Moisture In Pressboard Research Articles

Partial discharge behaviour due to irregular‐shaped copper particles in transformer oil with a different moisture content of pressboard barrier under uniform field

This study presents the investigations on Partial Discharge (PD) behaviour of irregular-shaped copper particle in oil with pressboard barrier placed in two positions under uniform field with different moisture contents. Frequency Domain Spectroscopy (FDS) was used to estimate percentage moisture in pressboard. Recovery Voltage Measurement was used to confirm obtained FDS results. PD characteristics like magnitude, number, discharge power, rise time and duration time of PD pulse were analysed. Electric field due to particle in oil with moisture content pressboard barrier was studied by simulations. Phase Resolved PD (PRPD) pattern for different moisture pressboard observed to have similar patterns with variation in PD inception voltage (PDIV), magnitude and number of discharges. It was observed that PD characteristics as a function of voltage, increases nearly quadratic rate with respect to percentage moisture in pressboard. Simulation results complimented experimentally measured PDIV values for particle in oil with moisture in pressboard. Filtering and conditioning of oil is mandatory practice in a new transformer before going to any high voltage (HV) applications. PRPD pattern and PD characteristics data obtained in this study has great practical significance to assess the condition of transformer insulation systems before proceeding for HV application to avoid any potential failure.

Influence of moisture on the interface charge of oil–pressboard composite insulation under DC voltage

The distortion of the electric field in the oil–pressboard composite insulation caused by the accumulation of the interface charge is detrimental to both the insulation design and operation of converter transformers. The influence of moisture content on the surface charge accumulation of oil–pressboard insulation under DC voltage was studied in this study. In accordance with the Kerr electro-optic effect, the electric field strengths in transformer oil and the surface charge density were acquired after applying the positive and negative DC voltages in three oil–pressboard insulation models with different moisture content, respectively. The resistivities of the oil and pressboard in three models, namely Model 1# with 3.8–4.2 ppm moisture in oil and 0.35–0.37% moisture in pressboard, Model 2# with 7.6–7.9 ppm moisture in oil and 0.79–0.82% moisture in pressboard and Model 3# with 14.9–15.4 ppm moisture in oil and 1.39–1.42% moisture in pressboard, was also measured. The results indicate that: (i) as negative charges in oil accumulated on the pressboard surface in a much greater speed than the positive ones, the electric field in transformer oil under negative DC voltage decreases more rapidly with time than that under positive DC voltage; (ii) the increase of the moisture content in both oil and pressboard, under either positive or negative DC voltage, leads to the decrease of both the electric field strength in transformer oil and the charge density with time; and (iii) the increase of moisture content could not only decrease the resistivity of both oil and pressboard, but also the ratio of the resistivity between the pressboard and the oil. On the basis of the Maxwell–Wagner theory, the decrease of the ratio between the pressboard and oil could lead to the decrease of the interfacial charge density, leading to the slow transient process of the electric field in transformer oil under DC voltage.

Condition Assessment of Power Transformers Using Loss Tangent Measurements

Mineral oil, cellulose paper and pressboard are the main insulation materials in a power Transformer. During service, increase of moisture, as the by -products ageing and increases the risk of insulation failure. This paper presents the possibility to use loss tangent (or power factor) and capacitance (tanδ and C) measurements as an estimate of the 'wetness' of aged transformer insulation. Firstly, oil, paper and pressboard samples were aged under laboratory conditions and their tan S and dielectric constant (er) values were measured. Applying the tanδ and er obtained onto a transformer insulation model, the tanδ and C of transformer were calculated and the effects of insulation geometry, moisture content (MC) of pressboard, oil conductivity (σ oil) and insulation temperature under test were established. It was found that tanδ and C values of a transformer strongly dep ended on its insulation geometry and the temperature. If the temperature is fixed, the change of tanδ value of a transformer can be a direct measure of moisture in pressboard for 'wet' and 'extremely wet' conditions. For a 'dry ' transformer, the σoil oil value is needed as an additional known parameter in order to estimate accurately the moisture in pressboard.