Verifying suspected insulation faults in power transformers

TEIAS, an electrical transmission utility in Türkiye, relies on MONTESTO 200 as part of its holistic approach to assessing the insulation condition of power transformers in its repair workshop and the field.

Introduction

Power transformers are one of the power grid’s most important high-voltage (HV) assets. They are the key elements in generation, transmission, distribution networks and industrial sites. With the advancing age of power transformers worldwide, it is vital to check their insulation condition by continuously assessing the health of various components to ensure reliability.

A power transformer is exposed to many stress factors during its life cycle due to environment and load conditions, high electrical field stress, mechanical vibrations, and chemical contamination. A lack of a regular diagnostic testing and maintenance plan can result in eventual undetected insulation faults that can shorten the expected service life of a power transformer. 

Assessing insulation condition in power transformers

A holistic approach is generally adopted for the overall check of insulation condition in power transformers based on Dissolved Gas Analysis (DGA), various electrical tests, monitoring, and visual inspections.

DGA results from the insulating oil are used as a first alarm for any evidence of problems that can occur inside oil-filled power transformers. Abnormal levels of certain gasses can indicate potential problems, triggering additional investigations to identify and localise developing faults using electrical testing, monitoring, and a direct visual inspection inside the transformer.

Advanced electrical testing and diagnostic tools, such as Dielectric Frequency Response (DFR) testing, Sweep Frequency Response Analysis (SFRA) testing and Partial Discharge (PD) testing and monitoring are now widely used to assess and diagnose the insulation condition of power transformers by detecting certain faults that conventional testing methods are unable to detect. In this article, we will focus on PD testing and monitoring.

Partial discharge testing on power transformers

According to the IEC 60270 standard, a PD is a localised dielectric breakdown of a small portion of a solid or liquid electrical insulation system under high-voltage stress. PD is a reliable indication and an accelerator of fault development in electrical asset insulation. This is why PD measurement and monitoring play an important role in assessing insulation conditions to maintain the health and reliability of electrical equipment, such as power transformers.

PD measurements on power transformers are initially performed at the factory as a quality control tool to assess transformer insulation condition according to specified acceptance criteria. During commissioning, PD measurements are performed to check the power transformers’ installation condition after transport. During the power transformer’s service life, PD measurements and monitoring are regularly performed for troubleshooting to detect and localise potential faults.

Many utilities worldwide have established maintenance programmes based on periodic PD testing and monitoring of their power transformer fleets since PD is a reliable indication of ageing and developing faults in the insulation. Corrective actions can be taken before a developing fault eventually causes the asset to fail.

PD testing, inspection and repair of a 100 MVA transformer

I am the Chief Engineer responsible for field test evaluation and test method development at TEIAS Turkish Electricity Transmission Corp. Me and my team have developed a holistic approach to condition assessments and a solid maintenance plan for our grid assets, particularly power transformers. If potential faults are detected in the field, the transformers are taken out of service and sent to our central workshop facility in Ankara, Türkiye. Medium and large power transformers are tested and diagnosed in our workshop’s HV lab to determine if repairs and refurbishments are needed to return the transformers to reliable service.

In one case, our test engineers investigated a 154/31.5 kV (100MVA) 3-phase power transformer which has been in service since 1996. A DGA analysis initially performed on the transformer in the field displayed a continuous increase of hydrogen (H2) and methane (CH4) gasses, which indicate potential PD activity and damage to the insulation in the transformer.

Increasing hydrogen (H2) levels were indicated after performing a DGA analysis on a 154/31.5 kV (100MVA) 3-phase power transformer.

Due to these increasing gas levels, we removed the transformer from the grid and sent it to our central workshop for further inspection and repair. In our workshop’s HV lab, our test engineers started by performing various electrical tests, including a DFR test, but no results were obtained to verify their suspicions of PD activity.

PD testing revealed various PD sources

As a next step, a series of PD tests were performed with OMICRON’s MONTESTO 200 temporary PD monitoring system using the induced voltage method. The PD tests were done over extended periods of time at different central frequencies and bandwidths to obtain the best signal-to-noise ratio. The patterns obtained clearly indicated high PD activity from various PD sources.

Different 3PARD diagrams were obtained that indicated various potential PD sources.

Thanks to the 3PARD diagram (3-Phase-Amplitude-Relation-Diagram), which is integrated into the MONTESTO 200 monitoring software capabilities, our test engineers could see many clusters, indicating various potential PD sources in the transformer, which could be separated for individual analysis.

Analysing different 3PARD clusters

When applying the 3PARD filters on some clusters displayed around the three phases, symmetrical discharges around the zero crossing characterised a predominant PRPD (Phase-Related PD) pattern. The shape of the pattern indicated it was probably related to voids in the insulation material, possibly in the oil. Still, even after applying the vacuum to remove the oil in the transformer tank, the same discharges remained with the same PRPD diagram, which means that the voids might be on or inside the solid insulation. On the other hand, according to CIGRE 676, this PRPD diagram is very close to the one related to voids in glue.

3PARD diagrams showing signs of possible carbonisation and floating particles in the transformer.

Besides, another PD source could be seen very clearly with the 3PARD filtering, and it was most probably due to carbonisation on the pressboard barrier – mostly around phase W. Still, it could also be seen in the other phases. Filtering other clusters revealed yet another type of PD related to floating metallic particles around phases U and V.

Visual inspection confirms PD testing results

Based on the PD test results, our test engineers decided to open the transformer for a visual inspection to identify suspected damaged components. After the transformer was opened, it became clear that our assumptions based on the PD test results were correct, and the extent of the damage was immediately apparent. After removing the oil from the tank, many copper dust particles deposited in different places inside the transformer were visible. This copper dust was likely the floating metallic particles suspected when the oil circulated.

Additionally, traces of carbonisation were found in almost all the phases (HV coarse, HV fine, LV coarse) on the pressboards and wedges. In addition, our inspection team also found many traces of damaged and burned glue on the upper side of the coarse-tuning windings of phases B and C. The transformer was then repaired in our workshop. All the damaged parts were replaced with new components and materials.

Copper dust particles were deposited inside the transformer tank.

Traces of carbonisation were found on various components.

Traces found of damaged and burned glue.

PD monitoring verifies the success of repairs

Additional PD tests were performed in our workshop’s HV lab after the repairs were made, and no suspicious PD activity was detected. Once back in service, continuous on-site PD monitoring was performed for many months with the MONTESTO 200 device used in our repair workshop. No additional PD activity was recorded that indicated remaining or developing faults in the transformer. This was a verification to us that the repairs were successful. The transformer was back in reliable service.

Visit us at- omicronenergy.com/montesto200

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Quote: “After we opened the transformer for visual inspection, it became clear that our assumptions based on the PD test results were correct.”-

Authored by: Mükremin Yanan Chief Engineer Field Tests Evaluation and Test Methods Development

TEIAS Turkish Electricity Transmission Corp.

Co-Author : Sofiane BAKKAY Regional Application Specialist Transformers/Rotating Machines OMICRON Middle East