Process for SSTI Studies and Mitigation of Interactions between HVDC and Thermal Turbine-Generators

Date: 2015-06-15Prepared by: ATCO Electric-AltaLink-AESO Task Force Version: Guidelines for SSTI Studies and Mitigation R1B7

Table of Contents

1 Introduction......................................................................................................................................... 21.1 Background...................................................................................................................................................21.2 Objective........................................................................................................................................................21.3 Scope............................................................................................................................................................21.4 General Steps for Assessing the Potential of SSTI.......................................................................................2

2 Pre-Screening for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal Turbine-Generators............................................................................................................................. 52.1 Pre-Screening................................................................................................................................................52.2 Type of Machine............................................................................................................................................52.3 Size of Machine.............................................................................................................................................52.4 Electrical proximity and system topology.......................................................................................................52.5 Tools, models and data required...................................................................................................................6

3 Formal Screening for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal Turbine-Generators..............................................................................................................73.1 Background...................................................................................................................................................73.2 The UIF Methodology for Assessing the Potential for SSTI..........................................................................73.3 Tools, Models and Data Required for UIF Calculations................................................................................73.4 General Guidelines for Conducting an UIF Study:........................................................................................8

4 Detailed Studies for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal-Turbine Generators..............................................................................................................94.1 Purpose of Detailed Studies..........................................................................................................................94.2 Perturbation Analysis.....................................................................................................................................9

4.2.1 Tools, Models and Data Required for Perturbation Analysis..........................................................94.3 Time Domain Analysis...................................................................................................................................9

4.3.1 Tools, Models and Data Required for Time Domain Analysis.........................................................95 Mitigation and Protection for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC

and Thermal Turbine-Generators.....................................................................................................105.1 Background.................................................................................................................................................105.2 HVDC Control & Protection System Changes.............................................................................................105.3 Generator Protection...................................................................................................................................105.4 Use of Filters...............................................................................................................................................105.5 Operational Measures/Awareness..............................................................................................................105.6 Generator Design........................................................................................................................................11

6 List of Turbine-Generator Models and Data Required for Detailed SSTI Simulations.................126.1 Data Required for Sub-Synchronous Studies..............................................................................................12

7 References:........................................................................................................................................ 19

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1 Introduction1.1 BackgroundSub-synchronous oscillations (< 60 Hz) in power systems can be amplified and sustained due to

the interaction between major transmission system devices such as HVDC converter terminals

and the torsional modes of vibration of turbine-generator shaft system. This phenomenon, called

sub-synchronous torsional interaction (SSTI), is well understood and can be readily analyzed

and mitigated. The first reported experience of SSTI between a classic line-commutated-

converter (LCC) HVDC, and nearby thermal turbine-generators, was in 1977 at Square Butte,

North Dakota, USA [1]. Since then, extensive studies and research have been conducted to gain

an understanding of this phenomenon and to develop analytical methods ranging from simple

screening methodologies to estimate the potential of SSTI, to detailed analytical techniques to

quantify the potential for SSTI and to explore various mitigation and protection schemes.

The existing thermal turbine-generators in Alberta were checked for SSTI with the planned

EATL and WATL HVDC systems. Mitigation and protection systems were installed at the HVDC

for all of the turbine-generator interactions identified in those studies.

1.2 Objective The main purpose of this document is to provide guidance to Generation Facility Owners (GFO)

on the basic analytical steps required to evaluate the potential of SSTI between a new or

modified thermal turbine-generator and the HVDC system. The document also gives an

overview of some of SSTI mitigation and protection measures.

1.3 Scope The scope of this document is twofold. First, it provides a general description of the various

steps that need to be followed in assessing the potential of SSTI between an HVDC system and

thermal turbine-generators, which includes steam, gas and combined cycle turbine generators.

Second, it gives an overview of the methodologies to be followed in executing each step based

on the latest state-of-the-art SSTI technology. The AESO will work with the GFO throughout the

process and coordinate the needed communication and activities with the TFOs.

1.4 General Steps for Assessing the Potential of SSTI Figure 1 depicts the four steps that are generally followed in assessing the potential of SSTI and

determining if mitigation and/or protection measures are required.

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Step 1: Pre-screening:

In this step the AESO is able to make a determination, without the need for in-depth analysis,

that the proposed turbine-generator is not exposed to the potential of SSTI with the HVDC

system. The general guidelines which need to be followed to arrive at this conclusion are

presented in Section 2.

Step 2: Formal Screening:

If the proposed turbine-generator does not qualify for exclusion in Step 1, then it has to be

further examined using the formal screening methodology to determine if it could potentially be

subject to SSTI. This formal screening analysis covers a wide range of possible system normal

and contingency conditions. The methodology and guidelines that are used in the formal

screening is presented in Section 3.

Step 3: Detailed SSTI Studies:

If a turbine-generator is identified in Step 2 as a unit with a potential for SSTI, then it has to be

subjected to further detailed analysis to quantify the likelihood and identify the system conditions

which may lead to SSTI. This step involves detailed modeling of the HVDC systems and

turbine-generators and typically uses Electromagnetic Transient Simulation Software such as

PSCAD or similar packages. This does not a detailed network representation. Normally two

types of analyses are involved here, the perturbation analysis and time-domain simulations, as

described in Section 4.

Since the HVDC controllers for EATL and WATL have sub-synchronous damping controllers

(SSDC) the detailed SSTI analysis conducted by the AESO and TFO will check if the SSDC,

with its existing settings, is capable of damping the torsional modes of oscillations of the turbine-

generator under consideration. If the torsional modes of oscillations are not damped with

existing settings (i.e., are negatively damped), then further analysis using detailed network

representation will be carried out by the AESO and TFO.

Step 4: Mitigation and/or Protection:

The outcome of the detailed analysis of Step 3 could be one of the following:

a) High potential for SSTI, requiring mitigation and protection.

b) Medium potential for SSTI, requiring mitigation and/or protection.

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c) Low potential for SSTI, where it is recommended to have either mitigation or protection.

d) No potential for SSTI, therefore mitigation or protection is not needed. This outcome

occurs when torsional modes are shown to be sufficiently damped in the detailed studies.

Section 5 lists a number of mitigation/protection options that need to be considered based on

the level of potential for SSTI identified in Step 3.

Figure 1: The Process for SSTI Investigation and Mitigation/Protection

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2 Pre-Screening for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal Turbine-Generators

2.1 Pre-ScreeningThe pre-screening process will be used to identify if the proposed turbine-generators need to be subjected to the formal screening process, described in Section 3. The following considerations need to be assessed in the pre-screening process:

Type of machine, Size of machine, Electrical proximity to HVDC system and, System topology

2.2 Type of Machine

Based on the study and analysis conducted by EPRI [2], the following types of generators must be included in the formal screening:

1. Steam turbine-generator2. Gas turbine-generator

Hydro-electric turbine-generator units may be excluded from SSTI study due to the large ratio of generator to turbine inertia [3, 4]. Viscosity of the water also provides additional damping.

The latest research [5, 6] shows that large wind farms that have a radial or nearly radial connection to Line Commutated Converters (LCC) HVDC may be exposed to SSTI. This is because the HVDC may introduce negative damping to the torsional modes of the wind turbine-generator therefore these wind farms should be studied in detail to check for possible SSTI.

2.3 Size of Machine

As will be illustrated in later sections discussing the Unit Interaction Factor (UIF) methodology for formal screening, a machine or a group of identical machines can be excluded from the formal screening if the aggregate MVA rating is greater than 10 times the rated capacity of the HVDC converter. Also, small turbine-generators, like distributed generation (DG) which has a negligible short circuit contribution relative to the system at the HVDC converter station can be excluded from formal screening.

2.4 Electrical proximity and system topology

Good engineering judgment will be used to exclude a unit from the formal screening process based on electrical proximity and system topology. As the system develops, the prescreening process as well as the limiting topology thresholds will be further refined by the AESO in the future as more of these studies are performed. Informed judgment will consider the findings of past studies, the electrical path impedance between the turbine-generator and the HVDC station and the network configuration/outages under which the generator may become radially or close to radially connected to the HVDC station. In the absence of sufficient justification for determining that there is no potential for SSTI at the pre-screening stage then formal screening must be used.

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2.5 Tools, models and data required Tools : No software tools are required for the prescreening. Models and data : The type and MVA rating of the machines being considered for the study are

required to complete the pre-screening. A Single Line Diagram (SLD) indicating the electrical connections of the turbine-generator with respect to the HVDC converter terminal is also required.

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3 Formal Screening for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal Turbine-Generators

3.1 BackgroundThe Unit Interaction Factor (UIF) methodology has been well-established by industry [2, 7] as a preliminary

screening tool to check how closely coupled the turbine-generator is with the HVDC and to decide

whether detailed studies of SSTI are needed.

3.2 The UIF Methodology for Assessing the Potential for SSTIThe UIF is a value that indicates the coupling between a particular generator and the HVDC in relation to

other generators. The UIF is calculated as follows [2, 7]:

UI Fi=(MV AHVDCMV Ai )(1− SC iSC tot )

2

UIFi: Unit Interaction Factor of the i-th generator

MVAHVDC: MVA rating of the HVDC (same as the MW rating of the HVDC)

MVAi: MVA rating of the i-th generator

SCtot: Short-circuit capability ( M V A ) at t h e HVDC commutating bus including all

generators (Subtracting AC filters and shunt capacitors)

SCi: Short-circuit capability ( M V A ) at t h e HVDC commutating bus excluding the i-

th Generator (Subtracting AC filters and shunt capacitors)

A calculated UIF is compared against the value of 0.1, recommended by EPRI, as an

indicator of potential SSTI.

If the UIF of a particular generator is considerably smaller than 0.1, SSTI between this

generator and the HVDC converter has a low potential to occurrence.

If the UIF of a particular generator is close or higher than 0.1, SSTI between this generator

and the HVDC converter cannot be excluded and has to be studied in detail.

A UIF higher than 0.1 cannot be used as a confirmation of SSTI until detailed studies are

completed.

The scope of the formal screening study should cover normal system conditions and all

possible critical contingencies that could result in the turbine-generator being connected radially

or close to being connected radially with the HVDC converter terminal.

3.3 Tools, Models and Data Required for UIF Calculations Tools: The UIF formula is based on the 3-phase short circuit levels at the HVDC commutating

bus. Therefore a short circuit analysis tool is all that is required.

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Models and Data: To calculate the UIF requires the MW rating of the HVDC and the

MVA rating of the generator under examination for SSTI, the source impedance and the

step-up transformer impedance. In addition a short circuit study case is required.

Note that the UIF methodology does not require detailed models and data of the turbine-generator,

excitation control or governor or other generation or transmission facilities. Also, it does not require

modeling of the HVDC.

3.4 General Guidelines for Conducting an UIF Study: In general the UIF is higher with a lower number of generators (lower short circuit level) in

service and vice versa. Therefore UIF calculations on the AIES should be conducted under

Summer Light Load conditions with no export.

Sensitivities to the generation dispatch for units in the vicinity of the HVDC station will be

considered.

The list of contingencies for the UIF analysis will be prepared by the AESO, with input from the

TFOs and GFO as required, as part of the project’s Connection Study Scope.

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4 Detailed Studies for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal-Turbine Generators

4.1 Purpose of Detailed StudiesIf the screening for a given turbine-generator results in an UIF value of 0.1 and above, then additional analysis is required to further investigate the potential SSTI between the HVDC and that particular generator. As described in the previous sections, the UIF only provides a general indication of coupling between the HVDC and the generator. For units that have high SSTI potential detailed simulations that capture the HVDC behaviors as well as the generator behaviors need to be performed. Because this analysis requires access to proprietary HVDC modeling information this analysis will be conducted by the TFO or a consultant under AESO’s direction.

4.2 Perturbation AnalysisPerturbation analysis (also known as ∆T /∆ω analysis) is a small signal analysis method of calculating the electrical damping. The model required to perform the study is a detailed time domain model which captures the behavior of the machine response, machine controllers and accurate HVDC behavior. For the EATL/WATL case, this is most likely to be performed in an Electromagnetic Transient Simulation Program such as PSCAD.

Perturbation analysis is typically used to tune the SSDC of the HVDC to ensure that positive damping is present at the torsional modes of the turbine-generator under study.

4.2.1 Tools, Models and Data Required for Perturbation AnalysisTools: A simulation tool that is capable of detailed representation of the controllers is required.

Models and Data: Accurate details of all the controller parameters are required. This applies to HVDC, SVCs and generator controllers such as exciters, governors and stabilizers.

The torsional modes of oscillations of the turbine-generator need to be known. In the event that the torsional modes of oscillation of the turbine-generator are not known or available, the mechanical parameters of the turbine-generator train will be required.

4.3 Time Domain AnalysisTime domain analysis is an extension of the perturbation analysis to include the impact of large disturbances.

4.3.1 Tools, Models and Data Required for Time Domain AnalysisTools: An Electro Magnetic Transient Program such as PSCAD.

Models and Data: Accurate details of all the controller parameters for generator controllers including the exciter, governor and stabilizer are required. Accurate mechanical parameters of the machine (i.e., turbine masses and shaft stiffness, inertia and damping) are required.

The list of turbine-generator models and data required for detailed SSTI analysis is given in Section 6.

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5 Mitigation and Protection for Sub-Synchronous Torsional Interaction (SSTI) Between HVDC and Thermal Turbine-Generators

5.1 Background Mitigation and protection systems can be used to address the SSTIs between turbine-generators and the HVDC links under various operating conditions. Some of the options may vary depending on the system conditions in which SSTIs occur as outlined in references 8, 9, and 10.

5.2 HVDC Control & Protection System ChangesThe SSDC in the HVDC control system may be re-tuned to dampen potential oscillations caused by interactions with turbine-generators. Initially, an evaluation of the performance of the existing damping controller is required which will be completed during detailed studies.

If the existing SSDC does not provide sufficient damping, the damping controller may be re-tuned. This re-tuning will require an evaluation of the SSDC performance against oscillations caused due to interactions with existing generators in addition to the new generators.

The HVDC systems in Alberta are also equipped with sub-synchronous protection, which blocks the HVDC if the SSDC fails to damp the sub-synchronous oscillations. Therefore changes to the SSDC may require changes to the sub-synchronous protection of the HVDC.

Studies required for this option will be the accountability of the TFO in coordination with the HVDC manufacturer and under AESO direction.

5.3 Generator Protection

There are two protection measures that may be considered to protect generators from SSTI. The first protection measure uses Torsional Stress Relays (TSRs) which monitor the torsional oscillations of a turbine-generator and trip it. The second protection measure is to trip a transmission element/generator to avoid certain system topology or a specific operating condition.

Either protection measure must be coordinated with the AESO and TFO to avoid nuisance tripping and adverse system impacts.

TSRs can offer generator protection due to all causes of high torsional stresses [8]. Manufacturer data for the generator under investigation is required for design of the TSR and it will be the responsibility of the generator owner to evaluate the need for generator protection.

5.4 Use of FiltersA static blocking filter [9] may be used to filter out frequencies which coincide with the complement of natural torsional frequencies of the generator unit. Since each filter is tuned to protect an individual unit, changes in system topology do not have a great impact on the effectiveness of this mitigation measure. These filters are typically installed at or near the generator terminals.

5.5 Operational Measures/AwarenessIf studies indicate that specific system configurations make a generator more susceptible to SSTI, it may be prudent to implement operational measures to ensure that the system does not enter this state. However, these measures may only be considered under multiple outage conditions, as indicated in reference 9.

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5.6 Generator DesignThe parameters of the mechanical shaft system for a turbine-generator determine the torsional modes of oscillation. If a new generating facility is known to have a potential for SSTI with an HVDC terminal, then these parameters may be adjusted to avoid undamped frequencies during the design stage. The dynamic stabilizer control and the excitation system may also be used to enhance the damping properties of the torsional modes of the turbine-generator.

Table 1: Mitigation and Protection options based on the potential for SSTI (this applies to new or modified generators only)

System Conditions where SSTI Occurs (As per Detailed Studies)

Mitigation/Protection Options

N-0 Mitigation o Re-tune SSDC in HVDC control systemo Install filterso Consideration of turbine-generation parameters during the

design/procurement stageo Dynamic stabilizer controlo Machine excitation system damping

Protection o Generator protection (TSRs), as an optional backup. This protection must

be coordinated with the TFO protection scheme to avoid nuisance tripping and adverse system impacts.

N-1, N-2 Mitigation o Remedial action schemeo Install filterso Re-tune SSDC in HVDC control systemo Consideration of turbine-generation parameters during the

design/procurement stageo Dynamic stabilizer controlo Machine excitation system damping

Protection o Generator protection (TSRs), as optional for consideration. This protection

must be coordinated with the TFO protection scheme to avoid nuisance tripping and adverse system impacts.

N-1-1, N-1-2, N-2-1, N-2-2

Above N-4

Mitigation o Operational measures/awarenesso Remedial action schemeo Install filterso Re-tune SSDC in HVDC control systemo Consideration of turbine-generation parameters during

design/procurement stageo Dynamic stabilizer controlo Machine excitation system damping

Protection o Generator protection (TSRs), as optional for consideration. This protection

must be coordinated with the TFO protection scheme to avoid nuisance

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tripping and adverse system impacts.

6 List of Turbine-Generator Models and Data Required for Detailed SSTI Simulations

6.1 Data Required for Sub-Synchronous StudiesThis data will be required for all new turbine-generators. This data is essential for conducting detailed analyses of sub-synchronous torsional interaction (SSTI) with HVDC facilities.

Table 2: Required Data for Turbine-Generator Shaft System

Basic Requirements Data

Number of poles of the Generator

Mechanical frequencies as calculated by the manufacturer

Table 3: Torsional Data for the Turbine-Generator Shaft System

Mass No. (i)Shaft Section

(i -> i+1)Rotor Section

Torque Fraction

S

Moment of Inertia

J

Stiffness constants

K

Damping Constant

(Min Load)Dmin

Damping Constant

(Max Load)Dmax

[pu][kg.m²]

Or[lb.ft²]

[N.m/rad]Or

[lb.ft/rad]

[N.m.s/rad]Or

[lb.ft.s/rad]

[N.m.s/rad]Or

[lb.ft.s/rad]1

1 ->22

2 -> 33

3 -> 44

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Appendix AExamples of Torsional Models for Turbine-Generators

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Example 1: Shaft System of a Combustion Turbine-Generator

Torsional data of the turbine generator

MassNo.

Rotating Mass Section

Power

Fractions of

Turbine

Section(s)

[pu]

Inertia Constant HMW.s/MVA

(s)

Moment of

InertiaJ

[kg.m²]

Shaft Stiffness Constant

Kbetween Masses

[pu torque/rad]

Damping constant

[pu torque/pu

speed]

1 Turbine 1.0 3.63 0.125

67.01

2 Generator -- 1.11 0.02

Other Required Data:

Number of poles of the generator = 2 Number of masses = 2 (Generator and Combustion Turbine)Number of Torsional modes= 1Mechanical frequencies as calculated by the manufacturer: 19.4 Hz

Generator

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Example 2: Shaft System of a Steam Turbine-Generator

Torsional data of the turbine-generator

MassNo.

Rotating Mass Section

Power

Fractions of

Turbine

Section(s)

[pu.]

(i)

Inertia Constant HMW.s/MVA

(s)

(ii)

Moment of Inertia

J[ton.m²]

Shaft Stiffness Constant

Kbetween Masses

[kN.m/rad]

Damping constant

[kN.m.s per rad]

1 Turbine HPa 0.30 0.67 2.98

549300

2 Turbine HPb 0.24 0.96 1.04

205800

3 Coupling#1 -- 0.15 0.79

104800

4 Turbine LPa 0.23 7.41 21.57

3071300

5 Turbine LPb 0.23 7.38 1.29

198400

6 Coupling#2 -- 0.21 1.32

209600

7 Generator -- 6.18 1.6 Notes: i) The summation of power fractions of turbine sections =1.0 ii) H (MW.s/MVA) can be derived from J (ton.m2) using the following formula [11]:

H = 5.48x10-06 J (n)2 / MVA Rating Page 15

iii) K (pu torque/rad) can be derived from K (kN.m/rad) using the following formula:

{K(kN.m/rad) x (4 x 377)/ (no. of poles)2}/MVA Rating

Where: J is the moment of inertia in [ton.m2] n is the rated speed in rpm. MVA is the rated MVA of the Generator Other Required Data:

Number of poles of the generator = 2 Rated speed (n) = 3600 MVA rating of the Generator = 586 MVANumber of masses = 7:(HPa , HPb , Coupling1, LPa, LPb, Coupling2 and Generator)Number of Torsional modes= 6

Mechanical frequencies as calculated by the manufacturer: 23.4, 34.2, 145.53, 183.26, 223.84, 242.30 Hz

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Example 3: Shaft System of a Single Shaft Combined Cycle Gas Turbine with SSS Clutch

Torsional data of the turbine-generator

Operation Mode 1: Steam Turbine is Engaged

MassNo.

Rotating Mass Section

Power

Fractions

of Turbine

Section(s)

[pu]

(i)

Inertia Constant HMW.s/MVA

(s)

Moment of Inertia

J[ton.m²]

Shaft Stiffness Constant

Kbetween Masses

[pu torque/rad]

Damping constant

[pu power/pu speed]

1 Gas Turbine 0.6 0.869 0.25

37.25

2 Generator -- 1.428 0.02

31.31

3 SSS Clutch -- 0.7 0.1

27.66

4 Steam T Section 1 .25 1.427 0.2

17.78

5 Steam T Section 2 .15 0.176 0.2 Note: i) The summation of power fractions of Gas Turbine and Steam Turbine sections =1.0 ii) No. of torsional modes = 4

Mechanical frequencies as calculated by the manufacturer: 8.26, 17.66, 22.44, 24.23 Hz

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Torsional data of the turbine-generatorOperation Mode 2: Steam Turbine is Disengaged

MassNo.

Rotating Mass Section

Power

Fractions

of Turbine

Section(s)

[pu.]

(i)

Inertia Constant HMW.s/MVA

(s)

(ii)

Moment of Inertia

J[ton.m²]

Shaft Stiffness Constant

Kbetween Masses

[pu torque/rad]

Damping constant

[pu power/pu

speed]

1 Gas Turbine 1.0 0.869 0.25

37.25

2 Generator -- 1.428 0.2

31.31

3 SSS Clutch -- 0.4 0.1

Note: i) No. of torsional modes = 2

Mechanical frequencies as calculated by the manufacturer: 16.35, 23.24 Hz

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7 References:

[1] M. Bahrman, E.V. Larsen, R.J. Piwko, and H.S. Patel, “Experience with HVDC – Turbine Generator Torsional Interaction at Square Butte,” IEEE Trans. Vol. PAS-99, pp. 966-975, May/June 1980.

[2] Electrical Power Research Institute. “HVDC System Control for Damping of Sub-Synchronous Oscillations”, EPRI, EL-2708. Report, NY, USA, October 1982.

[3] IEC/TR 60919-3 Performance of high-voltage direct current (HVDC) systems with line-commutated converters – Part 3: Dynamic conditions, Edition 2.0, 2009-10

[4] CIGRE 119 Interaction between HVDC Convertors and Nearby Synchronous Machines, Working Group 14.05, October 1997.

[5] Subsynchronous Torsional Interaction Behaviour of Wind Turbine-Generator Unit Connected to an HVDC System, by Y Choo, A.Agalgaonkar and K. Muttaqi, 36th Annual Conference of the IEEE Industrial Electronics, 2010, pp. 996-1003.

[6] Overview of Sub-synchronous Oscillation in Wind Power System, Han Chen, Chunlin Guo and Jiating Xu, Energy and Power Engineering, 2013,5,pp. 454-457.

[7] Electric Power Research Institute: High-Voltage Direct Current Hand Book, 1994.

[8] Application of Torsional Stress Relays.ppt, GE, March 12, 2010.

[9] Countermeasures to Subsynchronous Resonance Problems, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-99, No. 5 Sept/Oct 1980

[10] NERC Special Protection Systems (SPS) and Remedial Action Schemes (RAS): Assessment of definition, regional practices, and application of related standards. Rev.01 April, 2013.

[11] Power System Stability and Control- Chapter 15: Sub-Synchronous Oscillations, Book by Dr. Prabha Kundur, Mc Graw-Hill,1994..

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