Electrical transmission and distribution systems are the heart of every economy. Demand for electricity rapidly increases as a country becomes more developed and industrialised. Transformers are an essential part of the power grid that supplies electricity to homes, offices and factories. However, you may look at them as “potential weak links” in the grid. If one fails, the network is rendered vulnerable. If a power surge occurs across the network, knocking out a number of transformers, the electrical grid could be crippled for months, even years.
As an example, a transformer fire in June 2007 at Krümmel nuclear power station in Germany caused this plant and another nuclear plant at Brunsbüttel to be shut down. The resulting repairs put Krümmel – the largest reactor of its kind in the world at this time – out of action for two years. Days after the plant was restarted in June 2009 further transformer problems forced it to be shut down again, creating power shortages across Hamburg. Following that, Vattenfall, the plant’s owner, decided to replace two of Krümmel’s important transformers, causing another lengthy shutdown1.
The problem is that these transformer units are big, expensive to build and largely custom made, meaning that it can take up to three years to replace a transformer if no spare transformer is available.
Power transformers contain large quantities of mineral oil for insulation and cooling purposes. To prevent faults and to minimise the damage in case of a fault, they are equipped with both protective relays and monitors. However, roughly half of the fire incidents are related to the bushings, which are not covered by any protective devices. Bushings are made of porcelain filled with Oil Impregnated Paper (OIP). There is simply no way to attach detecting equipment on such a porcelain body without destroying its insulation properties.
Bushings may develop cracks, lose the oil and catch fire due to arcing. The bushings also may lose their insulation properties due to aging, build up gas and explode as a result of arcing. The result is a burning oil film all over the transformer’s oil tank. In such a case, instantly disconnecting the transformer and extinguishing the fire is essential to the saving of the transformer. This can only be achieved by applying ultra-fast responding heat or flame detectors. These detectors are therefore substantial to reducing the damage of a fire incident and thus very valuable for asset protection purposes.
While the probability of a transformer failure due to an explosion is low, it cannot be considered as insignificant. In the event of an explosive fire occurring in a bushing, in a cable box or within the oil-filled transformer, there is a high probability that it will develop into a serious disaster, causing loss of the transformer, loss of supply and, in the worst case, loss of life. Therefore, a fire is a serious risk and the awareness among operators of power transformers is generally high.
Risk
The probability of a transformer failure varies considerably among types of transformers, but it is typically in the range of 1% per transformer service year. In practice, this means that 3% of all transformers will cause a fire during a 40-year service life.
Cause
Transformer fires are predominantly mineral oil fires. Erroneously tank rupture is often assumed as the dominant cause. In particular for voltage levels below 300 kV, failure of oil impregnated paper bushings and air/oil insulated cable boxes account for 70% to 80% of transformer fires. On-load tap changers account for further 10% to 15% of transformer fires; even in the remaining 15%, a range of causes other than tank rupture are causes for fires. In fact, tank ruptures are rare for voltage levels below 245 kV, as the arcing fault energy is often below the critical level of energy required to cause tank rupture2.
The main root causes for fires initiated by failure of OIP bushings are:
- 80% leakage (defective seals)
- 13% deterioration of insulation
- 7% mechanical damage (cracks in the porcelain body)
Leaking bushings may spread oil over the transformer main body. This oil will partially evaporate and may be ignited by the arc created by a switching process.
Damage
When a transformer does fail, the result is often catastrophic. A power substation by its nature contains all of the ingredients to generate the perfect firestorm: A typical transformer bank is comprised of three or more transformer tanks, each containing thousands of litres of highly flammable mineral oil. The ignition of the oil can come from a variety of sources during a failure or short circuit electrical arcing inside the tank, any of which can generate heat and pressure sufficient to cause the tank to rupture. Once a rupture has occurred, air rushes into the tank and the tank explodes, resulting in a blast of intense radiation scattering oil, steel shrapnel, gaseous decomposition products and molten conductor material onto the surrounding area. The duration of a transformer fire can range from 4 to 28 hours, the time it takes the fire to burn out by itself3.
Transformer fires can quickly result in the partial or total loss of the entire electrical substation. However, the higher cost by far is the replacement of energy, which must be purchased from the spot market at premium prices. The rates for maintaining business continuity can spike up to 200’000 per hour during peak hours.
Risk mitigation
Specific transformer fires are difficult to anticipate and prevent, however, additional damage is preventable. During the transformers lifespan, structural strength and insulation properties of materials used for electrical insulation deteriorate. Aging reduces both mechanical and dielectric strength. For this reason, fire damage is largely preventable with the proper maintenance and fire controls in place.
For example, replacing aged bushings will significantly reduce the fire risk since these components are classified as the single leading cause of transformer fires. Similarly, doing regular checks of the transformer’s integrity by performing frequency response analysis will uncover winding and tap changer problems at an incipient stage4.
Early warning as damage mitigation measure
Transformer tank and tap changer failures
Electrical faults within the transformer tank and the tap changer are detected by a gas relay (known as the Buchholz relay). It detects electrical faults in oil-immersed transformers. Usually there are two Buchholz detectors installed: One between the transformer’s main tank and the oil conservator and the other above the tap changer5.
The Buchholz protection is a fast and sensitive early warning fault detector as it accumulates gases produced by minor faults rising from the fault location to the top of the transformer. A gas bubble build-up within the relay housing finally activates the relays (mercury switches).
The Buchholz relay has a second triggering device consisting of a pivot vane. This vane triggers the shutdown of the transformer in case of a steep build-up of pressure (typical for major faults) due to short circuits either to earth or between phases or windings. Such faults rapidly produce large volumes of gas and oil vapour, which cannot escape. This sets up a rapid flow of oil from the transformer towards the conservator. The Buchholz vane responds to this high oil flow by closing a mercury switch. Both the early warning Buchholz switch for minor faults and the vane switch for major faults may be used to release extinguishing agents5.
Heat Detection
The fire initiated by failure of OIP bushings have a limited impact if a fire extinguishing agent is released instantly. Detecting the fire as near as possible to the zone of ignition is therefore imperative for triggering the extinguishing system. However, this is most likely not the case for standard heat bulb activated sprinkler heads, because these components are part of the extinguisher tubes, which are placed a few meters away around the transformer. In an outdoor scenario, rain and wind may delay the build-up of enough heat energy to cause bursting of the initiating device to such an extent, that the trigger is factually disabled.
The same comparatively slow heat detection applies for standard point type heat detectors, which face additional challenges, such as operating temperature range (day, night, winter, summer), humidity changes (rain, snow, fog), lightning strikes, dust, dirt, etc.
A water (often a water and foam mixture) spray system usually involves heat actuated detectors operating an automatic mechanical flooding valve that supplies the extinguishing agent to the spray nozzles. Transformer fire detectors must be able to cope with these challenges and in addition, must respond extremely fast to an incident in order to keep the damage as low as possible. This can be achieved using industrial grade flame or heat detectors. However, because detectors should be mounted as close as possible to the potential flame zone, the sensing element must be able to withstand harsh conditions.
Ultrafast responding heat detectors
The fastest responding heat detector in this category is Securiton’s ADW 535, a sensor offering rate-of-rise heat detection within a few milliseconds for releasing extinguishing valves instantly. It is based on a sealed metal tube whose inherent pressure is evaluated at a rate of 2’000 samples per second at 0.2°C precision. This sensing tube is placed in the ultimate proximity of the transformer tank.
The tube material is copper or stainless steel. Special care is therefore not required when cleaning the transformer with pressurised air or cleaning chemicals. Lightening protection is simple: A connection of the tube to earth will do.
For more information, go to www.securiton.com
References
- Risk and consequences of transformer explosions and fires in nuclear power plants, Heinz-Peter Berg, Nicole Fritze, Bundesamt für Strahlenschutz, Salzgitter, Germany, www.degruyter.com/downloadpdf/j/jok.2012.23.issue-1/jok-2013-0034/jok-2013-0034.xml
- Risk Equals Probability Times Consequences, Arne Petersen, AP Consulting, April 1, 2014 Transmission & Distribution World http://tdworld.com/substations/risk-equals-probability-times-consequences
- Electrical Transformer Fire and Explosion Protection, KAFACTOR Group, 12 Bram Court, Unit 20, Brampton, ON L6W 3V1, Canada www.kafactor.com
- Transformers Continue to Fail, Irfan Akhtar, Lahore, January 2016 Science & Technology Articles: http://hamariweb.com/articles/70798
- 4 power transformer protection devices explained in details, Edvard Csanyi, EEP Electrical Engineering Portal, April 20, 2015 http://electrical-engineering-portal.com/4-power-transformer-protection-devices-explained-in-details
About the Author
Markus Mueller is Director for Vertical Application Management at Securiton AG.
