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Abstract-- Six areas of focus for turbine-generator torsional vibration monitoring are discussed: 1) The value case for generator torsional vibration monitoring, 2) Environmen-tal factors leading to vibration exposure, 3) Experience with torsional monitoring and protection, 4) Monitoring and Protection system design, 5) Evaluating response, stress and fatigue, and 6) the case for standards of turbine-design for torsional withstand. It is concluded that more work is needed on standards for turbine-generator capa-bility for off-nominal operation. That the art of torsional monitoring and protection is well advanced and can be profitably fostered by owners of turbine-generators given the state of the standards, the environment factors, and the margins for operation.

Index Terms— Fatigue, Subsynchronous Resonance (SSR), SSR Relay (Protection), Transmission Capacitor Series Compen-sation, Torsional Interaction, Torsional Monitoring, Torsional Response, Turbine-Generator Protection.

I. INTRODUCTION Instrumentation for detection of torsional vibration has been available for a long time [1] and such equipment is in a high state of evolution with regard to integration with monitoring algorithms that are required to determine stress and resulting fatigue. The reason for this capability to exist has been the relatively large number of incidents where torsional fatigue cracks have actually propagated, and on the large number of transmission projects that utilize series capacitor impedance compensation in the vicinity of highly rated turbine-generators [2]. In the former case the instrumentation has been mostly temporary for test, while the latter are of a permanent nature used to provide protection against high torsional vibration [4]. More recent applications of instrumentation and monitoring are for the case of the long blade dynamic response of LP tur-bines where the zero nodal diameter blade modes and flexible rotor modes combine to allow excitation from the generator electromagnetic torque. These situations are covered by the general subject -“Turbine-Generator Interaction with the Elec-trical Grid,” where torsional response is just one interface is-sue.

C.E.J. Bowler is with Instrumentation Technology Inc., Cary NC USA. (e-

mail cbowler@inst-tech.com)

II. THE VALUE CASE FOR MONITORING & PROTECTION The value case for monitoring and protection for torsional vibration is no different than that we use for generator electri-cal protection, or turbine lateral vibration. The case is actually stronger than this owing to the fact that the effects are not al-ways obvious until it is too late. There are no outward physical effects available to the senses while vibration is taking place. It is not until the vibration gets so large that nonlinear effects begin to show in the form of lateral vibrations due to detach-ment of blades or lifting and dropping of the generator stator in its mountings. Below are enumerated the basic values de-livered by torsional monitoring and protection.

1) Protect for Unstable Resonance a. Backup Human Error in Calculation of Stability Margin b. Backup Inadequacy in Physical Modeling (electrical

and mechanical) c. Trip the turbine when stability margin is compromised d. Reevaluate operating conditions for mitigation e. Reevaluate and modify the power transmission design

2) Monitor for Conditions Leading to Crack Initiation and Propagation

a. Establishment of Cause and Effect leading to cracks b. Modify turbine operations for prevention of cracks c. Feedback evidence for warranty concerns d. Feedback of evidence for Insurance concerns e. Coordinate inspection intervals to prevent crack separa-

tion

The viewpoint above for monitoring can be looked upon as a first in a series of actions that may be taken to mitigate vi-bration as follows:

1) Monitor, evaluate and inspect 2) Increase torsional damping 3) Rebalance transmission impedance 4) Re-tune turbine mechanical structures 5) Control system frequency

III. ENVIRONMENTAL AND DESIGN FACTORS LEADING TO HIGH VIBRATION

Looking back at the history of turbine-generator design and operation we see that the plant equipment currently in service are dominated by single OEM designed systems where each new generation of equipment have evolved using consistent design criteria. I am referring particularly to the large genera-tors that have gone into service over the past 40 years and the

Grid Induced Torsional Vibrations in Turbine-Generators – Instrumentation, Monitoring, and

Protection Colin E. J. Bowler, Life Member IEEE

978-1-4673-2729-9/12/$31.00 ©2012 IEEE

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utility gas turbine plant that have gone in service since the mid 90’s. This consistency is no longer a factor either for equipment design or operation. The reasons for this change are related to the competitive environment both for generation operation and equipment supply. Large turbine-generators are not being built in favor of renewable generation, particularly wind power, and the existing generation fleet is undergoing renewal rather than replacement. Both of these factors will lead toward the need for generation surveillance and protection for the reasons cited above. In the case of renewal-projects, the turbine-rotors in question are being replaced by the supplier of lowest price. This leads to a turbine-generator design that can only be characterized as a hybrid system, where no one OEM has responsibility for future operations. In this condition the operator of generation can expect troubles he has never seen before because there is no consistency in the design process that was available with the single OEM system purchases of old. In addition the new renewable resources; chiefly wind-generation is being placed where the wind blows, which tends to be remote from existing transmission. The implication of this is the proliferation of transmission series capacitors on both new and existing trans-mission [5]. There are at least two problems that will result from these changes, 1) the thermal generation will be asked to deal with generation dispatch that recognizes when the wind stops blow-ing or the sun is not shining, leading to increased fatigue duty, 2) the existing thermal generation will be exposed to subsyn-chronous resonance (SSR) from the added series capacitor transmission compensation, leading to torsional fatigue and unstable resonance of the torsional dynamics. So the future is likely to see increased use of torsional vibration instrumenta-tion and protection.

IV. EXPERIENCE WITH TORSIONAL MONITORING AND PROTECTION SYSTEMS

The majority of experience with torsional vibration surveil-lance monitoring is because of SSR. Where SSR is a factor in the transmission/generation design there must be protection for the generator against unstable resonance and torsional fa-tigue. There are a decent number of applications word-wide, to make an opportunity for equipment development for both pro-tection and surveillance. This experience has lead to the availability of commercial systems that specifically detect and respond to torsional vibra-tion of generation equipment. The specification of such equipment includes the need to track vibration amplitudes as large as +-10% variation of generator velocity over a frequency bandwidth range of 0 to 150 Hz, with a resolution from 0.005% for rotor modes down to 0.001% velocity for blade modes. With this specification it is possible to monitor worst-case response over the useful range of rotor material mechanical stress capability, while at the same time provide early onset detection of instability and fa-tigue. Generally speaking there have been few if any operations of torsional protection. This condition is based on the require-ment that protection be used only as back up for other means of control. In general this other means of control has been in

the design of the series compensation system to avoid SSR, with protection as back up when all skill is in vain. Such com-pensation design may include:

1. Operating practices to recognize and avoid dangerous network configurations

2. Direct mitigation means such as controlled power electronic devices and passive filters

3. Coordination of compensation application to lower probability of SSR.

For many of the SSR protection systems in service today there also include surveillance equipment. By this we mean the ability to collect and analyze the vibration information. Such information will include the vibration information itself, and the precursors of those vibrations such as the disturbance to generator voltage and current and/or the generator electro-magnetic- torque. Precursors also include the digital indica-tions of operation of other protection systems such as over current relays and breaker operation. To be useful as a network device monitoring system the torsional protection and monitoring systems are, or should be equipped with the ability to combine the surveillance output with other data collection systems. This facility is accom-plished using a uniform data format and time stamping that facilitates combining of data sources. The IEEE COMTRADE standard is the main vehicle that has been used to achieve this synchronization of surveillance data sources.

V. TORSIONAL VIBRATION PROTECTION AND MONITORING SYSTEM DESIGN

The elements of torsional response instrumentation, Figure 1, consist at a minimum of one or more transducers of torsional velocity; these generally use a turbine-generator shaft mounted tooth-wheel and magnetic reluctance sensor in conjunction with a velocity transducer. Such sensors provide a pulse train of tooth passing indication that is FM demodulated in a trans-ducer to recover the underlying velocity modulation of the pulses due to motion both absolute and differential. The torsional velocity sensors must be located properly so as to detect motions due to oscillatory twisting of the rotors. For most purposes the measurement of velocity at the out-board ends of the turbine-generator meet the need for detecting sub-synchronous torsional oscillation. In the case of super syn-chronous modes it may be advantageous to make additional measurements at the couplings, which participate with the rotor flexible modes if these cannot be seen at the outboard ends. The location of sensors and the transducer output -velocity calibration to mode participation is evaluated by solving for the torsional mode shapes and frequencies of the turbine-generator mechanical structure. In Figure 2 are plotted typical example rigid and flexible rotor mode shapes of sub and super synchronous modes against the backdrop of the physical turbine structure. Super-imposed on the rotor motion is the relative deflection of the last and near last stage (L-0, L-1 etc.) blades. In the rigid rotor modes, such as Mode 1, shown here there is no relative motion between blade and rotor. The higher order flexible rotor modes do have blade deflection relative to the rotor. The blade

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The blade model additions of figure 9 represent the blades rows explicitly only two blade rows are represented in the figure for clarity. This kind of model is for zero nodal diame-ter blade row modes only. Such a representation would add significantly to the order of the model and requires too much detail to be useful for extended computational studies. This would be especially true considering that there may be dozen or so inertia for the blade row model and up to six flows of LP stages each with two blade rows per rotor. Instead resort is made to a modal synthesis process to represent the blades that have significant benefit in model order reduction.

Figure 9: Turbine Rotor Train Inertia Stiffness with Blade Inertia Stiffness Distribution In the synthesis process the blade row is considered first in isolation with the root grounded resulting in the following Inertia stiffness distribution of Figure 10, with equations of representation as in 2) for tangential displacement relative to blade root.

Figure 10: Blade Row Mass Stiffness Distribution Grounding Node 1 Fixed-Free (Cantilever Model -Interpret m’s and k’s here as for the blade row)

2) The modal synthesis process starts with a physical to modal transformation of the blade row model based on the eigenvec-tors of 2) , given in 3).

3) Equation 2) transformed to Modal Coordinate Form

4) Matrix becomes diagonal. The three or more independent equations can be solved separately as in 5). In more typical higher order blade models we may only need to solve one or two of the modes that respond to generator electromagnetic torque, as the remaining modes do not participate in the struc-ture response.

5) The physical model view of the blade row now is represented by multiple grounded blade modes fixed in the same blade row root position as shown in Figure 11.

Figure 11: Multiple Blade Row Modes Physical Interpretation The original inertia and stiffness model of figure 9 is now transformed to that shown in Figure 12, a reduced order modal synthesis model. Some adjustments to the original model iner-tias of Figure 8 for the basic rotor train are required. These are a reduction of the rotor inertia at the blade attachment point by the sum of the modal inertias being represented in the particu-lar blade row.

Figure 12: Reduced Order Modal Synthesis Model - 1st and 2nd L-0 and L-1 Blade Row Modes Represented attached to the physical rotor train The reduced model of figure 12 can now be easily evaluated for overall mode frequency and mode shape, and time re-sponse of generator torque disturbance. The answers returned for the blade response are modal and so an inverse transform operation is required based on 3) to return the actual blade displacement response. Since the reliability criteria for the blade is stress and fatigue the blade displacement response information must be trans-formed to stress. This is in the form of a constant influence coefficient multiplier for modal displacement for the maxi-mum stress location in that mode similar to that used currently for shaft torque in SSR studies. For the case of physical stress evaluation the modal synthe-sis model root response can be used as base excitation in an FEM blade model to get stress information at any location on the blade.

VI. DATA REQUIREMENT FROM THE OEM In order for the user to pursue evaluations described here for simulated response evaluation and for monitoring applications for shaft and blade interaction with the electrical grid a stand-ard set of data should be required from the OEM along with scale drawings of the steam path and generator, and dimen-sioned drawings of the blade root structures and vane. A pro-posal for the form of data required was presented in [4].

VII. TURBINE-GENERATOR DESIGN STANDARDS In the US their have existed standards for turbo-generator op-eration, currently C50.13-2005 [6], that relate to fault with-stand severity, and coincidence of voltage and frequency vari-ation from nominal. This standard has its harmonized counter-part in IEC60034. These standards are typical in the language of standards that use the action words like “Shall, and Must” directives. Listed below are the items that related to off-nominal conditions and withstand requirements.

1. Voltage and frequency permissible coincident varia-tions: see figure 12.

2. Stator short-time over-current without damage: 150% rated current for 30 seconds; table/formula for other times.

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3. Routine Synchronizing Capability: ±10 degrees; 0% to 5% overvoltage; ±0.067 Hz

4. Negative sequence currents without damage: Steady State I2 < 5% to <10% depending upon rating; {5 to 30 (per unit I2)2 x t} seconds depending upon rating, for times up to 120 seconds; table/formula in standard

5. Sudden Short Circuit Mechanical Withstand: 3 phase short circuit at generator terminals with 1.05 per unit voltage not exceeding short time armature thermal ca-pability

6. Line Switching Capability ≤ 0.5 per unit sudden power change

One would expect that standards for a generator would also be a requirement for the connected turbine, yet this has not necessarily been embraced in practice. What is clear from the practice in this authors experience is that worst case terminal faults and synchronizing out of phase (SOP) withstand has been interpreted as to not cause catastrophic failure, whereas the affected generating set may require rebalance and/or rea-lignment in order to be operable. The implication of this is that for short circuits the shafts may yield at the surface, while for worst case SOP event the shaft may yield to the bore. These interpretations are reasonable especially for the SOP case, which is preventable and not generally an act of nature. No exceptions to the (I2)2 and (I2)2 x t requirement have been noted for the turbine, however exception to extended off-frequency operation has been taken, The off nominal operating region for voltage and frequency for the generator capability is without regard to simultaneous fault events, however it is clear that the continuous region of frequency from 98% to 102% of rated has been a target of the turbine designer at least for oper-ation in steady state without regard to negative sequence cur-rent from non-fault sources. A more recent standard is ISO 22266-1, 2009 [7] that covers “turbine-generator set” torsional vibrations for steady state and transient conditions.

While this standard is a start it does not require any specific capability, and is devoid of the action words like “Shall and Must” as in the C50.13 standard above. Instead it provides criteria for a frequency exclusion zone based on an “Allowa-ble Grid Frequency Variation” which instead of being a spe-cific range it is to be what is customary for the particular sys-tem. This type of standard puts the onus on the system opera-tor instead of the OEM.

Figure 12: C50.13 Voltage and frequency permissible coincident variations

Our point of view is that capability for off-nominal frequen-

cy operation should be specific like + 2Hz or some other val-ue, something that can be heeded and designed for in the sys-tem that is good for all time. This approach puts the onus for design on the OEM who is the supplier of the equipment that is generally under warranty for extended periods. The question also should be asked why not perform full field test verifica-tion it has to be a small cost compared to the cost of a single new LP rotor let alone a complete turbine.

Off-nominal frequency operation is not confined to a specif-ic turbine all generating units in the interconnection are affect-ed similarly regardless of type.

Off nominal frequency operation is for the most part normal operation and should allow for the coincidence with fault events, which are also normal. There is extreme off normal frequency operation, which would not be covered by the standard other than it alerts the affected utility entity or system regulator to take the necessary action to prevent extended op-eration in this frequency range or pay the consequences.

Such action would include increased frequency regulating (governor) capacity, intentional islanding and load shedding, and paying strict attention to the size of capacity additions relative to the size of the system under regulation. The greater this ratio the larger is the frequency excursion on loss of gen-eration.

While operation in the forbidden zone of off-nominal fre-quency cannot be totally avoided the entity affected by such operation needs to have the tools to make assessment of the damage from such operation, this could include recommenda-tion for inspection interval based on frequency excursion dura-tion, and/or the use of monitoring to allow more detailed eval-uation of the effects. This latter point is to provide inspection guidance, rather than blind faith based on anecdotal recom-mendations. As the standard is currently written it behooves the owner of the equipment to support any warranty claims with compre-hensive records of off-frequency duration and coincident faults.

The ISO 22266-1 continuous with § 6.1, Calculation of tor-sional vibration in the case that the frequency margin require-ments are not met. Here is stated that for forced excitation mechanism for stated types of disturbance (listed) and stress levels should be in accordance with the suppliers (OEM) expe-rience. This is clearly not a standard as the suppliers experi-ence is both variable and unknown to the owner of the turbine or anyone else. So neither the owner nor the utility regulating agency has a provable recourse in the event of damage.

VIII. CONCLUSION It is concluded that the standards for system interface for the turbine regarding frequency, short circuit withstand, and nega-tive sequence current, should be the same as for the generator. To allow otherwise makes a mockery of the standards process. These devices see the same duty they must be rated for this duty. Such harmonizing is achievable by making reasonable fixed frequency limits for the turbine and especially by power frequency control means including: 1) fault anticipation valve operation of the turbine, 2) increasing frequency regulating capacity, 4) islanding and load shedding. Additionally retun-

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ing mechanical structures, and rebalancing transmission im-pedance solve the majority of steady state vibration problem. Finally all turbine-generator sets should be subject to full-scale field test to prove tuned condition. This should be looked upon as a kind of type test, typical of any electrical apparatus.

IX. REFERENCES Papers from Conference Proceedings (Published):

[1] DN Walker, RJ Placek, CEJ Bowler, JC White "Turbine Generator Shaft Torsional Fatigue and Monitoring," CIGRE meeting 1984.

[2] CEJ Bowler, DN Ewart, C Concordia, "Self Excited Torsional Frequen-cy Oscillations with Series Capacitors," IEEE TPAS-92 No. 5, Sept/Oct 1973, pp. 1688-1895.

[3] SL Adams, DN Walker, CEJ Bowler "Torsional Vibration Monitoring and Torsional Fatigue," 1982 South Eastern Electric Exchange Engi-neering and Operation Division (GER-3258 Company Position Paper).

Papers Presented at Conferences (Unpublished): [4] CEJ Bowler "EPRI International Conference on Grid Interactions with

Steam-Turbines and Generators – SSR Mitigation, Protection and Moni-toring,” Las Vegas NV 01/16/2006 Web Sites:

[5] Gateway West Transmission Line Project. http://www.gatewaywestproject.com

Standards: [6] IEEE C50.13-2005 IEEE Standard for Cylindrical-Rotor 50 Hz and 60

Hz, 2-Pole and 4Pole Synchronous Generators [7] ISO 22266-1:2009 Mechanical Vibration – Torsional vibration of rotat-

ing machinery – Part 1: Land-based steam and gas turbine generator sets in excess of 50 MW.

X. BIOGRAPHIES

Colin Bowler (M’1970). Graduated from the Universi-ty of Aston, in Birmingham UK and Rensselaer Poly-technic Institute USA.

His employment experience includes General Elec-tric Co., ABB Power Systems, and Instrumentation Technology Inc. His special field of interest includes system interface issues of turbine-generators, torsional monitoring and protection system design and applica-tion.

Bowler received the GE Steinmetz Award and Medal in 1985 for his work in reliability evaluation of turbine-generator expo-sure to system faults and reclosing operations and related work on SSR.

Bowler formed Instrumentation Technology Inc. in 1999.