PD Sensors for Power Transformers
Dry Type and Cast Resin Transformer PD Testing
A Dry-Type Transformer can be tested or monitored relatively easily, it is a similar application to Air Insulated Switchgear testing. PD occurring in components within the transformer will emit a high-frequency signal which travels along the earthed panels to the exterior surface of the equipment. A TEV sensor placed on the outer switchgear panel will detect the PD signal.
Ultrasonic sensors can detect surface tracking activity in locations with exposed HV components. Highly sensitive microphones can be directed at potential defect points, often terminations or connection points, to pick up signals in the 40kHz range. Sensors can be installed externally looking through vents and gaps in the housing.
Oil Filled & Transmission Transformers
Monitoring of oil transformers can be trickier than other substation assets, primarily due to the fact that traditional external PD sensors can not be used. PD signals can attenuate rapidly as they transit through oil, and the sealed nature of oil filled transformers will prevent surface current and voltage pulses from easily escaping.
Traditional HFCT and TEV Sensors can be used on cable terminations of the transformers. For PD detection in the main tank, however, these sensors cannot be used.
UHF sensors installed internally are the only effective sensors for PD Detection in oil filled transformers. Usually for existing transformers, these are installed via the drain valve, or a dedicated PD sensing valve specifically designed for this application. When PD occurs inside the main oil chamber, the UHF sensor will direct detect the Ultra-high-frequency radiated energy of the Partial Discharge. Some transformer manufacturers are beginning to install UHF windows to new transformers.
Contact Acoustic Sensors are sometimes used externally to test for PD. This is possible, though it must be noted that the signal may attenuate before it can be detected, so you cannot be sure the transformer is PD-free.
