by IAR Gray, Oilwatch Transformer Laboratories

Wind turbine step-up transformers, which boost turbine outputs from a few hundred volts to medium voltage distribution levels, have a high failure rate. This is not unexpected as transformers follow the Bathtub failure curve. This trend affects both liquid filled and dry type transformers.

Most wind projects use pad mount liquid-insulated transformers, and the most common models which have been installed, do have their shortcomings. Many are designed and rated as distribution transformers rather than generator step-up units, which has created a high level of early failures.

This article presents data indicating that a significant percentage of these mineral oil insulated transformers have elevated gas levels. Along with this data, case studies will be discussed on the most likely causes of wind turbine step-up transformer faults detected by Dissolved Gas Analysis (DGA).

The assessment of a transformer’s magnetic core and windings can be carried out in three ways, i.e., thermal, dielectric and mechanical assessment. Condition assessment techniques like DGA through testing at a laboratory or on-line DGA monitors are used to analyse the symptoms of an incipient fault being developed in transformers based on IEEE/IEC standards and numerous sources of information.

Transformer asset managers are trying to achieve the required levels of safety and reliability from their fleet of transformers at minimum cost. Knowledge of condition is therefore essential for efficient transformer asset management decisions. Without this information only the most basic activities are possible, such as time-based maintenance, replacement before end of life, or repair after failure.

For asset managers, determining the minimum required budgets for maintenance and replacement and determining the most effective and targeted way of spending, is an important task. Often this must be justified to stakeholders and regulators in an increasingly competitive environment.

Insulating-oil testing is typically a critical first step in any power transformer analysis.

The use of dielectric liquid in a transformer provides a valuable way of assessing changes in the internal condition of the transformer. Removing a dielectric liquid sample for dielectric liquid testing and dissolved gas analysis works similarly to a human “blood test” for the transformers. The detection of abnormal patterns of behavior of the dielectric liquid properties offers information about the dielectric liquid itself, as well as transformer components immersed in that dielectric liquid. These dielectric liquid tests are essential for the condition assessment, for the predictive maintenance and for preventing unexpected outages, but they are much more of an investigation than just a test.

Dissolved gas analysis

Dissolved gas analysis (DGA) is a powerful diagnostic technique used to analyse dissolved gasses that are generated during the dielectric fluid and solid insulation decomposition process. This technique has been an industry standard for the detection and determination of faults in transformers for over 30 years and is recognised worldwide as the main tool to prevent catastrophic failures of power transformers. DGA has become the most informative test for power transformers in our era. See Figure 1.

Industry experts say that DGA is the most powerful tool in the industry. Laboratory-based DGA tests are usually the preferred choice, since their low cost, efficiency and capabilities are in most cases superior to the best available DGA portable and online monitors.

DGA is a multidisciplinary field and to become an expert on DGA, one must study all available DGA related material, literature, and other sources of information. It would be good to remember the age-old adage: “There is no substitute for experience”.

Importance of accurate data for improving asset reliability

Data is the most valuable commodity in today’s world, and it is no different in reliability engineering. Inaccuracies can quickly aggregate and escalate from a minor niggle into something that compromises all the efforts that have been previously invested, therefore, to obtain a reliable sample plays a critical role.

Transformer oil sampling

A programme of training the staff with this important aspect had a positive outcome. This upskilling and certification involved the understanding of the role of data within the company, included what data will be collected, how often, and for what purpose it will be used. The importance of capturing the correct nameplate information and location was also highlighted as this provides a primary source of information in creating an accurate asset register, which is a requirement of ISO 55000.

The first step in any asset management process is to work out what you have and keep the information current. This may sound easy, but there are challenges, as many sites may be involved, existing asset registers may conflict, with different names for the same item. Some types of equipment may be over-looked therefore site visits are necessary to confirm data. Data hygiene is an issue that requires constant attention and communication, where there must be clear definitions of roles, consistent with objectives.

The procedure in the use of dissolved gas syringes was also covered, as their use improved the accuracy of the DGA data and limited the oil wastage due to the increased sampling frequency. It is important to remember that the test results are only as good as the sample taken.

Transformer oil testing laboratories (ISO/IEC/SANAS 17025)

In South Africa there are a limited number of transformer oil testing facilities with accreditation to the ISO/IEC 17025 standard, however there is a local proficiency testing scheme, with the specific objective to improve the quality of insulating fluid results. This article makes use of DGA data from various sources, and it is encouraging to note that minimal DGA data was rejected as outliers. There is a lot of confusion about “accredited calibration”, why it is required and what it is really about.

The correct standard used for calibration and testing laboratory accreditation is ISO/IEC 17025. Beyond a quality management system, accreditation is a global standard that recognises the technical competence of a laboratory to perform specified tests.

The ISO/IEC 9001 standard only requires that test equipment used in a facility must be calibrated. The third edition, published in November 2017, considers numerous changes in market conditions that have occurred since 2005. There is an increased focus on impartiality, where commercial, financial, or other pressures must not compromise this. To comply with this clause, only authourised personnel may release opinions and interpretations based on relevant standards.

DGA interpretation

DGA is an important indicator of dielectric breakdown failure (considered essential).

The main gas signatures are:

• Hydrogen: Hydrogen is the key gas for partial discharges in oil or gas. Typically, there will be methane gas present as well.
• Carbon oxides: Dielectric breakdown inside the solid cellulose insulation material will generate carbon monoxide and carbon dioxide.
• Sparking gasses: Partial discharges of the sparking type (IEC 60599 D1 [43]) will create hydrocarbon gasses
• Arcing gasses: Complete breakdown of the insulation material, particularly from winding to winding or winding to ground, will result in an electrical arc with high levels of acetylene as well as other hydrocarbon gasses and hydrogen.

Since the 1970s numerous diagnostic schemes have been proposed, all with advantages and disadvantages. Duval triangles and Duval pentagon’s fault interpretation techniques for mineral oil are now a part of IEC 60599 – 2015 and IEEE c57.104-2019 standards. Using fault categories of Duval triangles and Duval pentagons, the six basic types of faults (PD, D1, D2, T3, T2 T1) are detectable with Duval triangle 1 and pentagon 1, and the five sub-types of faults (T3-H, C, O, S, PD) are detectable with Duval triangles 4, 5 and pentagon 2.

Stray gassing

To complicate the interpretation further, there are cases of “stray gassing oils” which can produce gases in transformers at temperatures from 105°C. Stray gassing is a non-damage fault and can be indirectly evaluated using the Duval methods. To achieve a >90 % Confidence Normality in DGA interpretation, a combination of the relevant standards and methods is required, where the rates of gas rise are of overriding importance.

Transformer failures

The risk of transformer failure is two dimensional, namely the severity and frequency.

The consequence of transformer failure is significant due to the following factors:

• Damage or injury from fire or explosion
• Environmental damage
• Size and type of load interrupted
• Duration of the possible interruption
• Time to repair or replace the transformer

Generally, the frequency of transformer failures is relatively low, but the failure rate within the Renewable Energy Industry is cause for concern due to several factors. There could be many initiators which cause a transformer failure, but a study like the CIGRE Transformer Reliability Survey WG A2.37 survey, would provide some helpful statistics to improve reliability. Historically, the wind turbine generator (WTG) transformer function has been handled by conventional, off-the-shelf distribution transformers. But a relatively large number of recent failures has convinced many that WTG transformer designs must be substantially more durable.

Key characteristics of WTG step-up transformers that wind farm owners and developers should pay attention to include transformer loading, harmonics and non-sinusoidal loads, transformer sizing, voltage variations, and special requirements to withstand faults.

Case studies

The following case studies of transformer faults detected by DGA will provide some insight to the challenges faced by wind farm owners.

Example of a unit affected by improper manufacturing from a wind farm

Consider the case of a pad mount transformer for a 2,7 MVA, 33 kV WTG, without an on-load tap changer (OLTC). The transformer was manufactured in 2014 according to SANS 780 and installed in 2016. The nameplate information is given in Figure 2.

The Duval pentagon showed a thermal fault (T3), (Figure 3), as did the gas signatures (Figure 4), with significant rates of gas rise (Figure 5). Note that the abnormal gassing pattern was evident after energisation on load. The internal inspection revealed several quality issues. Figure 6 shows one the findings.

Partial discharge and harmonics

In this case study of 55 transformers at a wind farm, the DGA showed a significant number with abnormal gas levels when compared to sister units, as seen in Figure 6. The fault category in most cases was partial discharge (PD) activity. According to CIGRE 761 Assessment Index (TAI) Scoring Matrix (Risk), the results are shown in Figure 7 (Transformer scored as Category D: E: F), while Figure 8 shows the gas signatures.

The main cause of the PD was identified as sharp edges on the foil windings as well as bad workmanship in the factory exacerbating the PD, due to stop-blocks damaging the paper. The process followed to properly cure the diamond dotted paper had not been followed correctly resulting in the paper not curing to the foil properly or even at all. Harmonics was also a role player.

Harmonics

Unfortunately, the subject of harmonics is complex and extensive but there is compelling evidence of a link to the gassing issues in windfarm transformers. It’s certainly worthy of additional investigation in respect to the PD activity detected by DGA.

PD and safety concerns.

PD activity typically occurs within insulation voids, on ungrounded metal objects lying in an electric field, or as corona due to the intense electric stress on the insulation surrounding sharp edged electrodes.

Safety concerns of high levels of hydrogen and hydrocarbons in active transformers

Due to the volatility and buoyancy of hydrogen and hydrocarbons (CH₄: C₂H₄: C₂H₆:C₂H₂), it is generally recognised that concentrations of 4-74% are flammable and at 18-59% the mixture is explosive. It could be expected that the concentration of H₂ and the hydrocarbons may be even higher in the transformer's gas space. This means that a spark in the gas space must be avoided at all costs.

Safety concerns of gas bubble formation

Gas bubbles lower the effective dielectric constant of the oil which in turn increases the electric stress in the oil and in particular across the gas bubbles. The high stress causes voltage breakdown across the gas bubbles; these gas bubbles elongate in the direction of the electric field resulting in flash-over-voltage stress.

Standards

Clearly there is much work to be done before all the issues associated with wind farm transformers can be understood. The industry needs to move from outdated standards to accepted International best practice, like the new Dual Logo Standard IEC/IEEE 60076-16, which might help guide wind farm owners to more trouble-free and reliable systems. To support this effort, the cooperation and input of all interested parties throughout this industry is needed.

Conclusions

This head-long rush to install more and more wind turbines has outstripped the usual developmental learning curve, where new technologies mature by a process of trial and error, but the use of properly applied DGA has already made a positive contribution in improving reliability.

Providing a quality service

Michael Venter, the business unit manager at Consolidated Power Projects had this to say: “I would like to thank you for the valued input we receive from you and your team. We are ‘making history’ as we are slowly but surely resolving the transformer gassing issues experienced in the industry at the moment in South Africa. Please thank your team on our behalf for their level of commitment and service to resolving this issue. We are of the belief that the latest design will finally resolve this issue. The role you and your team played in this process cannot be ignored and I would like to thank you for that.”

Further considerations

The lack of knowledge sharing and collaboration on best practices of design specifications, condition assessment, maintenance and maximizing lifecycles of power transformers, is an issue that needs attention. Knowledge and data sharing among end-users can also help to compensate for unfamiliarity with a specific type of critical component, technology, or supplier. Sharing the post-mortem and failure pattern data would provide the ability to anticipate a failure mechanism without any prior experience. The current Non-Disclosure Agreements (NDA) circumvent the ideal of knowledge sharing and should be addressed by the industry.

Acknowledgment

The author expresses his gratitude to all contributors.

About Ian Gray

Ian Gray is a member of the South African Chemical Institute and the South Africa mirror committee for IEC TC 10(International Electro technical Commission). He also serves on the SANAS/SABS 290 committee (Mineral insulating oils - Management of polychlorinated biphenyls (PCBs). His over 35 years of experience includes transformer oil testing, diagnostics and internal inspections coupled with the ISO17025 Quality management system.

Contact Ian Gray, Oilwatch Transformer Laboratories, Phone 087 238-2188, support@oilwatch.co.za, https://oilwatch.co.za