Measured Phase Currents Research Articles

Distribution Systems State Estimation for Distributed Generation Re-synchronization

This paper presents an application of a state estimation algorithm as a tool for re-synchronization of distributed generation in a distribution system. The estimation results can be used to operate a distributed generation in grid and off-grid mode. The estimated state variables are utilized as input data to control a Diesel generator while resynchronizing as well as for a grid mode loss predictor. The proposed state estimator, with branch currents as state variables, was solved by the weighted least squares method using singular value decomposition. To ensure the observability of the distribution system, a detailed numerical observability analysis was carried out with complex power measurements and unconventional branch current phasor measurements. In the algorithm, phasor measurement units and pseudo-measurements can be incorporated. Conditions that ensure system observability and redundancy increasing are derived when those measurements are included. The approach was tested in the IEEE 13 node test feeder, to which a distributed generation was connected for external network feeding. The algorithm shows stability, convergence, allowing the distributed generation re-synchronization to the distribution system. This feature could help improve self-healing capability in microgrids that use synchronous generators.

Monitoring and analysis of electronic current transformer’s field operating errors

The energy metering accuracy and relay protection reliability in power grid systems critically depends on the phase current measurement accuracy. Commonly manufactured electronic current transformers (ECT) may exhibit unacceptable errors during field operation. Our research platform targets to remotely monitor ECT’s errors caused by ambient temperature variation, load current, and adjacent phase interference. A 0.2% accuracy, 110kV, Rogowski coil-based ECT prototype was selected for field operation in over 2years of monitoring tests. Correlated field and laboratory tests showed ambient temperature-related errors of less than 0.1% in current ratio and 2′ in phase, findings consistent with the type tests. The load current and adjacent phase interference introduce negligible errors for load currents larger than 0.2In (where In represents the phase rated current). However, the phase error exceeds 20′ for load currents less than 0.2In, possibly leading to false conclusions. Based on the above influencing factors, standardize the 0.2% accuracy ECT’s error less than half of the error limit ensures acceptable field error levels.

Transmission system wide-area back-up protection using current phasor measurements

Zone-3 of distance relays might maloperate during stresses frequently encountered in power systems, such as power swing, load encroachment, and voltage instability. This paper proposes a new protection algorithm for discrimination between short-circuit faults and other stresses in the transmission networks. The proposed method compares the sum of currents at the predetermined buses before and after the disturbance occurrence using synchronized current phasor measurements. The faulted area and line are identified as well. The optimal placement of phasor measurement units (PMUs) is tackled using a mathematical model. One of the main advantages of the proposed algorithm is decreasing the number of required PMUs in comparison with those of existing wide-area back-up protection schemes. In virtue of its computational speed, the proposed method can be exploited as a practical back-up protection cooperating with conventional protection schemes. The extensive simulation studies carried out on the IEEE 57-bus test system verify applicability of the proposed algorithm as a reliable back-up protection scheme for lines.

An Inverter Nonlinearity-Independent Flux Observer for Direct Torque-Controlled High-Performance Interior Permanent Magnet Brushless AC Drives

This paper introduces a novel flux observer for direct torque controlled interior permanent magnet brushless AC (IPM-BLAC) drives over a wide speed range including standstill. The observer takes machine nonlinearities into account and is independent of inverter nonlinearities, dead time, and armature resistance variation at steady states since such inaccuracies are compensated quickly by measured phase currents. Magnetic saturations in the stator and rotor cores, cross-coupling effects of flux linkages of the motor, and spatial harmonics in the magnetomotive force are all considered in the novel scheme. There is no filter; hence, no delays and oscillatory responses like in conventional schemes where filters are employed to prevent integrator drift issue. Superiority of the observer when compared to the state-of-the-art schemes has been illustrated by both extensive simulations and experimental results of a 10-kW IPM-BLAC machine designed for traction applications.

2상 유도전동기용 벡터제어 인버터를 위한 전류측정 오차 보상 방법

The phase currents must be accurately measured to achieve the instantaneous torque control of AC motors. In general, those are measured using the current sensors. However, the measured current signals can include the offset errors and scaling errors by several components such as current sensors, analog amplifiers, noise filter circuits, and analog-to-digital converters. Therefore, the torque-controlled performance can be deteriorated by the current measurement errors. In this paper we have analyzed the influence caused by vector control of 2-phase induction motor when two errors are included in measured phase currents. Based on analyzed results, the compensation method is proposed without additional hardware. The proposed compensation method was applied vector-controlled inverter for 2-phase induction motor of 360[W] class and verified through computer simulations and experiments.

Hybrid Pulse Width Modulation Strategy for Wide Speed Range in IPMSM with Low Cost Drives

The control performance of hybrid PWM inverter using a phase current measurement is presented in this paper. The hybrid PWM technique consists of space vector pulse width modulation (SVPWM) and six-step voltage control operation. The SVPWM is performed to reduce the harmonic components in the low speed region, and the six-step modulation is applied to increase the maximum speed of the IPMSM in the high speed region. Therefore, it is possible to obtain a great performance in both the low speed range and high speed range. However, the six-step modulation cannot be completely implemented, since the inverter that includes the lag-shunt sensing method has an immeasurable current region. In this paper, a quasi-six-step modulation using a modified voltage vector is proposed. The validity and usefulness of the proposed PWM technique is verified by MATLAB/Simulink and experimental results.

Electrical Resistance Heating of Clay Layers in Water‐Saturated Sand

AbstractElectrical resistance heating (ERH) experiments were performed in a two‐dimensional water‐saturated porous medium comprising an electrically conductive, low‐permeability clay lens embedded within a less electrically conductive, higher permeability silica sand. These were compared to experiments performed in homogeneous silica sand. All experiments were performed in the absence of a non‐aqueous phase liquid (NAPL) or dissolved volatile organic compound (VOC). Temperature monitoring showed preferential heating in the clay lens and higher overall heating rates throughout the test cell compared to the homogeneous case. Gas production was localized around the sand–clay interface due to high temperature and low capillary displacement pressure. Above the clay lens, unexpected temperature plateaus were observed, similar to those observed in previous experiments during NAPL–water co‐boiling. A conceptual model based on the consumption of thermal energy as latent heat of vaporization in the highly localized heating and gas production region adjacent to the clay lens is proposed to explain the temperature plateaus. Supporting data is drawn from images of the gas phase and electric current measurements. These results show that the use of co‐boiling plateaus as an indicator of NAPL–water co‐boiling could be misleading during applications of ERH at sites containing electrically conductive, low‐permeability clay lenses embedded within less electrically conductive, higher‐permeability sands.

Hybrid state estimator considering SCADA and synchronized phasor measurements in VSC-HVDC transmission links

This paper proposes a practical approach for implementing a hybrid state estimator by considering SCADA and PMU measurements in VSC-HVDC transmission links under a unified framework of reference. A proposed formulation for the PMU measurements associated with the VSC-HVDC state variables is derived from the first principles and is implemented into a weighted least square-based state estimation algorithm. In order to avoid numerical problems of convergence of the proposed state estimation approach, the set of current phasor measurements is represented in rectangular coordinates, such that their corresponding variances are also recalculated according to the uncertainty propagation theory. The state estimation process uses the measurement of one voltage phase angle as the global reference for the rest of voltage phase angles estimated at network buses and VSC-HVDC's voltage source converters. The proposed formulation improves the accuracy at which both network and VSC-HVDC state variables are simultaneously estimated based on the measurements provided by a SCADA system and PMUs. A Mexican interconnected 190-bus equivalent system is used as the test system in order to validate the effectiveness of the proposed hybrid state estimator.

A SVM Based Condition Monitoring of Transmission Line Insulators Using PMU for Smart Grid Environment

A new methodology for the detection and identification of insulator arc faults for the smart grid environment based on phasor angle measurements is presented in this study and the real time phase angle data are collected using Phasor Measurement Units (PMU). Detection of insulator arcing faults is based on feature extraction and frequency component analysis. The proposed methodology pertains to the identification of various stages of insulator arcing faults in transmission lines network based on leakage current, frequency characteristics and synchronous phasor measurements of voltage. The methodology is evaluated for IEEE 14 standard bus system by modeling the PMU and insulator arc faults using MATLAB/Simulink. The classification of insulator arcs is done using Support Vector Machine (SVM) technique to avoid empirical risk. The proposed methodology using phasor angle measurements employing PMU is used for fault detection/classification of insulator arcing which further helps in efficient protection of the system and its stable operation. In addition, the methodology is suitable for wide area condition monitoring of smart grid rather than end to end transmission lines.

Applications of Czech PMU / WAMS Systems in Distribution Systems

Abstract: The paper presents several advanced applications of the wide area monitoring system (WAMS),which are implemented in the WAMS designed by the AIS company, for online power network monitoring, such as on-line ampacity monitoring and frequency oscillations monitoring. It is also shown, that there are significant systematic errors in measured current phasors negatively influencing the calculation of actual line parameters. Hence, the method for calibration of the current phasor measurements is proposed. Finally, two case studies are devoted to the on-line ampacity monitoring and frequency oscillations detection, respectively.

Novel Technique for a Dynamically State Estimation In Contemporary Power Substations

This paper presents implementation and performance evaluation of states estimation in contemporary power substations. The estimation is based on streamed IEC 61850-9-2 Sampled Measured Values (SMV) that provide by the Merging Units (MUs) at process level (Level-0) . With the reference to the most challenging problem of tracking dynamic phenomena, the Unsented Kalman Filtering (UKF) is proposed to be used in the centralized computation of synchrophasors in a substation to enhance the power system state estimation. Such system can supersede the current Phasor Measurement Units (PMUs) and mitigating the problems of installing new dedicated PMUs due to the high cost. TrueTime, a Matlab/Simulink-based simulator that developed by ABB Corporate Research Group, Sweden to emulate the IEC 61850-9-2 process bus communication has been used for the simulation. In this simulation, evaluating the effects of latency introduced by the communication network, the offset and the clock drift at each sensor node (MU) on simulated Dynamic Phasor Estimation performance were given more attention. Representative results in evaluating the Phasors computation performance are presented in the paper.

Online Fault Identification Based on an Adaptive Observer for Modular Multilevel Converters Applied to Wind Power Generation Systems

Due to the possibility of putting a large number of modules consisting of switches and capacitors connected in series, the modular multilevel converter (MMC) can easily be scaled to high power and high voltage power conversion, which is an attractive feature for filter-less and transformer-less design and helpful to achieve high efficiency. However, a significantly increased amount of sub-modules in a MMC may increase the requirements for sensors and also increase the risk of failures. As a result, fault detection and diagnosis of MMC sub-modules are of great importance for continuous operation and post-fault maintenance. Therefore, in this paper, an effective fault diagnosis technique for real-time diagnosis of the switching device faults covering both the open-circuit faults and the short-circuit faults in MMC sub-modules is proposed, in which the faulty phase and the fault type is detected by analyzing the difference among the three output load currents, while the localization of the faulty switches is achieved by comparing the estimation results by the adaptive observer. In contrast to other methods that use additional sensors or devices, the presented technique uses the measured phase currents only, which are already available for MMC control. In additional, its operation, effectiveness and robustness are confirmed by simulation results under different operating conditions and load conditions.

Online Voltage Sensorless High-Resistance Connection Diagnosis in Induction Motor Drives

A new method for online high-resistance connection (HRC) diagnosis in induction motor (IM) drives is presented in this effort. This voltage sensorless method is based on a signal injection strategy applied to the IM during its normal operation. The signal is injected in the $d$ -axis in such a way that it provides the phase currents with a dc component while avoiding torque perturbation. By measuring phase currents, it is possible to detect and isolate HRC problems. Experimental results showing high sensitivity to HRCs and immunity to symmetrical stator resistance variations have been obtained in a laboratory setup. The low perturbation produced by the injection method over the IM variables has also been experimentally demonstrated.

Correlation of phase resolved current, emission and surface charge measurements in an atmospheric pressure helium jet

The interaction of an atmospheric pressure plasma jet with two different surfaces (conducting and dielectric) is investigated using a setup with two ring electrodes around a dielectric capillary. For diagnostics, phase resolved ICCD-imaging, current measurements and surface charge measurements are applied. The results show the correlation of plasma dynamics with the deposition of surface charge and electrical current signals. Further, the influence of the distance between surface and jet capillary on the surface charge distribution is presented. A complex discharge dynamic is found with a dielectric barrier discharge between the ring electrodes and back-and-forth bullet propagation outside the capillary. A conducting channel connecting the jet nozzle and the surface is found. This correlates well with the observed charge exchange on the surface. The number of formed channels and the average deposited charge density on the surface is found to be strongly sensitive to the jet distance from the surface.

Dead-Time Effect Compensation Based on Additional Phase Current Measurements

This paper proposes a new method of dead-time effect compensation. The proposed solution is based on additional phase current measurements realized by analog-to-digital converters. These measurements are carried out at the time instants specified by a pulsewidth-modulation (PWM) strategy. This makes it possible to estimate inverter currents at the commutation instants and, finally, to estimate the voltage error caused by dead time. This voltage error is compensated during the next switching period by modification of a reference voltage. The proposed solution can be used to compensate the voltage error in multilevel multiphase voltage source inverters. The experimental research studies were carried out on three-phase two-level and three-level neutral-point-clamped inverters supplying 55- and 160-kW motors, respectively. The results of the experimental investigations are presented in this paper.

An Effective Dynamic Current Phasor Estimator for Synchrophasor Measurements

Accurate voltage and current phasor measurements are required to be provided by phasor measurement units placed at various buses in the system. This paper presents an effective method for estimation of alternating current signal fundamental phasor under dynamic conditions. The model-order estimation of the current signal has been carried out to separate the signal space and the noise space in the signal correlation matrix. The Vandermonde matrix of the total least square estimation of signal parameters via rotational invariance techniques method is extended to second-order Taylor's series approximation to estimate the dynamic phasor of the current signal. The suggested technique for the phasor estimation has been tested on simultaneous multimode amplitude and phase oscillations with decaying dc offset and noninteger harmonics, in the presence of noise and also for a fault scenario in New England 39-bus system. This paper also provides a few suggestions for a few possible amendments in the IEEE synchrophasor standard.

Real-Time Multiple Open-Switch Fault Diagnosis in Three-phase AC/DC PWM Converter for PMSG Based Grid-Connected Wind Power Generation System

This paper proposes a new multiple open-switch fault diagnosis method and faulty switch localization algorithm in three-phase AC/DC PWM converter for permanent magnet synchronous generator (PMSG) based grid-connected wind power generation system. In the proposed scheme, the open-switch fault is detected by using normalized three-phase RMS currents and faulty switch location is identified by using normalized three-phase RMS currents as well as average phase currents. Various kinds of fault may occur in AC/DC PWM converter, which decreases the generator output power and degrades the stability in wind power generation system. In this paper, multiple as well as single open-switch fault conditions are investigated. The proposed fault detection and faulty switch localization algorithms do not require additional sensors or hardware since they use only the information on measured phase currents. In addition, they are quite simple, cost-effective and easy to use. To verify that the proposed methods are effective to detect faults and to identify the faulty switches position, the simulation results under twenty-one cases of open-switch faults are presented on different operating conditions.

Magnetic circuit model and finite‐element analysis of a modular switched reluctance machine with E‐core stators and multi‐layer common rotors

This study presents a novel switched reluctance machine (SRM) with modular E-core stators and multi-layer common rotors for high reliability applications. The stator is composed of six independent modular E-cores with the windings wound around the yoke of the E-cores. The rotor consists of three magnetically independent common rotors which are place in the same shaft without any angular shift. The equivalent magnetic circuit (EMC) models of this new SRM at the aligned and unaligned positions are described. To evaluate the motor performance, two types of analysis, namely, approximate simplified two-dimensional (2D) model and 3D model finite-element analysis (FEA) have been utilised. The 2D FEA static and dynamic results are compared with 3D FEA results. Furthermore, comparisons between the novel modular SRM and a conventional 6/4 SRM are made. Finally, a prototype of this modular SRM has been fabricated and tested in the laboratory. Comparison between FEA and EMC analysis for flux linkage are made and compared with measured characteristics. The simulated and measured phase currents and average torque are also presented and compared to verify the analytical and simulation results.

Compensation of Current Measurement Error for Current-Controlled PMSM Drives

Accurate measurement of phase current is crucial in a current-controlled permanent-magnet synchronous machine (PMSM) drive system. Current measurement error directly deteriorates torque control performance. This paper analyzes effects of the current measurement error on the phase current and the output voltage of the current controller. Based on the analysis, a compensation method is proposed. The proposed method compensates the offset and scaling errors separately without any additional hardware but using the commanded voltage reference of the current controller. The proposed method can be applied to general current-controlled PMSM drives in the whole operation range. Experimental results verify the effectiveness of the proposed compensation method.

A New Diagnostic Technique for Real-Time Diagnosis of Power Converter Faults in Switched Reluctance Motor Drives

Accurate fault identification allows the adoption of procedures that minimize the fault impact on the machine operation. Switched reluctance motors (SRMs) have been recognized for their fault-tolerant capabilities, and they present a very robust configuration, particularly due to the absence of windings or magnets in the rotor. All these attributes make the SRM a competitive solution for aircraft and automotive applications, where system reliability is a crucial feature. Therefore, in this paper, a new algorithm for real-time diagnosis of power converter faults in SRM drives is proposed. In contrast to other methods that use additional sensors or devices, the presented technique uses the measured phase currents only, which are already available for the drive control. The proposed algorithm effectively detects the inverter faulty phase and is capable of localizing the faulty power switch. Power switch open- and short-circuit fault occurrences in an asymmetric full-bridge converter are considered and analyzed.