Zero-sequence Voltage Component Research Articles

Characteristics of symmetrical components for high impedance faults in distribution networks with photovoltaic inverters

This work models a photovoltaic (PV) inverter connected to an IEC microgrid system. The purpose of this study was to find the characteristics of symmetrical components before and after a high impedance single phase short-circuit fault. The IEC microgrid system is tailored to the distribution system in Indonesia, where the applied voltage is 20 kV. Only parameters related to the short-circuit study are included in the model. A second-order generalized integrator frequency-locked-loop is utilized to represent the 3-phase PV inverter. The inverter model, previously treated as an ideal current source, and positive sequence current are no longer employed, as they fail to depict the characteristics of symmetric components during a phase-to-ground short-circuit fault. The IEC microgrid system connected to the PV inverter is simulated using ATPDraw-electro magnetic transient program (EMTP). Simulation results reveal that the changes in the symmetric component values of current after a fault experience a very insignificant increase. Meanwhile, the positive sequence voltage values following the short-circuit fault exhibit a negligible decrease. In contrast, the negative sequence and zero sequence voltage components after the short-circuit fault undergo a significant increase.

A novel multi-resolution network for the open-circuit faults diagnosis of automatic ramming drive system

The open-circuit fault is one of the most common faults of the automatic ramming drive system (ARDS), and it can be categorized into the open-phase faults of Permanent Magnet Synchronous Motor (PMSM) and the open-circuit faults of Voltage Source Inverter (VSI). The stator current serves as a common indicator for detecting open-circuit faults. Due to the identical changes of the stator current between the open-phase faults in the PMSM and failures of double switches within the same leg of the VSI, this paper utilizes the zero-sequence voltage component as an additional diagnostic criterion to differentiate them. Considering the variable conditions and substantial noise of the ARDS, a novel Multi-resolution Network (MrNet) is proposed, which can extract multi-resolution perceptual information and enhance robustness to the noise. Meanwhile, a feature weighted layer is introduced to allocate higher weights to characteristics situated near the feature frequency. Both simulation and experiment results validate that the proposed fault diagnosis method can diagnose 25 types of open-circuit faults and achieve more than 98.28% diagnostic accuracy. In addition, the experiment results also demonstrate that MrNet has the capability of diagnosing the fault types accurately under the interference of noise signals (Laplace noise and Gaussian noise).

Development and testing midpoint voltage balance algorithms of three-level inverters

Nowadays, the use of inverters is increasing in more and more applications. Environmental awareness is becoming a trend, so more and more people install solar energy systems on their houses, which also require the use of inverters. Higher power levels have led to the emergence of three-level inverters which has opened up new challenges, such as the issue of midpoint voltage balancing. This paper discusses voltage balancing and voltage ripple reduction techniques for three-level inverters with Neutral-Point Clamped (NPC) topology. The balancing is based on injecting zero-sequence voltage component in carrierbased modulation. The viability of the modulation techniques are verified by computer simulation and then tested on hardware.

Analysis of equilibrium points and optimal grid support of grid-forming modular multilevel converter for balanced and unbalanced faults

This short-communication presents a steady-state analysis of a grid-forming Modular Multilevel Converter (MMC) providing optimal voltage support to the AC network under normal and constrained conditions. The analysis is performed based on a multi-objective function (OF) optimization problem which prioritizes to maximize the positive-sequence and to minimize the negative- and zero-sequence voltage components at the point of common coupling (PCC) while it also considers the minimization of the arm impedance losses, respectively. If the voltage condition at the PCC is satisfied, the optimization attempts to reduce the arm impedance losses; otherwise, the algorithm prioritizes the PCC’s voltage components in order to minimize the error. Different network voltage and internal fault scenarios are evaluated, where it is shown that the suggested problem formulation can be used to obtain the optimal MMC’s quantities, providing voltage support during the faults.

Uneven Demagnetization Fault Diagnosis in Dual Three-Phase Permanent Magnet Machines Based on Electrical Signal Difference

Dual three-phase permanent magnet (DTPPM) machines are suitable for applications with high power density and high reliability requirements. However, uneven demagnetization fault (UDF) can occur due to harsh working conditions. This article proposes a non-invasive, online detection method of UDF in DTPPM machines based on the electrical signal difference between two sets of three-phase windings, which can be more conducive to UDF diagnosis with higher sensitivity. Firstly, the influence of dual three-phase winding arrangements on phase permanent magnet (PM) flux linkage is investigated in DTPPM machines under UDF. It shows that the fault characteristic harmonics appear in fractional-slot DTPPM machines and integral-slot DTPPM machines where a single set of three-phase windings is rotationally asymmetric. Further, according to the three-phase relationship of harmonic vectors, it is analyzed whether the fault characteristic harmonics are reflected in stator currents or zero-sequence voltage components (ZSVCs). Notably, taking advantage of the structural feature in DTPPM machines, the proposed method obtains higher sensitivity of UDF detection, as the amplitudes of fault characteristic harmonics in electrical signal difference are larger than that in stator current or ZSVC in a single set of three-phase windings. Finally, experiments are conducted to validate the effectiveness of the proposed method.

Direct Power Control of a Bipolar Output Active Rectifier for More Electric Aircraft Based on an Optimized Sector Division

This paper presents a novel direct power control (DPC) strategy based on an optimized sector division for a three-phase coupled inductor-based bipolar output active rectifier (TCIBAR) applied in more electric aircraft (MEA). First, based on the instantaneous power theory, the power model of the TCIBAR is built in the synchronous rotating coordinate system. Second, to implement the hysteresis power control of TCIBAR without causing the runaway of the zero-sequence current in the three-phase coupled inductor (TCI), a set of new voltage vectors that have the same zero-sequence voltage (ZSV) component are synthesized and adopted in the proposed DPC strategy. Third, by quantitatively analyzing the effect of the new synthesized voltage vectors on the power variation of TCIBAR, an optimized sector division method is proposed to improve the accuracy of power control and reduce the phase current harmonics in TCIBAR. Finally, to maintain the voltage balance of the bipolar dc ports in TCIBAR, voltage balance control is studied in the proposed DPC strategy. The proposed DPC strategy is researched on an experimental platform of TCIBAR, and the results show that the proposed DPC strategy is feasible and has good static and dynamic performance.

Research on flexible uninterrupted neutral section passing system based on V-clamp multilevel converter

This paper proposes a novel flexible uninterrupted neutral section passing system based on four-leg V-clamp multilevel converter (VMC). It can better solve a series of problems such as overvoltage, overcurrent and speed loss caused by the interruptible over-phase scheme used in electrified railway, and compensate the negative sequence and reactive current in traction power supply system at the same time. VMC is a new type of multilevel converter with common DC-link capacitors. VMC has advantages in the number of devices to some extent, and its expansion from odd level to even level is very simple. Compared with schemes based on modular multilevel converter (MMC), this system has the advantages of low converter hardware cost and no energy loss resulting from hot backup. The system configuration, working process and operating principle are analyzed in detail, and a control strategy for the whole process of neutral section passing is designed. Next, the method based on zero-sequence voltage component injection is used to solve the voltage-balancing problem existed in multilevel converter with common DC-link capacitors, which does not need additional switching times and is easy to realize in three-phase three-wire system. Finally, the simulation results are given to verify the feasibility and effectiveness of the system.

Detection and Evaluation of the Interturn Short Circuit Fault in a BLDC-Based Hub Motor

A brushless dc (BLDC) motor with an outer rotor has great potential for emerging electric vehicle (EV) applications as an in-wheel hub motor. Interturn short circuit faults (ISCFs) are common electrical faults in these applications, and their fault diagnosis is of critical importance. This article proposes a signal analysis-based method to detect and quantitatively analyze the ISCF in a BLDC motor under a six-step commutation control strategy. The zero-sequence voltage component (ZSVC) and three-phase currents are simultaneously determined while the fundamental frequency amplitude of the ZSVC is developed as a fault indicator for diagnostic purposes. Thereafter, the faulty phase is identified, and the fault severity is estimated by jointly analyzing the ZSVC and faulted phase current. The proposed technique entails creating an analytical model, a simulation model, and experimental tests. Experimental results demonstrate that the proposed technique has excellent accuracy, quick response, and real-time fault diagnosis for BLDC motors. This technique will accelerate the use of BLDC-based hub motors in EVs and the widespread application of EVs to reduce the carbon emissions.

Development of an algorithm for identifying single-phase ground fault conditions in cable and overhead lines in the networks with isolated neutral

The paper addresses the issue of identification of the type of single-phase ground fault in medium voltage networks operating mainly with an isolated neutral. By analyzing the published research, the most promising methods for detection of a single-phase ground fault were selected. Further, using the selected parameters, algorithms were designed to determine the parameters, process them, and use them to identify the type of a single-phase ground fault. The algorithm developed by the authors was formalized for the application on the basis of microprocessor-based protection systems using digital signal processors. The operation process was modeled using real oscillograms of single-phase ground fault. The use of high-frequency zero-sequence current and voltage components, as well as the root-mean-square value of the zero-sequence voltage, made it possible to design an algorithm that identifies any of the possible single-phase ground fault types (bolted, arc, or contact resistance ones). The first part of the article gives an overview of the state-of-the-art of research in the field of detection and identification of single-phase-to-earth faults. In the second part, the parameters of emergency operating conditions are analyzed and the key parameters are determined based on them. The third part contains a description of the single-phase fault identification algorithm developed by the authors and the output oscillograms. The fourth part makes conclusions on the applicability of the algorithm.

Integration of Interturn Fault Diagnosis and Fault-Tolerant Control for PMSM Drive System

This article first studies the integration of interturn fault diagnosis and fault-tolerant (FT) control of permanent magnet synchronous machine (PMSM) drive system. In this article, interturn fault is diagnosed by zero-sequence voltage component and the torque ripple due to interturn fault is reduced by the current injection-based FT control strategy. The main novelty of this article is that the injection currents for FT control are directly calculated based on the already obtained information during the fault diagnosis process, thus reducing the complexity of FT control and making interturn fault diagnosis and FT control convenient integration. Both the simulation and experimental results validate the effectiveness of the proposed integration method for the PMSM drive system.

Integration of Interturn Fault Diagnosis and Torque Ripple Minimization Control for Direct-Torque-Controlled SPMSM Drive System

This article proposes an integration method for interturn fault diagnosis and torque ripple minimization control of direct-torque-controlled surface-mounted permanent magnet synchronous machine (SPMSM) drive system without requiring the specific fault severity (short-circuited turn ratio, current). In this method, the interturn fault is detected and located based on the amplitude and initial phase angle of the zero-sequence voltage component (ZSVC). Both the torque injection and the improved flux observer are applied to reduce the torque ripple caused by the interturn fault. Furthermore, the torque injection and the improved flux observer are directly calculated by using the already obtained amplitude and initial phase angle of the ZSVC during the fault diagnosis process, without requiring the specific values of short-circuited turn ratio and fault current. The experimental results validate the effectiveness of the proposed integration method for the direct-torque-controlled SPMSM drive system.

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.

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.

Phase-Shifted Model Predictive Control to Achieve Power Balance of CHB Converters for Large-Scale Photovoltaic Integration

Cascaded H-bridge (CHB) converters are attractive candidates for next generation photovoltaic (PV) inverters. CHB converters present a reduced voltage stress per power switch and a high modularity. Therefore, the plant can be divided in several PV strings that can be connected to each H-bridge cell. However, due to variability on solar irradiance conditions, each PV string may present different maximum available power levels, which difficult the overall converter operation. To address this issue, this article presents a model predictive control (MPC) strategy, which works along with a phase-shifted pulsewidth modulation (PS-PWM) stage; hence, its name phase-shifted MPC (PS-MPC). The novelty of this proposal is the way both interbridge and interphase power imbalance are directly considered into the optimal control problem by a suitable system reference design. Thus, the interphase imbalance power is tackled by enforcing the converter to operate with a proper zero-sequence voltage component. Then, by exploiting the PS-PWM working principle, PS-MPC is able to handle each H-bridge cell independently. This allows the predictive controller to also deal with an interbridge power imbalance using the same control structure. Experimental results on a 3-kW prototype are provided to verify the effectiveness of the proposed PS-MPC strategy.

Discontinuous Space Vector Modulation Schemes for Modular Multilevel Converters

In this article, a new, simple, and generalized discontinuous pulse width modulation (PWM)-based submodule capacitor voltage balancing approach is proposed and generated within the carrier-based PWM for modular multilevel converters (MMCs). In this scheme, different variants of zero-sequence voltage (ZSV) component is injected into the pulse width modulator so that each arm of the MMC is virtually clamped to positive or negative dc bus during a specific interval to achieve discontinuous modulation. A simple approach based on the location of the voltage vector mapping to a two-level space-vector diagram is presented to determine various ZSV components. The effectiveness of the proposed scheme with a voltage balancing algorithm is demonstrated through PLECS simulations and experimentally verified using a hardware-in-the-loop-based system. The performance of MMC with different ZSV injections is holistically evaluated to determine the best possible sequence over a wide range of modulation index.

Detection and Discrimination of Incipient Stator Faults for Inverter-Fed Permanent Magnet Synchronous Machines

The online detection of incipient faults for permanent magnet synchronous machine (PMSM) has been a great challenge in recent decades, where the fault features are extremely trivial and the machine fault type is normally unknown in advance. An online fault detection and discrimination of incipient faults for PMSM, including stator interturn short-circuit and resistive unbalance fault, is proposed in this article. First, zero-sequence voltage signal is collected and the zero-sequence voltage component (ZSVC) is used to detect the abnormal status of the PMSM. Then, high frequency (HF) signals are injected to the PMSM to discriminate the fault type. It shows that the amplitude of the HF component in ZSVC is proportional to the frequency of the injected signal under interturn short-circuit while it is constant under the resistive unbalance fault, and due to this difference, the fault type may be discriminated. Finally, the effectiveness of the theoretical analysis is validated by the simulation and experimental results.

A New Effective Method of Induction Machine Condition Assessment Based on Zero-Sequence Voltage (ZSV) Symptoms

Non-invasive diagnostic methods for electric machines’ diagnostics, which can be used during their operation in a drive system, are needed in many branches of the production industry. For the reliable condition assessment of electric machines, especially those operating in drive systems, various tools and methods have been suggested. One diagnostic method that has not been fully recognized and documented is a diagnostic method based on zero-sequence voltage component (ZSV) applications for the condition assessment of induction machines. In this paper, the application of ZSV in induction machine diagnostics is proposed. A factor that speaks in favor of applying this signal in such diagnostics is the high sensitivity of the signal to damage occurrence, and the distinct change of extracted symptoms in the case of asymmetry. It is possible to obtain a high signal amplitude, which simplifies its processing and the elaboration of reliable diagnostic factors. This ZSV-based method is also able to be applied to big machines used in industry. Due to the saturation effects visible in the ZSV signal, new diagnostic symptoms can appear, which allows for an easier condition assessment of certain machines. The usefulness of the described diagnostic method in machine condition assessment was shown through an equivalent circuit modeling process, finite element analysis, and laboratory tests of the machine.

Detection of Partial Demagnetization Faults in Five-Phase Permanent Magnet Assisted Synchronous Reluctance Machines

This paper analyzes partial demagnetization faults in a five-phase permanent magnet assisted synchronous reluctance motor (fPMa-SynRM) incorporating ferrite permanent magnets (PMs). These faults are relevant because of the application of field weakening, or due to high operating temperatures or short circuit currents, the PMs can become irreversibly demagnetized, thus affecting the performance and safe operation of the machine. This paper proposes fault indicators to detect such fault modes with low demagnetization levels between 5.0% to 16.7% relative demagnetization. Four partial demagnetization fault detection methods are tested, which are based on the analysis of the harmonic content of the electromotive force (EMF) under no load conditions, the harmonic content of the line currents, the harmonic content of the zero-sequence voltage component (ZSVC) and the analysis of the power factor (PF). This work also compares the sensitivity and performance of the proposed detection methods. According to the fault indicators proposed in this paper, the results show that the analysis of the EMF, ZSVC and PF are the most sensitive detection methods. Experimental results are presented to validate finite element analysis (FEA) simulations.

A Two-Step Strategy for Online Fault Detection of High-Resistance Connection in BLDC Motor

High-resistance connection (HRC) is one of the common stator faults in motors. The HRC fault detection method for brushless direct current motors (BLDCMs) has not been reported yet. This study proposes a two-step strategy for the detection, localization, and evaluation of HRC fault in a BLDCM. In the first step, the zero sequence voltage component (ZSVC) of the motor is acquired and analyzed, and the HRC fault can be detected from the amplitude of the fundamental frequency component of the ZSVC spectrum. In the second step, once the HRC fault has been detected, three indicators are calculated from the ZSVC and three-phase currents to localize the fault phase. The resistance value of the HRC is then calculated. Theoretical, simulated, and experimental analyses have been performed, and the results of them are consistent with one another, thereby validating the effectiveness of the proposed strategy. Both the HRC fault detection algorithm and the motor control algorithm are implemented on a single microcontroller unit in real time, which makes the proposed strategy efficient and flexible in industrial applications.