Above 1,000 m the air is less dense and IEC 61869-1 corrects two things on an instrument transformer: the arcing distance of the external insulation, which is multiplied by a factor k, and the temperature-rise limits, which drop 0.4% or 0.5% per 100 m. The internal insulation does not change.
Short answer
The higher the altitude, the less dense the air and the less voltage it withstands before flashing over. IEC 61869-1, 2007 edition, takes 1,000 m as the normal altitude (clause 4.2.2) and corrects two things when the site is higher: the arcing distance of the external insulation (clause 6.6.2) and the temperature-rise limit (clause 6.4.2). This correction is part of the special service conditions of clause 4.3.2.
External insulation: the k factor
For a site above 1,000 m, the arcing distance, measured under standard atmosphere conditions, is determined by multiplying the withstand voltages required at the site by a factor k (clause 6.6.2).
k = e^( m · (H - 1,000) / 8,150 )- H is the site altitude in meters.
- m = 1 for power-frequency voltage and for lightning impulse.
- m = 0.75 for switching impulse.
Figure 2 of the standard plots k from 1,000 to 4,000 m. The formula gives these values:
| Altitude (m) | k with m = 1 | k with m = 0.75 |
|---|---|---|
| 1,000 | 1.000 | 1.000 |
| 1,500 | 1.063 | 1.047 |
| 2,000 | 1.131 | 1.096 |
| 2,500 | 1.202 | 1.148 |
| 3,000 | 1.278 | 1.202 |
| 3,500 | 1.359 | 1.259 |
| 4,000 | 1.445 | 1.318 |
Internal insulation: no change
The standard warns that the dielectric strength of internal insulation is not affected by altitude. That is why the correction is only for the external insulation. The method to verify the external insulation must be agreed between the manufacturer and the purchaser.
Another option: a higher Um
Clause 5.2 gives an example. If the equipment is installed above 1,000 m, a Um higher than the next standard value equal to or above the highest system voltage can be chosen. This compensates for the drop in withstand voltage of the external insulation.
Temperature rise: lower limits
The thermal behavior of the equipment also changes with air density (clause 4.3.2.2). Clause 6.4.2 applies when the transformer is specified for service above 1,000 m but tested below 1,000 m. In that case, the temperature-rise limits of Table 5 are reduced for every 100 m the site exceeds 1,000 m:
| Transformer type | Reduction per 100 m |
|---|---|
| Oil-immersed | 0.4% |
| Dry-type and gas-insulated | 0.5% |
Figure 1 of the standard gives the temperature-rise correction factor K₀, which is the ratio between the temperature rise allowed at the site altitude and the Table 5 limit.
Example at 2,600 m
An instrument transformer will be installed at 2,600 m. That is 1,600 m above 1,000 m, that is, 16 steps of 100 m.
- External insulation. The exponent of the formula is 1,600 / 8,150 = 0.196. With m = 1, k = 1.217. With m = 0.75, k = 1.159. If the lightning impulse voltage required at the site is 325 kV, the arcing distance must be sized for 325 × 1.217 = 395 kV at reference conditions.
- Temperature rise, oil-immersed transformer. The reduction is 16 × 0.4% = 6.4%. A limit of 60 K drops to 56.2 K.
- Temperature rise, dry-type or gas-insulated transformer. The reduction is 16 × 0.5% = 8%. A limit of 60 K drops to 55.2 K.
The reduction applies to each Table 5 limit. With the same altitude of 2,600 m, some examples are these:
| Part (Table 5) | Limit at 1,000 m | Reduction | Limit at 2,600 m |
|---|---|---|---|
| Top oil, oil-immersed | 50 K | 6.4% | 46.8 K |
| Average winding, oil-immersed | 60 K | 6.4% | 56.2 K |
| Dry-type or gas-insulated winding, insulation class B | 85 K | 8% | 78.2 K |
| Dry-type or gas-insulated winding, insulation class F | 110 K | 8% | 101.2 K |
These calculations are an arithmetic example of the rule in the standard. For your equipment, use the values of your specification and of your Um.
- Is the site above 1,000 m?
- No: No correction: normal conditions of IEC 61869-1
- Yes: Is the transformer tested below 1,000 m?
- Yes: Correct the temperature rise (0.4% or 0.5% per 100 m) and the arcing distance with k
- No: Correct the arcing distance with k; the temperature-rise rule of clause 6.4.2 does not apply
What not to do
- Do not apply the k factor to the internal insulation.
- Do not use m = 1 for switching impulse: the standard sets m = 0.75.
- Do not forget to state the altitude in the purchase specification. It is a special service condition.
- Do not apply the temperature-rise reduction if the test was not made below 1,000 m: clause 6.4.2 covers only that case.
What data to give the manufacturer
For the manufacturer to apply the corrections, the specification must include:
- The altitude of the installation site, in meters above sea level.
- The highest system voltage, Usys, and the chosen Um.
- The required insulation level, with the power-frequency, lightning impulse and, if applicable, switching withstand voltages.
- The insulation type of the equipment: oil-immersed, dry-type or gas-insulated.
- The agreement on the method to verify the external insulation.
Related topics
Altitude is one of the special service conditions of IEC 61869-1, along with the other service conditions. It affects the insulation level, the temperature-rise limits and the creepage distance of the insulator. If the transformer is verified on site, see the on-site withstand voltage test.
When to call us
At Tiza Group we define the Um, insulation level and service conditions of your instrument transformers with you for high-altitude sites. Learn about our primary engineering design service. Write to us and we will quote it.
Related guides
- Service conditions of an instrument transformer
- Um and insulation level of an instrument transformer
- Temperature-rise limits of an instrument transformer
- On-site withstand voltage test: AC and VLF
- High-voltage current transformer (CT): types
References
- IEC 61869-1, Instrument transformers - Part 1: General requirements, 2007 edition, clauses 4.2.2, 4.3.2, 5.2, 6.4.2 (Figure 1) and 6.6.2 (Figure 2).
Frequently asked questions
How is the altitude factor k of an instrument transformer calculated?
With k = e^(m·(H - 1,000)/8,150), where H is the altitude in meters and m is 1 for power-frequency voltage and lightning impulse, and 0.75 for switching impulse. At 2,600 m, k is 1.217 with m = 1 and 1.159 with m = 0.75.
Does altitude affect the internal insulation of the transformer?
No. The standard states that the dielectric strength of internal insulation does not depend on altitude. That is why only the external insulation is corrected, and the verification method is agreed between the manufacturer and the purchaser.
Can the correction be avoided by choosing a higher Um?
The standard gives this as an example in clause 5.2: for equipment above 1,000 m, a Um higher than the next standard value may be chosen to compensate for the lower withstand voltage of the external insulation.