Marinko Kovačić

Measurement of Excitation Winding Temperature on Synchronous Generator in Rotation Using Infrared Thermography

A new measurement method for infrared (IR) surface temperature measurement of excitation winding in rotation is presented. The method is experimentally verified on a 400-kVA salient pole synchronous generator. This method uses an industrial IR thermometer which represents an alternative to expensive fast IR thermometers or cameras. The target application of this method is the determination of the dynamic limit in the P-Q diagram of a synchronous generator due to excitation winding overheating. The measurement error model which shows the way how to minimize measurement error has also been derived. The effect of the interpolar surface can be cancelled if the IR thermometer is positioned at a certain angle with respect to the machines main axis. Digital temperature sensors have been mounted on the rotor to measure the excitation winding surface temperature for comparison.

Analytical Wideband Model of a Common-Mode Choke

This paper presents an analytical model of a common-mode choke suitable for accurate calculation of the choke impedance over a wide frequency range. The model consists of lumped parameters (resistances, inductances, and capacitances) related to individual turns of the coils wound on the core. It takes into account the mutual interactions between the turns and the core with respect to their inductive and capacitive links. The variation of the core permeability and losses with frequency up to 100 MHz is also included. The open-mode impedance characteristic calculated analytically for a VAC 6123x425 single-phase common-mode choke shows very good agreement with the 3-D finite-element model and the measured characteristic from 150 kHz up to 30 MHz, thus confirming the accuracy of the model over a wide frequency range.

Bluetooth wireless communication and 1-wire digital temperature sensors in synchronous machine rotor temperature measurement

This paper addresses the usage of digital temperature sensors in rotor temperature measurement. A rotor temperature measurement system had to be developed because of the complete synchronous machine thermal analysis. Rotor temperatures are measured using multiple DS18B20 digital thermometers and sent via a wireless link to the host. The measurement system has been mounted on a 400 kVA synchronous generator and the temperature measurements in rotation are presented.

Influence of winding design on thermal dynamics of permanent magnet traction motor

This paper presents summarized electromagnetic and thermal design of interior permanent magnet motor for a low-floor tram TMK 2200. Motor geometry is optimized for maximum torque density using differential evolution algorithm. The influences of winding configuration on power losses and thermal transients in the motor during one driving cycle of the tram have been analyzed. The results indicate that an optimal number of turns per slot and parallel paths can be found to yield minimum temperatures in various parts of the motor considering the intermittent character of the load.

Small-Signal Calculation of Common-Mode Choke Characteristics Using Finite-Element Method

This paper presents a finite-element (FE) method approach to calculation of the common-mode choke (CMC) impedance over a wide frequency range. The proposed method involves a 3-D electrostatic FE calculation of each turn-to-turn and turn-to-core capacitance, and their usage as electric circuit lumped parameters in the 3-D time-harmonic magnetic FE calculation of the CMC impedance. It also takes into account the variation of the nanocrystalline core permeability and losses with frequency up to 100 MHz. The proposed methodology is used for calculation of the common-mode, open-mode, and differential-mode impedance characteristic for single-phase and three-phase CMC. Results are compared with measurements.

Centrifugal fan design for permanent magnet synchronous motor in a traction application

The requirements of high torque density and high efficiency, which are particularly pronounced in electric traction applications, often result in substantial thermal loading of electric machines for driving trams, electric multiple units (EMU) or electric cars. Permanent magnet synchronous machines are suitable candidates for traction applications due to their inherently high torque density and high efficiency. At the same time they are sensitive to temperature rise, especially in permanent magnets, highlighting the need for implementation of efficient cooling system. The performance of the cooling system and its ability to remove heat directly affect the attainable torque and efficiency of the electric machine. In this paper, the selection and sizing of the cooling system for an interior permanent magnet motor designed to drive a low-floor tram is presented. The procedure for selecting the basic dimensions of the centrifugal fan according to the analytical formulas in combination with computational fluid dynamics (CFD) analysis are explained. In addition to the geometry of the centrifugal fan itself, the geometry of the passive system components (e.g. air flow router) which have a significant impact on the performance of the cooling system, are also considered. The results of computer aided CFD analysis, which is taken as a benchmark of system performance in the design stage of the cooling system, have been confirmed with measurements on the machine prototype.

The analysis of the infrared excitation winding surface temperature measurement on a synchronous hydro-generator in rotation

Excitation winding temperature is used for determination of the dynamic limit in the P-Q diagram of a synchronous generator. The surface temperature of an excitation winding can be measured by an infrared (IR) thermometer. Due to rotation, the IR thermometer absorbs thermal radiation from the excitation winding on the salient pole and from the interpolar space as well. These two surfaces can have different temperature or emissivity, which both affect the amount of the thermal radiation radiated towards the sensor of the IR thermometer. Irradiation read by the sensor is therefore the average irradiation of a rotating rotor. This irradiation results in the temperature that differs from the pole surface temperature. The difference depends on the interpolar space size, emissivity and temperature. The effect of the interpolar space can be cancelled if the IR thermometer is positioned at a certain angle with respect to machines main axis. Temperature sensors have been mounted on the rotor to measure the excitation winding surface temperature for comparison.

Influence of mounting temperature probes on the excitation winding on its temperature field

Accurate and reliable information about the temperature of the synchronous generator excitation winding hot spot is necessary to determine the dynamic limit caused by excitation winding overheating in the PQ diagram. For good estimation of a position and the hot spot temperature it was decided to mount 19 temperature probes on one pole of the 6-pole, 400 kVA, 50 Hz synchronous generator. Due to a large number of the probes and because the probes should be glued with the metal epoxy it was assumed that mounting of the probes will disrupt the temperature field of the excitation winding. To get the answer to this question the excitation winding resistance was measured before and after mounting the probes, in a hot and a cold state. Temperature rise can be estimated if the resistance ratio in the hot and the cold state is known. The paper also addresses the analysis of the measurement accuracy.

Measurement Systems for Electrical Machine Monitoring

Monitoring systems on electrical machines, nowadays largely present on new electrical rotating (synchronous generators) and non rotating machines (transformers) have to endure harsh conditions such as vibrations and electromagnetic disturbances. Also, especially for rotating machines, rotor physical quantities (temperature, strain, current…), have to be transferred to the standstill part of the measurement system without galvanic contact. Therefore, monitoring systems should be, and usually are, designed for a specific purpose and for a specific machine. Some essential parts are the same but most of the monitoring system should be custom made especially if one takes into consideration that there is a small serial production of large electrical machines in the world and that most of the large units are unique. Modern monitoring of big transformers covers permanent on-line monitoring of electrical, magnetic and mechanic quantities. It is specific when compared to rotational electrical machines because it also usually includes monitoring of dissolved gasses in transformer oil. This information, with proper interpretation, represents an important element for transformer state determination. However, transformer monitoring systems are not the focus of this chapter. This chapter deals with all problems, obstacles and solutions applied to two measurement systems installed on a 35 MVA hydro-generator and a 250 MVA turbo-generator. Large electric machines justify investment in measurement systems for several reasons. Usually, measurement systems for bearings are installed. In such systems vibrations and oil parameters (pressure and temperature) are monitored to help maintenance. Measurements of stator and less often rotor currents are also present as parts of different systems. For example, a differential stator current measurement is present for protection purposes and enables detection of current leakage (short circuit between phase windings). Monitoring systems that incorporate all necessary measurements and, of course, interpretations of those measurements are almost non existing and very rare. This chapter will concentrate on synchronous generators.

Flying Start of a Permanent Magnet Wind Power Generator Based on a Discontinuous Converter Operation Mode and a Phase-Locked Loop

Flying start of a wind generator is a common requirement in wind turbine applications. This paper proposes a novel method for allowing the flying start of a permanent magnet generator (PMG). The proposed method is based on the discontinuous mode of operation of the converter and the phase-locked loop (PLL). During the discontinuous mode of operation, a series of zero-voltage pulses is applied to the generator, causing the occurrence of the short-circuit current, and thereby employing the PLL structure to estimate the rotor speed and angle. Also, a short-circuit current control structure with automatic adjustment of the reference value is proposed. The input signal to a PLL structure is adequately selected in order to enable a continuous mode of operation of the PMG. Simulation results demonstrate the applicability of the proposed method on the surface and interior PMGs, as well as on low- and high-power PMGs. The estimation accuracy does not depend on changes in the PMG parameters. The proposed method was implemented in a digital control system and experimentally verified on a 375 kW interior PMG. Experimental responses referring to the flying start of the PMG demonstrate the effectiveness of the proposed method.