Torsional Stress Relay

The Torsional Stress Relay (TSR) is a digital protective relay that continuously monitors the turbine generator shaft for torsional oscillations, and provides trip output contacts when shaft fatigue reaches predetermined levels. Torsional event data capture is also provided.

The TSR is an updated digital version of the proven GE SMF relay. GE technology has been used since 1976 to protect turbine-generators exposed to risk of harmful torsional interaction.

The TSR is a complete cabinet, suitable for a generating station relay room or other room with a reasonably controlled environment.

Typical connections are:

  • Inputs from dedicated magnetic speed pickup(s).
  • Status contact input from generator breaker(s).
  • Trip contact output(s).
  • Status/alarm output contacts for plant operators.
  • IRIG-B to synchronize the TSR’s SOE record.
  • Input power from 125Vdc station battery.
  • 120Vac for convenience usage.

Versions of the TSR are available that can protect more than one turbine-generator, for combined cycle plants or small units.

Product Application Information Overview

Power systems can include equipment that can interact with torsional vibrations of turbine-generators at the shaft natural frequencies, including stimulation or resonance. The amplitude of these vibrations can be large enough to cause cumulative fatigue damage to turbine-generator shaft components. In several cases these adverse interactions have led to sustained or growing torsional vibrations that caused severe damage (cracks, etc.) to rotors or couplings, resulting in long repair outages for those units. The known equipment of concern includes:

  • Series capacitors in transmission lines (SSR – Sub-synchronous Resonance)
  • HVDC converters (SSTI, see next section)
  • Large industrial motor drives (SSTI)
  • Other relatively large power electronic loads
  • Nearby steel mills with arc furnaces
tor-1.jpg

Since the early 1970’s, GE has pioneered the analysis and mitigation of torsional problems and has offered protective relays and monitoring equipment to address these problem areas.  Our latest torsional protection products build on GE’s extensive experience and feature digital processors, built-in monitoring, and higher reliability.

The Torsional Stress Relay (TSR) is a digital protective relay designed to continuously monitor turbine-generator shafts for torsional vibrations and provide trip output contacts when shaft fatigue reaches predetermined levels.  Torsional event data capture is also provided.

The first TSR model was introduced in 1997 as an updated digital version of the proven GE SMF relay.  These relays have been applied successfully since the 1970’s to many turbine-generators (including many non-GE units) exposed to risks of damaging torsional interactions. 

Application Studies / Screening

tor-2.png

SSR due to series capacitors is the most usual situation that may need torsional protection.  An SSR study should be performed if transmission line series capacitors (fixed or TCSC) are being added, the amount of compensation is being changed, or if other system (or shaft) changes will be done that may affect a turbine-generator's exposure to SSR.  The study evaluates the risk of damage due to SSR for normal and contingency system conditions. 

Depending on the level of risk it may be prudent to add torsional protection (TSR).  In some cases the risk is high enough to justify redundant protection. Large rated power electronic converters near to turbine generators are the next most common situation that may need torsional protection.  HVDC is especially of concern, due to its typically very large power rating. 

Large variable speed motor drives can also be of concern, particularly when smaller turbine-generators are nearby or operate in islanded conditions.  In such cases a Sub-synchronous Torsional Interaction (SSTI) screening study should be performed.  If screening determines risk, torsional protection and more detailed studies including control effects may be prudent. In rare cases exposure to SSR or SSTI can be so severe that active damping or even resonance blocking filters may be needed, in addition to torsional protection. Other cases such as nearby steel mill arc furnaces or high-speed reclosing typically have a lower risk.  GE can determine the need for a study in such cases.  New units being installed into electrical environments of concern should also be evaluated for risk. 

Torsional Stress Relay

tor-3.png

The GE TSR is a cabinet, suitable for generating station relay room or other room with a reasonably controlled environment.  Typical connections are:

  • Input from dedicated magnetic speed pickup(s)
  • Status contact input from generator breaker(s)
  • Trip contact output(s)
  • Status/alarm output contacts for plant operators
  • IRIG-B to synchronize the TSR’s SOE record
  • Input power from 125Vdc station battery
  • 120Vac for non-critical usage
  • Versions of the TSR are available that can protect more than one turbine-generator, for combined cycle plants or smaller units.  For applications that need Supplementary Excitation Damping Control the TSR/SEDC cabinet is available that provides both functions, the SEDC output signal is connected to a properly equipped static exciter.

    tor-4.png

    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 80 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.

    For more information and/or for price quote for SSR, SSTI studies OR TSR protection, please contact us today.

    Resources

        BROCHURE

    TSR Applications