In the daily operation of power systems, there is a “golden duo” that silently safeguards every current and voltage measurement—they are CT and PT. Whether you are a power industry professional or an engineer selecting transformers for a project, understanding the differences and functions of CT and PT is fundamental. Today, we’ll talk about these instrument transformer siblings and also explore how they relate to oil immersed transformer, cast resin transformer, and Dry transformer—the main power transformers they work with.
CT and PT are essentially instrument transformers—a special type of transformer. Their purpose is clear: convert the high voltage and large current on the primary side into safe, low‑voltage and low‑current signals on the secondary side that can be fed to measuring instruments, protective relays, and automation devices. In simpler terms, CT and PT act as the “sensors” of the power system, turning dangerous high‑voltage, high‑current quantities into safe low‑voltage, low‑current quantities for easy monitoring and control.
CT (Current Transformer) is designed to step down large currents to smaller, standardized values.
The primary winding has few turns, with thick conductors, and is connected in series with the circuit.
The secondary winding has many turns, with thin conductors, and its rated secondary current is typically 5A or 1A.
For example, a 600/5 CT means that 600A on the primary corresponds to 5A on the secondary—a reduction factor of 120.
Critical rule for CT: The secondary side must NEVER be open‑circuited! If the secondary circuit opens while the primary is carrying current, dangerously high voltages can be induced.
PT (Potential Transformer) is designed to step down high voltages to safe, standardized low voltages.
It works like a step‑down transformer: the primary winding has many turns and is connected across the high‑voltage line.
The secondary winding has fewer turns, and its rated secondary voltage is standardized at 100V or 100/√3 V.
For instance, a 6kV PT with a ratio of 60 will indicate 6700V when the voltmeter reads 105V.
Critical rule for PT: The secondary side must NEVER be short‑circuited! A short circuit on the PT secondary can cause extremely high currents, damaging equipment and endangering personnel.
For clarity, here’s a quick comparison:
| Aspect | CT (Current Transformer) | PT (Potential Transformer) |
|---|---|---|
| Measured quantity | Large current | High voltage |
| Connection | In series with the circuit | In parallel across the circuit |
| Primary winding | Few turns, thick wire | Many turns, thin wire |
| Secondary winding | Many turns, thin wire | Few turns, thick wire |
| Secondary rated value | 5A or 1A | 100V |
| Strict prohibition | Secondary must not be open | Secondary must not be shorted |
| Nature | Step‑up transformer | Step‑down transformer |
Although CT and PT have different roles, they share the same goals: isolate high voltage, protect equipment, and ensure personnel safety.
Now that we’ve covered the “measurement assistants” (CT and PT), let’s turn to the power transformers that truly handle bulk energy transmission. Today, the market offers three main types: oil immersed transformer, cast resin transformer, and Dry transformer.
An oil immersed transformer is the most widely used type in power systems. Its core and windings are immersed in insulating oil (mineral oil or natural ester), which provides both cooling and insulation.
Key advantages:
High cooling efficiency and strong overload capability
Smaller footprint and lower cost for the same capacity
High efficiency and low losses, ideal for large outdoor applications
Typical applications: Substations, factories, renewable energy plants, and other large outdoor projects. China is a major manufacturing hub for oil immersed transformer, and numerous oil immersed transformer manufacturers are concentrated in provinces like Jiangsu and Zhejiang.
A cast resin transformer belongs to the dry‑type family. Its windings are completely encapsulated in epoxy resin and rely on natural air convection or forced‑air cooling.
Key advantages:
Oil‑free – no leakage risk, inherently safe
Excellent fire resistance – flames will not spread
Withstands moisture, dust, and chemical corrosion
Low maintenance – virtually maintenance‑free
Typical applications: Hospitals, airports, commercial buildings, data centers, and other indoor locations with strict fire‑safety requirements.
Dry transformer is a broader category that includes cast resin transformer and any transformer that uses air or solid insulation instead of liquid oil. Compared with oil immersed transformer, a Dry transformer is more environmentally friendly and safer, but it typically costs more and has slightly higher losses for the same rating.
This is a classic question that every project faces. The simple rule is:
Choose oil immersed transformer if you need large capacity, high overload capability, outdoor installation, and lower initial investment.
Choose cast resin transformer or Dry transformer if your application is indoors, fire safety is a top priority, and you want to eliminate oil‑leakage risks.
The global oil immersed transformer market continues to grow, with a projected CAGR of 5.32% from 2025 to 2033. As a professional power equipment supplier, xinhong electrical keeps a close eye on industry trends and is committed to providing high‑quality oil immersed transformer, cast resin transformer, and Dry transformer products and solutions to our customers.
Back to the original question: What are CT and PT in transformers?
CT (Current Transformer) – steps large currents down to small, standard values; connected in series; secondary must never be open‑circuited.
PT (Potential Transformer) – steps high voltages down to safe, standard levels; connected in parallel; secondary must never be short‑circuited.
Together with the main power transformer (whether oil immersed transformer, cast resin transformer, or Dry transformer), they form a complete “measurement + transmission” system. CT and PT are responsible for “seeing” and “measuring,” while the main transformer handles “converting” and “delivering” – each plays its part, and neither is dispensable.
Whether you are sourcing an oil immersed transformer for a substation or evaluating a cast resin transformer for a commercial complex, understanding CT and PT fundamentals will help you better grasp the overall logic of the power system.
xinhong electrical – your professional partner in power equipment, offering comprehensive products and technical support for CT, PT, and all types of main transformers.
