Mechanisms Governing the occurrence of Partial Discharges in Insulation Liquids

• Main Author: Grav, Torstein
• Format: Others
• Language: English
• Published: Norges teknisk-naturvitenskapelige universitet, Institutt for fysikk 2013
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-22385

Breakdown of electrical equipment is unwanted and the use of reliable methods of insulation are therefore necessary. Liquid insulation is one preferred material for electrical devices used in subsea installations due to outstanding qualities related to insulation, heat transfer, safety and incompressibility properties. To ensure that the selected insulating liquid is suitable for the specific equipment it will be necessary to test its insulating quality. This is verified by stressing the liquid with voltages higher than the partial discharge inception voltage (PDIV). While there are existing tests for these properties, such as use of partial discharge measurements, this thesis examines an experimental approach to testing the properties of different liquids in a point-plane gap through the use of high AC voltage.Charges, like ions, in an electrical field creates a current flow. In a liquid, dissociative ionization under applied voltage is the mechanism that generates the most charges. Field emission originating from the negative polarity contributes in some liquids. Moving charges in the liquids results in measurable currents of the order 10 - 80 nA under applied AC voltage of 20 kV$_{peak}$ in a 20 mm point-plane gap in different liquids. Space charges are also found to affect the partial discharge (PD) behaviour in liquids. Different liquids have different chemical and physical properties and therefore different PD behaviour. It has been observed that PDs are stochastic, and in some sense correlated to earlier stress due to residual ions from previous half periods and permanently change of the chemical structure of some liquid molecules after a PD. The PD rate increases exponentially with increasing voltage, while the maximum charge per half period tends to increase linearly. This observed behaviour is in agreement with what is found in earlier reports.The presence of free electrons is important. A significant increase in the rate of PD at low magnitudes in both polarities occurred when the test cell was exposed to X-rays. PDs start under the influence of a strong field or an electron avalanche. It is essential to be able to test different liquids in laboratory conditions, for economical reasons, in order to identify the most suitable insulating media. Power electronics utilizing quality insulation is more reliable with a lower probability of downtime. The test setup and methods used in this thesis may easily be adapted for test of electrical components and equipment instead of the simple point-plane gap method.Different liquids have different PD phase patterns. This thesis intends to describe the reasons to explain this difference. It is found to be the result of different properties in the liquids for charge creation and differences in their electrical field threshold for PD initiation. PDs of a certain size are rarely occurring events and are strongly dependent on the prehistory of the stressed liquid gap. We do not know whether PDs in itself is harmful to insulating properties or not. It is therefore uncertain whether the IEC 61294 test method, based on PDs, provides any useful information or not. More research is therefore needed in order to fully understand the PD phenomenon.