The impact of Wind Farms on Subsynchronous Resonance

The impact of Wind Farms on
Subsynchronous Resonance
in Power Systems
Massimo Bongiorno, Evert Agneholm and Andreas Petersson
Gothia Power AB
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Outline
What is a subsynchronous resonance?
Risk for SSR in different generation units
Risk for SSR in variable speed wind turbines
Impact of controller parameters
Simulation results
Conclusions
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Subsynchronous Resonance in Power Systems
Subsynchronous Resonance - IEEE definition
Subsynchronous Resonance (SSR) is an electric power
systems condition where the electric network exchanges
energy with a turbine generator at one or more of the
natural frequencies of the combined system below the
synchronous frequency of the system
Famous incidents:
Mohave Power Station in 1970 and 1971
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Subsynchronous Resonance in Power Systems
Rotor
Spectrum
ofspeed.
rotor speed.
∆Tm
+
Spectrum
Resultingofvoltage
voltageatatmachine
machineterminals.
terminals.
∆ω
Ψ
es i
Transmission
system
_
Mechanical system
Electrical system
∆Te = f (∆i, ∆Ψ)
SSR may cause serious stress on the shaft systems and, in the
worst case, its breakdown.
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Subsynchronous Resonance in Power Systems
SSR is divided into two main categories:
Steady-state SSR:
• Induction Generator Effect (IGE);
• Torsional Interaction (TI).
Transient torques (or Torque Amplification, TA)
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Risk for SSR in different generation units
Generation type
Yes
No
Hydro
X
Steam
X
Nuclear
X
Wind turbines
(fixed speed)
X
Wind turbines
(variable speed)
?
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Event in Texas – October the 22nd 2009
The series capacitors are at Rio Hondo
They provide 50% line compensation of the Z of the Rio Hondo to Lon Hill line
There are two stages of capacitors (17% and 33%)
AJO
Both stages are usually in service
To Rio
Zorillo
Gulf
67 miles
To
Hondo
ERCOT
Nelson
Metering
Sharpe
37 miles
Wind
100
100
Fault occurred on this line
Mvar Mvar
ERCOT
93.6 MW 96 MW
Gulf Wind
South
Metering
Sarita
Penascal
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Description of the event
A single line to ground fault occurred on the Ajo to Nelson
Sharpe line due to a downed static wire.
The initial fault was cleared in 2.5 cycles.
The wind farms became radially connected to the to the Rio
Hondo series compensated line.
The system experienced overvoltages (up to about 195%)
which resulted in the trip of the Rio Hondo to Ajo line and the
shunt reactors at Ajo.
The series capacitors bypassed approximately 1.5 seconds into
the event.
The series capacitor controls indicated subsynchronous currents
during the event.
This event caused numerous failures of crow bar circuits at the
two wind farms.
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Recordings at Zorillo
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Cause of SSR in wind farms
At different frequencies, the wind turbine can be seen as a
controllable impedance. The parameters of this impedance (thus
its resonant frequency) depend on the controller settings.
WT resonance
Network resonance
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Cause of SSR in wind farms
resistance [pu]
1
0.5
0
-0.5
-1
0
5
10
15
5
10
15
20
25
30
35
40
45
30
35
40
45
reactance [pu]
1
0.5
0
-0.5
-1
0
20
25
frequency [Hz]
Trend of DFIG harmonic impedance (real and imaginary part) as a
function of the modulation frequency.
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resonance frequency [Hz]
Impact of controller parameters
42.5
42
41.5
41
40.5
2
3
4
5
6
7
8
9
10
3
4
5
6
7
8
curren controller bandwidth, α cc [pu]
9
10
resistance [pu]
-0.5
-1
-1.5
-2
2
Trend of DFIG resonance frequency (top) and corresponding equivalent
resistance (bottom) as a function of the current controller bandwidth.
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Impact of operating point
1.2
1.1
rotor speed [pu]
1
0.9
0.8
0.7
0.6
resonance frequency [Hz]
0.5
0.1
0.2
0.3
0.4
0.5
0.6
Output power [pu]
0.7
0.8
0.9
45
44.5
44
43.5
2
3
4
5
6
7
8
9
10
3
4
5
6
7
8
curren controller bandwidth, α cc [pu]
9
10
resistance [pu]
-0.4
-0.6
-0.8
-1
-1.2
-1.4
2
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Trend of harmonic impedance for Pout=0.9 (blue) and Pout=0.2 (red)
resonance frequency [Hz]
Impact of operating point
46
45
44
43
42
0
0.1
0.2
0.3
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.4
0.5
0.6
Output power [pu]
0.7
0.8
0.9
resistance [pu]
-1
-1.5
-2
-2.5
-3
-3.5
0
Trend of DFIG resonance frequency (top) and corresponding equivalent
resistance (bottom) as a function of the DFIG output power.
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Simulated system
The investigated system has been simulated using PSCAD/EMTDC
The wind farm has been modeled as an aggregated wind turbine
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SSR – Dynamic simulation analysis
Detail of the modeled wind turbine.
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4000
2000
0
αcc=2pu
-2000
-4000
5
5.5
6
6.5
2000
0
αcc=4pu
-2000
active power [MW]
active power [MW] active power [MW]
SSR – Simulation results
5
5.5
6
6.5
820
810
800
αcc=10pu
5
5.5
6
6.5
time [s]
Transmitted active power after line disconnection.
Series compensation level equal to 68%.
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Conclusions
The problem of subsynchronous resonance (SSR) in power
systems has been discussed.
It has been shown that variable speed wind turbines (DFIG)
might experience SSR when radially connected to a seriescompensated line.
The risk of unstable conditions is dependent on the
controller
settings
(current
controller,
dc-link
voltage
controller, phase-locked loop) as well as on the operating
conditions.
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