Core Insulation Resistance and Inadvertent Core Ground Test (Megger®)
Do not attempt to run excitation or SFRA tests on a transformer immediately after using dc test equipment. Residual magnetism will remain in the core and ruin the excitation current and SFRA test results.
Core insulation resistance and core ground test is used if an unintentional core ground is suspected; this may be indicated by the DGA. Key gases to look for are ethane and/or ethylene and possibly methane. These gases may also be present if there is a poor connection at the bottom of a bushing or a bad tap changer contact.
Therefore, this test is only necessary if the winding resistance test (section 9.1) shows all connections and tap changer contacts in good condition.
The intentional core ground must be disconnected. This may be difficult, and some oil may have to be drained to accomplish this. On some transformers, core grounds are brought outside through insulated bushings and are easily accessed. A standard dc Megger® (1,000-volt Megger® is recommended) is then attached between the core ground lead (or the top of the core itself and the tank [ground]). The Megger® is used to place a dc voltage between these points, and the resistance is measured. A new transformer should read greater than 1,000 megohms. A service-aged transformer should read greater than 100 megohms. Ten to one-hundred megohms is indicative of deteriorating insulation between the core and ground. Less than 10 megohms is sufficient to cause destructive circulating currents and must be further investigated . A solid, unintentional core ground may read zero ohms; this, of course, causes destructive circulating currents and must be corrected before energization.
Some limited success has been obtained in “burning off” unintentional core grounds using a dc or ac current source. This is a risky operation, and the current may cause additional damage. The current source is normally limited to a maximum of 40 to 50 amps and should be increased slowly to use as little current as possible to accomplish the task. This should only be used as a last resort and then only with consultation from the manufacturer, if possible, and with others experienced in this task.
This will generate gases which will be dissolved in the oil and will show up in the DGA! Take a sample for DGA with in 72 hours after burning off the unintentional core ground and compare this DGA with the prior one to determine what gases were created by this task.