Wind is one of the keys to the future of our energy supply, and a front-runner in the quest for renewable, cost-effective energy. Let’s take a look at the challenges, as well as the place of cast resin dry-type transformers in the wind power industry.

1. WHAT IS WIND POWER & HOW DOES IT WORK?

Energy harvested from the wind = wind power.

Harvesting wind power, involves converting the energy produced by the motion of wind turbine blades (driven by the wind) into electrical energy.

2. THE VALUE OF WIND POWER.

Wind power is a clean, carbon-free, and continuous source of energy. It is inexhaustible, reduces the use of fossil fuels, and doesn’t pollute the natural environment. In addition, wind energy never generates waste and never contaminates water. Also, considering the scarcity of water, producing wind energy has one of the lowest water-consumption rates.

The growing global energy consumption places immense strain on our natural resources. As a result, renewable energy sources are playing a growing and essential role. Wind power is one of these sources. And with an enormous potential for the new energy age, the wind power industry is expanding fast.

3. CHALLENGES FACING THE WIND POWER INDUSTRY.

The first turbines introduced for wind energy generation, presented transformer reliability concerns. This was due to problems with inconsistent winds, varying loads, vibrations, electric switching surges, gassing, and overloading.

After examining these problems, transformer designs continued being adapted. Since then, enough operational and failure data on transformers became available. Which still enable engineers to continue fine-tuning its design for the wind industry.

4. CAST RESIN TRANSFORMERS IN WIND POWER GENERATION.

So, where do cast resin dry-type transformers feature in the wind energy landscape?

Wind power generation and the harvesting thereof, uses electrical transmission equipment. For this infrastructure, a safe and reliable solution is key. Cue the cast resin dry-type transformer.

Transformers convert the lower voltage of the electricity generator, to the higher voltage of the grid. That is, from the wind turbine to the public supply grid. Therefore, wind turbines require a generator step-up (GSU) transformer design.

Remember, this design also requires full integration into the turbine design and performance specifications. Through years of continued efforts, the electrical, technical, and thermal know-how became available. This knowledge facilitated the development of transformers, which adapts to both onshore and extreme offshore conditions.

Cast resin transformers are acceptable for installation on the nacelle platform. The nacelle refers to the head of the turbine, which houses all generating components. It is also suitable for installation inside the turbine tower base, or outside the tower base. Keep in mind – the closer the transformer is to the load, the lower the losses are. Also, closer installation required less cabling and saves costs. (image)

For nacelle and in-tower installation, transformers have a compact design. This allows an easy fit through the tower door without disassembling the transformer. Furthermore, the design works to reduce losses while operating in high vibration conditions.

Additionally, cast resin transformers are flame-retardant and self-extinguishing. By adhering to the highest fire protection and environmental requirements, it promotes optimal operational safety. It also requires little maintenance and is easy to repair.

Installed in wind turbines, this transformer’s strengths are truly demonstrated in every aspect.

5. GSU TRANSFORMER DESIGN CONSIDERATIONS.

Turbine conditions expose step-up transformers to stresses, which are not present in normal conditions. So using conventional off-the-shelf distribution transformers as a low cost solution, is unwise. It simply does not belong in this field, and causes frequent failures and outages.

Normal distribution transformers are designed as step-down transformers. Here, the power source connects to the HV primary winding, and the load to the LV secondary winding. In step-up transformers, the power source connects to the LV winding, and the load to the HV winding.

GSU transformers play a unique role in the wind industry, and its design should be equally unique. Reliability and lower total cost of ownership, should not be traded for low initial cost. Consider the specific requirements, to ensure that transformers are solid links in the chain.

Acceptable GSU transformer design for wind turbine application, considers the following:

Transformer loading.

Wind turbines are dependent on local wind and other climatic conditions. Its yearly average load factor can be as low as 35%. The rather light loading of the transformer presents two problems. And this must be factored into the design.

First, wind farm transformers’ low average load factors skew purchasing decisions. When lightly loaded or idle, the core losses become a more significant economic factor. While the winding losses become less significant.

Generally, estimations consider the total transformer idle time and running time. It then compares the ratio of these two totals.

But applying this formula to wind farm estimations, require greater consideration of the total idle time. Thus, the typical cost estimation formula does not apply to wind farms.

Second, fluctuating turbine loads expose the transformer to thermal-cycling. Meaning from load, to no-load, back to load. This causes recurring thermal stress on the turbine’s electrical components. A scenario which is synonymous with someone bending a wire back and forth until it breaks. Metal fatigue, heat, and strain cause the wire to deteriorate and break. For electrical connections that endure recurring fluctuating loads, the same is true.

Recurring thermal-cycling damages insulation, by creating hot-spots and partial discharges. It also causes accelerated aging of internal and external electrical connections.

Transformer sizing and voltage variation.

GSU transformers are typically designed so transformer voltage exactly matches wind turbine output voltage. The design must therefore be uniquely robust to absorb any over-speeding of the generator.

Harmonics and non-linear loads.

Generator step-up transformers are switched with solid state controls as to limit inrush currents. Though it potentially aids the initial energization, it also contributes detrimental harmonic voltages. Harmonic voltages, coupled with the non-linear loads, is dangerous from a heating perspective.

When frequency disturbances occur, the transformer must be able to handle the higher load.

If using wind turbine rectifiers, the transformer must be designed for harmonics similar to rectifier transformers. Wind farm owners don’t want to pass high frequencies to the power grid, because it will impact other equipment. It may result in a defensive fault, causing transmission grid equipment to protect itself by shutting down.

Transformers must handle high loads, and provide electrostatic buffers that prevent the transfer of harmonic frequencies between the primary and secondary winding. Thus, it must handle the energy without transmitting it to the grid.

Potential sources of gas generation.

+ Winding.
Since heat travels layer to layer, minimizing the distances will cool it most effectively. Designing the coils with extra cooling ducts, promotes a smooth and convective flow. It shortens the distance heat energy travels to the coolant medium, eliminating hot-spot formation.

+ Core & Coil Design.
Temporary surges may expose the transformer to ongoing stresses, expanding the winding outward. But implementing circular shapes for the core leg and coil design, disperses radial forces evenly at 360°.

+ Core Frame.
At times, the transformer is also exposed to axial forces as a result of external events. Designing the core frame to contain these forces extends the transformer life.

+ Coil End-Blocking.
Adding end-blocking without affecting coolant flow, preserves the integrity of the insulation. Coil end-blocking by means of pressure plates, suppresses axial forces produced during faulty conditions. These forces may cause telescoping of the coils which shortens the transformer life.

What is telescoping?
Telescoping occurs when forces cause the primary and secondary winding to slide past one another, causing it to come apart. This damages the coil and ultimately results in transformer failure.

+ Tap Changers.
Over time, the rapid thermal-cycling present in wind farm transformer duty cycles, can impact no-load tap changers. If contact areas are impacted, it creates an arcing point and generates gas. Take care to establish whether the taps will ever need to changing in this application.

Special requirements to withstand faults.

When subjected to faults, conventional distribution transformers, and other types of transformers will stop. It only goes back online, after the fault has cleared.

But maintaining wind farm network stability means, disallowing turbine generators to disconnect from the system during disturbances.

Thus, generators must be designed to handle faults while still staying online.

6. CONCLUSION.

Transformers (GSU’s) required for wind power application, differ from its counterparts in other industries. So, when purchasing for wind farm developments, take care to only include acceptable components.

Generator step-up transformers are critical, and its design must be continually examined and re-evaluated. Consideration must be given to new technology that’s available in the field.

Purchasing decisions need to move away from lowest initial cost. Instead, focus on solutions that deliver the best results in terms of total cost of ownership, network stability, less down time, and lost revenue from high maintenance issues.

The equipment must meet the unique requirements of the individual wind farm. Also, its turbine type and related electrical components (harmonic contribution and load profile).

Cast resin dry-type transformers have been optimally designed to meet all the requirements of the wind power generation industry.
 

SOURCES:

1. Ahktar, RI. (Feb. 2016). “Wind Farm Transformers Are Different.” Retrieved from https://www.linkedin.com/pulse/wind-farm-transformers-different-akhtar-pe-msc-eng-bsc-eng

2. Steeber, T. (Nov. 2011). “Wind Turbine Step-Up Transformers.” Retrieved from https://www.pddnet.com/article/2011/01/wind-turbine-step-transformer