Transformer Core Faults: Hazards, Causes, Types, Testing, and Remediation

Transformers play a crucial role in the transmission and transformation of electromagnetic energy, with their windings and cores being the primary components involved. Ensuring the normal operation of transformers requires accurate diagnosis, detection, and elimination of core faults. This article provides a detailed exploration of these aspects.

Transformer Core

I. Hazards, Causes, and Types of Transformer Core Multiple Ground Faults

1. Hazards of Core Multiple Ground Faults

During normal transformer operation, the core should not experience multiple ground faults. The presence of alternating magnetic fields around the windings induces parasitic capacitance between the high-voltage windings, low-voltage windings, core, and the outer shell. This results in a floating potential of the core concerning the ground. Variations in distance between the core components lead to potential differences. When the potential difference between two points reaches the insulation breakdown level, intermittent spark discharges occur. Prolonged exposure to such discharges adversely affects transformer oil and solid insulation.

To mitigate this phenomenon, the core is reliably connected to the outer shell. However, if the core or other metal components experience two or more ground points, a closed loop is formed, causing circulating currents, localized overheating, oil degradation, and decreased insulation performance. In severe cases, it can lead to catastrophic failures, such as the burning of core silicon steel sheets. Hence, the core of the main transformer should only have a single grounding point.

2. Causes of Core Ground Faults

Common causes of transformer core grounding faults include:

Short circuits in grounding strips due to construction and design deficiencies.
Multiple grounding points caused by accessories and external factors.
Presence of metal foreign objects left inside the main transformer, leading to burrs, rust, and welding slag, causing grounding.

3. Types of Core Faults:

There are six common types of transformer core faults:

Core-shell and core-clamp collisions during installation.
Excessive length of through-core bolt steel sleeve causing a short circuit with silicon steel sheets.
Presence of foreign objects in the oil tank causing localized short circuits.
Moisture or damage to core insulation causing high-resistance multiple grounding.
Wear of pump bearings leading to metal powder entering the oil tank, forming bridges under electromagnetic forces, resulting in multiple grounding points.
Poor maintenance practices and irregular inspections.

II. Testing and Remediation Methods for Transformer Core Faults

1. Testing Methods for Transformer Core Faults

Clamp Ammeter Method (Online Measurement): Accurate measurement of core multiple ground faults without power interruption.
Gas Chromatography Analysis (Live Oil Sampling): Analysis of gas components in transformer oil. Changes in methane and ethylene indicate potential core faults.
Insulation Resistance Test (Power-Off Test): Testing the resistance between the core and the outer shell. High resistance indicates good core insulation.

2. Remediation Methods for Transformer Core Multiple Ground Faults

For cores with external grounding wires: Install resistors in the core grounding circuit to limit grounding current. This is an emergency measure.
For faults caused by metal foreign objects: Inspect using cover removal to identify and rectify the issue.
For faults caused by burrs and metal powder: Use methods like capacitor discharge impact, AC arc method, and high-current impact for effective treatment.

III. Quality Standards for Transformer Core Maintenance

1. Inspection Criteria for Transformer Core

Flat and even core surface.
Intact insulation paint, closely stacked laminations.
No warping or waving of silicon steel sheets on the sides.
Clean core surfaces, free from oil and impurities.
Absence of short circuits or overlaps between laminations, meeting specified gap criteria.
Adequate insulation between core and upper/lower clamps, side irons, pressure plates, and base plates.

2. Other Quality Standards

Ensure uniform gaps between steel pressure plates and the core.
The insulation pressure plate should be intact, without damage or cracks, and securely fastened.
Steel pressure plates must not create closed circuits and should have a grounding point.
Insulation resistance measurement between core and upper/lower clamps, and between steel pressure plates and the core should remain stable during routine tests.
Adequate tightening of bolts and nuts, ensuring insulation washers are in good condition.
Unobstructed oil passages, clean oil channel blocks, and orderly arrangement.

3. Single Grounding Point Standard

Use a 0.5mm thick, 30mm wide copper strip as the grounding strip, inserted 3-4 levels between the core.
For large transformers, insertion depth should not be less than 80mm.
The exposed part of the grounding strip should be insulated to prevent core short circuits.

4. Overall Integrity and Reliability

Adequate mechanical strength, ensuring that insulation does not form a closed loop and does not contact the core.
Reliable grounding with good insulation.

Temporary Remedies When Transformers Cannot Be Shut Down

1. Opening Ground Lines

If the transformer cannot be shut down, a temporary solution involves opening the ground line if the fault current is significant. However, close monitoring is crucial to prevent the occurrence of a floating potential in the core once the fault point disappears.

2. Resistance Limitation in Working Ground Lines

For unstable multiple ground faults, introducing a sliding resistor into the working ground line can limit the current to below 1A. The selection of the sliding resistor involves dividing the voltage measured when the normal working ground line is open by the current on the ground line.

3. Gas Production Rate Monitoring

Employ chromatographic analysis to monitor the gas production rate at the fault point.

4. Relocating Normal Grounding Pieces

After precisely identifying the fault point through measurements, if addressing proves challenging, relocating the normal grounding pieces of the core to the fault point can significantly reduce circulating currents.

Comprehensive Maintenance Measures

1. Prompt Shutdown and Fault Elimination

Upon detecting multiple ground faults in transformers that can be shut down, immediate shutdown and comprehensive elimination of the faults are necessary. The methods for addressing such faults depend on the type and cause of the multiple grounding points, requiring corresponding maintenance measures.

2. Diagnostic Techniques When Fault Point Is Not Immediately Visible:

In cases where the fault point is not immediately apparent after power-off and core lifting, the following diagnostic techniques can be employed:

a. Direct Current Method:

- Open the connection pieces between the core and clamps. Apply 6V DC to the silicon steel sheets on both sides of the yoke. Measure the voltage between various levels of silicon steel sheets using a DC voltmeter. The point where the voltage equals zero or shows reverse indication can be considered the fault point.

b. Alternating Current Method:

Connect the low-voltage winding of the transformer to an AC voltage of 220-380V. With magnetic flux present in the core, multiple ground faults will result in current (core and clamp connection pieces should be open). Use a milliampere meter to measure points along the yoke, considering the point where the current is zero as the fault location.

In conclusion

In conclusion, adhering to these testing and maintenance standards ensures the reliability and safety of transformer cores. Routine inspections, precise testing methods, and effective remediation strategies play a crucial role in maintaining the optimal performance of transformers and preventing potential hazards.

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