Table of Contents
High Altitude Application – Design Needs
All the design practices which are more frequent and widely used are for standard service conditions. But here we will be discussing design aspects for High Altitude Application for MV Switchboards. With increased developmental activities in hilly areas, the need for studying the behavior of electrical equipments has become of paramount importance.
Design Considerations
The relevant standard for MV Switchboard Design above 1 kV and up to 52 kV is IEC 62271-200. This standard has laid down certain standard service conditions for design of Switchboard. These service conditions are like maximum average ambient temperature +35⁰ C and a lower limit of -5⁰ C for indoor installation, maximum humidity of 95% for 24hrs and an altitude of ≤1000 meter. Other parameters like pollution level, air pressure can also be referred from IEC 62271-1. These service conditions are being called normal service conditions which does not cover High Altitude Application.
Service conditions for high altitude application are defined as special service conditions by the IEC. Here we will focus on special service conditions especially for MV Switchboard’s Design aspects for High Altitude Application to be more precise. We can take an example of Leh in India which is situated at an altitude of 3500 Meter (11562 ft) and temperature varies between +30⁰ C to -15⁰ C which is not a normal operating condition as per IEC.
The design and application of electrical systems at elevations in excess of 1000 meter (3000 ft) requires knowledge about effects of atmospheric condition on each component. Relative air density of air decreases by 1% for every 100M above mean sea level. Failure to understand the phenomenon and effects of high altitude in the design and application of the equipment may result in its poor performance, premature aging, and/or failure.
But what changes at high altitude which impacts the life and performance of the MV Switchboard? There are four major atmospheric condition viz relative air density, atmospheric pressure, temperature and relative humidity which impact the switchboard design and performance.
For special service conditions IEC advises for use of correction factor as detailed below.
Impact of Altitude on Current Carrying capacity
Temperature and barometric pressure together determines the relative air density. The relative air density decreases with an increase in Altitude. Lower air density reduces its convective capability and overall heat transfer capacity therefore heat transfer through air at high altitude will be lower compared to air below an altitude of 1000 meter. All air insulated switchboards are mainly dependent on air for heat removal. That means current carrying capacity of the system will decrease on account of reduced heat transfer, so need to apply correction for current rating of the system.
As per IEC 60071-2 we need to apply current correction factor as given below:
Current correction factor = 1-0.02*(H-1000)/1000 for system voltage more than 1KV
Where H is altitude over mean sea level in meter
Impact of Altitude on Insulation Level
We all know that Air is a good insulator and vacuum is even better, so the analogy is that, thinner air is better insulator but the fallacy is, it’s not. As per Paschen’s law dielectric properties of the air changes with altitude. It proved that at higher altitude air is not good as an insulator until it reaches a good vacuum. So creepage and clearances also needs to be taken care of, on account of reduced insulation level. Due to reduction in insulation properties of air, as per IEC 60071-2, we need to apply voltage correction factor.
Voltage correction factor = em(H-1000)/8150 for system voltage more than 1KV
Where H – Altitude above mean sea level
m – 1 for industrial frequency voltages supported by clean insulators
So we need to apply suitable corrections for current and voltage at high altitude as stated above. The best preferred solution at high altitude for medium & high voltage application is Gas or Solid Insulated Switchgear (SIS). In India “Central Electricity Authority” also recommends for Gas or Solid Insulated switchgear for high altitude application.
References
https://engineershangout.com/solid-insulation-switchgear-technology/
All switchboard engineers are invited to put their thought on this topic. You can also post your queries which I will try to answer. Even you can put new perspective to the subject.