The Value of a Transformer Bushing Monitoring System

Without bushings, power apparatus such as Switchgear and Power Transformers could not exist. All high voltage bushings have a built-in capacitor or condenser consisting of many dielectric layers with a conductor on one side that creates an equipotential gradient that prevents a flashover to the grounded tank. The number of layers is a function of voltage applied and physical design.

Approximately, 15% of transformer failures are due to bushing failures. Bushing failures rank as the 3^rd leading cause of transformer failures. Such failures may lead to catastrophic damage to the Power Transformer as arcing may take place along with a tank rupture that may lead to a fire. Given the implications, the value of a transformer bushing monitoring system is clear.

Transformer Bushing Monitoring Technology

The quality of transformer bushing monitoring systems varies greatly. For example, one large electric utility had approximately 400 bushing monitors installed in their system. They found that they were getting false alarms when the online bushing monitoring system indicating a bushing is bad. When offline testing was performed, the bushing was found to be good. In another case, the same bushing monitoring system missed detection of a bushing that eventually failed creating an unplanned outage. Subsequently because of the inability for the bushing monitoring technology selected to reliably and accurately predict if a bushing was good or bad, this utility decided to take all 400 bushing monitors out of service. For years there have been pivots but no significant improvements have been made to the underlying issues with online Power Factor and Capacitance measurement technology until now.

To understand bushing monitoring technology, one needs to understand how bushings are constructed and how offline bushing testing is performed.

How a Bushing Monitoring System Works: Bushing Construction

Below is a brief introduction to the construction of a high voltage power transformer bushing followed by a review of the failure modes that offline or online methods need to consider as well as an explanation of offline testing methods.

Figure 1 illustrates a high voltage transformer bushing with a Voltage Tap in conformance with IEEE Std™ C57.19.01. Lower but still high voltage bushings will not have a C2 capacitor and will be hence called a Test Tap. Important components include:

• A draw lead which allows connection from a single phase to the lead of the winding within the Transformer’s tank.
• Capacitor layers that are in essence in series where the total capacitance is equal to:

• The Capacitor layers are previously discussed and described above.
• The Insulating material impregnates the dielectric capacitor layers and is either oil for Oil Impregnated Paper (OIP bushings) or Resin Impregnated Paper (RIP bushings). These materials if they remain uncontaminated maintain the dielectric integrity of the bushing capacitor.
• The Insulator is constructed from either porcelain or resin and use concentric rings that improve the insulator’s intrinsic dielectric integrity.

Bushing Failure Modes

Several factors can cause bushing failure. Degradation of the insulating properties due to moisture ingress because of a faulty gasket, contamination, or a breakdown of the insulating materials are all possible causes. Capacitor layer failure is another. Sometimes a capacitor layer fails, putting added dielectric stress on the other layers causing them to fail. Failure can even result from a bushing tap not being properly grounded – a factor often overlooked.

Often it is almost impossible to determine exactly why the bushing failed. Hence the importance of both offline and reliable and accurate online testing of bushings.

Offline Transformer Bushing Monitor Testing

Offline bushing testing has been used for years and is a reliable method for determining the condition of a bushing because the bushing is tested in isolation from the transmission or distribution circuit the transformer is connected. A fixed test voltage of approximately 10 kV is applied. It is the Gold Standard of bushing testing because it allows accurate measurement of the bushing’s dielectric dissipation factor also known as tan δ or power factor also known as cos ϕ as well as the bushing capacitance. However, offline transformer bushing monitor testing requires a transformer outage and outages are inconvenient and expensive.

Online Transformer Bushing Monitor Testing

For over 20 years, users of high voltage bushings have been looking to manufacturers to develop an accurate, reliable, and intrinsically safe online bushing monitoring system. However, the product offerings available today are based on an imbalance of current. This is an unreliable measurement to determine the Bushing’s capacitance, the Bushing’s Dissipation Factor also known as tan δ , or the Bushing’s Power Factor also known as cos ϕ because of the following technical reasons:

1. The voltage applied to the bushing according to ANSI/NEMA C84.1 allows the voltage through the bushing to vary. To calculate the capacitance, one must know the voltage applied to the bushing.
2. A precise measurement of Power Factor or Dissipation Factor cannot be computed without the accurate value of C1.
3. In older transformers the bushings may not be identical. If they are not identical there will be an impedance mismatch and calculations of C1, Power Factor, and Dissipation Factor will be in error.

APT’s Transformer Bushing Monitoring: The Total ECLIPSE

APT’s intrinsically safe bushing monitoring system included with our Total ECLIPSE, addresses issues of accuracy, reliability, and safety. Regarding accuracy our Patented technology allows retrieval of the Phase voltage remotely or locally if available. Thereby allowing accurate measurement of Capacitance, Power Factor and Dissipation Factor without any black magic. Regarding safety, APT’s bushing coupler interface connects directly via fiber making Advanced Power Technologies bushing monitor the only intrinsically safe bushing monitor on the market.

APT’s bushing monitoring products address the issues of accuracy, reliability and safety and will protect against transformer failure. Hence, why old technology bushing monitoring systems produced poor results and why APT’s products can be trusted to find a bad bushing while at the same time keeping your field personnel safe by connecting to the bushing via a fiber optic connection.