Initial Voltage Distribution Research Articles

Comparison of initial voltage distributions in layer and disc type power transformer windings

Comparison of initial voltage distributions in layer and disc type power transformer windings

Direct Cell-to-Cell Equalizer for Series Battery String Using Switch-Matrix Single-Capacitor Equalizer and Optimal Pairing Algorithm

In series battery strings, cell-inconsistency is caused by the state-of-charge (SOC) mismatch, nonidentical battery impedance, or different self-discharging rates, and this leads to overcharge and overdischarge. Practically, switched-capacitor equalizers are the most promising means to eliminate the cell inconsistency by virtue of automatic equalization, but the performance is heavily dependent on the initial cell voltage distribution and the number of series connections due to inefficient switch utilization. This article proposes a direct cell-to-cell equalizer for a series-connected battery using a switch-matrix single-capacitor converter to further improve the switched-capacitor equalizer in term of performance consistency and Coulomb efficiency. By adopting one extra current sensor and an optimal pairing algorithm, energy is transferred directly between the highest-SOC cell and the lowest-SOC cell to eliminate the impact of battery-impedance difference and false voltage measurement in battery monitoring integrated circuit caused by polarization effect. The experimental results verify the feasibility of the proposed scheme. Real-time tests are also implemented to fairly compare the proposed with the conventional methods. It is found that the performance of the proposed method is independent from initial voltage distribution in a series string, where the performance indices are consistent regardless of initial conditions. Besides, the energy loss of the proposed equalizer is further reduced and its overall efficiency is high in all test scenarios.

Effect of shield placement for transient voltage mitigation due to switching surges in a 33/11 kV transformer windings.

This study presents an investigation on the effect of shield placement for mitigation of transient voltage in a 33/11 kV, 30 MVA transformer due to Standard Switching Impulse (SSI) and Oscillating Switching Impulse (OSI) surges. Generally, the winding and insulation in transformers could experience severe voltage stress due to the external impulses i.e. switching events. Hence, it is important to examine the voltage stress and identify the mitigation action i.e. shield placements in order to reduce the adverse effect to the transformer windings. First, the resistances, inductances, and capacitances (RLC) were calculated for disc type transformer in order to develop the winding RLC equivalent circuit. The SSI and OSI transient voltage waveforms were applied to the High Voltage (HV) winding whereby the transient voltages were simulated for each disc. The resulting voltage stresses were mitigated through different configurations of electrostatic shield placements. The resonant oscillations generated due to switching surges were analysed through initial voltage distribution. The analyses on the transient voltages of the transformer winding and standard error of the slope (SEb) reveal that the location of shield placement has a significant effect on the resonant switching voltages. The increment of the shield number in the windings does not guarantee optimize mitigation of the resonant switching transient voltages. It is found that the voltage stress along the windings is linear once a floating shield is placed between the HV and Low Voltage (LV) windings of the disc-type transformer under the SSI and OSI waveforms. These findings could assist the manufacturers with appropriate technical basis for mitigation of the transformer winding against the external transient switching overvoltage surges.

Elegant procedure to estimate series capacitance of a uniform transformer winding from its measured FRA: implementable on existing FRA instruments

A simple procedure to estimate series capacitance of a uniform transformer winding from its measured frequency response analysis (FRA) and shunt capacitance is presented. Unlike previously published approaches, this method does not involve any cumbersome and time-consuming curve-fitting nor running optimisation/search algorithms, and neither does it require data of winding geometry. The procedure relies on a property that is observable in the impedance function of a lossless winding, viz., the ratio of the product of squares of open circuit natural frequencies to the product of squares of short circuit natural frequencies bears a special relation to impedance function coefficients. Its feasibility was initially verified by simulation, and then by experiments on small-sized continuous-disk and interleaved-disc windings, followed by a large-sized 33 kV, 3.5 MVA continuous-disc winding, and finally on a 315 kVA 11/13.8 kV transformer. After measuring FRA, the process involves just finding roots of a polynomial, from which the initial impulse voltage distribution constant and series capacitance can directly be determined. Given these attractive features, authors believe that this method is implementable on existing FRA instruments, so that, along with routinely measured FRA, these two important constants of a winding can be displayed.

High-Frequency Impulse Modeling and Longitudinal Insulation Analysis of Bifilar Superconducting Coil

Bifilar coil is one of the most applicable structures used for resistive-type superconducting fault current limiter. However, the turn-to-turn (longitudinal) insulation of the bifilar coil under lightning and switching impulse is rather different from the conventional pancake coil. The longitudinal insulation is characterized as the distributed circuit of the coil under high-frequency excitation, and the capacitance, inductance, and resistance parameters are solved by finite element model. The influence of those resistance, inductance and capacitance (RLC) parameters on the accuracy of the model is analyzed, and two samples of one bifilar pancake coil and two series connected bifilar coils are fabricated for high-frequency impulse voltage testing. The experimental results are consistent with the simulation results. The initial voltage distribution and impulse response of two samples are calculated under standard lightning impulse. In the initial voltage distribution calculation, the interturn channels of one bifilar coil and two series-connected bifilar coils are subjected to a maximum voltage of 1 and 0.5 times the lightning impulse voltage.

Longitudinal Insulation Design of Solenoid Type Superconducting Fault Current Limiter

The longitudinal insulation analysis method is developed for the solenoid type windings. The distributed lumped model is proposed to calculate the response under high-frequency transient impulse for the solenoid type superconducting fault current limiter (SFCL). Various simulation results according to the proposed model are consistent with the experimental results, which indicates that the developed model is effective for describing such kind of “noninductive” type solenoid windings. Therefore, the longitudinal insulation of the 20 kV/400 A SFCL has been analyzed by this method. The initial voltage distribution and the standard lightning impulse response for different turn-to-turn distances of the superconducting windings are calculated. According to the simulation results, the most severe impact is at the time when the lightning impulse wave is applied to SFCL, and the first and last turns of the winding endure a very high voltage drop at this moment. Considering the insulation margin and size, the turn-to-turn distance of 4.5 mm is taken as the longitudinal insulation distance of the SFCL.

Investigation on the Resonant Oscillations in an 11 kV Distribution Transformer under Standard and Chopped Lightning Impulse Overvoltages with Different Shield Placement Configurations

This paper presents an investigation on the resonant oscillations of an 11 kV layer-type winding transformer under standard and chopped lightning impulse overvoltage conditions based on calculated parameters. The resistances, inductances and capacitances were calculated in order to develop the transformer winding equivalent circuit. The impulse overvoltages were applied to the high voltage (HV) winding and the resonant oscillations were simulated for each of the layers based on different electrostatic shield placement configurations. It is found that the placement of grounded shields between layer 13 and layer 14 results in the highest resonant oscillation and non-linear initial voltage distribution. The oscillation and linear stress distributions are at the lowest for shield placement between the HV and low voltage (LV) windings.

Multicell-to-Multicell Equalizers Based on Matrix and Half-Bridge LC Converters for Series-Connected Battery Strings

Due to the low switching loss, compact size, and low price, the LC series resonant converter (LCSRC) as an energy carrier is promising in battery equalizers. However, the conventional equalizers based on LCSRC only can realize the one-cell-to-one-cell balancing for a high-voltage series-connected battery pack, causing a low equalizing speed and efficiency. Therefore, two direct multicell-to-multicell (MC2MC) balancing topologies based on matrix LC converter and half-bridge LC converter are proposed, respectively, which achieve the direct energy transfer from a consecutive more-charged-cell group to a consecutive less-charged-cell group, significantly improving the equalizing speed and efficiency. An analytical model for the LCSRC is developed to provide a guidance for the optimal design of the proposed equalizers. The prototypes for the proposed two balancing topologies are constructed. The MC2MC balancing performances are verified though the comparative experimental results at different initial voltage distributions. In particular, the proposed equalizers simultaneously achieve the fast balancing speed and high equalization efficiency. The measured peak efficiency is about 87.9% at the four-cell-to-four-cell equalization.

Experimental studies on power transformer model winding provided with MOVs

Surge voltage distribution across a HV transformer winding due to appearance of very fast rise time (rise time of order 1μs) transient voltages is highly non-uniform along the length of the winding for initial time instant of occurrence of surge. In order to achieve nearly uniform initial time instant voltage distribution along the length of the HV winding, investigations have been carried out on transformer model winding. By connecting similar type of metal oxide varistors across sections of HV transformer model winding, it is possible to improve initial time instant surge voltage distribution across length of the HV transformer winding. Transformer windings with α values 5.3, 9.5 and 19 have been analyzed. The experimental studies have been carried out using high speed oscilloscope of good accuracy. The initial time instant voltage distribution across sections of winding with MOV remains nearly uniform along length of the winding. Also results of fault diagnostics carried out with and without connection of MOVs across sections of winding are reported.

Impulse voltage distribution and frequency response of intershield windings

Impulse voltage distributions in continuous disk windings are nonlinear, due to the small series capacitance of the winding. Consequently, transformer manufacturers often design large power transformers with interleaved windings to increase series capacitance, but in doing so reduce the value of the initial impulse voltage distribution coefficient α given by α = √(Cg/Cs), (1) where Cg is the shunt capacitance and Cs is the series capacitance of the winding. Sometimes fully interleaved windings are employed because they are more reliable, ensuring that the transformer will operate properly under lightning strike. However, fully interleaved windings require longer assembly times, and manual welding is necessary. Manufacturers have therefore been using series combinations of continuous and interleaved windings, known as multiple-α type windings [1]. However, this approach also has disadvantages, e.g., the design and analysis of performance under normal and abnormal transient conditions are complicated, and the impedances of continuous disks differ from those of interleaved disks [2], causing reflection of surge traveling waves at connection points. Transformer designers may therefore have to adjust inter-disk distances and repeat transient simulation studies several times, to find an optimum arrangement.

EMTP를 이용한 철도차량용 주변압기 고압권선의 공진특성 분석

철도차량용 주변압기 고압권선은 고압인통선에 유기되는 이상전압과 내부공진을 발생 시킬 수 있는 다양한 주파수 대역의 고유주파수를 가진다. 공장시험은 다양한 파형과 지속시간을 지닌 이상전압의 여자가능성을 판단하는데 한계가 있다. 본 연구는 전자계과도해석프로그램(이하 EMTP)을 이용하여 고압권선 모델을 설계하고 초기 전압분포와 전압-주파수 관계 측면에서 내부공진특성을 모의분석한다. 턴 단위의 집중매개변수는 변압기 형상정보부터 계산되며, 각 레이어 단위로 합산된 서브모델은 네트워크 모델로 결합되어 EMTP의 라이브러리를 통해 구현한다. 사례연구는 주파수 변동에 따른 레이어 단위의 전압-주파수 관계 특성과 시계열 영역에서 전압 확대 및 분포 양상을 보여준다. The primary windings of the main transformer for rolling stock have several natural frequencies that can occur internal resonance with transient voltages induced on a high voltage feeding line. Factory testing is limited in its ability to determine whether or not transient voltage with various shape and duration can be excitable. This study presents the design of a high voltage windings model and simulation and analysis of the internal resonant characteristics in terms of the initial voltage distribution and voltage-frequency relationship using the electromagnetic transients program (EMTP). Turn-based lumped-parameters are calculated using the geometry data of the transformer. And, sub-models, being grouped into the total number of layers, are composed using a ladder-network model and implemented by the library function of EMTP. Case studies are used to show the layer-based voltage-frequency relationship characteristics according to the frequency sweep and the voltage escalation and distribution aspects in time-domain simulation.

Voltage distribution constant versus losses in medium-voltage distribution transformer: A polynomial interpolation approach

This paper aims to calculate an estimate of the initial stress voltage distribution in the transformer windings through their losses (that can be reduced by varying the construction parameters such as conductor section and size of the core window) through the numerical interpolation applied in capacitances of distribution transformers 30, 45, 75 and 112.5kVA, 15kV class. The calculations of the projects were organized into sets, with simultaneous variation of three construction parameters, totaling analysis of 10,648 projects of each power transformers arranged in three-dimensional arrays. The equation of the constant voltage distribution (α-factor) according to the losses through the quadratic polynomial and cubic splines for the of LV and HV windings is formulated.

Analysis of fast transient voltage distributions in transformer windings under different insulation conditions

Evolution and operation of modern grids and networks necessitate reconsideration of several characteristics. For example transformers operating in power grid or in industrial environment are subjected nowadays also to transients or stimulus with different waveforms. Traditionally, in case of power transformers the stresses are related to lightning and switching phenomena. In industrial networks distribution transformers are exposed to many switching operations generating often overvoltages. Transformer windings subjected to such system disturbances are stressed due to the non-linear transient voltage distribution along the winding and potential resonance phenomena. Initial voltage distribution, reflects mainly the capacitive coupling between turns, windings, core and tank. The non-uniform distribution of voltage results in concentred stress in certain parts of the winding. Winding resonance might occur, if the frequency spectrum of incoming surge matches the winding natural frequencies. Therefore external transients occurring in power systems might trigger internal overvoltages with large maximum value in transformer windings. The frequency characteristic of the transformer winding may determine those regions in the frequency spectrum. The paper presents results of voltage distribution measurements performed on model windings with different constructions subjected to voltages with different rise time. The experimental setup allows for investigation of influence of the dielectric (air/oil) and temperature on the stress distribution along the winding. Also the impact of oil and temperature on frequency characteristics of internal voltages are shown. The presented results might be used both for design and optimization of transformers windings.

FEM 시뮬레이션을 이용한 2MVA 몰드변압기 권선간 써지전압 분배 해석 기법 연구

This paper presents an analyzing method of the capacitance of the power transformer for initial voltage distribution and insulation design. When a high incoming surge voltage is accidently occurred in windings of transformer, it does not distribute equally in the windings. This phenomenon makes electric field concentration and the insulating material could be break. Initial voltage distribute mostly depends on capacitances between winding to winding or winding to core in the transformer. If the C network can be structuralized into the equivalent circuit model and be calculated each capacitance element value by circuit analysis and FEM(Finite Element Method) simulation program, the transformer designer could know the place where the structure is to be modified or the insulation to be reinforced. This method quickly provides the data of the voltage distribution in each winding to the designer.

Rotor-Position Estimation of Switched Reluctance Motors Based on Damped Voltage Resonance

This paper proposes a method to obtain the rotor position of switched reluctance motors (SRMs) by means of voltage measurements. It is shown that the combination of a motor and a power-electronic converter defines a resonant circuit, comprising the motor phase inductances and the parasitic capacitance of converter switches, power cables, and motor phase windings. For salient machines, in general, the associated resonance frequency of the circuit depends on the rotor position. In the position-estimation method, an initial voltage distribution is imposed over the impedances of the resonant circuit after which the circuit is let to oscillate freely. During this phase of free oscillation, the induced voltage over a phase winding exhibits a damped oscillatory behavior, from which position information can be retrieved. An overview is given of the different possibilities to trigger the voltage resonance. It is shown that the proposed position-estimation method has favorable characteristics such as measurement of large-amplitude voltages, robustness against temperature deviations of motor and power semiconductors, very high update rates for the estimated position, and absence of sound and disturbance torque. Experimental results are given for a sensorless commutation scheme of an SRM under small load.

Investigation of impact of shallow trench isolation on SONOS type memory cells

The impact of shallow trench isolation (STI) on non-volatile memories becomes much more severe with the CMOS technology scaling down to sub-90 nm. In this work, the impact of STI on a polysilicon–oxide–nitride–oxide–silicon (SONOS) type memory has been investigated based on the experiments and TCAD simulation analysis. It has been found edge cells adjacent to STI have the lower channel-hot-electron (CHE) injection programming efficiency than center cells. In addition, edge cells exhibit different initial threshold voltage ( V t) distribution compared with center cells. STI impact is thought to be the main reason for these problems. To reduce the impact of STI, an additional boron implantation in STI BL contacts region is developed as a new solution. As a result, the performance differences between edge and center cells have been substantially minimized.

Numerical simulation of secondary electron emission charging at insulator surfaces

The charging process of insulator surfaces in vacuum at HV due to secondary electron emission has been calculated using numerical simulation. The systems considered consist of two electrodes at a HV difference, separated by a glass plate or a hollow glass channel. The simulations give insight into the critical HV positions and into the influence of the geometry, the initial voltage distribution and the material properties on the charging profiles and the leakage current paths. A HV performance increase of these systems can be obtained by decreasing the impact of free electrons, adding a coating on all insulating surfaces which have a high value of E/sub 1/ (i.e. the first crossover point of the secondary electron yield curve) and/or are weakly conductive, and breaking up the total voltage difference in smaller parts by adding electrodes to all surfaces at intermediate voltages.

96/03925 Transient analysis of three-phase transmission lines with initial voltage and current distributions

96/03925 Transient analysis of three-phase transmission lines with initial voltage and current distributions

Transient analysis of three-phase transmission lines with initial voltage and current distributions

An efficient numerical inverse Laplace transformation algorithm is proposed for the transient analysis of a three-phase transmission line with arbitrary initial voltage and current distributions (known as the initial conditions) on the line. We apply the Laplace transformation to the transmission line terminals which account for the initial conditions. The distributed nature of the transmission line makes the two-port admittance matrix and the current generators irrational functions of the complex frequency, which can be approximated by the Taylor series expansions. The phase voltages and phase currents on the transmission line can then be obtained as rational functions of the complex frequency. The numerical inverse Laplace transformation can be applied to obtain the time domain waveforms.

A rational complex frequency domain model for the completely transposed transmission line with initial voltage and current distributions

The transmission-line equations for a completely transposed transmission line are decoupled so that the Laplace transformation can be applied for each phase sequence. We then obtain the equivalent two-port admittance matrix with current generators connected to the transmission line terminals which account for the initial voltage and current distributions. This circuit model is suitable for power network analysis and can be rationalized so that analytical time-domain waveforms can be obtained by applying the Heaviside partial fraction expansion method in Laplace inversion. The three sequence networks can be combined to give the relation between phase voltages and phase currents when this transmission line is embedded in an unbalanced power network. >