Case Analysis: Economy and Reliability of 110kV All-Aluminum Winding Power Transformers

Currently, the market price of copper remains high, fluctuating in the range of 70,000 to 80,000 yuan per ton. In contrast, the price of aluminum stays low, lingering in the range of 18,000 to 20,000 yuan per ton. For power transformers, replacing copper windings with aluminum windings in design will undoubtedly significantly reduce the material cost of products, bringing substantial cost savings to end customers.

For a long time, it has been widely believed in the industry that aluminum windings can only be used in power transformers with a voltage level of 35kV and below. In fact, this is a major misunderstanding. As a matter of fact, aluminum windings can exert greater advantages when applied in high-voltage power transformers. The core factor that truly restricts the popularization and application of aluminum windings is that the yield strength of aluminum conductors can only reach about 70MPa at present, which may lead to insufficient short-circuit withstand capacity of transformer windings in some scenarios.

1. Current Situation and Standards
1.1 Current Situation of Aluminum Winding Transformers

Abroad, aluminum winding transformers are widely used in the field of distribution transformers and have a small number of applications in main transformers. In China, although aluminum windings have been applied in distribution transformers, main transformers with a voltage level of 110kV to 1000kV have not yet been legally applied.

1.2 Relevant Standards for Aluminum Winding Transformers

Both the international standard IEC and the national standard GB clearly allow power transformers to use copper or aluminum as conductor materials for windings. In addition, the National Energy Administration issued industry standards for aluminum winding transformers in January 2016, including Technical Parameters and Requirements for 6kV～35kV Oil-immersed Aluminum Winding Distribution Transformers and Technical Parameters and Requirements for 6kV～35kV Dry-type Aluminum Winding Transformers. This fully indicates that, from the standard perspective, the application of aluminum winding transformers is legal.

2. Quantitative Cost Comparison

According to conventional design experience, on the premise of ensuring consistent transformer performance parameters (such as no-load loss, load loss, short-circuit impedance, short-circuit withstand capacity margin, etc.), combined with the current raw material prices (the market price of bare copper is about 70,000 yuan per ton, and the market price of bare aluminum is about 20,000 yuan per ton), the main material cost of transformers using aluminum windings can be saved by more than 20% compared with those using copper windings.

The following is a specific comparison with a SZ20-50000/110-NX2 power transformer as an example.

It can be seen from the above comparison results that, on the premise of ensuring the same performance parameters, for a 50MVA/110kV double-winding Class II energy-efficient power transformer, the cost of the aluminum winding is about 23.5% lower than that of the copper winding, and the cost reduction effect is very significant.

Qualitative Comparison of Performance

The qualitative comparison of the main performances of power transformers with aluminum windings and copper windings is divided into the following aspects:

3.1 No-load Loss

The size of the iron core of the aluminum-winding transformer is relatively large. To ensure the same no-load loss, it can be achieved by appropriately reducing the magnetic flux density or the diameter of the iron core or selecting silicon steel sheets with lower unit loss.

3.2 Load Loss

Since the resistivity of aluminum conductors is about 1.63 times that of copper conductors, to ensure the same load loss, the current density of the aluminum winding conductors is generally reduced.

3.3 Short-circuit Withstand Capacity

Under the conditions of conventional short-circuit impedance and a rated capacity below 100MVA, as long as the design is reasonable, the aluminum-winding transformer can also have sufficient short-circuit capacity. However, when the rated capacity of the transformer is above 100MVA or the impedance is significantly low, the aluminum-winding transformer may show the characteristic of insufficient short-circuit withstand capacity.

3.4 Insulation Margin

Due to the generally larger size of the aluminum conductor gauge and the larger conductor curvature radius, the aluminum winding will obtain a more uniform electric field compared with the copper winding. Under the same main insulation distance of the winding and oil gap division, there will be a larger main insulation margin. In terms of the longitudinal insulation of the winding, the large size of the aluminum conductor means a larger inter-turn capacitance, which is also more conducive to the distribution of the wave process. This is the basic principle that makes aluminum windings particularly suitable for high-voltage transformers.

3.5 Temperature Rise Level

Due to the generally larger size of the aluminum conductor gauge, the aluminum-winding transformer will have a larger heat dissipation surface compared with the copper-winding transformer. On the premise of the same heat source, a lower copper-oil temperature rise will be obtained. Moreover, since the skin effect of the aluminum conductor is significantly weaker than that of the copper winding and the eddy current loss is smaller, the aluminum winding will have a lower hot-spot temperature rise.

3.6 Overload and Service Life

Due to the weaker skin effect of the winding itself and the lower hot-spot temperature rise, under the same conditions, the aluminum-winding transformer will have a longer service life and stronger overload capacity.

4 Summary

On the premise of ensuring the same performance parameters, according to the current market prices of copper and aluminum, power transformers with aluminum windings generally reduce the cost by more than 20% compared with those with copper windings. Objectively speaking in terms of technology, except for the short-circuit withstand capacity, the comprehensive performance of power transformers with aluminum windings is undoubtedly comprehensively leading that of those with copper windings.

Fundamentally speaking, the limited application of power transformers with aluminum windings does not lie in high voltage, but in large capacity. Essentially, it lies in the natural insufficiency of the yield strength of aluminum conductors, making it inherently difficult to meet the short-circuit withstand capacity of some large-capacity or low-impedance power transformers. The emergence of conductors for aluminum alloy transformer windings is precisely an attempt to solve this problem.

However, increasing the short-circuit impedance of power transformers can quickly solve this problem. After increasing the short-circuit impedance of power transformers, the short-circuit current will be reduced. Even for large-capacity (such as above 180MVA) power transformers, the short-circuit withstand capacity of aluminum windings may no longer be a restricting problem.