Stationary Reference Frame Research Articles

Control strategy for three-phase four-switch active power filter based on positive sequence extraction in stationary coordinate fundamental reference frame

Traditional three-phase four-switch active power filter often uses the method of phase-locked loop (PLL) and low-pass filter for harmonic detection, which seriously affects the detection accuracy. In order to improve the detection accuracy, a lock-free three-phase four-switch active power filter based on fundamental positive sequence extraction is proposed. The harmonic detection adopts the fundamental positive sequence extraction method, and the rotation coordinate axis transformation adopts the phase-locked loop (PLL) free method. Compared with the traditional three-phase four-switch active power filter, this method does not require a phase-locked loop and a low-pass filter, and does not need complex coordinate transformation[2].The control method is simpler. Finally, simulation results prove the effectiveness of the control method.

Optimized Distributed Cooperative Control for Islanded Microgrid Based on Dragonfly Algorithm

This study introduces novel stochastic distributed cooperative control (SDCC) in the context of island microgrids (MGs). A proportional resonant (PR) controller and virtual impedance droop control in stationary reference frames are employed in cooperation with distributed averaging secondary control optimized by the dragonfly algorithm (DA). The suggested approach demonstrates the capability to achieve mean-square synchronization for the voltage and frequency restoration of distributed generators (DGs) to ensure efficient active power sharing. Therefore, a sparse communication network has been used to avoid data congestion and reduce the need for extensive communication and information exchange. The proposed system offers an instinctive compromise between voltage regulation and reactive power sharing. A conventional centralized secondary control with PR droop control is simulated for performance evaluation and comparison purposes. In this study, empirical evidence is demonstrated to support the MG’s ability to confront communication failure and its ability to work reliably during plug-and-play operations.

A High Precision Initial Position Estimation Method for Low Saliency Ratio Machine Based on Image Tracking

In sensorless control of permanent magnet synchronous motor (PMSM) drives, initial rotor position detection is critical to ensure smooth start-up operation. However, extracting initial position information for machines with ultra-low saliency ratios is challenging. To address this issue, this paper proposes an estimation method for rotor position using image tracking based on high-frequency (HF) rotating voltage vector injection. Specifically, this paper extracts positive- and negative-sequence currents in the stationary reference frame and reconstructs the induced current vector. By utilizing the support vector machine (SVM) algorithm, the resulting current vector images, which contain amplitude and phase information of the position, are trained to establish the correlation between the current image and rotor position information. Ultimately, the electric position can be obtained from image recognition. Since this method employs image tracking for position estimation, it accounts for cross saturation and multiple saliencies, leading to the relatively high precision of position estimation. Experimental evaluation shows that the proposed method achieves higher precision compared to conventional observers.

Optimization of design parameters of the vibration protection system of a motor grader seat with quasi-zero stiffness

BACKGROUND: Vibrations occurring during operation of land transport-technological machines deteriorate the quality of control, reduce the productivity of the work performed, adversely affect the health of machine operators. In order to reduce the impact of vibrations on the operators, vibration protection systems of the cab and seat are used.
 AIMS: For practical application of the developed design of the passive vibration protection system of the motor grader operators seat on the basis of a parallelogram mechanism, tensile spring, cable and rollers, it is necessary to develop an algorithm for optimizing the values of design parameters.
 METHODS: The mean square value of seat acceleration in a stationary reference frame, determined as a result of simulation of the motor grader motion over a set of stochastic microprofiles of the support surface with various characteristics and velocities, was used as an optimization criterion. Comprehensive simulation mathematical model of a motor grader with cabin vibration protection supports and vibration protection system of a seat was used. All parameters of the optimization algorithm and model were divided into fixed, random and varying ones. The latter include a number of dimensions of the parallelogram mechanism of the seat vibration protection system, the coefficient of viscous friction of the mechanisms shock absorber, the height of the quasi-zero stiffness zone. The dependence of the criterion on the design parameters of the seat vibration protection mechanism, which has an implicit pattern and is determined by means of a simulation mathematical model, served as the target function. Boundary conditions were imposed on a number of parameters to ensure the operability of the mechanism, manufacturability of its parts as well as ergonomic considerations. Optimization of the values of the varying parameters was performed with the simplex method. At the same time, a part of the parameters unambiguously influencing the value of the target function was preliminarily maximized or minimized within the boundary ranges.
 RESULTS: An algorithm for assignment and optimization of design parameters of the vibration protection system of a motor grader seat with quasi-zero stiffness is developed, one of the key features of which is the possibility to adjust the vibration protection system of the seat to the weight of the current operator. Examples of algorithm application are given.
 CONCLUSIONS: The algorithm makes it possible to determine the optimal values of design parameters of the seat vibration protection system based on a parallelogram mechanism such as: the dimensions of the parallelogram mechanism, including the attachment points of rollers and cable, the coefficient of tensile spring stiffness, the coefficient of viscous friction of a shock absorber, the parameter of adjusting the mechanism to the weight of the current operator. The key feature of the developed vibration protection system and the optimization algorithm of its design parameters is the possibility of adjustment to the weight of the current operator.

PQ-Based Local Voltage Regulation for Highly Resistive Radial Distribution Networks

Modern distributed energy resources (DER) consisting of energy storage coupled with a voltage source converter (VSC) provides flexible and dynamic operations in active and reactive power quadrants. These technologies present significant opportunities to provide distribution network support instead of traditional approaches if effective controls are in place. Although voltage regulation using reactive power is typical, active power is more effective at manipulating voltage in highly resistive distribution networks. This paper proposes a local controller that regulates VSC terminal voltage and allows fast switching of the VSC power factor output, with minimized disturbance on the terminal voltage. This improves the effectiveness of the VSC in managing voltage in highly resistive distribution networks using both active and reactive power and provides a strategy to manage upstream power flow. The approach uses gain equations derived from the voltage sensitivity to power injection and the point on wave switching in the stationary reference frame. The control strategy is verified through detailed simulation in MATLAB/Simulink using a modified IEEE 4-node test feeder, including performance during a switched load, DER disturbance and controller interaction via voltage droop. The small-signal model and stability analysis of the proposed approach is also presented.

The Truths of Space-time Contractions of Special Relativity

This paper points out that there is no any experimental evidence for the length contraction of a moving object in special relativity. It is just a theoretical prediction based on the Lorentz transformation formula. Einstein put forward the concept of simultaneous relativity in order to explain the length contraction. In this paper, an example called Ji Hao's bridge-breaking paradox is provided to prove that this kind of paradoxes cannot be explained by simultaneous relativity. A completely symmetric method is introduced to prove that the famous twin paradox is unsolvable. The time delay experiments of special relativity, such as the life time of μ meson and the atomic clocks moving around the Earth are discussed. It is proved that time slows down of a moving clock does not exist too. It is a misunderstanding to use the lifetime of μ mesons to prove the time delay of special relativity because μ mesons decay prematurely due to strong collisions with other nuclei in the Earth's surface atmosphere. What calculated in theory is the time difference between two atomic clocks flying east and west observed in the stationary reference frame of the Earth's mass center. But the measurement of time difference is on the surface of the Earth. Because of the symmetry of motion speed, there is no time difference caused by the motion speed between the two atomic clocks observed on the Earth’s surface, so the experiment of atomic clocks moving around the Earth is invalid. The experiment is also suspected of fabricating experimental data. The conclusion of this paper is that the space-time contraction of special relativity and its relativity cannot happen in real nature, time and space are absolute concepts, and the Lorentz transformation cannot be correct.

Predictive Power Control of PMSG based WECS: Development and Implementation for Smooth Grid Synchronisation, Balanced and Unbalanced Grid

In this paper, the distributed generation unit is controlled to operate in both stand-alone (SA) and grid-connected (GC) modes and offers a smooth transition between these modes using the model predictive control scheme. The permanent magnet synchronous generator (PMSG), along with a 2-level back-to-back converter, is named a DGU. A simple and intuitive approach using predictive control is presented in a stationary reference frame for a load-side converter (LSC). A voltage control loop is used during SA mode to fix the voltage and frequency at the load end. The smooth synchronization technique is developed without a phase-locked loop. During GC, a predictive power control regulates active and reactive power during the symmetry and unbalanced grid voltage conditions. The designed strategy makes the power factor improvement, reduction of grid current harmonics, and elimination of twice grid frequency ripple from power during grid unbalance possible. This work has proposed a solution to reduce the overall computational burden on the processor by eliminating the state prediction in each sampling period. The machine-end converter control is formulated in the rotor flux reference frame to keep the power flow across the dc-link capacitor constant. Comprehensive experiments are conducted with the designed scaled laboratory prototype to successfully validate the control’s theoretical claim.

Pulse Train Control Strategy for Autonomous Three-Phase Voltage Source Inverter in Stationary Reference Frame

Pulse train (PT) control strategy is proposed for autonomous three-phase voltage source inverter (3ph-VSI) in stationary reference frame. Compared with classical control strategies for 3ph-VSI system, the proposed PT control strategy of 3ph-VSI utilizes the combination of two different duty cycles to approximate the desired duty cycles of 3ph-VSI. The load voltage can therefore be regulated. Without error amplifier and its corresponding compensation circuit, the proposed control scheme features fast transient performance and simple operation. The control loop of the proposed control strategy is simple to design. The operation principle and structure of the proposed PT-controlled 3ph-VSI are introduced. Its implementation and control parameter design are presented as well. The effects of the control parameter on the performance of the proposed control method are verified by simulation results. Besides, the fault-tolerance analysis of the circuit parameters of the proposed control scheme is provided. A 1.0-kVA experimental prototype of 3ph-VSI is built to verify the proposed control technique. Experimental results of the proposed control technique are provided to compare with conventional Ziegler Nichols (ZN)-PI control method, which show that the steady-state, load transient and output frequency transient performance of the proposed PT-controlled 3ph-VSI can be maintained over a wide output power range of 0-1000VA.

Multimode Control for Power Quality Improvement in an Electronically Coupled Distributed Generation Unit Under Grid Connected and Autonomous Modes

This paper develops a new multimode control strategy for power quality (PQ) improvement in distributed generation (DG) units that employs a voltage source converter (VSC) as the electronic interface medium. The devised control strategy seamlessly operates for both the grid connected (GC) and autonomous/stand-alone (SA) modes of microgrid (MG). A novel computationally efficient comb-filter driven discrete time oscillator is developed for current reference generation (CRG) under GC operation. It constitutes a cluster of zeros to introduce notch peaks at integer multiples of the fundamental frequency thereby ensuring improved harmonic rejection capability. Consequently, it ensures unconditionally balanced and sinusoidal grid currents with improved PQ to comply with grid interconnection codes prescribed in the IEEE std-519 and std-1547 even under electrical anomalies. In the SA mode, the CRG scheme is developed in a stationary reference frame with an improved vector proportional integral (VPI) compensator. Despite the uncertainty of load parameters, the proposed VPI guarantees zero steady-state error and superior transient response. A finite state machine-based synchronization framework is proposed to ensure secure and seamless transfer of MG between GC and SA even under disturbances. The practical viability of the proposed multimode control has been demonstrated by applying various switching rules in both GC and SA modes of MG operation.

Sensing Mechanism and Real-Time Bridge Displacement Monitoring for a Laboratory Truss Bridge Using Hybrid Data Fusion

Remote and real-time displacement measurements are crucial for a successful bridge health monitoring program. Researchers have attempted to monitor the deformation of bridges using remote sensing techniques such as an accelerometer when a static reference frame is not available. However, errors accumulate throughout the double-integration process, significantly reducing the reliability and accuracy of the displacement measurements. To obtain accurate reference-free bridge displacement measurements, this paper aims to develop a real-time computing algorithm based on hybrid sensor data fusion and implement the algorithm via smart sensing technology. By combining the accelerometer and strain gauge measurements in real time, the proposed algorithm can overcome the limitations of the existing methods (such as integration errors, sensor drifts, and environmental disturbances) and provide real-time pseud-static and dynamic displacement measurements of bridges under loads. A wireless sensor, SmartRock, containing multiple sensing units (i.e., triaxial accelerometer and strain gauges) and a Micro Controlling Unit (MCU) were utilized for remote data acquisition and signal processing. A remote sensing system (with SmartRocks, an antenna, an industrial computer, a Wi-Fi hotspot, etc.) was deployed, and a laboratory truss bridge experiment was conducted to demonstrate the implementation of the algorithm. The results show that the proposed algorithm can estimate a bridge displacement with sufficient accuracy, and the remote system is capable of the real-time monitoring of bridge deformations compared to using only one type of sensor. This research represents a significant advancement in the field of bridge displacement monitoring, offering a reliable and reference-free approach for remote and real-time measurements.

Current Measurement Offset Error Compensation for IFOC Induction Motor Drives

In vector control, the accuracy of the current measurement is crucial for ensuring good control performance. However, current measurement offset error is caused by various reasons in the current sensors, the op-amplifiers, and the A/D conversion circuits, such as thermal drift and nonlinearity. The measurement offset error results in undesired current ripples, which deteriorates the speed and torque control performance of the vector-controlled drive systems. This paper proposes the PI-type observer-based current offset error compensation method for the induction motor (IM). The proposed PI-type observer can estimate the DC offset error during the motor operation, which is designed in the stationary reference frame. This study analyzes the effect of the parameter mismatch and confirms the robustness of the parameter mismatch. Simulation and experimental results are provided to verify the performance of the current measurement offset error compensation method.

Estimation of Position and Shorted Turns Percentage of an Inter-Turn Short Circuit in Interior Permanent Magnet Synchronous Machines Based on a Current Observer and Stationary Reference Frame Tracking

This paper presents a method to detect and diagnose an inter-turn short circuit in interior permanent magnet synchronous machines. The proposed method detects the inter-turn short circuit fault using a current observer to estimate a disturbance related to the fault, defined as a fault index current. The fault index current estimates in the rotor reference frame have information about the location and the percentage of shorted turns. A novel stationary frame tracking technique is proposed to simultaneously determine the fault index current in the stationary frame and the fault position. Look-up tables are implemented to correlate the stationary frame fault index current magnitude to the percentage of shorted turns, providing an estimate of the percentage of shorted turns. Simulation and experimental results are shown throughout the paper to analyze and validate the proposed technique.

An adaptive control strategy for grid-forming of SCIG-based wind energy conversion systems

One of the crucial issues in the expansion of isolated wind energy conversion systems (WECSs) is the design of a grid forming control structure that is robust to changes of the system’s operating point in the steady-state as well as transient conditions. This paper has addressed the voltage/frequency control of a local microgrid as well as flux and slip control of a squirrel cage induction generator that feeds the microgrid via a back-to-back converter in an isolated WECS. An adaptive nonlinear voltage control structure has been proposed for load side converter (LSC) consists of a constant frequency voltage regulator and a current controller based on the Lyapunov function method (LFM), which are connected in cascade. In an outer loop, the reference current of LSC is generated by the voltage regulator designated for maintaining the voltage/frequency of the microgrid. In an inner loop, the reference signals are robustly tracked by the current controller. In a novel reference frame that its angle difference with the stationary reference frame is determined by a dual loop DC link voltage controller, an LFM-based nonlinear flux controller is proposed for the machine side converter (MSC). Using the proposed flux control structure, in addition to maintaining the flux magnitude at a nominal value, by controlling the generator slip through adjusting the speed of the proposed reference frame, the power balance is established and the DC voltage is maintained at a pre-set value. The performance of the proposed controllers has been investigated through simulation in the MATLAB® software. The obtained results confirm the robustness and stability of the proposed control structure with respect to various disturbances and uncertainties.

Sliding Mode with Equivalent Control for Induction Motor Drive Based on Multi-Pulse VSC

In this work, the application of a sliding mode equivalent control for an induction motor drive based on a multi-pulse Voltage Source Converter is proposed. This is accomplished by transforming the squirrel-cage induction motor mathematical model into a block control form. The equivalent control method is applied to the sliding manifold, where the voltages are obtained in the stationary reference frame to control the angular velocity of the motor. In addition, a proportional-integral control is designed and applied to the motor represented in this form in order to make performance comparisons with the equivalent control method. Multi-pulse converters of 6, 12, and 84 pulses are used directly to feed the plant that is being controlled. The outcomes of using these power electronics devices are used to perform comparisons among the velocity tracking performance, control voltages in stationary reference frame, tracking errors, and rejection of external disturbances. Furthermore a dynamic and steady-state analysis of the velocity tracking performance is executed. The energy profiles for the startup, torque variations, and steady-state are also obtained. Additionally, total harmonic distortion values are added. All comparisons are carried out using the established plan for the motor, both with and without the use of power electronics converters, and with both control algorithms. The obtained results demonstrate that the use of a high-quality voltage source converter, along with a good control strategy, allows for a general improvement in the overall system and a significant reduction in energy usage, whilst also reducing controller complexity.

NKG2020 transformation: An updated transformation between dynamic and static reference frames in the Nordic and Baltic countries

Abstract Coordinates in global reference frames are becoming more and more common in positioning whereas most of the geospatial data are stored in registries in national reference frames. It is therefore essential to know the relation between global and national coordinates, i.e., the transformation, as accurately as possible. Officially provided pan-European transformations do not account for the special conditions in the Nordic and Baltic countries, namely crustal deformations caused by Glacial Isostatic Adjustment. Therefore, they do not fulfill the demands for the most accurate applications like long-term reference frame maintenance. Consequently, the Nordic Geodetic Commission (NKG) has developed customized and accurate transformations from the global ITRF to the national ETRS89 realizations for the Nordic and Baltic countries. We present the latest update, called the NKG2020 transformation, with several improvements and uncertainty estimates. We also discuss its significance and practical implementation for geodetic and geospatial communities.

Voltage Control Strategy for Improving Disturbance Rejection Performance in Stationary Reference Frame During Parallel Operation of Multimodular UPS

This paper proposes a voltage control strategy to improve the disturbance rejection performance during the parallel operation of multi-modular uninterruptible power supply (UPS). In the standalone operation of UPS, feed-forward control can improve the dynamic performance of output voltage regulation. However, a circulating current can occur when feedforward control is applied in the parallel operation of UPS modules, causing uneven load sharing among the modules. In this study, the reason for the occurrence of the circulating current was analyzed using a circulating current impedance model. A partial feedforward control technique was proposed, which compensates for a portion of the load current by considering the circulating current impedance in the parallel operation of the UPS. However, this reduced the disturbance rejection performance compared with the standalone operation of the UPS, which uses full feedforward compensation gain. Therefore, an additional feedforward control technique was proposed to enhance the disturbance rejection performance. The effectiveness of the proposed control strategy was verified through the experimental result of the parallel operation of 500 W modular double-conversion UPSs.

Improved Power Control of DFIGs Driven by Wind Turbine under Unbalanced Grid Voltage

AbstractUndesired oscillation components appear in active and reactive powers, electromagnetic torque and DC-link voltage of doubly fed induction generators (DFIGs) connected to unbalanced grid voltage. These components oscillate at double source frequency as a result of negative sequence components in voltage and current. Different direct power control (DPC) techniques were studied in literatures to damp these oscillations. However, these techniques require sequence decomposition process, axes transformation of stator voltage/current and estimation of different power components which complicate the overall control system. This paper presents a simplified DPC of DFIGs in stationary reference frame under normal and unbalanced grid voltage. Decomposition process, axes transformation and compensation power terms are totally eliminated. Vector proportional- integral (VPI) controllers are designed to regulate stator active and reactive powers. The performance of the proposed DPC scheme using VPI and proportional-integral-resonant (PIR) controllers is analyzed and compared under different operating conditions. Bode diagram of open loop and closed loop control using VPI and PIR are studied to illustrate stability, steady state and transient response of the two controllers. Also, the performance of proposed technique and previous DPCs designed in synchronous reference frame is compared to prove the validity of proposed one. The results show that proposed DPC using VPI has superior performance in steady state and transient conditions with simple implementation.

Low voltage ride through control strategy for grid-tied solar photovoltaic inverter

Abstract This paper presents a low voltage ride through (LVRT) control strategy using an active power oscillations based reference current generation approach for grid tied solar photo voltaic (SPV) system under line-to-ground (LG) and double line-to-ground (LLG) faults. Proposed control strategy minimizes the harmonics injected into the grid, as well as oscillations in injected reactive power and DC-link voltage. Further this control strategy provides a dynamic control of active and reactive powers, the flow of sinusoidal currents into the grid, and avoids the disconnection between the utility grid and SPV system during faults. A proportional resonant (PR) current controller is developed in a stationary reference frame to achieve the power quality by fulfilling the grid codes. Simulation and experimental results are presented to highlight the merits of the proposed control scheme.

Development of enhanced direct torque control for surface‐mounted permanent magnet synchronous motor drive operation

AbstractDirect torque control (DTC) is one of the most prominent control techniques used by permanent magnet synchronous motor (PMSM) drives in industry applications. Nevertheless, the presence of hysteresis controllers and inaccurate voltage switching table in traditional DTC results in higher torque and flux ripple. This study proposes an enhanced DTC functioned Surface‐mounted PMSM (S‐PMSM) drive with mitigation of torque and flux ripple. The operation relies on generating the reference voltage vector (VV) in a stationary reference frame, which supports control of torque and flux without hysteresis controllers. The reference VV generation is simple and does not affect control robustness. The position of reference VV in a sector is used to build the voltage vector (VV) switching table. As a result, the application of nearest discrete VV to reference VV produces optimal torque and flux control. Moreover, redundant switching combinations of null VV are effectively used for possible minimization of switching frequency of two‐level voltage source inverter (VSI) supplied S‐PMSM drive. Therefore, proposed DTC gains improved S‐PMSM drive response along with switching frequency reduction. In dSPACE‐RTI 1104 platform, experimental response of S‐PMSM drive under various operating conditions have been depicted to highlight the proficiency of proposed DTC in comparison with existing DTC.

A Multiobjective Control Strategy for Harmonic Current Mitigation With Enhanced LVRT Operation of a Grid-Tied PV System Without PLL Under Abnormal Grid Conditions

This article proposes a novel multiobjective control strategy (MOCS) for improving the performance of grid-tied photovoltaic (GTPV) system without phase-locked loop (PLL) operating under abnormal grid voltage conditions such as harmonically distorted voltages, grid faults, and weak grid. The proposed control strategy effectively mitigates the harmonics in grid injected current, eliminates the oscillations in grid injected power, and enhances the low-voltage ride-through (LVRT) capability simultaneously during abnormal grid conditions. The LVRT capability of the GTPV system is enhanced by incorporating an active power regulator (APR) with dynamic reactive power support (DRPS) controller in the proposed control strategy. The proposed MOCS is designed in the <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> stationary reference frame. Furthermore, the proposed MOCS consists of an advanced phase compensated multiresonant (APCMR) current controller and a phase-shifted reference current generator (PSRCG) integrated with the APR with DRPS controller. The APCMR suppresses the grid current harmonics under weak and distorted grid voltages and the PSRCG eliminates active and reactive power oscillations at the grid side caused by grid faults. The synchronization mechanism of the GTPV system is included in the PSRCG, and therefore, the requirement of PLL is avoided. Consequently, the associated computational burden on the digital controller is reduced. The effectiveness of the proposed control strategy is validated using the OPAL-RT digital simulator and offers superior performance compared to the conventional control strategy (COCS).