How do you select a current transformer for current measurement? - Blog

When selecting a current transformer (CT), it is essential to comprehensively consider various technical parameters and operating conditions to ensure safe, accurate, and reliable performance. The following are the key selection criteria:

1. Selection of Rated Primary Current

• The selection should be based on the normal operating current of the circuit being measured. Generally, the normal operating current should be approximately 60% of the rated primary current, but it should not fall below 30%, in order to ensure measurement accuracy.

• If the actual current frequently drops below 30%, consider selecting a CT equipped with primary winding taps or one with a lower rated current; alternatively, consider using a specialized CT designed for high dynamic and thermal stability.

2. Selection of Rated Secondary Current

• The most commonly used rated secondary currents are 5A and 1A.

• 5A: Predominantly used in traditional applications; suitable for scenarios involving shorter transmission distances and higher secondary circuit loads.

• 1A: Suitable for long-distance transmission, electronic instrumentation/protection devices, communication systems, etc., as it helps reduce power losses within the secondary circuit.

• When making a selection, ensure that the chosen rated secondary current is compatible with the input requirements of the downstream secondary equipment (e.g., energy meters, protective relays, etc.).

3. Selection of Rated Transformation Ratio

• Rated Transformation Ratio = Rated Primary Current / Rated Secondary Current (e.g., 100/5A, 400/5A, etc.).

• This ratio should be determined based on the rated current of the primary side and the selected secondary current, while also taking into account the expected range of load variations to ensure a sufficient safety margin.

4. Selection of Accuracy Class (Precision Class)

• Different accuracy classes are selected based on the specific application:

• Metering (Energy Measurement): Typically requires Class 0.2S or 0.5S (high precision, used for commercial billing or precision metering).

• Measurement (Indicating Instruments): Commonly uses Class 0.5 or Class 1.

• Protection (Relay Protection): Commonly uses Class 5P, 10P, or PR-class protective transformers; the primary focus here is on transient characteristics and reliability, rather than absolute precision.

• A single current transformer may feature multiple windings, dedicated separately to metering, measurement, and protection functions, each corresponding to a different accuracy class.

5. Rated Output (Load Capacity) or Secondary Load Impedance

• The total impedance of the instruments, relays, and other devices connected to the secondary side must not exceed the rated burden (typically expressed in VA) permitted for the current transformer at its specified accuracy class.

• If the load impedance is excessive, it will result in increased errors within the current transformer, thereby compromising measurement accuracy or the operational reliability of protection systems.

6. Voltage Rating

• The rated voltage of a current transformer (CT) must correspond to the rated voltage of the power grid at its installation location-for example, 10 kV, 35 kV, 110 kV, etc.

• This voltage rating refers to the rated line voltage at the CT's installation point, not the magnitude of the primary current.

7. Mounting Method and Structural Form

• Select the appropriate structural type based on the specific on-site installation conditions:

• Pass-through Type (Busbar Type): Suitable for applications where a busbar passes directly through the CT.

• Post Type: Commonly used for fixed installation within switchgear cabinets.

• Bushing Type: Integrated into high-voltage equipment, such as transformer bushings.

• Wall-through Type: Designed for installation through walls or partitions, serving both insulating and supporting functions.

• Additionally, units are classified as either indoor or outdoor types; the choice should be made based on the operating environment.

8. Insulation Type and Protection Class

• Based on the installation environment (indoor/outdoor), pollution degree, altitude, and other factors, select products possessing the appropriate insulation level and **protection class (e.g., IP rating).

9. Transient Characteristics (Particularly for Protection-Class Instrument Transformers)

• For instrument transformers utilized in relay protection-especially rapid protection schemes (such as differential protection)-particular attention must be paid to their transient saturation characteristics, frequency response, and similar parameters. Select a type that meets the specific protection requirements, such as PR-class (Protection, Transient) instrument transformers.

10. Special Requirements

• If the system involves special operating conditions-such as the presence of harmonics, impulse loads, or DC components-select instrument transformers capable of accommodating these specific conditions.

• In digital substations, it may be necessary to utilize Electronic Current Transformers (ECTs), which differ from traditional electromagnetic instrument transformers.