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

Date:2015-06-15

Prepared by:ATCO Electric-AltaLink-AESO Task Force

Version:Guidelinesfor SSTI Studies and Mitigation R1B7

Page 1

Table of Contents

1Introduction

1.1Background

1.2Objective

1.3Scope

1.4General Steps for Assessing the Potential of SSTI

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

2.1Pre-Screening

2.2Type of Machine

2.3Size of Machine

2.4Electrical proximity and system topology

2.5Tools, models and data required

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

3.1Background

3.2The UIF Methodology for Assessing the Potential for SSTI

3.3Tools, Models and Data Required for UIF Calculations

3.4General Guidelines for Conducting an UIF Study:

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

4.1Purpose of Detailed Studies

4.2Perturbation Analysis

4.2.1Tools, Models and Data Required for Perturbation Analysis

4.3Time Domain Analysis

4.3.1Tools, Models and Data Required for Time Domain Analysis

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

5.1Background

5.2HVDC Control & Protection System Changes

5.3Generator Protection

5.4Use of Filters

5.5Operational Measures/Awareness

5.6Generator Design

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

6.1Data Required for Sub-Synchronous Studies

7References:

1Introduction

1.1Background

Sub-synchronous oscillations (< 60 Hz) in power systems can be amplified and sustained due to the interactionbetween major transmission system devices such as HVDCconverter 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.

1.2Objective

The main purpose of this document is to provide guidanceto 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.

1.3Scope

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.

1.4General Steps forAssessing 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.

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 stepinvolves detailed modeling of the HVDC systems and turbine-generators and typically uses Electromagnetic Transient Simulation Software such as PSCADor 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, withits existing settings, is capable of damping the torsional modes of oscillations of the turbine-generatorunder consideration. If the torsional modes of oscillations are not damped with existing settings (i.e., arenegatively 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 potentialfor SSTI, requiring mitigation and protection.

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

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

d)No potential for SSTI, thereforemitigation 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 forSSTI identified in Step 3.

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

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

2.1Pre-Screening

The pre-screening process will be used to identifyif the proposedturbine-generatorsneed 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 proximityto HVDC system and,
  • System topology
  • 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-generator
  2. 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 thatlarge 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 introducenegative damping to thetorsional modes of the wind turbine-generator therefore these wind farms should be studied in detail to check for possible SSTI.

2.3Size 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 aggregateMVA ratingis 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.

2.4Electrical 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 andthe 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.

2.5Tools, 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 respectto the HVDC converter terminal is also required.

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

3.1Background

The 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.

3.2The UIF Methodology for Assessing the Potential for SSTI

The 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]:

UIFi:UnitInteractionFactorofthei-thgenerator

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

MVAi:MVAratingofthei-thgenerator

SCtot:Short-circuitcapability (MVA) atthe HVDCcommutatingbusincludingall generators(Subtracting ACfilters and shunt capacitors)

SCi:Short-circuitcapability (MVA) atthe HVDCcommutatingbus excludingthe i-th Generator (Subtracting ACfilters and shunt capacitors)

  • Acalculated UIFiscompared againstthe valueof0.1, recommended by EPRI,as an indicatorofpotentialSSTI.
  • IftheUIFofaparticulargeneratorisconsiderably smallerthan0.1,SSTIbetweenthisgeneratorandtheHVDCconverterhas a low potentialto occurrence.
  • IftheUIFofaparticulargeneratoris close or higher than0.1,SSTIbetween thisgenerator andtheHVDCconverter cannotbeexcludedandhastobestudied in detail.
  • A UIF higher than 0.1 cannotbeused 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 withthe HVDC converter terminal.
  • 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.
  • 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.

3.4General 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.

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

4.1Purpose of Detailed Studies

If the screening for a given turbine-generator results in an UIF value of 0.1 and above, then additional analysis is required to further investigatethe 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.

4.2Perturbation Analysis

Perturbation analysis (also known as 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 SSDCof the HVDC to ensure that positive damping is present at the torsional modes of the turbine-generator under study.

4.2.1Tools, Models and Data Required for Perturbation Analysis

Tools: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.

4.3Time Domain Analysis

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

4.3.1Tools, Models and Data Required for Time Domain Analysis

Tools:An Electro Magnetic Transient Program such asPSCAD.

Models and Data:Accurate details of all the controller parameters for generator controllers including the exciter, governor and stabilizerare 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 inSection 6.

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

5.1Background

Mitigation and protectionsystems 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.

5.2HVDC Control & Protection System Changes

The 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.

5.3Generator 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-generatorand trip it. The second protection measure is to trip a transmission element/generator to avoid certain system topology or aspecific 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.

5.4Use of Filters

A 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.

5.5Operational Measures/Awareness

If 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.

5.6Generator Design

The 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
  • Re-tune SSDC in HVDC control system
  • Install filters
  • Consideration of turbine-generation parameters during the design/procurement stage
  • Dynamic stabilizer control
  • Machine excitation system damping
  • Protection
  • 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
  • Remedial action scheme
  • Install filters
  • Re-tune SSDC in HVDC control system
  • Consideration of turbine-generation parameters during the design/procurement stage
  • Dynamic stabilizer control
  • Machine excitation system damping
  • Protection
  • 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
  • Operational measures/awareness
  • Remedial action scheme
  • Install filters
  • Re-tune SSDC in HVDC control system
  • Consideration of turbine-generation parameters during design/procurement stage
  • Dynamic stabilizer control
  • Machine excitation system damping
  • Protection
  • 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.

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