1. CIGRE Study Committee B5 Colloquium September 2005, Calgary, Canada PS1 - Question 7 What would be the future practice for selecting harmonic restraint algorithm instead of another? Sylwia Wróblewska Institute of Power Engineering, Warszawa, Poland

2. 2 CIGRE Study Committee B5 Colloquium September 2005, Calgary, Canada For the conventional CT’s the second harmonic algorithm has the advantage that it makes it possible to restrain the differential protection both - in case of the transformer inrush currents as well as at the CT’s unsymmetrical saturation. Sometimes the protection differential current is the result of both these phenomena.

3. 3 CIGRE Study Committee B5 Colloquium September 2005, Calgary, Canada For both of these phenomena the distinctive feature is the contents of the second harmonic in the differential current. Transformer inrush Unsymmetrical CT saturation

4. 4 CIGRE Study Committee B5 Colloquium September 2005, Calgary, Canada The disadvantages of the second harmonic algorithm can be reduced by: the limiting of second harmonic restraint current or limiting of the sum of blocking currents (I bl.=k h I h +k 2h I 2h +k 5h I 5h ), optimization of the second harmonic restraint current setting with taking into consideration the transformer and CT’s construction. The operation time of the differential protection with the second harmonic algorithm is in range (8  34) ms.

5. 5 CIGRE Study Committee B5 Colloquium September 2005, Calgary, Canada The algorithm based on wavelet transform will be better then the second harmonic for differential protection with the unconventional, electronic CT’s, because it allows to reduce the protection operation time. What are the issues that users must look at when a transformer is protected by duplicate protection devices manufactured by different manufactures especially when the devices use different restraining and blocking principles? The duplicate protection devices that use different restraining and blocking principles would give a possibility to compare the restraining methods. The criterion recorder, described in the paper No111, would allow to optimizing the settings of both these protection devices.