Negative Sequence Current Components Research Articles

Control Solution to Over-Modulation of DFIG Converter for Asymmetrical Fault Ride Through

This letter proposes a control solution to mitigate the problem of converter over-modulation in doubly fed induction generator (DFIG)-based wind turbine generators (WTGs). Overmodulation is a critical reason in failing to ride through asymmetrical faults and inject positive- and negative-sequence reactive currents (I1R and I2R) per grid code requirements. Analytical expressions are first proposed to independently calculate the transient, positive- and negative-sequence current components. The I2R injection factor is then optimized to mitigate the problem of steady-state over-modulation. The transient components are eliminated in the inner-loop control to mitigate the problem of transient over-modulation. The proposed control solution is validated with EMT-type simulations. The results show that the transient performance and the fault ride through (FRT) capability are significantly improved.

Research on the Control Method of the PV Grid-Connected Inverter under an Asymmetrical Power Grid Fault

The negative-sequence current component and harmonic components generated when an asymmetrical fault occurs in the power grid seriously affect the normal operation of the photovoltaic (PV) grid-connected inverter. In order to suppress the negative-sequence current component and the harmonic component of the grid-connected current, and to meet the normal grid-connected operation requirements of the PV grid-connected inverter when asymmetrical faults occur in the grid, this paper proposes the proportional integral double-resonant (PI-DR) current controller, which consists of the PI controller and double-resonant controllers. The PI-DR current controller can directly control the negative-sequence current component and the grid-connected current harmonic component in the output current in the forward synchronous rotating coordinate system, without decomposing the positive and negative sequence currents. The PI-DR current controller ensures that the PV grid-connected inverter can realize normal grid-connected operation and improves the quality of the power when an asymmetrical fault occurs in the power grid. MATLAB/Simulink experiments show that the PI-DR current controller can improve the dynamic characteristics of the PV grid-connected inverter and improve the operating capability of the system when an asymmetrical fault occurs in the power grid.

Theory and simulation analysis of parallel operation after main transformer replacement

As one of the most important main equipment in the power system, the safe and stable operation of transformer plays a vital role in ensuring the reliability of power supply. Because the three-phase parameters of the main transformer are not the same, the parameters of each phase and the three-phase load of the power grid are also different, resulting in a certain degree of asymmetry between the three-phase current and voltage of the transformer during normal operation. There will be negative sequence and zero sequence current components. The degree of asymmetry is generally small for the high-voltage power grid, but it has a large impact on the low-voltage side of the main transformer. In this paper, the parameters of two main transformers in a substation are quite different, but when they need to run in parallel, whether they can run in parallel, what are the conditions of parallel operation, these problems have been studied in theory and simulation. The results show that the two main transformers can be operated in parallel under suitable conditions. The actual operation result of a substation’s main transformer is normal, which further verifies the correctness of theoretical calculation and simulation.

Novel busbar protection scheme for impedance-earthed distribution networks

Due to the vast number of substations at the distribution level and increased costs of differential busbar protection, DSOs are in search of cost-effective protection schemes for busbar protection. This includes the use of various communication-based protection schemes, such as the reverse-blocking schemes used at Stedin. However, due to impedance grounding, the single-phase-to-ground short circuit currents have small values in medium voltage impedance-earthed distribution grids. As a result, the reverse-blocking scheme fails to detect this type of fault. This paper introduces a novel distributed protection scheme based on the detection of zero-sequence components of the currents and voltages and the negative-sequence current component. The proposed scheme successfully detects single-phase-to-ground busbar faults by using the standard settings of the widely available overcurrent IEDs, and an IEC 61850 communication between them. Firstly, the detection of the zero- and negative-sequence current components is used to distinguish between a busbar and a feeder fault. Secondly, zero-sequence voltage detection is used to distinguish between the faulty and healthy sections of the busbar when the busbar coupler is opened. This also increases the proposed scheme’s reliability by avoiding miss-operation due to human errors during maintenance or testing. The grid is modeled in a Real Time Digital Simulator (RTDS), and a Hardware-in-the-Loop (HiL) simulation is carried out to test the protection scheme. The extensive simulations show the strengths and the limitations of the proposed scheme. Based on the research results, the developed protection scheme is implemented as a standard protection scheme in all of Stedin’s new distribution substations.

Stator Fault Detection and Faulty Phase Identification in Network/Inverter-fed Induction Machines using Negative Sequence Current Component

This paper proposes a non-invasive negative sequence impedance-based technique to detect stator turn-to-turn faults (STTFs) and dentify the related faulty phase at early stages based on the tracking the magnitude and angle variations of the negative sequence current component generated due to STTFs. To extract these indicators, a simplified steady-state negative sequence equivalent circuit of the induction motor is used. To neutralize the effect of various produced disturbances by the inherent non-ideal construction of the machine and also unbalanced feed voltage to the STTF diagnosis, they will be estimated and removed from the main obtained component. It is shown experimentally that the introduced technique is independent of mechanical loading level (load variations) and is applicable for network or inverter-fed motors as well. Online fault detection and faulty phase identification, as the most important goals of the protection plan, are accessible by defining an appropriate threshold for the magnitude and allowable range of angle variation of the introduced criterion, respectively. The performance of the method is evaluated by simulation as well as multiple experimental tests. The experimental results have shown that from the sensitivity point of view, even weak faults are detectable by such a technique. Also, the obtained tests showed that such technique is robust, reliable and secure in the face of unbalanced voltage sources and load level variations. In addition, the performance of this method for the inverter-fed mode showed that the related sensitivity will be increased in such a condition.

Current limiting strategy for grid-connected inverters under asymmetrical short circuit faults

Grid-connected inverter plays an essential role as an interface between energy resources and the power grid. The performance of the inverters is adversely affected by the grid disturbances such as imbalances and asymmetrical short circuit faults. Then, it is necessary to enhance the functionality of the inverter under such conditions. This paper proposes an unbalance current limiting strategy for grid-connected inverters under asymmetrical short circuit fault conditions. In the proposed current limiting strategy, two main features are included: (i) second-order harmonic elimination from instantaneous active power injected into the grid, and (ii) reactive current injection according to the grid code demand. In the proposed current limiting strategy, proper positive and negative sequence current components are calculated, by which the aforementioned goals are achieved. Furthermore, to be compatible with the well-known control structure of the power converters, the current references are obtained in the form that is applicable with the commonly-used double-synchronous-references-frame control structure. To verify the effectiveness of the proposed method, the behavior of the current limiting strategy is examined by simulation of an asymmetrical short circuit fault condition in a test system. In the simulation, both the transient response upon fault occurrence and steady-state of fault condition are considered and analyzed. Furthermore, the simulation results are supported by experimental test results, verifying the effectiveness of the proposed strategy.

Stator Winding Fault Diagnosis Method Based on Dc Side Current of Inverter

This paper analyzes the influence of the fundamental negative sequence component of the grid voltage on the output voltage of the variable frequency power supply, and points out that the fundamental negative sequence component of the grid voltage will not generate the fundamental negative sequence current component in the stator current of the induction motor, so when the stator winding fault is diagnosed, the influence of the negative sequence voltage of the grid does not need to be compensated. Based on the characteristic that the fundamental negative sequence current component in the stator current is converted into two times of the inverter output frequency component in the DC side current of the inverter, a stator fault diagnosis method based on the DC side current of the inverter is proposed. The method of connecting the short-circuit resistance between the two phases of the motor is used to simulate the nonmetallic short-circuit fault of the stator coil. The experimental results show that this method does not need to compensate the negative sequence voltage, and only needs to collect one current signal, and the algorithm is relatively simple, which is a simple and effective method.

Enhanced Performance of Charging Stations via Converter Control Under Unbalanced and Harmonic Distorted Grids

This article proposes a new control structure for grid-interfaced converters to enhance ancillary services from emerging electric vehicle (EV) charging stations (CS) under unbalanced and distorted grid conditions. Compared to existing methods, the proposed control structure is relatively simple yet effective for both regulating powers and minimizing harmonics under unbalanced and distorted grid conditions. A simple direct predictive control methodology is proposed where the positive fundamental component of currents is controlled for power regulation while other negative sequence or harmonic current components are set to zeros. Consequently, the charging station operation is enhanced with efficient ancillary services to the electric grid. The novelty originates from a simple predictive control structure in an <i>abc-</i>natural frame without adding any decoupled or separated current-component controllers. Comparative studies on both Matlab&#x002F;Simulink platform and OPAL-RT-based real-time system among proportional integral (PI) control, finite predictive control, and the proposed method are conducted to verify the efficacy of the proposed structure. The comparative results show that the control structure improves the control performance and significantly enhances the support for the local loads under unbalanced, harmonic distorted grid conditions.

Dual Synchronous Rotating Frame Current Control of Brushless Doubly Fed Induction Generator Under Unbalanced Network

This article investigates the modeling and control of brushless doubly fed induction generator (BDFIG) under an unbalanced network. The system behavior of BDFIG under the unbalanced grid voltage is analyzed. The control strategies and different control targets of BDFIG in balanced and unbalanced networks are discussed. In order to achieve control targets, a current control strategy consisting of a primary controller and a secondary controller for the control winding is proposed. The primary controller is implemented in the positive-sequence synchronous rotating frame (SRF) to ensure the dynamic response of the system without sequence components separation, whereas the secondary controller is implemented in the negative-sequence SRF to control the negative-sequence current component only. A feedforward compensation for the negative-sequence current control is used to mitigate the sequence components separation influence on the dynamic response of the secondary controller. Experimental results demonstrate the effectiveness of the control strategy in suppressing the power and current oscillation of a BDFIG system under the unbalanced network.

Active Control Method for Torsional Vibration of DFIG Drive Chain Under Asymmetric Power Grid Fault

When the asymmetric fault occurs in power grid, it will cause the oscillation of the electromagnetic torque of the doubly-fed induction generator(DFIG). The oscillation of the electromagnetic torque will cause the torsional vibration of the drive chain, and the long-term torsional vibration will shorten the service life of DFIG. The analysis shows that the positive and negative sequence current components generated by DFIG terminals are the main factors that cause electromagnetic torque oscillation when the asymmetric faults occur. Therefore, the suppression of positive and negative sequence current components is the key to achieve active control of torsional vibration of drive chain, the traditional PID control can only adjust the direct-current(DC) component without static error, and cannot suppress the negative sequence alternating-current(AC) component. In this paper, based on the traditional PID regulator, the resonance(R) current regulator with the specific frequency is incorporated into the grid-side converter(GSC) to form the PID-R current controller. The PID-R current controller can realize accurate and effective control of the positive and negative sequence components in the forward rotation synchronous speed rotating coordinate system without decomposing the positive and negative sequence components, and can actively suppress the electromagnetic torque oscillation, thereby realize the active control of the drive chain torsional vibration. MATLAB/Simulink verifies the effectiveness of PID-R current controller.

Single-Phase Consensus-Based Control for Regulating Voltage and Sharing Unbalanced Currents in 3-Wire Isolated AC Microgrids

Chilean National Commission for Scientific and Technological Research/Formation of Advanced Human Capital Programme (CONICYT/PFCHA)/Doctorado Nacional/2017-21171858 Comision Nacional de Investigacion Cientifica y Tecnologica (CONICYT) CONICYT FONDECYT 1170883 National Agency of Research and Development/Associative Research Program (ANID PIA)/Basal AFB180003 Solar Energy Research Center (SERC), Chile ANID/FONDAP/15110019 National Agency of Research and Development (ANID)/Basal FB0008 National Fund for Scientific and Technological Development (FONDEQUIP) EQM160122

Sequence component based approach for fault discrimination and fault location estimation in UPFC compensated transmission line

This study presents an integrated approach for discrimination of internal and external fault, and to obtain pin-point location of the fault in a transmission line connected to unified power flow controller (UPFC). To detect internal fault, this method uses positive sequence component of voltage and current at the local and remote end bus. Fault location estimation technique requires both positive and negative sequence voltage and current components measured from both ends of the line. The performance of the proposed technique has been evaluated by simulating a 400 kV double circuit UPFC compensated transmission line. This technique can detect internal fault under stressed conditions, viz. high impedance fault (HIF), change in source strength, change in load angle, and power swing. Precise fault location has been achieved under wide variation of system and fault parameters. The comparative assessment has been provided to prove the efficacy of the proposed approach.

A Novel Unbalance Compensation Method for Distribution Solid-State Transformer Based on Reduced Order Generalized Integrator

Owing to its modular construction, ability for bi-directional power flow and suitability for AC/DC grids, solid-state transformer (SST) is expected to be the backbone of the future smart grids. One of the main drawbacks of SST is the generation of negative-sequence current component at its input stage under unbalanced distribution system which causes adverse impacts on the power quality of the electricity grids. This paper is aimed at proposing a novel unbalance compensation method based on reduced order generalized integrator to suppress the negative-sequence current. Unlike the conventional sequence compensation method that is based on dual synchronous reference frames, the new proposed method does not involve complex calculation of the command current and sequence decomposition. As such, the response speed of the compensation controller is significantly improved. Additionally, the proposed method is easy to implement when compared with the current conventional compensation technique as there is no need to inject sequence components into the grid. A simulation model of three-module cascaded SST with three-phase star connection is established in Matlab/Simulink. Several case studies are carried out under different operating conditions. Simulation results validate the feasibility of the proposed method.

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.

Robust Model-Predictive Control for a Compound Active-Clamp Three-Phase Soft-Switching PFC Converter Under Unbalanced Grid Condition

Two main problems, including unbalanced grid input and circuit parameters uncertainty in the three-phase soft-switching converter control system, are considered in this paper. First, the constant-switching-frequency model-predictive control (CF-MPC) is improved for application in compound active-clamp zero-voltage-switching three-phase power factor correction converter. Second, a robust-constant-frequency model-predictive control (RCF-MPC) method is proposed for current loop; the novel contributions are: 1) the negative-sequence current component is introduced into the cost function of the model-predictive control to decrease the influence of unbalanced grid input; and 2) a robust item is added into the predictive model to reduce the influence of the circuit parameter uncertainty. Third, a robust variable structure controller is proposed for voltage loop considering the capacitor and load uncertainty. The simulation and experiment results show that, compared with the original CF-MPC method and the traditional proportional–integral control method, the proposed RCF-MPC method decreases the total harmonic distortion under the unbalanced grid condition. Meanwhile, the voltage loop robust variable structure controller has better transient and steady-state performance with respect to the load perturbation.

A novel controller for harmonics reduction of grid-tied converters in unbalanced networks

This paper proposes a novel controller for a grid-tied converter interfaced to a network with unbalanced voltage conditions. Due to the influence of negative sequence voltage and current components created by unbalanced voltages, conventional PI controller alone cannot adequately deal with the fluctuations in active and reactive powers generated from converters. In order to overcome this inadequacy, a systematic and comprehensive controller, namely Linear Parameter Varying (LPV) controller is designed and presented in this work. The proposed LPV controller is implemented using the well-known voltage oriented control (VOC) approach. The unbalanced network voltages are first decomposed using a signal delay technique with the support of Clarke’s transformation in order to filter out the negative sequence components. An enhanced phase-locked loop (ePLL) is then developed to reduce the higher-order harmonics. The feasibility of the proposed controller and the effectiveness of ePLL are verified by experimental studies.

Fundamental Sequence Component Based Hysteresis Current Controlled MSALC

Harmonic and power factor reduction is caused by non-linear component and current unbalance in the system. It is very often identified power quality issue. Fundamental negative sequence current tends to rise the temperature of electrical rotating machine, static electrical devices and unnecessary open circuit of electrical protection devices. Harmonic current causes rise in line losses and skin effect. In this paper, stationary reference frame control is proposed and executed based on fundamental positive and negative Sequence Component Evaluation. Fundamental positive sequence current is accounted as instruction parameter for Electric Potential Source Inverter. Modified Shunt Active Line Conditioner is employed as current conditioner. MSALC is modeled to eliminate fundamental negative sequence current component thereby balancing currents. Ultimately low Total Harmonic Distortion and low Harmonic Current Ratio Factor and fall in lines losses and enhancement of power factor are analyzed in the power distribution system.

Ride‐through strategy of quasi‐Z‐source wind power generation system under the asymmetrical grid voltage fault

Asymmetrical grid voltage will introduce negative-sequence current components on the grid side and cause large voltage ripple of quasi-Z-source capacitors. It will lead to the torque ripple of generator. This study proposes a novel control method based on super-capacitor (SC) to improve the capability of asymmetric fault ride through in quasi-Z-source wind power generation system (WPGS). The stream of system power, in the condition of unbalanced grid voltage, is discussed in detail in this study. To limit the voltage rise and suppress the 2-order voltage ripple of quasi-Z-source capacitors, the throughput power of SC is controlled by proportional–resonant controller to track the output power subtraction between generator and quasi-Z-source inverter. Meanwhile, the negative-sequence component of the grid-connected current is decreased significantly. Finally, simulation and experimental results are given to verify the theoretical analysis.

Evaluation of Superimposed Sequence Components of Currents based Islanding Detection Scheme during DG Interconnections

Abstract A new islanding detection scheme for distribution network containing different types of distributed generations (DGs) is presented in this paper. The proposed scheme is based on acquiring three phase current samples for full cycle duration of each simulation case of islanding/non-islanding conditions at the point of common coupling (PCC) of the targeted DG. Afterwards, superimposed positive &amp; negative sequence components of current are calculated and continuously compared with pre-determined threshold values. Performance of the proposed scheme has been evaluated on diversified islanding and non-islanding events which were generated by modeling standard IEEE 34-bus system using PSCAD/EMTDC software package. The proposed scheme is capable to detect islanding condition rapidly even for perfect power balance situation for both synchronous and inverter based DGs. Furthermore, it remains stable during non-islanding events such as tripping of multiple DGs and different DG interconnection operating conditions. Therefore, the proposed scheme avoids nuisance tripping during diversified non-islanding events. At the end, comparison of the proposed scheme with the existing scheme clearly indicates its advantage over the existing scheme.

전압 불평형 조건에서 스위칭 소자의 전류용량을 고려한 MMC-HVDC 순환전류 제어기법

This paper proposed a new control method which is capable of controlling circulating current considering current capacity of switching device. In the unbalanced voltage conditions, active power and reactive power have double line frequency. Thus, in order to provide active power without ripple, it is necessary to inject the negative sequence current components. However, when the negative current components is injected, it increases the total current flowing in the Arm, and in the Sub-module(SM) the current more than rated is impressed, which leads to destroy the system. Also, in impressing the circulating current reference of each arm, conventional control method impressed applicable iSUBdck/SUB/3 in the case of balanced voltage conditions. In the case of unbalanced conditions, as arm circulating current of three phase show difference due to the power impressed to each arm, reference of each arm is not identical. In this study, in the case of unbalanced voltage, within permitted current, the control method to decrease the ripple of active power is proposed, through circulating current control and current limitations. This control method has the advantage that calculates the maximum active power possible to generate capacity and impressed the current reference for that much. Also, in impressing circulating current reference, a new control method proposes to impress the reference from calculating active power of each phase. The proposed control method is verified through the simulation results, using the PSCAD/EMTDC.