When it comes to high-voltage power systems, current transformers (CTs) play a critical role in measurement and protection. CTs come in two main designs—Live Tank and Dead Tank? While both serve the same fundamental purpose, they differ significantly in construction. The main differences between a live tank and a dead tank current transformer (CT) primarily lie in how the high voltage components are insulated from the enclosure. This blog will help you understand the key differences between Live Tank and Dead Tank CTs, their advantages, disadvantages, and which one suits your needs best.
What is a Live Tank Current Transformer?
A Live Tank CT is a design where the primary winding, core and secondary winding are housed inside a tank that is at system voltage (live system potential). The entire structure is mounted on a porcelain or polymer insulator, which electrically insulates it from the ground. In other words, the entire current transformer is kept in a metal container or tank. This container is directly connected to the live line via the primary terminals of the CT. The CT then fitted on the top of a insulator column.
Key Features of Live Tank Current Transformers
- The primary winding, core, and secondary winding are all at high voltage (live potential).
- The main mass of the CT is located at the top of the equipment; hence, the CG is in the upper portion of the equipment.
- The tank is insulated from the ground by being mounted on an insulating cylinder.
- Typically smaller, lighter, and more compact compared to Dead Tank CTs.
- Commonly used in 132 kV and above due to cost and insulation advantages.
Advantages of Live Tank Current Transformers
✅ Compact & Lightweight – Since the primary and the container are in same live potential this type of CT uses less insulation material like insulating papers, insulating oil or SF6 gas compare to Dead Tank CT. It also makes the CT more space-efficient.
✅ Better Cooling & Heat Dissipation – The primary winding is typically a copper or aluminum rod or a few turns of copper or aluminum strip and is generally not covered with insulation paper. Hence, it is in direct contact with insulating oil or SF6 gas, which improves heat dissipation.
✅ Lower Magnetic Leakage – The core is smaller and efficiently surrounded the primary rod (winding), leading to better magnetic coupling. This reduces flux leakage, ensuring that more of the magnetic field is effectively utilized.
✅ Cost-Effective for High Voltage – As already mentioned the primary and the container are in same live potential hence it requires less insulation material, reducing costs for 132 kV and higher rated current transformers.
✅ Easy to Design for short-circuit current – As the primary is straight a bar only, it is more easy to design for higher short circuit withstand capacity.
✅ Faster Response to Transients – Due to better flux linkage a live tank CT provides better transient performance in fault conditions.
Disadvantages of Live Tank Current Transformers
❌ Maintenance Complexity – Since the entire tank is at high voltage, so working near or on a live tank CT requires total shutdown. Accidental contact with the tank is extremely dangerous. Accessibility might be more restricted in some designs due to the high voltage tank being elevated on insulators.
❌ Less Mechanical Stability – The entire high-voltage tank assembly is supported by an insulator. This design becomes critical for mechanical stability. The heavy tank being raised on the insulator column results in a higher center of gravity compared to dead tank CTs. This can make a live tank CT potentially more susceptible to overturning moments, especially from seismic or wind forces. The tank and insulator assembly effectively acts like a cantilever beam. Forces applied at the top (tank) can create bending moments at the base of the insulators and the supporting structure. During severe short circuits, some mechanical movement of the high-voltage tank and its components may be possible due to electromagnetic forces. This needs to be considered in design too.
❌ Higher Insulation Stress – The top head meaning tank is at live line potential and the base is at ground potential and there is an insulator support in between. So this support insulator requires high-quality insulation coordination to prevent flashover. It may make the CT taller compared to a dead tank CT of same rating.
What is a Dead Tank Current Transformer?
A Dead Tank CT is a design where the core, secondary winding and a portion of primary winding are housed in a grounded metal tank. Only the primary conductor inside the tank is at high voltage, making the exterior safer and easier to maintain.
Key Features of Dead Tank Current Transformers
- The primary winding loop extends down to a grounded metal tank, where the cores and secondary windings are enclosed within that grounded tank.
- The entire tank is at ground potential. The secondary terminals are connected to the secondary studs fixed on the tank. This makes the maintenance and other work on the secondary terminals side easier without requiring a shutdown.
- The primary winding requires thick insulation since it must withstand the voltage stress between the live winding and the grounded metal tank. As a result, these CTs become larger and bulkier due to the need for thicker insulation.
- They are commonly used for 33 kV systems, where the size and cost of insulation are less significant.
Advantages of Dead Tank Current Transformers
✅ Safer for Maintenance – Since the tank is grounded, technicians can safely inspect, work and maintain the CT secondary terminal side without de-energizing a dead tank current transformer.
✅ Better Mechanical Stability – As the main mass lies at the bottom of the CT structure, it is mechanically mone stable to wind pressure and other cantilever forces.
✅ Better Protection Against External Magnetic Field – The grounded tank shields the CT from external magnetic field.
✅ Lower External Insulation Requirements – No need for complex insulation coordination as the tank is at ground potential.
✅ More Stable in Polluted Environments – Works better in areas with high pollution, salt deposits, or extreme weather conditions.
Disadvantages of Dead Tank Current Transformers
❌ Bulky & Heavy – Requires more insulation material, making it larger and harder to install.
❌ Higher Core Saturation Risk – In a dead tank CT, the primary conductor enters and exits the grounded metal tank, creating an asymmetric magnetic circuit around the core. Unlike Live Tank CTs, where the core is evenly distributed around the primary conductor, the magnetic flux in Dead Tank CTs is non-uniform due to the off-center primary conductor placement. Unequal flux distribution leads to higher core saturation in certain regions, affecting the accuracy of secondary current measurement.
❌ Higher Cost for High Voltage – More insulation material means increased costs for 132 kV and above applications.
❌ Slower Transient Response – As the primary winding is longer it offers more reactance. More reactance means this CT takes longer to respond to sudden changes in current.
❌ Susceptible to Partial Discharge – In a dead tank CT the primary winding is insulated from the earthed tank, which can be more challenging to achieve than in a live tank CT where the primary winding is at the same voltage level of the tank. This can make the insulation in a dead tank CT more susceptible to partial discharge. Also, the primary winding in a dead tank CT is longer and has to pass through the porcelain or polymer insulator cylinder which can make it more vulnerable to mechanical stress during short circuits. This can damage the insulation and increase the risk of partial discharge. The primary winding in a dead tank CT has a larger volume of copper, which can lead to more heat generation during short circuits. This can make it more difficult to dissipate the heat and the trapped heat can degrade the insulation. It can also contribute to partial discharge. Overall, dead tank CTs can be more susceptible to partial discharge.