Connection of Wind Farms to Weak AC Network

Marian PiekutowskiHydro Tasmania,

Australia30 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. SCR > 3 indicate strong system 2. 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 sources4. 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 Yes• Symmetrical faults Yes• Asymmetrical faults ?• Dynamics ?• TOV/transients ?

• 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