Establishment of Test System for Electronic Voltage Transformers’ Accuracy of Harmonics

Transformers are generally designed in order to deliver the required power to the connected loads with minimum losses at nominal fequency. Nowadays , power quality affected by non-linear loads and unbalanced condition and causes deflection from a perfect sinusoidal waveform. This condition leads to additional losses, increase temperature and reduction of the useful life of the transformers. In order to avoid of these difficulties, the nominal capacity of transformer should be reduced. This paper studies the increase of losses and Derating of Distribution transformer, working under Different harmonic load and unbalanced supply voltage conditions. In this paper, the three dimensional finite element method (3-D) is utilized as a Instruments for viewing magnetic flux density on core and windings of distribution transformers. The computer simulation according to the use of the FEM has been improved in Ansoft - Maxwell. In section 2, a 3-D time stepping finite element method (TSFEM) is used in order to modeling the transformer under different harmonic load with (THD I =12% and 23%). The maximum flux density in the transformer windings under sinusoidal and different harmonic load condition has been investigated in this section. In section 3, the effect of harmonic load on transformer load losses Has been examined. The result shows that under harmonic load current , copper loss an eddy current losses significantly increases. In section 4, derating of transformer under harmonic load based on IEEE c57.110 standard and TSFEM by using k-factor and harmonic loss factor (H FL ) Has been done. The result shows that IEEE standard is a Conservative method for transformer derating under high harmonic current (THD I ) and TSFEM is a Convenient method for transformer derating in this condition. In section 5, The Performance of the transformer when the supply voltage are balanced compared to the case in which supply voltage is unbalance , the result shows that , the unbalanced voltage Causes increase in the both copper loss and core loss. Afterwards, Based on the data obtained from the TSFEM, a distribution transformer under different unbalanced condition has been derated

Harmonics in electric power systems can affect the measurement, quality analysis and relay protection of the power grid, making the accurate measurement of harmonics a necessity. The broad frequency range of electronic transformers makes them suitable for measuring harmonics. The requirements of accuracy on harmonics for electronic transformers are put forward in the standards GB/T 20840.8-2007 and IEC60044-8:2002. But because there was no system for this measuring in China, the accuracy test on harmonics haven't been done yet. So the measuring system for electronic voltage transformer's (EVT) accuracy on harmonics had been studied and found in this article. Then the preliminary measuring of accuracy on harmonics of different kinds of electronic voltage transformer has been done. And the test results have also been analysed. All the results shows that: under the condition of voltage from 0 to 80kV, frequency from 50Hz to 650Hz, the measuring system for accuracy on harmonics we found can meet the requirements of standards. In terms of accuracy on harmonics, the variation of the active electronic voltage transformers' ratio error is less than the passive electronic voltage transformers' with the increase of the harmonic order. And the variation of the active electronic voltage transformers' phase displacement is more than the passive electronic voltage transformers' with the increase of the harmonic order. The test results of accuracy on harmonics of passive electronic voltage transformer can meet the requirements of standards. And the active electronic voltage transformer need to improve the production craft to meet the requirements.

The accuracy and stability of electronic transformers (ETs) is the guarantee of the safe and reliable operation of a power system. As an emerging technology, however, the ETs’ failure rate is higher than that of traditional transformers. As a result, the calibration period should be shortened. Existing calibration methods address the verification on condition that the ETs and lines are powered off, which has a long calibration period and results in a power-off loss. As a result, an online calibration system for electronic voltage transformers (EVTs) is proposed in this paper. Employing a specially designed standard voltage online measuring unit, the proposed system can obtain the high voltage signals without powering off and do the calibration when the EVTs are working. The power-off loss can be avoided, and problems of the running EVTs can be detected in time. The main factors influencing the system accuracy are addressed. The system has been validated by the Quality Inspection and Testing Center of the China Electric Power Research Institute, and field tests have been conducted in Guizhou Electric Power Company. The results show that the proposed system has good reliability and can reach an accuracy class of 0.05 in complicated electromagnetic circumstances.

Electronic voltage transformer (EVT) and electronic current transformer (ECT) are important equipments in smart grid. In order to meet the requirements of power reliability in the grid, the annual failure rate of EVT and ECT should be less than 0.2% during normal operation. Power transformer is the electrical device, its annual failure rate is only 0.01%, and the comprehensive reliability of secondary relay protection devices can be improved to more than 99.8% by redundancy design. EVT and ECT's reliability can be hardly improved to 99.8% due to the limit of failure relevance, unless designed and manufactured according to aerospace standards. EVT and ECT's utilization is limited in the digital substation considering of reliability and economy. In the digital substation, Non-traditional transformer is an economical and rational design, because the reliability of 99.8% can be guaranteed by sending analog small-signals from non-traditional transformers to the relay unit. Non-traditional transformer's analog output is sent to the merged cell of transformer, and a digital signal is converted into the data bus serving for substation monitoring and control systems, so the reliability can reach 99%. Thus non-traditional transformer's analog output and merged cell's output can respectively meet the reliability requirements of relay protection and monitoring system's informatization in the digital substation.

Peculiarities of use of conductivity meters in the oil and gas industry. For the ecological monitoring of conductometry allows us to constantly monitor the maximum allowable concentrations of harmful substances in industrial waste waters, as well as to prevent instantaneous discharges into water bodies and rivers. The advantage of contactless conductivity meters with liquid communication circuit. The deficiencies of these devices associated with the influence on the measurement result of factors such as the instability of the source of harmonic oscillations, instability frequency, ambient temperature. Fundamentals of the theory of invariance of measurement and its main principle is the principle of multi-channel. The proposed scheme three-stage transformer conductivity meter, invariant to disturbing influences. The invariance principle is implemented in a three-stage transformer conductivity meter by introducing an exemplary conduction through the additional coil and the change gear ratio in one of the bars. Studies of such a device. The analysis shows that three-stage conductivity meter allows to eliminate the influence of the errors associated with the instability of the voltage and frequency of power supply, the inductance liquid loop, and also to eliminate stationary crosstalk that occurs in the core of transformer due to the flow of scattering of the exciting transformer. The dependence of the relative error of the instrument from the exemplary values of conductivity and resolution analog-to-digital Converter. Given the graph of this dependence. It is established that the main factor affecting the measurement error, is the resolution of the ADC. Construction of invariant measurement devices using the multichannel principle allows to obtain a high measurement accuracy, reduce the stability requirements of the units of these devices and to eliminate the influence on the accuracy indicators disturbing influences, including those in which single-channel measuring device virtually incapacitated.

Flexible phase-locked loops and millimeter wave PLL components for 60-GHz wireless networks in CMOS

In the last years, static power converters have become widely used in various applications. They can be found in domestic applications, railways, urban and ship transport, and even in several industrial systems. Some of these applications require a high or medium voltage power supply that is easily adjustable while providing good spectral performances. To overcome the maximum blocking voltage limits of the main power switches, multilevel techniques and other new power conversion topologies have been developed. They are series/parallel associations of existing power semiconductors, and allow generating an output voltage with many levels. The number of power semiconductors needed in these topologies increases as the number of levels increases. The power converter circuit becomes more complex and its reliability decreases. This thesis focuses on three-phase multilevel converters based on a series connection of single phase inverters (partial cells) in each phase. It's shown that, feeding partial cells with unequal DC-voltages (asymmetric feeding), increases the number of levels of the generated output voltage without any supplemental complexity to the existing topology. Voltage resolution is increased through interpolating the generated output voltage phasor (three phase voltage in α-β frame). This is achieved by seeking the non redundant switching states of the power switches. The resultant converter can generate a very high resolution voltage phasor up to the possible maximum resolution. This approach is generalized for any number of partial cells. Each partial cell is fed through a three-phase diode rectifier, fed itself through the windings of a multi-secondary low frequency power transformer. From analytical expressions in continuous and discontinuous current conix duction modes, it is shown that, from a supply network point of view, a symmetrical multilevel converter has a smaller total harmonic distortion than an asymmetrical multilevel converter with the same number of partial cells per phase. An asymmetrical multilevel converter is not more interesting than a classical three-phase converter, but its total harmonic distortion is compatible to the recommended IEEE std 519-1992. It is also shown that the advantages of an asymmetric multilevel converter from a load point of view (generation of a high resolution voltage phasor, possibility to choose the number of redundant switching states, reduction of the number of power semiconductors for the same voltage resolution, flexibility for the DC-voltage feeding choice) and the advantages of a symmetrical multilevel converter from a supply network point of view (smaller total harmonic distorsion) can be combined. Simulation results and the experimental test setup showed the reliability of the suggested approach.

In the analysis of harmonic power flows, the transformer is generally represented as a passive component. However, due to the magnetisation nonlinearity, the transformer constitutes a source of harmonic currents which can aggregate and cause substantial waveform distortion. This paper describes the development of a Norton equivalent circuit which accurately represents the transformer magnetising branch in harmonic power-flow studies. It is shown that even in the absence of other nonlinearities in the system, the transformer magnetising current can give rise to considerable harmonic voltage levels.

A major change in the electrical transmission and distribution system is taking place in Europe at the moment. The shift from a centralised energy production to a distributed generation profoundly changes the behaviour of the grid. Environmental or social issues associated with the construction of new power lines to relieve bottlenecks, together with aged equipment dating from the 1960s, pose some serious challenges to government, the research community and the economy. Concepts of reactive compensation, harmonic cancellation, voltage stability, power quality and bulky low-frequency transformers need to be redefined for power exchange and transmission in the future. Photovoltaics, wind turbines, fuel cells, storage systems and uninterruptible power supplies use many power electronic interface circuits, where DC intermediate levels already exist. Large photovoltaic- or wind- powered installations, which are connected to a cable network, are characterised by non-negligible distances due to their low power-by-surface density. On the side of the consumer, current trends show an increasing use of DC in end-user equipment. In such a context, the numerous advantages of power electronics and DC cables may sometimes out-weigh their higher cost. In the future, high-power semiconductor devices that allow higher switching frequencies of the converters may make it possible to down-size even more the passive components. This would significantly reduce raw material consumption and therefore cost, something that is crucial for the market to accept the technology. In the first part of this PhD thesis, the advantages of DC distribution in terms of transmission losses are illustrated with the help of three case studies. The second part and the main contribution of this thesis is the analysis of a promising candidate for a power electronic transformer, the key component of any DC based grid. It is a bidirectional isolated DC/DC converter based on modular multilevel converters, which are well suited for medium or even high voltage range. The motivation was to investigate a converter operation with important voltage elevation ratios, capable of adapting the voltage level between low, medium and high voltage. A medium-frequency isolation stage provides the possibility of downsizing the passive components. Two modulation methods, a multilevel and a two-level operation, were analysed and compared in terms of losses. The modular DC/DC converter is an attractive solution for the sensitive aspect of the short-circuit behaviour of classical DC links and power lines. The converter can also handle short circuits without the need for additional protection devices, such as circuit breakers. Given the many advantages of DC systems (reduced environmental impact, reduced space requirements, reduced raw material use, high power quality, power flow control, low transmission losses), this new technology must, at least, be considered when assessing the extension or the renovation of conventional AC grids.

Electronic transformers had excellent dynamic and frequency response properties, and played an important role in the intelligent construction of power systems. Since they worked in a serious electromagnetic environment, electromagnetic interference could result in many failures of the electronic current transformer. A new true-type platform was developed to test electromagnetic interference features, which included the busing, voltage detector, electronic voltage transformer, earthing switch, disconnecting switch, electronic current-voltage transformer, and voltage divider. Voltage and current data were measured and applied to assess the electromagnetic interference by adjusting the distance between the collector and gas-insulated switchgear at the first and secondary sides. The method integrated time domain and frequency domain was proposed to perform feature analysis. The results showed that the feature parameters on the first side were not significantly affected by distance, while the secondary side had certain interference, especially on the peak-to-peak value, maximum value, minimum value and other feature parameters. Through this experimental test and feature analysis, it built a good basis for future design and optimization.

This paper intends to optimize the control fashion of PWM inverters of Intelligent Universal Transformer (IUT) using Genetic Algorithm (GA). IUT is the state of art proposed as an electrical key point of Advanced Distribution Automation (ADA) technology. It comprises the power electronic construction and high frequency transformer. ADA will be raised for tomorrow's distribution system to perform the exchanging platform for both data and information which will be fully monitored and automatically controlled. IUT is introduced as intelligent electronic devices in lieu of traditional transformer with the controllable architecture that enhances the reliability and system performances and leading to the various services and user benefits like desirable output voltage, level and frequency, DC voltage options and sag correction. In case of oil elimination benefit it also yield to size and weight reduction. In this approach current and voltage source controllers based on GA optimization are developed for controlling the firing angle of switching pattern of PWM inverters which are recommended for voltage level and frequency control of IUT. The GA smart optimization fashion with the proposed three levels IUT topology emphasizes on the very smooth control and voltage regulation in both input and output inverters of IUT. GA improves the system characteristics under load and input disturbances with regulating the voltage and harmonic elimination.

In order to improve the reliability of power supply, a large number of electronic voltage and current transformers are used in digital substations. In this paper, the mathematical model of the electronic transformer is analyzed firstly, and its circuit model is given. According to the difference of working characteristics between voltage transformer and current transformer, the circuit model of voltage type electronic transformer and current type electronic transformer is given respectively. By analyzing their broadband transmission characteristics, the accuracy of the model is verified, and their lightning analysis models are obtained.

Power quality is a very important topic of the recent power system network topics. It refers to maintaining a sinusoidal waveform of bus voltages. Unfortunately, the power system faces many disturbances which affect the supply waveform, such as under voltage, notching, harmonics, sag and swell voltages. In practice, sag voltage and harmonics are the major problems which can cause malfunctioning or tripping of equipment and many other problems on the system. One of the causes of these events is the inrush current during the energization of no-load transformers. This paper presents an effective and economic solution for the protection of sensitive loads from sags and harmonics using the dynamic voltage restorer (DVR). It also highlights the full functions of the DVR, showing that it can be considered as a proper solution to overcome the inrush current effects. Sag indices concepts on the distribution system are also presented, showing the performance of the DVR to get better results. The increasing of the DVR effectiveness as well as its simplicity and low cost are reasons for considering it as the best solution for many quality disturbances. Simulation results using MATLAB/SIMULINK are presented to verify the effectiveness of the proposed scheme.

Travelling wave pilot protection for LCC-HVDC transmission lines based on electronic transformers’ differential output characteristic

During the restoration process of primary restorative transmission system, the over voltage may happen due to nonlinear interaction between the unloaded transformer and the transmission system. This over voltage is caused by harmonic resonance and can be prevented by damping loads that are connected before restoration process of primary restorative transmission system. But it is very difficult to predict the occurrence possibility of the harmonic resonance previously. This paper analyzes the relationship between the harmonic resonance and the amount of damping loads to prevent the harmonic resonance. This paper calculates the minimum amount of damping loads to prevent harmonic resonance while changing the length of primary restorative transmission line. And this paper confirmed that the amount of damping loads is not proportional to length of transmission line. The result of this paper will be used as important experiment data to predict the occurrence possibility of harmonic resonance previously.