Three-phase Grid Research Articles

Battery Storage System as Power Unbalance Redistributor in Distribution Grids Based on Three Legs Four Wire Voltage Source Converter

This article discusses the application of battery energy storage systems (BESSs) as power redistributors in three-phase distribution grids as an add-on functionality to typical BESS applications, such as congestion management and energy arbitrage. Combining those ancillary services into a single power unit is not yet performed in practice but may constitute an emerging business opportunity to increase the BESS revenues. The unbalanced operation of the BESS voltage source converter (VSC) leads to the circulation of low-frequency current harmonics in the dc-link through the capacitors and the battery cells. Therefore, it is particularly interesting whether relatively large 50- and 100-Hz currents can safely circulate within these components. Analytical modeling and design guidelines for the dc-link of a three-leg four-wire two-level VSC operating under unbalanced loads are detailed. Furthermore, a low-power VSC prototype is used to demonstrate the working principle of the BESS, providing power unbalance redistribution and symmetric power exchange. Additionally, the ICR18650-26F Lithium-ion cells are cycled to reach end-of-life with different current profiles and C-ratings. The analysis shows that charging with a 100 Hz ripple superimposed to the dc current leads to a 10% increment in degradation.

Data Analytics for Admittance Matrix Estimation of Poorly Monitored Distribution Grids

Smart grid operations require accurate information on network topology and electrical equipment parameters. This paper proposes estimating such information with data from the smart grid. Assuming that the availability of bus voltage data is restricted to their magnitude, a linear model of the relationship between these data and the parameters of the admittance matrix is derived in a way that does not involve bus voltage angles. A regression optimizer is then proposed to minimize the deviation between data and values estimated by the linear model. Results on the IEEE 33 bus system are presented to illustrate the model accuracy and efficiency when used to estimate parameters of medium-voltage, three-phase balanced grids.

A novel Phase-Lock-Loop algorithm for Three-Phase grid voltage Distortion-Connected electric vehicle chargers

A new phase-lock-loop (PLL) method is proposed to deal with the unusable angle for feeding into the current control in three-phase grid voltage distortion-connected electric vehicle (EV) charger systems. The development of the proposed PLL method is the modification by means of doubly cascading the PLL modules, called the double PLL (DPLL) method. The superiority of the proposed DPLL method is that it provides a more correct approach to the phase angle for the control system. This makes this system a good candidate for bidirectional power quality control with grid-to-vehicle and vehicle-to-grid modes and constant current, voltage, and power modes for charging batteries. With its third-order system, it is more attractive due to the ability to design a better cut-off frequency and better oscillator filtering performance. A mathematical transfer function of the proposed DPLL method is fully derived and compared with the existing PLL methods by numerically analyzing them in the theory of the bode plot schematic. To verify the feasibility of the proposed DPLL method, the simulation of the 12.5 kW-battery EV charger systems under the ideal and non-ideal grid voltage conditions with the two-level and multi-level converters is tested by comparing between the proposed and existing PLL methods.

Control Design for the NPC Three-Level Grid-Connected Converter with Non-Ideal Grid System

Background: The large-scale distributed new energy integrations bring about various non-ideal grid situations, which could deteriorate the control performance of grid-connected converters. Objective: This study aims to improve the control performance of the grid-connected converters when the grid is non-ideal. Methods: Firstly, a neutral point clamped (NPC) three-level grid-connected converter was studied, and the characteristics of the non-ideal grid were analyzed in detail. Then, a real-time signal extraction method called generalized sliding discrete Fourier transform (GSDFT) has been proposed to extract the non-ideal signals, e.g., harmonic components and negative sequence components, based on which a double closed-loop control strategy was designed to improve the control performance of the grid-connected converters when the grid is non-ideal. Results: The experimental results show that the converter double closed-loop control strategy in the non-ideal grid environment is suitable for a variety of non-ideal grid environments, and the gridconnected effect is good. Conclusion: Firstly, this paper analyzes the characteristics of non-ideal grid and establishes the mathematical model of NPC three-level grid-connected converter in non-ideal grid environment. Secondly, aiming at the problem of characteristic harmonics and negative sequence components in non-ideal power grid, a real-time and fast extraction method of non-ideal signals based on generalized sliding discrete Fourier transform is proposed. Finally, taking the three-phase grid current of grid-connected converter as the control target, the closed-loop control strategy of non-ideal signal extraction based on GSDFT is designed, and a good control effect is achieved.

Variable-Switching-Frequency Single-Stage Bidirectional GaN AC–DC Converter for the Grid-Tied Battery Energy Storage System

This article presents a 10-kW novel gallium-nitride (GaN)-based three-phase grid to 48-V battery energy storage system (BESS). The BESS utilizes a single-stage ac–dc dual-active-bridge (DAB) converter with dual-phase-shift (DPS) and variable-frequency (VF) control. 600- and 80-V GaN power transistors, as well as planar magnetics, are used to achieve 96.6% efficiency and 50-W/in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^3$</tex-math></inline-formula> power density. The operation principle and power transfer characteristics are derived based on time-domain analysis of the DAB inductor current. VF and DPS are utilized to achieve power factor correction and zero-voltage switching simultaneously while minimizing the DAB rms current. An online calculation algorithm is presented with short execution time, which enables the closed-loop control of the BESS. Experimental results are presented to validate the novel control.

Comparative Analysis of Unbalanced Three-Phase Grid Current Compensation Methods for the Scott Transformer Based Grid

Comparative Analysis of Unbalanced Three-Phase Grid Current Compensation Methods for the Scott Transformer Based Grid

An Enhanced Reactive Power Compensation Scheme Using a Synthesis Segmental Multilevel Converted for Three-Phase Grid Systems

Solar photovoltaic (PV) systems have gained significant attention due to their easy implementation and availability, where proper energy management should be highly concentrated for a successful PV power utilization. In the traditional works, various controlling techniques have been developed for reactive power compensation. But, it lacks with the issues of reduced system performance, increased loss, and high harmonics. Hence, this paper aims to develop a new controlling methodology, named the Synthesis Segmental Multilevel Converter (SSMC) for reactive power compensation in a three-phase grid system. Initially, it extracts the maximum amount of power from the solar PV systems by using an Enhanced Perturb and Observe (EPO) method. Then, the panel separation is performed and the three-phase power input is given to the SSMC converter, where the synchronization and switching pulse generation processes are performed. During synchronization, integrated techniques such as Proportional Integral (PI), Fuzzy Logic Controller (FLC), and Improved Artificial Neural Network (IANN) are utilized to maintain the voltage, magnitude and phase angle in the same level. Consequently, the Inductance Capacitance (LC) filtering technique is applied to reduce the harmonics distortion in the signal. After that, the Park transformation is used to perform the dq0 to abc transformation, which is implemented for reducing the high volume of error. Finally, the error-free signal is fed to the three-phase grid system with reduced harmonics. During experimentation, both the simulation and analytical results have been taken for analyzing the performance of the proposed technique. Moreover, it is compared to the existing algorithms for proving the betterment of the proposed methodology.

Three-Level SOC Equalization Control Strategy for MMC-BESS Based on Feedforward Sliding Window Integral Method

The modular multilevel converter of the battery energy storage system (MMC-BESS) not only is suitable for the large-scale energy storage and dispatching of AC and DC grids, but also has a strong ability to suppress power fluctuations caused by the new energy output or grid failures. When an asymmetric voltage or a sudden change in DC load occurs in the AC grid, in order to compensate for the power difference between AC and DC sides, the energy storage submodule of the MMC-BESS will have a large unbalanced charging and discharging current, destroying the equalization state of SOC and seriously affecting energy storage capacity utilization and battery service life. In order to deal with the above problems, in this paper, the characteristics of the power difference between the MMC-BESS phase unit and the upper and lower bridge arms are analyzed. It is found that when considering the fluctuation of the submodule capacitor voltage, the phase unit power has the fundamental frequency AC circulating current component, and the power difference between the upper and lower bridge arms has the DC circulating current component. Therefore, the three-level SOC equalization correction control strategy is proposed based on interphase, upper and lower bridge arms, and submodules, and the feedforward sliding window integral method is introduced into the SOC equalization correction control layer of upper and lower bridge arms, so as to achieve the purpose of more balanced and accurate power distribution among phases and among upper and lower bridge arms of each phase. The simulation results show that the MMC-BESS has effective compensation ability when there is a large power difference in the AC and DC power grid. Under the unbalanced working conditions of the three-phase power grid, the three-phase AC current can quickly reach equalization, and the total harmonic content is 1.38%, and the unbalance degree is 2.4%. Under the same operating conditions, compared with the traditional optimal one-third average method, the SOC equalization correction control strategy proposed in this paper has smaller submodule capacitor voltage fluctuation rate, harmonic distortion rate, and three-phase system circulating current. And the SOC of each phase has a faster equalization convergence speed.

Decoupling Control Applied to the Smart Grid Power Dispatching Problem

This paper presents controller designs of the decoupling issue dedicated to renewable decentralized generators or new generation grids known as smart grids. The control methods were based on a three-phase voltage source grid representation that integrated wind and photovoltaic generators. The utilized decoupling strategies (Boksenbom and Hood, Zilkind and Luyben, and inverted decoupling) allowed the optimization of energy transfers and costs through the routing grid distribution path. Simulations were conducted in Matlab/Simulink. The trustworthiness of the multivariable control system design in addition to the tuning method was established using three test batches, totally decoupled and stable from input dependencies.

Phase locked-loop with decaying DC transient removal for three-phase grids

Frequency and phase of the power grid, which are critical for reliable control and protection of grid-tied devices, are generally detected by the closed-loop phase locked-loop (PLL). In highly inductive high-voltage transmission systems, decaying DC (DDC) components with large amplitude can be easily introduced by load disturbances and/or grid abnormalities, leading to severe performance degradation of the PLL during the transient. Focusing on this issue, in this paper, modifications to the conventional synchronous reference frame (SRF)-PLL have been made to address the short-term disturbances including the DDC component, and the system operation is divided into the normal state and the DDC-transient state. The SRF-PLL is only adopted for the normal state where the DDC component is negligible. In the presence of a significant DDC component, as well as disturbances including negative-/zero-sequence components and harmonics, the weak effectiveness of the conventional SRF-PLL is proved, and an efficient DDC component extraction method, with a detection time of 0.5 grid cycle, is introduced for the three-phase system. The real-time amplitude and phase of the positive-sequence component can be efficiently extracted via the proposed scheme, by exploiting the transient signal properties in the dq-frame and assuming a constant grid frequency during the short transient. Finally, a proper design of switching logic has been proposed to allow for the fast and precise transition between the normal and the DDC-transient state, thereby ensuring high steady-state accuracy as well as short-term DDC transient immunity. Hardware-in-the-loop based experiments have been used to verify the effectiveness of the proposed PLL technique.

Generalized Clarke Transformation and Enhanced Dual-Loop Control Scheme for Three-Phase PWM Converters Under the Unbalanced Utility Grid

Three-phase grids imbalance causes serious alternating input current distortion in three-phase pulsewidth modulation (PWM) rectifiers where the conventional control scheme is employed. In this article, a generalized Clarke transformation is proposed to convert three-phase unbalanced sinusoidal quantities to two orthogonal sinusoidal quantities with equal amplitudes. Then, these two orthogonal sinusoidal quantities could be converted to two direct quantities by the Park transformation. Furthermore, based on the generalized Clarke transformation, an enhanced dual-loop control (EDLC) scheme is proposed to control the three-phase PWM rectifier in the synchronous rotating frame. Finally, a 5-kW Vienna-type rectifier prototype was built to verify the proposed generalized Clarke transformation and the proposed EDLC control scheme. Experimental results show that, by using the proposed EDLC scheme, the low current total harmonic distortion is achieved even both the amplitude and phase of the utility grid voltages are unbalanced.

Integration of multi voltages, multi electric vehicle spots based three phase photovoltaic array charging station to the modern distribution grid with improved electric vehicle charging capability and power quality

This paper proposes the solar photovoltaic (PV) array-based three-phase modern grid integrated multi-voltage and multi electric vehicles (EVs) spots-based charging station (CS). It provides the charging spot of the EVs as per the required voltage level of the EVs battery. The presented CS is multifunctional. Besides, the EV charging capability, it also mitigates the power quality issues due to EVs and nonlinear loads tied at the point of common coupling (PCC). The voltage source converter (VSC) control is developed using the gradient adaptive variable step least mean square (GAVS-LMS) based technique. The presented control extracts the fundamental weight of load current to improve the operation of grid integrated charging stations and the power quality issues. The DC bus voltage is stabilized through the bidirectional converter using the cascaded proportional-integral (PI) control under heavy multi-voltage EV loads. As the charging station is subjected to heavy multi-voltage EV loads, which causes the unstable operation of the CS. Therefore, the small-signal modeling and stability analysis is also carried out in charging/discharging modes. The vehicle to the grid (V2G) and grid to vehicle (G2V) mode transfer are also carried out. These operations are also shown with and without the availability of solar PV generation. The efficacy of the presented CS and its controllers are tested with simulated as well as Opal-RT workbench based experimental results.

Identification of the phase connectivity in distribution systems through constrained least squares and confidence-based sequential assignment

This paper addresses the customer-phase identification problem in three-phase distribution grids including three-phase customers characterized by aggregated energy measurements. The proposed technique first solves a relaxed problem, in which the binary nature of the variables is ignored, which leads to a constrained, least-squares estimation, using as inputs the active and reactive energy readings provided by the smart meters, along with the energy delivered by each phase at the head of the feeder. With the estimated values of the decision variables, and their corresponding variances, a confidence-based selection technique is then applied for the sequential assignment of the customer with the highest joint probability of being connected to one of the three phases but not to the other two. The performance of the proposed procedure is assessed with five different scenarios in terms of accuracy for increasing number of loads and measurement errors. The robustness of the algorithm is additionally tested in the presence of model errors, and its performance is compared to that of existing methods.

Multilevel Converter With Nearest Level Control for Integrating Solar Photovoltaic System

This article presents a new Scott-ternary solar multilevel converter (ST-MLC)-based solar photovoltaic (PV) system for medium voltage applications. This system employs a single solar PV array as an input source and two voltage source converters (VSCs) to feed a three-phase grid. Solar power is fed from a two-phase to a three-phase medium voltage grid with balanced grid currents. The legs of VSCs are cascaded to form a two-phase system. The multiwinding transformers are connected in a Scott manner to develop a nine-level three-phase power conditioning system. The turn ratios of the transformers have ternary progression for the generation of higher levels. The modeling of the system is presented, and a closed-loop nearest level control strategy is incorporated to have minimum losses. Simulation results show the performance of the solar PV system under steady-state and dynamic irradiance. Varying solar profile is considered on basis of the daily average global solar irradiance from the geographical information system for New Delhi, India. The harmonic performance is analyzed for the converter voltage and grid current. Moreover, a detailed comparative analysis is covered for the ST-MLC to evaluate its superiority over its existing counterparts. Performance validation is done in a real-time test bench to validate the control and operation of the large-scale solar PV system.

A Method Without Stability Issue for Robust Estimation of Three-Phase Grid Frequency

The grid frequency estimation using a recursive signal or an integrator-based method may suffer from a stability issue due to an accumulation or discretization error. To avoid these shortcomings and also to obtain a robust estimation of three-phase grid frequency under unbalanced and distorted conditions, this letter proposes a demodulation-based method without using a recursive signal and integrator. The proposed demodulation-based approach relies on the Clarke transform and fixed transport delay-based relatively simple finite-impulse-response filters. It includes an open-loop structure, thus unconditionally stable. It also does not demand the real-time evaluation of the trigonometric or inverse trigonometric function. Moreover, it generates improved accuracy as compared with a frequency-locked loop and phase-looked loop.

Predictive Current Control of Six-Phase IM-Based Nonisolated Integrated On-Board Battery Charger Under Different Winding Configurations

Integrated on-board battery charging (IOBC) constitutes one of the future trends and the potential state-of-the-art technologies proposed for high-power chargers of electric vehicles. Model predictive control has recently been favored in different applications due to its simplicity in defining new control objectives and the straightforward handling of nonlinear constraints. In this article, the predictive current control (PCC) is applied to a six-phase induction-machine-based IOBC with three different winding configurations. From the grid perspective, this article introduces the required winding connections that maximize the charging grid current. Under PCC, different stator phases are controlled to draw balanced three-phase grid currents through controlling the machine nontorque-producing <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>xy</i></b> current components while ensuring zero average/ripple torque production. This article also discusses the effect of winding configuration on the mapping of the 64 available voltage vectors to the <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><i>αβ</i></b> , <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <i>xy</i></b> , and <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0⁺ 0⁻</b> subspaces. The optimal subset voltage vectors of each configuration that achieve the highest possible dc-link utilization, zero torque production, minimum total harmonic distortion (THD), and unity power factor are then introduced. The feasibility to employ the concept of virtual voltage vectors to improve the current quality is also investigated. The three six-phase configurations are obtained from an externally reconfigured 1 kW 12-phase induction motor, which has been used to experimentally validate the theoretical findings.

CPT-Based Control Strategy for a Two-Phase Three-Wire Pv Inverter: a Multifunctional Perspective and a Comprehensive Review

Multifunctional inverters play an important role in electrical grids due to their capability to provide active power conversion from photovoltaic (PV) systems, as well as by concomitantly offering ancillary services that provide grid support and power quality improvement. Although control strategies for such inverters have been extensively explored for applications in single- and three-phase grids, their consideration for the two-phase three-wire topology has been superficially discussed in literature. Thus, first, this paper contributes to the literature by presenting a comprehensive review about the control of power electronic inverters in two-phase three-wire grids. As second contribution, this paper proposed a flexible control strategy based on the Conservative Power Theory capable of adequately synthesizing control references for a PV-based multifunctional inverter operating in a two-phase three-wire grid. Additionally, power quality services can be selectively offered by the inverter to achieve compensation of reactive, harmonic and unbalance current terms, as well as providing reduction in neutral currents. Hardware-in-the-loop experimental results comprising a multifunctional PV inverter under several scenarios of operation are shown to demonstrate and validate the flexibility of the method.

Water Cycle Algorithm Based Parametric Tuning of Non-Negative LMMN Control of Grid Tied Renewable Energy Systems

ABSTRACT This work proposes a hydrological cycle-based optimisation technique for performance enhancement of a non-negative least mean mixed norm (NNLMMN) based voltage source control (VSC) for a three-phase grid integrated wind/photovoltaic (PV) system. Water cycle algorithm (WCA) is employed to tune the proportional–integral (PI) controller parameters of VSC to regulate voltage variations at DC link generated during dynamic load and wind conditions. This results in generation of a specific loss component (Iloss) of current to boost the performance of VSC. The NNLMMN control deals with system identification troubles with a non-negativity limitation in varied Gaussian noise conditions. It generates accurate Iloss to upgrade the ability of VSC to accurately extricate weight signals and the fundamental load current component in terms of improvement in DC link voltage (vdc) and total harmonic distortion. The proposed algorithm mitigates problems like voltage fluctuations besides performing operations like reactive power compensation, enhancement of power quality, load and power balancing at the point of coupling during steady-state and dynamic conditions. In conjunction with the NNLMMN algorithm, the proposed technique reduces the surplus mean square error iteratively, resulting in a compact expression of step size. The proposed topology stands out with an improved performance of vdc under wind gusts and the well-known bell-shaped irradiance curve. For relative evaluations, the performance of non-optimised PI and particle swarm optimised PI has been added to analyse the improved response of vdc and Iloss. Extensive simulation results obtained in MATLAB/Simulink are exhibited to corroborate the efficacy of the suggested system.

Rapid Extraction of the Fundamental Components for Non-Ideal Three-Phase Grid Based on an Improved Sliding Discrete Fourier Transform

In order to make an effective extraction of the fundamental components for a non-ideal three-phase grid, an improved sliding discrete Fourier transform (ISDFT) has been proposed in this paper. Firstly, the non-ideal signal characteristics are studied in detail, which reveals that there are not only typical harmonic components, but also double frequency components, that exist in dq coordinates when the three-phase grid voltages are unbalanced. Then, the structure form of the conventional sliding discrete Fourier transform (SDFT) has been redesigned to form the ISDFT algorithm, in which a special offset link is introduced to reduce the extraction time while the effectiveness is guaranteed. The experimental results show that this proposed ISDFT is suitable for types of non-ideal signals extraction and can keep a nice dynamical and steady performance in cases of grid or load disturbance. For the average extraction time, ISDFT is saving about 44.56% more of the time than SDFT and about 65.32% more than discrete Fourier transform (DFT).

Model-Free Predictive Power Control for PWM Rectifiers under Asymmetrical Grids

Conventional model predictive power control (MPPC) features a simple concept and quick dynamic response. However, it relies heavily on the system model and its parameter accuracy. Furthermore, the steady ripples are still high due to the use of one voltage vector during one control period. Recently, model-free predictive current control (MFPCC) has been proposed in the current control of PWM rectifiers. Despite the strong parameter robustness, the principle of MFPCC cannot be directly applied to power control, because the relationship between power and converter voltage is more complex. This paper first proposes a basic model-free predictive power control (MFPPC), which successfully extends the principle of MFPCC to power control. Subsequently, an improved MFPPC is proposed, which uses an extended finite control set of voltage vectors to improve the steady-state performance. Furthermore, by using the online updated ultralocal model of PWM rectifiers, the problem of stagnant power updating in basic MFPPC is solved. The ideal three-phase grid voltages are symmetrical and sinusoidal, but the actual grids are usually unsymmetrical. In this paper, the proposed method is extended to asymmetrical power grids by adding an appropriate compensated power to the original power references. The proposed basic MFPPC and improved MFPPC are compared to conventional MPPC. The presented experimental results confirm the effectiveness of the proposed methods.