Switching surge overvoltages on the feeding system of a MAGLEV train

This paper investigates switching surge overvoltages due to a section switchgear between coils and a feeding cable, and surge overvoltages due to a ground fault in the feeding cable. The maximum overvoltage due to switchgear closing is about 25 kV on the coil and 15 kV on the cable sheath. The coil overvoltage becomes greater as a source voltage reaches its peak. The overvoltages due to switchgear breaking is dependent on the breaking current and they reach 115 kV on the coil and 15 kV on the cable sheath when the current is 50 A. The fault surge overvoltage is about 24 kV on the coil which is smaller than the switching overvoltage. On the other hand, the cable sheath overvoltage reaches 32 kV, which is the largest among various overvoltages investigated in this paper, and could result in sheath insulation breakdown. The coil overvoltage due to current breaking by a section switchgear may cause coil insulation breakdown when the breaking current is large. Both the coil and sheath overvoltages can be controlled to less than the insulation level by arresters. © 1998 Scripta Technica, Electr Eng Jpn, (4): 58–66, 1998     

Switching of thyristors in AC voltage relay-pulsed control circuits

Switching processes are considered and their parameters are calculated for two circuits for ac voltage relay-pulsed control. A comparative analysis of these circuits is conducted for their use in a resistance welding machine.     

Resonances and overvoltages in a medium-voltage fan motor drive with long cables in an underground mine

Underground mining is characterized by space constraints forcing the use of long cables to feed large machines, where the application of variable speed drives can generate overvoltages at the machine terminals. This paper presents the analysis and mitigation of harmful overvoltages detected in inverter-fed 1400-hp 13.8-kV motors used for air ventilation in an underground mine. The possible causes of the overvoltages, namely wave reflection and resonance, are described theoretically and simulated. On-site direct voltage measurements at the 13.8-kV motor terminals confirmed that resonance was the dominant phenomenon. Based on this study, a suitable low-pass filter was designed, constructed, and installed. Finally, the adequate operation of such filter is demonstrated by further on-site measurements.     

The impact of winding type on overvoltages in an induction motor operating with a frequency converter

A technique for calculation of wave parameters and overvoltages in one- and two-layer windings of an induction motor powered by a pulse-width modulation converter is introduced. A mathematical model and simulation results for wave processes in these windings are presented.     

Results of an investigation on the overvoltages due to a vacuumcircuit-breaker when switching an HV motor

The results of a study of overvoltages arising when an HV motor is switched by a vacuum circuit breaker are presented. A laboratory test circuit was devised that would reproduce the actual network conditions in a generating plant, and special measurement and acquisition systems were used to enable the switching-surge amplitude and waveform to be recorded with accuracy. Statistical processing of the data was followed by estimation of the probability of occurrence of overvoltages and by evaluation of the number of the surges impinging on the motor-winding during its operating life. The results are discussed, and a number of conclusions are drawn     

Diagnosis of stator inter-turn short circuits in DTC induction motor drives

This paper addresses the subject of stator fault diagnosis in direct torque control (DTC) induction motor drives. The particularities of the behavior of a DTC drive, when in the presence of inter-turn short circuits, are exploited, thus allowing the proposal of two different approaches for the diagnosis of this type of fault. One of the proposed diagnostic techniques is the well known motor current spectrum analysis. It is demonstrated that in a DTC induction motor drive, with stator inter-turn short circuits, the action of the torque and flux controllers introduce a strong third harmonic in the motor supply currents, which can be used to detect this type of fault. The other diagnostic technique is based on the multiple reference frames theory, recently proposed by the authors for the diagnosis of stator faults in line-connected motors. Here, its use is exploited in the diagnosis of stator faults in DTC induction motor drives. Simulation and experimental results demonstrate the effectiveness of the two proposed techniques for the diagnosis of stator faults but emphasize the fact that the multiple reference frames technique is the preferred one when the residual asymmetries of the motor are not negligible.     

Control of wound rotor induction motor using thyristors in thesecondary circuits

An investigation was made of three-phase induction motor operation in the presence of a thyristor-controlled resistive network in each rotor circuit. The experimental realization of the drive posed some serious problems, notably with regard to the successful firing of thyristors over a wide speed range. Motor performance characteristics of output power, stator current, power factor, and efficiency were obtained as functions of speed over the entire range of thyristor firing-angles. An equivalent circuit is used based on the single-phase equivalent of a balanced, sinusoidal three-phase system. Such an equivalence is not directly justifiable because the per-phase performance of the nonlinear, thyristor controlled star-connected load is not the same as that of the corresponding single-phase circuit. Proof of the appropriateness of the chosen equivalent circuit approach is in the high level of agreement between the measured and calculated performance characteristics. The effective value of the external rotor “resistance” was calculated using the principle of power invariance. This resistance value proved to be a function of the motor speed and thyristor firing-angle as well as of the resistor values, for the configurations investigated. Analysis of the external network was extended to calculate the total input “impedance” (i.e., voltage/current ratio)     

Wavelength constants and interturn overvoltages in a multiturn random winding of an induction motor supplied from a PWM converter

A calculation procedure for wavelength constants and interturn overvoltages in a multiturn random winding of an induction motor supplied from a frequency converter is described. A mathematical model and modeling results for wave processes in sections (turns) of the multiturn random winding of the induction motor with a cage rotor are presented.     

Evaluation of surge-transferred overvoltages in distribution transformers

The paper presents an analysis of very fast-transient overvoltages that occur because of the capacitive surge transfer from the high-voltage (HV) transformer winding to the low-voltage (LV) transformer winding.     

Transient overvoltages in low-voltage systems-a field study inGermany

The “Overvoltage Measurements” field study was undertaken with the aim of establishing regulatory measures based on information from metered transient overvoltages occurring in low-voltage networks, installations, and telecommunication lines. Some 40 companies and institutions from industry and the power-utility, insurance, telecommunications, and research sectors contributed to the study. The overvoltages occurring between phase conductors and protective conductors were also measured     

Capacitive voltage sensor for measuring very fast transient overvoltages in GIS

A reliable technique is needed for accurate measurement of very fast transient overvoltages such as disconnector-induced surges occurring in GIS which have very steep front and high-frequency oscillation. Recently, capacitive voltage sensors with very short response times have been proposed for this purpose. However, few studies have been made on the dielectric properties of materials used for a low-voltage-side capacitor, as well as the frequency bandwidth of the measuring system. In this paper a thin mica plate is proposed for a low-voltage-side capacitor which has a surface coated with silver on both sides. This material has been shown to have stable and high static capacitance up to a high-frequency range. A quantitative estimate also is given for the frequency bandwidth of the measuring system. Furthermore, a practical-scale sensor of this type has been developed which can successfully measure simulated disconnector-induced surges of several hundred kilovolts under similar conditions in GIS.     

Harmonics and transient overvoltages due to capacitor switching

This work presents a study of the steady-state and transient effects of power factor correction capacitors on the utility and on the customer. In the presence of harmonic-producing loads, capacitors used for power factor correction can cause parallel or series resonance problems which tend to increase the total harmonic distortion (THD) of the voltage and current waveforms. The cases studied in this work considers the addition of a power factor correction capacitor, in the presence of downstream harmonic loads and at the harmonic load site. In both cases the resonance created by the addition of the capacitor caused the harmonic distortion of the voltage and current waveforms to increase. Another problem is transient overvoltages created by switching the capacitor. A case study is reported where the operation of a semiconductor controlled motor drive is effected by transient overvoltages     

Suppression of lightning-induced overvoltages by distribution line surge arresters

The primary aim of surge arresters in power distribution lines is to protect lines and equipment from the voltage induced by nearby lightning strokes. To further improve power systems, methods to protect distribution lines against direct lightning strokes are still needed. An effective measure against direct lightning strokes is to increase the number of arresters. However, if the surge current is too large, some surge arresters absorb energy in excess of their capability and may break; this leads to a line fault. To evaluate the protective effect of the surge arresters against direct lightning strokes to overhead ground wire, the authors measured both the voltage across the surge arresters and the energy absorbed by them using a full-scale model line and a 12 MV impulse generator. The results were compared with simulation results by EMTP. There have been no previous studies making a comparison of this kind.     

Electric circuits and the performances of the brushless DC motor with tooth-pole winding

In this paper an electric circuit analysis of the circuits of brushless DC motors with tooth-pole windings is given. Additionally, a comparison is made in terms of starting, torque, and speed at no load under both impulse and shaped sinusoidal controls.     

Experimental and analytical studies of lightning overvoltages in wind turbine generator systems

This paper presents the results of the experimental and analytical studies undertaken for the lightning protection of wind turbine generator systems by using a reduced-size wind turbine model. In the analytical studies, the FDTD (Finite Difference Time Domain) method is used. This study focuses on the overvoltages observed at the wavefronts of lightning surges. The lightning strokes on one of the blades and on the nacelle were considered, and the experiments and analyses were carried out by considering the cases of summer and winter lightning. The voltages and currents at various positions on the wind turbine model were considered.     

Insulation co-ordination for continuous operating voltages and temporary overvoltages

This paper presents an approach to the insulation co-ordination of electric-power installations in the scope of the continuous operating voltages and temporary overvoltages; this approach opens the possibility to calculate the necessary required withstand voltages of unit insulations for short-duration (1 min) power-frequency test voltage, with respect to continuous operating voltages and power-frequency temporary overvoltages and the necessary reliability of the operation of high-voltage power system installations.