Zero-sequence Voltage Research Articles

Reference Voltage Self-Equalization-Based Modulation Strategy for Open-End Winding PMSM Fed by Dual Three-Level Inverters With Common DC Bus

For the open-end winding permanent magnet synchronous motor (OEW-PMSM) fed by dual three-level inverters with a common dc bus, it is necessary to balance the neutral point (NP) voltage and suppress the zero-sequence current to improve system performance. The traditional methods to solve these two problems are usually contradictory and are difficult to obtain satisfactory results. This article proposes a novel modulation strategy to deal with this dilemma. In this strategy, the NP voltage balance is achieved by automatically equalizing the distribution proportion of the reference voltage of OEW-PMSM between the two inverters. At the same time, the strategy makes full use of the control freedom brought by dual inverters to control the three-phase voltages of OEW-PMSM independently, in order to ensure that the average value of zero-sequence voltage in a sampling period is zero in any case, and thus greatly reducing the zero-sequence current. The experimental results verify the effectiveness and feasibility of this strategy.

A novel model predictive control with an integrated SOC and floating DC-link voltage balancing for 3-phase 7-level PUC converter-based MV BESS

Compared to conventional multilevel topologies, the Packed U-Cell multilevel topology utilizes the minimum number of components and provides both maximum voltage levels and high reliability. When used in a battery energy storage system, its dc-link consists of a top branch battery storage and bottom branch flying capacitors, whose state of charge and voltages must be balanced.This paper proposes a novel control scheme with two cascaded controllers: the inner loop regulates the output filter current and dc-link voltages using a novel finite-set model predictive control, the outer one is fully integrated with the model predictive controller and regulates the battery state of charge by injecting a zero-sequence voltage. The proposed control scheme was successfully implemented and tested through simulation and real-time testing. The obtained results highlight good dynamic performance and robustness of the proposed scheme during active power injection/absorption and low voltage ride-through capability.

A Novel Capacitor-Voltage Balancing Strategy for Double-Y STATCOM Under Unbalanced Operations

This article proposes a novel capacitor-voltage balancing strategy for the double-Y modular multilevel converter static synchronous compensator operated under unbalanced current conditions of the system. The proposed method, based on zero-sequence voltage injection, manipulates the current distribution between the converter arms to minimize the converter ratings. The proposed balancing strategy is compared with the classical approach, based on controlling the circulating dc currents. For the same unbalanced conditions, the proposed strategy leads to lower energy rating for the converter as compared with the classical one, whereas the semiconductor ratings remain the same in both strategies. Theoretical findings are verified through experimental results.

Open‐phase fault tolerant driving operation of dual‐inverter‐based traction drive

A method for achieving open-phase fault tolerant operation of a dual-inverter-based traction drive with charging capability is presented. This method utilises a contactor to connect the switching nodes of the two half bridges which are normally used for AC charging in this dual inverter configuration. These half bridges are then modulated to provide a path for zero-sequence current and enhance the available voltage vector space of the drive by injecting a controllable common-mode voltage between the two inverters. A control method and five-level modulator based on zero-sequence voltage control and phase shifted carriers are introduced to accommodate this novel remedial method. Using this approach, the open-phase faulted dual inverter drive is able to operate with twice the speed range of the conventional open-phase fault tolerant approach, which is to provide a path for zero-sequence current by connecting the negative DC rails of the two inverters with a contactor. Simulation and experimental results with a prototype dual inverter drive are presented to validate the theory of the new control and modulation method. The experimental results showcase the proposed algorithms performance by dynamically responding to an open-phase fault and starting from standstill, thereby proving its validity for an electric vehicle application.

Design Methodology for Three-Phase Four-Wire T-Type Inverter With Neutral Inductor

Three-level (3L) converters have been widely used in industry for decades. Compared to the three-phase-three-wire (3P3W) 3L inverter, the three-phase-four-wire (3P4W) one is able to supply the unbalanced loads but has to afford much larger filter inductors because the neutral wire provides a path for high-switching zero-sequence currents. To save filter inductances, a neutral inductor is proposed to insert in the neutral wire. Meanwhile, a complete design methodology is put forward to design the filter inductors and the neutral inductor. With low-frequency zero-sequence currents flowing through the neutral wire, the three-phase load voltages might become unbalanced and/or distorted. To improve the voltage quality, a resonant controller, with the resonant frequency at fundamental output frequency (fo), is presented to add into the zero-sequence voltage loop for balancing load voltages; concurrently, the other resonant controller, with the resonant frequency at 3fo, is presented to insert in the zero-sequence voltage loop or neutral current loop for mitigating voltage distortion. Finally, all of the proposed works are verified on a 3P4W T-type inverter.

Attenuation of Zero Sequence Voltage Using a Conventional Three-Wire Dynamic Voltage Restorer

Voltage sags/swells and harmonics are recurring problems in electric energy distribution systems. In order to solve these issues, several dynamic voltage restorer (DVR) topologies, such as the conventional three-wire DVR, have been proposed in the literature. Despite its capability of mitigating voltage disturbances, many researchers have established that conventional three-wire DVR cannot compensate for zero sequence voltage disturbances. In this paper, an in-depth study of the conventional three-phase DVR is presented, which shows that this DVR topology can also be used to attenuate zero sequence voltage components without increasing control complexity. The necessary conditions for this to occur are discussed in details and a brief comparison between the conventional three-wire DVR and other DVR topologies that can compensate for zero sequence voltage disturbances is made. Experimental results are included to validate the theoretical study.

Zero-Sequence Voltage Injection Method for DC Capacitor Voltage Balancing of Wye-Connected CHB Converter under Unbalanced Grid and Load Conditions

Leg capacitor energy balancing control is one of the crucial issues for stable operation of a cascaded H-bridge (CHB) converter. Because this topology inherently consists of numerous submodule cells with DC capacitors, the cell voltages and leg capacitor energy instantaneously fluctuate depending on operation sequence of the CHB converter. In general, a wye-connected CHB-converter-based static synchronous compensator (STATCOM) utilizes a zero-sequence voltage component for leg capacitor energy balancing. In this paper, to improve the dynamics of leg energy balancing control, a feedforward calculation method of the zero-sequence voltage injection is proposed. The feedforward term can be instantaneously calculated by using the information from the measured leg voltages and leg currents, and the method ensures successful regulation of the leg energy balance even under unbalanced grid and load conditions. Moreover, the verification of the proposed method is supported by the mathematical vector theorems. A 50MVA full-scale wye-connected CHB–STATCOM system simulation was performed to verify the proposed feedforward calculation method considering unbalanced grid as well as unbalanced load conditions.

3-D Vector-Based Model Predictive Current Control for Open-End Winding PMSG System With Zero-Sequence Current Suppression

In this article, 3-D vector-based model predictive current control (MPCC) for the open-end winding permanent magnet synchronous generator (OEW-PMSG) system driven by a dual inverter with a common dc bus is proposed to suppress the zero-sequence current. First, the 3-D voltage vector distribution of this system is analyzed and their zero-sequence voltages are investigated. Then, in order to reduce the computation burden of prediction control caused by vector enumeration, an improved current prediction control is developed, in which candidate voltage vectors for the dual inverter are determined according to the position of the zero-sequence reference voltage vector and the sector location of the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\alpha \beta $ </tex-math></inline-formula> -axis reference voltage vector. Moreover, to further improve the steady-state control performance of the whole generation system, a separate vector-selection method for the dual inverter is proposed. In this method, the first inverter applies one voltage vector in a control period and the voltage vector of the second inverter is synthesized by two vectors and selected in 3-D space. Finally, experiments are carried out to validate proposed methods, and the comparison experiments indicate that the proposed method has better control performance compared to conventional methods.

IEC 61850-Based Centralized Protection against Single Line-To-Ground Faults in Ungrounded Distribution Systems

We developed an International Electrotechnical Commission (IEC) 61850-based centralized protection scheme to prevent single line-to-ground (SLG) faults in the feeders and busbars of ungrounded distribution systems. Each feeder intelligent electronic device (IED) measures its zero-sequence current and voltage signals and periodically transmits zero-sequence phasors to a central IED via a Generic Oriented Object Substation Event message. Using the zero-sequence phasors, the central IED detects SLG faults in feeders and busbars. To achieve centralized protection, angle differences between the zero-sequence currents and voltage phasors are exploited, and their calculation compensates for data desynchronization. The feeder IEDs were implemented using the MMS-EASE Lite library, while the transmitted zero-sequence phasors were calculated based on fault signals simulated by Power System Computer Aided Design / Electro-Magnetic Transient Design and Control (PSCAD/EMTDC). The central IED determined if the SLG fault was in a feeder or busbar by aggregating and analyzing the zero-sequence phasors received from the feeder IEDs. The results confirmed the validity and efficiency of our centralized protection scheme.

Active Power Backflow Control Strategy for Cascaded Photovoltaic Solid-State Transformer During Low-Voltage Ride Through

Different from the conventional photovoltaic (PV) inverters, a three-phase PV solid-state transformer (SST) based on the cascaded H-bridge (CHB) topology can be regarded as consisting of three single-phase CHB inverters in essence. During the low-voltage ride through (LVRT), some phases of three-phase PV SST may inversely absorb active power from ac grid affected by negative-sequence voltages, resulting in that the dc bus voltages of all H-bridges in these phases continue to rise, and that the system will shut down due to overvoltage protection. To deal with this problem, in this article, we study the mechanism of active power backflow during LVRT, analyzes and compares the main methods for suppressing active power backflow, and then proposes an LVRT control strategy based on the zero-sequence voltage compensation, which not only ensures that all modules in three-phase PV SST transmit the almost same active power but also can avoid effectively active power backflow problem even if the total output active of PV array is relatively low. The validity of the proposed method is verified by the simulation and experimental results.

A Hybrid Control Method for Neutral-Point Voltage Balancing in Three-Level Inverters

Neutral-point (NP) voltage fluctuation is an inherent problem in three-level inverters. Without any control measures, the NP voltage will deviate, and the dc bias and ac oscillation can be observed, which may lead to the failure of power devices and capacitors, so as to damage the system security. In this letter, a hybrid control method is proposed, which it combines zero-sequence voltage regulation with the decomposition of modulation waveform organically. This carrier-based method is simple with lower switching loss due to the unchanged switching events per switching cycle. Experiments are carried out in a 10-kVA three-level platform, which demonstrate the validity of proposed method.

Construction and Modeling of Multi-Circuit Multi-Voltage HVAC Transmission Lines

A transmission network’s main objective is to continuously supply electrical energy to consumers. This article presents an analysis of the use of multi-circuit, multi-voltage overhead lines as a compromise between ensuring the system’s safe operation by increasing the transmission network capacity and managing the constraints related to its expansion. The considerations presented in this work include the construction of such lines, their operation, and modeling aspects. As part of the study, the potential for improving the environmental conditions around the lines is discussed in terms of the necessary area for their construction and the peak electromagnetic field strength in their vicinity. We also present a mechanical analysis of stress and sag coordination in the individual circuits of these lines. Then, we detail the method for determining the electrical parameters of multi-voltage lines’ series impedances and capacitance. Specific attention is given to the possibility of zero-sequence voltage that occurs in the systems despite the symmetric supply and load of circuits—especially in the circuits with the lowest voltages—that result from the line’s geometric asymmetry. We evaluate the impact of the line’s geometric asymmetry on the power system’s correct operation by determining the asymmetry factors. Finally, the accuracy of using a simplified symmetric model for lines with various geometric asymmetries is analyzed by studying the error of the short-circuit currents.

DC Voltage Balancing Strategy of a Bipolar-Output Active Rectifier for More Electric Aircraft Based on Zero Vector Redistribution

The ±270 V bipolar dc distribution has become an attractive power supply scheme in more electric aircraft (MEA). In this paper, a dc voltage balancing strategy based on zero vector redistribution (ZVR) is proposed for a three-phase coupled inductor-based bipolar-output active rectifier (TCIBAR) to achieve balanced ±270 V power supply for MEAs. First, the mathematical model of the TCIBAR is developed in the rotating reference frame and the zero-sequence equivalent circuit of the three-phase coupled inductor (TCI) branch is derived. On this basis, the effect mechanism of the zero-sequence voltage is analyzed in balancing the output dc voltages of the TCIBAR. Second, a voltage balancing control scheme is designed to control the zero-sequence voltage and maintain the dc voltage balance of the TCIBAR under unbalanced load condition. Finally, a ZVR - space vector pulse width modulation (ZVR-SVPWM) method is proposed. With the ZVR-SVPWM method, the duration time of zero vectors can be redistributed to modulate the desired zero-sequence voltage for the TCIBAR. The proposed dc voltage balancing strategy for the TCIBAR is implemented on an experimental platform, and the experimental results verify the feasibility and effectiveness of the proposed control scheme and ZVR-SVPWM method.

An Active Hybrid Modulation Strategy for a Si/SiC Hybrid Multilevel Converter

This paper proposes a hybrid modulation strategy for a medium-voltage hybrid seven-level (7-L) converter, which consists of a three-phase silicon (Si) IGBT-based active-neutral-point-clamped (ANPC) stage and a cascaded silicon carbide (SiC) MOSFET-based H-bridge (HB) stage in each phase. Within the framework of the proposed modulation strategy, three floating HB converters are controlled using a high-frequency three-level (3-L) space-vector-modulation (SVM), while a low-frequency modulation is applied to the ANPC converter. To regulate the DC voltages across floating capacitors, a zero-sequence voltage (ZSV) injection method is adopted. The proposed method possesses three major advantages. First, it enables the simplification from a 7-L to a 3-L SVM, which significantly reduces the computational burden. Second, with all the high-frequency switching actions assigned to SiC devices in the HB stage, the power loss of the hybrid 7-L converter is reduced. Third, the hybrid modulation together with the ZSV injection enables a wider stable operating range compared to the existing 7-L SVM method. Finally, both simulation and experimental results performed on the scale-down and the full-scale 1 MVA prototype validate the effectiveness of the proposed hybrid modulation strategy.

Common-Mode Voltage Reduction Method for Three-Level Inverter With Unbalanced Neutral-Point Voltage Conditions

The three-level inverter has been widely researched and adopted in industry. This article further proposes a carrier-based modulation method to reduce the common-mode voltage (CMV) and obtain high-quality output currents for three-level inverter with unbalanced neutral-point voltage conditions. In this method, the zero-sequence voltage is calculated by three-phase modulation waves, three-phase output currents, and voltages across the dc-link capacitors. The limitations of the zero-sequence component are carefully considered in order not to generate the basic voltage vector with high CMV magnitudes. In addition, a deadbeat control scheme for controlling the dc-link voltages asymmetrically with the proposed pulsewidth modulation method is presented, and asymmetrical control of dc-link voltages can be realized with fast response. Compared with the conventional space vector modulation (SVM) and the virtual SVM methods, the proposed scheme can reduce the CMV by half. Simulated and experimental results validate the effectiveness of the proposed method.

Accurate Fault Location Modeling for Parallel Transmission Lines Considering Mutual Effect

A new accurate algorithms based on mathematical modeling of two parallel transmissions lines system (TPTLS) as influenced by the mutual effect to determine the fault location is discussed in this work. The distance relay measures the impedance to the fault location which is the positive-sequence. The principle of summation the positive-, negative-, and zero-sequence voltages which equal zero is used to determine the fault location on the TPTLS. Also, the impedance of the transmission line to the fault location is determined. These algorithms are applied to single-line-to-ground (SLG) and double-line-to-ground (DLG) faults. To detect the fault location along the transmission line, its impedance as seen by the distance relay is determined to indicate if the fault is within the relay’s reach area. TPTLS under study are fed from one- and both-ends. A schematic diagrams are obtained for the impedance relays to determine the fault location with high accuracy.

Analysis and Simulation of Reactive Power Theory for Harmonic Elimination using Shunt Active Power Filter

A distinct type of load draws non-sinusoidal current from the main supply, which cause harmonic disturbance and reduce the power quality. The major cause of harmonic distortion is this nonlinear load in a power system. In addition, the nonlinear loads causing harmonic distortion can interrelate harmfully with power system equipment, mainly motors, capacitors and transformers causing extra losses. In order to minimise these extra loses due to harmonic disturbance; the shunt active power filters (SAPFs) is investigated to maximize power quality. This paper presents the control method for extracting the reference currents for SAPF using instantaneous reactive power theory (IRPT). It has been evaluated and their performance is brought to a comparison under distorted supply and nonlinear load circumstances. This new speculation gives a general description of IRPT that is applicable for balanced or unbalanced loads and sinusoidal or non-sinusoidal waveforms in three-phase power systems compensation or non-compensation zero-sequence currents and voltages.

A Carrier-Based Discontinuous PWM Strategy for T-Type Three-Level Inverter with Reduced Common Mode Voltage and Neutral Point Voltage Control

A novel carrier-based discontinuous pulsewidth modulation (CB_DPWM) strategy for T-type three-level converter is proposed in this article. Because one significant advantage of the proposed CB_DPWM is the reduction of common mode voltage (CMV), it is named as RCMV_CB_DPWM hereafter. It also can reduce switching loss and remove dc offset from the neutral point voltage (NPV) simultaneously. RCMV_CB_DPWM has two design freedoms, one is modulation wave and the other is carrier wave. By injecting zero sequence voltage properly, the corresponding clamping mode can be acquired. Moreover, reduced CMV can be achieved by arranging carrier wave flexibly. The integrity of RCMV_CB_DPWM is analyzed as well as its NPV self-equilibrium ability, and an active NPV control method is then introduced. Moreover, its realization is discussed briefly. Finally, the experiments are carried out to verify the effectiveness of the proposed method.

Discrimination Method of Interturn Short-Circuit and Resistive Unbalance Faults for Synchronous Condenser

In this paper, a discrimination method is proposed for the interturn short-circuit and resistive unbalance faults of the synchronous condenser. The mathematical models of the synchronous condenser are established under these two faults. Especially, the zero-sequence voltage components (ZSVC) under two faults are analyzed. Due to the direct connection between the synchronous condenser and the power grid, the fundamental component of ZSVC under the interturn short-circuit fault is not affected by the stator currents, while the resistive unbalance fault is just the opposite. Hence, the amplitude of fundamental component in ZSVC is used to detect the fault occurrence. Then, the change of the fundamental component in ZSVC is observed by changing the excitation current to discriminate the fault type. The effectiveness of the proposed method is verified by the simulation and experimental results.

A Novel Linear Optimization Method for Section Location of Single-Phase Ground Faults in Neutral Noneffectively Grounded Systems

The existing four linear section location methods are all established based on the three-phase fault current direction. When a single-phase grounding fault occurs in a neutral noneffectively grounded system, these methods are no longer applicable. This study proposes a linear optimization method for section location suitable for single-phase ground fault in the neutral noneffectively grounded system. It presents an improved transient zero-sequence current direction identification algorithm based on line voltage to accurately determine a transient zero-sequence current direction under single-phase grounding faults without zero-sequence voltage information. Second, it proposes a suitable linear section location model for single-phase ground faults as per the transient zero-sequence current direction’s distribution characteristics after the single-phase ground fault. Finally, it is verified in different feeders that the proposed method can quickly and accurately locate the fault section after the single-phase ground fault occurs in the neutral-point ineffectively grounded distribution system.