and low remanence type. The results may not always be valid for non remanence type CTs(TPZ).The performances of the protection functions have been checked in the range fromsymmetrical to fully asymmetrical fault currents. Primary time constants of at least 120 mshave been considered at the tests. The current requirements below are thus applicable bothfor symmetrical and asymmetrical fault currents.Depending on the protection function phase-to-earth, phase-to-phase and three-phase faultshave been tested for different relevant fault positions for example, close in forward andreverse faults, zone 1 reach faults, internal and external faults. The dependability and securityof the protection was verified by checking for example, time delays, unwanted operations,directionality, overreach and stability.The remanence in the current transformer core can cause unwanted operations or minoradditional time delays for some protection functions. As unwanted operations are notacceptable at all maximum remanence has been considered for fault cases critical for thesecurity, for example, faults in reverse direction and external faults. Because of the almostnegligible risk of additional time delays and the non-existent risk of failure to operate theremanence have not been considered for the dependability cases. The requirements below aretherefore fully valid for all normal applications.It is difficult to give general recommendations for additional margins for remanence to avoidthe minor risk of an additional time delay. They depend on the performance and economyrequirements. When current transformers of low remanence type (for example, TPY, PR) areused, normally no additional margin is needed. For current transformers of high remanencetype (for example, P, PX, TPX) the small probability of fully asymmetrical faults, together withhigh remanence in the same direction as the flux generated by the fault, has to be kept in mindat the decision of an additional margin. Fully asymmetrical fault current will be achieved whenthe fault occurs at approximately zero voltage (0°). Investigations have shown that 95% of thefaults in the network will occur when the voltage is between 40° and 90°. In addition fullyasymmetrical fault current will not exist in all phases at the same time.18.1.3 Fault current M11613-3 v1M11613-4 v3The current transformer requirements are based on the maximum fault current for faults indifferent positions. Maximum fault current will occur for three-phase faults or single phase-to-earth faults. The current for a single phase-to-earth fault will exceed the current for a three-phase fault when the zero sequence impedance in the total fault loop is less than the positivesequence impedance.When calculating the current transformer requirements, maximum fault current for therelevant fault position should be used and therefore both fault types have to be considered.18.1.4 Secondary wire resistance and additional load M11614-3 v1M11614-4 v4The voltage at the current transformer secondary terminals directly affects the currenttransformer saturation. This voltage is developed in a loop containing the secondary wires andthe burden of all relays in the circuit. For earth faults the loop includes the phase and neutralwire, normally twice the resistance of the single secondary wire. For three-phase faults theneutral current is zero and it is just necessary to consider the resistance up to the point wherethe phase wires are connected to the common neutral wire. The most common practice is touse four wires secondary cables so it normally is sufficient to consider just a single secondarywire for the three-phase case.The conclusion is that the loop resistance, twice the resistance of the single secondary wire,must be used in the calculation for phase-to-earth faults and the phase resistance, theresistance of a single secondary wire, may normally be used in the calculation for three-phasefaults.Section 18 1MRK 504 158-UEN ARequirements264Application manual
