Connection of Wind Farms to Weak AC Network

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 • SCR (ESCR) works well in case of current source converters, • SCR > 3 indicate strong system • SCR < 2 indicate potential problems with commutation failures, harmonics, phase unbalance, instability (v and/or f) • Is this measure applicable to voltage source converters? • But some voltage sourced converters are designed to act as current sources • 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

Connection of Wind Farms to Weak AC Network