Positive Sequence Components Research Articles

A low voltage ride-through approach for virtual synchronous machines under asymmetric fault conditions

Abstract With the advancement of renewable energy generation technologies, virtual synchronous machine (VSG) technology has garnered significant attention for enhancing power system stability. However, under asymmetric grid faults, the operational stability of VSMs under low voltage conditions remains a challenge. To address this, this paper presents a low-voltage ride-through strategy that utilizes dual-loop control for managing both positive and negative sequence components, aiming to improve the dynamic response of VSG during asymmetric faults. By thoroughly analyzing the VSM topology and mathematical model, an improved control scheme is developed. The effectiveness and practicality of the proposed strategy are ultimately validated through simulations under complex fault conditions.

Power quality enhancement of three-phase solar photovoltaic system-based generalized integrator controlled UPQC

Abstract As distributed generation becomes increasingly prevalent, providing reliable and stable electrical energy to the grid is of paramount importance. Therefore, precise monitoring of grid voltage is necessary in order to synchronize compensating current and voltage with the grid. The Dual Second-Order Generalized Integrator (DSOGI) can extract the fundamental positive sequence component from three-phase grid signals, enabling accurate determination of grid phase and current signals for rapid and precise grid compensation. This paper presents a control algorithm for extracting and compensating UPQC reference signals based on the Second-Order Generalized Integrator (SOGI). The improved photovoltaic-fed Unified Power Quality Conditioner System (PV-UPQC) is verified and analyzed under conditions of unbalanced grid voltage, current harmonics, as well as voltage sags and swells. It can be demonstrated that the enhanced system exhibits superior adaptability to the grid.

Power Quality Enhancement in Grid-Connected Solar PV Systems Using Adaptive Control and Multilevel Inverter Topologies Under Dynamic Grid Conditions

This paper introduces a control strategy that uses an integrator-based positive sequence estimator to improve the power quality of a grid-connected double-stage solar PV system. The control extracts positive sequence components from distorted and unbalanced grid voltages to calculate unit templates and grid reference currents during grid anomalies like distortion, unbalance, sag/swell, and DC offset. A feedforward term is introduced to the photovoltaic array to ensure rapid transient response, and the DC-link voltage is adaptively regulated to reduce the voltage source converter (VSC) losses during weak grid conditions. The paper compares the performance of two-level and three-level inverters in this system configuration, highlighting the differences in power quality improvement, harmonic reduction, and system efficiency under various grid disturbances and intermittent solar conditions. Simulation results, performed in MATLAB, validate the superior performance of the three-level inverter in terms of lower total harmonic distortion (THD) in grid currents, in compliance with IEEE standard 519, while demonstrating the satisfactory response of both inverter types during challenging grid conditions like intermittent solar irradiation, DC offset, voltage distortions, unbalance, and voltage sag/swell.

Modeling of Grid Tied PV Inverter to Improve its Performance During Unbalanced Load and Unbalance Fault

Integrating solar through inverter brings about various challenges in the microgrids and the inverter itself. This paper aims to the study of problems in the operation of inverter during unbalance load and unbalance fault condition along with the possible control measures to mitigate those problems to ensure the safe and reliable operation of the inverter. A grid connected PV inverter is modeled using hysteresis band controller and implementing the MPPT of the PV system with the buck boost converter. During unbalance load and unbalance fault condition, the negative and zero sequence components of current exists. All the zero-sequence component of current is delivered by the grid. The negative sequence component of current also flows through the inverter resulting in high value of current and loss of stability. Also, the voltage across the dc link capacitor increases during fault. The high value of current flowing through the inverter and high voltage across dc link capacitor may damages the inverter and capacitor respectively. An inverter dual current controller is introduced replacing the hysteresis band control such that the negative sequence component of current flowing through the inverter is almost mitigated from 600A to 10A and positive sequence current is also reduced relatively to a tolerable value 150A from 550A. The total current flowing through the inverter is limited to 200A from 950A and voltage across dc link capacitor is limited to 700V from 1050V during LG fault at the inverter side thus ensuring safe operation of the inverter during voltage dip condition. A further study can be done on limiting the positive sequence component of current to a rated value and improving the performance while operating in islanded mode.

Converter-based multi-frequency power transfer system for efficient energy packet transmission in the energy internet

The energy internet concept involves the transmission of energy in discrete packets, akin to the information internet's principle. The energy internet must adopt a decentralized configuration and enable bidirectional and simultaneous power transfer. This raises the problem of congestions during the energy packet dispatching process. To address potential congestion issues, we propose a three-phase multi-frequency power transfer system (MFPTS) where the positive-sequence fundamental frequency component is supplemented with the zero-sequence third and the negative-sequence fifth harmonic components. By using such signals with proper amplitudes, the harmonics are added to the voltage without changing the peak amplitude but increasing the rms value. To exert complete control over the different frequencies employed within the MFPTS, a decoupling method is necessary to separate these frequencies in the control system. This leads to creating three independent channels of power transfer by three frequencies without changing the existing infrastructure as these powers can be transferred within the same lines. This paper explores the feasibility of three-phase three-frequency power transfer by using three parallel power transfer channels, achieved through the utilization of three-phase grid-forming and grid-feeding converters. Using power converters provides the advantage of bidirectional power transfer and facilitates decentralization by serving as an interface between distributed power generation systems and the utility grid. These properties are essential for achieving optimal energy internet performance. The simulation and practical results prove that the proposed MFPTS could solve the energy packet dispatching congestion problem to have a reliable energy internet for the future.

Positive sequence component based directional relaying for lines integrated with solar photovoltaic plant

Different fault current characteristics of converter interfaced solar photovoltaic plants as compared to the synchronous generator based sources impose challenges to the commonly used directional relaying methods. In this work, the issues with phasor based directional relay algorithms are analysed for the solar plant connected lines. A directional relaying method is proposed for such connectivity using the phase angle difference between positive sequence voltage and the line current measured at the local bus relay. Using PSCAD/EMTDC simulations the proposed method is tested for solar plant connected 39 bus system and validated using field data. The accuracy of the proposed method is also evaluated on a real-time simulation platform. Comparative analysis with the conventional directional relaying methods reveals the superiority of the proposed method.

A Pilot Protection of Negative Sequence and Additional Network Considering Photovoltaic Integration

Background: Introducing pilot protection in active distribution networks containing PV can improve the reliability and selectivity of protection. However, the basic communication facilities of the existing distribution network make it difficult to meet the requirements of data synchronization, and the PV T-connection to the network leads to sudden changes in the impedance angle. Methods: Therefore, pilot protection of a negative sequence and additional network considering PV is proposed. The scheme is based on the feature that the PV model only outputs positive sequence components after a fault. For asymmetrical faults, the negative sequence impedance detected at both ends of the protection is utilized to construct a comparative negative sequence impedance protection criterion. For symmetrical faults, the voltage characteristics of the faulty additional network are utilized to construct a protection criterion. Results: Finally, an actual distribution network model with PV is constructed in PSCAD to verify the effectiveness and reliability of the protection method. Conclusion: The protection method is selective and reliable and is not affected by high penetration rate and PV fault characteristics.

Harmonic Sequence Component Model-Based Small-Signal Stability Analysis in Synchronous Machines during Asymmetrical Faults

Power systems are complex and often subject to faults, requiring accurate mathematical models for a thorough analysis. Traditional time-domain models are employed to evaluate the dynamic response of power system elements during transmission system faults. However, only the positive sequence components are considered for unbalanced faults, so the small-signal stability analysis is no longer accurate when assuming balanced conditions for asymmetrical faults. The dynamic phasor approach extends traditional models by representing synchronous machines with harmonic sequence components, making it suitable for an unbalanced condition analysis and revealing dynamic couplings not evident in conventional methods. By modeling electrical and mechanical equations with harmonic sequence components, the study implements an eigenvalue sensitivity analysis and participation factor analysis to identify the variable with significant participation in the critical modes and consequently in the dynamic response of synchronous machines during asymmetric faults, thereby control strategies can be proposed to improve system stability. The article validates the dynamic phasor model through simulations of a single-phase short circuit, demonstrating its accuracy and effectiveness in representing the transient and dynamic behavior of synchronous machines, and correctly identifies the harmonic sequence component with significant participation in the critical modes identified by the eigenvalue sensitivity to the rotor angular velocity and rotor angle.

A Fault Direction Criterion Based on Post-Fault Positive-Sequence Information for Inverter Interfaced Distributed Generators Multi-Point Grid-Connected System

In response to the poor reliability in identifying fault direction in distribution networks with Inverter Interfaced Distributed Generators (IIDGs), considering the control strategy of low-voltage ride-through, a fault direction criterion based on post-fault positive-sequence steady-state components is proposed. Firstly, the output steady-state characteristics of IIDGs considering the low-voltage ride-through capability are analyzed during grid failure, and the applicability of existing directional elements in a distribution network with IIDGs connected dispersively is demonstrated. Subsequently, for the typical structure of an active distribution grid operating under flexible modes, the positive-sequence voltage and current are examined in various fault scenarios, and a reliable direction criterion is suggested based on the difference in post-fault positive-sequence impedance angles on different sides of the lines that are suitable whether on the grid side or the IIDG side. Lastly, the reliability of the proposed direction criterion is verified by simulation and the results indicate that the fault direction can be correctly determined, whereas phase-to-phase and three-phase short circuit faults occur in different scenarios, independent of the penetration and grid-connected positions of IIDGs, fault location, and transition resistance. It is suitable for fault direction discrimination of an IIDGs multi-point grid-connected system under a flexible operation mode.

A developed DQ control method for shunt active power filter to improve power quality in transformers.

Power transformers are the most important component in power system. Exposing these transformers to the harmonic distortions causes additional heat losses, insulation stress, decrease in lifetime of insulation, and reduced power factor with decrease in efficiency of the system. The lifespan of distribution transformers is influenced by the fragility of power quality in power networks. Harmonic mitigation filters with robust control technique are required to reduce the harmonic effects on power transformer. Traditionally, synchronous reference frame (DQ) is employed to control the shunt active power filter (APF) for mitigation of harmonics in power transformers. DQ control of Shunt APF lacks merits of fast response, delayed operation due to phased lock loop under abnormal grid conditions leading to insufficient harmonic elimination. A developed DQ method based on detecting the positive and negative sequence components is proposed to precisely control the shunt APF for reliable operation of power transformer. This detection technique improves the response time, mitigate the harmonics effecting the operation of transformer and overall power factor. The proposed control system is evaluated under different abnormal operating scenarios and compared with traditional DQ method. The results and analysis confirm the efficacy of the developed DQ method in improving the power transformer performance.

Research on electromagnetic torque and fault parameter identification of pumped storage machine under power generation state based on optimized network parameter method

Because of the complex and frequent multi-operation conditions of pumped storage machine and the difficulty of its numerical analysis, an optimized network parameter method (ONPM) is given to identify the second harmonic electromagnetic torque of large salient-pole generators. Different from the former research which only considers the positive and negative sequence components, the second harmonic electromagnetic torque of pumped storage machine under power generation state is identified in neutral point grounded system, which are displayed by the lumped parameters. Then, in order to test the validity of the theoretical analysis among power generation salient-pole generators, the finite element model of 300 MVA pumped storage machine under power generation is established. Through the comparison and analysis of the finite element result data with the actual experimental data in the steady state of no-load test and short-circuit test, the correctness of the finite element model is completely verified. Finally, in the case of small current asymmetry and large current asymmetry, the second harmonic electromagnetic torques obtained by ONPM and finite element method (FEM) are found respectively, and their relative errors are elaborately displayed. The results display that ONPM can accurately identify the electromagnetic torque parameters which indicate the specific operation and fault state of pumped storage machine under power generation state. This fault parameter identification has practical significance for monitoring and improving the operation state and stability of large salient-pole generator.

Neural Network Quadrature Signal Generator-Based Virtual Flux Estimation for Predictive Control of PWM Converters

In this paper, a frequency-adaptive neural network-based virtual flux (FANN-VF) estimator is developed for sensorless control of a pulse-width modulation converter under unbalanced and distorted grid conditions. This method exploits two parallel FANNs configured as quadrature signal generators. The VF fundamental positive and negative sequence components (PNSCs) are inherently separated in the estimator without employing any cascaded filters. A frequency-locked loop is introduced to accurately track the frequency variations. The estimated VF PNSCs are directly exploited to compute the power references in VF-based predictive direct power control scheme. The developed strategy ensures constant active power and sinusoidal current waveforms under nonideal grid conditions. Simulation and experimental tests are performed to verify the effectiveness of the proposed method.

A novel sequence component based fault detection index for microgrid protection

The paper introduces a novel protective relaying method using symmetrical components of current to detect asymmetrical faults in microgrid. This approach is focused on utilizing the complex magnitude-phase plane (MPP) of the fault detection index (FDI), which is computed using the positive and negative sequence components of the current signal retrieved at one end of the faulty line. The MPP obtained from the settling values of the FDI's magnitude (MFDI) and FDI's angle (AFDI) after the fault inception, is considered as the key indicator for fault detection. The scheme is comprehensively evaluated for a wide range of fault situations including variations in fault types, fault locations, fault resistances and DG penetrations in grid-connected (GC) and non-grid-connected (NGC) modes of operation. Several critical no fault cases are also tested to show the efficacy of the proposed logic for false operation. The fault study is conducted extensively on a modified low voltage standard Consortium for Electric Reliability Technology Solutions (CERTS) microgrid and a standard IEEE-34 bus distribution system including DGs. Furthermore, the proposed FDI-based scheme is tested on MATLAB/Simulink platform and validated on Typhoon HIL platform to assess the real-time performance. The test results show that this FDI-based scheme using the MPP concept can be a potent solution for the protection of microgrid.

Model Predictive Control on Transient Flux Linkage and Reactive Power Compensation of Doubly Fed Induction Wind Generator

For weak grid scenario with high new energy proportion, large fluctuations of load are prone to cause low-voltage ride through. Moreover, stator transient magnetic flux will cause overvoltage and overcurrent problems in the rotor of doubly fed induction generator. Based on model predictive control, a control strategy for transient flux linkage and reactive power compensation is proposed. Firstly, regarding the issue of reactive power allocation of grid side converters (GSC) and rotor side converters (RSC), an allocation strategy is derived under minimizing winding energy loss on case of low-voltage ride through, enabling the wind energy conversion system to provide reactive power support during grid voltage recovery process. Meanwhile, an improved mixed second- and third-order generalized integrator (MSTOGI) phase-locked loop (PLL) is used to extract the positive and negative sequence components of the power grid voltage, further for RSC control. Secondly, in response to power grid faults, considering the influence of stator DC transient flux and negative sequence flux components on rotor current, by injecting rotor transient compensation current and stator flux feedforward compensation into RSC, the rotor impulse voltage and loop current are reduced. Moreover, combined with model predictive control algorithm, a control strategy of rotor current is designed. Finally, a simulation platform is built to validate the effectiveness of the proposed method based on comparing with several traditional vector control low-voltage ride through methods.

Fault Location Method for Distribution Network Using an Additional Inductance Strategy

In distribution networks, time asynchrony exists between the phasor measuring unit (PMU) at both ends of a line, and the effective measurement time of the devices is short, leading to insufficient accuracy in phasor measurements. This paper proposes a fault location method for distribution networks that employ an additional inductance strategy to address the limited location accuracy caused by time asynchrony and the inadequate accuracy of phasor measurement devices. The method enhances the stability and accuracy of phase measurement by connecting an additional inductance after the online circuit breaker, thus extending the effective measurement time. It uses the symmetrical component method to obtain the positive-sequence and negative-sequence networks following a fault. Time asynchrony is treated as an equivalent asynchronous phase angle, which is then applied to the positive and negative-sequence voltage components. The impact of time asynchrony is mitigated by compensating for the phase angle difference using the ratio of the positive-sequence voltage component to the negative-sequence voltage component. This approach provides the fault location function and has improved the accuracy of fault location, which is advantageous for rapid fault repair.

Environment and Power Quality International Conference on Renewable Energies and Power Quality (ICREPQ’09) Valencia (Spain), 15th to 17th April, 2009 Evaluation of Different Methods for Voltage Sag Source Detection Based on Positive-Sequence Componen

This paper evaluates methods for voltage sag source detection, which are based either on energy, current or impedance criteria. It is shown that some methods known from the literature do not work well, particularly in cases of asymmetrical voltage sags. Furthermore, some methods require measurements of instantaneous values, which is not always feasible. Therefore, symmetrical component transformation is applied, whereas only positive-sequence components are used. All the discussed methods were tested by applying extensive simulations and field tests. The obtained results show that all methods which are based on positive-sequence components are highly effective also in cases of asymmetrical voltage sags.

Control Strategy of PWM Rectifiers Connected to Unbalanced Grids

Current-controlled voltage source converters are widely used in grid-connected applications, for example at ac drives with indirect frequency converters. The negative sequence component of the grid voltage causes ripples of the dc voltage in the intermediate circuit. Some sophisticated topologies of current controllers that can cope with grids containing both the positive and negative sequence components are summarized and compared for this purpose. A new dual current control algorithm was developed and tested in the Matlab/Simulink environment.

Power Control Strategies During Voltage Sags According to Spanish Grid Code

In order to connect any power converters into the grid there are some grid requirements to insure the safe operation of the grid. So, the control of the converters especially during abnormal condition-e.g. during voltage sags- is a very important key to guarantee the good behavior of the distributed generation system. In this paper four control strategies,will be stated in the literature, are discussed in order to ensure their ability to match the grid requirements when unsymmetrical voltage sags are produced in the network. The Spanish grid code did not give any information about the negative sequence, and it only represents the positive sequence components. Therefore, the main contribution of this paper is to verify the grid code with not only the positive sequence but also with the negative sequence. Moreover, the system is tested by simulation to show that the results cope well with the analytical equations.

An island detection methodology with protection against cyber attack

Unplanned islanding of micro-grids is a significant barrier to supplying continuous power to key customers. The identification of the islanding moments must be rapid to enable the distributed generators (DG) to perform control measures in the shortest possible period. Micro phasor measuring units (µ-PMU) are gaining popularity in distribution systems and micro grids as a result of their ability to produce high-quality data at a high speed. These µ-PMUs can be utilized to detect islands. However, the µ-PMU relies heavily on the communication system for transmission of data, which is vulnerable to cyberattacks. In consideration of the previous technique, this research provides a smart island detection application with µ-PMU having lowered cyberattack probabilities. This representation is equipped with a µ-PMU implemented on the relevant DG’s bus. The voltage data acquired from these µ-PMUs are processed using the sequence transformation in order to simulate the sequence component angle. The angular sum of the negative and positive sequence components is evaluated and the maximum value is deployed for detection of islanding. MATLAB/Simulink tests the proposed approach through an IEEE-34 node distribution network. Multiple simulations demonstrate the robustness of the technique.

A New Control Method for Operation of D-STATCOM under Unbalanced Conditions

This paper presents a new control method for D-STATCOM operation under unbalanced condition. The aim of this method is to balance the load currents. The proposed method extracts the negative and positive sequence components for generating reference values of current that should be injected in order to compensat the load. The proposed method offers structural simplicity and less calculation complexity. Simulation results indicate that this method is effective and D-STATCOM has good performance under unbalanced conditions.