Transformer impedance is a critical parameter in the design and operation of electrical power systems. Understanding its complexities is essential for engineers to ensure optimal transformer performance and system stability.

In this comprehensive guide, we will delve into the intricacies of transformer impedance, exploring its fundamental concepts, calculation methods, and practical implications.

What Is Transformer Impedance

Transformer impedance represents the total opposition to the flow of alternating current (AC) in the transformer windings. This opposition is composed of two main components: the resistance of the windings and the leakage reactance.

• The winding resistance is the ohmic value of the conductor material used in the primary and secondary windings. It is determined by factors such as the cross-sectional area, length, and resistivity of the wire.
• The leakage reactance arises from the magnetic fields that leak out of the transformer core and do not contribute to the transfer of electrical energy between the primary and secondary windings. This leakage flux induces a voltage drop in the windings, which opposes the flow of current.

The combination of winding resistance and leakage reactance forms the total impedance of the transformer. It is typically expressed as a percentage of the rated voltage and current of the transformer, known as the percent impedance.

A higher impedance value results in better voltage regulation and lower short-circuit currents, but also leads to increased voltage drop and power losses.

On the contrary, a lower impedance value allows for higher short-circuit currents and improved efficiency, but may result in poor voltage regulation and increased stress on the transformer components during fault conditions.

Formula for Calculating Transformer Impedance

Z = √(R^2 + X^2)

Where:

• Z is the transformer impedance in ohms (Ω)
• R is the winding resistance in ohms (Ω)
• X is the leakage reactance in ohms (Ω)

%Z = (Z × I_rated) ÷ V_rated × 100%

Where:

• %Z is the percent impedance
• Z is the transformer impedance in ohms (Ω)
• I_rated is the rated current of the transformer in amperes (A)
• V_rated is the rated voltage of the transformer in volts (V)

Measuring Transformer Impedance

Two standard tests used to measure transformer impedance are the short circuit test and the open circuit test.

Short Circuit Test

The short circuit test is a routine test performed on transformers to determine their impedance values.

During this test, the secondary windings of the transformer are short-circuited while a reduced voltage is applied to the primary winding.

The voltage is adjusted until the rated current flows through the primary winding. The voltage, current, and power input are measured during the test.

The short circuit test helps determine the transformer’s series impedance, which consists of the winding resistance and leakage reactance.

Open Circuit Test

The open circuit test, also known as the no-load test, is another routine test performed on transformers. This test helps determine the transformer’s core losses and magnetizing current.

During the open circuit test, the secondary windings of the transformer are left open, and the rated voltage is applied to the primary winding. The voltage, current, and power input are measured during the test.

The open circuit test helps determine the transformer’s parallel impedance, which consists of the core loss resistance and magnetizing reactance.

Typical Transformer Impedance

Factors Influencing Transformer Impedance

Winding Resistance

Winding resistance is determined by the material and thickness of the wire used in the primary and secondary windings.

• Copper and aluminum are commonly used materials for transformer windings. Copper has a lower resistivity compared to aluminum, resulting in lower winding resistance and improved efficiency.
• The thickness, or cross-sectional area, of the wire also affects resistance. Thicker wires have lower resistance, allowing for more efficient current flow and reduced power losses.

Leakage Reactance

Leakage reactance, another component of transformer impedance, arises from the magnetic leakage flux between the primary and secondary windings. This leakage flux does not contribute to the transfer of electrical energy between windings.

Core Material and Sizes

Commonly used core materials include silicon steel, amorphous alloys, and ferrites.

• Silicon steel cores are widely used due to their high permeability and low losses.
• Amorphous alloys offer even lower core losses, making them suitable for high-efficiency transformers.

The size of the core also influences impedance and losses. Larger cores generally have lower impedance values and reduced core losses compared to smaller cores, as they provide a larger cross-sectional area for the magnetic flux to flow.

Temperature and Frequency

As the operating temperature of a transformer increases, the resistance of the windings also increases. This temperature-dependent change in resistance affects the overall impedance of the transformer.

Frequency also plays a role in transformer impedance. At higher frequencies, the leakage reactance becomes more dominant, while the winding resistance has less impact.

Impact of Impedance on Transformer Performance

Efficiency

The efficiency of a transformer is directly influenced by its impedance value. Higher impedance results in increased energy losses, primarily in the form of heat dissipation. These losses can be attributed to both the resistive and reactive components of impedance.

Winding resistance contributes to copper losses, while leakage reactance leads to magnetic flux leakage and associated energy losses.

Voltage Regulation

Transformer impedance significantly impacts voltage regulation, which refers to the stability of output voltages under varying load conditions. A higher impedance value results in a larger voltage drop across the transformer windings when current flows through them.

This voltage drop leads to poor voltage regulation, as the secondary voltage decreases with increasing load current. Conversely, lower impedance transformers exhibit better voltage regulation, maintaining a more stable output voltage even under fluctuating load demands.

Short-Circuit Current

Transformer impedance plays a critical role in limiting short-circuit currents during fault conditions. The higher the impedance, the lower the fault current that can flow through the transformer windings.

Inrush Current

Transformer impedance also influences the magnitude of inrush currents during transformer energization or startup. Inrush currents are high-magnitude, short-duration current surges that occur when a transformer is first connected to a power source.

Lower impedance transformers tend to experience higher inrush currents, as the impedance provides less restriction to the initial current flow. These inrush currents can potentially stress the transformer windings, cause voltage dips, and trigger protective devices.

Harmonics

Harmonics are voltage or current waveforms with frequencies that are integer multiples of the fundamental frequency. The presence of harmonics can lead to increased losses and heating in transformer windings and magnetic core. The impedance of the transformer at harmonic frequencies influences the flow of harmonic currents and the resulting harmonic losses.

Higher impedance at harmonic frequencies can help limit harmonic current flow and reduce associated losses. However, it is essential to consider the overall impact of harmonics on the transformer and the electrical system, as well as compliance with relevant harmonic standards and guidelines.

High Vs. Low Impedance Transformers

FAQs

What Is the Z Rating of a Transformer?

The Z rating, or impedance rating, of a transformer is the percent impedance voltage drop at rated load and frequency. It represents the transformer’s ability to limit short-circuit currents. Typical Z ratings range from 1% to 10%, with higher ratings indicating greater impedance and better short-circuit current limitation.

What Is the Impedance of a 1000 Kva Transformer?

Typical values range from 4% to 6% on the transformer’s base rating. For example, a 1000 kVA transformer with a 5% impedance would have an impedance of 50 ohms on the primary side at the rated voltage.

What Is the Impedance of a Current Transformer?

Current transformers (CTs) have very low impedance, typically in the range of 0.1 to 1 ohm.

Is Higher or Lower Impedance Better for Transformer?

Higher impedance transformers have better short-circuit current limitation, which can protect downstream equipment. However, they also have higher voltage regulation, meaning the output voltage varies more with load changes. Lower impedance transformers have better voltage regulation but less short-circuit protection.

What is the IEEE standard for transformer impedance?

IEEE C57.12.10 is the standard that specifies the impedance values for power transformers. It defines the preferred impedance values for various transformer sizes and types. For example, a 500 kVA, three-phase, liquid-filled transformer would have a standard impedance of 4.5% or 5.75%.

What is the impedance tolerance of a transformer ANSI?

According to ANSI C57.12.00, the impedance tolerance for power transformers is ±7.5% of the specified value for impedances less than 2.5%, and ±10% for impedances of 2.5% or higher. This means that a transformer with a specified impedance of 5% could have an actual impedance between 4.5% and 5.5%.

How many ohms should a transformer read?

To calculate the expected ohmic value, multiply the transformer’s percent impedance by its rated voltage squared, then divide by its rated power in watts. For example, a 100 kVA transformer with 5% impedance and a 480V primary would have an expected ohmic reading of 1.15 ohms on the primary side.