Unbalanced Grid Conditions Research Articles

Enhanced Control Scheme for a Three-Phase Grid-Connected PV Inverter under Unbalanced Fault Conditions

This paper presents an improved control strategy to cancel the double grid frequency oscillations in the active power, reactive power, and DC-link voltage of a three-phase grid-connected photovoltaic (PV) system under unbalanced grid condition. To achieve these goals, an enhanced positive–negative-sequence control (PNSC) to remove oscillations of active power and an instantaneous active–reactive control (IARC) to mitigate the fluctuations of active and reactive power, simultaneously, are suggested. These methods are also effective to reduce the oscillations of the DC-link voltage. To track the desired unbalanced or harmonic reference currents, improved proportional resonant (PR) current controllers have been designed using the Bode frequency analysis. Simulation studies are carried out via Matlab/Simulink® software to verify the effectiveness of the suggested control strategies.

Fix-frequency robust power model predictive control method for three-phase PWM rectifiers under unbalanced grid conditions

Under unbalanced grid conditions, the controller design of a three-phase pulse width modulation (PWM) rectifier is based on an instantaneous power model. By calculating the current references of the converter according to the instantaneous power model, traditional voltage-oriented control (VOC) methods realize the positive-sequence and negative-sequence active and reactive current control of the converter separately using the proportional-integral (PI) controller. However, due to the inner current loop control structure of the traditional VOC method, it is impossible to realize the regulation of all six power components in a common instantaneous power model under an unbalanced grid. Meanwhile, the control performance of the traditional VOC method with a PI controller is degraded under the circuit parameters uncertainty condition. In this paper, a fixed switching frequency robust power model predictive control method (FRP-MPC) is proposed for three-phase PWM rectifiers under the unbalanced grid condition. The proposed control strategy has a number of advantages. An improved instantaneous power model is used for the fixed switching frequency model predictive controller design under the unbalanced grid condition, which has less power variables than the common instantaneous power model. The robustness of the MPC controller is improved by adding a robust item into the predictive model under circuit parameters uncertainty. Simulation and experiment results verify the effectiveness of the proposed control method.

New AC–AC Modular Multilevel Converter Solution for Medium-Voltage Machine-Drive Applications: Modular Multilevel Series Converter

Due to its scalability, reliability, high power quality and flexibility, the modular multilevel converter is the standard solution for high-power high-voltage applications in which an AC–DC–AC connection is required such as high-voltage direct-current transmission systems. However, this converter presents some undesired features from both structural and operational perspectives. For example, it presents a high number of components, which results in high costs, size, weight and conduction losses. Moreover, the modular multilevel converter presents problems dealing with DC-side faults, with unbalanced grid conditions, and many internal control loops are required for its proper operation. In variable-frequency operation, the modular multilevel converter presents some serious limitations. The most critical are the high-voltage ripples, in the submodule capacitors, at low frequencies. Thus, many different AC–AC converter solutions, with modular multilevel structure, have been proposed as alternatives for high-power machine-drive applications such as offshore wind turbines, pumped-hydro-storage systems and industrial motor drives. These converters present their own drawbacks mostly related to control complexity, operational limitations, size and weight. This paper introduces an entirely new medium-voltage AC–AC modular multilevel converter solution with many operational and structural advantages in comparison to the modular multilevel converter and other alternative topologies. The proposed converter presents high performance at low frequencies, regarding the amplitude of the voltage ripples in the submodule capacitors, which could make it very suitable for machine-drive applications. In this paper, an analytical description of the voltage ripples in the submodule capacitors is proposed, which proves the high performance of the converter under low-frequency operation. Moreover, the proposed converter presents high performance under unbalanced grid conditions. This important feature is demonstrated through simulation results. The converter solution introduced in this paper has a simple structure, with decoupled phases, which leads to the absence of undesired circulating currents and to a straightforward control, with very few internal control loops for its proper operation, and with simple modulation. Since the converter phases are decoupled, no arm inductors are required, which contributes to the weight and size reduction of the topology. In this paper, a detailed comparison analysis with the modular multilevel converter is presented based on number of components, conduction and switching losses. This analysis concludes that the proposed converter solution presents a reduction in costs and an expressive reduction in size and weight, in comparison to the modular multilevel converter. Thus, it should be a promising solution for high-power machine-drive applications that require compactness and lightness such as offshore wind turbines. In this paper, simulation results are presented explaining the behavior of the proposed converter, proving that it is capable of synthesizing a high-power-quality load voltage, with variable frequency, while exchanging power with the grid. Thus, this topology could be used to control the machine speed in a machine-drive application. Finally, experimental results are provided to validate the topology.

Model predictive direct power control for modular multilevel converter under unbalanced conditions with power compensation and circulating current reduction

In this paper, a model predictive direct power control (MP-DPC) with power compensation and circulating current reduction is proposed for modular multilevel converter under unbalanced grid conditions. The main objectives of this paper are to achieve the circulating currents reduction and the power ripples elimination under unbalanced grid-side voltage conditions. In order to realize the aforementioned objectives, a triple-stage MP-DPC method based on a decoupled mathematical model of circulating current and DC-link current components is presented, which results in the elimination of weighting factors. Meanwhile, a power compensation term is embedded in the proposed design so as to eliminate the power ripples. In this sense, the power compensation terms are expressed by grid voltages and their quadrature signals without the complex positive/negative sequences extraction of grid voltage/currents and phase-locked loop. The proposed methodology can not only relieve the effect of circulating currents but also eliminate the active and reactive power ripples while remaining computationally feasible under unbalanced grid conditions. Additionally, a reduced switching frequency approach is introduced to decrease the power loss. Finally, the effectiveness of the proposed MP-DPC method is validated by both simulation and experimental results.

A Switching Control Strategy Based on Switching System Model of Three-Phase VSR Under Unbalanced Grid Conditions

The power electronic converters are typical nonlinear systems. The conventional linearized modeling methods cannot accurately describe the working process of the converter. The control strategies based on the linearized model can only achieve good performance near the working point. By considering the switching nonlinear system nature of the converters, the main contributions of this article are as follows: a current error switching model is developed for the three-phase voltage source rectifiers (VSRs) under unbalanced grid for the first time; and an improved stability analysis theorem is proved for the switching control of the three-phase VSRs under unbalanced grid condition. These contributions are used to design a simple current switching controller for VSRs. The main advantages of the proposed modeling and control strategy are as follows. The switching model of the converter is built without any approximation. It accurately describes the physical working principle of the three-phase VSR under unbalanced grid condition. The proposed current switching controller has a very simple control structure. The switching states of the converter are directly produced according to the switching rule. There are no control parameters, no complicated rotation transformation, and no voltage vector modulation in the proposed switching controller. The proposed switching controller is not sensitive to the circuit parameters. The robustness of the control system is improved without any extra compensators or observers. Simulation and experiment verifies the correctness, the effectiveness, and the superiority of the proposed theory and methodology.

A simple modulation approach for interfacing three-level Neutral-Point-Clamped converters to the grid

Multilevel converters are nowadays an enabling key in the integration of electric power into the grid as they introduce less distortion and, thus, they are more compliant with the grid standards, among other benefits. A well-known topology is the three-level Neutral-Point-Clamped whose control requires to deal with the capacitors voltage unbalance. This paper presents a modulation approach where the injection of a common component in the modulated voltage is studied in order to achieve such voltage balance. An optimization problem that, apart from the voltage balance, aims the lowest number of commutations and can be solved very efficiently with up to five computations of the cost function is formulated. The main advantages of the proposed modulation strategy are its simplicity and its flexibility, since it is also valid for unbalanced grid conditions and, with little added complexity, for low (and even zero) power factor conditions. Simulation results under unbalanced grid conditions are provided in order to show its validity under this scenario. The strategy is evaluated and compared with a space-vector-based approach in an experimental setup, yielding similar total harmonic current distortion, a 30% reduction in the number of commutations, and better voltage balance performance for lower power factor conditions.

Research on the Control Strategy of PWM Rectifier under Unbalanced Grid

Background: Aiming at the three-phase voltage source PWM rectifier (VSR), there have been some researches on the neural network control strategies, but the unbalanced power grid condition is always neglected. Meanwhile, about the self-detection technology of the unbalanced power grid voltage, there are still few researches. Methods: Under the unbalanced power grid conditions, this work presents a power grid voltage sensorless control strategy of the three-phase VSR. Based on the radial basis function neural network (RBF) theory, a newly PI controller with parameter self-tuning function is studied, and the RBF-PI controller is used for the closed-loop controls of the voltage and current variables. By means of T/4 delay method, the positive- and negative-sequence components of the power grid side current, and those of the equivalent virtual flux-linkage of unbalanced power grid are extracted. From the equivalent virtual flux-linkage, the unbalanced power grid voltage is reconstructed online. Results: Under the load mutation condition, the power grid voltage sensorless control of threephase VSR is achieved, the system response characteristics are analyzed. From the simulation experimental results, it can be seen that the unbalanced power grid voltage can be quickly tracked, and the sinusoidal control of the power grid side current can be achieved. In addition, the control system has a series of advantages, such as a faster dynamic response speed, a stronger robustness and a smaller DC side voltage ripple. Conclusion: The proposed unbalanced power grid voltage sensorless control strategy of threephase VSR is effective.

Multi-Target Control Strategy of DFIG Using Virtual Synchronous Generator Based on Extended Power Resonance Control under Unbalanced Power Grid

Virtual synchronous generator control is considered as an effective solution to optimize the frequency response characteristics of doubly fed induction generator. However, due to the insufficient control bandwidth of the original virtual synchronous generator, it has little control effect over the oscillating components of the power caused by the unbalanced grid voltage. Therefore, long-term unbalanced voltage will cause a series of problems, such as distortion of stator and rotor currents, as well as oscillations of power and electromagnetic torque, which seriously affect the power quality and the operating performance of the doubly fed induction generator. To solve these problems, the concept of extended power is introduced, and the second-order generalized integrator-based resonant controller is used to control the extended power and traditional power. Control targets of the extended power method are discussed and extended, so that the doubly fed induction generator system using extended power resonant control-based virtual synchronous generator control can realize three different control targets under the unbalanced grid condition. The three control targets are: balanced and sinusoidal stator current, sinusoidal stator current and constant active power, and sinusoidal stator current and constant reactive power and electromagnetic torque. The three control targets can also be flexibly switched according to the real-time requirements of the grid with unbalanced voltage. The simulation results verify the effectiveness of the control method.

Deadbeat Predictive Power Control with Fuzzy PI Compound Controller and Power Predictive Corrector for PWM Rectifier Under Unbalanced Grid Conditions

Under unbalanced grid conditions, the DC side voltage of pulse width modulation (PWM) rectifier will overshoot during start-up and reference voltage transients, which may lead to system instability. In this paper, a deadbeat predictive power control (DPPC) with fuzzy PI compound controller (FPCC) and power predictive corrector (PPC) is proposed to solve that problem. Firstly, the parameters of the PI controller are adjusted online by the fuzzy control rule of the FPCC to eliminate the overshoot of the DC side voltage and contribute to faster dynamic responses, which thereby could correct the reference active power. Secondly, the static error between the reference and system active powers is reduced by accumulative predictive errors in the PPC. The simulation is carried out under ideal and unbalanced grid conditions. The result shows that the proposed control scheme can effectively eliminate the overshoot of DC voltage, reduce the static error of active power, and improve the dynamic response and anti-interference ability of the rectifier.

Operation and control of MMC station with constraints of internal variables under unbalanced grid conditions

Modular multilevel converter (MMC) station is expected to be able to operate under unbalanced grid conditions (UGCs) and to satisfy all kinds of limits under UGC. To investigate the allowable region of MMC under UGC, this study presents a point-scanning method, which directly solves the steady-state values of MMC from non-linear dynamic equations for each possible operating point, and therefore, can access the constraints of internal variables including the sub-module capacitor voltage, arm current and modulation index. First, the complete 28th-order dynamic model of MMC, which contains the zero-sequence second-order modulation signal is derived. Next, as the premise to solve the steady state of MMC, point of common coupling voltages are solved from a proposed unbalanced grid model, which incorporates the effect of ac fault. Following that, a peak-value seeking algorithm is proposed to seek the peak values of internal dynamics of irregular waveforms to implement the constraint of internal dynamics. Next, the allowable region of MMC under UGC is depicted and analysed. Also, the power limiting control is proposed to avoid MMC under UGC, violating the limits of internal dynamics. Finally, the simulation validates the effectiveness and correctness of the proposed point-scanning method and power limiting control.

Small-Signal Modeling and Analysis of MMC Under Unbalanced Grid Conditions Based on Linear Time-Periodic (LTP) Method

As in grid-tied Modular Multilevel Converters (MMC), the steady-state operation under asymmetric grid conditions is rather general, especially for the overhead line of the AC network. Therefore, the small-signal dynamics of MMC under unbalanced grid conditions have become a great important research topic. In this paper, the inherent periodic time-varying characteristic of MMC's original relationships is given priority consideration. Based on the linear time-periodic (LTP) method, a general small-signal LTP model under unbalanced grid conditions is built considering the internal dynamic of submodules (SMs). Then based on the eigenvalue locus analysis method from Floquet- Lyapunov theory, the effects of controller parameters and interactions between different controllers on small-signal dynamics are studied, respectively. Finally, the results from the detailed nonlinear MMC model are provided to verify the effectiveness of the proposed analysis.

Control of Grid-Following Inverters Under Unbalanced Grid Conditions

This paper proposes a new control scheme to eliminate the 3rd harmonic in the output currents of grid-following inverters under unbalanced grid conditions. Unbalanced grids adversely affect the performance of grid-following inverters due to the oscillations appearing on the DC-link voltage with a frequency twice the line frequency. The paper is based on instantaneous active reactive control (IARC) technique due to its advantages over other existing methods. However, the presence of severe asymmetrical 3rd harmonic distortions in the inverter output currents is the main challenge with IARC method, which impairs the power quality requirements. This paper enhances the IARC scheme by proposing a 3rd harmonic elimination approach by adopting a current reference generation method using the symmetric sequence components concept. Furthermore, the proposed scheme complies with the grid code requirements by injecting the requested reactive current. Finally, the proposed approach is evaluated by theoretical and simulation analysis and validated experimentally using a hardware setup.

Enhancing the performance of grid synchronization in DFIG-based wind turbine under unbalanced grid conditions

Grid synchronization has always been a major challenge for grid-connected converters when the grid voltage is unbalance. Hence, this paper proposes an improved synchronize method for DFIG generator connected to an unbalance grid. In the proposed method, the stator voltage is directly controlled in order to follow the grid voltage. The grid voltage is divided into positive and negative components, and each one is entered in a separate control loops to observe the unbalanced grid voltage in the stator. Then, the stator voltage precisely follows the grid voltage. The rotor current is also applied in the inner control loop to control the rotor current before and after synchronization. The proposed method is tested on a 100 kW DFIG prototype; simulation and experimental results show that this improved control method works well with a fast response time and the DFIG stator voltage follows the grid voltage accurately under unbalanced conditions.

DSP‐based comparison of PFC control techniques applied on bridgeless converter

A comparison of fuzzy self-tuning (FST), sliding mode control (SMC) and conventional proportional-integral (PI) control methods is performed under unbalanced grid conditions using a bridgeless power factor correction (PFC) converter. The bridgeless PFC converter operates the correction of AC line current to obtain a DC output voltage without full bridge rectifier based on these control methods. These control methods generate the duty cycles to provide higher unity power factor and lower total harmonic distortion of input current, even if AC line voltage is distorted. This study focuses on the performance analysis of FST and SMC methods adaptation to the input current for the bridgeless PFC converter and comparing with the conventional PI controller. Although the SMC method is only used in the current control loop, the FST method is used in both current and voltage control loops for eliminating harmonics of input current and regulating of output voltage. The performance of control methods is evaluated with a TMS320F2812 digital signal processor. The simulation and experimental results are compatible with the limits for harmonic current emissions, IEC 61000-3-2 and show that the FST and SMC are better than the conventional PI control method and FST is superior to the SMC.

Virtual Flux Estimation for Sensorless Predictive Control of PWM Rectifiers Under Unbalanced and Distorted Grid Conditions

In this article, a new neural network-based virtual flux (NN-VF) estimator is proposed for sensorless control of a pulsewidth modulation rectifier under unbalanced and distorted grid conditions. In this estimator, the VF vector is reconstructed through an emulated ideal integrator. Thereafter, positive and negative sequence (PNS) VF components are extracted using an NN-based PNS components separator. A Lyapunov's theory-based convergence analysis is performed for optimal tuning of the NN-VF estimator. Accordingly, accurate and fast estimation is achieved. A VF-based predictive direct power control (VF-PDPC) model, including the estimated VF positive sequence components, is formulated. A startup procedure is considered for smooth starting under unbalanced grid conditions. Feasibility and robustness of the developed VF-PDPC are verified by experiments. Mainly, obtained results demonstrate that the startup procedure reduces settling time and avoids over currents; the VF-PDPC achieves better current waveforms than those obtained by the conventional PDPC under unbalanced grid conditions; the NN-VF estimator presents similar dynamic performances as the second-order generalized integrator that uses grid voltages measurement.

A novel control scheme for three-phase seven-level packed U-Cell based DSTATCOM

With the advent of SiC devices, the packed U-Cell converter has become a promising topology among the multilevel converters for medium voltage DSTATCOM application. It utilizes fewer components while achieving the maximum voltage levels with higher reliability. However, as a reactive power compensator, its dc-link consists of flying capacitors, whose voltages must be balanced which presents a challenging problem for a seven-level packed U-Cell inverter, as there are no-redundant switching states. This paper proposes a novel control scheme to keep these flying capacitors balanced under normal/abnormal grid conditions. It consists of a standard cascaded: inner and outer controllers. The inner current controller regulates both the active and reactive currents and the outer controller regulates the upper flying capacitor voltage. In parallel with the cascaded controllers, a novel model predictive voltage balancing controller is proposed to maintain the lower flying capacitor voltage at a given setpoint. To cope with the unbalance grid conditions, an additional zero-sequence injection regulator is also integrated into the designed control scheme. The proposed control scheme was simulated and real-time tested on the Hardware-In-the-Loop system. The results show that the proposed controller effectively balances the two flying capacitor voltages while controlling the reactive power.

Current and power quality multi-objective control of virtual synchronous generators under unbalanced grid conditions

In recent years, with the large-scale application of distributed power sources in the power grid, the power grid is moving toward low inertia and low damping. The virtual synchronous generator (VSG) has become a hot topic for scholars since it can simulate the moment of inertia, damping and frequency modulation of synchronous generators. Due to its unbalanced load distribution and the random variation of power loads, grid voltage is asymmetrical. Under grid voltage asymmetry, a VSG experiences output current imbalance and power oscillation. The grid current imbalance further reduces the transformer’s three-phase voltage, which makes it unequal, cyclic, and very easy to cause electricity accidents. Aiming at this problem, this paper proposes three modes of current balance, reactive power balance and active power balance without changing the characteristics of VSGs. The output current balance and power are constant when the grid voltage is asymmetrical. The effectiveness and feasibility of the control strategy are verified by simulation and experimental results, which provides an effective scheme for balanced and stable operation of the grid.

Control of a BDFIG Based on Current and Sliding Mode Predictive Approaches

The Brushless Doubly Fed Induction Generator (BDFIG) is a great attraction nowadays thanks to its high reliability and high low voltage ride-through capability. In variable speed generation systems, the most used control techniques are the vector control, direct torque/power control. Vector control is highly sensitive to parameters variations, while the two others offer high amount of ripples reducing power quality. This paper therefore proposes a Sliding Mode Power Predictive Control (SMPPC) and an Indirect Model Power Predictive Control (I-MPPC) for a simple and efficient power control of the BDFIG. The SMPPC takes current components in the αβ frame as sliding surfaces derived from the I-MPPC based on current predictions to track the desired active and reactive powers in the BDFIG. A cost function defined as currents’ quadratic errors is used to choose the appropriate voltage vector for control winding supply, and the Particle Swarm Optimization algorithm is used to determine the optimal gain for the SMPPC. Simulations on MATLAB/Simulink are carried out to show the effectiveness of the control schemes as well as their robustness to some parameters variations. More so, power compensation is used to improve the capability of the control strategy to provide sinusoidal and balanced currents under unbalanced grid conditions. The proposed methods compared to direct power control show good performances in terms of total harmonic distortion reduction, low ripples and robustness, strengthening the idea of using the BDFIG as an alternative to new generation of wind energy conversion systems.

Passivity-Based Control With Active Disturbance Rejection Control of Vienna Rectifier Under Unbalanced Grid Conditions

In this paper, a practical passivity-based control (PBC) with active disturbance rejection control (ADRC) is proposed to improve performance of the Vienna rectifier under unbalanced grid conditions. In general, the traditional double-loop control based on positive and negative sequence transformation is used in Vienna rectifier under unbalanced grid conditions. However, it could not fundamentally solve the additional time delay caused by the second harmonic filter and loss of performance caused by a linear weighted sum of proportional integral (PI) controller. What's more, the complexity of the controller is high for the positive and negative sequence currents need to be controlled separately. The PBC is a nonlinear controller based on energy dissipation and it has strong robustness to interference. Further, the line voltage based PBC in inner current loop can deal with the voltage unbalance effectively and easily without negative sequence transformation. To improve the disturbance rejection ability, the ADRC is applied in outer voltage loop, which could overcome PI's drawbacks of step overshoot and slow response. Under unbalanced grid conditions, the proposed control strategy has good performance, easy implementation and less consuming time with PBC control in inner current loop, and it has strong robustness and fast track performance with ADRC control in outer voltage loop. The detailed mathematical model, control principle and controller design of the Vienna rectifier are thoroughly analyzed. In addition, simulation results based on SIMULINK are also given in the paper. Finally, a downsize 5kW Vienna rectifier prototype is built to validate the correctness and effectiveness of the proposed strategy.

Sliding-Mode Control Strategy for Three-Phase Three-Level T-Type Rectifiers With DC Capacitor Voltage Balancing

A sliding mode control (SMC) strategy with dc capacitor voltage balancing is proposed for three-phase three-level T-type rectifiers. The proposed SMC strategy is designed in the abc frame rather than the dq frame. In this case, the necessity of three-phase current transformations is eliminated. The proposed SMC is based on the errors of the line currents. The amplitude of line current references is generated by controlling the dc voltage using a proportional-integral (PI) controller. In order to obtain unity power factor, the generated reference amplitude is multiplied by the corresponding sinusoidal waveform obtained from the phase locked loop (PLL) operating with grid voltages. The dc capacitor voltage balancing is achieved by adding a proportional control term into the line current reference obtained for each rectifier leg. The performance of the proposed control strategy is validated by simulations and experiments during steady-state, transients caused by load change, and unbalanced grid conditions. The results show that the proposed control strategy offers excellent steady-state and dynamic performances with low THD in the line currents, zero steady-state error in the output voltage, and very fast dynamic response.