Inter-turn Insulation Research Articles

A Permeance-Based Transformer Model and Its Application to Winding Interturn Arcing Fault Studies

This paper investigates the behavior of power transformers under the occurrence of permanent or intermittent winding insulation faults. For the study of these phenomena, a simple and efficient permeance-based electromagnetic transformer model is proposed, which is based on the simultaneous consideration of magnetic and electric equivalent circuits. To incorporate the internal faults in this model, a suitable equivalent circuit of the faulty winding is described. With the aid of this transformer model, the onload exciting current Park's Vector Approach will be applied for diagnosing the occurrence of permanent and intermittent winding faults. Experimental and simulation tests results are presented in this paper, which demonstrate not only the adequacy of the digital transformer model for winding fault studies, but also the effectiveness of the proposed technique for detecting winding interturn insulation faults in operating three-phase transformers.

ANN based real time incipient fault detection and protection system for induction motor

Artificial neural network (ANN) has its unique advantage in the area of incipient faults detection. This article presents an ANN based real time fault detection and protection system for two types of incipient faults viz. inter-turn insulation failure and bearing wear in single-phase induction motor. The ANN fault detection (ANNFD) program is developed in C++ and implemented using a PC based DSP controller board. The ANN is trained and tested by collecting the online experimental data for five input parameters viz. motor intake current, rotor speed, winding temperature, bearing temperature and noise. The results are compared with the signature of measurable parameters. The results of evaluation indicate that the system produces satisfactory performance for the fault detection as well as for the protection of motor.

Partial Discharge Activity of Inter-turn Insulation under Different Parameters of Square Wave Voltage

Partial discharge (PD) activity of inter-turn insulation of electric motor winding is investigated under different parameters of square wave voltage, and the memory effect of deposited space charge in a cavity on PD activity is also studied. Under different frequency and rise time of square wave, discharge number, maximum discharge magnitude and phase resolved PD pattern is investigated. Increase in frequency and decrease in rise time both results in increase in maximum discharge magnitude and discharge number at square wave. Comparing the results of phase resolved PD patterns at different frequency of square wave, most of PD moves to zero point of square wave voltage with the increase in frequency. Meanwhile, with the reduction in rise time of square wave, a similar phenomenon can be observed. Through the analysis of a PD process at square wave voltage, the memory effect of deposited space charge after a discharge in cavity leads to superposition of electric field when the polarity of square wave voltage rapidly reverses. With the increase in frequency and reduction in rise time at square wave, superposition of electric field enhances. As a result, available discharge time prolongs during the rise time of square wave and overvoltage across the cavity increases. They result in the increase in discharge number and maximum discharge magnitude. Furthermore, for this reason, the position of PD shifts to zero point of square wave voltage.

Stator Winding Inter-turn Insulation Fault Detection in Induction Motors by Symmetrical Components Method

The theory of instantaneous symmetrical components is used for the detection of insulation faults in a three-phase induction motor. Based on the experimental data of motor intake currents and the system voltages, the loci of positive- and negative-sequence components of currents are plotted. These loci overlap with each other under healthy winding conditions as their major and minor axes coincides. When an inter-turn short circuit occurs, these axes do not coincide with each other and displacement angle ϕ appears between two major axes. Thereby, the non-overlapping area comes into existence. The proposed system considers the effects due to unbalance in voltages, asymmetry in stator winding design, and measurement errors. Experimental results are reported to demonstrate the effectiveness of the proposed technique at different load conditions.

ANN Based System for the Detection of Winding Insulation Condition and Bearing Wear in Single Phase Induction Motor

This paper deals with the problem of detection of induction motor incipient faults. Artificial Neural Network (ANN) approach is applied to detect two types of incipient faults (1). Interturn insulation and (2) Bearing wear faults in single-phase induction motor. The experimental data for five measurable parameters (motor intake current, rotor speed, winding temperature, bearing temperature and the noise) is generated in the laboratory on specially designed single-phase induction motor. Initially, the performance is tested with two inputs i.e. motor intake current and rotor speed, later the remaining three input parameters (winding temperature, bearing temperature and the noise) were added sequentially. Depending upon input parameters, the four ANN based fault detectors are developed. The training and testing results of these detectors are illustrated. It is found that the fault detection accuracy is improved with the addition of input parameters.

Adaptive Neural Fuzzy Inference System for the Detection of Inter-Turn Insulation and Bearing Wear Faults in Induction Motor

The positive features of neural networks and fuzzy logic are combined together for the detection of stator inter-turn insulation and bearing wear faults in single-phase induction motor. The adaptive neural fuzzy inference systems (ANFISs) are developed for the detection of these two faults. These faults are created experimentally on a single-phase induction motor in the laboratory. The experimental data is generated for the five measurable parameters, viz, motor intakes current, speed, winding temperature, bearing temperature, and the noise of the machine. Earlier, the ANFIS fault detectors are trained for the two input parameters, i.e., speed and current, and the performance is tested. Later, the three remaining parameters are added and the five input ANFIS fault detector is trained and tested. It observed from the simulation results that the five input parameter system predicts more accurate results

Evaluation of medium voltage enameled wire exposed to fast repetitive voltage pulses

The paper describes the investigation of the failure mechanisms of medium voltage inter-turn insulation, as a consequence of pulse aging. To study the performance and failure behaviour of enameled wires, models of inter-turn insulation are prepared with enameled wires having polyimide and nano-filled coatings. The aging of such bar samples is attained by sinusoidal (60Hz), fast repetitive unipolar voltage pulses, and high frequency AC waveforms. To establish the residual strength of the bar samples, DC breakdown voltages are determined before and after the aging. In addition, the thermally stimulated depolarization current (TSDC) is used to provide information about the relaxation processes, such as the disorientation of the dipoles and the release of charges from the trapping sites in enameled wires. The various trapping levels, which have discrete energy differences with distinct peaks in the thermograms, are discussed in terms of space charge polarization

Induction Motor: Fuzzy System for the Detection of Winding Insulation Condition and Bearing Wear

In this article, fuzzy logic method is used to detect two types of incipient faults viz., interturn insulation failure and bearing wear in single-phase induction motor. The fuzzy rules are framed depending upon the experimental data for five input parameters viz., motor intake current, rotor speed, winding temperature, bearing temperature, and noise. The simulation and experimental results are given to verify the proposed method. Initially the performance is tested with two inputs, motor intake current and rotor speed; later the remaining three input parameters were added and the performance is tested with five inputs. It is found that the performance with five inputs gives better results.

Measurements of Temperature Dependence of Partial Discharge in Air Gaps Between Insulated Bi-Sr-Ca-Cu-O Tapes

Partial discharge (PD) in trapped air-filled gaps between two adjacent insulated high temperature superconducting (HTS) tapes made of Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ (BSCCO) was investigated experimentally. PD measurements were performed at several temperatures ranging from 295 K to 41 K. At each temperature a series of AC voltages of equal duration and increasing magnitude was applied to the inter-turn insulation until PD was initiated and observed on the insulation condition monitor ICM digital PD system. The PD data are presented in the form of phase and amplitude resolved PD (PRPD) patterns. The voltage at which PD initiation (PDIV) and extinction (PVEV) is observed at a particular temperature was measured on an oscilloscope. Experimentally obtained PDIV and PDEV values are compared with the Paschen curve values for air and the effect of temperature on PDIV is discussed. The data show that relatively low PD onsets can occur even when the insulation is cooled to the 40 to 50 K range which may be due to the presence of ice layers on the surface of the voids.

Impulse Volt-Time Characteristics of Oil and OIP Insulation

Estimation of insulation strength of transformer oil and Oil Impregnated Paper (OIP) insulation, which constitutes major portion of insulation in power transformer, is an important task. Also, it is often necessary to evaluate the breakdown strength of oil and OIP under non-standard impulse voltages, since the oil and OIP insulation is subjected to non-standard waveshapes between inter-disc and inter-turn insulation. This necessitates for a generalized model to estimate the insulation strength of it. In this study impulse strength of transformer oil and OIP insulation have been extensively analysed for very small electrode gap distances ranging from 0.1 to 2.5 mm, which represents the inter-turn and inter-disc thickness of the insulation. The statistical mean volt-time characteristics for uniform and highly non-uniform electrode configurations are obtained experimentally for few gap distances. A Hyperbolic model is developed based on the Disruptive Effect (DE) model parameters, namely onset voltage (Uo) and Critical Disruptive Effect Area (DE * ) to predict the volt-time characteristics. The DE parameters are also utilised to predict the impulse breakdown characteristics of oil and OIP under non-standard impulse voltages of standard and unidirectional oscillatory impulse waveshapes for all the gap distances and the errors are found to be less than 10%.

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.

Inverter statistics for online detection of stator asymmetries in inverter-fed induction motors

A simulation model of an induction machine that enables the analysis of the influence of stator fault conditions on the machine control of inverter-fed drives is presented. Based on this model, the influence of the breakdown of the stator windings insulation on the behavior of the machine and especially on the current control scheme is shown. A new online method to detect such asymmetries caused, for example, by an interturn insulation failure in the stator windings is proposed and investigated. This new method utilizes the influence of stator asymmetries on the inverter current control scheme. By evaluating the statistical distribution of the different inverter switching states and switching times, asymmetries in the stator can be detected and isolated. The measurements required to implement this method are already available in modern inverter fed drives as they are used to realize the current control loop. Thus, no additional sensors are necessary. The practical realization of the fault detection algorithm is demonstrated in combination with a predictive single step current controller. Measurements performed on a drive test stand verify the applicability of the proposed online method to detect and isolate stator faults.

Some effects of grain boundaries on the field distribution on the surface of grain oriented electrical steels

A non-destructive system using a Hall effect I.C. sensor has been used to measure the distribution of the vertical component of stray magnetic field on the opposite surfaces of a magnetised specimen of high permeability, grain-oriented electrical steel. An optical microscope was used to establish positions of grain boundaries in order to explain differences in field profiles outside the systematic accuracy of measurements. It was found that unlike previously assumed, grain boundaries may not necessarily be perpendicular to the sheet surface so different field distribution may occur. This should be considered when evaluating local power loss from surface sensor movements. Future studies are also necessary to quantify the effect of this phenomenon in the interpretation of surface or bulk properties of electrical steels using surface field sensors. Electrical steels are used in many transformer and motor core applications where a.c. magnetic permeability or losses are important. They are usually used in sheet form typically 0.23 mm to 0.65 mm thick. Grain oriented 3.25% Silicon Steel has a strong (001)(110) texture with grains of up to 10 mm in diameter. Its surfaces are usually coated with complex non-magnetic layers to impose beneficial tensile stress in the steel as well as providing the necessary interlaminar or interturn insulation when built into cores. The energy loss under a.c. magnetisation is proportional to the integral of the product of the instantaneous tangential component of surface field and the instantaneous spacial averaged rate of change of flux density. In grain oriented electrical steel the bulk of the material in the demagnetised state comprises anti- parallel bar domains with 180 ◦ walls. The domain, structure which depends on the grain size and orientation as well as several other factors is changed by application of an external field and during the process magnetic losses occur. The surface magnetic field in the magnetised or demagnetised state can give an indication of material structure which in turn has a large influence on the losses and permeability. The surface magnetic field can be measured in several ways. Hall sensors have been used for the measurement of the tangential component of surface field of the coated material at scanning steps of around 0.3 mm, sensor length being typically 3 mm (1,2). More recently smaller magneto resistive sensors are used with greater flexibility (3). Close relationships are found between grain structure orientation and tangential field (4).

Characteristics of an induction motor with reduced turns of terminal coils

AbstractThe use of PWM inverters with high switching characteristics for the adjustable speed drive system causes an overvoltage with steep‐fronted waveform on the motor terminals. As this steep‐fronted transient voltage in inverter drive loads heavily on the terminal coils of motor winding, the insulation failure in the motor winding occurs mostly on the terminal coils. Therefore, it is reasonable to reduce the turns of terminal coils only and to increase the dielectric strength of interturn insulation.This paper presents a comparison of measured characteristics of normal induction motors and induction motors with reduced turns of terminal coils and also a comparison of the results measured and calculated by equivalent circuit. The appropriate rate of reduced turns of terminal coils is about 20%. © 2002 Wiley Periodicals, Inc. Electr Eng Jpn, 142(3): 49–56, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.10093

Tests on JET TF coil insulation

The epoxy glass insulation from a JET toroidal field (TF) coil was extensively tested as part of a study to permit operation of JET at a field 20% higher than the design value. The measurements were performed on specimens taken from a coil that had been in use for several years. The JET TF coils are D-shaped and made of two pancakes of 12 copper turns, with 2 mm inter-turn insulation and 6 mm inter-pancake insulation, all wrapped by 7 mm of ground insulation (the total coil cross-section area being about 0.1 m/sup 2/). Most of the tests were carried out on specimens taken from the straight leg of the used coil. Tests on inter-turn insulation included shear strength and shear modulus, fracture tests, micrographic analysis, void and glass-resin content and second order glass transition temperature. Tests on ground insulation included tensile strength and elastic moduli, shear strength and shear modulus, inter-laminar shear strength and elastic modulus, mode 1 and mode 2 fracture tests. The results were not as consistent as might be achieved using laboratory specimens, but we consider that they are more representative of the quality achievable in actual manufacture.

The coil of the MBI bending magnets for the LHC injection transfer lines

All MBI bending magnets in each of the two LHC injection transfer lines will be powered in series. The limited output voltage of existing power converters lead to an unusual coil design avoiding external return bus-bars by combining two overlapping half-coils, electrically separated, with 3 1/2 turns each in a monolithic structure. The voltage between turns in one coil can reach up-to 3.6 kV. The coil has been designed with particular care for obtaining high interturn and ground insulation. Flux-free soldering of connections with plug-in cone sleeves is applied, allowing to execute water cooled current connections as prolongation of the coil conductor. Epoxy compound polymerization in the impregnation mould is obtained by passing overheated water in regulated cycles through the water circuit of the coil conductor. We describe the design basics as well as various test results of pre-series and series produced coils.

Voltage Distribution in the Winding of PWM Inverter Motor

The output voltage waveform of PWM inverter to use for the adjustable speed drive of the motor is the voltage which the pulse widths differ. When surge voltage of PWM inverter is applied to the motor, the surge voltage propagates in the motor winding. The propagation of surge voltage in the motor winding differs with the propagation in the cable, because interturn capacitance is much larger than ground capacitance. In case of the steep-fronted inverter voltage, the larger voltage is developed in the interturn and it might cause the interturn insulation breakdown of stator winding, which often leads to earth insulation failure. This paper states about the result of calculation and measurement for 600kW measurement motor to examine for the surge voltage propagation in the motor winding.

Classical and fracture mechanics analysis of the tails of the JET TF coils

The JET toroidal field (TF) coils are made of two 12-turn copper pancakes. The tails are designed to strengthen the inter-pancake joint and to transfer the tension between the tail and the bulk of the coil through shear in the inter-turn insulation. Stresses in the tail region, due to the combination of thermal and mechanical loads, have been found to be close to the measured material strength. In the high tensile stress region at the truncated tip of the tail, the computed tensile strain release energy is close to the critical value typical for glass-fibre/epoxy insulation. In contrast, the computed strain release energy for a crack extending to the region of the peak shear stress is smaller than the critical shear strain release energy measured on specimens taken from a used coil. This indicates that even if a crack opens at the tail tip, it would propagate slowly, if at all.

Quench modeling of the ATLAS superconducting toroids

Details of the normal zone propagation and the temperature distribution in the coils of ATLAS toroids under quench are presented. A tailor-made mathematical model and corresponding computer code enable obtainment of computational results for the propagation process over the coils in transverse (turn-to-turn) and longitudinal directions. The slow electromagnetic diffusion into the pure aluminum stabilizer of the toroid's conductor, as well as the essentially transient heat transfer through inter-turn insulation, is appropriately included in the model. The effect of nonuniform distribution of the magnetic field and the thermal links to the coil casing on the temperature gradients within the coils is analyzed in full.

High‐frequency voltage oscillation in transformer windings and electrical breakdown properties of interturn insulation immersed in oil at VFT voltage

High‐frequency voltage oscillation in transformer windings and electrical breakdown properties of interturn insulation immersed in oil at VFT voltage