Ground-breaking monitoring and protection methods for OLTCs can ensure power delivery

Monitoring and protection methods for on-load tap changers, OLTCs, could predict and minimize unplanned outages and ensure uninterruptible power delivery. Tord Bengtsson and Nilanga Abeywickrama have analysed the extra power loss and circulating current during tap operations and the research showed ground-breaking results for electricity suppliers.

To ensure uninterruptible power delivery, relying on the functionalities of power transformers is of utmost importance. More than one-third of the transformer failures every year are caused by so called on-load tap changers, OLTC.   

OLTCs are used for voltage regulation. They are the only moving parts of a transformer and require frequent care and maintenance. A tap charger with resistor type transition impedance generates an extra power loss during its operation, which must be limited to a short time. Otherwise, the tap changer compartment can get overheated which could lead to catastrophic consequences. To handle situations like these, a protection function is needed.  

Tord Bengtsson and Nilanga Abeywickrama propose a new approach to tap changer monitoring and protection in their paper “On-load tap changer monitoring and protection by extra power loss and circulating current analysis”. By analysing the extra power loss and circulating current during a tap operation, their results showed ground-breaking outcomes. With the appropriate signal processing techniques, installing additional sensors or dedicated acquisition hardware is not needed in modern substations that provide digitized voltage and current signals.   

Nilanga Abeywickrama will present the results of this study at the CIGRE Biennial Session in Paris in August. The results of this research are considerable for the industry. Simply put, the methods make it possible for companies to save the additional cost that would have been required to install additional sensors or hardware. Instead, they can use the infrastructure in the already existing systems, in most cases.   

– I think one of the reasons behind the good remarks this research has gotten is because it’s a pleasant surprise that one could dig out this kind of information, says Tord Bengtsson and Nilanga Abeywickrama.  

In the paper, Tord Bengtsson and Nilanga Abeywickrama propose monitoring and protection methods that are applicable to resistance type tap changers. The methods have been verified with more than 20 000 field recorded tap changer operations from a few different tap changer models.  

The duration of the circulating current, one of the estimated parameters, is an important indication of how well the transition is performed. Also, significant deviations from the design value are a cause of concern that must be acted upon.  

By applying appropriate signal processing techniques, the steady part of power loss of a transformer can be predicted and removed. Thereby, the changes during a tap operation can be obtained with an accuracy better than 1 % of nominal value and a time resolution of about 1 ms   

– At first, one would say this is impossible because these changes are of the order of 1% signal level. Usually, normal conventional signal analysis techniques would not allow you to dig anything below. So, we’ve been able to dig further than one would normally do with traditional techniques, says Tord Bengtsson and Nilanga Abeywickrama.