What Is The Purpose of The Current Transformer (CT) Ratio?

A current transformer (CT) is an instrument transformer whose purpose is to stepping down large alternating currents in a primary circuit to a smaller, safer, standardized current level in its secondary circuit for measurement, protection, or control.

The CT ratio (also called current ratio) is the mathematical relationship between the primary current and the secondary current under rated (or full-load) conditions. In other words:

CT Ratio = (Primary current) : (Secondary current)

For example, a CT rated 300:5 means that when 300 A flows through the primary side, the secondary will produce 5 A. If only 150 A flows in the primary, then ideally 2.5 A appears in the secondary (150/300 × 5) under linear conditions.

Why the CT Ratio Matters — Purposes & Functions

The CT ratio is fundamental because it:

1. Scales High Currents to Safe Measuring Levels
   High currents in power systems (hundreds or thousands of amps) cannot be directly handled by typical meters, relays, or monitoring devices. The CT ratio ensures that such large currents are proportionally reduced (e.g. into 1 A or 5 A) so that the connected instruments can safely and accurately measure them.

2. Maintains Proportional Accuracy over Range
   Provided the current transformer is properly designed and loaded, the CT secondary current remains proportional to the primary current throughout its operating range (within specified accuracy limits). This proportional behavior enables precise measurement, metering, and protective relay operation.

3. Standardizes the Interface for Instrumentation and Protection
   Because secondary currents are standardized (typically 5 A or 1 A), CTs across various systems and installations can present a consistent input to meters, energy analyzers, protective relays, and control systems. This makes instrumentation design and interchangeability easier.

4. Ensures Safety and Isolation
   The CT provides electrical isolation between high-voltage primary circuits and low-voltage secondary instruments. The CT ratio ensures that dangerous currents never reach measuring devices or operators.

5. Facilitates Protection and Coordination
   In protective systems, the ratio of the CT must match the expected fault currents and relay settings. If the ratio is mismatched, fault detection may be inaccurate, leading to delayed or incorrect trip operation. Correct CT ratios help ensure proper calibration of protective relays and coordination among protection zones.

   
   

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Design & Considerations of CT Ratio

Some key aspects and caveats surrounding CT ratios are:

• Standard Secondary Ratings
  Most CTs provide standard secondary currents of 5 A or 1 A, so CT ratios are expressed accordingly (e.g. 1000:5, 2000:1).

• Turns Ratio vs. Current Ratio
  The CT’s actual physical windings define a turns ratio (primary turns : secondary turns). The current ratio is inversely related to the turns ratio, following transformer principles (i.e. more turns in the secondary yield a lower secondary current for the same primary current).

• Linear Operating Range & Saturation
  A CT must be operated within its linear (unsaturated) range. If the primary current exceeds the CT’s design (or burden), the core may saturate, breaking the proportional relationship and causing measurement errors or malfunctions. Thus, the CT ratio must be chosen so that even during overloads or fault currents, the CT can maintain acceptable performance.

• Burden & Accuracy
  The CT’s accuracy at its ratio depends on the burden (i.e. the total load, in VA or impedance, connected to the secondary). The CT must be capable of driving that burden while maintaining accuracy within specified limits. If the burden is too high for the selected CT ratio, errors increase.

• Multi-tap CTs
  Some CTs allow multiple ratio options via taps on the secondary, giving flexibility to match different load levels or protection schemes.

• Polarity & Phasing
  Correct polarity (i.e. the orientation of primary and secondary terminals) is essential to ensure the CT ratio works meaningfully with instruments. Reversed polarity would result in negative readings or misoperation of directional relays.

  
  

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Practical Example & Implications

Suppose a power system line carries a nominal current of 1200 A, and we want to monitor it using standard 5 A instrumentation. We choose a CT with ratio 1200:5. Under normal load, the CT secondary will produce 5 A, which is directly measurable by the meter or relay. If the line current doubles to 2400 A during a fault, the CT would attempt to produce 10 A (if within its linear range). Protective relays are set accordingly to interpret that 10 A as 2400 A and initiate trip actions. If the CT’s ratio had been improperly chosen (e.g. 2000:5), then at 2400 A the CT might saturate or misrepresent the fault level, leading to relay errors.

Thus, the CT ratio links real-world power system currents with the internal, manageable currents of protection and measurement devices.