Negative-sequence Control Research Articles

An adaptive limiter for unbalanced operation of doubly fed induction generator-based wind energy system

Actual limits on parameters of type-3 wind generation unit, i.e., doubly fed induction generator (DFIG), are in terms of peak values of phase quantities but enforcement of limits in DFIG is achieved by positive and negative sequence controllers. The limits are generally enforced considering the scenario where positive and negative sequence vectors are collinear. However, this restricts the capability regions of the DFIG. The objective of this paper is to implement an adaptive limiter which conforms to all possible variations in phase angle between positive and negative sequence components. This will improve the range of operation irrespective of the level of unbalance of the connected distribution system. An adaptive limiter for voltage and current, considering different pulse width modulation (PWM) techniques for the voltage source converters (VSC), is implemented for unbalanced operation of DFIG and compared with generalised collinear technique.

An adaptive limiter for unbalanced operation of doubly fed induction generator-based wind energy system

Actual limits on parameters of type-3 wind generation unit, i.e., doubly fed induction generator (DFIG), are in terms of peak values of phase quantities but enforcement of limits in DFIG is achieved by positive and negative sequence controllers. The limits are generally enforced considering the scenario where positive and negative sequence vectors are collinear. However, this restricts the capability regions of the DFIG. The objective of this paper is to implement an adaptive limiter which conforms to all possible variations in phase angle between positive and negative sequence components. This will improve the range of operation irrespective of the level of unbalance of the connected distribution system. An adaptive limiter for voltage and current, considering different pulse width modulation (PWM) techniques for the voltage source converters (VSC), is implemented for unbalanced operation of DFIG and compared with generalised collinear technique.

Dynamic phasor based frequency scanning for grid-connected power electronic systems

Frequency scanning is a method of obtaining the frequency response of a system by injecting a small-amplitude wide-band signal as an input in a time domain simulation of the system. This is an alternative to analytical derivation of small-signal models, especially for complex grid-connected power electronic systems (PESs). These models are required for the study of adverse interaction of PES with lightly damped oscillatory modes in a power system. The use of the frequency scans for conventional small-signal stability analysis is predicated upon the time-invariance of the underlying model. Since PES are generally time-periodic, time-invariance may be achieved in some transformed variables. Although the DQ transformation is suitable in many situations, it is not so for systems with low-order harmonics, individual-phase schemes, unbalanced or single-phase systems, and PES with negative-sequence controllers. This paper proposes the use of dynamic phasor variables in such situations since the underlying model in these variables is time-invariant. The procedure for dynamic phasor based scanning is, however, intricate because wide-band signal injection results in the simultaneous presence of harmonic dynamic phasor components. The paper outlines this procedure and presents illustrative case studies of Thyristor Controlled Series Compensator (TCSC) and STATCOM. For the TCSC, a comparison of the frequency response obtained from the scanning method and the one obtained from an approximate analytical dynamic phasor model is also presented .

A Novel Centralized PV Power Plant Controller for Reducing the Voltage Unbalance Factor at Transmission Level Interconnection

This paper presents a novel centralized positive and negative sequence control strategy for enhancing the unbalanced operation of electric transmission networks. The proposed control strategy is deployed as an effective ancillary service provided by a photovoltaic power plants. The excess capacity of grid-tied inverters is utilized based on a novel centralized positive and negative sequence reactive current division algorithm. The control strategy dynamically allocates the amount of reactive power compensated by each inverter in the power plant according to its active power generation level. The performance of the proposed control strategy is evaluated for a photovoltaic power plant connected to the IEEE 12-bus Flexible AC Transmission Systems test system in steady-state and transient operation. This is demonstrated through unbalanced loads and asymmetrical grid faults. The proposed control strategy effectively reduced the voltage unbalance factor within the acceptable operating limits, and enhanced the transient response at the transmission level interconnection. This is achieved in order to adhere to grid codes’ requirements and the IEC 61000-3-13 standard.

Positive and negative sequence control of DFIG based wind turbines and its impact on grid voltage profile concerning converter control capability

Positive and negative sequence control of DFIG based wind turbines and its impact on grid voltage profile concerning converter control capability

Unbalanced PCC voltage regulation with positive‐ and negative‐sequence compensation tactics for MMC‐DSTATCOM

Unbalanced voltage fluctuation at the point of common coupling (PCC) is a challenging power quality problem in power distribution system. This study presents an unbalanced PCC voltage regulation scheme with positive- and negative-sequence control for modular multilevel converter (MMC) in medium-voltage distribution static synchronous compensator (DSTATCOM) application. The proposed positive-sequence control is utilised to regulate the positive-sequence component of PCC voltage to rated value, while the negative-sequence control aims to cancel out the negative-sequence component of PCC voltage completely. Compensation commands whether in positive- or negative-sequence control are generated from unbalanced PCC voltage without utilisation of unbalanced load currents. DC circulating currents transferring active power among three phases of MMC are also analysed to illustrate power balance issue resulting from negative-sequence control. Then comparison of negative-sequence compensation capacity between MMC and full-bridge cascaded converter with delta-connection is illustrated. Experiment results acquired from a downscaled 85 V/1.5 kVA apparatus validate the effectiveness of the proposed control scheme.

Study on the Current-Limiting-Capable Control Strategy for Grid-Connected Three-Phase Four-Leg Inverter in Low-Voltage Network

The three-phase four-leg inverter can produce balanced voltages even with unbalanced loads, yet its controller design is quite complicated. Based on the analysis on time domain equations, a decoupled sequence control strategy for the three-phase four-leg inverter in a low-voltage network is proposed. A negative sequence controller and a zero sequence controller are added to the control strategy besides the positive sequence controller. Furthermore, considering the output limit of the inverter, a current limit design scheme is raised through the analysis on each sequence current in synchronous rotating frame. In the case of asymmetry, the limit value of each sequence current can be adjusted dynamically according to the design scheme. The output currents in each sequence can be controlled and limited for different purposes respectively. Finally, simulation results based on PSCAD/EMTDC V4.5.0 verify the validity of the control strategy.

Indirect Torque and Stator Reactive Power Control of Doubly Fed Induction Machine Connected to Unbalanced Power Network

The paper deals with the methods of either stator current or rotor current vector control of doubly fed induction machine connected to the unbalanced power network. Several alternative control targets, such as fixed electromagnetic torque, symmetrical stator current, or symmetrical rotor current, have been achieved. In spite of the selected target, the stator current is sinusoidal, however in some cases, it is unbalanced. The proposed vector control methods do not use separate negative sequence calculation and control. In some targets, filtration of signals is required in order to achieve symmetrical reference variables (positive sequence), which can be either stator or rotor current depending on the assumed target. Laboratory tests results have been presented in the steady state, as well as in transients. Short comparison of stator and rotor current controllers has been made on the basis of the laboratory test results.

Analysis of fault current contribution of Doubly-Fed Induction Generator Wind Turbines during unbalanced grid faults

The fault current contribution of the Doubly-Fed Induction Generator Wind Turbines (DFIG-WTs) under unbalanced grid faults did not draw researchers' attention due to the absence of any related requirements in the former grid codes. However, in 2015 new requirements were established dictating the DFIG-WT/generating unit to remain connected to the grid during phase-to-phase short-circuit and to perform reactive power provision. Thereupon, the knowledge of the fault current magnitude as well as the dynamic response of the DFIG-WT are crucial for the power system component design and for the validation of its commitment to the grid codes requirements. There are concrete control objectives regarding negative sequence control that cannot be fulfilled simultaneously but rather set according to the owner's preferences. Therefore, in this paper, a detailed analysis of the negative sequence current response under each control objective were carried out, considering all the controller parameters. The influence of each controller structure on the short-circuit quantities is investigated and based on it an approximated mathematical models of the short-circuit quantities and currents were proposed. Finally, a brief analysis, due to space limitation, of the response during the voltage limitation period is also performed where the influences of voltage limitation on the negative sequence current were shown.

Assessment of control strategies for fault ride through of SCIG-based wind energy conversion systems

ABSTRACTWith increasing penetration of wind energy into the power grid, researchers have started focusing more on control and coordination of wind energy conversion systems (WECS) with the other components at system level, especially during fault. It is important to implement a suitable fault ride through control strategy to avoid tripping of the generators when the power system is subjected to voltage dips normally below 90% of nominal voltage. The dips below 90% may lead to a significant loss of generation and frequency collapse, followed by a blackout. This article implements and assesses the methodologies to deal with such situations for squirrel cage induction generator-based wind energy conversion systems employing fully rated power electronic converters. Three distinct control techniques—namely, balanced positive sequence control, positive negative sequence control, and dual current control—have been simulated and applied to grid side converter of SCIG-based WECS. The performance of all the three c...

Control strategy for microgrid under three-phase unbalance condition

Microgrid (MG) is generally developed at utility terminal which contains lots of unbalanced loads and distributed generations (DGs). The interaction between MG and the unbalance loads or DGs will degrades the control performance of interfaced inverter in MG and dramatically leads to MG voltage unbalance. In this paper, a negative-sequence compensation based three-phase voltage unified correction strategy is proposed. While MG operates in islanded mode, a positive virtual impedance control is used to eliminate the negative voltage resulted from the negative-sequence current, and then a positive-sequence voltage control loop and negative-sequence control loop are used to improve the inverter control performance. While MG operates in grid-tied mode, the inverter operates as a negative-sequence current source to compensate the negative-sequence currents of loads to guarantee the point of common coupling (PCC) voltage balance. By using the proposed strategy, the voltage control performance of inverter can be improved; the MG power quality can be enhanced significantly. Simulation and experimental results verify the effectiveness of the proposed method.

MicroRNA-26b inhibits cell proliferation and cytokine secretion in human RASF cells via the Wnt/GSK-3β/β-catenin pathway.

BackgroundRheumatoid arthritis (RA) is a chronic systemic auto- immune disease characterized by joint synovitis. Recent evidence suggests that rheumatoid arthritis synovial fibroblasts (RASFs) promote joint destruction. In this study, we investigated the role of microRNA-26b (miR-26b) in cell proliferation and inflammatory cytokine secretion using patient-derived Rheumatoid arthritis fibroblast-like synoviocyte (RAFLS) to understand pathways influencing rheumatoid arthritis.MethodsRAFLS were cultured in vitro and transfected with miR-26b mimics (experimental group) and negative sequence (control group). The protein levels of Wnt4, Wnt5ɑ, GSK-3β, CyclinD1, Ser9-GSK-3β and β-catenin were detected by western blot analysis. Tumor Necrosis Factor-ɑ (TNF-ɑ), IL- 1β, and IL-6 levels were quantified by Enzyme-linked Immunosorbent Assay (ELISA). RAFLS proliferation and apoptosis were measured by 3-[4, 5-dimethylthiazol-2-yl]-2, 5-diphenyl tetrazolium bromide (MTT) assay and flow cytometry, respectively.ResultsGSK-3β and CyclinD1 expression levels were lower in miR-26b mimic group compared to Mock group and negative control (NC) group. Conversely, GSK-3β and CyclinD1 expression levels were markedly higher in the miR-26b inhibitor group compared to Mock and NC group (P < 0.05). Transfection of miR-26b mimics significantly increased the, levels of Ser9-GSK-3β and β-catenin in comparison to Mock and NC groups, while transfection of miR-26b inhibitors showed the opposite effect. In miR-26b mimic group, TNF-α, IL- 1β and IL-6 levels were lower than the Mock and NC groups, while in miR-26b inhibitor group, these cytokine levels were higher than the Mock and NC groups (P < 0.05). Transfection of miR-26b mimics significantly reduced the cell proliferation of RAFLS, compared to the Mock and NC groups, and miR-26b inhibitors increased the proliferative capacity of RAFLS compared to Mock and NC groups (P < 0.05). The miR-26b mimic group exhibited higher RAFLS apoptosis rate compared to Mock and NC group and miR-26b inhibitor group showed significantly lower RAFLS apoptosis rate compared to Mock and NC groups (P < 0.05).ConclusionsMiR-26b regulates β-catenin and CyclinD1 levels by inhibiting GSK-3β expression, which in-turn alters the Wnt/GSK-3β/β-catenin pathway to lower RAFLS proliferation and elevate cell apoptosis and the secretion of TNF-α,IL-1β and IL-6 cytokines. Therefore, our results show that miR-26B plays a central role in inhibiting the inflammation associated with rheumatoid arthritis.Virtual SlidesThe virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/9063056861547150

계통 불평형시 과도 응답 특성이 개선된 고압 이중여자 유도형 풍력발전 시스템의 제어 전략

【This paper investigates control algorithms for a doubly fed induction generator with a back-to-back three-level neutral-point clamped voltage source converter in a medium-voltage wind power system under unbalanced grid conditions. Negative sequence control algorithms to compensate for unbalanced conditions have been investigated with respect to four performance factors: fault ride-through capability, instantaneous active power pulsation, harmonic distortions, and torque pulsation. The control algorithm having zero amplitude of torque ripple indicates the most cost-effective performance in terms of torque pulsation. The least active power pulsation is produced by a control algorithm that nullifies the oscillating component of the instantaneous stator active and reactive power. A combination of these two control algorithms depending on operating requirements and depth of grid unbalance presents the most optimized performance factors under generalized unbalanced operating conditions, leading to a high-performance DFIG wind turbine system with unbalanced grid adaptive features.】

Comparison between separated and not separated positive and negative sequence control in a high voltage direct current transmission system during unbalanced grid faults

Especially during unbalanced grid faults the behavior of voltage source converter based generation or transmission units is highly dependent on the implemented control of the converter. For the control strategy of the converter the two general alternatives are on the one hand a separated control of positive and negative sequence quantities and on the other hand a not separated control in which voltages and currents are not decomposed in the symmetrical components.Furthermore the quickness of the converter control allows the injection of reactive current during grid faults in order to stabilize the voltage locally. The possibilities and limitations of this so called dynamic voltage control depend highly on the implemented control strategy as well.In this publication the advantages and disadvantages of the two control strategies with separated and not separated positive and negative sequence are explained and demonstrated during an unbalanced grid fault. For this an AC-DC hybrid transmission test system in which the converter control of the high voltage direct current transmission system is equipped with both control strategies forms the basis for this comparison.

Novel Configuration and Transient Management Control Strategy for VSC-HVDC

This paper presents a novel system configuration for voltage source converter (VSC)-based high-voltage direct current (HVDC) transmission connected to a large-scale offshore wind power plant (WPP). The proposed scheme is reconfigured at the onshore end to achieve shunt and series compensation, which is named as `Unified-VSC-HVDC' (U-VSC-HVDC). A mathematical model of the proposed configuration is derived to determine the rating of the employed series and shunt converters. To achieve a flexible control strategy for balanced and unbalanced fault conditions, the proposed transient management scheme employs positive and negative sequence controllers for the series compensation. The negative sequence voltage components are determined in such a way as to minimize power oscillations caused by asymmetrical faults, and hence to reduce DC link voltage overshoots. A test system comprised of a detailed representation of the proposed configuration is simulated and evaluated using PSCAD/EMTDC. A comprehensive study validates the capability of the proposed configuration and transient management scheme for achieving smooth power transfer and superior transient performance of the electrical grid. Also, it minimizes the possibilities of electrical network propagations in response to symmetrical and asymmetrical grid faults.

Dual current control strategy to fulfill LVRT requirements in WECS

Purpose – The purpose of this paper is to investigate a control strategy to fulfill low-voltage ride through (LVRT) requirements in wind energy conversion system (WECS). Design/methodology/approach – This paper considers an active front-end converter of a grid connected WECS working under grid fault conditions. Two strategies based on symmetrical components are studied and proposed: the first one considers control only for positive sequence control (PSC); the second one considered a dual controller for positive and negative sequence controller (PNSC). The performance of each strategy is studied on LVRT requirements fulfillment. Findings – This paper shows presents a control strategy based on symmetrical component to keep the operation of grid-connected WECS under unsymmetrical grid fault conditions. Research limitations/implications – This work is being applied to a 2 kVA laboratory prototype. The lab prototype emulates a grid connected WECS. Originality/value – This paper validate the PNSC strategy to LVRT requirements fulfillment by experimental results obtained for a 2 kVA laboratory prototype. PNSC strategy allows constant active power delivery through grid-voltage dips. In addition, the proposed strategy is able to grid-voltage support by injection of reactive power. Additional features are incorporated to PNSC: sequence separation method using delay signal cancellation and grid frequency identification using phase locked loop.

Fault Ride-Through Configuration and Transient Management Scheme for Self-Excited Induction Generator-Based Wind Turbine

The paper proposes a novel fault ride-through (FRT) configuration with a transient management scheme to provide dynamic grid supports for self-excited induction generator (SEIG)-based wind turbines. The new configuration is capable of supporting both shunt and series compensation modes for enhancing FRT capability and transient stability. The proposed control scheme, utilizing positive and negative sequence controllers, provides a transient management solution for both balanced and unbalanced fault conditions. An active damping control loop for the shunt compensation configuration is developed to mitigate system oscillation and improve system performance during and following faults. A comprehensive simulation has verified the FRT capability of the new configuration and transient management scheme, and showed that the proposed topology increases system stability under various short circuit ratios, even in case of a weak grid. The experimental results are also carried out to show the effectiveness of the proposed shunt-series reconfiguration topology.

Voltage Booster Schemes for Fault Ride-Through Enhancement of Variable Speed Wind Turbines

This paper presents series grid interface topologies for enhancing the fault ride-through (FRT) performance of doubly-fed induction generator (DFIG)-based wind turbines (WTs). Two voltage booster schemes, 1) dynamic voltage restorer (DVR) and 2) resistive type high temperature superconducting fault current limiter (HTS-FCL), are designed and implemented. The test system represents a WT connected to an electric grid with alternatively employing DVR and HTS-FCL. Both schemes provide fast mitigation of voltage dip that maintains the nominal operating conditions for DFIG-WT. To achieve a flexible control solution for balanced and unbalanced fault conditions, the DVR employs positive and negative sequence controllers while the HTS-FCL is designed to perform fast quenching for each phase individually. The potential of the two booster schemes is evaluated and analyzed in positive and negative sequence reference frames. Comprehensive simulation studies are presented to verify the capability of the series grid interface schemes for ensuring the normal operation and smooth wind power evacuation with effective isolation from grid faults. Furthermore, the grid code requirements of reactive current support are evaluated for both schemes at various fault scenarios.

Fault ride through capability for grid interfacing large scale PV power plants

Integration of dynamic grid support is required for distributed power systems that are interconnected with medium voltage grids. This study proposes a comprehensive control solution to enhance fault ride through (FRT) capability for utility-scale photovoltaic (PV) power plants. Based on positive and negative sequence control schemes and PV characteristics, the approach alleviates dc-bus double-line-frequency ripples, reduces voltage stress on inverter power switches and DC-link capacitors, and minimises undesirable low-order voltage and current harmonics that are presented on the ac side. The study proposes a new feature to achieve superior FRT performance by using the overload capability of grid-tied inverters. A weak electric grid is used for the test case including a wind turbine induction generator, diesel engine driven synchronous generators and various loads. A comprehensive simulation verified the capability of the proposed control schemes for mitigating the voltage dip, enhancing the voltage response and further improving the stability of interconnected distributed generation in reaction to severe unbalanced voltage conditions because of asymmetrical grid faults.

The Performance of a New Hybrid PLL in an Interconnected Renewable Energy Systems under Fault Ride Through Operation

Modern renewable energy systems (RES) require fault ride through (FRT) operation in order to provide voltage and frequency support to the power grid when faults occur. The grid synchronization of a RES could be ensured by the appropriate operation of a phase-locked loop (PLL). A new hybrid PLL (dαβPLL) has recently been suggested by the authors and could operate accurately and faster than the other existing PLLs. The dαβPLL could be a very useful tool in the FRT operation of a RES, since the faster performance of the new PLL could boost the time performance of the FRT algorithm when a disturbance occurs. This paper investigates the performance of the flexible positive and negative sequence control (FPNSC) when the new dαβPLL is used. The paper also deals with the improvement on the FRT operation of the RES that could be obtained from the outstanding performance of the dαβPLL.