Zoran Lazarevic

A novel approach for temperature estimation in squirrel-cage induction motor without sensors

In this paper, the authors present a coupled thermal and electromechanical model for squirrel-cage induction motor simulation and analyses. The effect of iron saturation, rotor parameter variation due to current displacement (skin-effect) and temperature variations of both stator and rotor resistances are taking into account simultaneously. A new device for the temperature measurement in the stationary as well as rotating parts of electric machines is built. The unique construction of the device for continuous rotor temperature monitoring is briefly described. This device is a part of a complete acquisition system, which is used for precise testing and investigating of a coupled thermal and electromechanical phenomena. An original thermal observer based on the direct stator temperature measurement under normal running condition with no additional sensors is proposed.

Triggered vacuum and gas spark gaps

This paper presents a comparative analysis of the characteristics for the gas insulated three-electrode spark gaps and vacuum insulated three-electrode spark gaps. The experimental part of this paper includes the testing of spark gap models. Two spark gap types were studied: one having the third electrode inside the main electrode and one having a separate third electrode, both being insulated by vacuum or gas (under pressure, providing the same operating voltage as for a vacuum insulated spark gap). Both types of spark gaps were theoretically sized in the optimal way. Several characteristics are determined experimentally: the influence of the gas and vacuum insulation parameters on the spark gap functioning, the influence of the rate of rise and injected energy of the triggering pulse on the spark gap functioning, and the degree of spark gap erosion vs. number of operations (long-time-stability). Two types of gases were applied: SF/sub 6/ gas, N/sub 2/ gas and three vacuum (residual) pressures: 10/sup -1/ Pa, 10/sup -4/ Pa, and 10/sup -6/ Pa. Three electrode materials were used: copper, steel and tungsten. The spark gap switching time and delay time are measured. It was found that the switching time decreases with application of pressure decrease, and the statistical dispersion of switching time raises with the pressure decrease. By comparison of results obtained for the vacuum insulated spark gap and the SF/sub 6/ or N/sub 2/ gas insulated spark gap, it was found that the vacuum spark gap has a slightly shorter switching time and a significantly higher corresponding statistical dispersion.

Numerical and experimental design of three-electrode spark gap for synthetic test circuits

The development of a three-electrode spark gap used with a synthetic circuit for circuit breaker testing is considered in this paper. Two types of three-electrode gas insulated spark gaps have been tested: a spark gap with a third electrode being inside the main electrode, and a spark gap with a separate third electrode. Both types of spark gaps were theoretically sized in the optimal way. Several characteristic are determined experimentally: (1) the influence of the gas insulation parameters on the spark gap functioning, (2) the influence of the working and triggering voltages polarities on the spark gap functioning, (3) the influence of the triggering voltage rate-of-rise on the spark gap functioning and (4) the degree of spark gap erosion vs number of operations. Three types of gases were applied: SF/sub 6/ gas, N/sub 2/ gas and a mixture of 60% SF/sub 6/ with 40% N/sub 2/. Also, three electrode materials were used: copper, steel and tungsten. The spark gap switching time and delay time are measured. The statistical analysis of results is presented. >

On the determination of characteristic temperatures in power oil transformers during transient states

ContentsUnder usual assumptions the equations describing thermal phenomena in a transformer are established. Without neglecting any substantially important factor, algorithms are developed for predicting the temperatures in critical spots in non-stationary states occurring either in practice or in load/time diagrams. This paper suggests the determinaton of conductances dependent on temperature and constant capacitances of equivalent thermal circuits of a transformer. They are based on a small number of experiments (as simple as possible) and on a parameter estimation procedure. In such a way the temperatures of transformer parts in different transient states can be determined by computer simulation established on equivalent circuits, so the operators in power and sub-stations will be able to load their transformers close to the permitted limit. It is felt that this paper may give a stimulation for a possible revision of IEC rules.ÜbersichtDie Gleichungen des Temperaturverhaltens eines Transformators werden unter den üblichen Annahmen aufgestellt. Es werden Algorithmen entwickelt, welche die Vorausbestimmung der Temperatur an kritischen Stellen bei nichtstationärem Betrieb erlauben. Vorgeschlagen wird die Bestimmung der temperaturabhängigen und-unabhängigen Parameter auf Grund von wenigen und einfachen Messungen. Damit kann das Personal in Verteilerstationen in die Lage versetzt werden, mit Hilfe von PCs die Belastung der Transformatoren bis zu den zulässigen Grenztemperaturen auszudehnen. Die Autoren möchten damit einen Anstoß zu einer möglichen Überarbeitung von IEC-Vorschriften geben.

A new thermal observer for squirrel-cage induction motor

In the present paper the authors built a device for the temperature measurement in the stationary as well as rotating parts of electric machines. The effectiveness of the device is tested in real working condition on squirrel cage induction machine. The unique construction of the device for continuous rotor temperature monitoring is briefly described. This device is a part of a complete acquisition system which is used for precise testing and investigating of a thermal process. A new thermal observer based on direct stator temperature measurement under normal running condition with no additional sensors is proposed.

Transient Electric Field Measurement in the Liquid Dielectrics Using Computerized Laser Tomography

This paper presents an original method to measure ultrafast voltage transient responses and electric field distribution in liquid dielectrics. The capacitive probe is employed to record the fast voltage transient responses. The electric field is reconstructed with multiangular laser scanning of the space between electrodes and a modified arithmetic reconstruction technique. The finite-difference method (FDM) is used to compare and verify experimental results for symmetrical and asymmetrical electrode configurations. Numerical verification confirmed that the experimental results are accurate enough to be used in the design of oil-immersed dielectric systems.

Effect of Insulation Construction Bending on Stator Winding Failure

This paper investigates electrical breakdown of interconductor insulation, which presents an obstacle to the increase of the rated voltage of electrical machines as a means to improve efficiency of electrical energy exploitation. We have investigated a large number of samples searching for weak points in interconductor insulation along the coil of a high voltage asynchronous machine. The experiments were performed on lacquer-glass-glass-lacquer conductor insulation. Location of weak points and breakdown voltage levels have been determined.

Measurement of the transient electrical and shaft torques in an induction motor drive by the indirect two-speed method

ContentsThis paper presents a new method, called Indirect Two-Speed Method (ITSM), for obtaining electrical and shaft torques in an induction motor drive by means of measuring the speeds of both lumped-inertia elements assigned to a second order model. The measuring is realized by using high-resolution digital tachometers (represented by incremental encoders), two frequency measurement systems based on the Double-Buffered Method, digitalfiltering of the raw speed data and numerical differentiation of the filtered speed data.ÜbersichtIm Beitrag wird ein neues Verfahren zur Ermittlung des elektrischen Moments und des Wellen-Drehmoments eines Induktionsmotor-Antriebs vorgestellt. Das als Indirekte Zwei-Drehzahlen-Methode (ITSM) bezeichnete Verfahren geht aus von einem mechanischen Modell zweiter Ordnung und mißt die Drehzahlen beider als konzentrierte Parameter betrachteten Drehmassen. Für die Durchführung werden hochauflösende digitale Tachometer eingesetzt. Die Frequenzmessung erfolgt durch ein Verfahren mit doppelter Pufferung, und es werden die Rohdaten nach digitaler Filterung numerisch differenziert.