Experimental Investigating on On-line Monitoring Of Insulation System of Power Transformers

The insulation system of the transformer is a critical aspect of its health condition and the failure of the insulation system can make the transformers out of service. HV bushings and windings are the most important components of the transformer insulation system and failure statistics show that bushings and windings are the two of the major reasons for transformer failures, which motivates operators for continuous on-line monitoring of their insulation condition. Partial discharge (PD) measurements are also an important tool to assess the condition of the insulation system of the transformers, including the bushings and windings. The paper adopts a simple but effective method to not only monitor the bushings and the windings' health condition but detect the PD of the transformer. The 600 KHz similar to 1.2MHz sinusoidal signal based on direct digital synthesis (DDS), is applied on the bushings and the windings along with power frequency signal to assess the bushings and windings' health condition. Then the line-end and neutral-end high frequency currents are continuously measured by isolated precise current probes based on wide bandwidth Rogowski coils and related digital filtering methods. Meanwhile, the wide bandwidth Rogowski coils, which have double-channel outputs, can also detect the PD signals in the bushings and the windings. With the sinusoidal current data obtained from one channel of the probes, both the bushings' and the windings' insulation condition can be determined. And with the pulse current signal from PD obtained from the other channel of the probes, the PD of the bushings and the windings can be monitored all the time. Some simulations and experiments were made whose results showed that this detection method had high sensitivity and anti-interfere capability and it could wildly be used in insulation diagnose and partial discharge detection of power transformers.