Analysis of Sub-synchronous Frequency
Interactions in Power Systems Using TGSSR
Presented by: Chandana Karawita, TransGrid Solutions
Team:
Prof. Udaya Annakkage (U of M / TGS)
Chandana Karawita (TGS)
Hiranya Suriyaarachchi (U of M / TGS)
Dan Kell (TGS)
IEEE PES Winnipeg Section Luncheon Meeting - January 2012
1
Outline

Introduction to TGSSR
 Applications
Generator-turbine - series capacitor SSR
 Generator-turbine – HVDC torsional
interactions
 Generator-turbine – VSC torsional
interactions
 Wind plant - series capacitor SSR


Applicability in Modern Power Systems
2
Introduction to TGSSR

Dynamic phasor based small signal
stability assessment.





Not another small signal stability program meant for
electromechanical oscillation analysis
Network dynamics are modelled using dynamic
phasors.
Generator stator dynamics are modeled.
Models are accurate up to the frequency at which the
system harmonics and the frequency dependency of
network components can be ignored.
Main focus is on sub-synchronous frequency range.
3
Introduction to TGSSR

Process
Input
Process-1
Output
Process-2
Output
• Read PSS/E raw data
• Read Dynamic data
• Create linearized models of dynamic devices
• Combine with dynamic phasor network model
• Save A and B matrices (ΔẊ=A ΔX + B ΔU) to text files
• Save state and input data to text files
• Eigen analysis of A matrix
• Small signal time domain simulations
• Eigen values and vectors, participation factors
• Polar plots, frequency responses
4
Introduction to TGSSR

Present Capabilities




Synchronous generator models including
exciters, governors, stabilizers (PSS) and
multi-mass turbine units.
Single and double cage induction
generator/motor models.
Network components – Tx lines, two and
three winding transformers, series capacitors
and zero impedance lines.
Static and dynamic load models.
5
Introduction to TGSSR

Present Capabilities
Monopole and Bipole HVDC models
including detailed controllers and DC
transmission system.
 Monopole and Bipole VSC models.
 SVC and STATCOM models – Detailed and
PSS/E type.
 Wind plant models (DFIG type)


The models have been validated against PSCAD
6
Introduction to TGSSR

Applications





Generator-turbine – Series capacitor subsynchronous resonances.
Generator-turbine – HVDC torsional interactions.
Wind turbine – Series capacitor sub-synchronous
resonances.
HVDC control interactions and DC resonance
issues.
Multi-in-feed HVDC interactions.
7
Introduction to TGSSR

Applications
All other sub-synchronous frequency
interactions in power systems (Interactions
of FACTS devices, Network resonances
etc.)
 Controller tuning and sub-synchronous
damping controller design.
 Analysis of Eigen properties to determine
the locations for damping controllers.

8
Introduction to TGSSR

Largest System Analyzed

Manitoba Hydro System
More than 400 buses
 About 100 current injection devices
 Bipoles 1, 2 and 3 (proposed)
 About 5000 state variables

9
Applications – Generator-Series Cap SSR

Series cap – generator electrical
resonance (self excitation)
 Network (series cap-gen) interaction with
torsional oscillations.
10
Applications – Generator-Series Cap SSR

A network mode interacts with a torsional mode.
SSI occurs when Network mode is
close to one of the torsional modes
in frequency
- weak resonance condition
11
Applications – Generator-Series Cap SSR

A network mode interacts with a torsional mode.
12
Applications – Generator-Series Cap SSR

When Network mode is close to 16 Hz torsional mode.
13
Applications – Generator-HVDC Torsional
Interactions

Torsional interactions occur when there is a
lightly damped oscillatory mode in the HVDC
system close to one of the torsional modes –
weak resonance condition
14
Applications – Generator-HVDC Torsional
Interactions

Under normal operating conditions
Generator
HVDC
AC Filters and Network
State participations in
16 Hz torsional mode
15
Applications – Generator-HVDC Torsional
Interactions

Current controller gains were adjusted to create a oscillatory
mode close to 16 Hz.
Generator
HVDC
AC Filters and Network
State participations in
16 Hz torsional mode
16
Applications – Generator-HVDC Torsional
Interactions

Sub-synchronous Damping Controller (SSDC) Design


The torsional modes of nearby generators can be controlled
through a damping controller added to HVDC controllers.
Current controller is the most effective location.
17
Applications – Generator-VSC Torsional
Interactions


It is believed that VSC systems provide
positive damping to the torsional modes of
nearby generators.
Our analysis showed that this is not always
correct.
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Applications – Generator-VSC Torsional
Interactions
0.25
16.3 Hz Torsional Mode
Damping (%)
0.20
0.15
0.10
0.05
Damping (%)
0.00
12.0
14.0
12.0
10.0
8.0
6.0
4.0
2.0
0.0
12.0
14.0
16.0
18.0
20.0
22.0
24.0
Controller Mode Frequency (Hz)
26.0
28.0
30.0
32.0
20.0
22.0
24.0
26.0
Controller Mode Frequency (Hz)
28.0
30.0
32.0
Controller Mode
14.0
16.0
18.0
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Applications – Generator-VSC Torsional
Interactions
When controller mode is at 16 Hz
When controller mode is at 18 Hz
20
Applications – Wind Generator – Series
Cap SSR


Torsional oscillations are in low frequency range
(<5Hz) – not possible to have torsional interactions.
The problem is identified as a Sub-Synchronous
Resonance in the electrical system.
21
Applications – Wind Generator – Series
Cap SSR

Frequency scans with equivalent circuit
(change in slip is calculated for each
frequency)

Analysis shows sub-synchronous
resonance (stable).
 Self excitation according to
conventional definition is not
Dynamic behaviour of controllers
present.
and rotor voltage is not included.
22
Applications – Wind Generator – Series
Cap SSR

Damping of resonance is very sensitive to the rotor
side converter controllers.
1
1
6
Kpdr=Kpqr=0.025,Kpωr=30,KpQs =0.5
ωr
+
5
Kpqr=0.0375
4
Damping(%)
3
2
1
0
x2
-
IQr,ref
Kpωr
+
Kpqr
VQr
Kiqr/s
IQr
Kpqr=0.05
Kpdr=0.05
Kpqr=0.0625
IDr,ref
Qs,ref
Kpdr=0.0625
+
Kpqr=0.075
1
1
Kpdr=0.075
KpQs
1
-x
+
x
- 3
Kpdr=0.0875
IDr
Qs
Kpdr=0.100
Kpqr=0.100
-2
Kpdr
-3
Kpqr
Kpqr=0.125
24.5
24.6
24.7
Kpqr=0.125
24.8
24.9
Frequency(Hz)
25
25.1
VDr
Kpdr
1/(sTidr)
1/(sTiQs)
Kpqr=0.0875
-1
-4
24.4
x4
Kiωr/s
ωr,ref
Kpdr=0.0375
-
25.2
25.3
Small signal analysis shows
an electrical resonance in
which the damping can be
controlled through the rotor
side converter controllers.
23
Applicability in Modern Power Systems

A new era of power systems






Most of the HVAC lines are series compensated.
A large percentage of wind generation.
Wide usage of HVDC and DC grids.
Involvements of FACTS devices are high.
Possibilities of having sub-synchronous
frequency interactions are high.
A systematic approach using time domain
simulations and small signal stability is
essential to identify and solve these problems.
24
Publications
1.
2.
3.
4.
5.
6.
7.
Chandana Karawita, D.H.R. Suriyaarachchi and U.D. Annakkage , “A Case Study on the Vulnerability of VSC
HVDC Systems for Sub-synchronous Interactions with Generator-Turbine Units”, CIGRE SCB4 Colloquium
2011, Brisbane, Australia, October 2011
D.H.R. Suriyaarachchi, U.D. Annakkage and Chandana Karawita, A Procedure to Study Sub-synchronous
Interactions of Wind Integrated Power Systems, submitted to review, ”, IEEE Transactions on Power
Systems.
D. H. R. Suriyaarachchi, U. D. Annakkage, C. Karawita, D. Kell, R. Mendis, and R. Chopra, “Application of an
SVC to Damp Sub-synchronous Interaction between Wind Farms and Series Compensated Transmission
Lines, Accepted to present in IEEE PES meeting, 2012
Chandana Karawita, U. D. Annakkage, “A Hybrid Network Model for Small Signal Stability Analysis of Power
Systems”, IEEE Transactions on Power Systems, Vol.25, No. 1, Feb. 2010
Chandana Karawita, U. D. Annakkage, “Multi-In-Feed HVDC Interaction Studies Using Small Signal Stability
Assessment”, IEEE Transactions on Power Delivery, Vol.24, No. 2, April 2009
Chandana Karawita, U. D. Annakkage, “Control Block Diagram Representation of an HVDC System for SubSynchronous Frequency Interaction Studies” The 9th International Conference on AC and DC Power
Transmission, Oct 2010.
Chandana Karawita, U. D. Annakkage, “HVDC-Generator Torsional Interaction Studies Using A Linearized
Model with Dynamic Network Representation”, International Conference on Power Systems Transients
(IPST), June 3-6 2009, Kyoto, Japan
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Questions
26