Download Sub-Synchronous Interactions with Wind Turbines

Simulation and Analysis Methods
for SSR/SSTI/SSCI
PUCT Panel Session
Austin, Nov 19, 2014
Garth Irwin
Electranix Corporation, Winnipeg, Canada
Electranix Corporation
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Presentation Overview
• SSR/SSTI/SSCI - Definitions
• SSCI
• 2009 event of a wind farm near a 345 kV series compensated line
• Simulation/Analysis Techniques:
•
•
•
•
Screening Methods – Impedance Scans
Perturbation Analysis
Advanced Multi-Port Perturbation Analysis
Time Domain Non-Linear Analysis
• Mitigation Methods
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SS Phenomena - Definitions
• “SSR”: Sub-Synchronous Resonance
• Interaction between the mechanical/torsional masses in a generator
(or wind turbine) and the electrical resonance from a series
capacitor.
• “TA”: Torque Amplification: Increase in peak shaft torques
leading to higher fatigue.
• “SSTI”: Sub-Synchronous Torsional Interaction
• Interactions between the mechanical/torsional masses in a
generator (or wind turbine) and a power electronic device (such as
an HVDC link, SVC, wind turbine etc…).
• “SSCI”: Sub-Synchronous Control Instability
• Interactions between a power electronic device (such as an HVDC
link, SVC, wind turbine etc…) and a series compensated system.
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Sub Synchronous Interactions
Device
Series
Capacitor
Power Electronics
Gas Turbine
or
Wind Shaft
Series Capacitor
---
SSCI
SSR
Power Electronics
SSCI
CI (control
SSTI
interactions can be at
any frequency)
Gas Turbines
or Wind Shaft
SSR
SSTI
Electranix Corporation
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SSCI Event in Texas
I
V
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Real System SSCI Event Trace
SSCI Event - PSCAD Studies - Comparison of Simulation to Measured Waveforms (SLG on 345 kV line)
600
Simulated:High_Side_Volts,_A
Real System Measured:VA_345
Simulated:High_Side_Volts,_B
Real System Measured:VB_345
Simulated:High_Side_Volts,_C
Real System Measured:VC_345
400
kV
200
0
-200
-400
-600
600
400
kV
200
0
-200
-400
-600
600
400
kV
200
0
-200
-400
-600
Sec
0.00
Electranix Corporation
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
Real System Traces and PSCAD Simulation
1.00
...
...
...
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Wind Projects with Series
Compensated Transmission Lines
•
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•
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* Texas 2009 SSCI Event
* North Dakota series capacitor/wind turbines
* Alberta-Montana 230 kV series compensated line
ERCOT CREZ expansion 345 kV series compensated lines
Alberta southern system expansion
Project in Texas with a N-0 radial 345 kV series compensated line
UK large scale transmission expansion
Twin circuit 275 kV series capacitor expansion Australia
...
* Indicates real-system SSCI events.
Electranix Corporation
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SSCI - Description
• Voltages and currents are distorted due to the series
capacitor and electrical resonance
• Difficult to filter:
• Can be close to 60 Hz
• Resonant frequency changes
• Distorted inputs signals are processed by turbine controls,
and ultimately fire IGBTs/power electronics (creating a
feedback loop).
• Overall controller response can introduce negative
damping, resulting in instabilities (growing or sustained
oscillations)
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SSCI - Description
Doubly Fed Induction Generator Wind Turbine
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SSCI - Description
Rotor Side Currents IRA
IRB
IRC
P
QOrder
D + -
I
F
P
IRDOrdered D + -
P
POrder
D
IRDMeasured
Q
IRQMeasured
0
URDOrdered
I
F
IRDMeasured
QMeasured
D + -
A
B
C
I
F
PMeasured
P
IRQOrdered D + -
D
Q
A
B
0
C
URAOrdered
URBOrdered
URCOrdered
PWM Comparator
Firing Pulse Generator
IGBT_Pulse_A
IGBT_Pulse_B
IGBT_Pulse_C
URQOrdered
I
F
IRQMeasured
Cascaded PI Controller – Outer and Inner PI Loops
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SS – Analysis Methods Overview
1.
2.
3.
4.
Screening Studies
• SSR/SSCI: Harmonic Impedance Scans
• SSTI: Unit Interaction Factors
Perturbation Analysis
• SSR/SSTI: Used to determine generator electrical damping vs freq
• SSCI: Used to determine Effective Dynamic Impedance of a power
electronic device
Advanced Screening Studied
• SSCI: Uses a combination of Harmonic Impedance Scans (linear
portion of a system) and Perturbation Analysis (Effective Dynamic
Impedance)
Full Time Domain Analysis
• SSR/SSTI/SSCI: Uses fully detailed models of all devices
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SSR/SSCI – 1) Screening Studies
• Harmonic Impedance Scans
•
•
•
Determine net system impedance (as seen from behind the generator equivalent
impedance) as a function of frequency
Determines approximate frequency of electrical resonance
Impedance “dip” an approximate indicator of the likelihood of SS interactions
(large dips indicate “closer to radial” connections – transition from positive to
negative reactances)
• Limitations:
•
•
•
•
How should nearby SVC/Statcom/HVDC/non-linear-devices be represented?
What is the equivalent impedance of a wind turbine?
Does SSCI depend on the magnitude of a disturbance/oscillation?
If there are 2 or more wind farms nearby, how does a turbine non-linear controller
affect the impedance as seen from the other farm?
• Screening Method – used to determine system condition (for later
study with more accurate methods).
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SSR/SSCI – 1) Screening Studies
Harmonic Impedance from Generator Bus
for SSR Screening Analysis
MC_0_SC - 1 series comp. line, 35 ohms
MC_0_2xSC_1 - 2 series comp. line, 35 ohms each
MC_0_2xSC_2 - as above, 2nd line not terminated
MC_0_2xSC_3 - 2 series comp. lines, 2nd cap is 47 ohms
Ohms
MC_0_SC_4 - as above, 1st line out of service
Frequency (Hz)
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SSR/SSTI
2) Perturbation Analysis
• Consider open loop transfer function from generator rotor
speed to electrical torque…
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SSR/SSTI
2) Perturbation Analysis
• Enable PSCAD Multi-mass feature on a generator
• Force the speed to be 1 pu plus a small oscillation at 5 Hz (this will
rock the full system at 5 Hz)
• Run until steady state in the time domain
• Measure the relative magnitude and angle between the electrical torque
and delta W
•
•
•
•
•
•
May require a variable/tuned filter on both Te and W to remove noise and DC, or
(better) use FFT methods.
The electrical damping is the real part of dTe/dW
Compute and store the damping factor for this frequency
Increment the frequency to 6 Hz and repeat
Use multiple run features to sweep from 5 Hz to 55 Hz
Plot damping vs frequency
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SSR/SSTI
2) Perturbation Analysis
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SSR/SSTI
2) Perturbation Analysis
• HVDC links can directly affect torsional damping
•
•
Not a problem however, as SSDC stabilizers are easy to design and very effective
Should be studied using detailed models (PLL and main PI controls are critical)
• SVCs are usually not an SSR concern, however indirect effects
through nearby loads can cause interactions
• Interpolation in SVC and HVDC firing controls is essential!
• Exciter and governor models are often not validated at torsional
frequencies – simplified PSS/E models often are not valid.
• Impact with and without mitigation methods can be tested
• We always recommend TSRs (torsional stress relays) for all generators
near series caps, HVDC links or SVCs.
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SSR/SSTI
2) Perturbation Analysis
Torsional Damping With and Without SVC
Electrical Damping (pu/rad/sec)
(N-0, Constant Power Loads)
wo SVC
w SVC
Frequency (Hz, Mechanical)
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SSR/SSTI
2) Perturbation Analysis
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3) SSCI Advanced Screening
Dynamic Effective Impedance
• Perturbation Analysis can be used to determine the Dynamic Effective
Impedance of a non-linear device (wind farm):
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•
•
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Perturb voltage with sub-synchronous components
Measure sub-synchronous magnitude and phase of measured terminal current
Impedance (Z) = V/I (performed with complex vectors at each frequency)
Table of Z (R + jX) as a function of frequency
• Calculation for wind turbines (Dynamic Effective Impedances) can be
added to linear system impedance (including the series capacitor).
• Relatively easy for 1 wind farm connected through 1 port to a series
compensated system.
• Can be applied for a 2 port (2 wind farm) scenario:
•
•
Requires 3 system harmonic impedance scans for each system conditions
Solve 3 equations in 3 unknowns to device a 2x2 two port linear network
equivalent
• Add wind Dynamic Effective Impedances, solve 2x2 network as seen from each 20
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device
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3) SSCI Advanced Screening
Dynamic Effective Impedance
Dynamic Impedance as seen from the 345 kV Bus
(Varying Perturbation Magnitudes)
700
600
R - 1 kV
500
R - 2 kV
400
R - 3 kV
R - 4 kV
Ohms
300
R - 5 kV
200
R - 6 kV
R - 10 kV
100
R - 20 kV
0
0
10
20
30
40
50
60
R - 30 kV
R - 40 kV
-100
R - 20 kV (Damped)
-200
-300
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Frequency (Hz)
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Perturbation Analysis
Limitations for SSCI
• Some devices have a damping characteristic which is
“magnitude sensitive”
• Non-linearities in the system and device models are not
considered
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SSR/SSTI/SSCI Analysis
4) Complete System Time Domain Analysis
• Ultimate Simulation… Model the entire system including
multi-mass shaft models, HVDC/SVC/Statcoms, wind
farms etc…
• Apply a small signal disturbance and measure logdecrement (quantify damping)
• Apply faults and observe large signal disturbances (and
watch for tripping/ride through)
• Time consuming (varying loadflow conditions,
contingencies, wind turbine combinations, two segment
series capacitors…)
• Used in conjunction with Screening Studies (to focus on
most-concerning cases)
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Mitigation Methods
SSR SSCI SSTI
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Electranix Corporation
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Description
Select other transmission or generator options (higher
voltage AC lines etc…)
Select Series Compensation Level to Avoid Problems
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Use transfer trips to avoid trouble conditions
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Design damping controllers (SEDC, SSDC…)
SSCI resistant wind farm controllers
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Mitigation Methods
SSR SSCI SSTI
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Description
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Add generator step up transformer filters
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Add shunt compensation with stabilizers (SVC/Statcom)?
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FACTS Devices (TCSC, SSSC, UPFC…)
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Combinations of all of the above
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Series Capacitor Bypass Filters
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New Simulation Products
• E-TRAN Plus for PSCAD
Parallel Processing of PSCAD Simulations
- break the PSCAD simulation into several cases and
run them in parallel talking with each other
• E-TRAN Plus for PSS/E
Hybrid Simulation
- PSCAD and Transient Stability simulations are run
in parallel talking with each other
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Thanks!
Garth Irwin
Electranix Corporation – Engineering Consultants
Winnipeg, Manitoba, Canada
1-204-953-1831
[email protected]
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