Decoupled Power Control Research Articles

Adaptive perturb and observe maximum power point tracking with current predictive and decoupled power control for grid-connected photovoltaic inverters

In order to improve maximum power point tracking (MPPT) performance, a variable and adaptive perturb and observe (P&O) method with current predictive control is proposed. This is applied in three-phase three-level neutral-point clamped (NPC) photovoltaic (PV) generation systems. To control the active power and the reactive power independently, the decoupled power control combined with a space vector modulation block is adopted for three-phase NPC inverters in PV generation systems. To balance the neutral-point voltage of the three-phase NPC grid-connected inverter, a proportional and integral control by adjusting the dwell time of small voltage vectors is used. A three-phase NPC inverter rated at 12 kVA was established. The performance of the proposed method was tested and compared with the fixed perturbation MPPT algorithm under different conditions. Experimental results confirm the feasibility and advantages of the proposed method.

Power-Decoupling of a Multiport Isolated Converter for an Electrolytic-Capacitorless Multilevel Inverter

This paper presents a generalized power-decoupling control scheme using a multiport isolated bidirectional converter for a multilevel inverter, which has multiple dc links inside. In the proposed method, a single power-decoupling capacitor is needed for all the dc links in the multilevel inverter cell. First, a prototype of the power-decoupling concept of individual H-bridge cells in the multilevel inverter is proposed, using a separate power-decoupling circuit. Then, a more advanced one-step power-decoupling method is proposed. The lifetime and reliability of the multilevel inverter is improved as film capacitors replace the large capacitance electrolyte capacitors. A multi-input ports/single output voltage-fed dual half-bridge converter (MDHB) is used for the power-decoupling circuit. A steady-state analysis for the peak and root mean square of the MDHB current is carried out for the loss breakdown. The currents are functions of the switching frequency, phase shift, leakage inductance, turn ratio, and output voltage, which make the multiport transformer design complex. A design methodology is proposed, which takes into account the design of the copper and core losses as functions of the switching frequency and number of turns. Furthermore, a special winding method for the input port is illustrated to obtain identical leakage inductances for the uniform current distribution in the multiport transformer. The proposed MDHB employs a current-sensorless power-decoupling control that contributes to the spontaneous ripple rejection of all the dc links without individual link current information, as well as to the cost and size reduction. Hence, the ripple-rejection controller is independent of the control configuration of the multilevel inverter, and also available for universal applications of various inverter topologies. Since the primary-input ports of MDHB share a single magnetic core for interfacing the ripple power to the unified secondary ripple capacitor, the controller design becomes difficult in considering the dynamic interaction among the ports, along with the average-voltage control loop design. In this paper, the dynamic analysis and controller design procedure of the circuit is also presented. The power decoupling is achieved even when the ripple frequency is other than the double frequency of the inverter output, since the single-pole transfer function of the small-signal model of the MDHB allows sufficient phase margin, along with high bandwidth. The proposed power-decoupling method for the multilevel inverter is validated with the help of simulation and 1.2-kW hardware prototype experimental results.

Sensor Fault Tolerance Enhancement of DFIG-WTs via Perturbation Observer-Based DPC and Two-Stage Kalman Filters

This paper presents a sensor fault-tolerant control (SFTC) strategy, combining perturbation observer-based direct power control (PODPC) and two-stage Kalman filter (TSKF) to enhance the fault tolerance of a doubly fed induction generator-based wind turbine (DFIG-WT) subject to rotor and stator current sensor faults. In the PODPC scheme, the interactions between active and reactive power control loops are represented by newly introduced perturbation states, and the feedback linearization control is realized with the state estimations derived from perturbation observers to achieve the decoupled power control. No rotor current sensors nor parameters of DFIG-WT are required in the implementation of PODPC. Stator current TSKFs are designed to generate residuals for fault detection and isolation, and provide current estimations to replace the faulty current measurements for system reconfiguration under stator current sensor faults. Simulation studies undertaken on a grid-connected DFIG-WT system reveal that the proposed SFTC strategy is immune to rotor current sensor faults, and it provides strong fault tolerance to stator current sensor faults.

Robust Rotor-Current Sensorless Control of Doubly Fed Induction Generators

This letter presents a robust rotor-current sensorless control (RCSC) scheme for decoupling control of active and reactive powers of a doubly fed induction generator (DFIG). The RCSC scheme does not involve power control loops and rotor-current measurements, which simplifies the implementation and improves the sensor fault tolerance of the system. Besides, RCSC provides fully decoupled power control, and it is robust to model uncertainties. Case studies are undertaken on a DFIG system to verify the control performance of the scheme.

Model Predictive Control of Dual-Mode Operations Z-Source Inverter: Islanded and Grid-Connected

This paper presents model predictive control of dual-mode Z-source inverters (ZSIs) with capability to operate in islanded and grid-connected mode. The transition from islanded to grid-connected mode and vice versa can cause significant deviation in voltage and current due to mismatch in phase, frequency, and amplitude of voltages. The proposed controller offers seamless transition between the two modes of operations. The main predictive controller objectives are direct decoupled power control in grid-connected mode and load voltage regulation in islanded mode. The proposed direct decoupled active and reactive power control in grid-connected mode enables the dual-mode ZSI to behave as a power conditioning unit for ancillary services such as reactive power compensation. The proposed controller features simplicity, seamless transition between modes of operations, fast dynamic response, and small tracking error under steady-state condition of controller objectives. The operation of the proposed system is verified experimentally.

Enhanced Automatic-Power-Decoupling Control Method for Single-Phase AC-to-DC Converters

Existing control schemes for single-phase ac-to-dc converters with active power-decoupling function typically involve a dedicated power-decoupling controller. Due to the highly coupled and nonlinear nature of the single-phase system, the design of the power-decoupling controller (typically based on the small-signal linear control techniques) is cumbersome, and the control structure is complicated. Additionally, with the existing power-decoupling control, it is hard to achieve satisfied dynamic responses and robust circuit operation. Following a recently proposed automatic-power-decoupling control scheme, this paper proposes a nonlinear control method that can achieve enhanced large-signal dynamic responses with strong disturbance rejection capability without the need for a dedicated power-decoupling controller. The proposed controller has a simple structure, of which the design is straightforward. The control method can be easily extended to other single-phase ac-to-dc systems with active power-decoupling function. Simulation and experimental results validate the feasibility of the proposed control method on a two-switch buck–boost PFC rectifier prototype.

Direct control of active and reactive power of a doubly fed

This paper proposes a direct power control for doubly fed induction machine for variable speed wind power generation. It provides decoupled regulation of the primary side active and reactive power and is suitable for both electric energy generation and drive applications. In order to control the power flowing between the stator of the doubly fed induction generator and the network, a decoupled control of active and reactive power is synthesised using PI controllers. The obtained simulation results show the feasibility and effectiveness of the suggested method.
 Keywords: Doubly fed induction machine, decoupled power control, vector control, active and reactive power, PWM inverter.

Model Predictive Control of a Voltage-Source Inverter With Seamless Transition Between Islanded and Grid-Connected Operations

Inverter-based distributed generation (DG) system is becoming an attractive solution for high penetration of renewable energy sources to the main grid. DG system should be able to supply power to the local loads whenever necessary even in case of utility power outage. Thus, the inverters in DG systems are expected to operate in both grid-connected and islanded mode, where they are acting as a current source for the ac grid and a voltage-source for the load, respectively. Transition between modes of operation is nontrivial and can cause deviations in voltage and current, because of mismatch in frequency, phase, and amplitude between the inverter output voltage and the grid voltage. Thus, it is necessary to have seamless transition between grid-connected and islanded mode. This paper presents a new control strategy with seamless transfer characteristics for a grid-connected voltage-source inverter using model predictive control (MPC) framework. The main objectives of the proposed predictive controller are: 1) decoupled power control in grid-connected mode, which enables the proposed power electronics interface to provide ancillary services such as reactive power compensation; 2) load voltage control in islanded mode; and 3) seamless transition between modes of operation through proposed synchronization and phase adjustment algorithm. The proposed controller features simplicity to implement since only one cost function should be minimized for all modes of operation, and hence no ambiguity in the control algorithm that could cause mode transition problems. An autotuning strategy for weight factors in MPC cost function is proposed to simplify the weight factor tuning strategy. The stability analysis of the proposed controller is provided. Simulation and experimental results validate the expected performance and effectiveness of the proposed control strategy.

Analysis and implement of the single‐phase voltage‐controlled grid‐connected inverter

This study describes the design and implementation of an inverter control algorithm with both the inverter inner controllable impedance and governor-free characteristics. The inverter controlled as a voltage supply works with open-loop power control in grid-connected operation. It is realised that a pulse-width modulation inverter mimics the primary frequency and voltage modulation characterised similarly to a synchronous generator. On the basis of the proposed control algorithm, the inner controllable impedance and governor-free characteristics are analysed mathematically. Simultaneously, the authors analyse the output power characteristics of the proposed inverter and the level of the decoupled power control for designing the inner controllable impedance. When the inverter connects with the stiff grid, it will appear as the output current deviation problem. The generation mechanism of the dc component from the proposed inverter is clearly identified. The current deviation compensation control is analysed and implemented to solve it. Simulations and experimental results are given to verify the idea.

Current-Sensorless Power-Decoupling Phase-Shift Dual-Half-Bridge Converter for DC–AC Power Conversion Systems Without Electrolytic Capacitor

In this paper, a novel DHB power-decoupling control scheme without current sensor is proposed for single-phase inverters. As electrolyte capacitors are conventionally used; however, these capacitors limit ripple current capability and circuit reliability. Film capacitors improve the ripple current capability, size reduction, and circuit reliability. Conventionally, non-isolated topologies are used for power decoupling. The volume of the decoupling capacitor per unit energy in isolated bidirectional power decoupling topologies is reduced as compared to nonisolated due to unrestricted voltage across the decoupling capacitor. Voltage-fed Phase-Shift Dual Half-Bridge (DHB) is preferred with film capacitors as it has the least number of components in isolated bidirectional topologies. However, in DHB, power decoupling controller is a challenge as current control is conventionally required. The challenge has been overcome with novel current-sensorless dc link ripple rejection control. The controller has the advantage of current-sensorless configuration especially when it needs information from inverter phase-lock-loop (PLL). The proposed power decoupling control scheme is independent of the inverter control and universal. A new dynamic analysis has been carried out by taking into account the input-voltage dynamics. The advantage of the DHB is that it has single pole behavior and, hence, sufficient bandwidth can be obtained. Simulations and experiments have been performed to verify the analysis of the power decoupling control scheme and the capability of film-capacitor DHB for power decoupling.

Constrained decoupled power predictive controller for a single‐phase grid‐tied inverter

This study presents a decoupled active and reactive power control technique, for a single-phase grid-tied inverter, using model predictive control (MPC). The proposed technique does not use conventional phase-locked loop, pulse-width modulation nor a synchronisation transform, which makes the control algorithm well suited for an all-digital implementation. The proposed controller minimises the number of switching state transitions required to control the grid-side current while simultaneously constraining the harmonics distortions and protecting the inverter from overcurrent condition. In this study, the switching frequency is reduced by using a look-up table to minimise the number of switching state transitions, which helps lowering down the switching losses. The proposed technique uses an adaptive weight factor which gives more priority to commanded power tracking during transient, and minimises the tracking error and switching frequency in steady state while constraining the harmonics distortion. This method improves the tracking performance as well as reduces the switching losses by minimising the switching frequency compared to the MPC with fixed weight factor and conventional decoupled power control. Thus, the outcome of proposed controller is a constraint multi-objective optimisation between the switching frequency reduction and grid-side current harmonics in terms of cost function with weighting factor.

PWM Plus Secondary-Side Phase-Shift Controlled Soft-Switching Full-Bridge Three-Port Converter for Renewable Power Systems

In this paper, a pulsewidth modulation (PWM) plus secondary-side phase-shift (SSPS) controlled full-bridge three-port converter (FB-TPC) is proposed for a renewable power system, which is derived by integrating two interleaved buck/boost circuits (IBBC) and a SSPS full-bridge converter (FBC), to interface the energy source, energy storage element, and output load simultaneously. The two switching legs are shared by the IBBC and FBC, which improves the active devices sharing ratio and power density. With PWM plus SSPS control scheme, decoupled power control can be realized and the converter can achieve wide input voltage range power conversion. In addition, zero-voltage switching (ZVS) of both the primary and secondary switches are achieved in a wide range. Furthermore, the circulating current can be greatly eliminated, which helps to reduce conduction losses. Moreover, the voltage stresses on the secondary rectifier are effectively suppressed since the power switches and diodes are clamped to output voltage. The operation principles, output characteristics, ZVS performances, and control strategy are analyzed in detail. Experimental results of a 600-W prototype are provided to verify the effectiveness and the advantages of the proposed converter.

Linear and Sliding-Mode Control Design for Matrix Converter-Based Unified Power Flow Controllers

This paper presents the design and compares the performance of linear, decoupled and direct power controllers (DPC) for three-phase matrix converters operating as unified power flow controllers (UPFC). A simplified steady-state model of the matrix converter-based UPFC fitted with a modified Venturini high-frequency pulse width modulator is first used to design the linear controllers for the transmission line active (P) and reactive (Q) powers. In order to minimize the resulting cross coupling between P and Q power controllers, decoupled linear controllers (DLC) are synthesized using inverse dynamics linearization. DPC are then developed using sliding-mode control techniques, in order to guarantee both robustness and decoupled control. The designed P and Q power controllers are compared using simulations and experimental results. Linear controllers show acceptable steady-state behavior but still exhibit coupling between P and Q powers in transient operation. DLC are free from cross coupling but are parameter sensitive. Results obtained by DPC show decoupled power control with zero error tracking and faster responses with no overshoot and no steady-state error. All the designed controllers were implemented using the same digital signal processing hardware.

Adaptive decoupled power control method for inverter connected DG

The integration of renewable energy technology is making the power distribution system more flexible, but also introducing challenges for traditional technology. With the nature of intermittent and less inertial, renewable energy-based generations need effective control methods to cooperate with other devices, such as storage, loads and the utility grid. The widely used power frequency (P–f) droop control is based on the precondition of inductive line impedance, but the low-voltage system is mainly resistive, and also the different load character needs to be considered. This study presents an adaptive droop control method based on online evaluation of power decouple matrix for inverter connected distributed generations in distribution system. Traditional decoupled power control is simply based on line impedance parameter, but the load characteristics also cause the power coupling, and alter the equivalent system impedance characteristic. In this study, the decoupled matrix is evaluated by the ratio of the variations of active power and reactive power under a small perturbation on the voltage magnitude. As analysed, the proposed decoupled control method can take the effects both the transmission line and the load characteristics into account, and it will be more practical, and flexible in various network and load conditions. The performance has been validated by simulation and experimental results.

An improved sensorless decoupled power control scheme of grid connected variable speed wind turbine generator

This paper proposes an improved sensorless control for a variable speed-constant frequency generation system based on a doubly fed induction generator (DFIG). The rotor position and speed estimation method is based on a modified phase-locked-loop (MPLL). Using the measured stator voltages and rotor currents, the proposed algorithm can directly generates the rotor position and speed by simple arithmetic operations using the PLL basis. The speed is estimated independently of the machine parameters and the algorithm avoids using differentiation, which results in a substantial improvement in control robustness and improves its immunity to noise. This paper also presents the design of a space-vector-based hysteresis current control (SVBHCC) technique applied to a three level neutral-point-clamped (NPC) inverter to enhance the control system performance and achieve high power quality. The algorithm allows a systematic application of zero-voltage vectors that leads in reducing the average switching frequency. Furthermore, the space-vector-based approach reduces the interference between the commutations of the three phases. Simulation results confirm the effectiveness and validity of the proposed control approach.

Decoupled Direct Control of Natural and Power Variables of Doubly Fed Induction Generator for Extended Wind Speed Range Using Feedback Linearization

The electromagnetic torque, stator power, and the grid side converter power outputs of a doubly fed induction generator (DFIG) are differentiated to obtain a feedback linear relationship with the rotor voltage vectors serving as the control inputs. The derived model is such that can allow for a transition of DFIG to operate at low wind speed by shorting its stator terminal to one another. The torque and power quantities are the state variables of the system, which are insensitive to coordinate system orientation. Steady-state models are used to obtain optimal operating regimes with respect to minimal electrical losses. Decoupled torque control is developed by field orientation, while decoupled power control is derived from voltage orientation of the DFIG. Transients originating from transitioning into low wind speed operation are investigated. Robustness of each of the decoupled controllers against parameter variations is investigated by locating the poles of its closed-loop transfer function. The results for a 5-Hp machine are presented.

An Improved Direct Decoupled Power Control of Doubly Fed Induction Machine Without Rotor Position Sensor and With Robustness to Parameter Variation

In this paper, coupling between active and reactive powers in conventional direct power control (DPC) strategies is analyzed and a new direct DPC method for doubly fed induction machine without rotor position sensors is presented. Coupling analysis is done on an improved DPC strategy with rotor flux controllers in the stator reference frame. The presented control strategy is done by controlling the rotor flux in the grid flux reference frame. The rotor flux command is calculated using a predicted stator flux, the stator current command, and the stator resistance. Moreover, the rotor position is estimated by comparing measured and estimated values of the rotor current. Furthermore, to reduce the method's sensitivity to the parameter inaccuracies, the mutual inductance of the machine is updated during the machine operation by the error between the magnitudes of the measured and estimated values of the rotor current.

A Versatile Rotor Position Computation Algorithm for the Power Control of a Grid-Connected Doubly Fed Induction Generator

This paper proposes a simple yet versatile rotor position computation algorithm (RPCA) for the rotor position and speed of a doubly fed induction machine. The rotor position is computed in a straightforward manner obviating the need for estimation. The resolved components of stator flux vector are computed using measurable stator and rotor quantities. Although it is an open-loop technique, it does not involve integration, recursive techniques, recomputations or programmable low-pass filters, etc. Starting on the fly, accurate computation near and through the synchronous speed, immunity against fluctuations in the grid voltage, and frequency are the other advantages of the algorithm. Reduced complexity and the computational burden facilitate the easy implementation of the algorithm on a low-cost fixed-point processor. Furthermore, unlike most techniques reported so far, there is no need to assume a constant stator flux since the algorithm is versatile to allow for variations in the grid or stator flux. For the purpose of verification, the proposed RPCA, employed in decoupled power control, is implemented for the laboratory test bench with a 3-hp doubly fed induction generator (grid-connected) system and a TMS320LF2407A DSP controller. The efficacy of the algorithm is demonstrated through the extensive experimental results. Overall, the very encouraging results endorse the proposed algorithm.

Direct active and reactive power control of single-phase grid-tie converters

In this study, a simple digital power control technique for single-phase grid-tie converters is proposed. The suggested technique is based on the application of dead-beat control theory to the instantaneous powers in the virtual two-axis reference frame. A voltage estimation scheme is added to the proposed direct power control algorithm that allows grid voltage sensorless operation. The simulation and experimental results confirm that the proposed control strategy provides fast, accurate and decoupled power control with a lower alternating current distortion.

Decoupled Power Control of Doubly-Fed Induction Generator Based on an Implicit Position and Speed Estimation Algorithm without the Flux Computation

Owing to the increased attention on the need for deployment of renewable energy sources such as the wind energy, there is a growing interest in Doubly Fed Induction Generators (DFIG) which are suitable for variable speed constant frequency generation. Accurate information of the rotor position with respect to the stator is central to the effective decoupled control of real and reactive power in the DFIG. Typically sensorless estimation of the rotor position involves complex signal processing. This paper presents the implementation of a simple and implicit sensorless algorithm for estimating the rotor position and speed for decoupled control of the doubly-fed induction generator. The proposed algorithm does not involve the computation of stator magnetic flux directly or indirectly using the voltage integration, recursive techniques, re-computations or programmable low pass filters etc. Instead, it employs simple analytical equivalents for the resolved components of the stator flux which are derived from the basic equations governing the machine operation. Since the flux terms are replaced with other equivalent terms, the estimation process is devoid of computing any flux terms and requires only the measured stator voltage and the currents in stator and rotor windings apart from the machine parameters. Starting on the fly, accurate estimation near synchronous speed, immunity against variation in stator and rotor resistances or in the magnetizing inductance are the principal advantages of the algorithm. Reduced complexity and the computational burden facilitate the easy implementation of the algorithm on a low cost fixed point processor. Excellent agreement between the simulation and test results on a laboratory doubly-fed induction machine validates the proposed estimation algorithm. Further MATLAB/Simulink based simulation results are presented to validate the efficacy of the position estimation algorithm based vector control of a doubly-fed induction generator.