Suppression of common-mode voltage in a PWM rectifier/inverter system

This paper proposes a PWM rectifier/inverter system capable of suppressing not only supply harmonic currents but also electromagnetic interference (EMI). An active common-noise canceler (ACC) developed for this system is characterized by sophisticated connection of a common-mode transformer which can compensate for common-mode voltages produced by both PWM rectifier and inverter. As a result, the size of the common-mode transformer can be reduced to 1/3, compared with the previously proposed ACC. A prototype PWM rectifier/inverter system (2.2 kW) has been implemented and tested. Some experimental results show reduction characteristics of the supply harmonic current and EMI</p

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

A program of research and development of low input voltage conversion and regulation First quarterly report, 14 Jun. - 14 Sep. 1965

Switching and circuit studies for development of low input voltage converter and regulato

Are inductive current transformers performance really affected by actual distorted network conditions? An experimental case study

The aim of this work is to assess whether actual distorted conditions of the network are really affecting the accuracy of inductive current transformers. The study started from the need to evaluate the accuracy performance of inductive current transformers in off-nominal conditions, and to improve the related standards. In fact, standards do not provide a uniform set of distorted waveforms to be applied on inductive or low-power instrument transformers. Moreover, there is no agreement yet, among the experts, about how to evaluate the uncertainty of the instrument transformer when the operating conditions are different from the rated ones. To this purpose, the authors collected currents from the power network and injected them into two off-the-shelf current transformers. Then, their accuracy performances have been evaluated by means of the well-known composite error index and an approximated version of it. The obtained results show that under realistic non-rated conditions of the network, the tested transformers show a very good behavior considering their nonlinear nature, arising the question in the title. A secondary result is that the use of the composite error should be more and more supported by the standards, considering its effectiveness in the accuracy evaluation of instrument transformers for measuring purposes

Electrical transient interaction between transformers and the power system: case study of an onshore wind farm

A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements of the degree of Master of Science in Electrical Engineering June 2016Through the Renewable Energy Independent Power Producer Procurement Program (REIPPPP) the South African government has awarded opportunities for growth of renewable energy through bidding rounds. Round 1 saw a total capacity of 397 MW being awarded to independent power producers (IPP). Subsequently Rounds 2, 3 and 4 each had a total capacity of 333 MW auctioned. The advent of renewables on the market has brought upon its own associated problems with regards to power quality issues and failure of HV equipment. This thesis will address transformer failures that occurred at an onshore wind farm. The nature of the transformer failures suggest transient overvoltages are mainly to blame. A comparison between transformer failures in South African and Brazil suggest a common failure mechanism. The failure starts with an inter-turn insulation failure which propagates to an inter-layer insulation. In worst cases the failure mode results in a puncture through the LV-HV barrier and punctures through the LV winding. An extensive literature review was performed to find appropriate methods to predict and explain the failure mode in wind turbine LV-MV step-up transformers. Of the different models which were reviewed the most notable was the Multi-conductor Transmission Line (MTL) model which was chosen as the preferred model due to its ability to predict the inter-turn/inter-layer voltage stresses. Verification of the developed MTL model by the author was then compared to published results of an MTL model of a disc winding transformer. The results of the comparison revealed a relatively good agreement between the developed model and the published model. The application of the MTL model to represent the voltage stresses in transformer windings was then extended to two specially constructed wind turbine step-up transformer prototypes. The prototypes differed in the winding arrangement of the MV coil. The other used two separate MV coils separated by an oil gap whereas the other had a single MV coil. To validate the model accuracy, a comparison of measured results versus those obtained analytically was done for the two prototypes. The analytical and measured results also had a relatively good agreement for the two prototypes considered. Measurement of switching surges was done on-site at the wind farm to understand the nature of the transients. Using analysis tools such as FFT and frequency domain severity factor it was possible to understand the impact the nature of these transients would pose on the transformer insulation. Different mitigation techniques which can be used to alleviate the transient overvoltages to within safe levels were investigated. The most notable protection device considered was the RL choke device which offered a significant reduction of the pre-strikes and is virtually transparent under power frequency operation.MT201

Assessment of unintentional islanding operations in distribution networks with large induction motors

This paper is aimed at assessing the impact of unintentional islanding operations (IOs) in the presence of large induction motors (IMs) within distribution networks (DNs). When a fault occurs,followingthecircuitbreaker(CB)faultclearing,theIMsacttransientlyasgenerators,duetoits inertia, until the CB reclosing takes place. The present work is the outcome of a project carried out in a small DN, where ¿eld measurements were recorded over two years. This paper provides a detailed description of the test system, a selected list of ¿eld measurements, and a discussion on modeling guidelinesusedtocreatethemodeloftheactualpowersystem. Themaingoalistovalidatethesystem model by comparing ¿eld measurements with simulation results. The comparison of simulations and ¿eld measurements prove the appropriateness of the modeling guidelines used in this work and highlight the high accuracy achieved in the implemented three-phase Matlab/Simulink modelPostprint (published version

Calibration of Current Transformers in distorted conditions

In the context of modern power systems, where there are lots of non-linear loads and generators based on switching power electronics, the accurate measurement of voltage and current harmonics is a key task for the knowledge of the actual state of the network. Voltage and current transducers play a crucial role since they are always the first part of the measurement chain. Currently, classical voltage and current instrument transformers are the most installed transducers, but their performance not always is fully characterized in the presence of distorted waveforms. Therefore, in this paper a calibration setup for the accurate characterization of current transformers with distorted waveforms is presented. System implementation and characterization is presented; then it is employed for the evaluation of the performance of a commercial current transformer in distorted conditions

A Method for the Measurement of Digitizers’ Absolute Phase Error

A lot of engineering applications, from telecommunications to power systems, require accurate measurement of phase angles. Some of them, like synchrophasor measurement and calibration of instrument transformers with digital output, in order to reach high phase measurement accuracy, require the knowledge of phase error of digitizers. Therefore, in this paper a method for the measurement of digitizers’ absolute phase errors is proposed. It adopts a sinewave and two square waves, that are the digitizer sample clock and a phase reference signal. Combining the measurements of the relative phase differences between the adopted signals it is possible to accurately evaluate the absolute phase error of a digitize

ASDTIC control and standardized interface circuits applied to buck, parallel and buck-boost dc to dc power converters

Versatile standardized pulse modulation nondissipatively regulated control signal processing circuits were applied to three most commonly used dc to dc power converter configurations: (1) the series switching buck-regulator, (2) the pulse modulated parallel inverter, and (3) the buck-boost converter. The unique control concept and the commonality of control functions for all switching regulators have resulted in improved static and dynamic performance and control circuit standardization. New power-circuit technology was also applied to enhance reliability and to achieve optimum weight and efficiency

Calibration of Current Transformers in distorted conditions

In the context of modern power systems, where there are lots of non-linear loads and generators based on switching power electronics, the accurate measurement of voltage and current harmonics is a key task for the knowledge of the actual state of the network. Voltage and current transducers play a crucial role since they are always the first part of the measurement chain. Currently, classical voltage and current instrument transformers are the most installed transducers, but their performance not always is fully characterized in the presence of distorted waveforms. Therefore, in this paper a calibration setup for the accurate characterization of current transformers with distorted waveforms is presented. System implementation and characterization is presented; then it is employed for the evaluation of the performance of a commercial current transformer in distorted conditions