Selection And Installation Precautions for Low-Voltage Current Transformers

Selection and Installation Precautions for Low-Voltage Current Transformers

Low-voltage current transformers play a crucial role in power systems, not only for measuring and protecting circuits but also ensuring the stable operation of the power system. To ensure the efficiency and safety of low-voltage current transformers, proper selection and installation are vital.

I. Selection of Low-Voltage Current Transformers

When selecting low-voltage current transformers, several key factors need to be considered:

1. Determination of Rated Voltage: The rated voltage UN of the current transformer should be compatible with the voltage UL of the measured circuit, i.e., UN ≥ UL. Low-voltage current transformers are typically used in AC circuits with a rated voltage not exceeding 0.66kV.

2. Determination of Rated Ratio: The rated ratio is determined by the ratio of the rated primary current I1 to the rated secondary current. The rated primary current I1 is usually determined based on the primary power load P1 connected to the current transformer, i.e., I1 = P1 / (UN * cosψ), where cosψ is the average power factor, generally taken as 0.8. To ensure measurement accuracy, the primary current during normal operation should be approximately 60% of its rated value, and no less than 30%.

3. Determination of Rated Secondary Load: If the secondary load connected to the transformer exceeds the rated secondary load, its accuracy class will decrease. To ensure measurement accuracy, the secondary load S2 of the current transformer must be within 25% to 100% of the rated secondary load S2N.

4. Determination of Accuracy Class: The accuracy class of the current transformer should be selected based on the requirements of the electric energy metering device. The accuracy class of the current transformer varies depending on the amount of electrical energy metered and the importance of the metering object.

II. Installation Precautions for Low-Voltage Current Transformers

When installing low-voltage current transformers, the following points need to be noted:

1. Selection of Installation Location: The current transformer should be installed near the start and end of the circuit to accurately measure the current. At the same time, the installation location should be convenient for observation and operation, ensuring safety.

2. Mounting Method: The current transformer should be securely fixed in place using appropriate brackets or cable clamps. Ensure good contact between the current transformer and the measured circuit, but avoid excessive contact to prevent overheating.

3. Wiring Method: The secondary side of the current transformer must not be open-circuited. When connecting wires, ensure that the cross-sectional area of the connecting wires is sufficient to withstand the current in the circuit. At the same time, avoid wires forming eddy currents near the current transformer, which may affect its performance. Additionally, the secondary side of the low-voltage current transformer must not be grounded to prevent damage to the electric energy meter or transformer when high voltage is applied to insulation weaknesses.

4. Grounding Treatment: The casing of the current transformer should be reliably grounded for safety.

In summary, the selection and installation of low-voltage current transformers is a complex and meticulous process that requires comprehensive consideration of multiple factors. Proper selection and installation can not only improve the stability and safety of the power system but also ensure measurement accuracy and reliability.