5.6.202.2.2 Accuracy class designation
The accuracy class is designated using the highest permissible percentage of the composite error, followed by the letter “P” (standing for “protection”) and the ALF value.
5.6.202.2.3 Standard accuracy classes
The standard accuracy classes for protective current transformers are: 5P and 10P
5.6.202.2.4 Error limits for class P protective current transformers
At rated frequency and with rated burden connected, the ratio error, phase displacement and composite error shall not exceed the limits given in Table 205.
The rated burden shall have a power-factor of 0,8 inductive except that, when the rated output is less than 5 VA a power-factor of 1,0 shall be used.
5.6.202.3 Class PR protective current transformers
5.6.202.3.1 Standard accuracy limit factors (ALF)
The standard ALF values are: 5 – 10 – 15 – 20 – 30
5.6.202.3.2 Accuracy class designation
The accuracy class is designated by the highest permissible percentage of the composite error, followed by the letters "PR" (indicating protection low remanence) and the ALF value.
5.6.202.3.3 Standard accuracy classes
The standard accuracy classes for low remanence protective current transformers are: 5PR and 10PR
5.6.202.3.4 Error limits for class PR protective current transformers
At rated frequency and with rated burden connected, the ratio error, phase displacement and composite error shall not exceed the limits given in Table 205.
The rated burden shall have a power-factor of 0,8 inductive except that, when the rated output is less than 5 VA a power-factor of 1,0 shall be used.
5.6.202.3.5 Remanence factor
The remanence factor shall not exceed 10 %.
NOTE The insertion of one or more air gaps in the core is a method for limiting the remanence factor.
5.6.202.3.6 Secondary loop time constant
The secondary loop time constant may be specified.
5.6.202.3.7 Secondary winding resistance
The upper limit of the secondary winding resistance may be specified.
5.6.202.4 Class protective current transformers
The performance of class PX protective current transformers shall be specified in terms of the following: rated primary current; rated secondary current; rated turns ratio; rated knee point e.m.f. ; upper limit of exciting current at the rated knee point e.m.f. and/or at a stated percentage thereof; upper limit of secondary winding resistance.
In this case, the rated resistive burden and the dimensioning factor shall be specified, and the choice of Rct is left to the manufacturer.
For class PX, the turns ratio error shall not exceed 0.25 %.
For class PXR, the turns ratio error shall not exceed 1 %.
For class PXR, the remanence factor shall not exceed 10 %.
NOTE 201 To ensure a remanence facto 10 %, class PXR current transformers may comprise air gaps.
NOTE 202 For large class PXR cores with low ampere-turns, it may be difficult to meet the remanence factor requirement. In such cases, a remanence factor higher than 10 % may be agreed.
5.6.202.5 Protective current transformers for transient performance
5.6.202.5.1 Error limits for TPX, TPY and TPZ current transformers
With rated resistive burden connected to the current transformer, the ratio error and the phase displacement at rated frequency shall not exceed the error limits given in Table 206.
When the specified duty cycle (or a duty cycle corresponding to the specified transient dimensioning factor is applied to the current transformer connected to the rated resistive burden, the transient errors (for TPX and TPY) or (for TPZ) shall not exceed the limits given in Table 206.
All error limits are based on a secondary winding temperature of 75°C.
5.6.202.5.2 Limits for remanence factor
5.6.202.5.3 Specification Methods
The two specification methods are illustrated in Table 207.
In some cases, the choice of one specific duty cycle cannot describe all protection requirements. Therefore, the alternative definition offers the possibility to specify “overall requirements”, which cover the requirements of different duty cycles. The specifications shall not be mixed, otherwise the current transformer may be over-determined.
NOTE 1 For current transformers with tapped secondary windings, the given accuracy requirements can be fulfilled for one ratio only.
Note 2 For current transformers with primary reconnection, the accuracy requirements may be fulfilled for different ratios. In this case, attention should be paid to the factor of construction which may be influenced by the configuration of the primary conductors.
NOTE 3 In the alternative specification, it is usually given by the supplier of the protection devices. It has also to be specified, because it is the only parameter of the current transformer which is used in the calculation.
5.6.203 Class assignments for selectable-ratio current transformers
5.6.203.1 Accuracy performance for current transformers with primary reconnection
For all accuracy classes, the accuracy requirements refer to all specified reconnections.
