Online Bushing Monitoring – a Case Study

In one of our recent news articles we covered the topic ‘Why Monitor Circuit Breakers?’, which was based on a presentation by Dominique Legrand, GE Product Line Manager for M&D products, exploring the six key drivers for monitoring these core pieces of infrastructure. We’ve also recently shared a white paper on what to look for in transformer DGA monitoring, also created by Dominique, which explored the consideration factors that have emerged as important when it comes to determining the best monitoring solution.

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More recently, Transformer Technology – the print and online publication produced by APC Media – featured a case study on the benefits of online bushing monitoring at Southern Power, a leading wholesale energy provider based in the US that operates 49 generating facilities with more than 11,200 megawatts of electricity generating capacity. Online bushing monitoring is another key way critical infrastructure providers can utilise the technology that exists today and enable asset owners to detect impending failures and reduce their maintenance costs.

Monitoring transformer bushings is critical, because bushings are constantly under high stress due to the line voltage and heat effect of current flow. These stresses can be further aggravated by the presence of micro cracks from manufacturing, loss of mechanical strength due to ageing, repeated thermal cycling (load + sun), pollution and external flashover melting the porcelain, sludge and moisture in the insulating oil, and by the fact that new bushings have been made closer to design limits in order to reduce cost, size and weight.

When Southern Power began commercial operation in 2016 they installed a bushing monitoring system from GE to monitor a key 50 MVA transformer at their 102-megawatt Henrietta Solar Project in Kings County, California. An online bushing monitoring system continuously monitors the capacitance change of C1 of a condenser bushing, as the bushing capacitance measurement is the most common and one of the key parameters in diagnosing the health of a bushing. Condenser type bushings have a central conductor wound with alternating layers of paper insulation with wrapped tin foil (creating capacitive layers). The capacitive layers are housed in a protective weather casing (usually porcelain) and filled with insulating oil. The capacitive layers are designed to provide uniform voltage drops between each condenser layer. When one or more of the capacitive layers are damaged (shorted), the total capacitance of the condenser increases, indicating a potential fault within the bushing, and immediately track and alert personnel of such changes.

On 26 March 2019, the data being received from the bushing monitor through the SCADA system showed an alarm coming from the phase B bushing on the LV secondary side. It was indicating an increase of the relative percent change in Power Factor (PF) from 311% on February 27th to above 895% on March 26th, 2019.

Based on the data from the bushing monitor, Southern Power decided to take the transformer offline to avoid a possible catastrophic failure of the bushing. They replaced the suspicious bushing and returned the transformer to service. Subsequent offline tests on the suspicious bushing confirmed that the bushing had indeed significantly deteriorated from its original values and that the operational decision taken had been correct.

The Southern Power experience demonstrated how online monitoring enhances the safe, reliable operation of substation power equipment, measures performance, reduces failure rates and provides more consistent and frequent information of the existing fleet. For more information about how eliminating – or at a minimum mitigating – typical time-based maintenance and testing schedules using online monitoring can help you, contact CSE Uniserve.

Acknowledgement: the content in this blog is based on the original article from Transformer Technology which was authored by Neil Hutchins (Southern Company Services), Elm Costa (GE), Kwasi Yeboah (GE) and Alfonso Ambrosone (GE) and coordinated by Randy Cox (GE).