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This paper introduces an approach to calculating the minimum value of split DC link capacitance in three-phase three-level grid-connected DC-AC converters operating with unity power factor without either active balancing circuits or AC zero sequence injection. Due to the fact that partial DC link voltages and rectified mains phase voltages reach their maximum and minimum values, respectively, at different time instants, it is feasible to decrease the minimum value of the former below the maximum value of the latter while still maintaining proper functionality of the power stage. The minimum possible split DC link capacitance values are hence derived from the boundary condition where the above-mentioned voltages are tangent to each other. The accuracy of the analytical derivations is confirmed by simulations and experiments carried out on a 10 kVA T-type converter prototype, which show a high degree of agreement. [ABSTRACT FROM AUTHOR]

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With the increasing applications of grid-connected voltage source converters, it is important to analyze the impedance characteristic of the grid-connected system to implement the stability analysis of the grid connection operation. Compared with the three-phase three-wire system, an additional zero-sequence path exists in the three-phase four-wire system. Therefore, it is necessary to consider the influence of the zero sequence when analyzing the stability of the system. In this paper, an impedance model including positive-sequence, negative-sequence and zero-sequence impedance of the three-phase four-leg grid-connected inverter is established. Through impedance characteristic analysis, the relationships between zero-sequence impedance and positive-sequence, negative-sequence impedance can be found which are decupled. Based on the developed zero-sequence impedance model, the main factors affecting the zero-sequence impedance model are revealed. Then, the system stability consisting of the grid and three-phase four-wire inverter is investigated, and the experimental results validate the theoretical analysis.

Locating high impedance faults (HIF) in a distribution system increases the system’s reliability and improves the service’s quality for customers. However, this type of fault produces currents of low magnitude, making it difficult to locate using conventional techniques. Therefore, this paper proposes a method for the location of single-phase high impedance faults in distribution systems with photovoltaic cells in low voltage. The method is based on the use of the sequence component of the current circulating in the medium voltage feeders of the distribution system, which are obtained from synchronized phasorial meters positioned along the feeder. The method considers nonsymmetrical lines, unbalanced loads, capacitors banks, photovoltaic DGs, and load variation. The performance of the proposed method was evaluated and tested on several distribution systems using ATP or PSCAD® and the results show that the method can locate HIFs with high precision. Also, a comparison was made with recently published methods to show the efficiency of the proposed method. [ABSTRACT FROM AUTHOR]

The purpose of this article is to investigate the potential of obtaining an auxiliary dc output from the common-mode switching harmonics of a three-phase inverter without access to the neutral point of the ac load. To achieve this, the control of common-mode (CM) switching harmonics with space vector modulation is proposed. With this control, CM switching harmonics can be regulated independently from the differential mode power flow to the main three-phase load. To physically implement the CM power flow, a harmonic extraction circuit is placed in parallel with the main three-phase load. The extraction circuit is formed by series-connected inductance and capacitance. Due to the LC series resonance and the mutual coupling of the inductance, only common-mode switching harmonics can pass through the extraction circuit. The extracted harmonics are then rectified and delivered to the auxiliary dc output. The control and topology are verified in both simulations and experiments. In the end, it is shown in experiments that there is no significant difference of the inverter efficiency after the auxiliary dc load is connected. This method can be used for instance to drive an ac machine while charging a low voltage battery or powering a compressor simultaneously. [ABSTRACT FROM AUTHOR]

The multifunctional converter system (MFCS) is a special topology of three-phase ac motor drives, which inherently integrates a dc-voltage regulation function. First, in order to explain the mechanism of the MFCS, an average model is derived in this article. An equivalent circuit is developed, which explicitly reveals that the MFCS is equivalent to a cascaded dc/dc converter-dc motor drive system. Second, the inverter's mean duty cycle is identified as an independent control degree of freedom for the dc-bus voltage. Then, the influence of pulsewidth modulation (PWM) on the dc-bus voltage is studied. It concludes that space vector PWM is invalid for the MFCS. To address this problem, the zero-sequence voltage injection PWM (ZSVIPWM) is proposed and actually sinusoidal PWM is a special case of the ZSVIPWM. Third, an overall control strategy is proposed for the MFCS, which includes not only the vector control for the motor but also a closed-loop control for the dc-bus voltage. Finally, comparative experiments with the standard topology are carried out on a permanent magnet synchronous machine prototype to verify these theoretical analyses and the control strategy. [ABSTRACT FROM AUTHOR]

Abstract Current and voltage waveforms recorded by intelligent electronic devices (IEDs) are more useful than just performing post-fault analysis. The objective of this paper is to present techniques to estimate the zero-sequence line impedance of all sections of a three-terminal line and the Thevenin equivalent impedance of the transmission network upstream from the monitoring location using protective relay data collected during short-circuit ground fault events. Protective relaying data from all three terminals may not be always available. Furthermore, the data from each terminal may be unsynchronized and collected at different sampling rates with dissimilar fault time instants. Hence, this paper presents approaches which use unsynchronized measurement data from all the terminals as well as data from only two terminals to estimate the zero-sequence line impedance of all the sections of a three-terminal line. An algorithm to calculate positive-, negative- and zero-sequence Thevenin impedance of the upstream transmission network has also been presented in this paper. The efficacy of the proposed algorithms are demonstrated using a test case. The magnitude error percentage in determining the zero-sequence impedance was less than 1% in the test case presented.

This paper proposes an innovative method for dimensioning and designing the zero-sequence electromagnetic filter applied to three-phase four-wire distribution power systems. As premises, the optimum ratio between zero-sequence and positive-sequence impedances of the filter must be established in order to potentiate the filtering percentage of zero-sequence currents and ensure the quality of the supply system displacement power factor. The calculations are based on the sequential harmonic transmittances of the equivalent circuit reduced to the coupling point of the electromagnetic filter. To validate the proposed method, the results obtained with the application of the electromagnetic filter designed in a test system using the MATLAB software are compared to the results of the system modeled in the PSIM software. [ABSTRACT FROM AUTHOR]

Highlights • Zero-sequence network voltage compensation strategy. • Modular mulitlevel static synchronous compensator. • Control algorithm for four-wire static synchronous compensator. Abstract This paper presents a zero-sequence voltage compensation strategy for a three-phase four-wire modular multilevel static synchronous compensator used to regulate the network voltages. The control strategy is based on a synchronous reference frame mathematical modelling used to control the positive and zero-sequence voltages and currents of a MMC static synchronous compensator. The static compensator is controlled to synthesize AC currents in such a way to restore the positive-sequence voltage to its rated value and to mitigate the zero-sequence voltage at the point of common coupling. Digital simulations are presented to validate the static synchronous compensator mathematical analysis and to demonstrate the performance of the proposed control strategy. [ABSTRACT FROM AUTHOR]

Data recorded by intelligent electronic devices such as protective relays contain a wealth of valuable information that can be used beyond post-mortem analysis of fault events. The objective of this paper is to propose algorithms to estimate zero-sequence line impedance and fault resistance using protective relay data collected during short-circuit fault events. Protective relaying data may be available from one or both ends of the line. For the latter, they may be collected at different sampling rates with dissimilar fault time instants. Furthermore, they may be unsynchronized. This paper presents approaches which uses measurement data from one and both ends of the line. The efficacy of the proposed algorithms presented in this paper are demonstrated using a test case as well as field data. The error percentage of the zero-sequence line impedance magnitude estimated using the data from both ends of a two-terminal line was under 3% for all the test cases. The error percentage in estimating the fault resistance was less than 1% in the test cases presented. [ABSTRACT FROM AUTHOR]

This paper proposes an input voltage sensorless control algorithm for three-phase active boost rectifiers. Using this approach, the input ac-phase voltages can be accurately estimated from the fluctuations of other measured state variables and preceding switching state information from converter dynamics. Furthermore, the proposed control strategy reduces the input current harmonics of an ac–dc three-phase boost power factor correction (PFC) converter by injecting an additional common-mode duty ratio term to the feedback controllers’ outputs. This additional duty compensation term cancels the unwanted input harmonics, caused by the floating potential between ac source neutral and dc link negative, without requiring any access to the neutral point. A 6-kW (continuous power)/10-kW (peak power) three-phase boost PFC prototype using SiC-based semiconductor switching devices is designed and developed to validate the proposed control algorithm. The experimental results show that an input power factor of 0.999 with a conversion efficiency of 98.3%, total harmonic distortion as low as 4%, and a tightly regulated dc-link voltage with 1% ripple can be achieved. [ABSTRACT FROM PUBLISHER]

The paper proposed a novel single-phase grounding fault line selection method of small current grounding system based on EMD and differential spectrum. The method uses EMD method to decompose transient zero-sequence current and extract high-frequency IMF1 component of each line, obtained differential spectrum and fluctuation value of IMF1 component of each line according to definitions of differential spectrum and fluctuation value, and found out the fault line which has the maximum fluctuation value of IMF1 component by comparing fluctuation values of IMF1 component of each line. The experiment results showed that the method has higher accuracy of line selection and is suitable for fault line selection of small current grounding system under different grounding resistances and different fault initial phase angles.

This paper proposes a zero-sequence model-based sensorless control strategy for open-winding permanent-magnet synchronous machine (OW-PMSM) with common dc bus. The third harmonic back electromotive force (EMF) in zero-sequence is utilized to estimate the rotor position information. The novelty is that the third harmonic back EMF is reconstructed based on the zero-sequence model together with zero-sequence current, instead of using voltage transducer and three phase resistance network which can be eliminated from the control system. Due to the essence of back EMF, the developed method is appropriate for the applications including wind power generation and motor drives operating above certain speed. Meanwhile, the phase shift-based space vector pulse width modulation method for OW-PMSM is also presented for easier implementation to form zero voltage disturbance in zero-sequence from the inverter side and hence the corresponding influence can be consequently minimized. The torque ripple, loss, and parameter sensitivity analysis are also carried out, as confirmed by predictions of two-dimensional finite element analysis. Due to the essence of decouple, the estimation can be more robust than the conventional fundamental model-based sensorless methods. Finally, the effectiveness of the proposed method is validated experimentally on a 3-kW OW-PMSM drive system with 2.5 kg·m2 total inertia. [ABSTRACT FROM AUTHOR]

When the faulty cells of a nonregenerative cascaded H-bridge inverter are bypassed, it is necessary to inject a zero-sequence voltage into the inverter phase voltages so as to achieve the maximum balanced line-to-line voltage. However, the injected zero-sequence voltage may lead to a back flow of real power (BFRP) in at least one phase so that the dc voltage will rise to an intolerable level. To solve the problem, this paper proposes a new method of generating zero-sequence voltages. This method can not only maximize the available output voltage but also suppress the BFRP effectively. The zero-sequence voltage is always limited to an appropriate range to achieve the maximum output voltage through linear modulation. Closed-loop control is used to minimize the fundamental component of the zero-sequence voltage so that the power is less likely to flow back. Compared with the conventional methods, the proposed method allows the inverter to drive a load with a lower power factor not inducing BFRP or degrading the output voltage capability. As a result, the probability of overvoltage on the dc side is reduced. The experimental results show that the proposed method is effective and feasible. [ABSTRACT FROM AUTHOR]

• The paper constrains the zero-sequence currents of the ungrounded winding nodes to zero, and combines with the power equations and the Newton Raphon method to propose a three-phase power flow algorithm for ungrounded distribution networks. • The paper analyzes (a) the convergence problem of the method that constrains squares of zero-sequence voltages and currents to zero based on mathematical derivation and (b) the inapplicability problem of the method that constrains zero-sequence voltages to zero of the ungrounded winding nodes based on both a theoretical analysis and verification by examples. • The algorithm proposed in the paper is universally applicable, and has a low computation time, high efficiency and good convergence performance. It has the potential to be widely applicable to engineering practice. The existing research on three-phase power flow in ungrounded distribution networks primarily adopts the methods of presetting zero-sequence reference voltages and neutral point reference voltages. This makes it impossible to calculate the power flow in ungrounded network with non-zero neutral point voltages or zero-sequence voltages. Furthermore, a unified zero-sequence components constraint method of ungrounded distribution network does not exist. In this paper, a three-phase power flow algorithm is proposed to calculate three-phase power flow in an ungrounded distribution network. First, power flow equation variables, transformer admittance matrices and zero-sequence components are used to analyze the characteristic of three-phase power flow in an ungrounded distribution network. Second, a three-phase power flow algorithm for ungrounded distribution network is designed by constraining the zero-sequence currents of the ungrounded winding nodes to zero. Third, theoretical analyses and mathematical derivations are used to prove the applicability of the two methods, one of which constrains zero-sequence voltages of ungrounded winding nodes to zero and the other constrains squares of zero-sequence voltages and currents to zero. Last, a five-nodes network, modified IEEE123 network and practical network are used to demonstrate that the proposed method has a high accuracy and good applicability in calculating the three-phase power flow of ungrounded distribution networks. [ABSTRACT FROM AUTHOR]

This paper proposes a novel circulating-current control strategy applied in back-to-back parallel three-phase converters with individual dc links for permanent magnet synchronous motor (PMSM) drives. First, the zero-sequence circulating current and the cross current paths in the whole parallel system are analyzed. Then, the models of an n-paralleled subsystem with individual dc links in both the three-phase stationary coordinates and the synchronous rotating coordinates are built up, which explores the fundamental mechanism of circulating current. Based on the analysis, a novel control strategy can suppress the circulating current at the motor side. The control system consists of two main loops in the d-axis, q-axis, and zero-axis, which are defined in this paper as sum control and difference control. The proposed control strategy effectively suppresses the circulating current in both steady-state and transient operations. The zero-sequence circulating current is greatly reduced by the difference control on the zero-axis. In addition, the bandwidth of the controllers is greatly improved, especially when the current-sharing inductance is much smaller than the stator inductance. Finally, with a PMSM load, the simulation and experiment results are presented to verify the theoretical analysis and proposed control strategy. [ABSTRACT FROM PUBLISHER]

In this paper the performances of three reference currents and DC bus voltage control techniques for Three-Phase Four-Wire Four-Leg SAPF are compared for balanced and unbalanced load conditions. The main goals are to minimize the harmonics, reduce the magnitude of neutral current, eliminate the zero-sequence current components caused by single-phase nonlinear loads and compensate the reactive power, and on the other hand improve performances such as robustness, stabilization, trajectory pursuit, and reduce time response. The three techniques are analyzed mathematically and simulation results are compared. The techniques considered for comparative study are the PI Control, Sliding Mode Control and the Backstepping Control. Synchronous reference frame theory (SRF) in the dqo-axes is used to generate the reference currents, of the inverter. [ABSTRACT FROM AUTHOR]

• The basis for the selection of multi-criteria fault line selection is given. • The proposed method avoids the complex signal processing and analysis process. • Avoid misjudgments caused by unbalanced load current interference. • Simplifies the fault identification process by means of comprehensive criteria. Single-phase grounding faults often occur in low-current grounding systems, and the selection of fault lines is an important factor affecting the reliability and safety of the distribution system. Aiming at the problem that the existing line selection criteria may cause misjudgment, this paper makes full use of the changes in the amount of fault information during the occurrence of a fault and proposes a multi-criteria line selection basis that integrates fault information. Since different neutral grounding methods will affect the change of the fault state quantity, the characteristic changes of the fault state quantity under different neutral grounding methods are fully discussed, and different fault quantities are selected as the basis for line selection. In addition, a membership function and a weight coefficient function are respectively set to indicate the possibility and credibility of a single criterion line selection result. The weighted summation of the membership functions corresponding to multiple criteria completes the information integration of multiple criteria, and the formed composite criterion reduces the proportion of misjudgments. The simulation experiment data proves that the proposed method can correctly, effectively, and reliably select faulty lines, greatly reduce the possibility of misjudgment and improve the efficiency and universality of line selection. [ABSTRACT FROM AUTHOR]

This paper presents analysis and control of a hybrid filter that can suppress neutral harmonic currents in three-phase four-wire systems not only with harmonic voltage-source loads but also with harmonic current-source loads. The filter voltampere rating is significantly reduced compared to the conventional neutral-current-suppressing methods. The rectifier part of the neutral harmonic suppressor has been removed by controlling the dc capacitor voltage. The experimental results on a 50-kVA prototype validate the proposed control approach. [ABSTRACT FROM AUTHOR]

Current and voltage waveforms recorded by intelligent electronic devices (IEDs) are more useful than just performing post-fault analysis. The objective of this paper is to present techniques to estimate the zero-sequence line impedance of all sections of a three-terminal line and the Thevenin equivalent impedance of the transmission network upstream from the monitoring location using protective relay data collected during short-circuit ground fault events. Protective relaying data from all three terminals may not be always available. Furthermore, the data from each terminal may be unsynchronized and collected at different sampling rates with dissimilar fault time instants. Hence, this paper presents approaches which use unsynchronized measurement data from all the terminals as well as data from only two terminals to estimate the zero-sequence line impedance of all the sections of a three-terminal line. An algorithm to calculate positive-, negative- and zero-sequence Thevenin impedance of the upstream transmission network has also been presented in this paper. The efficacy of the proposed algorithms are demonstrated using a test case. The magnitude error percentage in determining the zero-sequence impedance was less than 1% in the test case presented. [ABSTRACT FROM AUTHOR]