Measured Phase Currents Research Articles

Parameter Identification With PMUs for Instability Detection in Power Systems With HVDC Integrated Offshore Wind Energy

Large-scale offshore wind farms can be integrated with onshore ac grids by the voltage source converter based high voltage direct current (VSC-HVDC) technology. The resulting impact on the security of ac grids can be significant. Therefore, it is important to develop an HVDC model for online stability monitoring of the integrated ac/dc systems. This paper proposes a new HVDC model based on a circuit-theoretic foundation. With the available phasor measurement units (PMUs) at VSC stations, the parameters of the HVDC equivalent model can be identified in real-time by synchronized voltage and current phasor measurements at VSC ac terminals. The proposed HVDC model is applied to online voltage instability detection for integrated ac/dc systems by the Thevenin impedance matching. The HVDC equivalent circuit enables the Thevenin equivalent impedance of the HVDC-connected offshore wind farm to be determined directly from the PMU measurements. Numerical simulations are performed on the IEEE 39-bus system with an HVDC-connected offshore wind farm to validate the effectiveness of the proposed HVDC model.

A New Approach for Current Sensor Fault Diagnosis in PMSG Drives for Wind Energy Conversion Systems

Fault diagnosis is a mandatory feature in fault-tolerant systems, since it provides the information necessary for the fault isolation and system reconfiguration. Recently, permanent-magnet synchronous generator (PMSG) drives have achieved prominence in wind energy conversion systems (WECSs), due to their reliability and availability. In this paper, a new approach is proposed for current sensor fault diagnosis in PMSG drives for WECSs. As opposed to the conventional state-observer-based methods for current sensor faults, which require a system model and the respective parameters, the proposed diagnostic method uses the measured phase currents only. Thus, its main merits are simplicity and reliability in the diagnosis, making it suitable for real-time implementation and to trigger remedial procedures. In addition, current sensor and open-circuit faults can be distinguished, and the affected phase is effectively identified in both cases. The proposed diagnostic technique is applied to the two power converters of a conventional back-to-back topology, and its performance is analyzed by means of several experimental results.

An Accurate Synchrophasor Based Fault Location Method for Emerging Distribution Systems

The accurate knowledge of fault location in distribution systems can aid in the repair process, expedite system restoration, and, thus, reduce outage duration. This letter presents a new method for locating a fault in distribution systems using synchrophasor measurements, which are often provided by phasor measurement units (PMUs) with high accuracy and a timestamp. Using voltage and current phasor measurements at substations and/or feeder heads obtained through suitable communication schemes, candidate fault locations are identified by iterating every possible line segment. The measurements from remote devices are used to eliminate all nonfaulted cases. This method works effectively for active and passive networks, radial and looped topologies, high impedance faults, and is not constrained to a certain pattern of PMU locations. The proposed method has been tested in a real distribution system. Results show that it can identify faults within 1% accuracy of the line length.

Impact of Fast-Chargers for Traction Batteries on Distribution Network

High-power converters with high frequency pulse transformers are used in fast-chargers for electric vehicles to decrease their size and price. These converters and their input rectifiers have significant impact on distribution network. Distribution net impact can be split into 3 basic domains: EMC issues, unusually high local concentration of consumed power and presence of higher harmonics in input current. The third domain is the main objective of the article. An advantageous solution of input rectifier was proposed. A conception with a passive diode input rectifier and a DC-link with an extremely low capacity is used in this conception. Dimensioning of this DC-link capacitor to ensure mentioned low impact on distribution net is presented. The final part of the paper demonstrates overall applicability of RN14-140 fast-charger pattern via presentation of high harmonics of phase current measurements.

A Switching Flux-linkage Reference-based Sensorless Method for Switched Reluctance Motor Drives

A switching flux reference estimation based method for sensorless control of switched reluctance motors is proposed in this article. The basic idea of the method is to estimate switching-on and switching-off flux linkage references from the giving switching angles range and measured phase current. Real-time comparing the estimated flux linkage with estimated switching flux references, the driving signals can be obtained for sensorless control and rotor speed estimation. For closed-loop control, switching-off angle is fixed for speed estimation, and switching-on angle is regulated by the speed deviation proportional-integrator controller. The process to estimate the switching flux references is greatly simplified compared with conventional flux/current methods; it is easily implemented and is suitable for low-cost industry applications. To verify the proposed method, experiments were conducted on a 1-kW, 3-phase 12/8 switched reluctance motor. Experimental results show that speed variable performance of the sensorless drive is comparable with that of position sensors.

An adaptive fault location algorithm for power system networks based on synchrophasor measurements

This paper presents an adaptive fault location algorithm for power system networks based on synchronized phasor measurements obtained by Phasor Measurement Units (PMUs). To enhance its accuracy, the proposed algorithm is made to be independent of any data that shall be provided by the electric utility. The proposed algorithm requires three different sets of pre-fault voltage and current phasor measurements at both terminals of the faulty line to be obtained through PMUs. The three sets of local PMU measurements at each terminal are used for online calculation of the respective Thevenin's equivalent (TE). Using the method of multiple measurements with linear regression (MMLR), the three sets of PMU measurements are also employed for online calculation of the transmission line parameters. Online determination of the TEs and line parameters ensures avoiding any possible mismatch with the actual parameters due to system loading and other environmental conditions. The proposed method is applied to a 115kV system selected from the Saudi Electricity Company (SEC) network. The simulation results obtained using PSCAD/EMTDC and MATLABreveal that the proposed algorithm is highly accurate and independent of fault type, fault location, fault resistance, fault inception angle and pre-fault loading.

State estimation of flexible AC transmission systems considering synchronized phasor measurements

This paper proposes a practical approach to incorporate synchronized phasor measurements into a weighted least squares state estimation algorithm suitable for electric networks containing flexible AC transmission systems (FACTS) controllers. Equations relating the branch current phasors to FACTS controllers’ state variables are derived from first principles in order to directly append synchronized phasor measurements of currents and voltages to the set of measurement data collected from the supervisory control and data acquisition system to estimate the equilibrium point of the electric power system. The phase angle measured by a perfect phasor measurement unit at one of the buses is selected as the global reference for the estimation, such that the voltage phasor measurements provided as data to the estimator correspond to the angle differences between all remaining voltage phasor measurements and the reference angle. Furthermore, the synchronized current phasor measurements are transformed to rectangular coordinates to enhance the convergence properties of the proposed state estimation approach. In both cases, the variance of the new set of synchronized measurements is calculated based on the uncertainty propagation theory. The proposed approach simultaneously upgrades the estimated values of the state variables of FACTS devices and the electric network for a unified estimation of the system state. Lastly, numerical simulations with two real-life electric power networks are reported to demonstrate the prowess of the proposed approach.

Optimized echo state networks using a big bang–big crunch algorithm for distance protection of series-compensated transmission lines

Distance protection of transmission lines compensated by Thyristor-Controlled Series Capacitor (TCSC) suffers malfunction due to series variable voltages injected by TCSC. To mitigate the problem, echo state networks (ESNs) are used in this paper through the voltage compensation technique by subtracting estimated TCSC voltages from measured phase voltages where fault loop observed by the relay contains TCSC. The design parameters of ESNs are optimized using a big bang–big crunch (BB–BC) algorithm with an especial random generation scheme in big bang phase. Main features of our study are: (a) only locally measured phase currents at the relaying point is used, assuming firing angles and other variables of TCSC are not accessible there; and (b) high accuracy of estimations is achieved using optimized ESNs for a large number of test cases. The method is tested on a 400km, 500kV line for all fault types using Matlab/Simulink. Tests for 7680 cases with varying fault resistance, fault inception-angle, fault location, load angle and compensation degree show much improved accuracy of estimations by ESN in comparison to time-delay neural network (TDNN), radial basis function neural network (RBFNN), nonlinear autoregressive network with exogenous inputs (NARX) and Elman network particularly at instants immediately after fault incidence. The effectiveness of the ESN-based method is examined using two distance relay algorithms.

Observability and Criticality Analyses for Power Systems Measured by Phasor Measurements

This paper proposes observability and measurement criticality analysis methods for any type of current phasor measurement configurations. Measurement Jacobian cannot be decoupled when using current phasor measurements and thus conventional methods of network observability analysis can no longer be used. Current phasor measurements may be classified into two types based on their configuration. Depending on whether there is a voltage phasor measurement at the sending end of the measured line or not, a current phasor measurement configuration may be referred to as of type-1 or type-2, respectively. Two methods which can handle conventional as well as phasor measurements, one being a modified version of the conventional method for type-1, and the other being a new method for type-2 phasor measurements, are developed and presented. Moreover, due to the fully coupled model used in the analysis, a method to handle voltage magnitude measurements is also developed.

Analysis of the Phase Current Measurement Boundary of Three Shunt Sensing PWM Inverters and an Expansion Method

To obtain phase currents information in AC drives, shunt sensing technology is known to show great performance in cost-effectiveness and therefore it is widely used in low cost applications. However, shunt sensing methods are unable to acquire phase currents in certain operation conditions. This paper deals with the derivation of the boundary conditions for phase current reconstruction in three-shunt sensing inverters and proposes a voltage injection method to expand the measurable areas. As the boundary conditions are deeply dependent on the switching patterns, they are typically analyzed on the voltage vector plane for space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM). In the proposed method, the voltage injection and its compensation are conducted within one sampling period. This guarantees fast current reconstruction and the injected voltage is decided so as to minimize the current ripple. In addition to the voltage injection method, a sampling point shifting method is also introduced to improve the boundary conditions. Simulation and experimental results are presented to verify the boundary condition derivation and the effectiveness of the proposed voltage injection method.

Centrifugal pump operation monitoring with motor phase current measurement

Centrifugal pumps often operate inefficiently because of their incorrect dimensioning and inefficient control. The economic potential of improving the pumping efficiency has generated a wide market for energy auditing products and services, which determine the prevailing energy efficiency of pump operation. Energy audits are commonly based on additional measurements that might be carried out for a short period of time, covering only a fraction of the pump operation cycle. Often the most feasible solution would be to use the existing measurements in the pumping system to determine its typical operation. Then, the correctness of pump dimensioning and the efficiency of the applied flow control method could be determined simultaneously for a large group of pumps, as there would be no requirements for additional measurement sensors on each device.In this paper, a current measurement-based approach to monitor centrifugal pump operation and its energy efficiency is studied. This approach allows a simple realization of energy audits for fixed-speed pumping systems, where only current consumption of the electric motor is measured. Approach is based on adjustable models for motor and pump operation, which in this case are configured for a laboratory pumping system. Factors affecting the estimation accuracy of the motor operation are also analyzed in the paper. According to the conducted tests with the laboratory setup, the motor shaft power can be estimated with the accuracy of 3.2%, and the pump flow rate with the accuracy of 16%. This accuracy class can be considered sufficient for energy auditing and monitoring purposes, since it allows detection of inefficiently operating pumps. After this, more accurate measurements with separate measurement instrumentation and more detailed pumping system energy efficiency analysis can be carried out.

Sensorless stator field orientation controlled induction motor drive with a fuzzy speed controller

A sensorless stator-field oriented control induction motor drive with a fuzzy logic speed controller is presented. First, a current-and-voltage parallel-model stator-flux estimator is established using measured phase currents and voltages of the induction motor. Then the estimated rotor shaft position is obtained from the magnitude and position of the estimated stator flux. The speed controller is developed by utilizing fuzzy logic control techniques. The control algorithms are realized by a DSP 6713 and, using a DSP F2812 to generate PWM signals to the power stage, drive the motor to experimentally validate the proposed approach.

A state estimator including conventional and synchronized phasor measurements

This paper presents an effective weighted least square formulation for the solution of the state estimation problem, considering conventional as well as synchronized phasor measurements. The proposed algorithm is based on a reference-free formulation, using both rectangular and polar coordinates for branch current phasor measurements, and alleviates any numerical problems encountered during initialization stage of the state estimation algorithm. Analytical equations are provided for the conventional and the phasor measurements and their corresponding partial derivatives with respect to state variables. The proposed algorithm is tested with a seven bus system and is compared with the traditional state estimator. Simulation results show that the proposed algorithm improves the precision greatly and gets better behavior as compared with the traditional state estimator.

Coupling effects in inductive discharges with radio frequency substrate biasing

Low pressure inductively coupled plasmas (ICP) operated in neon at 27.12 MHz with capacitive substrate biasing (CCP) at 13.56 MHz are investigated by phase resolved optical emission spectroscopy, voltage, and current measurements. Three coupling mechanisms are found potentially limiting the separate control of ion energy and flux: (i) Sheath heating due to the substrate biasing affects the electron dynamics even at high ratios of ICP to CCP power. At fixed CCP power, (ii) the substrate sheath voltage and (iii) the amplitude as well as frequency of plasma series resonance oscillations of the RF current are affected by the ICP power.

Study on Sensorless Control Model Based on a Model Reference Adaptive System with Parameter Optimization

A sensorless control model based on a model reference adaptive system (MRAS) with the theory of parameter optimization for the Permanent Magnet Synchronous Motors (PMSM) servo system is presented in this paper. The model can promptly work out the speed of the rotor without any other position/speed sensor. Different from the other MRAS-based speed observers which constructed according the theory of stability customarily, by using the error between the measured phase currents of actual motors and theoretic phase currents acquired by the mathematical model of motors as the parameter to be optimized, the proposed model can gain the speed of rotor with fast response and better performance. The stability of MRAS is analyzed subsequently. In the model, only a few parameters are needed and satisfactory performance can be achieved. So this model is very simple to be used as the position/speed observer of the PMSM servo system. Finally the simulation and experimental results confirm the validity and advantage of the proposed method.

State estimation and bad data processing for systems including PMU and SCADA measurements

Conventional state estimators (SE) are based on real-time measurements, consisting of bus voltages and active and reactive power flows and injections, and estimate the voltage phasors of the network buses. Until recently, these measurements were obtained only through SCADA. With the advent of GPS synchronized measurements obtained by phasor measurement units (PMU), effective techniques are required to incorporate the extremely accurate PMU measurements into state estimation, in order to improve its performance and observability. This paper develops a non-linear weighted least squares estimator by modeling the current phasor measurements either in rectangular or in polar coordinates and compares the two approaches. Any numerical problems arised at flat start or for lightly loaded lines, are resolved. The error amplification, due to the current phasor measurement transformation from polar into rectangular coordinates, is also investigated. The normalized residual test is used to effectively identify any bad data in the conventional and phasor measurements. The proposed techniques are tested with the IEEE 14-bus system.

An Estimation Method of Rotational Direction and Speed for Free-Running AC Machines Without Speed and Voltage Sensor

This paper presents an estimation method of rotational direction and speed for free-running ac machines driven by an inverter without speed and voltage sensor. The method has four estimation modes, and it utilizes only the measured phase current of machines. The performance of the proposed method is verified through the experiments for both permanent-magnet synchronous motor and asynchronous machines.

Phasor-assisted Hybrid State Estimator

This article proposes a simple and effective approach to incorporate phasor measurements in the existing traditional supervisory control and data acquisition measurements based state estimation software. A phasor assisted hybrid non-linear state estimator has been proposed, where both supervisory control, data acquisition, and phasor measurements are integrated together and utilized to enhance the accuracy of the conventional weighted least square state estimator. A sequential phasor-assisted state estimation approach is also presented, where, in the first stage, the conventional weighted least square state estimation is run using supervisory control and data acquisition measurements, and, in the second stage, voltage and current phasor measurements from phasor measurement units are utilized to enhance the output of the conventional weighted least square estimator. The results of both estimators are compared to establish the effectiveness of the proposed method on three test systems, namely, the IEEE 14-bus system, the New England 39-bus system, and the Northern Region Power Grid 246-bus Indian system.

Inclusion of PMU current phasor measurements in a power system state estimator

Three different methods of inclusion of current measurements by phasor measurement units (PMUs) in a power system state estimator is investigated. A comprehensive formulation of the hybrid state estimator incorporating conventional, as well as PMU measurements, is presented for each of the three methods. The behaviour of the elements because of the current measurements in the measurement Jacobian matrix is examined for any possible ill-conditioning of the state estimator gain matrix. The performances of the state estimators are compared in terms of the convergence properties and the variance in the estimated states. The IEEE 14-bus and IEEE 300-bus systems are used as test beds for the study.

New Designs of a Two-Phase E-Core Switched Reluctance Machine by Optimizing the Magnetic Structure for a Specific Application: Concept, Design, and Analysis

Three new magnetic structures for an E-core switched reluctance machine (SRM) comprising two segmented stator cores or a monolithic stator core are proposed for good manufacturability, mechanically robustness, ease of assembly, and electromagnetic-performance improvement. The E-core stator has four small poles with phase windings and two or four large poles (hereinafter referred to as common poles) in between. The common poles do not have copper windings or permanent magnets. The common poles are shared by both phases for positive-torque generation during the entire operation. The air gap around the common pole has constant and minimum reluctance, irrespective of rotor position, by its unique design, and the two small stator poles experience variable reluctance with respect to rotor position. The segmented stator can be constructed with two independent and physically separate E-shaped cores with L- or I-shaped yokes so that the straight back iron in the stator rather than the curved back iron is suitable for assembly with other parts like end bells or brackets. The monolithic stator also has an L-shaped yoke for the same reason, and the new E-core SRMs are designed by means of pole-arc optimization. The novel E-core SRMs are compared to a conventional two-phase SRM. The comparison includes cost savings, torque, copper and core losses, efficiency, and radial force in order to validate the distinct features of the E-core SRMs. For experimental verification, laboratory testing of a segmented E-core SRM is conducted, and the simulated and measured phase currents and voltages correlate well.