3.1.2 The ambient temperature, altitude, vibration or ground tremor of the current transformer installation site shall comply with the provisions of the current national standard "Mutual Instrument Part 1: General Technical Requirements" GB20840.1.
If the actual use conditions of the transformer are different from the normal use conditions, the transformer should be designed according to the special use conditions required by the user.
3.2 Primary parameter selection principle
3.2.1 The rated primary voltage of the current transformer should not be less than the rated primary voltage of the circuit.
3.2.2 The rated primary current of the current transformer should be selected according to the rated current or maximum working current of the primary equipment to which it belongs.
The standard value of the rated primary current is: 10A, 12.5A,15A, 20A, 25A, 30A, 40A, 50A, 60A, 75A and their decimal multiples or decimals.
3.2.3 The rated continuous thermal current, rated short-time thermal current and rated dynamic stability current of the current transformer should be able to meet the requirements of the maximum load current and short-circuit current of the primary circuit, and the development of the system should be considered.
3.2.4 The current transformer with selectable ratio can achieve different ratios by changing the series and parallel connection of the primary winding or the tapping of the secondary winding.
3.2.5 The rated primary current of the current transformer should be selected so that the secondary current under the rated current ratio condition meets the accuracy requirements of the measuring instrument and protection device of the circuit.
3.2.6 When the current transformer has multiple secondary windings, the secondary windings for protection and measurement can adopt different ratios.
3.2.7 The rated short-time thermal current of the current transformer shall meet the following requirements:
1 The standard value of the short-circuit duration of the rated short-time thermal current of the current transformer is 1S, and the rated short-time thermal current can be selected from the following values: 3.15kA, 6.3kA, 8kA, 10kA12,5kA, 16kA, 20kA, 25kA, 31.5kA, 40kA, 50kA, 63kA, 80kA, 100kA
2 The short-circuit duration of current transformers of different voltage levels should meet the following requirements: 2s for 550kV and above, 3s for 126kV~363kV, 4s for 3.6kV~72.5kV, and 1s for 1kV
3 When the primary winding of the current transformer can be switched in series or in parallel, it should be connected according to its The rated short-time thermal current shall be checked for the actual rated primary current and system short-circuit current under the line state
3.2.8 The rated dynamic stability current of the current transformer shall meet the following requirements:
1 The rated dynamic stability current shall be checked for the current transformer with the primary circuit conductor; the rated dynamic stability current may not be checked for the current transformer with the primary circuit conductor passing through the window and without a fixed plate
2 The short-circuit stability performance of the current transformer with optional ratio shall be determined according to the primary winding series connection method
3 The rated dynamic stability current should be 2.5 times the rated short-time thermal current
3.2.9 The temperature rise limit, insulation requirements and mechanical strength requirements of the current transformer shall comply with the relevant provisions of the current national standard "Transformer Part 1: General Technical Requirements" GB20840.1.
3.3 Principles for selecting secondary parameters
3.3.1 The rated secondary current of the current transformer should be 1A.
If it is beneficial to the production or expansion of the transformer, and in some cases to reduce the secondary open-circuit voltage of the current transformer, the rated secondary current can also be 5A
3.3.2 The selection of the rated output value should comply with the following principles:
1 The rated output value of the measurement level, P level and PR level is expressed in volt-amperes.
2 The rated resistive load value of the TPX level, TPY level and TPZ level current transformer is expressed in Ω.
3 The rated output value of the current transformer should be selected according to the rated secondary current value of the transformer and the actual load requirements.
In order to meet the requirements of transient characteristics, a larger rated output value can also be used.
3.4 Configuration requirements
3.4.1 The type of current transformer, the number of secondary windings and the accuracy level should meet the requirements of relay protection, automatic devices and measuring instruments.
3.4.2 The configuration of the protection current transformer should avoid the dead zone of the main protection.
The secondary winding distribution of the transformer should avoid the occurrence of a protection action dead zone for faults in the protected area when one set of protection is disabled.
3.4.3 When the high-voltage distribution device adopts 3/2 circuit breaker wiring, three sets of independent current transformers should be configured for each string.
3.4.4 For the neutral point effective grounding system, the current transformer should be configured as three phases; for the neutral point non-effective grounding system.
It can be configured as three phases or two phases according to specific requirements.
3.4.5 The relay protection and measurement meter should be connected to different secondary windings of the transformer.
If it is limited by conditions and must share a secondary winding, its performance should meet the requirements of both measurement and relay protection, and its wiring method should avoid affecting the relay protection work during instrument calibration.
3.4.6 The two sets of dual protection should use different secondary windings, and the main protection and backup protection of each set of protection should share a secondary winding.
3.4.7 The secondary circuit of the current transformer should not be switched. When necessary, measures should be taken to prevent open circuit.
