Unbalanced Network Conditions Research Articles

Improved Low Voltage Ride-Through Capability of DFIG with ESO Controller under Unbalanced Network Conditions

Under unbalanced grid condition, in a Doubly-Fed Induction Generator (DFIG), voltage, current, and flux of the stator become asymmetric. Therefore, active-reactive power and torque will be oscillating. In DFIG controlling Rotor Side Converter (RSC) aims to eliminate power and torque oscillations. However, simultaneous elimination of the power and torque oscillations is not possible. Also, Grid Side Converter (GSC) aims to regulate DC-Link voltage. In this paper, in order to regulate DC-Link voltage, an Extended State Observer (ESO) based on a Generalized Proportional-Integral (GPI) controller, is employed. In this controlling method, DC-Link voltage is controlled without measuring the GSC current, and due to using the GPI controller, the improved dynamic response is resistant against voltage changes, and the settling time is reduced. To improve the transient stability and Low Voltage Fault Ride Through (LVRT) capability of DFIG, Statistic Fault Current Limiter (S-FCL) and Magnetic Energy Storage Fault Current Limiter (MES-FCL) are proposed in this paper. The proposed FCL does not only limit the fault current but also fasten voltage recovery. The simulations are implemented by MATLAB software in the synchronous positive and negative sequence reference (d-q).

A Single Back-to-Back Converter Based System Emulator for Testing Unbalanced Microgrids

Renewable energy sources are typically interfaced to the main grid at the distribution level, through power electronic converter interfaces. Testing these microgrid systems is a crucial aspect which is gaining increased interest and importance among researchers. This article proposes a system emulator , for testing microgrids, with a special focus on replicating unbalanced network conditions . The proposed system uses a single back-to-back bidirectional converter for real-time emulation, and the user has the option to chose any node of their interest to study the dynamics. This is different from the ones proposed to-date - most of them focus on balanced systems, and use several converters to replicate the dynamics of the microgrid system, in which each converter mimics one entity at a time (source or load). A system with two generators is emulated for different control strategies and types of unbalances in the network. The crucial aspects related to the modelling and control are presented in detail supported by simulations and experimental validation on a prototype developed.

Grid-Voltage Sensorless Model Predictive Control of Three-Phase PWM Rectifier Under Unbalanced and Distorted Grid Voltages

The sensors in a three-phase pulse width modulation (PWM) rectifier control system are the basis of high-performance methods. Line voltage sensors are utilized to obtain information about the grid voltage amplitude and phase. To eliminate the grid voltage sensors and to decrease the cost of PWM rectifier control systems, various grid-voltage sensorless methods have been developed. In this article, a virtual-flux-based grid voltage estimation method is utilized in the model predictive control (MPC) of a PWM rectifier. The extended reactive power is introduced in the proposed MPC method; the MPC is then able to realize the control targets of eliminating input-side power oscillations and ensuring sinusoidal currents under unbalanced network conditions without any power compensation terms. The cascaded delayed signal cancellation (CDSC) method is applied to obtain the fundamental rectifier voltages in the virtual flux-based grid-voltage sensorless method. The proposed method can operate under unbalanced and distorted network conditions. Moreover, a startup current suppression method is applied in this method to avoid over-currents during the startup process. Experimental tests are performed to verify the effectiveness of the proposed method.

Direct Power Control of Doubly Fed Induction Generator Using Extended Power Theory Under Unbalanced Network

This paper proposes a novel direct power control (DPC) method for the doubly fed induction generators (DFIGs) power control system. The proposed DPC method is based on extended power theory rather than conventional instantaneous power theory. The proposed method calculates the rotor reference voltage based on the principle of deadbeat power control. Then, it synthesizes the rotor reference voltage using space vector modulation (SVM). As a result, the switching frequency is constant, and satisfactory control performance is achieved under both balanced and unbalanced network conditions. This technique achieves the control target of ensuring a constant stator-side active power, constant extended reactive power, and sinusoidal stator currents without power compensation terms or other measures under unbalanced network conditions. Different from other DPC-SVM methods, the phase-locked loop block is eliminated and the whole structure of the control system is simple and easy to use. No parameter needs to be tuned in the proposed method. The effectiveness of the proposed method is validated by the presented simulation and experimental results on a 1.5-kW laboratory DFIG control system.

Model Predictive Direct Power Control of Doubly Fed Induction Generators Under Balanced and Unbalanced Network Conditions

Model predictive direct power control (MPDPC) has been widely studied for the control of doubly fed induction generator (DFIG) systems because of its conceptual simplicity and multivariable control ability. However, conventional MPDPC suffers from the problems of high power ripples and intensive computational effort. Furthermore, this approach presents highly distorted currents under unbalanced networks. To address the problems mentioned above, this article proposes a universal and low-complexity MPDPC, which can work effectively under both balanced and unbalanced networks. On one hand, the proposed method unifies conventional MPDPC and multiple-vector-based MPDPC under a common framework with lower complexity. The optimal vectors and their respective durations in the proposed MPDPC are obtained in a substantially more efficient manner than conventional enumeration-based MPDPC. On the other hand, a flexible power control method with a universal power compensation expression is proposed. By adding the calculated power compensation value to the prior power reference value, the proposed universal MPDPC method can be applied to unbalanced networks. Three control objects under unbalanced network conditions can be realized. Current distortion and power ripple can vary smoothly among the three objects by regulating the coefficient determining the universal power compensation value. The presented experimental results confirm the effectiveness of the proposed method.

Gaussian Mixture Model-Based Ensemble Kalman Filter for Machine Parameter Calibration

This letter proposes a novel Gaussian mixture model-based ensemble Kalman filter approach to the accurate calibration of the parameters of machine dynamic models. This approach aims to overcome some practical challenges affecting parameter calibration accuracy. Results show the proposed approach can provide precise calibrated parameters even when the machine operates under unbalanced network conditions with nonGaussian measurement noises.

Disturbance Observer Based Power Control of DFIG Under Unbalanced Network Conditions

This article investigates a new current control strategy for grid connected doubly fed induction generators (DFIG) under unbalanced grid voltage conditions. DFIG dynamic model is represented in synchronously rotating symmetrical component dq frames of references. A proportional controller which feed forwards estimated disturbance terms is proposed for power control in unbalanced voltage conditions to accomplish positive sequence power demand and also to dissipate negative sequence components which cause double frequency oscillations in power. DFIG parameters are not required to be known since it estimates the machine parameters relevant nonlinear terms with a disturbance observer (DOB). The novel controller structure is implemented by using dSPACE ds1103 digital controller which controls a 1.1 kW DFIG experimental setup.

Analysis and Dynamic Performance Improvement of Grid-Connected Voltage–Source Converters Under Unbalanced Network Conditions

The energy sector is moving toward an extensive utilization of distributed and renewable energy resources. Such resources are usually interfaced to power grids via voltage–source converters (VSCs). Due to the increased penetration level of VSC-interfaced resources, the utilization of interfacing VSCs to support host grids under unbalanced conditions (e.g., due to grid voltage unbalance, unbalanced load conditions and unsymmetrical faults) becomes essential. However, detailed dynamic analysis and systematic design procedure to enhance the dynamic performance of grid-connected VSCs equipped with grid-support controllers are not reported in the literature. To fill in this gap, this paper presents a detailed small-signal model and analysis of the dynamics of a grid-connected VSC equipped with the recently developed balanced positive-sequence control and positive/negative-sequence control methods to support the grid under unbalanced conditions. The effects of the short-circuit ratio, angle of the ac system impedance, and phase-locked-loop parameters on the transient behavior of the VSC are thoroughly studied and characterized. Furthermore, to improve the dynamic performance of grid-connected VSCs, a simple yet effective current-control-based compensator is developed to mitigate possible instabilities associated with the low-voltage operation. Comparative simulation and experimental results validate the theoretical analysis and the effectiveness of the proposed compensation scheme.

Control Strategy for a Grid-Connected Inverter under Unbalanced Network Conditions—A Disturbance Observer-Based Decoupled Current Approach

This paper proposes a new approach on the novel current control strategy for grid-tied voltage-source inverters (VSIs) with circumstances of asymmetrical voltage conditions. A standard grid-connected inverter (GCI) allows the degree of freedom to integrate the renewable energy system to enhance the penetration of total utility power. However, restrictive grid codes require that renewable sources connected to the grid must support stability of the grid under grid faults. Conventional synchronously rotating frame dq current controllers are insufficient under grid faults due to the low bandwidth of proportional-integral (PI) controllers. Hence, this work proposes a proportional current controller with a first-order low-pass filter disturbance observer (DOb). The proposed controller establishes independent control on positive, as well as negative, sequence current components under asymmetrical grid voltage conditions. The approach is independent of parametric component values, as it estimates nonlinear feed-forward terms with the low-pass filter DOb. A numerical simulation model of the overall power system was implemented in a MATLAB/Simulink (2014B, MathWorks, Natick, MA, USA). Further, particular results show that double-frequency active power oscillations are suppressed by injecting appropriate negative-sequence currents. Moreover, a set of simulation results provided in the article matches the developed theoretical background for its feasibility.

Relationship Between Two Direct Power Control Methods for PWM Rectifiers Under Unbalanced Network

Active power control plays an important role in a pulse width modulation (PWM) rectifier under both balanced and unbalanced network conditions. The prior methods to achieve active power oscillation cancellation (APOC) under unbalanced grid voltages are usually implemented by forcing the grid current to track appropriate current references. They require fine tuning work of a current controller and positive-/negative-sequence extraction of grid voltage/currents and/or converter voltage, which complicates the design of the controller. This paper presents two APOC methods in the frame of direct power control (DPC) for the PWM rectifier and reveals their inherent relationship. In the first method, an appropriate power compensation is added to the original power references without modifying the internal control structure. In the second method, a novel definition of reactive power is employed to replace conventional reactive power, which achieves the aim of APOC automatically. Both methods can be easily integrated with existing DPC schemes with a slight modification. The sequence extraction required in prior methods is eliminated. The inherent relationship between these two methods is investigated, and it is found that they are completely equivalent. The two APOC methods are comparatively studied and implemented in the basic table-based DPC. Simulation and experimental results confirm the theoretical study and the effectiveness of the two methods.

Model Predictive Direct Power Control of PWM Rectifiers Under Unbalanced Network Conditions

Model predictive direct power control (MPDPC) has been proposed as an effective alternative to conventional direct power control for pulsewidth-modulation (PWM) rectifiers. However, the sampling frequency of MPDPC still has to be high to achieve satisfactory performance. Furthermore, the grid currents of MPDPC would become highly distorted under unbalanced network conditions. To cope with the problems above, this paper proposes an improved MPDPC for PWM rectifiers, which is able to operate under both balanced and unbalanced grid voltages. By using a new definition of instantaneous reactive power in the predefined cost function, the proposed MPDPC can obtain sinusoidal grid currents and eliminate twice grid-frequency oscillation in both active power and the new reactive power. Neither complicated positive/negative-sequence extraction of grid voltage/current nor power compensation is required. Depending on the desired performance, two variants of the improved MPDPC are proposed and comparatively studied. At the same sampling frequency, the first approach achieves relatively low switching frequency, whereas the second approach obtains lower power ripples. Both simulation and experimental results are presented to confirm the effectiveness of the proposed methods.

Improvement of the transient negative-sequence rating formula for large generators based on the negative-sequence current component

Summary This paper proposes an improved transient negative-sequence calculation formula for a large-capacity nuclear power generator system under unbalanced network conditions. Because of the disadvantages of traditional transient negative-sequence ratings, I2*2 t is concluded to be inadequate for rating the generator imbalance capability. In fact, many factors, such as the possible fault asymmetry, the duration of the fault and non-periodic components, should also be considered when rating a generator. According to the inherent relation between rotor heating and generator imbalance capability, the non-periodic component is added to the improved transient negative-sequence formula. In this paper, an AP1000 nuclear turbo-generator was examined as a model to systematically analyse the electromagnetic properties and generator transient negative-sequence capability affected by different types of typical short circuits. A comprehensive comparison based on accident examples, experimental results and extensive simulations shows that the improved transient negative-sequence formula not only significantly enhances the assessment accuracy of large-capacity generator imbalance capability but also provides a theoretical reference for the related research of large-capacity generator systems. Copyright © 2015 John Wiley & Sons, Ltd.

IJIREEICE - Electrical, Electronics, Instrumentation and Control

3 Abstract: This paper presents greater value to control a doubly fed induction generator (DFIG) - based wind power complete persons living time system with series grid side changer (SGSC) under unbalanced network conditions. The behaviours of the DFIG system with SGSC during voltage unbalance are described by injecting a series control voltage Produced from the SGSC. To balance the stator voltage against effects of voltage unbalance upon the DFIG such as stator and rotor current unbalances electromagnetic torque and power pulsations can be removed , and then the Conventional vector control strategy for the rotor-side converter remains in full force under unbalanced conditions. Mean while, three control targets for the parallel grid-side converter (PGSC) are identified, including eliminating the oscillations in the total active power or reactive power, or eliminating negative-sequence current injected to the grid. Furthermore, a precise current reference Generation strategy for the PGSC has been proposed for The PGSC to further improve the operation performance of the whole system. The simulation results of a mw DFIG based wind turbine 12with SGSC and testing results on a laboratory13scale based on experience trick prices under small unchanging state grid voltage unbalance.

해상풍력단지에서의 PMSG 풍력발전기를 활용한 계통연계점 불평형 전원 보상

This paper proposes a control algorithm for permanent magnet synchronous generators with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage off-shore wind power system under unbalanced grid conditions. Specifically, the proposed control algorithm compensates for unbalanced grid voltage at the PCC (Point of Common Coupling) in a collector bus of an off-shore wind power system. This control algorithm has been formulated based on symmetrical components in positive and negative synchronous rotating reference frames under generalized unbalanced operating conditions. Instantaneous active and reactive power is described in terms of symmetrical components of measured grid input voltages and currents. Negative sequential component of AC input current is injected into the PCC in the proposed control strategy. The amplitude of negative sequential component is calculated to minimize the negative sequential component of grid voltage under the limitation of current capability in a voltage source converter. The proposed control algorithm enables the provision of balanced voltage at the PCC resulting in the high quality generated power from off-shore wind power systems under unbalanced network conditions.

A Novel Adaptive Load Balancing Algorithm in Heterogeneous Wireless Networks

Based on WLAN and UMTS coexistence scenario, a novel adaptive load balancing algorithm is proposed, in order to achieve efficient network resources utilization under unbalanced network conditions. Firstly, we propose a Markov model-based adaptive load control algorithm to adaptively allocate new arriving calls. Secondly, considering the difference of the network load, we adaptively calculate the amount of vertical handoff. Finally, considering extra signaling for vertical handoffs, a cost function-based terminal selection algorithm is proposed to perform vertical handoffs. Simulation results show that, the proposed adaptive load balancing algorithm can effectively improve fairness in network load, reduce network congestion and signaling overhead, and thereby improving system performance.

DC‐bus voltage control of grid‐connected voltage source converter by using space vector modulated direct power control under unbalanced network conditions

Unbalanced grid voltage will cause large dc-bus voltage ripple and introduce high harmonic current components on the grid side. This will severely threaten the safety of the grid-connected voltage source converter (VSC) and consequently, affect the healthy operation condition of the load. In this study, a new proportional-integral-resonant (PI-RES) controller-based, space vector modulated direct power control topology is proposed to suppress the dc-bus voltage ripple and in the same time, controlling effectively the instantaneous power of the VSC. A special ac reactive power reference component is introduced in the controller, which is necessary in order to reduce the dc-bus voltage ripple and active power harmonics at the same time. The proposed control topology is implemented in the lab. Simulation and experimental results are provided to validate its performance and the analysis presented in this study.

Sliding-mode-based direct power control of grid-connected voltage-sourced inverters under unbalanced network conditions

This study presents an improved means of direct power control (DPC) of grid-connected voltage-sourced inverters (GC-VSIs) when the network voltage is unbalanced. The DPC is based on sliding mode control (SMC) approach, which directly regulates the instantaneous active and reactive powers in order to keep the features of DPC without requirement of synchronous coordinate transformation or phase angle tracking of grid voltage. The behaviour of GC-VSIs by the conventional DPC, which takes no negative sequence voltage into consideration, is analysed under unbalanced network conditions. A new power compensation method is proposed for the SMC-based DPC during network unbalance to achieve three selective control targets, that is, obtaining sinusoidal and symmetrical grid current, removing reactive power ripples and cancelling active power ripples. The active and reactive power compensation components are calculated briefly and the proposed three control targets can be achieved, respectively, without the need for extracting negative sequence current components. Simulation and experimental results on a 1 kVA prototype GC-VSI are presented to verify the correctness and validity of the proposed control strategy and power compensation method.

Adaptive Control Strategy for VSC-Based Systems Under Unbalanced Network Conditions

A new adaptive control strategy, intended to improve the ride-through capability of high-voltage direct current (HVDC) systems under unbalanced network conditions and parameter uncertainties, is introduced. The proposed strategy resorts to a model reference adaptive control plus a resonant filter. The resonant filter scheme is based on a unique synchronous reference frame that prevents the use of the customary sequence component detector, increasing the controller bandwidth accordingly. Several tests are conducted to compare the proposed scheme against existing HVDC controllers, showing an improved performance regarding: 1) elimination of the 2ω ripple on the dc voltage arising during ac-side imbalances; 2) accurate and decoupled active and reactive power tracking when converter parameters are not perfectly known.

Dynamic control of unified power flow controller under unbalanced network conditions

The effective and reliable use of existing transmission lines are critically important because high-voltage transmission networks met enormous power demands are very expensive investments in terms of their costs. Flexible Alternating Current Transmission Systems (FACTS) are quite efficient to control the power flow of the transmission lines and increase the current capacity of system. Unified Power Flow Controller (UPFC) is the more efficient among FACTS equipments which have the potential to increase the power flow and stability of the transmission line. This paper develops new control approaches for both series and shunt inverters of UPFC. The proposed controller algorithms of shunt and series inverters are based on fuzzy logic controller and rotating orthogonal-coordinate method, respectively. Dynamic control of power flow using proposed UPFC is analyzed as mathematically. Power System Computer-Aided Design (PSCAD) is used to simulate the system and test UPFC in the simulation environment. The test results are presented to show the increased stability of the system and improved dynamic response of UPFC during faults occurred in the transmission line.

DFIG wind generation systems operating with limited converter rating considered under unbalanced network conditions – Analysis and control design

This paper presents an analysis and operation of doubly-fed induction generator (DFIG)-based wind power generation systems under unbalanced network voltage conditions. The limited voltage and current ratings, and control capabilities of both grid side and rotor-side converters (GSC&RSC) for unbalanced network voltage compensation are investigated. A current control scheme consisting of a proportional integral (PI) controller plus a resonant (R) compensator is designed. The control scheme is implemented in the positive synchronous rotating reference frame and provides precise current control for both the GSC and RSC without involving the decomposition process of positive and negative sequence currents. By taking into account both the converters’ current ratings, a coordinated control strategy between the GSC and RSC is proposed and adjusted. Finally, simulation studies on a 1.5 MW DFIG generation system and experimental tests on a 1.5 kW prototype are carried out. The correctness of the analytical results as well as the feasibility of the proposed coordinated control scheme are confirmed when the network voltage is unbalanced.