5.1 General provisions
5.1.1 Under steady-state symmetrical short-circuit current, the error of the current transformer should not exceed the specified value.
5.1.2 When a fault occurs in the protection zone, the current transformer should meet the requirements for correct operation of the relay protection, and its error should not affect the reliable operation of the protection device.
5.1.3 When the most serious fault occurs outside the protection zone, the error of the current transformer should not cause false operation or non-selective operation of the protection.
5.1.4 When the protection device has the function of mitigating the influence of current transformer saturation, the requirements for transient saturation of the current transformer can be appropriately reduced according to the requirements of the protection device.
5.2 Type selection
5.2.1 The current transformer for protection should be selected with appropriate characteristics and parameters. The same group of differential protection should not use P-level and TP-level current transformers at the same time.
5.2.2 When there is a requirement for residual magnetism, PR-level current transformers can be used for current transformers of 220kV and below.
5.2.3 For special occasions where the accuracy limit of P-class current transformer is not suitable, PX-class current transformer should be used.
5.2.4 TPY-class current transformer should not be used for circuit breaker failure protection
5.2.5 TPX-class current transformer should not be used for line reclosing.
5.2.6 TPZ-class current transformer should not be used for main equipment protection and circuit breaker failure protection.
5.3 Rated parameter selection
5.3.1 In addition to meeting the requirements of Section 3.3 of this standard, the selection of parameters of the current transformer for protection shall also meet the following specific requirements of the circuit where the current transformer is located:
1 The rated primary current of the current transformer for the transformer differential protection circuit should be selected so that the secondary current of the transformers on each side is basically balanced;
2 The rated primary current of the current transformer for the protection of the high-voltage plant transformer of a large generator set should be selected so that the secondary current of the transformer meets the setting selectivity and accuracy requirements of the protection device under normal and short-circuit conditions;
3 The current transformers for each circuit of the busbar differential protection should have the same transformation ratio.
When the current transformers of the small load circuit use different transformation ratios, the minimum transformation ratio can be determined by negotiation with the manufacturer;
4 For current transformers with zero primary current under normal circumstances, the appropriate rated primary current should be selected based on the actual application conditions, the measured value or empirical data of the unbalanced current, and taking into account the protection sensitivity coefficient and the error limit of the transformer and dynamic and thermal stability and other factors;
5 For the current transformer for grounding protection of the neutral point non-effective grounding system, a dedicated cable type or bus type zero-sequence current transformer can be used according to the specific situation.
The transformer ratio and related parameters should be selected according to the sensitivity coefficient of the protection device.
5.3.2 The parameters of TPX, TPY and TPZ level current transformers shall comply with the following provisions:
The rated symmetrical short-circuit current multiple, the rated symmetrical short-circuit short-time thermal current, the rated primary symmetrical short-circuit current, and the rated primary time constant shall meet the provisions of the current national standard "Technical Requirements for Transient Characteristics of Protection Current Transformers" GB16847;
2 The standard values of the rated primary time constant are 40ms, 60ms, 80ms, 100ms and 120ms.
The circuit time constant of large generator sets should adopt a higher value.
The primary time constant should be determined by the ratio of the inductance to the resistance of the short-circuit branch.
When the short-circuit current is supplied by several branches with different primary time constants, the calculation method should comply with the provisions of Section 10.3 of this regulation;
3 The secondary circuit time constant should be optimized and determined by the manufacturer according to the characteristics of the transformer;
4 The secondary circuit time constant is inversely proportional to the total circuit resistance and can be corrected according to the actual secondary load when applied
5 The working cycle should include C-O cycle and C-O-C-O cycle.
