1. What is Current Transformer Ratio?

The Current Transformer Ratio (CTR) is a fundamental parameter in electrical engineering that represents the relationship between the primary current and secondary current in a current transformer. It indicates how much the primary current is reduced to a standardized secondary current for measurement and protection purposes.

2. How Does the Calculator Work?

The calculator uses the CT ratio formula:

Where:

• \( CTR \) — Current Transformer Ratio
• \( I_p \) — Primary current (A)
• \( I_s \) — Secondary current (A)

Explanation: The CT ratio represents how many times the primary current is greater than the secondary current. Standard secondary currents are typically 1A or 5A for measurement and protection devices.

3. Importance of CT Ratio Calculation

Details: Accurate CT ratio calculation is crucial for proper meter reading, relay protection settings, and overall electrical system safety. Incorrect CT ratios can lead to inaccurate measurements, improper protection coordination, and potential equipment damage.

4. Using the Calculator

Tips: Enter both primary and secondary current values in amperes (A). Both values must be positive numbers greater than zero for accurate calculation.

5. Frequently Asked Questions (FAQ)

Q1: What are common standard CT ratios?
A: Common standard ratios include 100:5, 200:5, 400:5, 600:5, 800:5, 1000:5, 1200:5, and 2000:5 for 5A secondary systems, with corresponding ratios for 1A secondary systems.

Q2: Why are CT ratios important in electrical systems?
A: CT ratios allow measurement of high currents using standard low-current instruments, provide electrical isolation, and enable protective relaying to function properly.

Q3: How does CT ratio affect meter readings?
A: Meter readings must be multiplied by the CT ratio to obtain the actual current values. For example, with a 200:5 CT ratio, a meter reading of 3A represents 120A actual current.

Q4: What happens if the wrong CT ratio is used?
A: Using the wrong CT ratio results in inaccurate measurements, potential overload of instruments, and improper operation of protective devices, which can compromise system safety.

Q5: Can CT ratio be changed after installation?
A: CT ratio is typically fixed based on the transformer's physical construction. To change ratios, either the CT must be replaced or multi-ratio CTs with tap connections must be used.