Positive-and Negative-sequence Research Articles

Novel Virtual Winding Current Controller for Suppression of Linear Permanent Magnet Machine Electromagnetic Thrust Ripple

Thrust force ripple, including no-load detent force and electromagnetic thrust ripple, worsens the precision of linear permanent magnet machine (LPMM) control system. Asymmetrical current injection algorithm has been proposed for the suppression of electromagnetic thrust ripple, and the key point of the algorithm is positive- and negative-sequence (PNS) current closed-loop control. To achieve better steady-state and dynamic performance, a novel high-bandwidth current controller is proposed in this paper. A dual three-phase control system composed of three-phase virtual winding and the three-phase LPMM is introduced. According to the dual three-phase topology, PNS components are decoupled by a novel coordinate transformation, and the proposed controller avoids the use of filters or sophisticated algorithms for extraction of PNS currents. Experimental results based on a linear vernier permanent magnet machine (LVPMM) are presented. It verifies that the asymmetrical current control strategy based on the novel PNS current controller reduces electromagnetic thrust ripple by more than 60% compared with symmetrical three-phase current control.

A Novel Sequence-Based Unified Control Architecture for Multiple Inverter Modes of Operation in Unbalanced Distribution System

Control of multiple inverters in the distribution grid can be achieved by adopting appropriate droop laws. However, in an unbalanced power distribution system with single-phase and two-phase laterals, droop-based control methods fail thus requiring dedicated inverter control methods that can supply required positive and negative sequence (PNS) components. In this article, a unified control architecture is proposed for stable multiple inverter operation that can improve the accuracy of active and reactive power sharing alongside maintaining the voltage and frequency of the system close to the nominal values. The proposed control strategy further ensures the supply of required negative sequence components to maintain balanced steady-state system voltage and mitigate the active power ripples during unbalanced faults. The unified control architecture is devised through a systematic definition of steady-state operating modes and the interaction among hierarchical grid entities, facilitating the coexistence of PNS controllers to meet the defined objective. The results are verified using a real-time simulator-based multi-rate model-in-the-loop set-up. Furthermore, appropriate communication network latency is considered for the verification of the performance of the proposed PNS controller. The verification for various grid events confirms the capability and effectiveness of the proposed control architecture, and suitability for 100% inverter-based grid operation.

Virtual Flux Estimation for Sensorless Predictive Control of PWM Rectifiers Under Unbalanced and Distorted Grid Conditions

In this article, a new neural network-based virtual flux (NN-VF) estimator is proposed for sensorless control of a pulsewidth modulation rectifier under unbalanced and distorted grid conditions. In this estimator, the VF vector is reconstructed through an emulated ideal integrator. Thereafter, positive and negative sequence (PNS) VF components are extracted using an NN-based PNS components separator. A Lyapunov's theory-based convergence analysis is performed for optimal tuning of the NN-VF estimator. Accordingly, accurate and fast estimation is achieved. A VF-based predictive direct power control (VF-PDPC) model, including the estimated VF positive sequence components, is formulated. A startup procedure is considered for smooth starting under unbalanced grid conditions. Feasibility and robustness of the developed VF-PDPC are verified by experiments. Mainly, obtained results demonstrate that the startup procedure reduces settling time and avoids over currents; the VF-PDPC achieves better current waveforms than those obtained by the conventional PDPC under unbalanced grid conditions; the NN-VF estimator presents similar dynamic performances as the second-order generalized integrator that uses grid voltages measurement.

Current control based power management strategy for distributed power generation system

The integration of renewable energy sources (RES) based distributed generation (DG) systems into electric grid has many challenges such as synchronization, control, power management (PM) and power quality problems. This paper proposes a references current generator (RCG) based PM strategy to control three phase inverter and manage power flow among the DG energy sources, electric grid and load demand under balanced and unbalanced grid conditions. The amplitudes of active and reactive power oscillations are also eliminated and controlled through only one flexible control parameter (FCP) under grid faults and harmonic distortions. Compared with previous similar studies, one of the important contributions is capable to inject maximum active power and minimum reactive power into electric grid and load at inverter power capacity under grid faults Another contribution is to extract the positive and negative sequence (PNS) voltage and current components with the improved fast and robust dual adaptive filters based phase locked loop (DAF-PLL). Fractional order proportional integral (FOPI) is selected as an attractive solution for AC current regulation to exhibit fast transient response and to achieve zero steady-state errors as compared with conventional current regulation controllers in synchronous or stationary frames.

A Control Strategy for the Grid-Side Converter of the Directly-Driven Wind Turbine with PM Synchronous Generator under Unbalanced Grid Voltage

A mathematical model of the full power grid-connecting converter at grid side for permanent magnet synchronous generator (PMSG) under unbalanced grid conditions is built, double-frequency oscillation mechanism on DC voltage is analyzed, and the influence of active power ripple of grid-connecting impedance on dual current control is discussed. To suppress the double-frequency oscillation on DC voltage, and take the grid-connected impedance on active power into account, a dual current control strategy, which based on respectively orientating positive-and negative-sequence voltages,is proposed, and introduced the correction value of active power ripple of grid-connecting impedance to correct reference current of proposed control strategy. Simulation results show that the proposed control strategy can effectively control positive and negative-sequence currents,eliminate the DC voltage double-frequency oscillation,and improve harmonic characteristic of grid-connected current.

Virtual-Flux-Based Voltage-Sensor-Less Power Control for Unbalanced Grid Conditions

This paper presents a virtual flux-based method for voltage-sensor-less power control of voltage source converters under unbalanced grid voltage conditions. The voltage-sensor-less grid synchronization is achieved by a method for virtual flux estimation with inherent sequence separation in the stationary reference frame. The estimated positive and negative sequence (PNS) virtual flux components are used as basis for calculating current references corresponding to the following objectives for control of active and reactive powers under unbalanced conditions: 1) balanced positive sequence currents, 2) elimination of double-frequency active power oscillations, and 3) elimination of double-frequency reactive power oscillations. For simple implementation and flexible operation, the derived current references are synthesized into one generalized equation where the control objectives can be selected by real coefficients. Since the converter has a limited current capability, a simple, generalized, method for current limitation is also presented with the purpose of maintaining the intended power flow characteristics during unbalanced grid faults. The proposed strategies for virtual flux-based voltage-sensor-less operation have been investigated by simulations and laboratory experiments, verifying the expected performance of active and reactive power control with different objectives.

Fast Single-Turn Sensitive Stator Interturn Fault Detection of Induction Machines Based on Positive- and Negative-Sequence Third Harmonic Components of Line Currents

Unambiguous detection of stator interturn faults for induction machines at their incipient stage, i.e., few turns' fault, has recently received great attention. Traditionally, interturn faults are detected using negative-sequence current and impedance. However, their effectiveness under supply imbalance conditions is questionable. Recently, line-current third harmonic (± 3f) has also been used in an attempt to achieve this goal. Nevertheless, issues such as inherent structural asymmetry and voltage imbalance also influence the + 3f. In this paper, the positiveand negative-sequence third harmonics (± 3f) of line current under different operating conditions have been explored by combining space and time harmonics. The suggested fault signature was obtained by removing residual components from tested quantities. Simulation and experimental results using 1 s of data indicate that the proposed ± 3f signatures are capable of very effectively detecting even a single-turn fault, and distinguish it from voltage imbalance and structural asymmetry.

Adaptive rotor current control for wind-turbine driven DFIG using resonant controllers in a rotor rotating reference frame

This paper proposes an adaptive rotor current controller for doubly-fed induction generator (DFIG), which consists of a proportional (P) controller and two harmonic resonant (R) controllers implemented in the rotor rotating reference frame. The two resonant controllers are tuned at slip frequencies ωslip+ and ωslip−, respectively. As a result, the positive-and negative-sequence components of the rotor current are fully regulated by the PR controller without involving the positive-and negative-sequence decomposition, which in effect improves the fault ride-through (FRT) capability of the DFIG-based wind power generation system during the period of large transient grid voltage unbalance. Correctness of the theoretical analysis and feasibility of the proposed unbalanced control scheme are validated by simulation on a 1.5-MW DFIG wind power generation system.

Time–Frequency Analysis of Complex Space Phasor in Power Electronics

This paper considers applying nonparametric and parametric methods for the calculation of the time-frequency representation of nonstationary signals in power electronics. A space phasor is proposed as a complex representation of a three-phase signal to calculate the spectrum of positive-and negative-sequence components. The developed methods are tested with nonstationary multiple-component signals occurring during the fault operation of inverter-fed drives. First, the main differences between parametric and nonparametric methods are underlined. Then, some aspects of additional kernel functions are introduced on the basis of Wigner and Choi-Williams distributions. An additional comparison of uncertainty of measurements using the described methods, as well as a widely used Fourier technique, can be useful for practitioners. The proposed methods allow tracking instantaneous frequency as well as magnitude that can lead to applications in the diagnosis and power quality areas.

A Unified Dynamic Model and Control for the Voltage-Sourced Converter Under Unbalanced Grid Conditions

This paper introduces and develops i) a novel unified dynamic model, and ii) new control schemes for a grid-connected static energy conversion system that is based on the voltage-sourced converter (VSC) configuration. The model and the controls are applicable to both the conventional two-level and the three-level neutral point diode clamped (NPC) VSCs. The model is unified since it represents the dynamic behavior of the with respect to both positiveand negative-sequence components, due to unbalanced grid conditions and disturbances. To develop the unified model, the concepts of generalized sinusoidal pulse-width modulation (SPWM) and VSC sequence subsystems are presented. Based on the negative-sequence subsystem control, a new control strategy, to either balance the ac line currents or to mitigate dc-bus voltage ripples, is introduced. Application of the developed model and the controls to a variable-speed wind-power unit is presented in the paper.

Symmetrical-Component Circuit Constants and Neutral Circulating Currents for Concentric-Neutral Underground Distribution Cables

Use of single-conductor concentric-neutral cables in underground distribution requires a method with new features for proper inclusion of the effects of the concentric- neutral upon the positive-and negative-sequence network impedances. Circulating currents in the concentric-neutrals cannot be ignored in any of the sequence networks, and the dissymmetry in cable spacing substantially increases the effects.

Derating of polyphase induction motors operated with unbalanced line voltages

The results of tests are presented which indicate the need for a severe reduction in the rating of motors when operated with unbalanced line voltages. Varying amounts of positive-and negativesequence applied voltages are considered. Also, results of one method of calculating reduced motor rating are presented, and are compared with measured values. As data are conservative in that only the most unfavorable conditions for unbalance are examined.

Voltage Induction in Parallel Transmission Circuits

General formulas are developed with calculation examples for determining the positive-, negative-, and zero-sequence voltage induced in transmission circuits paralleled either on the same tower or on adjacent towers. For nontransposed lines the induced voltages are a function of all three sequence currents. The positive-and negative-sequence mutual impedance is less than 10% of the self-impedance of the lines while the zero-sequence mutual impedance can be as high as 50 to 70% of the zero-sequence self-impedance of the lines. The conductor configuration should be symmetrical around a vertical axis to avoid differences in the induced voltages in the two circuits.

An instrument for complete symmetrical-component analysis

Established methods for the resolution of an unbalanced system into positive-and negative-sequence components are extended to give information about their relative phase-displacement. The method developed depends on the inherent property of dynamometer instruments that, for given currents in the coils, the deflection is proportional to the cosine of their phase difference. A dynamometer instrument of ordinary design was modified simply for the purpose by separating the coils and connecting the ends to external terminals, two similar resistance-capacitance networks being added. If it is assumed that the zero-sequence component is absent, or has been eliminated, three readings r1, r2 and r12 and one unrecorded test of a qualitative nature are sufficient for a complete analysis. The test serves to determine the sign of the phase angle, the sequence components r1 and r2 are given directly, and the expression cos ?=r122/(r1r2) enables the phase angle to be found. The only calculation is very simple, and the solution can be obtained quickly from nomogram.