid Under Unbalanced Grid Conditions

Small-signal models (SSM) of modular multilevel converter (MMC) based HVDC grid under unbalanced grid conditions in recent literatures do not consider interactions between MMC and dc network. Therefore, it is necessary to correctly model the double-fundamental-frequency zero-sequence circulating currents (DFF-ZSCC) existing under unbalanced grid conditions, although they can be suppressed to avoid penetrating into dc side. In view of that, a general and modular small-signal modeling method of MMC-HVDC grid under unbalanced grid condition is developed. Firstly, a general d-q transformation process for multi-sequence and multi-frequency components is introduced to build the state-space model of MMC electric part in d-q frame. Secondly, the SSM of dc network, which is divided into dc- and double-fundamental-frequency parts, is developed based on the existing frequency-dependent-cascaded-pi (FDC-PI) cable model. Then, to interface MMC stations with dc network under unbalanced grid conditions, a general method to synchronize the variables of double fundamental frequency at dc side of each MMC station is introduced to compose the SSM of the entire HVDC grid from SSMs of each MMC and dc network. Finally, taking a three-terminal MMC-HVDC grid as an example, two SSMs of the MMC-HVDC grid with FDC-PI cable model and single-PI cable model are developed in Matlab, respectively. The small-signal dynamics and stability of both SSMs of the grid under unbalanced grid conditions are compared to validate the accuracy and stability of the proposed SSM of the whole HVDC grid. The simulation results validate the correctness and effectiveness of the proposed modular modeling method.

Effective Control and Management Scheme for Isolated and Grid Connected DC Microgrid

This article addresses a voltage control and energy management strategy of active distribution systems with a grid-connected dc microgrid as well as for an islanded dc microgrid with hybrid energy resources. In the islanded mode, a control and management strategy using a backup diesel generator (DG), a renewable energy source (RES), and an energy storage system plays a vital role in maintaining the microgrid bus voltages within the limits. However, operating backup diesel generator (DG) has its own challenges including startup delay, frequent switching, and uneven loading when operated along with a RES. Additionally, fuel efficiency and emission characteristics vary with loading since most of DGs are driven by constant-speed diesel engines. Hence, an exhaustive power management scheme (PMS) is proposed by utilizing the hybrid energy storage system. Real-time simulation and experimental validation of the proposed scheme are provided using a real-time digital simulator (RTDS) and a laboratory-scale prototype, respectively. Extreme scenarios including DG failure/scheduled maintenance, low power generation, and battery charge are analyzed in the islanded mode. Furthermore, a dc microgrid is connected to an IEEE active distribution system feeder to analyze control and management challenges for the grid connected mode with contribution from a microgrid and with no contribution from a microgrid. These scenarios resemble more realistic unbalanced utility grid conditions. A centralized optimization problem is formulated at an advanced distribution management system level to maintain all the node voltages within limits in the IEEE test system. RTDS is used to simulate dc microgrid connected with the IEEE test system and an optimization algorithm is implemented in MATLAB. Superior performance of the developed algorithms are demonstrated and validated for coordination between centralized optimization at ADMS and the microgrid energy management system.

Control of Doubly-fed Induction Generator with Extended State Observer under Unbalanced Grid Conditions

Under unbalanced grid condition, in a doubly-fed induction generator (DFIG), voltage, current, and flux of the stator become asymmetric. Therefore, active-reactive power and torque will be oscillating. In DFIG controlling rotor side converter (RSC) aims to eliminate power and torque oscillations. However, simultaneous elimination of the power and torque oscillations is not possible. Also, grid side converter (GSC) aims to regulate DC-Link voltage. In this paper, in order to regulate DC-Link voltage, an extended state observer (ESO) based on a generalized proportional-integral (GPI) controller, is employed. In this controlling method, DC-Link voltage is controlled without measuring the GSC current, and due to using the GPI controller, the improved dynamic response is resistant against voltage changes, and the settling time is reduced. A versatile rotor position computation algorithm (RPCA) is utilized to measure the rotor speed. This algorithm is simple, yet it is accurate and is resistant to changes in the resistance of the rotor and stator. The simulations are implemented by MATLAB software in the synchronous positive and negative sequence reference (d-q).

Analysis and Control of an MMC-HVDC System Under an Unbalanced Grid Condition

Modular multilevel converter (MMC)-based high voltage direct current (HVDC) systems have been increasingly researched and installed in recent years. During operation, systems may be subjected to various unbalanced conditions, the single-line to ground (SLG) fault is one of the most common unbalanced faults. When a SLG fault occurs in a three-phase alternating current (AC) system, the double-line frequency positive- and zero-sequence harmonic components will be produced besides the double-line frequency negative-sequence harmonic component. And if the control system cannot suppress these harmonic components, they can propagate into the direct current (DC) side of the HVDC system and cause ripples in DC-line voltage and current as well as other unwanted consequences. Among the proposed controllers in the published papers on this topic, rarely controllers can both effectively suppress the circulating current and completely eliminate ripples in the DC-line voltage and current while still keeping the AC-side current balanced under unbalanced grid conditions. Therefore, this paper first analyses the propagation of harmonic components in a two-terminal MMC-HVDC system under a SLG fault. Then, based on the mathematical analysis, a proposed controller including an inner current controller for controlling the negative- and positive-sequence components and a controller for suppressing the zero-sequence component is designed to eliminate harmonic propagation from the faulty three-phase AC system to the DC side of the HVDC system, resulting in suppressing ripples in DC-line voltage and current. The theoretical analysis and proposed controller are verified by a simulation model in PSCAD platform.

A Novel Direct Power Control for DFIG With Parallel Compensator Under Unbalanced Grid Condition

This article presents a voltage-modulated direct power control (VM-DPC) with an additional parallel compensator for the doubly fed induction generator under unbalanced grid conditions. The proposed method not only guarantees a satisfying steady-state performance but provides also a regulating property of the negative-sequence output currents through designed negative-sequence parallel compensator. It can provide symmetrical stator currents and suppress the ripples in both active and reactive powers under unbalanced grid conditions. The performance of the proposed method are verified by comparing it with three different control strategies in simulations carried out in MATLAB/Simulink SimScape Power System. Finally, the effectiveness of the proposed method is evaluated in an experimental prototype, which proves the proposed VM-DPC with the additional compensator has a satisfactory steady-state performance and a fast power transient response under unbalanced grid conditions.

Sensorless Virtual Flux Combined Control of Grid Connected Converters With High Power Quality Under Unbalanced Grid Operation

This paper presents a novel sensorless combined control for grid connected power electronic converters (GCCs) under both balanced and unbalanced grid operation. Under unbalanced grid conditions, output power quality of GCCs would usually be deteriorated by increased current harmonics and power oscillations injected to grids. To overcome the problem, an improved sensorless combined control method with a new switching logic is presented in this paper based on instantaneous control of grid current components. The simplicity and robustness of combined control is fully preserved due to a current reference vector, which is calculated using the virtual flux components in the grid voltage, and virtual flux reference frames without any positive and negative sequence extraction. The proposed method, in comparison with voltage oriented control, avoids problems of tuning current controller, PWM sub-system, and decoupling circuit. Compared to the direct power control, the method lacks active and reactive power calculation and voltage sensors and enjoys improved power ripples and current THD.

Generalized predictive power control for grid-connected converter

The aim of the contribution is to present a new method for the control of the active and non-active instantaneous power for the grid-connected voltage source converter. The GPPC (Generalized Predictive Power Control) is examined in the paper. A design procedure, simulation results, and experimental validation of the GPPC are presented. This GPPC application belongs among predictive control strategies for multi-input multi-output systems with some time varying coefficients. The responses following changes in the power references as well as after disturbances in the grid voltage are presented and evaluated. The simple implementation, long prediction horizon, and also resistance to parameter changes and to unbalanced grid conditions belong among advantages of the method. The determination of the GPPC parameters is not dependent on the AC/DC converter circuitry and on the PWM applied. The developed GPPC used in the converter connected to the grid can be considered as an original contribution to the problem of controlling the dc/ac converter performance.

Hierarchical Model Predictive Control of Grid-Connected Cascaded Multilevel Inverter

This article presents a hierarchical finite-set model predictive control (MPC) scheme to enable autonomous operation and self-balancing cascaded multilevel inverter. The proposed approach is an alternative to MPC scheme based on a generic cost function, which in some applications is ill fit or challenging to design. The proposed controller has a hierarchical framework to eliminate the overall cost function optimization and associated weight factor design stage of the control objectives. The control formulation approach allows for multiobjective optimization with a cost-tolerance framework. The concept is well suited to simplify the control design stage of cascaded H-bridge inverters at the grid-edge with advanced functionality. The control scheme achieves active and reactive power control with switching event reduction while equalizing power draw from the independent voltage sources. The latter of these objectives is made possible by the proposed hierarchical approach to the control objective tracking. The control is modularized for each phase, making the system robust to unbalanced grid conditions. The concept is explained in depth in simulation, and then tested experimentally on hardware.