Inter-turn Voltage Research Articles

Influence of Connecting Cables on Stator Winding Overvoltage Distribution under High-Frequency Pulse Width Modulation

In the variable frequency motor drive system, because the cable impedance does not match the motor impedance, the reflection wave of the voltage wave will be generated. The superposition of reflected voltage waves can lead to overvoltage at the motor ends, which can damage the insulation structure. In this paper, the equivalent circuit models of cable and stator winding are established, respectively. The overvoltage distribution under different power supply frequencies and cable lengths is simulated and analyzed. The influence mechanism of power supply frequency and cable length on the overvoltage distribution of stator winding are studied. The simulation results show that the overvoltage of the first pulse falling edge will be superimposed on the overvoltage of the second pulse rising edge under high-frequency conditions, resulting in a further increase in the overvoltage. The voltage appears in all coils after the middle of the winding. The ground voltage is up to 1.32 times the input voltage, and the inter-turn voltage is up to 9.2 times the average voltage. The increase in cable length will lead to an increase in ground voltage, but the increase in speed will slow down after exceeding the critical length of 300 m. The maximum ground voltage can reach 1.93 times of the input voltage, which is 3.6% different from the calculation result under ideal conditions. The inter-turn voltage changes with the cable length in an N-shaped manner, up to 185 V. The results of this paper are of great significance to further study the insulation design of generator end input.

Interturn Voltage Stress Comparation and Analysis of Different Typology of Hairpin Windings for EV Traction Machines

Interturn Voltage Stress Comparation and Analysis of Different Typology of Hairpin Windings for EV Traction Machines

High-frequency transformer winding model with adequate protection

High local electric field intensity in transformer windings originating from transient signals is one of the reasons for transformer failures. Due to the integration of renewable energy sources into the power grids and the increased number of transients, the likelihood of transformer catastrophic failure increases accordingly. Therefore, to ensure the reliable performance of transformers and associated power networks studying their behavior during these events is required. Accordingly, there is a need for accurate modeling of transformer windings capable of simulating electromagnetic transients. Using these models, it is possible to identify frequencies that can be dangerous to the transformer windings and to study different protection schemes. This paper aims to find an accurate analytical model of transformer winding validated by experimental measurements and to study the performance of the R-L protection device during the transient phenomena. The protection device is designed based on the winding model to introduce an impedance comparable to that of the transformer winding at critical frequencies where voltage amplification in the winding is significant. This approach ensures enhanced protection against potential transformer damage to the transformer. By using this protection scheme, the high inter-turn voltage originating from transient signals may be mitigated. At the same time, it does not affect the grid's performance during normal conditions.

Varistor Insulation for HTS Magnets

A variable resistance thin dielectric insulation coating for REBCO tape HTS coils has been developed. This new type of insulation system switches between high and low resistance, after an increase in inter-turn voltage. Non-Insulated (NI), fully soldered, HTS coils have proven to be very reliable; NI coils are achieving high magnetic fields above 25 Tesla and are almost impossible to quench. Over-current operation simply redirects the excess current out of the superconducting tape, to flow radially through the coil then back to the power supply. The internal coil resistance can then run the current down when the power supply is switched off. The disadvantage with NI coils is, as the coil volume and inductance increase, the charging / discharging time can take many hours, even days. This is not compatible with magnet systems that need accurate and fast current to magnetic field control, such as accelerators or other systems. With the Varistor Insulation (VI) we aim to achieve both robust performances as seen in NI coils and fast ramping with controlled current to field transfer functions. In this paper we present the electrical characterization of the insulation at room temperature and cryogenic temperatures, along with simulated magnet operation during ramping, normal operation and failure modes. We discuss other features of the VI insulation such as, application methods to provide thin layers, and alternative formulations to tune its properties. Its ability to act as a distributed quench heater when the voltage threshold is exceeded is also discussed.

Voltage Stress Calculation and Measurement for Hairpin Winding of EV Traction Machines Driven by SiC MOSFET

SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> is a promising power device for electric drive system due to its various advantages such as high switching frequency and low loss. However, fast switching speed results in high voltage stress and can potentially cause insulation damage of the winding. Few studies have analyzed and evaluated the voltage stress, especially for the hairpin winding, which is the mainstream winding type for electric vehicle traction machines. For this purpose, this article proposes a high frequency equivalent circuit model to predict the interturn voltage stress of hairpin windings driven by SiC <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s. First, the working principle of the overvoltage at the winding ends is analyzed and validated by experiments. Then, the parasitic parameters are calculated before building the high-frequency equivalent circuit. A field-circuit coupled finite element method is used to consider the nonlinear parameters and to evaluate the voltage stress. Finally, a prototype is built, and the voltage stress is tested. The resultant error between the calculated and experimental maximum interturn voltage stress is only 1.3%, which supports the inference on the validity of the presented equivalent circuit and the method. Based on the above, a novel winding connection scheme is proposed to reduce effectively the voltage stress of hairpin winding.

Analysis of Turn-to-Turn Transient Voltage Distribution in Electrical Machine Windings

The interturn overvoltage during transient events in stator windings is an important feature to be considered during the machine’s design. In the literature there is no consensus about which turn is subjected to the highest voltage, once there are results of simulations and measurements that found the highest values across the last turns of the first coil, although, there are other that verifies the highest overvoltages across the first turns. In this context, this paper aims at verifying the transient interturn voltage distribution (turn-to-turn and turn-to-ground) in the stator windings of a large motor, using a method in which transient magnetic fields and electric circuit elements (resistance, inductance and capacitance) are coupled. The transient simulation and the calculation of the electric circuit elements used in the model are performed by means of Finite Elements Method (FEM). The end-windings parameters are calculated based on a 3D geometry. The model used only needs of the machine’s geometry, that is, the studies can be done at design phases of the machine. Moreover, the simulations consider all coils of one winding group from the 3 phases, which is different from the other simulation models found in the literature that consider only one or two coils in series of a single phase. To verify the interturn voltages, different numerical simulations are done by changing the surge rise time. According to the results, for the particular motor studied, the last turns were the ones submitted to the highest voltages.

Time-Domain Homogenization of Foil Windings in 2-D Axisymmetric Finite-Element Models

In this paper, an approach for the time-domain homogenization of foil windings in axisymmetric finite-element (FE) models is presented. The homogenized formulation is characterized by an axial current redistribution due to the skin effect and a radial inter-turn voltage given by the insulation in-between the turns. The method is successfully applied to an axisymmetric 20-turn foil-winding inductor. At low frequencies, the local and global results present an excellent agreement with those obtained by an accurate but expensive FE model in which all turns are explicitly discretized. The air-gap effect and the frequency limitations are studied as well.

Analysis of End-Windings Influence on the Transient Voltage Distribution in Machine Stator Windings by a Three Phase Model

Transient voltage distribution due to steep-fronted surges must be taken into account during the machine's design as it can lead to main and turn insulation failure. In medium voltage machines, the end-windings region has a considerable percentage of the coil length and plays an important role during transient studies. This article has the purpose of verifying the influence of end-windings on the transient voltage distribution by using a three-phase simulation model in which transient magnetic fields and electric circuit elements are coupled. The transient simulation and the calculation of the electrical parameters used in the model are done employing the Finite Elements Method (FEM). To verify the influence on the results, simulations for different surge rise times are performed, considering and neglecting the end-windings model. As main results, the intercoil and interturn voltages are analyzed for all turns from the three phases of one winding group. The model is validated by measurement results. It is shown that the end-windings, indeed, affect the transient voltage distribution for both, interturn and intercoil voltages, regarding the rate of rising (slope), instant, and the amplitude of the peak voltages. Moreover, all three phases are affected by the end-windings, especially for the turn-to-turn voltages.

Very Fast Transient Analysis of Transformer Winding Using Axial Multiconductor Transmission Line Theory and Finite Element Method

A transformer white box model based on multiconductor transmission line (MTL) theory in cylindrical coordinates has been developed. This model allows variable lengths in conductors to be incorporated into the MTL formulation. A 2D axisymmetric model for the windings is implemented using finite element method (FEM) to determine the per unit length capacitance and inductance matrices. The model is capable of producing voltage and current results in both the frequency and time domains for each turn in the winding. The results obtained using the proposed model has been compared with those from Numerical Electromagnetic Code (NEC) and measurements done by others. Applications of the proposed model in analyzing inter-turn voltages and the induced voltages on secondary windings have been investigated.

Comparison between suppressing approaches of very fast transient over voltages in gas insulated substation

Very Fast Transient Over-Voltage (VFTO), which is a unique electromagnetic transient phenomenon generated by SF6 gas discharge in disconnector gap in Gas Insulated-Substation (GIS), could bring damage to power devices, especially to power transformers. The peak magnitude and high-frequency oscillations of VFTO cause reduction of dielectric of gas insulation, high inter-turn voltages and internal resonance in the transformer windings. Due to all of these disadvantages related to VFTO, VFTO becomes a criterion to determine the insulation level of the GIS, and VFTO damping is rising as an essential issue to be solved. However, the researchers concern on finding the optimum technique for suppressing VFTO, this paper aims to discuss the most feasible methods for mitigation of the overvoltage magnitude of VFTO and determine the most efficient approach for VFTOs attenuation by presenting the advantages and disadvantages of each approach. In this review paper, some suppression methods of very fast transients in GIS are presented, including adopting a resistor-fitted disconnector, ferrite rings, surge arresters, and high-frequency resonator. The results show that the peak value of VFTOs can be decreased to a great level by employing the proposed damping methods. Finally, the best approach for the suppression of VFTOs is chosen based on deep discussion for each method with its features.

Voltage Stress in a Transformer Winding During Very Fast Transients Caused by Breaker Closing Event

Transformers connected at the medium-voltage level in cable grids, such as wind park collection grids and industrial grids, are exposed to the stress of very fast transients. These electric transients are mainly generated during breaker switching operations and the rise time of the transient voltage in such systems is much shorter compared to the rise times of transients generated in transmission systems at a high-voltage level. In this paper, the internal voltage stress is studied during very fast transients generated during transformer energization. Instead of using a breaker, the energizing tests are performed using a low-impedance pulse generator that can generate lightning impulse-shaped waveforms and voltage steps with rise times varying between 35 and 500 ns. Experiments show that during very fast transients with a 35-ns rise time and 1-p.u. magnitude, the interturn voltage exceeds the level obtained with a lightning impulse-shaped voltage waveform of 4.4 pu. Furthermore, during a specific switching scenario with delta-connected transformers, where the winding is excited from both ends, the same 1-p.u./35-ns voltage step generates an interturn voltage that exceeds the 1-p.u. level, which is more than 2.5 times higher voltage stress than during a lightning impulse test.

Analysis of Inter-Turn Insulation of High Voltage Electrical Machine by Using Multi-Conductor Transmission Line Model

In this paper, the inter-turn insulation of stator winding is comprehensively discussed. An equivalent circuit model and a multi-conductor transmission line (MTL) theory are established for the inter-turn voltage evaluation under the switching-impulse voltage, respectively. The distributed inductances are calculated by using small-signal analysis in finite element method (FEM) and the ac resistances of coils are analyzed considering the skin effect. The distributed capacitances of the coils are determined by using both electric field FEM and analytic equations. The electric potential distribution of inter-turn is calculated by the above two methods. The incident and reflect voltages are calculated by using the MTL model. The electric field of inter-turn calculated by using the FEM is presented. The FEM is used for the calculation of the inter-turn electric field distribution. The position where the inter-turn insulation may be more prone to breakdown is determined according to the result. Finally, the impact factors of inter-turn insulation including insulation materials and positions of failure in winding are analyzed in detail.

Experimental testing and modelling of a resistive type superconducting fault current limiter using MgB2 wire

A prototype resistive superconducting fault current limiter (SFCL) was developed using single-strand round magnesium diboride (MgB2) wire. The MgB2 wire was wound with an interleaved arrangement to minimize coil inductance and provide adequate inter-turn voltage withstand capability. The temperature profile from 30 to 40 K and frequency profile from 10 to 100 Hz at 25 K were tested and reported. The quench properties of the prototype coil were tested using a high current test circuit. The fault current was limited by the prototype coil within the first quarter-cycle. The prototype coil demonstrated reliable and repeatable current limiting properties and was able to withstand a potential peak current of 372 A for one second without any degradation of performance. A three-strand SFCL coil was investigated and demonstrated scaled-up current capacity. An analytical model to predict the behaviour of the prototype single-strand SFCL coil was developed using an adiabatic boundary condition on the outer surface of the wire. The predicted fault current using the analytical model showed very good correlation with the experimental test results. The analytical model and a finite element thermal model were used to predict the temperature rise of the wire during a fault.

Conversion of plasma energy into electrical pulse by magnetic flux compression

A conceptual study of magnetic flux compression inside a cylindrical coil by an expanding inertial fusion plasma sphere across the magnetic field produced by the coil itself have been performed numerically using a two-dimensional magnetohydrodynamic (MHD) simulations. The concept may find application in inertial fusion energy (IFE) system as a direct energy conversion scheme to convert a part of fusion plasma kinetic energy into pulsed electrical energy. Important theoretical and technical issues that has to be addressed are discussed. Preliminary theoretical analysis are given for the analysis of MHD interchange instabilities of expanding plasma across magnetic field. Overall efficiency of the system is determined numerically for a typical set of initial plasma and system parameters. Ultrahigh coil inter-turn voltages are predicted. Therefore, the application of magnetic self-insulation to avoid coil inter-turn break-down is considered. Also, we have analysed the system performance with different load conditions.

脉冲变压器充电过程中次级线圈匝间电压的分布

脉冲变压器充电过程中次级线圈匝间电压的分布

Application of Hybrid Modeling for Calculating Interturn Voltages in Transformer Windings

This letter deals with the hybrid modeling of transformers and the determination of interturn voltage distribution in transformers with layer-type windings. The hybrid model uses a combination of a multiconductor transmission-line model and a single transmission-line model. The good agreement between computed and measured voltage waveforms shows that this model can be used for the computation of interturn voltages with full success. Analysis is performed by making use of a step function with 100-ns rise time.

Voltage Distribution of Steep Fronted Transients in Stator Windings–Modelling, Simulation and Experimental Verification

Steep fronted surge voltages are produced at H.T. motor terminals due to various switching operations. These are distributed within the stator winding coils and turns of the phases in a highly nonuniform manner leading to frequent insulation failures. There has been considerable interest in the development of the computational methods for analysing such phenomenon. This paper presents the development of a circuit model to predict the interturn voltage distribution and its validation through comparison of the results of simulation using EMTP with those obtained experimentally on sample coils for a specific three phase 6.6 kV, 400KW H.T. motor. The correlation between the experimental and simulation results is found to be good.

Analysis of Very Fast Transients in Layer-Type Transformer Windings

This paper deals with the measurement, modeling, and simulation of very fast transient overvoltages in layer-type distribution transformer windings. Measurements were performed by applying a step impulse with 50-ns rise time on a single-phase test transformer equipped with measuring points along the winding. Voltages along the transformer windings were computed by applying multiconductor transmission-line theory for transformer layers and turns. Interturn voltage analysis has also been performed. Computations are performed by applying an inductance matrix determined in two different ways; by making use of the inverse capacitance matrix and by making use of the well known Maxwell formulas. The modeling of the transformer and the computations are verified by measurements

Investigation of very fast transient overvoltage distribution in taper winding of tesla transformer

In order to study the very fast transient overvoltage (VFTO) distribution in the taper winding of a tesla transformer under high-frequency steep-fronted voltage surge, we built a distributed line model based on multiconductor transmission line (MTL) theory. We used a new hybrid algorithm combining finite-element-method (FEM) and interpolation formulas to quickly evaluate the induction coefficient matrix K by utilizing some characteristics of the taper structure. The turn-to-ground and interturn voltage distributions can be obtained by solving the telegraphist's equations in the frequency domain. We measured the voltage distribution inside the taper winding to find some ways to weaken the voltage oscillations. Here, we compare the results with numerical values.

Influence of Very Fast Transients on Transformer

Very fast transients (VFTs) occur in a gas insulated switchgear (GIS) at a disconnecting switch operation. VFTs are high frequency surges and are characterized by a fast steep wavefront and an oscillating waveform. VFTs cause voltage oscillations in a transformer winding connected directly to a GIS. VFTs in the GIS are simulated using the Laplace transform. The fast steep wavefront and the oscillating waveform attaking on the transformer are shown as a function of GIS constitutional parameters. For a case study, influence of the fast steep wavefront and the oscillating waveform is investigated on 550kV class transformer. In the case of VFTs at the transformer, a voltage level of the fast steep wavefront is larger than an amplitude of the oscillation waveform. However the interturn voltage oscillation in the winding caused by the oscillation waveform can be larger than that by the fast steep wavefront. The authors deduce that the voltage oscillations do not threaten the reliability of the interturn insulation.