For most power plants, the probability of an unstable turbine-generator shaft torsional event is very small, even for units exposed to SSR, HVDC converters, or large (relative to unit size) variable speed motor drives.  However, should an unlikely torsional event occur, the consequences are tremendous.  A single event can cause serious damage to a unit, resulting in large repair costs and a unit outage lasting many months.  Torsional protection greatly reduces this risk. 

A machine experiencing high torsional stress gives off no telltale signs.  Operators see, hear, and feel nothing unusual, even when shaft torsional vibrations are at damaging levels.  Normal plant protection systems do not protect vs. torsional vibrations. 

Any protection system that can trip a turbine-generator must be secure.  It must trip when necessary, and not trip falsely.  Failed trips and false trips both yield very expensive consequences.  The GE Torsional Stress Relay (TSR) is based on simple, robust principles and is implemented with high quality components by a very experienced team.  The input signal is a direct measurement of shaft speed deviation (not generator electrical signals from which shaft vibration must be deduced or assumed).  The final trip decisions are supervised by diagnostic logic implemented in a separate IED (a GE Multilin Universal Relay, UR).  Settings are calculated by GE engineers based on 40 years of experience with torsional protection.  Extensive factory testing, including burn-in, and site commissioning by a GE TSR engineer assures that everything in the TSR operates as it should. 

Mitigation is used to help operate by reducing risk.  However, in some applications with risk of torsional interaction mitigation schemes have been implemented to “eliminate” the torsional problem.  For example, a few plants have an SSR blocking filter, several plants have operational “RAS” schemes, and some units have been equipped with “dynamic stabilizers” to add torsional damping. But what if these systems fail and the mitigation is lost?  Or what if someone inadvertently changes the mitigation equipment so that it is less effective (or even does the wrong thing)?

In any case, mitigation reduces the chance of a damaging torsional event, it cannot completely eliminate the risk.  Torsional protective relays are the final backup, to prevent damage to the turbine generators regardless of what circumstances lead to high torsional stress. 

The core digital hardware of the TSR is the same as in GE’s excitation system and turbine control products, along with the GE Multilin UR.  The TSR’s service life and parts availability is the same as for these other major GE products. 

Fleet experience over the last ~20 years shows the digital TSR’s reliability is outstanding.  Data is available upon request at the time of quotation. 

Since 2007 the TSR has included extensive built-in monitoring to capture data upon significant torsional events, as well as diagnostic data for maintenance. The torsional data captured by TSR has been used to later compute unit loss of life after a trip due to an SSR event, as well as to match grid SSR study analysis.

Power plant owners have been depending on GE engineers for solutions to torsional vibration problems for more than 40 years. GE engineers designed and built the first SMF analog torsional protection system in the early 1970s. Some of those relays may still be in service today, although many have now been replaced with GE’s digital TSR. There are presently more than 100 GE TSR’s in service. Our team of torsional experts provided and supports all those past projects. GE is committed to providing torsional protection and analysis services to our customers now and in the future. We will be here when you need us.