IEC 60076-7 measures insulation aging with a relative aging rate that depends only on the hot-spot temperature. For normal paper it equals 1.0 at 98 °C and doubles every 6 K; for thermally upgraded paper it equals 1.0 at 110 °C. Loss of life is obtained by adding up that rate over time.
What is measured
The solid insulation of a transformer is cellulose, in the form of paper. It degrades over time. As its mechanical properties get worse, the effective life of the transformer can be reduced, especially if it suffers short circuits (clause 5.4).
IEC 60076-7 does not define a single end-of-life criterion (clause 6.1). It therefore focuses on the aging process of the insulation and on its condition.
The standard model uses one control parameter: the insulation temperature (clause 6.2). Temperature is not uniform across the winding. The hottest part ages the most, so aging is referred to the hot-spot temperature.
The relative aging rate
The relative aging rate V compares aging at a given temperature with the aging that occurs at a reference temperature (clause 3.9).
- Non-thermally-upgraded paper: V = 1.0 at a hot spot of 98 °C.
- Thermally upgraded paper: V = 1.0 at 110 °C.
Thermally upgraded paper is cellulose paper chemically modified so that it breaks down more slowly (clause 3.12). The standard recognizes it as upgraded if it meets the life criterion of ANSI/IEEE C57.100: keeping 50% of tensile strength after 65,000 hours in a sealed tube at 110 °C.
Table 2 of clause 6.2 shows how sensitive V is to temperature:
| Hot-spot | V, non-upgraded paper | V, upgraded paper |
|---|---|---|
| 80 °C | 0.125 | 0.036 |
| 92 °C | 0.5 | 0.145 |
| 98 °C | 1.0 | 0.282 |
| 104 °C | 2.0 | 0.536 |
| 110 °C | 4.0 | 1.0 |
| 116 °C | 8.0 | 1.83 |
| 128 °C | 32 | 5.8 |
| 140 °C | 128 | 17.2 |
In non-upgraded paper, V doubles every 6 K. In upgraded paper, the increase per 6 K is somewhat smaller, but still fast: from 110 to 116 °C it goes from 1.0 to 1.83.
The standard sums it up with another fact: a hot spot only 6 °C below the reference value gives half the rated loss of life (clause 6.4).
How loss of life is calculated
The loss of life L over a period is the sum of the relative aging over time (clause 6.3). The period is divided into intervals, V is taken for each one according to its hot spot, and V multiplied by the duration of each interval is added up.
The percentage loss of life (clause 3.11) is the equivalent aging, in hours, over a period (normally 24 h), multiplied by 100 and divided by the expected life of the insulation. The equivalent aging in hours is V multiplied by the hours.
An example with non-upgraded paper, over 24 h:
- 12 h with the hot spot at 98 °C: V = 1.0, that is, 12 h of aging.
- 12 h with the hot spot at 104 °C: V = 2.0, that is, 24 h of aging.
- Total: 36 h of aging in 24 h. The transformer aged 1.5 times faster than at a constant 98 °C.
The normal cyclic loading of the standard (clause 3.5) is based on this idea: a period of high hot-spot temperature is offset by another of low hot-spot temperature, so that average aging equals the reference condition.
The life of the insulation
Table 3 lists four end-of-life criteria for a thermally upgraded insulation system, well dried and oxygen-free, at 110 °C:
| Basis of the criterion | Hours | Years |
|---|---|---|
| 50% retained tensile strength | 65,000 | 7.42 |
| 25% retained tensile strength | 135,000 | 15.41 |
| Retained degree of polymerization of 200 | 150,000 | 17.12 |
| Data from a functional life test of distribution transformers | 180,000 | 20.55 |
The standard clarifies that these are reference values. Most power transformers run well below full load almost all their life. A hot spot only 6 °C lower than the reference gives half the aging. That is why the real life of the insulation can be several times longer than the table values.
There is an exception: step-up transformers connected to base-load generators and others with constant load or an almost constant ambient need special consideration of their real life (note to clause 6.4).
Overload: three regimes and their limits
The standard (2005 edition) separates three ways of loading a transformer above its nameplate rating. Each accepts a different risk (clauses 3.5 to 3.7 and 5).
- Normal cyclic loading. During part of the cycle (normally 24 h) the current or the ambient temperature exceeds the design value. The rest of the cycle makes up for it with low load or cold ambient. The total ageing is equivalent to that of rated load at normal ambient temperature.
- Long-time emergency loading. It results from the prolonged outage of other system elements. It can last weeks or months and cause considerable ageing (clause 5.4).
- Short-time emergency loading. It is a very high, transient load of less than 30 min, caused by one or several unlikely events (clause 3.7). It takes the hot spot to a level that can temporarily reduce the dielectric strength. The standard accepts it only when it is preferable to losing supply (clause 5.3).
Table 4 sets the limits according to transformer size. The standard defines a distribution transformer as up to 2,500 kVA three-phase, a medium power transformer as up to 100 MVA three-phase and a large power transformer as above 100 MVA (clauses 3.1 to 3.3).
| Regime | Current (per unit of rated): distribution / medium / large | Winding hot-spot (°C) | Top oil (°C) |
|---|---|---|---|
| Normal cyclic | 1.5 / 1.5 / 1.3 | 120 | 105 |
| Long-time emergency | 1.8 / 1.5 / 1.3 | 140 | 115 |
| Short-time emergency | 2.0 / 1.8 / 1.5 | 160 (medium and large) | 115 (medium and large) |
Two clarifications on the table:
- The limits do not apply at the same time. The current may have to stay below the table value to respect the temperature, and the other way around (note to Table 4).
- For distribution transformers there is no temperature limit in short-time emergency, because it is usually impossible to control its duration (clause 7.2.1).
Above 140 °C at the hot spot, gas bubbles can appear that reduce the dielectric strength. That figure corresponds to insulation with about 2 % moisture and falls as moisture rises (clause 5.3). Bare metallic parts in contact with oil, aramid or fibreglass must not exceed 180 °C.
The windings are not the only limit. The standard warns that bushings, cable connections, the tap changer and internal conductors can restrict operation above 1.5 times rated current (clauses 7.2.2 and 7.3.2). In large transformers the leakage flux adds heating in metallic parts, and a temperature-rise test at rated current allows no conclusion about higher currents (clause 7.4.1). That is why the standard recommends stating in the enquiry or order how much overload capability is needed.
For whoever operates the network, the consequence is this: what uses up insulation life is the hot-spot temperature, not the current itself. An overload with cold ambient can age the insulation less than rated load with hot ambient.
What the model does not include
The standard model uses temperature as the only parameter. Clause 6.2 itself says that aging depends on time and temperature, moisture, oxygen and acidity.
Table 1 of clause 5.6 shows the effect of the medium. For wood-pulp paper at 98 °C, life is 15 years dry and air-free, and 0.8 years with air and 2% moisture. At 90 °C it goes from 38 to 1.9 years. The standard warns that the figures and tables in that clause are not intended to calculate aging or estimate life; they only show that upgraded and non-upgraded paper behave differently.
This explains why oil and paper moisture analysis accompanies any life calculation. See the guide on dielectric oil analysis.
Scope of the calculation
The model does not allow the life of a specific transformer to be predicted. That prediction needs the real hot-spot temperature over years, the paper type, the moisture and the condition of the oil. The Table 3 values are references from the standard, not a guarantee.
We can help with the study of your transformer
Our team evaluates plant transformers: we apply the loading guide with your equipment data, analyze the load measurements and tell you which options are worth considering. See engineering design and studies, delivered remotely and worldwide, and power transformer testing, in Colombia, or elsewhere on request, or contact us with the nameplate and the load history.
Related guides
- Dielectric oil analysis
- Elevated dissolved gases in the transformer
- Transformer online monitoring
- Transformer hot-spot temperature
- Loading a transformer beyond its nameplate rating
References
- IEC 60076-7, Power transformers – Part 7: Loading guide for oil-immersed power transformers, clauses 3.9, 3.11, 3.12, 5.6 and 6, Tables 1 to 3.
- ANSI/IEEE C57.100 (cited by the standard to define thermally upgraded paper).
Frequently asked questions
How many years does transformer insulation last?
The standard gives no single life. There is no simple, single end-of-life criterion (clause 6.1). Table 3 of IEC 60076-7 gives four reference lives, from 65,000 to 180,000 hours (7.42 to 20.55 years) for thermally upgraded paper, dry and oxygen-free, at 110 °C. They are reference values, and the real life of most transformers is several times longer because they operate below full load.
What happens if the hot spot rises 6 K above the reference value?
With non-upgraded paper, aging doubles. If it drops 6 K, it falls by half. Table 2 of IEC 60076-7 shows it: 1.0 at 98 °C, 2.0 at 104 °C, 4.0 at 110 °C and 128 at 140 °C.
Does moisture influence aging?
Yes, in reality. The standard model uses only temperature as the control parameter (clause 6.2), but the standard itself acknowledges that aging also depends on moisture, oxygen and acidity. Table 1 shows that non-upgraded paper at 98 °C lasts 15 years dry and air-free, and 0.8 years with air and 2% moisture.
How much current can a transformer carry above its nameplate?
It depends on the size and the loading regime. Table 4 of IEC 60076-7 (2005 edition) gives 1.5 times rated current for normal cyclic loading for distribution and medium power transformers, and 1.3 for large ones (above 100 MVA). Those values must also respect the temperature limits, which govern when they are stricter.
What hot-spot temperature is dangerous in an emergency?
The standard limits the hot spot to 120 °C for normal cyclic loading, 140 °C for long-time emergency and 160 °C for short-time emergency of medium and large transformers. Above 140 °C gas bubbles can form and lower the dielectric strength (clause 5.3).