Transformers are of the key equipment used for electrical power transmission and distribution. Hardly, ever could anyone find an industrial facility which does not take benefit from a form of this equipment. As essential and practical transformers are, they may pose significant risks to the facility and may need special safety considerations. In this article a review on this type of equipment, its makes, application, possible hazards and in line protection measures is presented.
A transformer is a device that transfers electrical energy between circuits using electromagnetic induction, allowing it to either increase (step up) or decrease (step down) voltage levels. They are widely used in power grids to step up voltage for long-distance transmission and step down for local distribution, in electronic devices like chargers to convert high voltage to usable levels, in industrial equipment to match voltage requirements, and in audio systems for impedance matching. Transformers come in various scales, from small ones in electronic devices to large, heavy ones in power grids, ensuring efficient and safe electricity delivery across different applications. Large-scale transformers are critical components in power grids and industrial applications, designed to handle high voltage and power levels. These transformers are essential for stepping up voltage for long-distance transmission and stepping down voltage for local distribution, ensuring efficient and reliable power delivery.
Oil-Filled vs. Dry-Type Transformers
Oil-filled transformers use mineral oil or other insulating liquids (such as silicone oil) for cooling and insulation. The oil circulates within the transformer, dissipating heat generated during operation. These transformers are highly efficient due to their excellent cooling properties, allowing them to handle higher loads and operate more efficiently. They also have a long lifespan, typically lasting 25 to 35 years, and can handle overloads more effectively. Additionally, oil-filled transformers are easier to recycle at the end of their service life. On the other hand, dry-type transformers use air or solid insulation systems for cooling and do not contain any liquid. They rely on natural or forced air circulation to dissipate heat.
Oil-filled transformers, pose a fire hazard due to the flammable nature of the oil and require regular maintenance, including oil sampling and filtration. There are also environmental concerns related to potential oil leaks, which can be harmful. They are generally not suitable for indoor use due to fire and safety risks. Dry type transformers are safer for indoor use as there is no risk of oil leaks or fire, making them ideal for environments where safety is a priority. They require less maintenance compared to oil-filled transformers and are environmentally friendly, with no risk of oil contamination. They can be installed closer to the load, reducing the need for long cables. However, they operate at higher temperatures, which can affect their efficiency and lifespan, typically lasting 15 to 25 years. They are also more expensive to manufacture and install and tend to produce more noise compared to oil-filled transformers.
In summary, oil-filled transformers are preferred for high-power, outdoor applications, while dry-type transformers are ideal for indoor or environmentally sensitive areas. Each type is chosen based on the specific requirements of the application, balancing factors such as efficiency, safety, maintenance, and environmental impact.
Hazards and Safety Measures
In majority of oil-filled transformers mineral oil is used, which has a flash point of around 150°C (300°F) and an auto-ignition temperature of approximately 350°C (655°F). While these temperatures are not typically reached during normal operation, an internal electrical fault can generate temperatures exceeding 540°C (1000°F), potentially igniting the oil. Internal faults can occur due to water infiltration, core insulation failure, external fault currents, and tap changer failures.
Protecting oil-filled transformers is crucial because they play a vital role in the transmission and distribution of electrical power. Any failure or fire in these transformers can lead to significant power outages followed by production loss, equipment damage, and other resulting hazards.
Oil-filled transformers have some safety measures in place to prevent fires and ensure safe operation among which the below could be listed.
- Buchholz Relay: This device detects gas accumulation and oil movement within the transformer, indicating potential faults. It can trigger alarms or shut down the transformer to prevent further damage.
- Pressure Relief Devices: These devices release excess pressure from the transformer tank, preventing explosions due to internal faults.
- Oil Temperature Indicators: These monitors track the temperature of the oil, triggering alarms if it exceeds safe levels.
While the built-in safety measures in oil-filled transformers are essential, they might not be sufficient on their own. Active and passive fire protection measures are crucial for providing an additional layer of protection. With this regard, the facility could benefit from automatic fire suppression systems such as fixed water spray or foam systems coupled with a fire detection system. Additionally, use of fire walls to separate adjoining equipment provides considerable advantage in terms of escalation prevention.
Above actions and be performed manually as well, via extinguishers and in place firefighting equipment yet care shall be given to the fact that before initiating manual firefighting, it is essential to have a qualified electrical equipment operator on-site to deenergize the involved equipment. This ensures the safety of the firefighting personnel and prevents further electrical hazards.
Is It Mandatory to Consider Automatic Water Spray System For Oil-Filled Transformers?
According to NFPA 850, oil-filled main, station service, and startup transformers that do not meet the separation or fire barrier recommendations of the relevant standard should be protected with automatic water spray or foam-water spray systems. Additionally, as per API 2030, large oil-filled transformers are typically installed with separation from process equipment, buildings, structures, or other transformers by distance or masonry walls. Water spray protection is generally not justified unless there is significant potential fire exposure to or from the transformer. When water spray systems are necessary, they should be designed and installed in accordance with NFPA 15, using an application rate of 0.25 gpm/ft² (10.2 lpm/m²) on all exposed surfaces.
Given the absence of specific criteria for implementing water or foam spray systems, it’s important to note that individual fire protection solutions can provide different levels of damage mitigation for specific applications. The owner must determine the acceptable level of fire loss to decide on the appropriate level of fire protection.
References
[1] IEEE Std 979- IEEE Guide for Substation Fire Protection
[2] NFPA 850- Recommended Practice for Fire Protection for Electric Generating Plants and High Voltage Direct Current Converter Stations
[3] API 2030- Application of Fixed Water Spray Systems for Fire Protection in the Petroleum and Petrochemical Industries
[4] IPS-M-EL-152- Material and Equipment Standard for Oil Immersed Power Transformers
