Double Synchronous Reference Frame Research Articles

A New Approach for Voltage Balancing and Appropriate Power-Sharing in Autonomous Microgrids

This paper suggests a new control method to modify the virtual impedance performance in unbalanced conditions. The proposed method compensates the voltage drop that occurred due to the virtual impedance and adjusts the voltage of the point of common coupling at a desirable level. To compensate the voltage drop, the reference voltage in the droop control varies according to the proposed algorithm. Moreover, a modified decoupled double synchronous reference frame is introduced to achieve appropriate active and reactive power sharing and voltage balancing, simultaneously. Finally, the simulation results in MATLAB/Simulink are provided to validate the accuracy and effectiveness of the proposed approach.

Voltage unbalance compensation using new structure of decouple double synchronous reference frame in stand‐alone microgrids

In distribution networks, the voltage unbalance occurrence is inevitable. The voltage unbalance can have negative impacts on the customer equipment and the network performance if it exceeds the permissible limit. This study proposes a novel control strategy to compensate for the voltage unbalance in stand-alone microgrids. The proposed control method applies to the inverter-interfaced distributed generation units. The suggested control strategy is based on the decouple double synchronous reference frame (DDSRF) that is modified to improve its effectiveness. The modified DDSRF can effectively decouple the positive and negative sequence components. The simulation results depict that the proposed control strategy provides faster dynamic response and more effective voltage unbalance compensation comparing to the DSRF-based control method.

Integrated Battery Charger for EV by Using Three-Phase Induction Motor Stator Windings as Filter

This article presents an on-board charger for the electric vehicles (EVs) by integrating the drive-train components into the charging process. The stator windings of three-phase induction motor (IM) can be used as a grid interfacing inductor with the constraint that the rotor should be in the stationary position during the charging mode. This article proposes the stator winding configurations of three-phase IM split into two equal parts at zero electrical degree for the on-board integrated battery charger. The effective filtering with these two three-phase windings combination, either in series or parallel while the rotor is in a stationary position, is analyzed. The equivalent inductance provided by the stator windings for filtering is increased by generating the pulsating magnetic field (PMF) in the motor. However, it results in the unbalanced impedances in filter causing odd harmonics in the grid current. This is resolved by using a modified double synchronous reference frame control algorithm, which maintains the balanced current operation with the unity power factor at the grid interface during the charging mode. This configuration is obtained by using the existing three-phase-based drive-train components, thereby providing the saving in space and reducing the weight of EV. Experimental results for the proposed winding configurations are presented with filter losses and system efficiency.

Recursive Least Squares-Based Adaptive Parameter Estimation Scheme for Signal Transformation and Grid Synchronization

Utility-interfaced power electronic systems use a grid synchronizing framework, known as phase-locked loop and need a transformation of sinusoidal signals to rotating dq reference frame for control purposes. The voltage or current signal parameters, including instantaneous fundamental frequency, phase angle, and amplitude, need to be captured in the presence of harmonics, noise, and dc offset. This article proposes an adaptive estimation scheme for the same, using recursive least squares with time-varying covariance gains. Proposed adaptation of gain presents faster transient response and noise-tolerant steady-state response, achieving optimal tradeoff between the two. The covariance resetting mechanism is presented for a better dynamic profile during step changes in the signal. The scheme for the single-phase signal transformation is extended for transforming three-phase unbalanced sinusoids to the decoupled double synchronous reference frame. Stability analysis, design guidelines, and discrete-time realization of proposed methods are provided for reproducibility. Theoretical deductions of the proposed method are supported with several comparative test cases simulated in MATLAB/Simulink and the experimental results.

A delta-connected MMCC-based active power conditioner for unbalanced load compensation and harmonic elimination

Modular multilevel cascaded converters (MMCC) are seen in medium and high voltage applications and have gained attention from industry. This paper presents new control techniques to improve accuracy and control response speed when applied to a delta-connected MMCC- based Active Power Conditioner used for negative sequence and harmonic current elimination. A novel reference current generation scheme combining a notch filter with Decoupled Double Synchronous Reference Frame (DDSRF) is proposed which can extract the negative sequence and a selection of dominant harmonic currents accurately with the minimum time delay. Fast and accurate tracking the extracted reference current is obtained by using a model-based predictive controller with a derivative reference current term. The control strategy is applied to an experimental delta-connected MMCC-APC and the resultant waveforms are shown and analysed, confirming that the grid current quality after compensation has improved significantly.

New PQ theory for power quality improvement using modular multilevel converter based UPFC system

The source current harmonics reduction techniques were found to be unpredictable and disparity under different loading conditions. The presence of uncertainity issue in harmonics elimination is due to nonlinear loads. Filters can be used to eliminate the harmonics and power quality issues. But these filters are not cost effective to provide dynamic performance under various loading conditions. The target of this paper is to minimize source current harmonics with optimum voltage stability under different loading conditions. A new Unified Power Flow controller is developed whose series compensator is replaced by modular multilevel converter to achieve high modular level with reduced harmonics and fast current limiting during the fault short circuit and shunt compensator is replaced with four switches and one capacitor combination to achieve the twin benefit of more reliable power system and good voltage stability for different loadings. DDSRF (Decoupled Double Synchronous Reference Frame) theory is utilized in the proposed converter for generating the reference current from the AC supply. DDSRF theory generates sinusoidal harmonics with the opposite phase to the load current. The UPFC can suck or injects the responsive power in the PCC. After DDSRF theory, hysteresis controller is used to produce PWM pulse for the shunt and series compensator. The proposed DDSRF theory is compared with existing dq theory to show its effectiveness in terms of THD analysis. The PI and fuzzy logic methodology is utilized to control the capacitor DC rail voltage. The proposed approach is simulated using Matlab under various loading condition and hardware is developed using Spartan 6E FPGA Controller.

An Improved Harmonic Suppression Control Strategy for the Hybrid Microgrid Bidirectional AC/DC Converter

The nonlinear load in AC/DC hybrid microgrid seriously affects the power transmission quality of bidirectional AC/DC interlinking converter (BIC). In this article, an improved Fryze-Buchholz-Depenbrock (FBD) harmonic current detection method combined with droop control is designed to suppress harmonic current control strategy. The decoupled double synchronous reference frame phase locked loop (DDSRF-PLL) with self decoupled network is used to eliminate the negative sequence fundamental frequency component, and the accurate phase detection can also be realized when affected by nonlinear load. The least mean square (LMS) algorithm adaptive filter can enhance the dynamic response ability when the load current suddenly changes. To solve the problem that the cut-off frequency of low pass filter (LPF) can not take into account the detection accuracy and response speed of FBD harmonic current detection method. Finally, Matlab/Simulink is used for simulation verification. The simulation results show that the improved control strategy not only realizes the bidirectional flow of power and maintains the DC bus voltage stability, but also reduces the total harmonic distortion (THD) at point of common coupling (PCC) by 0.45% compared with the traditional FBD control strategy. And the effectiveness and feasibility of the design method are proved.

Active Power Transfer Enhancement in VSC using Impedance Compensated DSRF-PLL during Weak Grid Condition

In last two decades the use of renewable sources has been increased tremendously. This has increased the number of inverters connected to grid for the integration of energy from renewable sources. The stability of grid tied inverter is very much important with increased number of inverters. Especially in weak grid, which leads to voltage fluctuation, stability of inverter needs more attention. For a stable operation of VSC it is very important to detect the phase of grid voltage at point of common coupling (PCC) with proper accuracy. In this paper a Double Synchronous Reference Frame PLL (DSRF-PLL) with grid impedance compensation is presented. It is shown that with grid impedance compensation in phase detection, a VSC can be virtually synchronized at a stronger point of common coupling (PCC) in grid and this will enhance the active power transfer capacity of VSC with stable operation in weak grid condition. MATLAB simulation results are presented to verify the findings.

Advanced Energy Flow Control Concept of an MMC for Unrestricted Operation as a Multiport Device

An advanced approach of the internal energy flow control of a multiport device based on a modular multilevel converter (MMC) was proposed. Such a multiport device offers various connecting points as ac terminals, the main dc terminal as well as the dc terminals of the MMC modules. State-of-the-art multiport concepts have restrictions concerning the flexibility of connecting the available ports. The proposed concept makes an unrestricted energy interchange between all ports possible without affecting the ac grid and the main dc terminal. It contains a microscopic and macroscopic view of the entire system and an analytic description in a double synchronous reference frame. This approach allows the identification of degrees of freedom for the internal energy flow control to overcome the mentioned restrictions of state-of-the-art multiport devices. Beyond numerous simulations calculations, the proposed concept has been implemented to a prototype of a multiport device which consists of an MMC with integrated batteries in power electronics laboratory. The approach has been successfully tested and verified with measurement results.

A control scheme for voltage unbalance compensation in an islanded microgrid

One of the major power quality issues in low-voltage (LV) microgrids is the voltage unbalance which has adverse effects on electrical devices. This paper presents a novel control scheme to mitigate the voltage unbalance by a photovoltaic (PV) system. In this study, load current sensors are eliminated and the PV system mitigates the voltage unbalance by analyzing its terminal voltage. This could effectively reduce the cost and complexity of compensation system because of the difficulties of load current sensors. The proposed control strategy is developed for a three-phase PV. As the inverter detects the voltage unbalance at its terminal, the proposed control algorithm calculates the compensation reference currents based on the double synchronous reference frame (DSRF) analysis of the PV terminal voltage. Therefore, the PV inverter injects the compensating currents to the microgrid. The effectiveness of the control scheme is verified by simulation studies in the PSCAD/EMTDC environment.

Dual Current Control of Inverter-Interfaced Renewable Energy Sources for Precise Phase Selection

The unique fault current signatures of inverter-interfaced renewable energy sources (IIRES) hinders the correct operation of the existing phase selection methods (PSMs) that operate based on the sequence currents. As a result, the system resilience during unbalanced faults is imperiled. In this paper, a new dual current controller is developed to regulate the relative angles of the IIRES sequence currents during unbalanced faults to enable accurate identification of the fault type by the existing PSMs. The proposed method consists of three design stages. In Stage I, the IIRES control system identifies the fault type using the sequence voltages at its terminal. Stage II aims at generating the appropriate reference angle for the IIRES negative-sequence current. In Stage III, the IIRES sequence currents are controlled in the double synchronous reference frame to track the reference angle. The proposed controller makes the existing PSMs immune against fault conditions and reactive current generation requirements imposed by grid codes. Consequently, the protection system can select and trip only the faulted phase(s), and thus, the system reliability is considerably enhanced. Time-domain simulations are used to confirm the successful operation of the proposed controller under different fault conditions and grid codes.

An Implementation of Improved PLL for Control of Grid-Connected Converters Under Grid Perturbations

In this article, a new method is proposed for calculating electric angle using a phase-locked loop (PLL) to perform these calculations correctly under disturbance conditions of the grid voltage. This work is based on using the decoupled double synchronous reference frame PLL (DDSRF-PLL), and this kind of PLL has been expanded to solve its problems in the presence of harmonics and high disturbance in the input voltage. In the DDSRF-PLL, only components with a frequency of double the fundamental frequency will be deleted, but in the expanded PLL all the harmonics will be eliminated. The effect of this method was tested using both simulation and practical experimentation. The simulation of this method was performed in MATLAB software. It has been shown that this method is robust, precise and fast for calculating the electric angle under grid voltage perturbations. Subsequently, the developed PLL was implemented in a FPGA-based experimental setup and its function was studied.

Islanding Detection Using RT-Lab

As renewable energy is widely used, distributed power generation systems are also used in wide range. However, some problems in renewable power systems have to be addressed. Among these problems, the islanding operation has the most important impact to the safety of utility workers and the service lives of equipment. This paper studies islanding detection for a microgrid system with unbalanced loads and its implementation on a real-time simulator (RT-Lab) to accelerate simulations. The presented islanding detection approach utilizes rate of change of frequency (ROCOF), under/over frequency, and negative sequence current injection methods. Decoupled double synchronous reference frame software phase lock loop (DDSRF-SPLL) is used to synchronize the grid-connected power converter with the utility voltages under unbalanced load conditions. Two cases are tested in real time. The presented approach detects the islanding in 0.09 seconds after the fault occurs, and the voltage at the point of common coupling (PCC) returns stable in 0.1 seconds after the fault occurs, satisfying the IEEE Standard 1547-2003.

A Terminal Voltage Estimation and Unbalance Compensation Strategy Based on Double Synchronous Reference Frame for Shore Power Supply

In this paper, a strategy of terminal voltage estimation unbalance compensation for shore power supply is proposed. In this strategy, based on double synchronous reference frame, sequence components are extracted from the measured current and voltage fast and precisely. And then, the terminal voltage could be accurately estimated under the equivalent circuit in the form of vector. At last, a simple and robust method is applied for voltage compensation, from which independent adjustability on the magnitude and the phase of both positive and negative sequence of terminal voltage can be achieved. The proposed strategy is simulated in PSCAD. The simulation results show that, by this strategy, the terminal voltage can be rapidly compensated to be fully balanced and adjusted to the demanded phase within 15ms.

Effects of post-filtering in grid-synchronization algorithms under grid faults

The spread of distributed generation systems has reinforced concerns and requirements on grid-tied power converters. The synchronization with the utility voltage vector is a major concern. In the literature, algorithms based on Phase Locked Loops are extensively presented. In spite of adequate performances under ideal and balanced grid conditions, under grid faults great inaccuracies arise. Shortcomings are overcome by advanced algorithms at the expenses of the complexity and computational cost. In this paper, grid synchronization algorithms are addressed. A solution is proposed by introducing a new post-filter stage in a Decoupled Double Synchronous Reference Frame not affecting, at the same time, the complexity of implementation, the detection time and damping of the conventional DDSRF solution. The post-filtering stage design criteria are described based on a mathematical derivation of the phase error in a conventional DDSRF algorithm under distorted grid utility. A comparison of system performances with the conventional DDSRF algorithm is carried out under distorted, balanced and unbalanced utility conditions, validating the benefits brought by the proposed solution.

Simple current control method for three‐phase VSC under unbalanced grid condition

Injection of both positive and negative sequence currents is desired for three-phase voltage source converters (VSC) under unbalanced grid fault condition. However, the second-order oscillation exists in the positive and negative sequence synchronous reference frame. Several approaches have been reported to cope with this second-order oscillation. Here, a simple direct proportional plus integral current control method in double synchronous reference frame is proposed. The proposed method does not require the additional filter or decoupling circuit, and does not require signal reconstruction for the current reference. Theoretical analysis and experimental study show that the proposed current control not only achieves zero steady-state error, but also exhibits good transient response.

Fault ride through strategy of inverter-interfaced microgrids embedded in distributed network considering fault current management

Abstract This paper presents a fault ride through (FRT) strategy specific for a microgrid (MG) with an inverter-based interface (IB-MG) to maintain power exchange with distribution networks during network faults or disturbances. The fault current injected by IB-MG is controlled by its back-to-back converter interface. Besides guaranteeing the power quality of local loads in MGs during faults, the proposed IB-MG FRT could realize the fault current management (FCM) ability – instead of using a fault current limiter – to maintain the short circuit current level in a dense-load network. As the traditional three-phase FRT techniques conducted on the double synchronous reference frame are no longer applicable on IB-MG FRT, especially for unbalanced faults, a newly developed reference current calculation method conducted on the stationary reference frame is adopted and modified. By changing the six control variables – amplitudes and phases of the three-phase currents – based on the information of the fault voltages and the fault currents from the host network, IB-MG FRT could achieve multiple effects: (1) controlling instantaneous energy flow between the MG and the distribution network; (2) limiting the short-circuit current increase; (3) improving the MG system power quality. The application range of the proposed FRT strategy is also discussed. A 27 kV distributed network connected with a 5 MW IB-MG system was built in the MATLAB/Simulink software environment and the proposed IB-MG FRT strategy was demonstrated under the different unbalanced fault scenarios.

Performance Analysis of Three-Phase Phase-Locked Loops for Distorted and Unbalanced Grids

This paper studies the performances of five typical Phase-locked Loops (PLLs) for distorted and unbalanced grid, which are the Decoupled Double Synchronous Reference Frame PLL (DDSRF-PLL), Double Second-Order Generalized Integrator PLL (DSOGI-PLL), Double Second-Order Generalized Integrator Frequency-Lock Loop (DSOGI-FLL), Double Inverse Park Transformation PLL (DIPT-PLL) and Complex Coefficient Filter based PLL (CCF-PLL). Firstly, the principles of each method are meticulously analyzed and their unified small-signal models are proposed to reveal their interior relations and design control parameters. Then the performances are compared by simulations and experiments to investigate their dynamic and steady-state performances under the conditions of a grid voltage with a negative sequence component, a voltage drop and a frequency step. Finally, the merits and drawbacks of each PLL are given. The compared results provide a guide for the application of current control, low voltage ride through (LVRT), and unintentional islanding detection.

Model predictive control strategy for NPC grid‐connected inverters in unbalanced grids

A model predictive control (MPC) strategy for a three-phase three-level neutral point clamped (NPC) grid-connected inverter is discussed. Conventional two-level inverters are effective when a symmetric fault occurs. But when a three-phase unbalanced fault occurs, they will generate errors and introduce delay in the power system because of the complex design or the neglected negative-sequence component of the grid. To solve this problem, a new method to control a NPC grid-connected inverter is proposed, which combines an MPC strategy with a decoupling double synchronous reference frame phase-locked loop technique. The proposed method reduces the use of PI controllers and enhances the system's response speed. Using simulations and experiments, the proposed MPC strategy for a three-level NPC grid-connected inverter is proven to be accurate and reliable.

Experimental Investigation of the Response of Different PLL Algorithms to Grid Disturbances

In this paper, the performance of three different phase locked loop (PLL) algorithms is experimentally evaluated in a grid connected photovoltaic inverter. Robust PLL is a key requirement for ensuring system compatibility with existing grid codes. The double second order generalized integrator (DSOGI) and the decoupled double synchronous reference frame (DDSRF) PLL are compared against the conventional synchronous reference frame (SRF) PLL under grid disturbances. All synchronization algorithms are initially simulated in Matlab and subsequently transformed in discrete time for implementation in the digital controller. Grid distortions, such as phase voltage sags, frequency fluctuations and voltage harmonic distortion are emulated via a controllable three phase generator.