Core Concepts
The presence of hydrogen gas bubbles, especially those in contact with metallic protrusions, significantly increases the risk of partial discharge ignition in high-voltage transformers, even under typical operating conditions.
Abstract
Bibliographic Information:
Kourtzanidis, K., Dimitrakellis, P., & Rakopoulos, D. (Year). Numerical analysis of partial discharge ignition in H2 bubbles floating in dielectric oils, for High-Voltage Solid State Transformer applications. [Journal Name].
Research Objective:
This study investigates the factors influencing partial discharge (PD) inception in high-voltage solid-state transformers (SSTs), focusing on the role of hydrogen gas bubbles and metallic protrusions within the dielectric oil.
Methodology:
The researchers employed a self-consistent plasma-fluid model using COMSOL Multiphysics and a custom-built solver (COPAIER) to simulate PD ignition. They considered various bubble sizes, positions, and the presence of protrusions under both high-frequency (50 kHz) and low-frequency (quasi-DC) voltage conditions.
Key Findings:
- Larger bubbles significantly increase the probability and severity of PD events, leading to lower inception voltages.
- Metallic protrusions in contact with bubbles drastically reduce the PD inception voltage, especially with sharper protrusions.
- Even under typical operating conditions (66 kV RMS), bubbles larger than 2.5 mm can initiate PDs.
- The bubble size effect is primarily attributed to charge gain/loss timescales rather than electrostatic field enhancement.
Main Conclusions:
Minimizing gas impurities and ensuring smooth winding surfaces are crucial for mitigating PD risk in SSTs. Even small bubbles can pose a hazard, particularly under transient overvoltage or in the presence of protrusions.
Significance:
This research provides valuable insights for designing reliable high-voltage transformers, highlighting the importance of considering even seemingly minor defects like gas bubbles and surface irregularities.
Limitations and Future Research:
The study assumes immobile, spherical bubbles. Future research could incorporate bubble deformation, movement, and interaction with fluid flow for a more comprehensive analysis. Additionally, coupling plasma dynamics with fluid and thermo-dynamics could provide a more realistic representation of the transformer environment.
Stats
Typically, more than 70% of transformer faults are internal, initiated via partial discharges inside transformer insulation.
Stray gassing in electrical insulating oils heated at relatively low temperatures favors the production of saturated carbon gas bubbles and especially hydrogen (H2) in temperatures below 120 °C.
The primary winding in the simulation is fixed at position z=0, whilst the secondary winding is fixed at z = -50 mm (overall gap 50 mm) and is considered grounded (V0 = 0).
The bubble is assumed spherical and the H2 plasma model is applied only inside the bubble.
The gas bubble diameter varies between 1 and 4.5 mm.
The bubble center is positioned at different coordinates in terms of x (radius r) and z, with fixed y = 0 (axis) and a sector angle 180° (half circle).
The bubble is placed inside the dielectric fluid (insulation medium) and a fixed relative permittivity value ε = 3 is considered.
The temperature is 360 K.
For the high-frequency simulations, an AC applied voltage with a frequency (f) of 50 kHz is considered.
For the low-frequency simulations, a DC voltage ramped up to its maximum value with a long rising time (~ 100 μs) is used.
The study considers a 66 kV operating RMS voltage (peak value 93 kV) for the low-frequency simulations.
The critical PD charge for PD inception is set to 10 pC.
Quotes
"The presence of bubbles in a dielectric fluid increases the breakdown probability."
"The initial stage of the transformer oil breakdown is caused by a gas bubble formed by evaporation due to local heating in high electric field regions of the electrode surface."
"As the PD is most probable when small amounts of gases are present, several models [8] have been developed to simulate the PD in voids (e.g., gas bubbles)."