4.1 Type selection
4.1.1 The type of current transformer should be reasonably selected according to the actual needs of the power system measurement and metering system.
When the working current changes in a large range, the S-type current transformer should be used for accurate measurement.
To ensure that the secondary current is within the appropriate range, a current transformer with a ratio selectable can be used.
4.1.2 The electric energy gateway metering device should be equipped with a special S-type current transformer or a special secondary winding.
For the analysis of economic and technical indicators within the power generation and supply enterprise, and the electric energy metering device for assessment, a secondary winding can be shared with conventional measuring instruments under the condition of meeting the accuracy.
4.2 Rated parameter selection
4.2.1 The secondary load of the current transformer for measurement should not exceed the load range specified to ensure the accuracy level.
4.2.2 The rated primary current of the current transformer for measurement should be close to but not lower than the normal maximum load current of the primary circuit.
For indicating instruments, in order to enable the instruments to have appropriate indications during normal operation and overload operation, the rated primary current of the current transformer should not be less than 1.25 times the rated current of the primary equipment or the maximum load current of the line.
For the current transformer used in the measuring instrument of the direct starting motor, the rated primary current should not be less than 1.5 times the rated current of the motor.
4.2.3 In order to adapt to the development and changes of the power system or the different requirements of the measuring instrument and relay protection, the measuring current transformer can use a smaller ratio transformer or a transformer with taps on the secondary winding.
4.2.4 In order to prevent the measuring instrument from being damaged by a large short-circuit current passing through the primary circuit for a short time during a fault, the measuring current transformer should use a transformer with an instrument safety limit, and the instrument safety factor should be selected as 10, or 5.
4.2.5 When using a current transformer with a current amplification characteristic, its continuous thermal current can be 120% of the rated primary current, and 150% or 200% can be used in special cases.
4.3 Accuracy level selection
4.3.1 The accuracy level selection of the current transformer for measurement shall comply with the following principles
1 The accuracy level of the current transformer for measurement shall be rated as a percentage of the maximum allowable current error specified for the accuracy level at the rated current.
Standard accuracy levels should be 0.1, 0.2, 0.5, 1, 3 and 5; for special purposes, 0.2S and 0.5S should be used;
2 For 0.1, 0.2, 0.5 and 1 current transformers for measurement, when the secondary load is any value between 25% and 100% of the rated load value, the current error and phase error at the rated frequency should not exceed the limits listed in Table 4.3.1-1;
3 For 0.2S and 0.5S current transformers for measurement, when the secondary load is any value between 25% and 100% of the rated load value, the current error and phase error at the rated frequency should not exceed the limits listed in Table 4.3.1-2;
4. For 3 and 5, when the secondary load is any value between 50% and 100% of the rated load value, the current error and phase error at the rated frequency should not exceed the limits listed in Table 4.3.1-3.
4.3.2 The accuracy level of the current transformer matched with the measuring instrument shall be selected in accordance with the following principles:
1 The accuracy level of the current transformer for measurement under the actual secondary load shall be compatible with the accuracy level of the matching measuring instrument.
Different measuring instruments for different purposes require different accuracy levels, and the accuracy level of the matching transformers is also different.
The accuracy level of the measuring instrument directly connected to the transformer and the matching current transformer shall comply with the provisions of Table 4.3.2-1;
2 The working current of the current transformer for electric energy metering should be above 60% of its rated value, and should not be less than 30%, and should not be less than 20% for S-level.
For the billing electric energy meter with a large range of working current changes, S-type current transformers should be used;
3 For all accuracy levels, the secondary load power factor should be 0.8 (after), when the load is less than 5V·A, the power factor should be 1.0, and the minimum value is 0.5V·A;
4 The accuracy level of the current transformer used for harmonic measurement should not be lower than 0.5;
5 When a current transformer loop is connected to several different types of instruments, the instrument with the highest accuracy level should be selected.
4.3.3 When a small ratio bushing current transformer with a rated current below 300A requires a higher accuracy level, it can be determined through consultation with the manufacturer.
4.4 Secondary load selection
4.4.1 The secondary load value of the measuring current transformer should not exceed the range specified.
4.4.2 For measuring-level current transformers with a rated output value not exceeding 15V·A, the load range can be expanded.
