The Core Insulation Test is a vital step in checking the condition of a transformer’s core insulation. This test is especially important for power transformers, as insulation problems can lead to serious issues over time. By identifying any weaknesses in the insulation early, you can address them before they cause major failures. Regular testing, especially during commissioning or maintenance, helps keep the transformer running safely and reliably.
Insulation is a critical part of how a transformer performs. The Core Insulation Test checks the electrical resistance between the core laminations and the grounding system. A high resistance value means the insulation is in good shape, while a low value could indicate a problem.
This test should be done at key points in the transformer’s lifecycle. These include when the equipment first arrives, before it’s put into service, after any major modifications, and during scheduled inspections. If a Dissolved Gas Analysis (DGA) shows high levels of gases like ethane, ethylene, or methane, it’s also a sign that the Core Insulation Test might be needed. Regular testing like this helps catch problems early and ensures the transformer stays in good working condition.
Test Procedure
To start the Core Insulation Test, the first step is to remove the cover from the transformer’s terminal block, which is usually located at the top. Once you have access, locate the CL (Core Lamination) and G (Ground) terminals, and disconnect the link that connects them. It’s also essential to make sure the transformer tank is properly grounded during the test to avoid any electrical hazards and ensure safety.
For the test itself, you’ll apply a direct voltage (DC) of 1.0 kV between two key points:
- Core Lamination (CL) and Ground/Tank (G)
- Core Clamp (CC) and Ground/Tank (G)
A reliable insulation resistance meter, like a megger, is used to measure the insulation resistance accurately.
The insulation resistance should be at least 1000 megaohms (MΩ) or 1 gigaohm (GΩ). If the resistance is lower than this, it could indicate problems with the core insulation that need attention.
When to Perform The Core Insulation Test
1. New Transformer Commissioning
When a new transformer is being commissioned, the core insulation test is necessary to ensure that the core insulation is intact and there are no faults during manufacturing as well as transportation.
For instance, applying a 1.0 kV DC voltage between the core lamination and the ground, a high insulation resistance reading (e.g., 10 GΩ) confirms that the transformer is safe and ready for operation at its rated voltage.
2. After Maintenance or Modifications
If routine maintenance such as cleaning or replacing parts of the transformer is done, it is necessary to do the core insulation test to ensure no damage is occurred to the insulation during the process of maintenance.
For example, if a core clamp is removed and reinstalled, the test can confirm that no electrical short exists between the core and ground.
3. Post-Overload or Fault Condition
After a transformer has experienced an overload or fault condition, the insulation test can help detect if the core insulation has deteriorated due to overheating.