GB/T 1094.7-2024 Power transformers — Part 7: Loading guide for mineral-oil-immersed power transformers
1 Scope
This document describes the effect of operation under various ambient temperatures and load conditions on transformer life.
This document is applicable to mineral-oil-immersed power transformers.
Note: For furnace transformers, the manufacturer is consulted in view of the peculiar loading profile.
2 Normative references
The following documents contain provisions which, through reference in this text, constitute provisions of this document. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.
GB/T 1094.2 Power transformers - Part 2: Temperature rise for liquid-immersed transformers (GB/T 1094.2-2013, IEC 60076-2:2011, MOD)
GB/T 1094.14 Power transformer - Part 14: Liquid-immersed power transformer using high-temperature insulation materials (GB/T 1094.14-2022, IEC 60076-14:2013, MOD)
GB/T 2900.95 Electrotechnical terminology - transformers, voltage regulators and reactors (GB/T 2900.95-2015, IEC 60050-421:1990, NEQ)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in GB/T 2900.95 and the following apply.
3.1
small power transformer
power transformer without attached radiators, coolers or tubes including corrugated tank irrespective of rating
3.2
medium power transformer
power transformer with a maximum rating of 100MVA three-phase or 33.3MVA single-phase
3.3
large power transformer
power transformer with a maximum rating of greater than 100MVA three-phase or greater than 33.3MVA single-phase
3.4
cyclic loading
loading with cyclic variations (the duration of the cycle usually being 24h) which is regarded in terms of the accumulated amount of ageing that occurs during the cycle
Note: The cyclic loading may either be a normal loading or a long-time emergency loading.
3.5
normal cyclic loading
loading in which a higher ambient temperature or a higher-than-rated load current is applied during part of the cycle, but which, from the point of view of relative thermal ageing rate (according to the mathematical model), is equivalent to the rated load at normal ambient temperature
Note: This is achieved by taking advantage of low ambient temperatures or low load currents during the rest of the load cycle. For planning purposes, this principle can be extended to provide for long periods of time whereby cycles with relative thermal ageing rates greater than unity are compensated for by cycles with thermal ageing rates less than unity.
3.6
long-time emergency loading
loading resulting from the prolonged outage of some system elements that will not be reconnected before the transformer reaches a new and higher steady-state temperature
3.7
short-time emergency loading
unusually heavy loading of a transient nature (less than 30min) due to the occurrence of one or more unlikely events which seriously disturb normal system loading
3.8
hot-spot
if not specially defined, hottest spot of the windings
3.9
relative thermal ageing rate
for a given hot-spot temperature, rate at which transformer insulation ageing is reduced or accelerated compared with the ageing rate at a reference hot-spot temperature
3.10
transformer insulation life
total time between the initial state for which the insulation is considered new and the final state for which the insulation is considered deteriorated due to thermal ageing, dielectric stress, short-circuit stress, or mechanical movement (which could occur in normal service), and at which a high risk of electrical failure exists
3.11
percent loss of life
equivalent ageing in hours over a time period (usually 24h) times 100 divided by the expected transformer insulation life
Note: The equivalent ageing in hours is obtained by multiplying the relative ageing rate with the number of hours.
3.12
non-thermally upgraded paper
kraft paper produced from unbleached softwood pulp under the sulphate process without addition of stabilizers
3.13
thermally upgraded paper
cellulose-based paper which has been chemically modified to reduce the rate at which the paper decomposes
Note: Ageing effects are reduced either by partial elimination of water forming agents (as in cyanoethylation) or by inhibiting the formation of water through the use of stabilizing agents (as in amine addition, dicyandiamide). A paper is considered as thermally upgraded if it meets the life criteria defined in IEEE Std C57.100 [8][Numbers in square brackets refer to the bibliography.]: 50% retention in tensile strength after 65 000h in a sealed tube at 110°C or any other time/temperature combination given by the equation:
(1)
Because the thermal upgrading chemicals used today contain nitrogen, which is not present in kraft pulp, the degree of chemical modification is determined by testing for the amount of nitrogen present in the treated paper. Typical values for nitrogen content of thermally upgraded papers are between 1% and 4% when measured in accordance with ASTM D-982 [2], but after the sealed tube test.
3.14
non-directed forced-oil flow
OF
flow indicating that the pumped oil from heat exchangers or radiators flows freely inside the tank, and is not forced to flow through the windings
Note: The oil flow inside the windings can be either axial in vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow.
3.15
non-directed natural-oil flow
ON
flow indicating that the oil from the heat exchangers or radiators flows freely inside the tank and is not forced to flow through the windings
Note: The oil flow inside the windings can be either axial in vertical cooling ducts or radial in horizontal cooling ducts with or without zigzag flow.
3.16
directed oil flow
OD
flow indicating that the principal part of the pumped oil from heat exchangers or radiators is forced to flow through the windings
Note: The oil flow inside the windings can be either axial in vertical cooling ducts or zigzag in horizontal cooling ducts.
3.17
design ambient temperature
temperature at which the permissible average winding and top-oil and hot-spot temperature over ambient temperature are defined
4 Symbols and abbreviations
For the purpose of this document, the following symbols and abbreviations apply.
5 Effect of loading beyond nameplate rating
5.1 General
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 document 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 [11], [12], [13].
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.
