VFD dimensioning: High altitude

High altitude

Majority of variable frequency drive (VFD) systems is installed at altitudes below 1’000 meters above sea level (masl). In such cases no additional considerations due to installation height apply. However, there are sites located at high altitudes. How does it impact the VFD and other drive components? What is the impact of high altitude on the drive components? Are there any design changes required? This article gives a small insight.

In our article about derating and uprating we listed several factors contributing to reduced or increased power rating of VFD [1]. Altitude is another factor having impact on VFD design.

The industrial plants that utilize VFD system are seldom at altitudes exceeding 1’000 masl. Such installations are fully covered by international standards such as IEC and generally do not require any special consideration in this perspective. However, there are also sites with installation altitude by far exceeding 1’000 masl. In this article we simply call them ‘high altitude’ installations.

Important part of the economy of many South American countries is the mining industry. For example Chile and Peru have large deposits of copper – an essential resource for electric equipment and apparatus. According to [2] the top 10 largest copper mines in the world are:

– Escondida (Northern Chile, opened since 1990, world’s largest copper mine by output)

– Cerro Verde II (Peru, second largest copper mine in Peru based on reserves)

– Collahuasi (Chile, opened since 1999, second largest copper mine in Chile)

– Antamina (Peru, scheduled to cease production in 2019)

– Las Bambas (Peru, possible largest copper mine in the future)

– El Teniente (Chile, world’s largest underground mine)

Note that six out of world’s top ten copper mines are located in Chile and Peru. They have one thing in common: high or very high altitude. VFDs are used in these mines for variable speed mills, pumps, conveyors or ventilation fans.

What are the technical challenges associated with high altitude?

Impact of high altitude on VFD

High altitude does not impact the human; it also impacts the electric equipment. VFDs and other drive components are not an exception. For the VFD there are three main areas where altitude has an impact: cooling, dielectric insulation and cosmic ray. Let’s look into each category.

a. Reduced cooling

With increasing altitude the air pressure and density are both being reduced (air density is a function of temperature and pressure). Lower density in turn means lower cooling capacity. In order to compensate this effect derating rules for altitudes above 1’000 masl are defined in standards and in installation manuals of manufacturers.

Note that at higher altitude the maximum air temperature is probably reduced as well (i.e. 40°C as per IEC will likely never be reached). The lower air temperature can principally be used to offset the lower specific thermal capacity. However, this approach is seldom used and more conservative derating rules are used instead.

Liquid cooled VFDs are less affected in terms of cooling. The liquid cooling system is typically closed-loop type and pressurized so the altitude hardly has any thermal impact. This surely holds for main power part. If auxiliary air-cooled systems are used inside the VFD then these components shall be checked separately.

b. Insulation coordination

Air with lower density has reduced dielectric strength. Since many VFD internal components are air insulated (e.g. busbars) the impact on insulation shall be checked. The manufacturers have internal rules which product is available up to which installation altitude. VFD system voltage might be reduced as one way to handle the insulation problem. For instance a standard VFD with 6 kV output can be used up to certain altitude without any modifications. Beyond that altitude the system voltage needs to be reduced. Feasible output voltage is correspondingly lowered as well, e.g. to 5.5 kV. Sometimes the manufacturer reduces the output voltage to next lower ‘standard’ level, e.g. 4.16 kV. This voltage derating usually means power derating as well. The maximum current loadability is the same as for installations below 1’000 masl (or even reduced as described above). The power reduction is therefore almost directly proportional to the voltage derating.

c. Cosmic ray / semiconductor FIT rate

Cosmic ray may sound a bit sci-fi for some readers. However, this is a phenomenon well-known for several decades. An excellent explanation is provided in this webinar from ABB Power Grids – Semiconductors: [5]. If you are interested in high altitude installations of VFDs or other power electronic based equipment, you should definitely watch that webinar. The experts from ABB Powr Grids – Semiconductors guide us in the webinar through the history of cosmic ray influence on power electronics, the origin of cosmic ray (high energy cosmic particles formed outside of our solar system), the failure mechanism of semiconductor caused by cosmic ray, the way how the semiconductors are tested and importantly how the ruggedness of power semiconductors can be ensured.

– FIT rate of semiconductors

Failure rate is commonly expressed as FIT = failure in time. FIT tells the number of failures within 10^9 element hours.

ABB developed mathematical models to consider cosmic ray. There are three most relevant factors for FIT:

– blocking voltage

– junction temperature

– altitude

Failure rate can be generally expressed as function of above main factors: λ(Vdc, Tvj, A)

Example of 6.5 kV HiPak IGBT as part of traction converter: λ(4’000 V, 80°C, 4’000 masl)

Without any measure the mean time between failure (MTBF) at 4’000 masl altitude would be reduced by factor 15 compared to operation at sea level.

Efficient measure to reduce FIT rate and increase the MTBF is to reduce the dc link voltage. As example, reduction of dc link voltage from 4’000 V to 3’700 V (-7.5%) increases the MTBF of semiconductor by factor 20 to 30!

Since the dc voltage might need to be reduced also for insulation coordination as explained in previous paragraph, it is a very effective measure to minimize failure probability.

All ABB semiconductors are tested for their ruggedness against cosmic ray and the effect of cosmic ray is considered since early design stage of a new product.

Impact of high altitude on transformers

Temperature rise

The IEC 60076-2 defines reduction of temperature rise for liquid filled transformers (top liquid and winding temperature rise):

– 1 K per each 400 m above 1’000 masl for natural cooling (ONAN, KNAN)

– 1 K per each 250 m above 1’000 masl for forced cooling (ONAF)

The IEC 60076-11 defines a reduction of temperature rise for dry type transformers. For altitudes exceeding 1’000 masl the temperature rise shall be reduced by applying power derating for each additional 500 m:

– 2.5% for natural-air-cooled transformers (AN)

– 5% for forced-air-cooled transformers (AF)

Impact of high altitude on motors

The impact of high altitude on motors is physically very similar like on transformers. The installation altitude influences the cooling and the insulation coordination.

Since the design insulation rules are less standardized and each manufacturer applies his own best practice, the manufacturer shall be consulted to confirm suitability for operation at high altitude.

Experience with drives at high altitude

ABB has a vast experience with drive installations at high and very high altitude:

Chile

– Collahuasi (4’200 – 4’700 masl)

– Caserones (3’000 – 4’000 masl)

– Escondida (3’200 masl)

Peru

– Toromocho (4’600 masl)

– Antamina (4’300 masl)

– Las Bambas (4’000 masl)

Medium voltage VFDs operate there successfully driving large SAG and ball mills, pumps, conveyors or fans.

Figure 4 shows a typical gearless mill drive (GMD) featuring so called ring motor (introduced in 1969 by ABB): the rotor poles are fixed directly to the mill shell and the stator is wrapped around it.

Two basic design types of ring motors are depicted in figure 5.

Figure 6 provides a look at the Collahuasi mine in Chile where several drive systems, among others large conveyors, operate at altitude close to 5’000 m above sea level.

Summary

There are several aspects to consider when dimensioning a drive system that shall operate at high altitude. Impact on cooling, air insulation and also the effect of cosmic ray on semiconductors shall be evaluated. Renowned manufacturers, such as ABB, test their equipment thoroughly and have clear proven design rules for installations at high altitude.