IEC 60076-7-2018 pdf free download.Power transformers – Part 7: Loading guide for mineral-oil-immersed power transformers.
The normal life expectancy is a conventional reference basis for continuous duty under design ambient temperature and rated operating conditions. The application of a load in excess of nameplate rating and/or an ambient temperature higher than design ambient temperature involves a degree of risk and accelerated ageing. It is the purpose of this part of IEC 60076 to identify such risks and to indicate how, within limitations, transformers may be loaded in excess of the nameplate rating. These risks can be reduced by the purchaser clearly specifying the maximum loading conditions and the supplier taking these into account in the transformer design.
5.2 General consequences
The consequences of loading a transformer beyond its nameplate rating are as follows.
a) The temperatures of windings, cleats, leads, insulation and oil will increase and can reach unacceptable levels.
b) The leakage flux density outside the core increases, causing additional eddy-current heating in metallic parts linked by the leakage flux.
c) As the temperature changes, the moisture and gas content in the insulation and in the oil will change.
d) Bushings, tap-changers, cable-end connections and current transformers will also be exposed to higher stresses which encroach upon their design and application margins.
The combination of the main flux and increased leakage flux imposes restrictions on possible core overexcitation [6], [7], [8].
NOTE For loaded core-type transformers having an energy flow from the outer winding (usually HV) to the inner winding (usually LV), the maximum magnetic flux density in the core, which is the result of the combination of the main flux and the leakage flux, appears in the yokes.
As tests have indicated, this flux is less than or equal to the flux generated by the same applied voltage on the terminals of the outer winding at no-load of the transformer. The magnetic flux in the core legs of the loaded transformer is determined by the voltage on the terminals of the inner winding and almost equals the flux generated by the same voltage at no-load.
For core-type transformers with an energy flow from the inner winding, the maximum flux density is present in the core-legs. Its value is only slightly higher than that at the same applied voltage under no-load. The flux density in the yokes is then determined by the voltage on the outer winding.
5.3 Effects and hazards of short-time emergency loading
Short-time increased loading will result in a service condition having an increased risk of failure. Short-time emergency overloading causes the conductor hot-spot to reach a level likely to result in a temporary reduction in the dielectric strength. However, acceptance of this condition for a short time may be preferable to loss of supply. This type of loading is expected to occur rarely, and it should be rapidly reduced or the transformer disconnected within a short time in order to avoid its failure. The permissible duration of this load is shorter than the thermal time constant of the whole transformer and depends on the operating temperature before the increase in loading: typically, it would be less than half-an-hour.
The main risk for short-time failures is the reduction in dielectric strength due to the possible presence of gas bubbles in a region of high electrical stress, that is the windings and leads. These bubbles are likely to occur when the hot-spot temperature exceeds 140 °C for a transformer with a winding insulation moisture content of about 2 %. This critical temperature will decrease as the moisture concentration increases.
NOTE Concerning the bubble generation, see also IEC 60076-14.
a) Gas bubbles can also develop (either in oil or in solid insulation) at the surfaces of heavy metallic parts heated by the leakage flux or be produced by super-saturation of the oil. However, such bubbles usually develop in regions of low electric stress and have to circulate in regions where the stress is higher before any significant reduction in the dielectric strength occurs.
Bare metallic parts, except windings, which are not in direct thermal contact with cellulosic insulation but are in contact with non-cellulosic insulation (for example, aramid paper, glass fibre) and the oil in the transformer, may rapidly rise to high temperatures. A temperature of 180 °C should not be exceeded.
b) Temporary deterioration of the mechanical properties at higher temperatures could reduce the short-circuit strength.
c) Pressure build-up in the bushings may result in a failure due to oil leakage. Gassing in condenser type bushings may also occur if the temperature of the insulation exceeds about 140 °C.
d) The expansion of the oil could cause overflow of the oil in the conservator.
e) Breaking of excessively high currents in the tap-changer could be hazardous.
6 Relative ageing rate and transformer insulation life
6.1 General
For the manufacture of paper and pressboard for electrical insulation, mainly unbleached softwood kraft pulp is used. The cellulose is refined from the tree by the so-called “sulphate” or “kraft” process. After processing, the typical composition of unbleached kraft pulp is 78 % to 80 % cellulose, 10 % to 20 % hemicellulose and 2 % to 6 % lignin.
Cellulose is a linear condensation polymer consisting of anhydroglucose joined together by glycosidic bonds, Figure 1.IEC 60076-7 pdf free download.