In the analysis of transformer faults, the degradation of the winding insulation is usually the largest share, while the deterioration of the oil condition normally comes first. Looking at the water, the gas and the particles separately is a useful starting point for the analysis, but the real difficulty is that they feed each other.
Water: from emulsification to hydrolysis
Water does harm the insulation in two ways. Emulsified water at the surface lowers the breakdown voltage of the oil directly, and that is the easiest effect to see in the measurements. The more insidious one is the migration of dissolved water into the solid insulation: insulation paper has an affinity for water, and the water gradually transfers out of the oil phase into the paper, and both the dielectric strength and the mechanical strength of the paper fall as a result. Water also takes part in the hydrolysis of oil and paper at high temperature, producing acidic products and accelerating the degradation. The process is slow, the equipment does not alarm at once, but after a few years the degree of polymerisation of the paper has fallen irreversibly.
Gas: a chain reaction that starts with a single bubble
The dielectric strength of a bubble is far below that of a solid insulating material. Under the electric field the bubble is ionised first, and the ionisation produces more gas, so the process itself is the starting point of a partial discharge. A rising gas content also means that a compressible medium is present in the oil, which changes the field distribution and concentrates the local field strength further. The ZYD-100 holds the gas content after treatment at ≤0.01% (GB/T 423), because once gas has entered the insulation system, the cost of tracing it is far higher than the cost of removing it.
Particles: potential carriers of a conductive path
Metal swarf and oxide particles are not poor insulators in themselves; the danger lies in the fact that, if they are conductive, they align along the field direction under the electric field, migrate and bridge the electrodes, forming a conductive path through the whole gap. The particles also act as a catalyst: they adsorb onto the oil-paper interface and accelerate the formation of ageing products, and at the same time they are themselves one of the products of the ageing cycle.
| Contaminant | Main mechanism | Direct consequence | Corresponding indicator |
|---|---|---|---|
| Water | Emulsification lowers the breakdown voltage, migration into the insulation paper, hydrolysis producing acids | Falling strength of the insulation paper, accelerated ageing | Water content ≤3 ppm (GB/T 7600) |
| Gas | Ionisation of the bubbles initiates partial discharge | Partial discharge, continuing deterioration of the insulation | Gas content ≤0.01% (GB/T 423) |
| Particles | Conductive particles bridge the electrodes and form a path | Sharp fall in the breakdown voltage, local overheating | Cleanliness NAS 1638 class 5, filtration accuracy ≤1 μm (GB/T 511) |
How the three feed each other
Water promotes hydrolysis, the product of hydrolysis is acid, the acid aggravates the degradation of the insulation paper, and the degradation produces yet more fine particles and gas; at the same time the particle surfaces adsorb water and raise the local water content; and the gas decomposes under the electric field to produce more gas. Once this cycle has started, a single filtration can only bring the indicators down, it cannot change the trend. Regular treatment combined with tracking of the indicators is therefore far more economical than waiting for a breakdown voltage alarm before treating the oil.
The treatment order cannot be reversed either: the particles are filtered out first, and the water and the gas are removed afterwards. Done the other way round, the atomiser and the separation unit are held back by the particles and the degassing efficiency suffers.