Instrument Transformers are essential components in electrical power systems, serving the critical function of measuring and monitoring electrical parameters. These are specifically designed to reduce voltage and current levels, enabling a safer and more accurate measurement for revenue metering and monitoring of electrical current and voltages.
With their ability to accurately transform, instrument transformers are the cornerstone of reliable power system measurement and control.
When it comes to instrument transformers, understanding the difference between Cast Resin (Dry type) and oil-immersed (Oil-impregnated paper insulated) transformers is crucial for selecting the right fit for your specific needs. Each type has its unique characteristics, applications, advantages, and disadvantages. In this post, we’ll break down the key differences between the two, helping you make a decision for your power system requirements.
Resin Cast Instrument Transformers (Dry Type)
Resin cast instrument transformers, also known as cast resin dry-type instrument transformers, utilize epoxy resin for insulation instead of oil. This type of transformer is widely appreciated for its robustness and minimal maintenance requirements.
There are 2 types of resin casting methods:
1. Automatic Pressure Gelation (APG) process:
Epoxy resin casting is performed under vacuum. During gelation, the resin is continuously delivered to each mold under pressure, which eliminates voids and compensates for the shrinkage of the material.
2. Gravity Casting method:
Gravity casting on the other hand where the resin is poured into the mold and pulled down into all the parts by gravity, creates voids / bubbles within the mold leading lowering the reliability of the insulation of the instrument transformer.
At Straton Electricals, we leverage state-of-the-art technology to manufacture premium-quality resin cast instrument transformers, commonly referred to as dry-type transformers. These employ epoxy resin as primary insulation to form a solid dielectric between the High Voltage and Earth. They are gaining prominence in indoor and outdoor applications due to their superior design and performance benefits, offering numerous advantages. Our resin casting process incorporates the Automatic Pressure Gelation (APG) methodology, guaranteeing void-free and reliable insulation with a monthly production capacity of 6000 nos. per month.
Applications of Resin Cast Current & Potential Instrument Transformers
Indoor Applications:
Epoxy Resin cast current transformers (CTs) and potential transformers (PTs) are commonly used inside 33kV HT Vacuum Circuit Breaker Panels (Switchgear) and Metering Cubicle. Their compact and maintenance-free nature makes them suitable for such confined and critical environments. Plus, they’re a game-changer for testing laboratories, offering a space- efficient solution that meets all your needs in one convenient package. Whether you need a standard or source, our Resin Cast CTs and VTs have got you covered!
Current Transformer
Epoxy Resin Cast
1.2kV to 36kV
Voltage Transformer
Resin cast / Air cooled
Up to 1.2kV
Current Transformer
Resin cast / Tape Wound
Round
Up to 1.2kV
Outdoor Applications:
Resin Cast Current and Voltage Transformers are an excellent alternative to Oil immersed instrument transformers in industrial electrical substations. They offer a fit-and-forget solution compared to oil immersed current transformers, eliminating issues such as leaks, tank corrosion, and maintenance hassles. Resin cast instrument transformers stand strong due to their hydrophobic nature, requiring minimal upkeep and excelling in challenging environments.
Can Cast Resin Transformers be used in Outdoor applications?
In contrast to oil cooled instrument transformers, resin CT’s require minimal maintenance, making them ideal for conditions where periodic upkeep is not feasible.Straton’s Cycloaliphatic Resin Cast Instrument Transformers undergone rigorous type-testing and have proven their mettle in outdoor environments
Additionally, Their robust design makes them an ideal choice for remote areas with ungrounded systems, where they provide a reliable auxiliary supply to auto-reclosers, withstanding frequent lightning and switching surges.
Straton’s CT/PTs have been successfully used in many projects, With their impressive capability to handle 1.9 times the rated voltage at the rated frequency for 8 hours and their resistance to ferro-resonance, these transformers make the perfect solution for challenging applications.
Performance Certificate: Supply of 33KV resin cast indoor type potential transformers to TOSHIBA.
Advantages of Resin Cast Current Transformers
● Resilience to Overloads: These transformers prevent overheating and ensure continuous operation.
● Low Maintenance: The absence of transformer oil simplifies maintenance, reducing costs and efforts.
● Clean Operation: No risk of oil leaks or spills, contributing to a cleaner environment.
● Flexible Installation: Suitable for various settings, catering to both high and low-voltage requirements.
● Reliable Voltage Rating: Consistently maintain their voltage rating.
● Resin Insulation: Enhances safety and reliability.
● Minimal Risk of Overheating: Effective insulation and design prevent overheating issues, ensuring optimal performance.
Straton’s Advantage
i) Availability of DIN-compliant indoor resin cast products for easy replacement and installation in standardized High-Tension panels.
ii) One-stop solution for instrument transformers from 415V to 36kV voltage level requirement in cast resin technology.
Case Study: Successful operation of resin cast PT’s at Tata Power – DDL & APTRANSCO
Our R&D team used FEMM – Finite Element Method Magnetics, performance analysis using simulation software to avoid the failure of CT’s/PT’s in challenging site conditions also, and to to improve the design and robustness identifying that the voltage factor has to be 1.9 times of rated voltage for 8 Hrs instead of 1.5 times.
Validation using prototypes and internal testing and evaluation was conducted before supply and delivery to client site.
Limitations of Resin Cast Instrument Transformers(Dry Type)
● Limited Voltage Capacity: Designed to handle small to medium voltage Level. Max. BIL is up to 110 kV, Straton Electricals can deliver resin cast transformers designed to handle up to 40 kA for 3 seconds Short-Time Current (STC) rating. Outdoor resin cast PT’s perform at 1.9 times the rated Voltage for 8 hours as per IEC 61869.
● Expensive: Customized Design and manufacturing is expensive, Straton Electricals can deliver with optimum pricing with our expert engineering team
Oil Cooled Instrument Transformer
Outdoor oil cooled instrument transformers are paper insulated oil impregnated type insulation & Winding is done by machines in controlled environment.
Outdoor oil cooled instrument transformers are hermetically sealed type in construction. The whole CT/PT/ unit is dried in an air heating oven under very high vacuum & strictly controlled conditions. Measurement of amount of moisture taken out from the CTs/PT/ is done at regular intervals during the vacuum drying process. After the process is over the CT/PT/ are filled with filtered, de-aerated EHV grade transformer oil under vacuum.
At Straton Electricals, oil immersed instrument transformers feature oil impregnated paper insulated windings crafted in a dust-free and moisture-free environment using automated machines. These hermetically sealed instrument transformers undergo rigorous drying and vacuum processing to reduce moisture from the typical 7-10% found in insulating paper to less than 0.5%, ensuring optimal performance.
Current Transformer
Oil Impregnated paper(OIP)
36kV to 245kV
Voltage Transformer
Oil Impregnated paper(OIP)
36kV to 245kV
Current Transformer
Oil Impregnated paper(OIP)
36kV to 245kV
We conduct in-house testing for filtered and degassed oil before the impregnation process. The oil is tested for BDV and Tan-Delta, a measure of oil’s dielectric properties, to guarantee its insulation properties meet the highest standards. Then, the instrument transformers in the autoclave are filled with the tested, filtered, and degassed EHV-grade oil under vacuum for impregnation with insulation paper to improve the dielectric strength of the insulation.
Types
Oil cooled current transformers are classified into Live Tank and Dead Tank designs, suitable for various applications, including substations, switchyards, railway and renewable. Stratons’ oil cooled instrument transformers are approved in various sectors such as renewables, railways and transmission / distribution utilities.
Oil immersed live tank current transformers are more economical, have lower losses, and have higher STC withstand capability compared to dead tank CTs, the simpler design with a shorter primary conductor translates to lower material and manufacturing costs. Whereas dead tank CTs stand out for their mechanical stability where they can be installed in seismic-prone areas. Dead tank CTs offer a more robust enclosure as the active part is located at the bottom, making them more suitable for transportation and handling.
Applications where Oil Cooled Type Instrument Transformer are used
● Substations & Switchyards: Operated by State Electricity Transmission Corporations or State Utility Distribution Boards.
● Railway Switchyards
● Renewables Sector
● Transmission & Distribution Utilities
Article: Straton Electricals receives CORE approval current transformers to railways
Core Approval Certificate: With this approval, Straton Electricals supplies 132 kV, 50 kV, 25 kV HT OCCT & OCPT to railway switchyards in Southern Railways, North Eastern Railways, and South Western Railways.
Performance Certificate: Supplying of 11 kV, 33 kV, 132 kV oil cooled CT&PT’s to POWER DISTRIBUTION Company of Telangana, Andra Pradesh, APTRANSCO, TSTRANSCO.
Advantages of Oil Cooled Instrument Transformer
● Higher reliability: Oil-immersed instrument transformers are more reliable due to the oil impregnated paper insulation with a hermetically sealed enclosure, providing uniform stress throughout the insulation due to the homogenous mix of liquid and solid dielectric. Thereby reducing the risk of partial discharges and prolonging the life of the product.
● Better thermal performance: Oil-immersed instrument transformers have a better thermal performance due to the presence of insulating oil, which also acts as a cooling agent.
● Feasibility of manufacturing for different voltage ranges: Oil-impregnated paper insulation has been in service and designed for low-voltage to ultra-high-voltage applications. It is considered one of the most proven and age-old technologies, with significant advancements in both technology and materials within this insulation design.
i) Our products are designed to have PD-free performance up to power-frequency voltage for a one-minute duration for all products.
ii) One-stop solution for instrument transformers from 6.6kV to 245kV voltage level requirement in Oil Impregnated Paper technology.
iii) Availability of Bar-Primary design for Current Transformers to withstand high Short-time currents that prevent local heating due to contact losses. It also brings mechanical stability to the product
Drawbacks of Oil Cooled Instrument Transformer
● Maintenance Demands: Regular maintenance of transformer oil is not necessary for instrument transformers. For 33kv and above Instrument transformers, Straton provides stainless steel expansion bellows for arresting oil expansion and contraction due to heat thus reducing the need for maintenance by 60%.
● Environmental Concerns: ‘Risk of oil leakage, as usage of oil is not environmental friendly.
● Complex Design: Complexities arise with high-power and high-voltage setups.
● Heat Generation & Safety Risks: Oil-filled transformers can generate heat during operation. If exceeds ambient temperature, possible of excess heating, cascading to failure.
Factors to Consider for Selecting Cast Resin or Oil Cooled Transformer :
Making an informed decision about selecting a transformer for your project is crucial. This could be done considering the below important factors:
Environmental Considerations
Cast resin dry type transformers are used for both indoor and outdoor applications like high moisture areas, due to the use of epoxy resin and they contain no oil which eliminates the risk of oil leakage, making them ideal for sensitive environments. Oil type transformers are used for outdoor installations which contain oil, which can harm environment if leaked.
Maintenance of the Transformers
Oil-cooled transformers require more maintenance than dry-type transformers. Annual oil filtration is essential to maintain optimal performance and increase the transformer’s lifespan. In contrast, dry-type transformers are resistant to chemical contaminants, requiring less maintenance.
Location of the Transformer
If the transformer is located in a remote area where routine maintenance is not feasible, or where the risk of fire or oil spillage can cause casualties, then resin cast transformers are recommended.
Voltage Capabilities
Resin cast dry transformers are designed for small to medium applications up to 33kV. Oil transformers handle heavier loads, with higher voltages of 132kv.
Conclusion
Both cast resin and oil-cooled instrument transformers have unique advantages and disadvantages. The choice between the two depends on the project’s specific requirements, such as maintenance, environmental considerations, location, and voltage needs. However, selecting a trusted and reliable provider is also important.
At Straton Electricals, we are committed to helping you make the best choice. Our high-performance oil-insulated instrument current and voltage transformers are designed for outdoor installations, and resin cast transformers for indoor and outdoor installations are customized to meet your specific requirements. We have in-house testing facilities in our state-of-the-art test lab in accordance with IEC and ISO standards.
Our 700KV high voltage partial discharge-free test lab ensures each transformer meets stringent quality standards. With a wide range of Voltage Options our Transformers undergo a five-stage quality inspection process, validated by international laboratories such as ERDA and CPRI (India), further underscores our commitment to excellence. Transformers are rigorously type tested for short circuit performance, thermal stability, multiple chopped wave impulse, wet lightning impulse, and partial discharge, all in compliance with IEC standards.
For those requiring quick solutions, we deliver within one week and with up to a five-year warranty. Whether you need maintenance-free resin cast transformers or robust oil-cooled transformers, straton offers custom-built solutions tailored to your exact specifications, with a proven excellent field performance in various applications including power utilities, solar and wind energy providers, railway electrification systems etc.
Check out our range of transformers or contact our support team online for more details. You can also request a quote to find the best transformer solution for your needs.
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