Estimation of Slip Frequency in Case of Fast Trip by Pole Slip Protection

Session Title: Condition monitoring and situational awareness

When a generator set gets tripped due to pole slip protection (PSP) function, it is quite useful for network operation staff as well as system design and development engineers to have an estimation from the slip frequency that caused tripping. This important information can be used not only for adjustment and optimization of existing network operation instructions, but also in modern networks, as an input to System Integrated Protection Schemes (SIPS), for real time analysis of new situation. In some cases the output from SIPS could include a series of control actions which may prevent further consequent protection trips in other areas of the network. This is especially more important in Distributed Generation system where there are numerous generation points with closer interactions.

In the existing generator protection systems, slip frequency can be defined only after at least one complete oscillation cycle has been reached. This is while most of utility companies do not want to allow the disturbance to remain in the system for one slip cycle more, therefore they usually set the protection function such that the trip command would be issued when only a segment of the slip cycle was gone through. With the trip signal issued, there will no longer remain a chance for protection function to measure the duration of complete oscillation cycle and therefore the slip frequency. 

This paper presents a method to estimate slip frequency with an acceptable accuracy if pole slip is detected and issues a trip signal even before a complete oscillation period is elapsed. Accuracy of measurements based on this theory was tested and assured in various system conditions with different slip frequencies.