Grid Voltage Disturbances Research Articles

Performance improvement in grid-connected fuel cell power plant: An LPV robust control approach

Abstract This paper presents a robust control scheme for power electronic interfaces of grid-tied fuel cell power plant. Firstly, a linear matrix inequality (LMI)-based PID controller is designed to control the duty cycle of the boost DC/DC converter, aiming to adjust the output DC voltage to an arbitrary reference value. Afterwards, linear parameter-varying (LPV) technique is utilized for robust control of grid-connected inverter to follow the desired real and reactive power demand of the grid, considering time-varying load and uncertain grid-impedance. Taking uncertainty of the system into account, all admissible variations are considered as a convex polytope. To guarantee a prescribed disturbance attenuation level and improve the transient response of the closed-loop system, H∞ approach is exploited with pole placement consideration in terms of LMIs. Simulation results of the proposed controller compared to those of conventional PI controller, in presence of grid-voltage disturbance, validate high efficiency of the designed control.

Novel grid voltage estimation by means of the Newton–Raphson optimisation for three‐phase grid connected voltage source converters

This study presents a proportional resonant current control scheme for three-phase grid connected voltage source converters without any grid voltage sensors. Towards this goal, the grid voltage sensors are replaced by a voltage estimation scheme based on the Newton–Raphson algorithm. In the proposed method, the grid connected converter behaviour is converted to an optimisation problem which relates the instantaneous grid voltage value to the filter parameters, the generated converter voltage and the exchanged powers in each sampling period. The proposed optimisation problem is then solved in each sampling period by means of the Newton–Raphson optimisation technique. Afterwards, the reference current for the current control loop is generated from the estimated grid voltages. Using an estimator instead of the voltage sensors not only reduces the size and cost of the converter system but also improves its reliability and protects it from grid voltage disturbances and noises. The excellent performance of the proposed grid voltage sensorless scheme is confirmed through experimental and simulation results.

Dynamic Voltage Restorer for Fault Ride through of Doubly Fed Induction Generator — A Solution for a Case Study at 110/11KV Substation in India

The wind power penetration has been increasing significantly year by year. Most of the Wind Turbine Generators (WTGs) are designed with induction generators. The induction generator based wind farms have poor fault ride through capability and any system disturbance may affect the wind power generation. The disturbances are very common, if the WTGs are connected to weak grids. One such problem of frequent tripping has been encountered at Anthiyur substation in India. Subsequently, a power quality study has been undertaken in a 110/11KV substation at Anthiyur for examining the causes for tripping. From the study, it is found that frequent grid voltage disturbances have caused the tripping. Since the contribution of wind power is in predominant percentage, maintaining its stability becomes an important issue. A simulation based solution is proposed to improve the reliability of Doubly Fed Induction Generator (DFIG) based wind farms. In the proposed strategy, Dynamic Voltage Restorer (DVR) is used to compensate the grid voltage disturbances. The DVR can compensate the faulty line voltage and enhance fault ride through capability of DFIG wind farms for a stable operation as demanded in actual grid codes. Especially, vector control technique is utilizedhere to control the DVR. The proposed control scheme is very effective to protect the DFIG and DVR from disturbances and faults.

Improving Low Voltage Ride Through Capability of Wind Generators Using Dynamic Voltage Restorer

Abstract The increasing wind power integration with power grid has forced the situation to improve the reliability of wind generators for stable operation. One important problem with induction generator based wind farm is its low ride through capability to the grid voltage disturbance. Any disturbance such as voltage dip may cause wind farm outages. Since wind power contribution is in predominant percentage, such outages may lead to stability problem. The proposed strategy is to use dynamic voltage controller (DVR) to compensate the voltage disturbance. The DVR provides the wind generator the ability to remain connected in grid and improve the reliability. The voltage dips due to symmetrical and unsymmetrical faults are considered for analysis. The vector control scheme is employed for fault compensation which uses software phase locked loop scheme and park dq0 transformation technique. Extensive simulation results are included to illustrate the control and operation of DVR.

Behavioral modeling of grid-connected photovoltaic inverters: Development and assessment

This paper describes and assesses a behavioral model for grid-connected photovoltaic inverters. The model allows us to simulate the electrical behavior of commercial single-phase and three-phase inverters in accordance with the limits of EN50160 power quality. Regarding power strategies for current inverters, both voltage and current control loops have been explicitly modeled, providing suitable simulations of the injected AC-current waveform under either power dynamics or grid voltage disturbances. Additionally, irradiance oscillations and dynamic Maximum Power Point Tracking performance have been also considered in the proposed solution. Internal inverter variables are not needed to fit the model parameters, being estimated from basic data-sheet information provided by manufacturers and simple AC-collected values from the PV power plants. This characteristic avoids any additional DC-side measurement, being a significant contribution in comparison with previous approaches.The proposed model is assessed using real data collected in Spanish photovoltaic power plants along several years. The obtained results are compared with previous behavioral model approaches and also included in the paper.

Direct Grid Current Control of LCL-Filtered Grid-Connected Inverter Mitigating Grid Voltage Disturbance

A direct grid current control strategy for LCL-filtered grid-connected inverters is proposed in this paper. The conventional current control strategies are analyzed and compared, and then the necessity of direct grid current control is presented to mitigate the grid voltage disturbance. In the proposed control strategy, the virtual resistance based on the capacitance current is used to realize active damping, zero compensation is brought in to enhance the stability, and the proportional resonant (PR) controller under two-phase static coordinate is designed to track the ac reference current as well as to avoid the strong coupling brought by the coordinate transformation. Under the distortion grid voltage, the PR plus harmonic compensator (PR+HC) structure is adopted to restrain the distortion of the grid current. Finally, the proposed control strategy is verified by the experimental results.

Power Feedward Strategy of Grid-Connected Inverter Based on Direct Power Decoupling

Take into account grid inverter that using direct power control is vulnerable to the disturbance of grid voltage, a control algorithm is proposed based on the relationship between the power and voltage, which is deduced according to the mathematic model of the inverter that using double-loop control strategy. The change amount of the output power of inverter is added to the reference of power closed-loop as power feedforward, which improves the dynamic response to the grid voltage change. Simulations with EMTDC/PSCAD are carried out and the comparison of output power response to grid voltage change between control with and without power feedforward verifies its effectiveness.

Control Strategy for Three-Phase Grid-Connected Converters under Unbalanced and Distorted Grid Voltages Using Composite Observers

This paper proposes a novel scheme for the current controller for the grid-side converter (GSC) of permanent-magnet synchronous generator (PMSG) wind turbines to eliminate the high-order harmonics in the grid currents under grid voltage disturbances. The voltage unbalance and harmonics in three-phase systems cause grid current distortions. In order to mitigate the input current distortions, multi-loop current controllers are applied, where the positive-sequence component is regulated by proportional-integral (PI) controllers, and the negative-sequence and high-order harmonic components are regulated by proportional-resonance (PR) controllers. For extracting the positive/negative-sequence and harmonic components of the grid voltages and currents without a phase delay or magnitude reduction, composite observers are applied, which give faster and more precise estimation results. In addition, an active damping method using PR controllers to damp the grid current component of the resonant frequency is employed to improve the operating stability of VSCs with inductor-capacitor-inductor (LCL) filters. The validity of the proposed method is verified by simulation and experimental results.

Two fuzzy-based direct power control strategies for doubly-fed induction generators in wind energy conversion systems

This paper proposes two novel (direct power control) DPC strategies for a doubly fed induction generator (DFIG)-based wind energy conversion system based on a (fuzzy logic controller) FLC. At first, the mathematical model of the DFIG in the synchronous reference frame is derived. Then, based on this model, two novel FLC-based DPC strategies, called (fuzzy-based DPC) FDPC and (fully fuzzy-based DPC) FFDPC are proposed. In the FDPC, the required rotor voltages to eliminate power errors within each fixed sampling period are directly calculated based on the FLC, the measured active and reactive powers, the stator voltage and some machine parameters. On the other hand, in the FFDPC, the rotor voltages are directly calculated from the FLC. The control structures of proposed methods are very simple. Compared to the conventional switching table-based DPC, in the proposed methods, the hysteresis comparator and the switching table are replaced by a simple FLC and a PWM (pulse width modulation) modulator. The converter switching frequency is constant which simplifies the practical implementation. Also the proposed methods are robust against machine parameters mismatches and grid voltage disturbances. Extensive simulations in Matlab\\Simulink are performed to confirm the effectiveness of the proposed methods under transient and steady state conditions.

Adaptive Current Control for Grid-Connected Converters With LCL Filter

This paper presents a discrete-time adaptive current controller for grid-connected pulse width modulation voltage source converters with LCL filter. The main attribute of the proposed current controller is that, in steady state, the damping of the LCL resonance does not depend on the grid characteristic since the adaptive feedback gains ensure a predefined behavior for the closed-loop current control. An overview of model reference adaptive state feedback theory is presented aiming to give the reader the required background for the adaptive current control design. The digital implementation delay is included in the model, and the stability concerning the variation of the grid parameters is analyzed in detail. Furthermore, current distortions due to the grid background voltage are rejected without using the conventional stationary resonant controllers or the synchronous proportional-plus-integral controllers. Simulation and experimental results are presented to validate the analysis and to demonstrate the good performance of the proposed controller for grid-connected converters subjected to large grid impedance variation and grid voltage disturbances.

Sliding-Mode Control for DFIG Rotor- and Grid-Side Converters Under Unbalanced and Harmonically Distorted Grid Voltage

Regarding doubly fed induction generator (DFIG) operation, unbalanced and harmonically distorted grid voltage conditions have been treated as two separate control problems. This paper reports a solution for the rotor- and grid-side power converters, which allows one to keep the DFIG successfully in operation under both grid voltage conditions. The proposed solution is based on sliding-mode control (SMC). The rotor-side converter is commanded so that the electromagnetic torque and the stator reactive power remain free of fluctuations that arise during grid voltage disturbances. Meanwhile, the grid-side converter ensures both constant DC-link voltage and steady active power output from the overall system. The developed algorithms turn out being robust against parameter variations and of fast dynamic response. In addition, none of the converters need either voltage or current positive and negative sequences extraction. The simulation results presented demonstrate the appropriateness of SMC to face such disturbed scenarios. Finally, the stability proof of both converters' control algorithms is provided in the appendices.

An Adaptive Synchronous-Reference-Frame Phase-Locked Loop for Power Quality Improvement in a Polluted Utility Grid

The proper operation of grid-connected power electronics converters needs using a synchronization technique to estimate the phase of the grid voltage. The performance of this synchronization technique is related to the quality of the consumed or delivered electric power. The synchronous-reference-frame phase-locked loop (SRF-PLL) has been widely used due to its ease of operation and robust behavior. However, the estimated phase can have a considerable amount of unwanted ripple if the grid voltage disturbances are not properly rejected. The aim of this paper is to propose an adaptive SRF-PLL which strongly rejects these disturbances even if the fundamental frequency of the grid voltage varies. This is accomplished by using several adaptive infinite-impulse-response notch filters, implemented by means of an inherently stable Schur-lattice structure. This structure is perfectly suited to be programmed in fixed-point DSPs (i.e., it has high mapping precision, low roundoff accumulation, and suppression of quantization limit cycle oscillations). The proposed adaptive SRF-PLL has been tested by means of the TI TMS320F2812 DSP. The obtained experimental results show that the proposed synchronization method highly rejects the undesired harmonics even if the fundamental harmonic frequency of a highly polluted grid voltage abruptly varies.

A Single Cycle Inverter Welding Power Control System

Single cycle control technique is applied in the field of welding power which makes waveform control in the various stages of arc and circuit in the welding process respectively based on the characteristics of the welding process. A new welding power control has been put forward which is able to adjust the grid voltage disturbance in a very short time. We choose dual single-ended forward topology as the main circuit and make flux reset for the transformer in each cycle to ensure that the transformer magnetic saturation dose not occur, meanwhile, controlling the condition of arcing and short circuit respectively to achieve a better welding effect.

UPQC for Power Quality Improvement in DG Integrated Smart Grid Network - A Review

Quality of power supply has become an important issue with the increasing demand of Distributed Generation (DG) systems either connected to the grid through some power electronics grid-tie inverters or to work in isolated (microgrid) mode towards the development of a smart grid network. In this paper a technical review of Integration of Unified Power Quality Conditioner (UPQC) in Distributed Generation Network has been presented. Though the primary task of UPQC is to minimize the grid voltage and load current disturbance along with the reactive and harmonic power compensation, additional functionalities like compensation of voltage interruption and active power transfer to the load and grid have also been identified. Connection methodologies with their pros and cons are also described. Recent improvements in capacity expansion techniques and future trends for the application of UPQC to cope up with the expanding DG capacity are also reviewed.

A Robust Natural-Frame-Based Interfacing Scheme for Grid-Connected Distributed Generation Inverters

This paper presents a robust natural-frame-based interfacing scheme for grid-connected distributed generation inverters. The control scheme consists of a dead-beat line-voltage sensorless natural-frame current controller, adaptive neural network (NN)-based disturbance estimator, and robust sensorless synchronization loop. The estimated uncertainty dynamics provide the necessary energy shaping in the inverter control voltage to attenuate grid-voltage disturbances and other voltage disturbances caused by interfacing parameter variation. In addition, the predictive nature of the estimator has the necessary phase advance to compensate for system delays. The self-learning feature of the NN adaptation algorithm allows feasible and easy adaptation design at different grid disturbances and operating conditions. The fact that converter synchronization is based on the fundamental grid-voltage facilitates the use of the estimated uncertainty to extract the position of the fundamental grid-voltage vector without using voltage sensors. Theoretical analysis and comparative evaluation results are presented to demonstrate the effectiveness of the proposed control scheme.

A novel synchronization scheme for grid-connected converters by using adaptive linear optimal filter based PLL (ALOF–PLL)

This paper proposes a novel grid synchronization scheme using a new phase-locked loop (PLL) scheme based on the adaptive linear optimal filtering (ALOF) technique. The problem formulation of the proposed ALOF is based on decomposing grid voltage signal into inner product of two vectors, namely, the vector of trigonometric functions and the vector of coefficients, corresponding to the input vector and the weight vector of the closed-loop adaptation algorithm by using least-mean-square (LMS) optimization algorithm. The coefficient of the fundamental component of the grid voltage is used as input signal for the PLL and the phase angles of the trigonometric functions are obtained from the output of the PLL recursively. The mathematical derivation of the weights updating law and the stability analysis of the ALOF–PLL are presented. Besides, the parameter selection for optimal performance is also discussed in terms of continuous domain ( s-domain) analysis, discrete domain ( z-domain) analysis and time domain simulations. The proposed ALOF–PLL shows the characteristic of band-pass filter at fundamental frequency and a notch filter at the harmonic frequencies. Finally, a detailed comparison with the existing single-phase and three-phase grid synchronization methods is also presented, and the proposed ALOF–PLL is found to have overwhelming advantages over the existing grid synchronization methods in terms of tracking accuracy, dynamic response and immunity to grid voltage disturbances, such as voltage sag/swell, phase-angle jump, harmonics, unbalance, random noises and frequency jump, etc. The validity and effectiveness of the ALOF–PLL is substantially confirmed by the extensive simulation results obtained from Matlab/Simulink.

Analysis, Control and Comparison of Hybrid Two-Stage Matrix Converters for Increased Voltage Transfer Ratio and Unity Power Factor

Matrix converters have no boost capabilities. Their output voltage amplitude is therefore reduced compared to back-to-back inverter topologies. Moreover, they are sensitive to grid voltage fluctuations and disturbances and the voltage transfer ratio is further reduced under such grid perturbations. The present paper analyses the behaviour and limits of the two-stage matrix converters. It proposes and investigates three alternative topologies that modify the DC-link voltage of the two-stage matrix converter, by the means of additional circuitry, in order to increase the voltage transfer ratio above unity and to obtain immunity to grid disturbances. A high-performance control of the grid current is required to obtain a power factor close to unity. An effective control method is proposed for each converter. Feed-forward model based controllers allow obtaining good performances. Finally, the performances of the topologies are analysed and compared.

Suppression of Line Voltage Related Distortion in Current Controlled Grid Connected Inverters

The influence of selected control strategies on the level of low-order current harmonic distortion generated by an inverter connected to a distorted grid is investigated through a combination of theoretical and experimental studies. A detailed theoretical analysis, based on the concept of harmonic impedance, establishes the suitability of inductor current feedback versus output current feedback with respect to inverter power quality. Experimental results, obtained from a purpose-built 500-W, three-level, half-bridge inverter with an L-C-L output filter, verify the efficacy of inductor current as the feedback variable, yielding an output current total harmonic distortion (THD) some 29% lower than that achieved using output current feedback. A feed-forward grid voltage disturbance rejection scheme is proposed as a means to further reduce the level of low-order current harmonic distortion. Results obtained from an inverter with inductor current feedback and optimized feed-forward disturbance rejection show a THD of just 3% at full-load, representing an improvement of some 53% on the same inverter with output current feedback and no feed-forward compensation. Significant improvements in THD were also achieved across the entire load range. It is concluded that the use of inductor current feedback and feed-forward voltage disturbance rejection represent cost-effect mechanisms for achieving improved output current quality.