Dq Current Control Research Articles

Comparative Study of Multilevel Converters using DQ Current Control

This paper presents a comprehensive comparative analysis of different multilevel converter topologies, including 2-level, Neutral Point-Clamped (NPC), Cascaded H-Bridge (CHB), and T-Type converters using the DQ current control strategy. The study aims to evaluate the performance of these topologies concerning current Total Harmonic Distortion (THD) and ripple in the direct and quadrature components of the current. A mathematical model of the grid is presented to facilitate the simulation study. The DQ current control scheme is implemented to regulate the current in both the direct and quadrature axes. The simulations are conducted under various operating conditions to capture the converters’ performance across different load scenarios and modulation techniques. Results indicate significant variations in the performance metrics among the different converter topologies. The 2-level converter exhibits higher THD and ripple compared to the more complex multilevel topologies.

A Transformerless Converter with Common-Mode Decoupling in Low-Voltage Hybrid Grids

Compared with isolated converters, transformerless converters are a preferred choice in low-voltage grids due to their efficiency and lower cost. However, leakage current and common mode (CM) voltage appear through the converter and ground in hybrid grids, which consist of AC and DC subgrids. The leakage current and CM voltage seriously influence operation and power quality in low-voltage distribution systems. This paper proposes a common-ground-type (CGT) converter equipped with a CM decoupling control strategy to eliminate the leakage current and CM voltage. A CM model is derived, and the leakage current and CM voltage are analyzed in detail. A CGT four-leg converter is constructed to eliminate the high frequency CM voltage. A dual DQ current control loop is developed to suppress the DC double-frequency ripple. Additionally, an active damping method is proposed, based on the neutral current feed-forward plus inductor current feedback, to attenuate the low frequency CM voltage. The proposed converter and control strategy guarantees excellent performance in suppressing leakage current and CM voltage. The DC voltage of the converter connected to the DC grid maintains stability and symmetry. The leakage current is significantly reduced, and the leakage current suppression performance is improved by 83%. The high frequency CM voltage is attenuated from 50%udc to 2%udc, and the low frequency CM voltage is suppressed from approximately 32%udc to 3%udc, which is a significant improvement compared with the traditional method. In addition, the proposed control strategy has good transient performance when the load changes abruptly. Finally, an experimental platform is established to validate the feasibility and performance. The experiment results showed that the proposed control strategy improves the system performance and power quality.

Three-Level T-Type qZ Source Inverter as Grid-Following Unit for Distributed Energy Resources

Distributed energy resources (DERs), such as solar photovoltaics (PVs), are required to achieve a high performance and an efficient use of generated renewable power in grid-integrated applications, both in normal and fault operating conditions. In this article, a grid-following (GFL) multifunctional control strategy for a three-phase three-level T-type quasi-impedance source inverter (3L-T-type qZSI) is studied through simulation and experimental tests. The GFL functionality is achieved by a proportional–integral (PI)-based dq current controller for the active and the reactive power tracking. A PI-based dc controller is used for the dc-link voltage control by taking advantage of the quasi-impedance source (qZS) network ability for input voltage boosting. The dc–ac power conversion is accomplished through a space vector pulsewidth modulation (SVPWM) with inner capacitors voltages balancing capability and minimum common-mode voltage (CMV) generation. Besides, a low-voltage ride-through (LVRT) strategy was implemented to fulfill the fault requirements imposed by the Spanish standard. The simulation and experimental results demonstrate the abovementioned functionalities and validate the stability and good dynamic response of the grid-connected 3L-T-type qZSI. Thus, this work supposes a novel contribution due to the few works previously reported in the literature concerning the performance of this relatively recent inverter topology in grid-tied applications.

Analysis and Design Guidelines for Current Control Loops of Grid-Connected Converters Based on Mathematical Models

Having a method for analyzing and designing regulators of controls that contain many current loops such as active filters is not a trivial task. There can be many parameters of regulators and filters that must be carefully selected in order to fulfill certain desired requirements. For instance, these can be stability, dynamic response, robustness under uncertainty of parameters, and rejection capability to switching harmonics. Hence, this paper provides general analysis guidelines for designing current control loops by using mathematical models in an αβ reference frame. Then, by using the proposed modeling tool, a multi-objective tuning algorithm is proposed that helps obtain all the control loops’ regulator and filter parameters, meeting all the desired requirements. Thus, the proposed analysis and design methodology is illustrated by applying it to three different controls conceived in a dq rotating reference frame with PI (Proportional Integral) regulators. The first control presents two current loops (simple dq current control), the second control uses four current loops (dual vector control, for unbalanced loads), while the third control presents eight current loops (active filter controlling current harmonics). Several experimental and simulation results show the effectiveness and usefulness of the proposed method. Since the mathematical model employed is in the αβ reference frame, it can also be easily applied to controls conceived in a αβ reference frame using resonant regulators, providing also a common comparative framework.

The Impact of VSFPWM on DQ Current Control and a Compensation Method

The variable switching frequency pulsewidth modulation (VSFPWM) is varying the carrier frequency of pulsewidth modulation (PWM) based on the prediction model to reduce the switching losses and lower the electromagnetic interference (EMI). However, it may cause some deterioration in digital dq current control of voltage source converters. This article takes the L -type grid-connected converter as the research object, and first presents the generation mechanism of the varying time delay caused by VSFPWM. In the case of proportional-integral current regulator implemented with VSFPWM, the varying time delay is accompanied by the frame rotation, which causes a varying phase lag in output voltages, degrading the system performance. Subsequently, an analytical method based on complex coefficient transfer functions is developed to provide an accurate explanation of the relation between the VSFPWM and the corresponding impacts. A compensation method is then proposed to eliminate the varying phase lag of output voltages when the VSFPWM is employed, improving the steady state and dynamic performance of dq current control. Finally, the simulation and experimental results are presented to verify the analysis and the effectiveness of the proposed compensation method.

Dynamic Modeling of Battery Energy Storage and Applications in Transmission Systems

In this paper, a Battery Energy Storage System (BESS) dynamic model is presented, which considers average models of both Voltage Source Converter (VSC) and bidirectional buck-boost converter (dc-to-dc), for charging and discharging modes of operation. The dynamic BESS model comprises a simplified representation of the battery cells, which allows to simulate the effects of battery degradation, dc-to-dc converter, VSC, and the dynamics associated with the filter and transformer connecting the BESS to the grid. A decoupled dq-current control is used for the VSC, allowing the operation of the BESS in several modes, i.e., constant active and reactive power, constant power factor, voltage regulation, frequency regulation, oscillation damping, and a combination of the latter two. The proposed model is implemented in DSATools and tested for different contingencies on a benchmark system, and compared with a industry-grade BESS model used in power system dynamic studies. The importance of modeling the current control and dynamics of the dc-to-dc are demonstrated, especially when the battery cells are degraded due to, for instance, aging.

Synchronous-Frame Modeling and dq Current Control of an Unbalanced Nine-Phase Induction Motor Due to Open Phases

Multiphase machines have several benefits compared with three-phase machines. Their inherently higher reliability makes them attractive in several applications. For instance, the nine-phase motor can operate with up to six open phases in its stator but with torque ripple production. However, the modeling and control schemes for a balanced motor cannot be used since now the stator is unbalanced. In order to allow the multiphase machine to work magnetically balanced even in open-phase conditions, an adequate dq current control should be employed. To achieve this control scheme, it is necessary to have a synchronous-frame modeling of a nine-phase induction motor under open-phase faults, which is developed here. Using it, a synchronous-frame control is derived, still using proportional controllers in the dq current loops. This is possible due to the use of an appropriate voltage decoupling considering the fault condition. Hence, the unbalanced multiphase induction motor can be controlled very similarly as a balanced one, additionally resulting in the torque ripple minimization. Experimental results characterize the operation with open phases. Simulation and finite-element results show the success of the proposed control scheme.

A Passivity-Based Control of Euler–Lagrange Model for Suppressing Voltage Low-Frequency Oscillation in High-Speed Railway

The traction network voltage low-frequency oscillation (LFO) in high-speed railways easily leads to the traction blockade of electric multiple units (EMUs), which seriously affects the normal operation of high-speed railways. A passivity-based control (PBC) strategy for single-phase EMUs rectifier is proposed in this paper. First, for the single-phase EMU rectifier, the Euler–Lagrange (EL) mathematical model in dq frame is built, which can realize the decoupling of active power and reactive power. Second, according to the unique characteristics of the rectifier, it is proven that the rectifier is strictly passive, which is the premise of PBC controller design. Third, using the deduced EL mathematical model and the passivity of rectifier, the PBC controller for the single-phase rectifier of EMUs is designed using the damping injecting method. Next, compared with dq current control and interconnection and damping assignment PBC (IDA-PBC), it can be verified that the PBC controller has better dynamic and static characteristics, and can significantly suppress the voltage LFO of traction network. In these strategies listed in this paper, using the PBC, the single-phase rectifier input current has minimal total harmonic distortion, and the dc-link voltage has minimal oscillation. Finally, the dSPACE semiphysical experiment platform of the cascade system of EMUs and traction network is built. Simulation results are validated by the dSPACE semiphysical experiments, which indicate that the PBC has the better inhibitory effect for LFO as compared with that of the IDA-PBC and dq current control in single-phase EMUs rectifier.

Large-Signal Impedance-Based Modeling and Mitigation of Resonance of Converter-Grid Systems

Large-signal impedance of grid-connected converters can be used to predict resonance-generated distortions in converter-grid systems. Note that the large-signal impedance of a network represents its impedance response for different magnitudes of perturbation injected at its terminals. This paper presents large-signal impedance-based modeling and mitigation of resonance of grid-connected voltage source converters. Challenges of large-signal modeling because of the inapplicability of the small-signal approximation are addressed by leveraging the dominating influence of hard nonlinearities (such as pulsewidth modulation saturation and limiters) over soft nonlinearities (such as Park's transformations and phase-locked loop (PLL)) in shaping the large-signal behavior of the converter. The paper develops large-signal gains of hard nonlinearities using different types of describing functions. The paper shows that the large-signal impedance of a voltage source converter (VSC) can be shaped to reduce resonance-generated distortions by inserting limiters in the control system of the VSC. Developed large-signal impedance models are validated using numerical simulations of a VSC with dq current control and PLL. Large-signal impedance measurements of a commercial 1 MW VSC-based inverter and a medium-voltage doubly-fed induction generator with approximate 4 MW rating are presented to experimentally demonstrate the influence of the injected perturbation magnitude on the impedance response.

Multistage energy management system using autoregressive moving average and artificial neural network for day‐ahead peak shaving

In this Letter, a multistage energy management system is developed and analysed incorporating the coordination between the tertiary stage and the primary stage. In the tertiary level, a day-ahead peak shaving strategy is developed using the autoregressive moving average and artificial neural network technique. These two techniques are used to predict customer's power demand and photovoltaic power generation, which are fed to a tertiary controller for the day-ahead power demand management. As the peak-power-demand management system is highly dependent on the demand-generation values, any fluctuations and errors in predicted values impact the performance of the peak shaving. The reference power generated from the energy management layer at the tertiary stage is communicated to the local inverter controller at the primary stage. The inverter implements a dq -current controller to track the reference power efficiently.

Low voltage ride through capability enhancement in a grid‐connected wind/fuel cell hybrid system via combined feed‐forward and fuzzy logic control

Fault ride through (FRT) capability is an essential practice as per the present grid code demands for grid-connected renewable energy-based distributed energy resources. Studies on FRT capability for grid-connected hybrid systems are rarely found. This study considers a wind energy conversion system and a fuel cell system interconnected at a common dc bus. It proposes a new feed-forward-based FRT control scheme for the inverter control where new current references in dq -axis frame are derived by tracking the positive sequence power. The newly derived references are fed forward to the input of the current regulator of the voltage source inverter. Second, fuzzy logic-based current controllers are suggested to improve the tracking capability of the current references in the inverter control scheme so as to enhance the FRT capability of the hybrid system as a whole. The proposed feed-forward-fuzzy control scheme for achieving an enhanced FRT capability is compared with the conventional dq current control and feed-forward FRT control for various grid voltage sag tests, where the performance of the combined feed-forward-fuzzy control is found better. The validation of the proposed FRT control scheme is performed in MATLAB-Simulink environment.

Analysis and Design of a Maglev Permanent Magnet Synchronous Linear Motor to Reduce Additional Torque in dq Current Control

The maglev linear motor has three degrees of motion freedom, which are respectively realized by the thrust force in the x-axis, the levitation force in the z-axis and the torque around the y-axis. Both the thrust force and levitation force can be seen as the sum of the forces on the three windings. The resultant thrust force and resultant levitation force are independently controlled by d-axis current and q-axis current respectively. Thus, the commonly used dq transformation control strategy is suitable for realizing the control of the resultant force, either thrust force and levitation force. However, the forces on the three windings also generate additional torque because they do not pass the mover mass center. To realize the maglev system high-precision control, a maglev linear motor with a new structure is proposed in this paper to decrease this torque. First, the electromagnetic model of the motor can be deduced through the Lorenz force formula. Second, the analytic method and finite element method are used to explore the reason of this additional torque and what factors affect its change trend. Furthermore, a maglev linear motor with a new structure is proposed, with two sets of 90 degrees shifted winding designed on the mover. Under such a structure, the mover position dependent periodic part of the additional torque can be offset. Finally, the theoretical analysis is validated by the simulation result that the additionally generated rotating torque can be offset with little fluctuation in the proposed new-structure maglev linear motor. Moreover, the control system is built in MATLAB/Simulink, which shows that it has small thrust ripple and high-precision performance.

DQ Current Control of Voltage Source Converters With a Decoupling Method Based on Preprocessed Reference Current Feed-forward

Axes cross-coupling in dq-frame will deteriorate the current control performance, especially for applications with low switching/sampling frequency. Decoupling methods based on inductor current state feedback and complex vector proportional-integrator controllers are usually performed to solve this problem. In this paper, an alternative decoupling method based on preprocessed reference current feed-forward is proposed, which can provide excellent decoupling performance even with considerable control delay. The current control of grid-connected voltage source converters is analyzed with complex-coefficient transfer functions. It is revealed that the decoupling performance of current control schemes is closely related to the symmetry of current closed-loop transfer function bode diagrams. The current closed-loop transfer function should show a standard symmetrical bode diagram with respect to 0 Hz, if axes cross-coupling is well decoupled. A quantitative index named cross-coupling function is then proposed to analyze and comprehensively compare the decoupling performance of the proposed method and existing ones in terms of the influence of the control delay, inductance estimation error, grid frequency deviation, and voltage disturbance. Experimental tests are implemented to validate the performance of the proposed method and comparison results.

Voltage control of unbalanced three-phase networks using reactive power capability of distributed single-phase PV generators

Summary In this paper, a developed voltage control method for distributed single-phase photovoltaic (PV) generators in unbalanced distribution networks is proposed. The reactive power capability of the PV inverters has been used to regulate bus voltages during irradiance or load variations. The amount of the reactive power required to optimize phase voltages is calculated using fuzzy logic control function, and hence, the power factor is precisely determined. The proposed control for the PV inverters is based on the dq-current control to regulate the DC bus voltage and the amount of the reactive power. A hysteresis current control is used as a pulse width modulation gate generator. The proposed control precisely determines the amount of the required reactive power to regulate the system bus voltages to lie within standard limits. The efficiency of the proposed control is evaluated with a 10-node, 5 kV/phase unbalanced three phase distribution system. The analysis includes different test cases under transient and steady state operations to ensure accurate representation of the actual system. The obtained results assess the quality of the network voltages of the studied system regarding voltage levels and the degree of the unbalance. Moreover, the proposed control is capable of operation under different power factors of the PV inverter.

Analysis of transient-to-island mode of power electronic interface with conventional dq-current controller and proposed droop-based controller

Distributed generation (DG) is predicted to play an increasingly important role in the future power system. DGs, however, can provoke technical problems when they are added to the existing system. An islanding is one of those problems while using DGs. During that mode, the power electronic interface of DGs has both roles of keeping the point of common coupling’s voltage within a required range and making islanded frequency arbitrarily at normal values. This paper investigates the operation response while keeping the conventional current controller in some cases of typical islanded loads. Then, it validates the feasibility of a proposed droop-based current controller for the islanded entity dealing with transient-to-island mode using PSCAD simulation. That proposed controller is fast switched from the conventional one using the negative-sequence impedance islanding detection method. The stability analyses of an islanded entity and islanded controller regarding frequency, voltage and transient period are also carried out in depth.

Grid‐connected photovoltaic power plants for helping node voltage regulation

This study presents a way of using the photovoltaic power plants for helping the grid to regulate the point of common coupling voltage, not only for injecting active power. This functionality of the photovoltaic resources has become a need as some countries’ standards propose renewable energy to increase the voltage when a sag happens. In addition, this philosophy could also be applied in smart grids where the power must be controlled by the distributed generation. In this study, active power injection is developed by a reference power point tracking and reactive power regulation is carried out by controlling the node voltage, both with dq current control under different power priority possibilities. The generic flow diagram of the control algorithm was implemented on a single-stage traditional inverter topology. The whole system has been checked with a simulation model, including the control algorithm, in order to tune the controller for obtaining the best performance. Finally, an experimental prototype was implemented for validating the proposed node voltage regulating method.

Internal Model-Based Current Control of the <italic>RL</italic> Filter-Based Voltage-Sourced Converter

Power system operates close to its operational limit to achieve a higher level of utilization of the grid infrastructure. As a result, its transient behavior is of great importance to prevent violation of the operation limits. This paper proposes an internal model control (IMC)-based approach for voltage-sourced converters (VSC) to improve their dynamic behavior. This IMC controller is designed based on a detailed model of the VSC. The proposed control approach has superior performance, i.e., faster step response and less overshoot, and higher axes decoupling than the conventional dq-current control method. The proposed IMC-based controller has a straightforward design procedure, and can be implemented using simple PI and PID controllers. Several case studies are presented to evaluate and compare the transient performance of the IMC-based controller with conventional dq current control subsequent to step changes in the set points of direct and quadrature axes current, single-phase fault, and three-phase fault.

Optimal Control of Traction Motor Drives Under Electrothermal Constraints

Peak torque and power density requirements for traction motor drives continue to increase, while demands on reliability are getting increasingly stringent as well. With the knowledge that most of the failure mechanisms are related to excessive temperature (cycling), thermal management is a key for increasing performance, without jeopardizing reliability. This paper proposes a control strategy for active thermal management of permanent magnet synchronous motor (PMSM) drives, based on real-time estimation and feedback of switching device and motor temperatures. By regulating the switching frequency and current control limit, critical components can be safeguarded from excessive temperature rise. Furthermore, optimal dq-current control vectors are calculated within the temperature and voltage constraints, to maximize the drive's efficiency and speed-torque envelope. Hence, the control strategy enables the drivetrain to operate safely at maximum attainable performance limits. The strategy is experimentally validated on an 11-kW PMSM drive for a number of representative vehicle loads, including a maximum standstill torque test, a maximum acceleration test, and a driving cycle test.

Voltage variation due to solar photovoltaic in distribution network

Grid integration of solar photovoltaic (PV) plant offers reduction in greenhouse emissions and independence from fossil fuels for power generation. The integration of such forms of power generation also brings with it a variety of policy and technical issues. One of the technical issues is the variation in grid voltages in the presence of solar photovoltaic (PV) plant, resulting in degradation of power quality. In this paper, the application of a dq current controller to limit the voltage variation at the point of common coupling (PCC) due to a 2 MW solar photovoltaic (PV) plant will be discussed. The controller's goal is to ensure that the voltage variation meets the momentary voltage change limits specified in TNB's Technical Guidebook for the connection of distributed generation. The proposed dq current controller is shown to be able to limit the voltage variation.

Control of an Electronically-Coupled Distributed Resource Unit Subsequent to an Islanding Event

This paper presents a new control strategy for islanded (autonomous) operation of an electronically coupled distributed generation (DG) unit and its local load. The DG unit utilizes a voltage-sourced converter (VSC) as the coupling medium. In a grid-connected mode, based on the conventional dq-current control strategy, the VSC controls real- and reactive-power components of the DG unit. Subsequent to an islanding detection and confirmation, the dq-current controller is disabled and the proposed controller is activated. The proposed controller utilizes (1) an internal oscillator for frequency control and (2) a voltage feedback signal to regulate the island voltage. Despite uncertainty of load parameters, the proposed controller guarantees robust stability and prespecified performance criteria (e.g., fast transient response and zero steady-state error). The performance of the proposed controller, based on time-domain simulation studies in the PSCAD/EMTDC software environment, is also presented.