Transformer Inrush Current

No tool or machine is without its limitations or drawbacks, just as every rose has thorns. Transformers are crucial parts of electrical power systems because they may increase or decrease voltage levels for effective power distribution and transmission. Even though they are generally effective and passive devices, they occasionally display some characteristics that need to be controlled for proper operation. Inrush current, a brief, high-magnitude current that enters the transformer during energization, is one example of this phenomenon. It can have a big impact on system protection devices and equipment. The causes, traits, effects, and mitigation strategies associated with transformer inrush current are examined in this article.

Table of Contents

• What is Transformer Inrush Current?
• Why Does Inrush Current Occur?
• Inrush Current Calculation
• Inrush Current Types
• Effects of Inrush Current
• Elements That Impact Inrush Current
• Methods to Reduce Inrush Current
• Transformer Inrush Current Limiter
• Conclusion

What is Transformer Inrush Current?

The first spike of current that happens when a transformer is turned on, particularly when it is first connected to a power source, is known as inrush current. This current usually lasts for a few cycles (milliseconds to seconds) and can be several times higher than the transformer's rated full-load current. Inrush current is a temporary occurrence that only happens during switching, in contrast to steady-state operating currents. Because magnetic flux needs to be established in the transformer's core, the current flowing is primarily a magnetizing current.

Why Does Inrush Current Occur?

The transformer's magnetic core behavior, specifically the remanent flux and the non-linear B-H features (hysteresis curve) of the core material, is the primary cause of inrush current.

Flux Imbalance:

A transformer's magnetic core must increase its flux when it is turned on. Saturation of the core may occur if there is residual magnetism (remanent flux) in the core and the switching occurs at a point on the voltage waveform where the flux is driven in the same direction. The transformer draws a significant amount of magnetizing current as a result of this saturation.

Angle of Switching for the AC Supply:

The instantaneous voltage angle at the moment of energization has a significant impact on the inrush current magnitude. Switching at the voltage waveform's zero-crossing point usually results in the largest inrush current because the flux must rapidly increase from zero to its maximum value.

Saturation of the Core:
The ferromagnetic materials used to make transformer cores saturate when the magnetic flux rises above a particular threshold. In order to sustain the magnetic field during saturation, the inductance decreases and the current sharply rises, creating an inrush current.

Depending on the transformer design and the system conditions, the inrush current's usual duration ranges from a few milliseconds to several seconds. Its magnitude can range from 5 to 30 times the transformer's rated current.

Having flux right away after turning on the transformer is not feasible. Before the electricity is applied, the core has no flux. It will take some time for the flux to achieve its steady state value. It takes a non-zero amount of time, even though it appears to be very quick to us. How quickly the circuit can absorb energy determines how quickly this process proceeds. This is due to the fact that a circuit's energy transfer rate cannot be infinite. When the transformer is turned on, the flux in the core will likewise begin at zero. The voltage produced across the winding is determined by Faraday's law of electromagnetic induction, which reads e = dφ/dt. where φ represents the core's flow. The flux will therefore be an integral part of the voltage wave, and this may be computed using the following formula:

Equation of flux in the core
Source:www.electrical4u.com

The flux wave begins at the same location as the voltage wave if the transformer is turned on while the voltage is zero. One can compute the flux value at the conclusion of the voltage wave's first half-cycle using:

Flux value at the end of the first half-cycle of the voltage  Source:www.electrical4u.com

where φm is the steady-state flux's highest value. Typically, the transformer core is saturated somewhat over the flux's maximum steady state value. However, in our example, the maximum flux value will double from its steady state maximum value when the transformer is turned on. The core becomes saturated after attaining the maximum steady-state flux, and it takes a lot of current to produce more flux. Transformer inrush current, also known as magnetizing inrush current, is the high peak current that is drawn by the transformer primary. The current that a transformer drowns while it is being powered on is known as the magnetizing inrush current. This current is only present for a few milliseconds and is considered transitory. Up to ten times the transformer's typical rated current may be the inrush current. Despite having a very high magnitude, inrush current typically does not cause a permanent transformer defect since it only lasts for a very brief period of time. However, inrush current in power transformers continues to be an issue since it disrupts circuits' ability to operate as intended.

Transformer inrush current   Source:www.electrical4u.com 

Inrush Current Calculation

Transformer inrush current can be calculated by:

Transformer inrush current calculation

I=Vpeak / Z 

This is another common equation where Vpeak is the peak value of supply voltage and Z is the transformer impedance.

Transformer inrush current is generally 5 to 10 times rated current.

Inrush Current Types

Energization Inrush: When the transformer is first turned on, inrush takes place. This is the most prevalent type of inrush current.

Recovery Inrush: When the transformer resumes operation following a voltage dip or brief interruption, this type of inrush occurs.

Fault Recovery Inrush: After a system issue has been fixed and the supply has been restored, this takes place.

When two transformers are powered simultaneously, a phenomenon known as sympathetic inrush occurs. A part of the inrush current can be temporarily supplied to the freshly connected transformer by the already-energized one.

Effects of Inrush Current

1. Disturbance Protection Device Tripping: Because inrush currents are mistaken for short circuits, they might cause circuit breakers or overcurrent protection relays to activate. Equipment disconnections or needless power outages may result from this.

2. Stress from Mechanical: The transformer windings experience mechanical stresses as a result of the abruptly high current, which may eventually cause displacement or damage from repeated stress cycles.

3. Dips in Voltage: The local power system may experience a brief voltage drop (dip) due to inrush current, which could impact adjacent sensitive loads and equipment.

4. Generation of Harmonics: Highly non-sinusoidal inrush currents have the ability to introduce harmonics into the power system, which may impair the functionality of other equipment or result in inaccurate meter readings.

Elements That Impact Inrush Current

The severity and behavior of inrush current are influenced by a number of design and operating parameters:

Core Material: Inrush current is decreased by high-permeability materials.

Winding Resistance: A higher resistance reduces the current's strength.

Point-on-wave switching: The precise moment of energization inside the AC cycle is known as point-on-wave switching.

Transformer Size: Inrush currents are generally higher with larger transformers.

Residual Flux: The inrush increases with the amount of flux left over from the prior operation in the core.

Source Impedance: Higher inrush currents are permitted in systems with lower impedance.

Methods to Reduce Inrush Current

Controlling inrush current is essential for transformer longevity and system stability. Here are a few methods:

1. Resistors before insertion: The initial current can be limited during energization by temporarily connecting resistors in series. These resistors are avoided once the flux has stabilized.

2. Point-on-Wave switching: By energizing the transformer at an ideal voltage point (such as the voltage peak), circuit breakers that allow for fine control over the switching angle can reduce flux imbalance.

3. The Management of Residual Flux: Inrush current and residual flux can be decreased by employing demagnetization methods prior to energization (e.g., load tap changers, controlled disconnection).

4. Utilizing Relays for Inrush Restraint

In order to differentiate between inrush current and real problems and avoid nuisance tripping, modern protective relays are outfitted with inrush restraint logic or harmonic filtering.

5. Switching Sequentially

Cumulative inrush effects can be avoided in multiple transformer systems by activating transformers one after the other rather than all at once.

Transformer Inrush Current Limiter

For minimizing the magnitude of inrush current, several types of inrush current limiters are used:

a. Pre-Insertion Resistors: At energization, they are temporarily inserted in series with the transformer. After a few milliseconds (once inrush dies), the resistor is bypassed.

b. NTC(Negative Temperature Coefficient) Thermistors (for small transformers): It limits initial current and then allows normal operation.

c. Series Reactors: To restrict initial current, high-impedance reactors are provided in series connection. After the starting is done, they are removed.

d. Controlled Switching Devices: As mentioned, point-on-wave switching avoids worst-case energization utilizing intelligent circuit breakers.

Conclusion

In power systems, transformer inrush current is a naturally occurring but potentially problematic event. A number of variables, including as system impedance, switching conditions, and core design, affect its amplitude and consequences. Despite being a brief occurrence, its effects on transformer health, voltage stability, and protective systems are significant. Fortunately, inrush current can be efficiently reduced or controlled with the use of protective relays, precise modeling tools, and contemporary control technology. To ensure dependable and secure operation, electrical engineers working in generating, transmission, and distribution systems must have a firm grasp of inrush behavior.