Online Capacitor Voltage Transformer Measurement Error State Evaluation Method Based on In-Phase Relationship and Abnormal Point Detection

The assessment of the measurement error status of online Capacitor Voltage Transformers (CVT) within the power grid is of profound significance to the equitable trade of electric energy and the secure operation of the power grid. This paper advances an online CVT error state evaluation method, anchored in the in-phase relationship and outlier detection. Initially, this method leverages the in-phase relationship to obviate the influence of primary side fluctuations in the grid on assessment accuracy. Subsequently, Principal Component Analysis (PCA) is employed to meticulously disentangle the error change information inherent in the CVT from the measured values and to compute statistics that delineate the error state. Finally, the Local Outlier Factor (LOF) is deployed to discern outliers in the statistics, with thresholds serving to appraise the CVT error state. Experimental results incontrovertibly demonstrate the efficacy of this method, showcasing its prowess in effecting online tracking of CVT error changes and conducting error state assessments. The discernible enhancements in reliability, accuracy, and sensitivity are manifest, with the assessment accuracy reaching an exemplary 0.01%.

Over Voltage Fault due to Disconnection of Consumer’s Transformer Neutral Wire

Practically,the load currents in three phases are asymmetric in the power system.It means that the impedances are different in all three phases.If the consumer’s transformer neutral cut off and/or was disconnected from the neutral of power supply source,then there will be some trouble and failure occurred.The current in the neutral wire drops down to zero when the neutral wire is cut off and the phase currents of all three-phase equal to each other since there was no return wire.The currents are equal but the voltages at the phase consumers are different.Especially for residential single-phase consumers,the voltage at the consumers of the phase varies differently for three phase systems when the neutral wire was disconnected at consumer side and even the voltage at the consumers one or two of those three phases becomes over nominal voltage or reaches nearly line voltage.In this case,the electronic appliances in that phase will be fed by high voltage than the rated value and they can be broken down.In the power system of UB(Ulaanbaatar)city,there are some occasional such kind of failures every year.Obviously,many electronic appliances were broken down due to high voltage and the electricity utility companies respond for service charge of damaged parts.

Smart Transformer-based Medium Voltage Grid Support by Means of Active Power Control

In the last decades the voltage regulation has been challenged by the increase of power variability in the electric grid,due to the spread of non-dispatchable generation sources.This paper introduces a Smart Transformer(ST)-based Medium Voltage(MV)grid support by means of active power control in the ST-fed Low Voltage(LV)grid.The aim of the proposed strategy is to improve the voltage profile in MV grids before the operation of On-Load Tap Changer in the primary substation transformer,which needs tens of seconds.This is realized through reactive power injection by the AC/DC MV converter and simultaneous decrease of the active power consumption of voltage-dependent loads in ST-fed LV grid,controlling the ST output voltage.The last feature has two main effects:the first is to reduce the active power withdrawn from MV grid,and consequently the MV voltage drop caused by the active current component.At the same time,higher reactive power injection capability in the MV converter is unlocked,due to the lower active power demand.As result,the ST increases the voltage support in MV grid.The analysis and simulation results carried out in this paper show improvements compared to similar solutions,i.e.the only reactive power compensation.The impact of the proposed solution has been finally evaluated under different voltage-dependence of the loads in the LV grid.

Development of a Medium Voltage, High Power, High Frequency Four-Port Solid State Transformer

The power and voltage levels of renewable energy resources is growing with the evolution of the power electronics and switching module technologies.For that,the need for the development of a compact and highly efficient solid-state transformer is becoming a critical task in-order to integrate the current AC grid with the new renewable energy systems.The objective of this paper is to present the design,implementation,and testing of a compact multi-port solid-state transformer for microgrid integration applications.The proposed system has a four-port transformer and four converters connected to the ports.The transformer has four windings integrated on a single common core.Thus,it can integrate different renewable energy resources and energy storage systems.Each port has a rated power of 25 kW,and the switching frequency is pushed to 50 k Hz.The ports are chosen to represent a realistic industrial microgrid model consisting of grid,energy storage system,photovoltaic system,and load.The grid port is designed to operate at 4.16 k VAC corresponding to 7.2 kV DC bus voltage,while the other three ports operate at 500 VDC.Moreover,the grid,energy storage and photovoltaic ports are active ports with dual active bridge topologies,while the load port is a passive port with full bridge rectifier one.The proposed design is first validated with simulation results,and then the proposed transformer is implemented and tested.Experimental results show that the designed system is suitable for 4.16 k VAC medium voltage grid integration.

Analysis of output voltage on a planar insulating core transformer

Output voltage is an important performance characteristic of planar insulating core transformer (PICT).In PICT magnetic cores are insulated from their neighboring magnetic cores by solid insulating materials.Solid insulating materials can increase leakage flux.This results in a low generated voltage in secondary coils,especially on the upper stages.Connecting flux compensation capacitors to secondary coils can compensate the flux loss.Design equations to calculate the flux compensation capacitors value and relevant simulation by CST and Protel software were presented.Simulation results of an actual PICT showed that output voltage increased by 19% after being connected to flux compensation capacitors and the voltage on every stage was equally distributed.Results of simulation were consistent with the following experimental test,which revealed that flux compensation capacitors were effective.

Application of Voltage Response Method to Condition Assessment of Power Transformer

Power transformer is one of the most important equipment in the power system.Its operating condition affects the reliability of power supply directly.Therefore,in order to guarantee transformer operation safely and reliably,it is necessary to assess condition of power transformer accurately.Return voltage method based on voltage response measurements is still a new non-intrusive diagnosis method for internal insulation aging of transformer.In this paper the technique and application of return voltage measurement and some results of voltage response measurements of several transformers was introduced.Voltage response measurements were performed on various transformers with different voltage grades,various operating periods,different moisture contents and aging degrees on site.Derived moisture contents from return voltage measurement were compared with the corresponding moisture contents obtained from the analysis of oil samples.Based on on-site experiments and theoretical analysis,the criteria for insulation state of transformer are proposed.Moisture condition of transformer insulation can be determined by using return dominant time constant,and a good correlation between aging degree and the return voltage initial slopes of the aged transformers.Field test performed on several transformers,its interpretation of results are also presented,which proves that return voltage measurements can be used as a reliable tool for evaluating moisture content in transformer insulation.

Analysis on the Influence Factors of Capacitor Voltage Transformer Dielectric Loss Measurement

Capacitor voltage transformer (CVT), which is with simple structure, convenient maintenance, functional diversity and high impact pressure strength, is widely used. And its capacitance and dielectric loss Angle measurement is an important test on testing the insulation of the equipment. This paper is mainly to introduce and discuss one of the CVT test methods——the “self excitation method”, combined with the actual situation encountered in the work, sums up and expounds the maintenance of CVT and preventive test of influence factors.

Optical fiber-based power and data delivery system for high voltage electronic current transformer

This paper describes a prototype power delivery system developed for high voltage electronic current transformer （ECT） that uses laser light to transfer power to and communicates with the primary converter. The design is based on optical-to-electrical power converters, solid-state diode lasers and optical fibers. Command signals are transmitted via the same up-fiber used to send power from secondary power supply to primary converter. The upward data transmission is completed during the brief interruption of power delivery without affecting steady power-supply. A simple comparator added to the primary converter can take the command data. Experimental results show that the fibers can provide reliable up-link for data transmission at 200 kb/s from the secondary to the primary converter. Based on the delivery system, the secondary converter can control three auxiliary channels to provide additional information. These monitoring channels are used in a time-multiplexing mode to provide information about the operation temperature, voltage and current at the remote unit for monitoring the ECT. This preventive maintenance or built-in test can increase reliability by giving early warning for necessary maintenance request.

Voltage Profile Enhancement and Power Loss Reduction with Economic Feasibility Using Small Capacity Distribution Transformers

Usually,rural areas can be electrified via three-phase distribution transformers with relatively large capacities.In such areas,low voltage lines are used for long distances,which cause power losses and voltage drop for different types of consumers.Reducing losses and improving voltage profiles in rural distribution networks are significant challenges for electricity distribution companies.However different solutions were proposed in the literature to overcome these challenges,most of them face difficulties when applied in the conventional distribution network.To address the above issues,an applicable solution is proposed in this paper by installing a number of small-capacity distribution transformers instead of every single large-capacity transformer in rural areas.The proposed approach is implemented in the branch network of Al-Hoqool village,which belongs to the Nineveh distribution network.The network has been inspected on-site,drawn,and analyzed using the electrical systems analysis program(ETAP).The analysis showed that using the single-phase pole-mounted transformers can improve the voltage in the network’s end by 29%and enhance the voltage profile for all consumers.The analysis has also demonstrated that the modification can reduce the total power losses by 78%compared to the existing network.Concerning the economic aspect,the payback period for the proposed network is assigned to be 20 months.

Optimal PV Allocation&Minimal tap-Changing Transformers Achieving Best Distribution Voltage Profile&Minimum Losses in active distribution networks

In distribution systems,voltage levels of the various buses should be maintained within the permissible limits for satisfactory operation of all electrical installations and equipment.The task of voltage control is closely associated with fluctuating load conditions and corresponding requirements of reactive power compensation.The problem of load bus voltage optimization in distribution systems that have distributed generation(DG)has recently become an issue.In Oman,the distribution code limits the load bus voltage variations within±6%of the nominal value.Several voltage control methods are employed in active distribution systems with a high share of photovoltaic systems(PV)to keep the voltage levels within the desirable limits.In addition to the constraint of targeting the best voltage profile,another constraint has to be achieved which is the minimum loss in the distribution network.An optimised solution for voltage of load busses with on-load tap-changing(OLTC)tarnsformers and PV sources is presented in this paper.This study addresses the problem of optimizing the injected power from PV systems associated with the facilities of tap-changing transformers,as it is an important means of controlling voltage throughout the system.To avoid violating tap-changing constraints,a method is depicted for determining the minimal changes in transformer taps to control voltage levels with distributed PV sources.The taps of a range+5 to-15%,can be achieved by tap-changing transformers.The OLTC operation was designed to keep the secondary bus within the voltage standard for MV networks.

Identification of voltage sag source location using S and TT transformed disturbance power

An alternative method was introduced for voltage sag source location based on S and TT transformed disturbance powers. It is done to avoid the wrong and inconclusive detection of conventional disturbance power method proposed in the literature. Unlike in the case of the traditional method, the proposed method first transforms the recorded voltage and current during the sag event to some special features before calculating the new version of disturbance powers. The effectiveness of the proposed method has been verified through simulation and actual data from an industrial power system. The results show that the presented method can correctly detect the location of voltage sag source.

Transformation optics for efficient calculation of transmembrane voltage induced on cells

We present a novel efficient approach in calculating induced transmembrane voltage（ITV） on cells based on transformation optics. As cell membrane is much thinner than the dimension of a typical cell, discretizing the membrane needs numerous meshes. Using an anisotropic medium based on transformation optics, the thickness of the membrane can be exaggerated by at least one order, which eliminates rigorous mesh refinement and reduces unknowns greatly. The accuracy and efficiency of the proposed method are verified by a cylindrical cell model. Moreover, the influence on ITV with bound water（BW） layers is also studied. The results show that when cells are exposed to nanosecond electric field, BW layers should be rigorously considered in calculating ITV.

考虑多绕组耦合的Sen Transformer电磁解析模型

为剖析Sen Transformer(ST)的内部特性,提出了一种适用于三相三柱式变压器结构的考虑多绕组耦合的ST电磁解析模型。一方面,从ST的电磁耦合关系出发,基于统一电磁等值电路推导了由自感系数和互感系数构成的ST电磁耦合模型。另一方面,从ST与外接等值系统的电气连接关系出发,推导了ST的内部电压和电流的电气解析模型。进而形成了由上述电磁耦合模型与电气解析模型构成的该类考虑多绕组耦合的ST电磁解析模型。最后,借助MATLAB软件,以一个三相三柱式ST为例,通过比较其电磁解析计算结果和现有ST串联电压补偿时域仿真结果发现,仅有个别绕组电压和支路电流有较明显的差异,但幅值差异≤6%,相角差异≤7°。该结果表明,多绕组磁耦合对于此类ST影响不大,但不宜忽视;且所提模型通过其磁路的不对称性表征,能更精确地反映其内部电磁特性。

Smart Transformer/Large Flexible Transformer

Solid-state transformer-based smart transformer(ST)can provide the dc connectivity and advanced services to improve the grid performance and to increase the penetration of the power electronics interfaced resources(e.g.,distributed generators and electric vehicle charging stations)in modern electricity distribution grids.Since the ST is a new and effective paradigm of the electricity grid evolution to well understand the ST,this paper systematically presents the basic architecture and the typical control schemes of the ST and then the advanced services that ST can provide to improve the electricity grids performances in terms of the power flow control,power quality improvement,active damping and active contribution to improve distribution grid resilience by means of enabling autonomous microgrids operation as well as launching a restoration procedure following a general blackout.

Improvement of Used Transformer Oils with Activated Bentonite

This research presents the evaluation of activated Bentonite material for treatment of used transformer oil. Different properties such as;electrical, physical, chemical and thermal of used transformer oil were meas-ured before and after purification and treatment. Two power transformers were used in this research (6.4: 4.6 MVA, 3 phases, 50 Hz). One of them was filled with purified oil and the other was filled with activated Bentonite treated oil after purification, and then the two power transformers were tested for one year under practical conditions of the operating field. Initial tests have indicated that the use of Activated BENTONITE in the treatment process for the aged transformer oil improved breakdown voltage, water content, total acidi-ty and flash point. Thus activated Bentonite gives an ideal treatment of aged transformer oil with its environmental and economic advantages. Moreover, activated Bentonite is available at many places in Egypt with low costs.

Key Techniques of High-voltage and Large-capacity Series Hybrid Active Power Filters

基于基波磁通补偿的串联混合型有源电力滤波器（active power filter,APF）应用于高电压、大容量的谐波抑制场合时,受变压器励磁支路谐波电压的影响,逆变器不能等效为基波电流源,其输出电流将含有大量的谐波,从而影响滤波效果。以逆变器为核心,推导出在励磁支路谐波压降影响下的串联变压器的谐波等效阻抗。并以此为基础,从理论上分析励磁电感和变比对串联变压器谐波等效阻抗和APF滤波效果的影响。仿真结果验证了理论分析的正确性。最后,针对广东某厂10kV、1MVA电力负荷,研制一套基于基波磁通补偿的串联混合型有源滤波器工程样机,取得非常好的滤波效果。