Is Higher Transformer Impedance Always Better?

Recently, a client engaged in the casting industry contacted us to customize a 4500kVA 10/0.575×4 oil-immersed transformer. During technical discussions, he explicitly required the short-circuit impedance to be designed above 9% to enhance the transformer's short-circuit resistance. Based on calculations according to load characteristics, we recommended a standard impedance of 7%.

 

This brings up a long-debated and easily misunderstood question in the industry: Is a higher impedance voltage (Ud%) truly better for transformers?

 

Many operators, especially owners of casting and smelting plants, believe that higher impedance means stronger resistance to surge and short-circuit faults, offering greater operational safety. But is this really the case?

 

 

This article thoroughly analyzes the double-edged sword effect of transformer impedance, and explains why excessively high impedance leads to increased power consumption and a sharp rise in electricity bills.

 

 

Simply put, impedance voltage (short-circuit impedance) refers to the internal resistance to electric current inside a transformer.

 

• Low impedance (4% - 6%): Similar to a wide straight road. Current flows smoothly and voltage remains stable. However, in the event of a short circuit, the unrestrained current will cause severe damage.

 

• High impedance (8% - 15%): Comparable to speed bumps on a road. It limits the peak short-circuit current and protects downstream equipment. The downside is higher power loss.

 

Conclusion: Neither excessively high nor excessively low impedance is ideal. The most suitable value delivers the best performance.

 

 

For this 4500kVA oil-immersed transformer for casting applications, we stick to 7% impedance instead of 9% for three key reasons:

 

1. Severe Voltage Fluctuations Reduce Melting Efficiency

Oil-immersed transformers for such working conditions experience drastic load changes, from high current surges at startup to steady operation during melting. The impedance directly determines the voltage regulation rate on the secondary side.

• 7% impedance: Voltage drop is kept within a reasonable range, ensuring stable operation of the medium-frequency power supply.
• 9% impedance: Output voltage fluctuates far more drastically with load changes. This causes unstable power output of the medium-frequency furnace, prolongs melting time, lowers production efficiency and impairs the quality of molten iron.

 

2. Soaring Reactive Power Loss 

Contrary to common misconceptions, high impedance is not just a minor waste of conductors. The reactive component (X) of impedance continuously consumes reactive power.

 

• Formula for reactive power loss: Q≈I2X

 

Raising the impedance from 7% to 9% increases the reactive component by 28.6%. The transformer will draw far more reactive power from the power grid to maintain its magnetic field.

 

As a result, the power factor will drop significantly. Power supply authorities impose power factor surcharges on users whose power factor falls below the standard (generally 0.9) to compensate for line losses. For a 4500kVA transformer, the additional annual electricity expense can reach tens of thousands of dollars.

 

3. Increased Copper Loss and Overheating Risk

To boost impedance, manufacturers usually increase winding turns or expand the magnetic leakage path. This leads to a sharp rise in load loss (copper loss). All excess power loss is converted into heat, forcing the cooling system to operate more frequently. In hot summer weather, the transformer may even trigger overtemperature alarms.

 

 

Below are industry-standard impedance references for various scenarios:

Application Scenario	Recommended Impedance Range	Selection Principle
General distribution transformer	4% - 6%	Prioritize voltage stability and low power loss
Medium-frequency furnace / Rectifier transformer	6% - 8%	Optimal balance between current limiting and energy efficiency
Large power transformer	8% - 12%	Focus on limiting short-circuit current to protect the power grid
Special high-impedance transformer	Above 15%	For special sites such as laboratories; must be equipped with dynamic reactive power compensation devices

 

Higher impedance never equals better performance. For this 4500kVA oil-immersed transformer, 7% is the efficient range, while 9% leads to excessive energy consumption. We are technically capable of designing 9% impedance, but for your long-term benefits, we sincerely recommend 7% - it is more energy-saving, stable and cost-effective.

Get a quote

 

When purchasing transformers for casting plants, steel mills or submerged arc furnaces, do not focus merely on impedance. Pay more attention to no-load loss, load loss and professional anti-short-circuit structural design. These factors are far more valuable than simply raising impedance by a small margin.