What Happens During a Short Circuit?

A short circuit is one of the most severe electrical faults a power system can experience. It occurs when an unintended connection allows current to travel along an unintended path with little or no electrical resistance. For transformers and power systems, this can lead to mechanical stress, overheating, and catastrophic failure if not managed correctly. Understanding the consequences and dynamics of a short circuit is essential for electrical engineers, operators, and maintenance personnel.

What Physically Happens in a Transformer During a Short Circuit?

Short circuits are among the most violent and dangerous events that can occur in power systems—and transformers, being critical grid components, are particularly vulnerable. When a short circuit occurs, the transformer experiences an instantaneous surge of current—many times its rated capacity—which can cause intense mechanical forces, magnetic stress, heating, and even internal arcing. If not properly designed to withstand these extreme conditions, a transformer can suffer catastrophic failure within milliseconds. Understanding the physical phenomena inside a transformer during a short circuit is essential for engineers, asset managers, and protection designers.

During a short circuit, a transformer experiences a rapid rise in fault current that produces intense electromagnetic forces acting on its windings. These forces can deform, displace, or collapse the windings if mechanical bracing is insufficient. The sudden current surge also causes extreme heat generation, possible insulation breakdown, and pressure waves that stress the core and tank. Internal arcing, dielectric fluid breakdown, and magnetic core saturation may occur. If not cleared promptly by protection systems, these effects can permanently damage the transformer or lead to explosion.

Electromagnetic and Mechanical Forces During Short Circuit

The most critical physical reaction in a transformer short circuit is the intense Lorentz force between current-carrying conductors.

These forces are proportional to the square of the short-circuit current:

$$
F \propto I^2
$$

If short-circuit current is 20× the rated current, the resulting mechanical force is 400× greater than during normal operation.

Heating and Dielectric Stress

If thermal limits are exceeded, cellulose insulation chars, bubbles form, and dielectric breakdown may occur.

Structural and Core Reactions

In severe cases, the entire transformer may vibrate or hum violently due to sudden core magnetostriction under saturated conditions.

Sequence of Physical Events During a Typical Fault

Protection relays are typically set to detect and isolate faults within 30–100 ms to prevent structural damage.

Design Requirements to Withstand Short Circuits

IEC 60076-5 specifies short-circuit testing to ensure transformer resilience:

Factory short-circuit withstand testing simulates worst-case events with fault currents injected under controlled conditions.

Data Table: Short-Circuit Effects Based on Transformer Size

The magnitude of force increases quadratically with current and linearly with conductor length and proximity.

Case Study: Windings Deformed by Repeated Faults

In a 132 kV substation transformer, undetected line faults caused repeated short-circuit events. Initial protection failures allowed current to persist for over 200 ms on each occasion. Inspection revealed:

• Axial compression of HV winding
• Radial bowing of LV winding, evident from DGA and sweep frequency tests
• Permanent displacement of leads and oil degradation

The unit had to be rewound at significant cost—highlighting that even “temporary” faults leave permanent scars.

What Are the Main Causes of Short Circuits in Power Systems?

Short circuits in power systems are sudden, high-current faults that bypass normal load paths—causing voltage collapse, equipment damage, and possible power outages. These faults are typically unintentional contacts between phases or between phase and ground. The root causes are diverse, ranging from environmental events like lightning and flooding to insulation failure, human error, and mechanical breakdown. If not identified and mitigated, short circuits can result in transformer damage, arc flash incidents, or cascading grid instability. Understanding the main causes is key to implementing effective protection and prevention strategies.

The main causes of short circuits in power systems include insulation failure due to aging or thermal stress, environmental events like lightning strikes or wildlife contact, mechanical damage from vibration or poor installation, contamination from moisture or dust, and operational errors such as miswiring or equipment misuse. These faults can occur anywhere along the network—from transmission lines to transformers, switchgear, and underground cables—and must be rapidly cleared by protective relays and breakers.

1. Insulation Failure in Equipment

Once dielectric breakdown occurs, voltage surges or load current can force arcing between conductive parts—leading to a short.

2. Environmental Factors

Outdoor insulators and unsealed enclosures are highly vulnerable in coastal, industrial, or rainy environments.

3. Wildlife and Vegetation Intrusion

Utilities often install wildlife barriers and tree-trimming programs to mitigate these common, yet often overlooked, causes.

4. Mechanical Damage or Wear

Even micro-vibrations can gradually degrade components, particularly in wind-prone or seismic areas.

5. Operational or Human Error

Human mistakes often trigger latent defects to evolve into full short-circuit conditions.

6. Aging Infrastructure

Legacy systems lacking modern insulation or monitoring are more prone to faults over time.

Real-World Example: Substation Short Circuit Due to Floodwater

• Location: Urban 33/11 kV substation
• Cause: Heavy rainfall flooded the control room
• Fault Path: Water entered LV switchgear panel through floor conduit
• Result: Earth fault caused busbar flashover and cascading outage
• Remedy: Replaced affected switchgear, added cable sealing, installed water detection alarm

This underscores the importance of environmental design in substation layout and cable routing.

Visualization of Common Short-Circuit Causes

Routine inspection and predictive maintenance tools like thermography, partial discharge monitoring, and insulation resistance tests can preempt many of these faults.

What Are the Effects of a Short Circuit on Transformer Performance?

A short circuit is one of the most severe electrical faults a transformer can experience. It leads to sudden and massive electrical currents, unleashing intense mechanical, thermal, magnetic, and dielectric stresses on the transformer’s internal components. Even if the fault is cleared quickly, residual damage can impair future performance—causing reduced insulation life, compromised mechanical integrity, or inefficient operation. In some cases, the effects are immediately catastrophic; in others, degradation occurs progressively, leading to latent failure years later. Therefore, understanding these effects is critical for determining post-fault transformer usability and ensuring long-term grid reliability.

A short circuit causes significant degradation in transformer performance by inducing mechanical deformation of windings, thermal damage to insulation, and dielectric stress that can lead to partial discharges or arcing. These impairments reduce the dielectric strength, increase internal losses, cause noise and vibration, and may shift impedance characteristics. Over time, the transformer becomes less reliable, prone to overheating, and may exhibit a higher failure rate unless thoroughly inspected, tested, or refurbished.

Immediate Effects of Short Circuit on Transformer Internals

Even a transient fault lasting <100 ms can initiate slow degradation of multiple components.

Long-Term Performance Effects After a Short Circuit

Transformers that survive short circuits without immediate failure may still fail prematurely due to hidden weakening of their insulation or structure.

Measurable Post-Fault Indicators

These tests are often used during post-fault investigations to determine if a transformer can return to service or requires repair.

Short Circuit Impact Table by Duration and Intensity

Prompt clearing by protection relays is essential to prevent long-term damage.

Simulation: Winding Shift and Impedance Change

In a 63 MVA transformer that underwent a 120 ms fault, the following post-fault data was collected:

This confirms how minor deformation impacts energy efficiency and performance.

IEC Standards on Short Circuit Withstand Performance

IEC 60076-5 outlines design and test requirements:

Transformers are tested at the factory for theoretical maximum stresses, but real-world system faults can exceed expectations.

Recommendations After a Short Circuit Event

Never re-energize a transformer after a major fault without testing, even if it looks outwardly intact.

How Are Transformers Protected Against Short Circuits?

A transformer is one of the most valuable and critical assets in a power system. Despite robust design standards, no transformer is immune to the dangers of a short circuit—either internally due to winding faults or externally from system conditions. When a short circuit occurs, the transformer's thermal and mechanical limits can be exceeded in milliseconds, leading to catastrophic insulation failure, tank rupture, or even explosion. That's why transformer protection is not optional—it’s an integrated system of devices and logic designed to detect, isolate, and report faults faster than damage can occur. These protection systems must be carefully selected, tested, and calibrated according to the transformer's rating, system configuration, and fault profile.

Transformers are protected against short circuits using a combination of differential protection relays, overcurrent and earth fault protection, Buchholz relays (for oil-filled units), surge arresters, pressure relief devices, and circuit breakers. These systems rapidly detect abnormal current, voltage, or internal gas generation, and isolate the transformer from the power grid before damage escalates. Protective devices are coordinated to act within milliseconds, adhering to IEC and IEEE standards to prevent thermal or mechanical failure during faults.

Primary Short Circuit Protection Systems for Transformers

These elements are often connected to SCADA systems for remote fault indication and event logging.

Core Protection Principle: Differential Protection (87T)

Differential protection is the most critical and sensitive method for transformer short circuit protection.

Differential current:

$$
I{\text{diff}} = I{\text{primary}} - \text{CT ratio} \cdot I_{\text{secondary}}
$$

If the differential current exceeds a set threshold and is sustained for a defined time (e.g., >30 ms), the relay sends a trip signal to the circuit breaker.

Protection Coordination and Tripping Sequence

Speed is critical: Most mechanical damage during a short circuit happens within 50–100 ms.

Secondary and Backup Protections

These systems work in tandem to ensure no fault goes undetected or unaddressed.

Mechanical and Thermal Fault Indicators

For oil-immersed transformers, non-electrical protective devices also provide critical safety margins:

All signals are wired to lockout relays that can prevent re-energization until inspection is completed.

Protective Device Mapping in an Example 132/33 kV Transformer

These form a complete protection architecture governed by IEC 60255 and IEEE C37 series standards.

Performance Chart: Fault Clearing Time vs. Transformer Damage Risk

Prompt tripping preserves the mechanical and dielectric integrity of the transformer.

Conclusion

A short circuit represents a high-risk event in any electrical system, particularly for vital assets like power transformers. If not swiftly interrupted, it can cause irreversible damage to windings, insulation, and the transformer core. Comprehensive protection systems, regular maintenance, and robust design are essential to minimize risk and ensure transformer reliability under fault conditions.

FAQ

Q1: What exactly is a short circuit in a transformer?
A1: A short circuit occurs when there is an unintended low-resistance path between conductors, causing excessive current flow. In a transformer, this can happen:

Internally: between winding turns or phases

Externally: in connected cables or equipment
This sudden surge can be many times the rated current, potentially causing severe thermal and mechanical stress on the transformer components.

Q2: What are the immediate effects of a short circuit?
A2: When a short circuit occurs, it leads to:

Massive overcurrent (often 10–25 times nominal current)

Rapid heating of windings and conductors

Mechanical deformation of windings due to magnetic forces

Voltage drops or outages in the electrical system

Triggering of protection systems like circuit breakers or relays
If not cleared quickly, it can cause permanent damage or explosion.

Q3: What transformer components are most vulnerable?
A3: The components most affected by short-circuit forces include:

Windings: Can deform or burn due to heat and magnetic forces

Core insulation: May break down, causing internal arcing

Tap changers: Prone to contact welding or damage under fault currents

Bushings and terminals: Subject to flashover or thermal cracking
Repeated or high-energy short circuits shorten the transformer's lifespan significantly.

Q4: How do protection systems respond to a short circuit?
A4: Transformers are protected using:

Differential relays: Detect imbalances in current

Overcurrent relays: Trip circuits at preset limits

Buchholz relays (in oil-immersed units): Detect gas or pressure changes

Circuit breakers or fuses: Interrupt current flow within milliseconds
These systems are designed to isolate faults quickly, minimizing damage and fire risk.

Q5: Can a transformer be reused after a short circuit event?
A5: That depends on the severity. After a short circuit:

The transformer undergoes electrical and mechanical inspections

Tests include winding resistance, insulation resistance, and SFRA

Minor faults may be repairable (e.g., bushing or tap changer replacement)
Severe damage—like winding collapse or core shift—often requires full rewinding or replacement. Post-fault diagnostics are essential before re-energization.

References

Electrical4U: Short Circuit in Transformer
https://www.electrical4u.com/short-circuit-test-of-transformer/

IEEE C57.109-2018: Guide for Transformer Short Circuit Withstand
https://standards.ieee.org/standard/C57_109-2018.html

Doble Engineering: Post-Fault Diagnostic Testing
https://www.doble.com/solutions/fault-analysis-and-testing/

NREL: Transformer Reliability and Fault Tolerance
https://www.nrel.gov/docs/fy21osti/transformer-short-circuit-failure.pdf

ScienceDirect: Mechanical and Thermal Effects of Short Circuits in Transformers
https://www.sciencedirect.com/science/article/pii/S0378779619303525