© ERLPhase Power Technologies Ltd. All Rights Reserved. © ERLPhase Power Technologies Ltd. All Rights Reserved. Mitigation & Protection of Sub-Synchronous Controller Interactions (SSCI) in DFIG Wind Turbine Systems Krish Narendra , P.hD. CTO ERL – Protection, Automation, Control & Smart Grid 14 April 2015, Hannover, Germanay
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© ERLPhase Power Technologies Ltd. All Rights Reserved. © ERLPhase Power Technologies Ltd. All Rights Reserved.
Mitigation & Protection of
Sub-Synchronous Controller
Interactions (SSCI) in DFIG Wind
Turbine Systems
Krish Narendra , P.hD.
CTO ERL – Protection, Automation, Control & Smart Grid
14 April 2015, Hannover, Germanay
Outline
• What is DFIG ?
• What is Sub Synchronous Controller Interaction (SSCI) ?
• Why does the Wind System Controller interact with the
system?
• How does the Wind System Controller interact with the
system ?
• Why it is important to Mitigate & Protect against SSCI?
• What is S-PRO Relay ?
• How to mitigate the SSCI Using S-PRO Relay?
• How to protect from SSC Using S-PRO Relay ?
• What are the consequences if we don’t mitigate or protect ?
• Conclusions
What is DFIG ?
• Doubly Fed Induction Generator
– Double Fed : There will be excitation (voltage & current) on
both STATOR & ROTOR windings
– This is different from normal induction machine, where the
STATOR is excited and the ROTOR is normally short
circuited or through a variable resistance
DFIG should produce CONSTANT POWER
to the electrical grid to which it is connected??
ROTOR excitation
is applied here STATOR excitation
is applied here
Wind Turbine
Electrical Equivalent Circuit of DFIG
S = Slip = w1 – wm : w1 = synchronous speed, wm = rotor mechanical speed
------------
w1 Pr = -s Ps; Pr = rotor power, s= slip, Ps= stator power
fr = s f1; fr = rotor frequency, s = slip, f1 = stator or network frequency
DFIG constant power – how to achieve?
• SHOULD KEEP THE RATIO OF
STATOR VOLTAGE / FREQUENCY A CONSTANT
• V/ f = constant = flux
What is SSCI (sub synchronous controller
Interaction)?
Sub Synchronous Controller Interaction (SSCI)
How does the controller interact
with the system ?
How Does Turbine Side
Dynamic Resistance Varies with Frequency ?
9
Turbine Side Scans, R & X, 100% Dispatch
-6
-4
-2
0
2
4
6
0 10 20 30 40 50 60
Frequency (Hz)
Oh
ms
Resistance
Reactance
Turbine Side Scans, R & X, 30% Dispatch
-15
-10
-5
0
5
10
15
0 10 20 30 40 50 60
Frequency (Hz)
Oh
ms
Resistance
Reactance
How does the controller interact
with the system.. ?
Turbine Side Scans, R & X, 100% Dispatch
-6
-4
-2
0
2
4
6
0 10 20 30 40 50 60
Frequency (Hz)
Ohm
s
Resistance
Reactance
Frequency Scan - CTG008
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
0 10 20 30 40 50 60
Frequency (Hz)
|Z1|
(ohm
s)
CTG008-current_injection
CTG008-Harmonic_Scan
Xcel Energy (USA) Utility Event
System Single Line Diagram- 2008
1- Breaker 1 & 2 opened for
regular system switching
procedure
2- CT1,CT2, and W start feeding
radially through series capacitor
3- Tripped the CT generator unit
Line 1 345 kV
345 kV
45 MVA
Utility Event: What Happened?
12
15 MW (20% of total generation)
To the
System
Utility Event - TESLA DFR Capture
13
9 Hz & 13 Hz dominant sub harmonics
High speed recording of 3 phase generator currents captured
by the DFR
Utility Event – Slow Speed Event
Analysis
14 Slow speed (swing ) recording of one of the phases
What are the consequences ?
• Damage to the wind turbine
• Damage to the series compensation system
• Damage to the electronics – convertor
• Possible saturation of the transformers at low frequency
sub harmonics
How to mitigate SSCI
• Use advanced Micro Processor Based Protection Relay
• Use high speed detection sub harmonic currents / voltages
using advanced microprocessor based system
• Improve existing controller performance – more positive
damping at critical sub harmonic frequencies
• Faster command to by pass the series compensation
system
Microprocessor Based Sub harmonic
Protection Relay (S-PRO)
SSCI - Sub harmonic calculations using
dual processor implementations in S-PRO
Anti Aliasing
Filter
A/D
Conversion
FPGA
Processor -1
(Field Programmable Gate Arrays)
High Speed Recursive Fourier Transform (RFT)
Using 2 mS Sliding Window from high speed
samples
(5760 / s on 60 Hz or 4800/ s on 50 Hz)
DSP
Processor -2
Sub Harmonic Detection Logic ( 5 – 45 Hz)
• Band Pass Filtering (5 – 45 Hz)
• Evaluation of sub harmonic logic using
MAX (f5 … f45) principle.
• Trip / Alarm based on user configuration
AC line currents
AC
Voltages
Microprocessor…Sub Harmonic Detection
Logic Diagram
Microprocessor…Sub Harmonic
Detection
20 Trip or Alarm: = max (f2, f3, f4, f5, f6, f7) > Lset
Mitigation: By Passing of Series Capacitor Using
S-PRO Relay
Other
Generators
(Combustion,
Steam etc.)
Series Capacitor
Wind
generators
• Direct Connection from
Relay output contact to
the bypass circuit
• IEC 61850 GOOSE
message through FO
• Use existing PLCC to
send close command
to bypass circuit
Mitigation: Change Wind Controller
gain using S-PRO Relay Logic
Other
Generators
(Combustion,
Steam etc.)
Series Capacitor
Wind
generators
• User defined Relay
logic to vary the gain of
the controller to damp
the oscillations
Mitigation: Transfer Trip Wind Generator
using S-PRO Relay Logic
Other
Generators
(Combustion,
Steam etc.)
Series Capacitor
Wind
generators
• Send transfer trip
command through
output contact or
GOOSE message to
remove Wind
Generator System from
the grid
Protection : Trip Wind Generator
/ Other Generators using S-PRO Relay Logic
Other
Generators
(Combustion,
Steam etc.)
Series Capacitor
Wind
generators
Xcel Energy (USA) – SSCI EVENT – 2008
Playback on S-PRO Relay
Xcel Energy (USA) – SSCI EVENT – 2008
Playback on S-PRO Relay
ERCOT – TEXAS (USA) – SSCI EVENT – 2009
Playback on S-PRO Relay
ERCOT – TEXAS (USA) – SSCI EVENT – 2009
Playback on S-PRO Relay
• With the increase use of wind generators (DFIGs) feeding
HV and EHV utility networks with series compensation, it is
necessary to ensure that sub harmonic oscillations are
monitored, and that the electrical grid is protected from any
resulting detrimental effects.
• Micro Processor Based S-PRO Relay can reliably mitigate /
protect from sub harmonics ( 5 – 55 Hz with 60 Hz system)
generated due to following phenomena with dual processor
( < 250 mS) :
– SSR – sub synchronous resonance (Generators, HVDC, FACTS, PV -
Convertor etc..)
• SSI – sub synchronous interactions
– SSCI – sub synchronous controller instability/interactions- New phenomena in Wind
Turbines controller interactions in Type 3 DFIG system.
– SSTI – sub synchronous torsional interactions
• IGE- Induction generator effect
– SSFR – sub synchronous ferro resonance (in distribution system
transformers)
Conclusions
THANK YOU!!
www.erlphase.com
knarendra@ erlphase.com
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