Current Transformer (CT)
Construction, Working & Types
A current transformer (CT) is an instrument transformer that produces a secondary current proportional to the primary current — enabling safe measurement of high currents using standard ammeters, protection relays and energy meters.
Purpose & Overview
A Current Transformer (CT) is an instrument transformer designed to produce an alternating current in its secondary winding that is accurately proportional to the current being measured in its primary winding. CTs allow safe and practical measurement of very high currents — thousands of amperes — using standard instruments rated for only 1A or 5A.
Without a CT, connecting an ammeter directly to a high-voltage, high-current power line would be impossible — both because of the dangerously high current and the high voltage isolation required. The CT solves both problems simultaneously.
A Current Transformer is an instrument transformer structured to induce an alternating current in its secondary winding. The magnitude of this secondary current is directly proportional to the primary current, with the ratio determined by the turns ratio N₁/N₂.
Construction Components
A current transformer consists of five key components, each with a specific role in achieving accurate, safe current measurement:
Primary Winding
The primary winding of a CT is fundamentally different from a conventional power transformer. Because CTs are connected in series with the line current, the primary winding carries the full line current and must be designed accordingly.
- Very Few Turns: Typically only 1 to 5 turns — or in many cases, just a single pass of the conductor through the core window.
- Heavy Conductor: Must carry full line current (can be thousands of amperes) — so cross-section is very large.
- Bar Type (Bus Bar): In many CTs, the primary is simply the existing bus bar or cable passing through a hole in the toroidal core — no winding at all.
- Single Flat Turn: In wound CTs, a single flat copper strip wound around the core serves as the primary.
- Series Connection: Always connected in series with the circuit — interrupting the primary opens the main line circuit.
A CT is used to step down current, not voltage. With N₁ = 1 (one primary turn) and N₂ = 200 (secondary turns), the current ratio is 200:1 — so 200A primary becomes 1A secondary. The turns ratio is the inverse of the current ratio: I₁/I₂ = N₂/N₁.
Secondary Winding
The secondary winding is where the CT delivers its output — a small, safe, measurable current that is proportional to the primary current. Unlike the primary, the secondary winding has many turns of fine wire wound on the laminated core.
- Many Turns: Typically hundreds of turns of fine copper wire, wound uniformly on the core to minimise leakage flux.
- Standard Output: Designed to produce either 1A or 5A at full rated primary current — matching standard instrument inputs.
- Low Impedance Load (Burden): Connected to a very low impedance load (ammeter, relay coil, burden resistor) — keeps the CT operating near short-circuit conditions for accuracy.
- Must Never Be Open-Circuited: If the secondary is open while primary current flows, dangerously high voltage appears across secondary terminals — a critical safety rule.
- Accuracy Winding: In metering CTs, the secondary winding is wound with extreme precision to achieve 0.1–0.5% accuracy class.
NEVER open-circuit the secondary of a CT while primary current is flowing. The full primary MMF drives flux into saturation and induces a dangerously high voltage (hundreds to thousands of volts) across the open secondary terminals. This can destroy insulation, damage equipment and cause fatal electric shock. Always short-circuit the secondary before disconnecting any burden.
Core & Insulation
Magnetic Core
The core is the most critical component for accuracy. It must have very high permeability to minimize the magnetizing current error, and very low core loss to minimize the phase angle error between primary and secondary currents.
- Material: Cold-rolled grain-oriented (CRGO) silicon steel, or high-permeability nickel-iron alloys (e.g. Mumetal) for precision metering CTs.
- Laminated Construction: Thin laminations (0.1–0.35mm) reduce eddy current losses, improving accuracy.
- Large Cross-Section: Keeps magnetic flux density low, ensuring the core operates in the linear region of its B-H curve.
- Toroidal Shape: Most CTs use a toroidal (ring-shaped) core — it provides a closed magnetic path with no air gap, minimising leakage flux and improving accuracy.
Insulation System
- Materials: Paper-oil insulation (high voltage CTs), cast resin/epoxy (medium voltage), moulded thermosetting plastic (low voltage).
- Primary-Secondary Isolation: Must withstand the full system voltage between primary and secondary — may be 11kV, 33kV or higher.
- Environmental Protection: Protects windings from moisture, dust, chemical contamination and mechanical damage.
Working Principle
The CT operates on the same electromagnetic induction principle as a conventional transformer — but with one critical difference: it is designed to maintain a constant current ratio rather than a constant voltage ratio.
Key CT Formulas
I₁ = primary current, I₂ = secondary
Printed on nameplate
Must not exceed rated burden
A CT has a ratio of 500/5A. If the ammeter reads 3.5A, what is the actual line current?
Line current = Ammeter reading × CT ratio = 3.5 × (500/5) = 3.5 × 100 = 350A
Types of Current Transformers
Current transformers are manufactured in three main physical configurations, each suited to different installation requirements and current measurement scenarios.
- Primary winding physically wound on the core
- 1 to 5 turns of heavy conductor
- Secondary current depends on turns ratio
- Used for lower primary currents
- More precise ratio control
- Higher cost than toroidal
- No primary winding on the CT itself
- Line conductor passes through the window
- Single pass = 1 primary turn
- Most common type — simple installation
- Split-core version clamps onto existing cable
- Used in switchboards, metering panels
CT Specifications & Ratings
| Parameter | Typical Values | Explanation |
|---|---|---|
| Ratio (Primary/Secondary) | 100/5, 400/5, 1000/1A | Nominal transformation ratio — printed on nameplate |
| Standard Secondary Current | 1A or 5A | IEC standard secondary output at rated primary current |
| Accuracy Class (Metering) | 0.1, 0.2, 0.5, 1, 3, 5 | Maximum ratio error at rated current (% deviation) |
| Accuracy Class (Protection) | 5P, 10P, 5PR, 10PR | Composite error limit for protection applications |
| Burden (VA) | 1, 2.5, 5, 10, 15, 30 VA | Maximum VA load on secondary without exceeding accuracy |
| Rated Voltage | 0.66kV, 11kV, 33kV | System voltage the CT is designed to insulate against |
| Short-Time Rating (IST) | 40× In for 1 second | Fault current the CT can withstand without damage |
| Knee Point Voltage | — | For protection CTs — minimum voltage before saturation |
Critical Safety Rules for CT Operation
The most important CT safety rule: NEVER open-circuit the secondary winding while primary current is flowing. Short-circuit or properly terminate the secondary at all times when primary current may be present. A shorting link or terminal block shorting bar must be applied before removing any secondary instrument or relay.
- Always short secondary before disconnecting: Apply a shorting link across secondary terminals before removing meters or relays.
- Check polarity markings: P1/P2 (primary) and S1/S2 (secondary) polarity dots must be observed for correct energy metering direction.
- Earth the secondary: One terminal of the CT secondary (S2) must always be earthed for safety — prevents floating high-voltage potential.
- Do not exceed burden rating: Excessive burden causes ratio error — keep total secondary circuit VA within the rated burden.
- Use correct accuracy class: Metering CTs (0.1–0.5 class) must not be used for protection — they saturate at high fault currents. Use 5P or 10P class CTs for protection relays.
- Demagnetise after fault current: After a high fault current event, CTs may retain residual magnetism — this must be removed before returning to service.
Conclusion
The current transformer is one of the most critical components in any electrical power system. Its ability to step down very large primary currents to safe, standardised secondary values enables accurate metering, reliable protection and safe system monitoring — all without any direct electrical connection between the high-voltage primary circuit and the measuring instruments.
Understanding CT construction — the few primary turns, the many secondary turns, the precision laminated core and the insulation system — makes it clear why CTs must never be open-circuited, and why selecting the correct ratio, accuracy class and burden rating is so important for both metering accuracy and protection system reliability.
Primary → few turns / heavy conductor / in series with line. Secondary → many turns / 1A or 5A output / feeds instruments. Core → CRGO steel / toroidal / low loss. Never open-circuit the secondary. Ratio: I₁/I₂ = N₂/N₁. CT types: Wound / Toroidal / Bar.
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