Tomasz Koltunowicz

The influence of square voltage waveforms on transformer insulation break down voltage

The use of Voltage Source Converters (VSCs) in HVDC transmission and, more generally, in flexible AC transmission systems (FACTS), raises concerns over the endurance of turn/turn insulation of grid/converter interface transformers. In this paper, tests performed on oil-impregnated paper specimens are discussed. The results indicate clearly that shorter rise times and higher repetition frequencies have a strong impact on both partial discharge inception voltage and breakdown voltage. These results suggest that deeper investigation might be important to understand the long term behavior of these insulation systems.

Investigation of the effects of fast transients on a transformer's paper insulation

Identifying future trends in the power grid is vital in the extension of the life of its already aging components. Fast transients are the new factors that need to be taken into consideration as their high repetitive frequencies and fast rise times accelerate the aging of the paper insulation in devices such as transformers. As will be outlined below, an aging set-up has been proposed to study these effects on paper insulation in relation to the time to failure. The aging waveform generation is discussed together with the steps taken to assess the influence of the fast transients. Initial test results will be presented on paper samples used in a transmission transformer showing that the transient waveform has a factor of 2 in shortening the life time of the cellulose.

Optimization of maintenance for power system equipment using a predictive health model

In this paper, a model-predictive control based framework is proposed for modeling and optimization of the health state of power system equipment. In the framework, a predictive health model is proposed that predicts the health state of the equipment based on its usage and maintenance actions. Based on the health state, the failure rate of the equipment can be estimated. We propose to use this predictive health model to predict the effects of different maintenance actions. The effects of maintenance actions over a future time window are evaluated by a cost function. The maintenance actions are optimized using this cost function. The proposed framework is applied in the optimization of the loading of transformers based on the thermal degradation of the paper insulation.

Repetitive transient aging, the influence of rise time

Power electronic devices are becoming very popular as they are linked with renewable energy sources. Their primary use is to switch between DC and AC waveforms so that the energy generated at locations such as wind farms and solar plants can be used by the power grid. Although cheap and reliable, power electronic converters generate repetitive transients that affect HV components such as transformers and cables. The three main features of these transients, repetition frequency, rise time and magnitude are the cause of premature failure in the insulation system of HV devices. Rise time is responsible for the energy injected into the paper insulation as it determines the speed at which cellulose fibers are polarized. This causes movement at a molecular level which weakens the insulation. Increasing the speed at which the transient reaches its maximum level causes bigger damages to the insulation. Polarity also has an effect. Results are presented in this contribution when transients of different rise time are applied to oil impregnated cellulose.

Repetitive transient aging, the influence of repetition frequency

Power electronic devices are required in the modern grid in order to make a “bridge” between the DC and AC waveforms. These power inverters perform fast switching operations thanks to IGBTs and create fast repeating transients that are injected to the next HV component in line, e.g. a transformer. The transients are characterized by three main parameters: high repetition rates, fast slew rates and high magnitudes. This contribution presents the findings related to the repetition frequency. It was observed that the dielectric quality and lifetime decrease considerably when the frequency is changed from 1 kHz to 10 kHz. For this purpose, an experimental waveform is presented in this paper together with a broad explanation of the various phenomena that occur in the paper impregnated insulation.

The effects of prolonged exposure of paper insulation to fast repeating transients

Paper insulation impregnated with oil has been stressed with fast repeating transients. the frequency was set to 5 kHz, 8kHz and 10 kHz and the times to failure showed a much faster breakdown rate then when compared with a clean AC waveform at 50 Hz. The times to failure were also different for each frequency showing that repetition frequency has a major effect on the failure time. The material was also analyzed with tan d measurements and these measurements have shown that the paper sample has degraded much faster with the addition of transients.

Exploring the feasibility of an aging model for paper insulation based on the repetition frequency of transients

Fast transients have become a considerable addition to the electric grid, especially on locations where DC generated current needs to be converted into AC. The full impact of fast transients on high voltage equipment is still unknown and the scope of this contribution is to show that fast repetition frequencies of fast transients decrease the life expectancy of paper insulation. An aging waveform has been applied to paper samples and two frequencies for the transients are analyzed, 5 kHz and 8 kHz. The times to failure are then compared against a pure sinusoid at 50 Hz. The results show that fast transients have a very clear effect on shortening the life time of oil impregnated cellulose by factors of 6 and 4 respectively.

A statistical aging model for paper oil insulation under repetitive transients

AC voltage has been applied on oil impregnated paper with superimposed transients of repetition frequency ranging from 1 kHz to 10 kHz. The time-to-breakdown has been recorded. Then the “accelerated life test analysis” (ALTA) approach is applied to model the lifetime vs. repetition frequency relationship with inverse power, Eyring or exponential laws. During the “accelerated life test analysis”, the recorded life data at multiple repetition frequencies can be inputted simultaneously into the statistical aging model, and the parameters of each life vs. frequency relationship can be estimated. The estimation shows that the inverse power, Eyring or exponential relationships can be applied in our test frequency ranges, and extrapolated to higher or lower frequencies.