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Experience of PSS Tuning in Indian Power System
Conference Paper · December 2020
DOI: 10.1109/NPSC49263.2020.9331918
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Experience of PSS Tuning in
Indian Power System
Presenter : Chandan Kumar (Author)
Co-Author : Saibal Ghosh, Saurav Sahay, Alok Pratap Singh, Akash Modi, Surajit Banerjee
Power System Operation Corporation Limited(POSOCO)
India
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
Outline
Paper summarises
• Regulatory Overview and Required Improvements
• Non-uniformity in Vendors practice
• Experiences and Challenges observed in field exercise for 129 Units
• Modified PSS tuning procedure and implementation
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
Power System Stabilizer (PSS)
Basic Principle
Some Key points :
• Part of Synchronous machine excitation system
• Additional Input to AVR to improve Damping Torque
• Control excitation through voltage regulator for damping
power oscillation
• Input signal : Speed, Frequency and Power
• Efficient and economical Mean to damp Oscillations (LFO)
and Small signal stability improvement
• Need proper tuning
Observed Issues in PSS tuning :
• Lack of Simulation study
• Widely varying practices among utilities & vendors
• Lack of procedure and evaluation guidelines
• Field implementation with standard tests for evaluation
Paper summarises
• Regulatory Overview and Required
Improvements
• Non-uniformity in Vendors practice
• Challenges observed in field exercise
• Best Practices for PSS tuning
• Modified PSS tuning procedure and
implementation
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
Regulatory Overview
PSS Mandatory for Excitation System
CEA Technical Standard for Construction of Electrical Plants and Electric Lines
(Published in 2010):
•
For Coal or lignite based Thermal Generating Stations (10.2. g.i: ), Gas turbine
based Thermal generating stations (18), Internal Combustion(IC) engine based
Thermal generating station (27) : Suitable Excitation System, as well as
Automatic voltage regulator (AVR), shall be provided with the generator as per
CEA (Technical standards for connectivity to the Grid) Regulation, 2007. Power
System Stabiliser (PSS) shall be provided in AVR for generator of 100 MW and
above rating.
•
For Hydro power Plan (37.3.e): All the performance requirements of AVR, PSS
shall be in accordance shall be in accordance with CEA (Technical standards for
connectivity to the Grid) Regulation, 2007 and CEA (Grid standard)regulation
as and when they come into force.
CEA (Technical standards for connectivity to the Grid) Regulation, 2007:
•
For New generating Units (part II. 1. C): The AVR of generator of 100 MW and
above shall include Power system stabiliser (PSS)
•
For Old Units: (part II. 2. 2): For thermal generating units of having rated
capacity of 200 MW and above and Hydro Units having rated capacity of 100
MW and above, following facility should be provided at the time of renovation
and modernization : Every generating unit of capacity having rated capacity
higher than 100 MW shall have PSS.
PSS Tuning Mandatory
• IEGC 5.2.k: All generating units shall normally have their automatic voltage
regulators (AVRs) in operation. In particular, if a generating unit of over fifty (50)
MW size is required to be operated without its AVR in service, the RLDC shall be
immediately intimated about the reason and duration, and its permission
obtained. Power System Stabilizers (PSS) in AVRs of generating units (wherever
provided), shall be got properly tuned by the respective generating unit owner as
per a plan prepared for the purpose by the CTU/RPC from time to time. CTU /RPC
will be allowed to carry out checking of PSS and further tuning it, wherever
considered necessary.
•
CEA (Technical standards for connectivity to the Grid) Regulation, 2007 6.g: The
requester and user shall cooperate with RPC and appropriate Load despatch centre
in respect of matter listed below, but not limited to: Cooperate with RPC for tuning
of PSS provided in the excitation system of generating Unit.
Report of the Task Force on Power System Analysis under Contingencies: Power
System Stabilizers (PSS) as part of the generators installed in the network are also
critical for damping the local area oscillations and imparting stability to the networks.
Optimal tuning of PSS also enhance effectiveness of other HVDC and FACTs controllers
in supporting overall/ inter-area stability. Necessary exercise to retune PSS should be
undertaken at interval of 3-4 years or even earlier depending on network additions in
vicinity of specific generators.
Missing Features : Damping Criteria , Procedure and
Periodicity of PSS tuning and Evaluation
Review of Damping Criteria : POSOCO, “Report on Low Frequency Oscillation in Indian Power System”, New Delhi, Tech.Rep. March 2016.
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
Vendors & Utility Existing Practices
Vendor
V1
PSS Tuning Study based on Dynamic Model
Model Validation during Actual PSS tuning
V2
V3
V4
V5
√
×
√
×
√
√
×
×
×
×
Challenges observed
• Non-uniform practice.
Gain and Phase margin Calculation using System
Study
√
×
√
×
√
PSS Tuning at Site: Step Test
√
√
√
√
√
PSS tuning at Site: Disturbance Test
×
×
×
√
√
• Focus only on step test response.
PSS tuning at Site: Impulse Test
√
×
×
×
×
• Lack of disturbance/impulse test.
PSS tuning at Site: Governor Test
×
×
×
×
×
PSS tuning at Site : OEL and UEL Test
×
×
×
×
×
• Missing Validated model based study
and
Parameter
determination
exercise.
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
PSS Tuning : Experience & Challenge
Summary of PSS tuning at 129 Units in Eastern and
Western Region in Indian Power system
• Thermal Power Plant Units: 97
• Hydro Power Plant Units: 21
Model validation
• Ensures the study results and actual site tests match
• Validation of the local mode of frequency with step test.
• Three vendors perform this activity
50 MW-800 MW
• Gas Based Power Plant Units : 11
Measurement Data
• Lack of high precision data from field testing.
• Response/tuning validation difficult.
Reference voltage step change
• Voltage step of 5 % is good for analyzing the PSS response.
• Evaluation based on lower steps (1%-3%) is difficult
Generation level
• 80-100 % of the rated capacity & All test at same loading
• PSS cutoff criteria
• Gets bypassed below certain generation level.
• Affects overall response evaluation.
PSS Gain
• Conservative Setting – Lack of study and field test.
Governor Status
• Free Governor Mode need to be kept off for ideal condition for
testing.
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
PSS Tuning Experiences…
Step response test
• Unit response for change in Excitation system Vref.
• Provide PSS response for system voltage change.
• Impact of other units in plant affect the testing
(irrespective of PSS on/off)
• Difficult to assess if plants have units with multiple rating
• Better evaluation if unit under test is only kept in service
• Response during the lowest possible fault level
• Can be done Hydro and Gas
• Non-economical in thermal plant.
• Does not ensure plant level response.
Disturbance test
• Provide overall plant response
• Most effective yet least performed
• Plants (Thermal plant) avoid it on apprehension of
disturbance.
• If transient study is performed then impact of
disturbance test known in advance.
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
PSS Tuning Methodology : Simulation Study
Study Requirement
• Time domain and Frequency domain study
• Data Requirement :
• Generator detailed model
• Oscillation Frequency Range (System Operator/
Planner)
Objective
• Study should demonstrate improved damping for :
• Voltage step change of 1% -5%.
• Defined frequency range.
• Within 2 cycles.
• No appreciable instability at 3 times proposed
PSS gain.
• Variable system operating conditions
• Network topologies changes.
• Short circuit on transmission lines from plant.
• No adverse impact/negative interaction on the
torsional mode.
Task
• Find local mode of oscillation for the plant and its
damping.
• Impact of external network impedance variation
• Phase and Gain margin for the given range of
frequency.
Study
• Eigen Value Analysis : Phase lead lag compensation
to provide positive damping.
• Root Locus Analysis : Gain constant determination.
• Time domain study : Perform step and disturbance
test at various power output.
Outcome of Simulation Study:
• Local mode of oscillation frequency
• PSS phase and gain margin
• Number of cycle required for damping (Damping
ratio)
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
PSS Tuning Methodology : Field Testing
Requirement : AVR tuning IEEE Std 421.2-2014
Gain margin Test
1. Keep gain as calculated from study and increase it
till oscillation starts.
2. Reduce gain to 1/3 of this gain value.
Step Response Test
• Provide time-domain performance
• Application of voltage step changes to VTref
• 1-3 % change –PSS ON/OFF
• Validates local mode and simulation study
Frequency Response Test
1. Injecting low frequency band-limited random
noise / sinusoidal signal in VTref
2. Provide PSS performance for a range of frequency
and ensures model validation
Disturbance Test
• Most effective : Opening of line/reactor
• All Unit PSS ONor OFF (Plant level response)
Impulse Test (Alternative to Disturbance Test)
• Low magnitude Impulse type signal superimposed
over the VTref for step-change in impulse input.
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
PSS Tuning Methodology : Field Testing
OEL/UEL Test:
• Ensure under excitation limiter (UEL) & over
excitation limiter (OEL) do not interact with PSS .
• Unit is under and overexcited till UEL/OEL
activation perform step/impulse response test
Governor Test:
• Test PSS performance during frequency events.
• Apply +0.5 Hz frequency injection to the governor
and keep unit it frequency sensitive mode.
Utilization of Synchrophasor
• Proved effective for PSS
performance evaluation.
• Helps in model validation
Precautions
• Alert during frequency injection close to the
resonant frequencies of the machine or
neighboring machines.
• Alert during frequency injection higher than 3 Hz as
can eb near to lowest shaft torsional mode. Consult
OEM
• Stop if error between Simulation and field test is
high .
• Stop if growing/sustained oscillation observed
PSS out at 4000 MW plant
PSS In at 4000 MW plant
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
Suggested Inclusion in Standard/Regulation
• Damping criteria Introduction in power system planning and operation criteria.
• Mandatory PSS tuning at periodic interval and criteria
• PSS tuning procedure and guideline.
• Generator model validation
Summary
Overall Paper provide a glimpse of :
• Challenges faced during PSS tuning study and field testing.
• Limited resources on real world experience sharing on PSS tuning.
• Challenge in deriving strength from Regulation/Standard on PSS tuning evaluation.
• Varying practices by utilities and vendors on PSS tuning.
• Field Experience & challenges faced during PSS tuning of more than 129 Units.
• Methodology for PSS tuning based on best practices.
• Suggested Regulation/Standard Inclusion on PSS tuning.
New Methodology and Guideline :
Approved in the Eastern Region
Operation Committee (OCC) Forum
in Sept 2020
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar
References
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Central Electricity Authority (Technical standards for connectivity to the Grid) Regulation, 2007.
Central Electricity Regulatory Commission, Indian Electricity Grid Code (IEGC), 2010
POSOCO, “Report on Low Frequency Oscillation in Indian Power System”, New Delhi, Tech.Rep.
March 2016.
P. K. Agarwal, Chandan Kumar. “Oscillation Detection and Mitigation Using Synchrophasor
Technology in the Indian Power Grid” in Power System Grid Operation Using Synchrophasor
Technology, 1st ed, vol. 1, Nuthalapati, S, Ed. Cham: Springer International Publishing, 2019, pp.195216.
C. Kumar et al., "Detection of LFO and Evaluation of Damping Improvement Using Synchrophasor
Measurement," 2018 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), Kota
Kinabalu, 2018, pp. 701-706, doi: 10.1109/APPEEC.2018.8566336.
C. Kumar et al., "Intra-Area Mode: Measurement-Based and Model-Based Assessment in Indian
Power System," 2019 IEEE Milan PowerTech, Milan, Italy, 2019, pp. 1-6, doi:
10.1109/PTC.2019.8810729.
National Grid Electricity Transmission, Grid Code, Issue 5, Revision 21, 2017.
WECC Power System Stabilizer Tuning Guidelines
ERPC,
164th
OCC
Minutes,
Jan-2020,
Available
[online]:
http://erpc.gov.in/wpcontent/uploads/2020/01/164-OCC-MINUTES.pdf
NERC, VAR-501-WECC-3 – Power System Stabilizer Standard
IEEE Std 421.2-2014: IEEE Guide for Identification, Testing, and Evaluation of the Dynamic
Performance of Excitation Control Systems
IEEE Tutorial Course Power System Stabilization via Excitation Control,2009
ERCOT Nodal Operating Guides
E. V. Larsen and D. A. Swann, "Applying Power System Stabilizers Part II: Performance Objectives
and Tuning Concepts," in IEEE Transactions on Power Apparatus and Systems, vol. PAS-100, no. 6,
pp. 3025-3033, June 1981.
E. V. Larsen and D. A. Swann, "Applying Power System Stabilizers Part III: Practical Considerations,"
in IEEE Transactions on Power Apparatus and Systems, vol. PAS-100, no. 6, pp. 3034-3046, June
1981.
POSOCO, “Synchrophasors Initiative in India”, New Delhi, Tech.Rep. June 2012.
P. Kundur, G. R. Berube, L. M. Hajagos and R. Beaulieu, "Practical utility experience with and
effective use of power system stabilizers," 2003 IEEE Power Engineering Society General Meeting ,
Toronto, Ont., 2003, pp. 1777-1785 Vol. 3
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Thank you
for your attention
Contact : chandan@posoco.in
21st National Power Systems Conference (NPSC 2020), December 17-19, 2020, IIT Gandhinagar