Understanding Transformer Impedance: Balancing Resistance and Reactance for Optimal Performance

What is Transformer Impedance?

Transformer impedance encompasses resistance and reactance, which oppose the flow of alternating current. It is represented as a percentage of the voltage required to drive the transformer to full load. The impedance value typically increases with the transformer’s capacity. For distribution transformers, the impedance usually falls within 4%.

Role of Impedance in Transformers

Impedance plays several key roles in transformer operation:

• Short Circuit Protection: It limits the maximum fault current during short circuits.
• Voltage Regulation: Helps manage voltage differences across the transformer.
• System Efficiency: Influences the voltage drop across windings and the core’s maximum flux density.

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Calculating Transformer Impedance

Impedance can be calculated using a short circuit test. The formula is:

Z%=(Short Circuit Voltage×100​)/Rated Voltage

For instance, with a short circuit voltage of 104V and a rated voltage of 4160V:

Z%=(104×100)/4160=2.5%

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Typical Impedance Values

The typical impedance values ​​of transformers vary according to different standards. The following lists the transformer impedance values ​​of IEEE, CSA, and AS standards.

According to IEEE C57.12.34 standards

• 45 - 75 kVA: 2.7%-5.75%
• 112.5 - 300 kVA: 3.1%-5.75%
• 500 kVA: 4.35%-5.75%
• 750-2500kVA: 5.75%
• 3750kVA: 5.75%-6%
• 5000-10000kVA: 6%-6.5%

Reading more about IEEE C57 Standard

According to CSA C227.4 standards

Minimum transformer impedance

• 0 - 150 kVA: 1.8%
• 225 - 300 kVA: 2%
• 500kVA: 3%
• 750 - 1000 kVA: 4%
• 1500 - 3000 kVA: 5%

Reading more about CSA C227.4

According to AS/NZS 60076.5 standards

• 25 - 630 kVA: 4%
• 631 - 1,250 kVA: 5%
• 1,251 - 2,500 kVA: 6%

Larger transformers generally have higher impedance to accommodate their increased capacity.

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High vs. Low Impedance

• High Impedance: Provides better protection against short circuits and poor power factors but can lead to increased voltage drops and overheating.
• Low Impedance: Minimizes voltage regulation and is typically more efficient but may result in higher short circuit currents and additional costs due to the need for more robust equipment.

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Factors Affecting Transformer Impedance

Transformer impedance is influenced by:

• Number of Turns: More turns increase impedance.
• Core Material: Ferrite cores generally have higher impedance.
• Core Size and Winding Resistance: Larger cores and different winding arrangements affect impedance.

Measuring Transformer Impedance

A short circuit test is commonly used:

• Setup: Apply a voltage to the HV windings while keeping the LV windings short-circuited.
• Measure: Record the voltage and current to determine impedance.

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Modifying Transformer Impedance

Impedance can be adjusted by changing the number of winding turns or altering core size and material. Increasing turns raises impedance while decreasing turns lowers it.

Conclusion

Transformer impedance is essential for safe and efficient transformer operation. It affects short circuit protection, voltage regulation, and system efficiency. Understanding and correctly managing impedance is crucial for optimizing transformer performance.

Daelim Transformer, with over 20 years of expertise, can design transformers with optimal impedance values, ensuring high efficiency and stability. Contact Daelim Transformer to meet your specific impedance needs.