Winding Insulation Material Research Articles

Thermal-flow characteristics analysis of a novel resilient capacity ceramic insulated winding dry type transformer

With the increasing demand for the resilient capacity of transformers in high-proportion renewable energy grids, the overload capacity of transformers and the thermal conductivity of winding insulation materials have been put forward as important requirements. In response to these requirements, a novel ceramic high temperature tolerance insulation winding is proposed in this paper and is applied in a ceramic insulated winding transformer. A combination of simulation and experimentation is used to analyze the thermal-flow characteristics of this transformer. The temperature and flow field distribution characteristics of transformers with both winding insulation methods (ceramic and Nomex paper) are compared. The internal mechanism of reflux near the iron core and high voltage winding under forced air cooling is clarified. A superior outside cooling condition is obtained. A prototype ceramic insulated winding transformer is developed and tested to verify the accuracy of the simulation model. The results show that the hotspot temperature of the ceramic insulated winding transformer under natural cooling is 10.7% lower than that of the Nomex paper insulated winding transformer. With forced air cooling of 2, 3, 4, 5, and 6 m/s applied at the bottom, the hotspot temperature of the ceramic insulated winding transformer is 11.4%, 10.5%, 9.7%, 14.0%, and 17.8% lower than that of the Nomex paper insulation winding transformer, respectively. Under forced air cooling, a reflux phenomenon is observed in both transformers in the vicinity of the iron core and high voltage winding due to the formation of adverse pressure gradient region and wall viscous force blocking. The strength of the reflux is affected by the magnitude of the forced air cooling flow velocity and the construction of the transformer. The optimal range of the external cooling flow velocity for the ceramic insulated winding transformer is 3–4 m/s. The errors between the simulated temperature and the measured temperature at the nine monitoring points of the prototype are all less than 5%, which verifies the effectiveness of the simulation model. The experimental and theoretical basis for the design and application of resilient capacity dry type transformers is provided by this research.

Synergistic effects of Micro-hBN and core-shell Nano-TiO2@SiO2 on thermal and electrical properties of epoxy at high frequencies and temperatures

Epoxy (EP) resin insulations are widely used in electronic devices and power systems. They are gradually finding their way as winding insulation material for medium frequency transformers (MFT) due to their attractive insulating and adhesive properties along with low cost and easy processing. However, their dielectric insulation properties are compromised at high frequencies and high temperatures due to poor thermal properties. In this context, the surface-treated hexagonal boron nitride (h-BN) micro fillers and core-shell TiO2@SiO2 nanofillers are doped in epoxy (EP) to enhance their thermal and electrical insulation properties under high frequencies and high temperatures. Results reveal that the thermal conductivity of the EP increases continually, whereas the electrical breakdown strength is reduced above the threshold content of micro/nano fillers. Particularly, the micro h-BN addition is beneficial for thermal conductivity enhancement, whereas the core-shell structure of TiO2@SiO2 improves the electrical insulation properties of the EP. The multi-layer core model is used to explain the dynamics of the fillers. The homogeneous dispersion and optimal content of suitable micro-nano fillers can produce efficient EP composites as the electrical insulation for medium frequency transformers.

Multi-rise time PWM: A way to reduce PD exposure in motor windings

Decreasing the rise time of pulse width modulated waveforms (PWM) has been observed to reduce the life length of high voltage motors. To solve this problem, the advantages of employing multilevel PWM waveforms with adjustable rise times operating above partial discharge inception are discussed in this paper. Examples of how the voltage supply circuits can be designed to enable investigation of this well as measured results are presented. To elucidate this approach, it is investigated what influence the choice of inverter levels has on the partial discharge (PD) characteristics within motor winding material applying adjustable rise times. The test objects were fed from either three- or four level inverter voltage waveforms to resemble realistic stress conditions as in field application. Presented experimental results show that the total peak and summed PD magnitude (exposure) drops considerably when applying a longer rise time at the critical voltage flanks and is shown to reduce the wear of the winding insulation material, showing the consistency between PD pattern and wear. It is further suggested that the presence of PDs can be eliminated by adjusting the rise times at the most critical voltage flanks and thus enhance the life of the machine winding. The presented principles can be implemented for various PWM pattern.

The Impact of Thermal Degradation on Properties of Electrical Machine Winding Insulation Material

Interturn stator short circuits can develop quickly leading to serious damage of an electric machine. However, degradation mechanisms of winding insulation material are not yet fully understood. Therefore, the main contribution of this paper is the analysis of the impact of thermal ageing on the electrical properties of the thin-film winding insulation. The insulation samples have been aged thermally at 200 °C–275 °C and for 100–1600 h. After ageing, impedance spectroscopy measurements were undertaken on the samples and equivalent-circuit model (ECM) parameters fitted for each measurement. This allows the impact of thermal ageing on ECM parameters to be analyzed, giving insight into the changes in the electrical properties of the insulation. Finally, high voltage was applied to the samples aiming to identify the breakdown-voltage (BV) characteristics of the insulation material.

Influence of rise time on dielectric parameters extracted from time domain spectroscopy in the context of generator stator insulation

The low-frequency dielectric response of generator stator winding insulation materials was calculated from high voltage time domain spectroscopy measurements. These results were used to calculate the theoretical response after the application of a voltage function consisting of a ramped voltage followed by a constant level voltage. The rate of rise of the ramped portion of the voltage function was shown to influence the well known and much used polarization index (PI) and insulation resistance (IR) parameters.

Historical development of rotor and stator winding insulation materials and systems

This article presents an overview of some of the significant developments in rotating machine insulation systems that have occurred in the past century. This article describes the chronological development of insulation materials and systems, together with some of the key innovations in manufacturing. For the most part, it was the innovation in materials and manufacturing from wound stator groundwall insulation that paced the development of machine insulation systems.

The feasibility to model a 3-leg core type medium frequency power transformer by use of a dual Cauer circuit

A model for medium frequency power transformers is suggested. The model is able to treat arbitrary topologies by use of Cauer circuits for core and windings. An implementation example is given for a 1-phase transformer with turn ratio 1:1. The model can be extended to include winding insulation materials by use of lumped capacitive and resistive circuit elements.

Considerations of distributed capacitance of high‐voltage transformers

AbstractAn effective means to achieve a small and light‐weight high‐voltage power supply is to raise the conversion frequency of the supply. At present, however, the conversion frequency is largely limited by the distributed capacitance of high‐voltage transformers and the switching time of semiconductor devices. Reduction of the distributed capacitance of a high‐voltage transformer and a new method of analysis which can quantitatively describe the relation between transformer structure and distributed capacitance were achieved. Using the proposed method of analysis, the distributed capacitance is calculated using the winding scheme, core dimension, number of bobbin splits, class and material for winding insulation, core size and impregnation material as the parameters. The calculated values are compared with the measured values and the effects of the parameters on the distributed capacitance are discussed. By using a four‐split bobbin, the distributed capacitance is reduced by 10 percent less than the nonsplit bobbin. The capacitance using the core with a circular center pole is reduced by approximately 10 percent compared with the square center pole. Using class 1 insulation winding, the capacitance is reduced by 10 percent compared with class 2 winding insulation material.