Connection of Wind Farms to
Weak AC Network
Marian Piekutowski
Hydro Tasmania,
Australia
30 May 2013
What is a weak network
• Long radial connection
• Low fault level (little change with the system
strength)
• Large size of connected wind farm – low SCR
• Other voltage control equipment connected along
the line – uncoordinated actions
• Other users connected to the radial connection line
impose limitations
Wind Farm Connection to Weak
Network – First Indications
• Initial modelling indicate poor solution convergence
• High TOV on the recovery from a fault
• Loss of stability if synchronous condensers are used, very
large voltage angles
• Interaction between different controls, hunting
Also consider large penetration of wind generation
• Impact on operation of existing LCC HVDC links
• Overall reduction on system strength and fault level
• Management of disturbing load with lower fault level
Selection of Critical Contingency
• Three phase fault (not causing a trip of radial connection
but) temporarily separating two systems with one
part slowing down and the wind farm accelerating
• Faults causing large voltage angle shifts
• Asymmetrical faults – how credible is their
simulation?
• Close by or remote fault
Is SCR Right Measure to Asses the Strength
of the System
1. SCR (ESCR) works well in case of current source converters,
1.
2.
SCR > 3 indicate strong system
SCR < 2 indicate potential problems with commutation failures,
harmonics, phase unbalance, instability (v and/or f)
2. Is this measure applicable to voltage source converters?
3. But some voltage sourced converters are designed to act as
current sources
4. What is the equivalent impact of VSC (statcom) on SCR
(ESCR)
Sources of possible connection problems
• Overvoltages at the wind farm or other locations
along connecting line due to too aggressive setting of
FRT on WTG
– They are set to supply reactive power into the fault but
after the fault is cleared the reactive power is back off too
slowly causing TOV
• Interaction of WTG controls with supporting control
equipment (eg: statcom, synchronous condenser)
uncoordinated responses
• Phase-Locked Loop problems
WTG Design Impacts
• Differences in response of DFIG and the inverter
(LVFRT and HVFRT) and different voltage support
requirements
• Design specification of the inverter, Is the inverter of
direct drive WTG fully controllable?
• Have WTGs been tested under low SCR (past
experience)?
• Can DFIG/full converter WTG control TOV on the
recovery from a fault – compatibility of HVFRT of
WTG with rules requirements
Interactions between multiple controllers
• Avoid use of too many controllers
• Coordination of set points in FRT sequence on
different devices
• Controlling devices with different time constant
(inverter vs rotating machine)
• Importance in timing coordination of different
controllers
Interaction with close-by wind farms
• Faults in weak systems may not necessarily cause
widespread large voltage changes in the network
therefore containing overall impact of energy not
supplied during a fault due to FRT action
• Interaction between voltage controllers
Are Current Analytical Tools Adequate
•
•
•
•
•
Load flow
Symmetrical faults
Asymmetrical faults
Dynamics
TOV/transients
Yes
Yes
?
?
?
• When do three phase studies become necessary ? Usually due
lack of confidence in positive sequence models
• Availability of verified models valid under wide range of system
conditions
• Verification against real cases not other models
• IP limitations imposed by manufacturers