Bushing Health Monitoring

It appears that all new capital equipment that is purchased for distribution and transmission systems is not with just the intent of replacing aging equipment but also with the intent to gauge the expected lifetime cycle of the new unit. This allows for an increase in planned outages to repair/replace parts that are soon to fail and a decrease in unplanned outages to address parts that have already failed. With transformers, bushing failures can be catastrophic to human life and to the integrity of the transformer. One way to monitor the health of a transformer bushing is to measure the capacitive reactance of the series capacitances within the oil impregnated, foil layers that surround the electrical conductor. This monitoring is done by advanced electronics such as performed by Dynamic Ratings. Sometimes the monitoring systems can give false alarms. The aim of this article is for the field technician to understand how the bushing health monitoring system works and how to perform manual in-service measurements to verify the legitimacy of an alarm.
 
Transformer bushings will typically have a connection point to the outer foil layer. This connection is called the C2 tap and gets shunted to ground via the installation of its cap cover. For out of service testing, when the cap is removed, the equivalent series capacitance from the conductor to the tap is called C1 and the capacitance from the tap to ground is called C2. Both C1 and C2 are given on the nameplate of the bushing in units of picofarads. Doble testing takes advantage of the C2 tap to measure the capacitances of C1 and C2 and the Watts loss of the bushing by applying a test voltage to the bushing terminal. Dynamic Ratings basically performs this Doble testing while a transformer is in service by removing the cap and shunting the C2 tap to ground via a resistor or "burden." The Dynamic Ratings system will constantly measure the voltage across and the current through all three resistors (three phase system) for a set of bushings such as H1-H2-H3 on a transformer. If one bushing begins to fail, the magnitude of the sum vector will breach a set circular zone called the bushing intolerance limit, which can be adjusted via the settings of the monitoring unit.

In this test, by knowing line voltage, frequency, burden resistance, and nameplate capacitances of C1 and C2, a field technician can determine to some degree the health of the bushing via voltage measurements taken by an Arbiter. In this example, line voltage is 69kV, frequency is 60 Hz, and the burden is 500 ohms. The reference chosen was A phase on the low side of the transformer. A potential transformer is connected to 7.6kV A phase and its 120V output provides a signal to the load tap changer controller.  Because C2 has a high impedance at 60 Hz and is shunted by only a 500 ohm resistor, C1 and the burden resistance can be essentially considered a series circuit and the voltage divider rule can be used to calculate the voltage across the burden. Therefore, by using Ohm's Law calculations, the voltage across the burden was calculated to be 1.8V. And since the equivalent impedance between a bushing terminal conductor and ground is mostly capacitive, it was expected that the burden voltage would lead the system phase-to-ground voltage by 90 degrees. The Arbiter proved these calculations. If one of the bushings were going bad, where foil layers within the bushing would begin shorting together, then the capacitance of C1 would go up, the capacitive reactance of C1 region would go down, and the voltage across the burden would go up. All burden voltages Ax, Bx, Cx are under 1.8V and evenly spaced about 120 degrees apart. Therefore, the magnitude of sum vector for all three burden voltages would likely be very small.

Article by D Scrobe III