Stable Renewable Plant
Voltage and Reactive Power
Control
NERC ERSTF
June 11-12, 2014
Sebastian Achilles
Nicholas Miller
Einar Larsen
Jason MacDowell
GE
Energy Consulting
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GE Proprietary /
May 10, 2008
Topics
•
Features of modern wind turbine generators and wind
plants
•
Voltage vs. power factor control in wind plants
•
Weak system control stability with PE sources
•
Response coordination of multiple wind plants
•
Summary
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GE Proprietary /
May 10, 2008
What makes a Wind Plant “Grid Friendly”?
•
Not trip during Faults and other System Disturbances …
ride through capability
•
Regulate Plant Voltage and Reactive Power
•
Limits the Rate of Change of Power from Variations in
Wind Speed … Ramp Rate Control
•
React to Changes in Grid Frequency … Frequency Droop
•
Provide reactive power all the time
•
Provide inertial response to large under-frequency events:
WindINERTIATM
•
Reliable operation with series compensated lines
•
Reliable operation with low system strength
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GE Proprietary /
May 10, 2008
Wind Turbines and Reactive Power Control
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GE Proprietary /
May 10, 2008
WTG Reactive Power Capability
Reactive Power for Voltage Support
•
•
•
•
•
Steady-state PF range - 0.90 under-excited/0.90 over-excited
Dynamic range meets or exceeds steady-state range
WTG reactive capability often sufficient to satisfy PF
requirements at POI
VAR capability reduced at low power due to units cycling off-line
GE offers reactive power capability with no wind
Terminal Bus
P gen
WTG
Q gen
Reactive Capability all the time is valuable!
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GE Proprietary /
May 10, 2008
WindCONTROL
Substation
Plant Level Control System
HV Bus
• Coordinated turbine and
plant supervisory control
structure
• Fast WTG controls
• Slower plant controls
QWP PWP
Point of Interconnection
(POI)
LTC
LV Bus
QL
Reactive
Power
Controller
QC
• Voltage, VAR, & PF control
• Active Power control
• PF requirements primarily
met by WTG reactive
capability, but augmented
by mechanically switched
shunt devices if necessary
• Integrated with substation
SCADA
Reactive Compensation
(if required)
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
QWTGGE Proprietary /
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May 10, 2008
Voltage & Reactive Power Controls
• Regulates Grid Voltage at
Point of Interconnection
Actual measurements from a
162MW wind plant
Wind Plant Voltage
Voltage at POI
• Minimizes Grid Voltage
Fluctuations Even Under
Varying Wind Conditions
• Regulates Total Wind Plant
Reactive Power through
Control of Individual
Turbines
Wind Plant Power Output
Average Wind Speed
Voltage and Reactive Power Regulation
Like A Conventional Power Plant
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GE Proprietary /
May 10, 2008
Network Stability: Voltage or Power Factor
Control?
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GE Proprietary /
May 10, 2008
Steady State considerations
Strong grid
Weak grid
PF
Control
Voltage
Control
Insufficient margin and challenging
operation in PF control
Plant level voltage control improves network voltage
stability performance in constrained transmission systems
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GE Proprietary /
May 10, 2008
Control Stability of Power Electronic
Sources in Weak Grids
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GE Proprietary /
May 10, 2008
Conventional and Power-Electronic sources
• Relevant electrical characteristics of Power-Electronic (PE)
sources
Performance aspect
Conventional generation
Power-Electronic Sources
Short circuit contribution
(system strength)
Around 3 pu
Small/none
Inherent to design
characteristics. Sharing
depends on size and
impedances of the
machines
Fast current controls
force current sharing
Current
sharing/distribution
• All current-controlled PE sources require grid strength to
operate reliably and stably
• Grid strength is high when electrically close to
conventional generation
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GE Proprietary /
May 10, 2008
Conventional and Power-Electronic sources
• Long transmission corridors (low system strength)
typically have power transfer limited below thermal limits
due to stability challenges
Conventional generation
Transient Stability
Dynamic stability
Power-Electronic Sources
Voltage stability
Fast control stability
Voltage stability
• Fast control stability refers to interactions between
transmission system and PE sources (Wind Turbine
Generators, SVCs, STATCOMs, etc.)
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GE Proprietary /
May 10, 2008
Wind turbine generators and system strength
Modern WTGs are PE sources
Some applications may need mitigations to achieve desired
system performance under very low system strength conditions:
• Transmission upgrades
» New lines
» Updated Topology (meshed vs. radial)
» Series compensation
» Synchronous condensers (System strength, dyn VARs)
» SVC, STATCOM (dyn VARs, control challenges)
• Special protection schemes (such as transfer trips)
• GE offers control features to improve performance in low SCR
conditions
More than one mitigation mechanism may be used and
system coordination of multiple mechanisms is highly
desirable and very effective (e.g. Wind Plant + Condenser)
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GE Proprietary /
May 10, 2008
SCR applied to Wind Plants
• In the case of Wind Plants, the characterization of system
strength has to take into account all electrically close
converters (Multi-infeed)
• Composite SCR (CSCR) considers the grid strength as
seen by all electrically close converters and is used for
wind plants.
Converter
locations
WP1
WP 2
WP 3
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GE Proprietary /
May 10, 2008
Composite SCR
• Composite short circuit level:
– 3Ph short circuit at 34.5 KV buses - all interconnected
– Low load conditions (low/realistic commitment of
conventional generation)
– Contingency conditions also considered
– No contribution from converters
WP1
WP 2
WP 3
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GE Proprietary /
May 10, 2008
Composite SCR: Recommended Practice
Composite SC MVA
CSCR 
 converter MW rating
•
Composite SCR is useful to characterize grid strength
and screen for system stability risks
•
Understanding of the grid parameters, system operation
and future wind projects is required to meaningfully
estimate CSCR
•
Grid entities should estimate this parameter for normal
and contingency operation and communicate to
developers.
•
For very low CSCR applications, dedicated detailed
analysis is recommended
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GE Proprietary /
May 10, 2008
Coordinated response of electrically close
wind plant level controllers
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GE Proprietary /
May 10, 2008
Plant Level Control
Interactions
Substation
HV Bus
Point of Interconnection
(POI)
QWP
MV Bus
VWP
QWP1
Plant Level
Wind CONTROL
Control
Plant Level
Control
Wind CONTROL
time
QWP2
QWP1
QWP2
VWP
time
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
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GE Proprietary /
QMay
10, 2008
WTG
Plant Level Control
Interactions
Substation
HV Bus
Point of Interconnection
(POI)
QWP
MV Bus
VWP
QWP1
Plant Level
Wind CONTROL
Control
Plant Level
Control
Wind CONTROL
time
QWP2
Circulating Q
VWP
time
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
QWTG
PWTG
PWTG
QWTG
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GE Proprietary /
QMay
10, 2008
WTG
Plant Level Control Interactions: Voltage Droop
Uncoordinated Without Voltage Droop
Effect of voltage setpoint differences or PT errors
QWP
Coordinated With Droop
Effect of system event
QWP
QWP1
QWP1 ≈ QWP2
time
time
QWP2
VWP
VWP
time
time
•
•
Faster control details neglected
for this discussion
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Dynamic response coordination
GE Proprietary /
also assumed
May 10, 2008
Summary
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GE Proprietary /
May 10, 2008
Summary
• Coordinated plant level voltage control improves voltage stability
performance in weak grid applications
• Fast terminal voltage control significantly improves the system
performance during contingencies in weak grid applications
• Providing Reactive power all the time is valuable for system stability
• Understanding and potential remediation of very low grid strength wind
plant operation is critical to maintain stability
• This requires collaboration between system operator, planners,
developers and OEMs. CSCR estimation is an initial step in such
collaboration
• Electrically close plant level controls require coordination to avoid
negative interactions of reactive power
These issues are resolvable if known and included in the
planning/development process
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GE Proprietary /
May 10, 2008
Thank you!
Questions?
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GE Proprietary /
May 10, 2008
Back-up
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GE Proprietary /
May 10, 2008
Fast or Slow Voltage Control?
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GE Proprietary /
May 10, 2008
Conventional Generation
• Fast acting Automatic Voltage Regulator (AVR)
– Quick an effective system support to improve stability
– Limits voltage variations at equipment
• Slower acting joint control
– Optimizes steady state operation
– Does not interfere with inner control loops
– Reduced risk of instabilities due to communication
delays
FAST Loop
FAST Loop
SLOW Loop
FAST Loop
A reasonable and valuable concept
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GE Proprietary /
May 10, 2008
Contingency operation of Wind Plants
• Contingency operation requires
higher reactive power for stable
operation with same power level
N-0 Steady state
• Joint control is typically slower than
WTG active power recovery
• Fast WTG terminal voltage control
keeps system stable for wider range
of operating conditions
N-1 before
joint control
reaction
• Joint control further optimizes
operating condition in steady state
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GE Proprietary /
May 10, 2008
How “strong” are grid conditions?
•
The industry has used the Short Circuit Ratio (SCR) to
assess the system strength for the connection of power
electronic converters
•
SCR varies with system conditions
•
There are few different SCR calculation methods
proposed in the industry
SCR 
short circuit MVA of AC System
converter MW rating
HVDC rectifier
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GE Proprietary /
March 2012
May 10, 2008