Transformer Design Scheme for High-Temperature Desert Environments | Meeting The Stringent Requirements of IEC 60076 And 55℃ Middle East

I am an engineer from the transformer design team of Zisheng Electric, with many years of experience in designing transformers for special environments. Today, I would like to have a practical discussion with our Middle East customers and friends — in that kind of desert heat, how should transformers actually be designed?

1. Why Does the Desert High-Temperature Environment Require Special Transformer Design?

Let’s first look at the data.

In Saudi Arabia’s Eastern Province, the extreme maximum air temperature has reached 53.5℃. The ground surface temperature? 79.8℃. Abu Dhabi in the UAE is slightly better, with 52.8℃ air temperature and 76.5℃ ground surface temperature. Kuwait and Qatar are similar. The annual number of sandstorm days is 28 days in Saudi Arabia and 35 days in Kuwait. The maximum daily temperature difference can reach 28 to 30℃.

The following data are based on publicly available meteorological information, as well as the actual site environmental conditions collected from multiple Middle East projects we have participated in.

Looking at these figures, it becomes clear that transformers designed for normal environmental conditions may experience insufficient temperature rise margins and accelerated insulation aging under continuous high temperatures and heavy dust conditions.

2. The Impact of a 55℃ Ambient Temperature on Transformers

This is not the end of the story. When the air temperature rises, the air density decreases. At 45℃, the air density is about 8% lower than at 20℃. No matter how powerful the radiator is, heat cannot be effectively carried away when the air becomes thinner. In addition, when sand and dust accumulate on the cooling fins, the heat dissipation efficiency will continue to decrease.

Conventional nitrile rubber sealing components may experience hardening, aging, and reduced sealing performance under long-term exposure to high temperatures, ultraviolet radiation, and sand and dust environments. Oil leakage may occur, the oil level may drop, and exposed windings may suffer from moisture ingress and oxidation. How many more years can such equipment continue to operate?

Then there is the issue of sand and dust. Conductive dust can accumulate on the surface of insulation components. Under dry conditions, the impact may be limited, but when condensation occurs in the morning and evening, surface discharge may develop. In severe cases, bushing flashover can occur, resulting in direct tripping.

Therefore, it is not enough to simply transport a standard transformer to the desert environment and put it into operation. The design requires major modifications.

3. IEC 60076-7 Thermal Aging and Hotspot Temperature Control

IEC 60076-7 introduces the six-degree rule. What does this mean? According to the IEC 60076-7 thermal aging model, insulation life decreases significantly as the winding hotspot temperature increases. In engineering practice, an empirical relationship of approximately an 8K temperature increase corresponding to a halving of insulation life is commonly used for evaluation. Under a 55℃ ambient temperature, the winding hotspot temperature may quickly approach 110℃ or even higher. If the hotspot temperature remains above the design limit for a long time, insulation aging will accelerate significantly.

4. Cooling Design in Middle East Desert Environments

First, let’s talk about heat dissipation area.

In conventional designs, 0.8 to 1.2 square meters of heat dissipation area per kilowatt of loss is typically used, which is suitable for temperate climates. For Middle East operating conditions, we directly increase this to 1.8 to 2.3 square meters per kilowatt.

Let’s look at an actual calculation. A substation project in Qatar used a 20,000kVA, 132kV transformer. The total loss was 142kW. The ambient temperature was considered as 50℃, and the allowable top oil temperature was set at 95℃ — rather than operating close to 105℃, leaving additional design margin.

Required heat dissipation area:
S = 142000 / (10 × 45) ≈ 316 m²

The final design used 338 m², providing an additional 7% margin. The radiator width was increased from 480mm to 600mm, and the spacing between fins was increased from 50mm to 70mm. Why? Because sand and dust are less likely to accumulate in the gaps, and airflow can easily remove them.

The oil tank uses a fully sealed corrugated wall design. The corrugated wall provides heat dissipation and also absorbs the thermal expansion and contraction of the oil through the elastic deformation of the corrugated plates. There is no need for a breather to draw outside air.

In this way, the insulating oil is isolated from the external environment. Moisture cannot enter, sand and dust cannot enter, and the oil quality can remain stable for more than ten years.

5. Cooling Method Selection: ONAN, ONAF, OFAF

For the cooling method, I will provide a direct reference table.

An Omani customer had a 12.5MVA transformer. Initially, they wanted to save the cost of installing fans. I asked them to calculate the actual operating impact. During the hottest three months, without fans, the top oil temperature was 13 to 15 degrees higher than when the fans were operating. According to the IEC aging model, long-term operation at elevated temperatures significantly increases insulation aging speed and reduces the design life margin of the equipment. Later, they modified the design, and the transformer has been operating for more than four years without any issues.

6. Sand, Dust, Moisture Protection and Sealing Design

Sand and dust protection — sealing strips are only the basic measure.

All external interfaces should be equipped with dust protection covers. The louver-type covers installed outside radiators and fans are designed with a 35° inclination angle. This angle has been verified through CFD analysis. It blocks sand while keeping airflow loss within 6%. The measured interception efficiency reaches 92%.

The pressure relief valve should also be equipped with a dust cap. Why? Because sand and dust may accumulate on the valve disc, preventing it from opening properly when pressure relief is actually required. As the internal pressure of the oil tank rises, the consequences can range from deformation to, in severe cases, rupture.

The dual-layer structure we developed consists of an outer stainless-steel 200-mesh screen. In a sandstorm environment, the mesh openings may gradually become blocked by fine sand; over time, the effective exhaust flow area continues to decrease, causing the pressure backpressure during fault pressure relief to gradually rise. The inner PTFE membrane blocks fine particles, while a 5mm cavity is reserved between the layers as a buffer zone.

The creepage distance of bushings should be increased. A value of 4100mm is slightly below the theoretical minimum value of 4495mm for IEC Class IV pollution level. It is a reinforced configuration approaching Class IV requirements, but it does not fully meet the complete IEC Class IV standard configuration. Therefore, tender documents should not directly state “meets Class IV pollution level”; instead, they should state: “Designed according to IEC Class IV pollution principles, with a creepage distance ratio not less than 28.3mm/kV.” An additional 800mm was added. Although the bushing height increased by 150mm, even under severe sandstorms, the surface flashover voltage can still withstand the harsh conditions.

For sealing components, fluororubber is used instead of nitrile rubber.

Fluororubber costs three times more than nitrile rubber. Is it worth it? Look at Middle East projects that have used nitrile seals for five years. Once spring arrives, oil leaks from cover joints like sweating. Maintenance workers spend all day cleaning oil. When labor costs, oil losses, and power outage losses are combined, the additional cost difference has already been recovered.

7. Selection of Insulation Materials and Insulating Oil

For insulation thermal class, we directly start from Class F, 155℃. For large-capacity transformers, we use Class H, 180℃. For projects with a 55℃ ambient temperature, higher thermal-class insulation is usually required to increase the hotspot temperature margin.

A seawater desalination project in Saudi Arabia had two 37.5MVA transformers, and the owner specified Class H insulation. Nomex aramid paper was used. It was expensive. But the cost of one day of downtime there was three million US dollars. Was this investment worth it?

The selection of insulating oil depends on the voltage level.

For transformers of 35kV and below, mineral insulating oil is used. ASTM D3487 Type II inhibited mineral oil is selected, offering excellent oxidation stability; the open cup flash point is not less than 155℃. The main difference between Type I and Type II is the content of antioxidant additives. Type II is more suitable for long-term operation in desert high-temperature environments, helping to slow down sludge formation and acid value increase.

For 66kV and above transformers, where budget allows and fire safety and environmental requirements are higher, natural ester insulating oil (IEC 62770) can be selected. It is classified as a Class K less-flammable fluid according to IEC 61100, with a fire point not lower than 300℃. Natural ester has much higher water solubility than mineral oil, which helps delay thermal aging of cellulose insulation and provides excellent biodegradability.

For critical loads — hospitals, airports, and data centers — synthetic ester is the most reliable choice. It is simply more expensive.

The magnetic flux density is controlled below 1.55T. Some designs push it up to 1.7T. We take a more conservative approach. Reducing the magnetic flux density increases core weight by approximately 7–12%, meaning several tens of thousands of yuan more in silicon steel cost. However, this core operates several degrees cooler than a high-flux-density design under 50℃ ambient conditions. A cooler core also benefits the surrounding windings.

8. Factory Testing and Field Reliability Verification

Actual measured data from a 20,000kVA/132kV desert transformer:

Key points during factory supervision:

Ensure single-point grounding of the transformer core and check whether the air gaps are uniform. Some manufacturers rush production schedules and cut corners. Local overheating of the core may reach more than 70℃, and once the transformer leaves the factory, it cannot be corrected.

Vacuum drying should last more than 72 hours, with the water discharge rate controlled below 0.3%. For units completed after only 48 hours of drying, moisture may not be fully removed. After commissioning and heating, the moisture can be released again, reducing insulation strength by one level.

During oil filling, maintain a vacuum level below 50Pa for 12 hours before starting. The oil filling speed should not exceed 1.5 tons per hour. If the speed is too high, a vacuum leakage rate test should be performed to prevent gas leakage in the system. Checking only the vacuum level without checking leakage rate can lead to misleading results. After filling, allow the transformer to stand for 72 hours before conducting oil analysis tests.

The breakdown voltage should not be lower than 60kV/2.5mm.

The previous statement “dielectric loss tanδ≤0.3%” is incorrect: for new mineral oil at 90℃, the standard limit for dielectric dissipation factor is tanδ≤0.005 (0.5%? No, it is 0.5‰, 0.005). A value of 0.3% (0.003) is already close to the level of aged operating oil; new oil should perform much better than this.

Additional Special Tests (Not Mandatory Requirements of IEC 60076, but Customized Special Tests for This Desert Project)

(1) High-Temperature Long-Term Simulated Load Test

The transformer, including the complete radiator and cooling system, is placed inside an environmental test chamber. The inlet air temperature inside the chamber is maintained at ≥45℃, and the transformer is operated continuously at 1.1 times the rated load for 72 hours.

During the entire test process, the top oil temperature and winding hotspot temperature are continuously monitored. The hotspot protection limit is set at no more than 120℃, and the test shall be terminated if the limit is exceeded.

Before and after the test, the following inspections are performed respectively: partial discharge, oil dissolved gas analysis (DGA), oil dielectric loss, and SFRA winding deformation analysis.

Acceptance criteria: All temperature parameters shall remain within the design allowable limits. No abnormal gas generation shall be detected by DGA before and after the test. Partial discharge shall show no significant increase, and no abnormal winding deformation shall be detected.

Explanation: This test verifies the thermal margin under high-temperature operating conditions and improves reliability for full-load operation at a 50℃ ambient temperature. It does not change the rated capacity indicated on the transformer nameplate.

(2) Sand and Dust Sealing Simulation Test

ISO 12103-1 A2 Arizona fine test dust is used. A sealed sand and dust chamber is used to simulate wind-blown sand conditions, combined with temperature and pressure breathing cycles to simulate the day-night breathing effect of the oil tank. The test duration is 2 hours.

After completion of the test, all sealing areas including flanges, manholes, valves, pressure relief valves, and other components are dismantled and inspected.

The inner surfaces of the flanges are wiped and inspected. No dust is allowed to enter the internal oil chamber of the tank. Continuous visible dust accumulation films are not allowed on the inner sealing surfaces; only extremely small amounts of isolated point-like floating dust are acceptable.

After the test, partial discharge and oil analysis tests are repeated to confirm that no dust has entered the oil chamber and contaminated the insulation system.

You say the equipment is expected to survive twenty years in the desert. Is it really excessive to spend two extra days conducting additional tests?

9. Middle East Project Cases

Several real projects:

Saudi Arabia Eastern Province petrochemical expansion project: Two units of 25,000kVA/115kV transformers. F-class insulation, ONAF cooling, Type II mineral oil. From June 2022 until now, they have completed their fourth summer. In July last year, the temperature exceeded 50℃ continuously for 12 days, and the maximum top oil temperature reached 87℃. The user’s operation manager said: “Your transformer is the only major equipment in the entire site that has never triggered a temperature alarm.”

Abu Dhabi photovoltaic power plant project, UAE: One unit of 40,000kVA/33kV transformer. H-class insulation, OFAF cooling, and natural ester insulating oil. It operates at full generation during the day and near no-load at night, resulting in significant load fluctuations. We configured an OLTC voltage regulation range of ±12%, with 1.5% step voltage adjustment. The transformer performs more than twenty automatic voltage adjustments every day and has operated without failures.

Qatar substation centralized procurement project: Seven transformers, ranging from 5MVA to 20MVA. Delivered in 2021 and operating until now, the overall availability has reached 99.87%. The only shutdown occurred because an external overhead line was damaged by a construction vehicle, which was unrelated to the transformer itself.

10. Common Questions and Selection Recommendations

Several frequently asked questions:

How much more expensive is a desert transformer compared with a standard transformer?

Around 20% to 35%. The reasons include larger heat dissipation areas, higher insulation classes, more expensive fluororubber sealing components, and more complex fully sealed oil tank structures. However, ordinary transformers may only last 8 to 10 years in desert environments, while our design target is 25 to 30 years. When averaged over the service life, which one is actually more economical?

What about delivery time?

For conventional transformers below 20MVA, the delivery time is usually 90 to 100 days, about 15 days longer than standard products. The additional time is mainly required for extra testing. For urgent projects, early communication is recommended; the shortest delivery time can be reduced to around 75 days.

Are there any special requirements for on-site installation?

Yes. Before oil filling, the inside of the tank should be purged with dry nitrogen to remove moisture introduced during transportation. After installing the radiator, compressed air should be used to blow the radiator fins from inside to outside to remove accumulated dust and sand. These two steps can reduce the initial operating temperature by 3 to 5℃.

How are spare parts supplied?

A warehouse is available in Jebel Ali Free Zone, Dubai. Sealing rings, oil level indicators, thermometers, pressure relief valves, and fan motors can be delivered within 48 hours across the six Gulf countries. Large components are shipped from China, with sea-air combined transportation taking 7 to 10 days.

Furan analysis only applies to cellulose paper insulation (Class A, with mineral oil transformers being the mainstream application).

For transformers using aramid paper insulation (Class H / Class F): aramid does not generate furan compounds, therefore furan indicators are not applicable and cannot be used to evaluate aramid insulation aging.

Furan can be adsorbed. If activated carbon filtration or oil regeneration treatment has previously been performed, furan may be removed, resulting in lower measured values. This can create the false impression that “furan levels are not high, but the paper insulation has already aged.”

A comprehensive multi-parameter assessment is required, including:

• Furan content

• Oil acid value

• Degree of polymerization (DP) — paper sampling during inspection, which is the most reliable method

• DGA analysis (CO, CO₂ content)

• DC resistance

• Dielectric loss

A decision should never be made based on a single furan indicator alone.

11. About Zisheng Electric and Technical Support

After talking so much, the conclusion is actually just one sentence.

Whether equipment can withstand the desert environment does not depend on whether one single aspect is done exceptionally well. It depends on whether every detail has been carefully considered without compromise.

When materials are selected properly, tests are performed thoroughly, and no shortcuts are taken, reliability comes with confidence.

We have been working in the Middle East market for 13 years, delivering 40 to 50 units every year. We are not the cheapest supplier, but our reputation has been built through one project after another and one transformer after another.

Our products cover oil-immersed transformers, substation transformers, pad-mounted transformers, pole-mounted transformers, dry-type transformers, and other power distribution equipment. We hold 22 utility model patents and 3 software copyrights, have passed ISO 9001 quality management system certification, and comply with relevant IEC standard requirements.

we can provide an initial solution within 24 hours, including:Equipment selection、Line loss calculation、Investment return estimation

Thirty degrees is a comfortable temperature for people. Fifty-plus degrees is the temperature at which equipment has to fight to keep operating.

For projects facing equipment selection challenges, or for equipment experiencing frequent minor issues during high-temperature periods, please contact us directly through our official website. Our technical team will provide an initial response within 24 hours.

Zisheng Electric. Born for extreme environments.