What are some common reasons why current transformers are unsuitable?

I. Selection Errors: Mismatched selection is the primary cause, accounting for over half of all cases. Selection errors are the leading reason for unsuitable current transformers, with the core issue being parameter mismatch:

1. Incorrect Turns Ratio Selection: Failure to select the rated turns ratio based on the actual operating current. A too-large turns ratio results in the operating current consistently falling far below the rated range, significantly reducing the transformer's measurement accuracy; a too-small turns ratio leads to prolonged current overload, causing core saturation errors to exceed limits.

2. Incorrect Accuracy Class Selection: Misusing a low-accuracy class (e.g., industrial measurement grade 0.5 instead of metrology grade 0.2S) in trade settlement scenarios leads to excessive settlement errors; misusing measurement-grade accuracy in protection scenarios results in excessive errors under short-circuit current, failing to meet protection requirements.

3. Incorrect Rated Capacity (Load) Selection: Failure to calculate the total load of the secondary circuit, resulting in an undersized capacity, causes the actual secondary load to exceed the rated allowable value, increasing measurement errors and even triggering core saturation.

4. Frequency Characteristic Mismatch: In high-frequency/harmonic scenarios (such as inverter output and new energy grid connection), conventional power frequency transformers are used, exceeding the transformer's frequency response range, leading to waveform distortion and large errors in measurements.

5. Incorrect Structure Type Selection: Incorrect use of closed-type transformers in existing line upgrades requires power outages and busbar removal for installation, affecting power supply; blindly choosing open-type transformers in new projects results in higher costs and lower accuracy compared to closed-type transformers.

II. Installation Operation Errors: Problems Caused by Improper On-Site Operations
Improper installation operations are the second most common cause, accounting for 30%-40% of actual problems:

1. Incorrect Number of Turns: This is the most common, accounting for 70%-80% of installation-related problems. The transformer label indicates a specific number of turns corresponding to the transformer ratio (e.g., a 75/5A ratio requires 2 turns), but in actual construction, only 1 turn is installed, making the actual ratio 150/5A. However, the calculated electricity is still based on the label ratio, directly leading to metering errors.

2. Incorrect Wiring Polarity: In three-phase circuit wiring, reversing the polarity of one or more phases can cause phase current indication deviation in incomplete star connections, reverse polarity in both phases can cause the energy meter to reverse, and reverse polarity in complete star connections can cause abnormal current in the common conductor, leading to metering errors.

3. Non-standard Installation Location: Installation locations too close to transformers, busbars, or other strong electromagnetic field sources introduce additional electromagnetic interference, causing unstable measurement data fluctuations. Insufficient heat dissipation channels within the distribution box can lead to prolonged overheating and accelerated insulation aging.

4. Non-standard Interface/Grounding: Indiscriminate use of BNC and SMA interfaces can result in loose connections and poor contact. Inadequate secondary grounding or excessive grounding resistance not only affects measurement stability but also poses a risk of electric shock. Lack of shielding during wiring, or parallel wiring with high-voltage lines, introduces interference and causes signal distortion.

III. Equipment Quality Issues

1. Non-Standard Manufacturers: Small manufacturers produce equipment without standard testing, resulting in excessive factory errors and substandard core material permeability, leading to significant errors from the outset.

2. Human Tampering: Some units replace the transformer labels, changing large-ratio labels to small-ratio labels, or vice versa, causing ratio mismatch during metering and artificially creating errors for illicit gain.

3. Transportation/Storage Damage: Impacts during transportation cause porcelain bushing cracks and loose cores, preventing normal operation after installation and resulting in excessive errors.

IV. Environmental Factors and Incorrect Adaptation

1. Operating Environment Exceeding Tolerance Range: Using ordinary indoor transformers in humid, high-temperature, or explosion-risk environments accelerates equipment aging and insulation damage; failure to perform derating treatment in high-altitude environments results in substandard insulation withstand voltage.

2. Insufficient Protection in Special Environments: In coastal areas, salt spray protection is not selected; in corrosive environments, IP65 or higher protection levels are not chosen, leading to equipment corrosion, reduced insulation, and premature damage.