Oil-Immersed Current Transformer (OICT)
A traditional CT design using insulating oil (e.g., mineral oil or synthetic ester) as the primary insulation and cooling medium.
Widely applied in high-voltage (HV) and extra-high-voltage (EHV) power systems due to its superior insulation performance.
Dry-Type Current Transformer (Dry-Type CT)
Uses solid insulation materials (e.g., epoxy resin, silicone rubber) without liquid dielectric, eliminating the risk of oil leakage.
Predominantly used in medium-voltage (MV) and low-voltage (LV) systems, as well as in environments requiring high fire safety or environmental friendliness.
| Component | Oil-Immersed CT | Dry-Type CT |
|---|
| Material | Metal casing (steel or aluminum) for oil containment. | Non-metallic (fiberglass-reinforced plastic, epoxy resin) or metal casing with sealed solid insulation. |
| Design Feature | Sealed tank with pressure relief devices (e.g., buchholz relay) to prevent overpressure from oil thermal expansion. | Compact, hermetically sealed casing without liquid, often with flame-retardant materials. |
| Environmental Impact | Potential oil leakage risk; requires spill prevention measures. | No oil, reducing environmental hazards and fire risks. |
| Aspect | Oil-Immersed CT | Dry-Type CT |
|---|
| Primary Insulation | Insulating oil combined with paper or pressboard for dielectric strength. | Epoxy resin casting or vacuum pressure impregnation (VPI) of glass-fiber reinforced materials. |
| Insulation Structure | Multi-layered oil-paper insulation system; requires periodic oil quality monitoring. | Monolithic solid insulation with uniform dielectric properties; less maintenance. |
| Temperature Resistance | Oil degradation at high temperatures (max. ~105°C); cooling dependent on oil circulation. | Epoxy resin withstands higher temperatures (up to ~155°C) with better thermal stability. |
| Component | Oil-Immersed CT | Dry-Type CT |
|---|
| Core Material | Grain-oriented silicon steel or amorphous metal for low core loss. | Same as oil-immersed CT; core design optimized for compactness. |
| Winding Construction | Copper or aluminum windings surrounded by oil-paper insulation, wound on a bobbin or core. | Windings embedded in epoxy resin or silicone rubber, often with concentric or toroidal configurations. |
| Electrical Clearance | Requires larger spacing between windings due to oil’s dielectric properties. | Tighter spacing possible with solid insulation, enabling smaller form factors. |
| Feature | Oil-Immersed CT | Dry-Type CT |
|---|
| Cooling Medium | Natural convection or forced circulation of insulating oil. | Air cooling or heat dissipation through the casing (passive cooling). |
| Heat Dissipation Efficiency | Effective for high-power applications but dependent on oil viscosity and temperature. | Limited by solid insulation’s thermal conductivity; suitable for lower power ratings. |
| Cooling System Complexity | Requires oil reservoirs, radiators, and seals, increasing system complexity. | Simple structure without moving parts or liquid systems. |
| Aspect | Oil-Immersed CT | Dry-Type CT |
|---|
| Seal Design | Multiple seals (gaskets, O-rings) to prevent oil leakage; susceptible to aging. | Hermetic seals in casting or housing, with fewer failure points. |
| Maintenance Requirements | Regular oil testing (dielectric strength, moisture content), seal replacement, and oil filtration. | Minimal maintenance; periodic visual inspection for casing cracks or surface tracking. |
| Service Life | 30–50 years with proper maintenance; oil degradation limits longevity. | 20–30 years; solid insulation resists environmental degradation better. |
| Parameter | Oil-Immersed CT | Dry-Type CT |
|---|
| Voltage Class | Typically used in 35 kV to 1,100 kV systems. | Predominantly applied in ≤35 kV systems; some advanced designs for 66 kV–110 kV. |
| Current Rating | Capable of handling high primary currents (up to 20,000 A) with robust winding designs. | Limited to lower currents (up to 5,000 A) due to heat dissipation constraints. |
| Category | Oil-Immersed CT | Dry-Type CT |
|---|
| Advantages | - Superior dielectric strength for high-voltage applications.
- Efficient heat dissipation in large power systems.
- Mature technology with proven reliability in outdoor and harsh environments. | - Fireproof and environmentally friendly (no oil).
- Compact size and lightweight for easy installation.
- Low maintenance and suitable for indoor or urban areas. |
| Disadvantages | - Risk of oil leakage, requiring strict sealing and environmental precautions.
- Higher installation and maintenance costs.
- Susceptible to moisture ingress, affecting insulation performance. | - Limited voltage and current ratings compared to oil-immersed types.
- Solid insulation may crack under thermal stress or mechanical impact.
- Higher cost for specialized high-voltage designs. |
Oil-Immersed CTs: Ideal for HV/EHV substations, transmission lines, and industrial settings where high reliability and voltage withstand are critical (e.g., offshore platforms, heavy industries).
Dry-Type CTs: Preferred in urban substations, commercial buildings, renewable energy plants (e.g., wind farms, solar parks), and areas with strict environmental regulations (e.g., underground cable systems).
Oil-Immersed CTs: Development of eco-friendly insulating oils (biodegradable esters) and intelligent monitoring systems for early detection of oil degradation.
Dry-Type CTs: Advancements in nanocomposite epoxy materials to enhance thermal conductivity and voltage resistance, enabling applications in higher voltage classes (e.g., 66 kV–110 kV).
The structural differences between oil-immersed and dry-type current transformers primarily stem from their insulation and cooling systems. Oil-immersed CTs excel in high-voltage, high-power applications due to their robust insulation and heat dissipation, while dry-type CTs offer safety, compactness, and low maintenance for medium/low-voltage and environmentally sensitive environments. The choice between the two depends on voltage requirements, installation constraints, maintenance budgets, and environmental considerations.
