Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress
Abstract
:1. Introduction
- a.
- Enhanced partial discharge (PD) intensity. The number of PDs per unit of time is proportional to the switching frequency. In addition, PD amplitudes are observed to increase with switching speed [11]. Thus, if PD is incepted, the total PD charge and its erosive effect on the insulation material is greatly enhanced when switching frequency and speed are increased.
- b.
- Systemic overvoltages. The broad spectral content of the terminal voltage can excite system resonances, generate standing wave phenomena on electrically long conductors (cable/load impedance mismatch), and lead to strongly nonlinear voltage drops along windings or field-grading layers due to parasitic capacitive couplings [4]. As a result, the electric field stress on certain parts of the insulation can be increased significantly.
- c.
- Excessive insulation temperature. Increased switching speeds broaden the spectral content of the terminal voltage, while a higher switching frequency shifts its frequency spectrum as a whole. Both effects typically entail enhanced dielectric losses. Losses associated with charge migration or the dielectric -relaxation have positive temperature coefficients and can lead to thermal runaway [17]. Moreover, increased capacitive currents can lead to significant heating of (field-grading) semiconductive layers [4]. Even in the absence of thermal runaway, a higher insulation temperature due to increased power (loss) densities enhances thermally activated degradation processes and—especially for organic materials—compromised (di)electric properties. Moreover, the problem of stress cracking and delamination is accentuated.
- d.
- Space charge. Structural aging due to electromechanical stress from the Coulombic interaction between the highly dynamic electrode field (due to repetitive impulse voltages of high slew rate) with space charge evolving under the DC-component of the applied voltage.
2. Methods
2.1. Test Bench
2.2. Sample Preparation and Electrode Geometry
- The stressed volume is small, which allows to avoid a significant heating of the insulation even at high pulse frequencies (quantitative analysis given in Section 2.2.3), and thus to investigate insulation failure outside the thermal aging/thermal breakdown regime;
- The configuration has a high degree of (macroscopic) symmetry, and thus is more easily amenable to numerical estimations of system properties (thermal, electric field, etc.) than more complicated systems (such as twisted enamelled wires);
- The arrangement can be used to investigate the breakdown of polymeric insulation in the presence or absence of PDs without the need for replacing the surrounding air with insulating oil. This is a consequence of the well-defined insulator-electrode configuration and the associated existence of a narrowly scattered value of the inception voltage.
2.2.1. Test Procedure
2.2.2. Statistical Analysis
2.2.3. Maximal Sample Temperature
3. Results
3.1. Dielectric Heating
3.2. Systemic Overvoltages
3.3. Partial Discharge Inception Voltage (PDIV)
3.4. Times to Failure
4. Discussion
5. Conclusions
- PDIV is determined by the pulse peak-peak voltage , but not significantly influenced by the applied DC bias ;
- While the spatial extent of the PDs is correctly predicted by the streamer criterion, the application of the “textbook value” for the streamer constant () significantly overestimates the measured PDIV;
- Repetitive PDs erode the polymer surface (thinning of the foil) and breakdown occurs when the applied DC field exceeds the breakdown strength of the thinned foil;
- In the presence of PDs, the insulation lifetime shows a clear “per pulse” degradation, and it is typically shorter than under a pure DC voltage of the same peak amplitude;
- In the PD regime, the lifetime is inversely proportional to the optical PD intensity (as quantified by a near-UV camera) and scales as with the switching frequency. The “per pulse deterioration” thus decreases with increasing ;
- In the absence of PDs, no evidence is found of a detrimental “per pulse” impact on insulation lifetime (it is observed to be independent of the switching frequency in the investigated range). The processes leading to failure under a pure DC voltage are merely modulated by the changing voltage levels introduced by the rectangular pulse train. Accordingly, the lifetime with pulse modulation is observed to be larger than under pure DC voltage of the same peak amplitude.
Author Contributions
Funding
Acknowledgments
Conflicts of Interest
Abbreviations
AC | Alternating Current |
DC | Direct Current |
MF | Medium Frequency |
MOSFET | Metal Oxide Field Effect Transistor |
MV | Medium Voltage |
PD | Partial Discharge |
PDIV | Partial Discharge Inception Voltage |
RH | Relative Humidity |
TTF | Time To Failure |
UV | Ultraviolet |
d | Foil thickness (23 μm) | |
Switching frequency | ||
h | W m−2K−1 | Thermal interface conductance |
k | W m−1K−1 | Thermal conductivity |
— | Streamer constant | |
°C | Temperature | |
°C | Room temperature | |
— | Effective dielectric permittivity | |
S m−1 | DC conductivity | |
Pulse rise/fall time | ||
Pulse jump voltage | ||
DC offset voltage |
Appendix A. Indentation of an Elastic Insulation Foil in a Sphere-Plane Electrode Configuration
Appendix B. Dielectric Heating: Estimation of Upper Limit on Temperature Rise
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Färber, R.; Guillod, T.; Krismer, F.; Kolar, J.W.; Franck, C.M. Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress. Energies 2020, 13, 13. https://doi.org/10.3390/en13010013
Färber R, Guillod T, Krismer F, Kolar JW, Franck CM. Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress. Energies. 2020; 13(1):13. https://doi.org/10.3390/en13010013
Chicago/Turabian StyleFärber, Raphael, Thomas Guillod, Florian Krismer, Johann W. Kolar, and Christian M. Franck. 2020. "Endurance of Polymeric Insulation Foil Exposed to DC-Biased Medium-Frequency Rectangular Pulse Voltage Stress" Energies 13, no. 1: 13. https://doi.org/10.3390/en13010013