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FACT SHEET: FACT SHEET: SYSTEM SYSTEM STRENGTH STRENGTH WHAT IS SYSTEM STRENGTH? System strength is an inherent characteristic of any power WHAT system – itISis SYSTEM a measure ofSTRENGTH? the stability of a power system under allstrength reasonably conditions. System System is anpossible inherentoperating characteristic of any power strengthSystem is important as itiscan materially the way a system. strength important as itimpact can materially power system impact the wayoperates. a power system operates. System strength is usually measured by the available fault System at strength usually measured by thecircuit available current a givenislocation or by the short ratiofault – the current givencircuit location or byat the short in circuit ratio.with Higher ratio of at thea short current a point the grid the fault current levels are typically a stronger and power current at that point under openfound circuitinconditions with system,voltages. while lower faultfault current levelslevels are representative normal Higher current are typically of a weaker system. A system, high fault level, or high found in apower stronger power while lower faultcurrents current following fault, could beofviewed as the generation onhigh the levels are arepresentative a weaker power system. A grid responding strongly tofollowing the drop ainfault, voltage at the fault fault level, or high currents could be viewed – trying to restore on thethe situation. Similarly astrongly high short circuit as the generation grid responding to the ratio at point in measure of the strength of drop in avoltage atthe thegrid faultis–atrying to restore the situation. the response toshort any faults that at area. Similarly a high circuitinratio a point in the grid is a measure of the strength of the response to any faults in Faultarea. currents vary around the grid both by location and by that voltage level. The currents areboth higher areas close to Fault currents varyfault around the grid byinlocation and by synchronous and lower generators. voltage level.generation The fault currents arefurther higher from in areas close to System strength reducesand withlower increasing of PEC1 synchronous generation furtheramounts from generators. connected generation, along the displacement System strength reduces with with increasing amounts of of PEC synchronous generationalong whichwith contribute more to the connected generation, the displacement of fault current. generation which contribute more to the synchronous fault current. EXAMPLES OF GENERATION SYNCHRONOUS: Coal, gas, hydro. POWER ELECTRONIC CONVERTER (PEC) CONNECTED GENERATION: Large-scale wind and solar. WHAT ARE THE CHARACTERISTICS OF STRONG AND WEAK SYSTEMS? WHAT ARE THE CHARACTERISTICS OF Power systems with a high quantity of on-line synchronous STRONG AND generation and very WEAK little PECSYSTEMS? connected generation providesystems larger fault and are of categorised as strong Power with current a high quantity on-line synchronous systems. This is very manifested by connected the ability of the powerprovide generation and little PEC generation systemfault to maintain stability in response as to strong varioussystems. types of larger current and are categorised disturbances. This is manifested by the ability of the power system to maintain stability in response to various types of Points of the power system with PEC connected generation disturbances. which are distant from synchronous more Points of the power system with PECgeneration connectedare generation likely to weaker. Low system strength generally which arebe distant from synchronous generation are leads more to increased network system normal likely to bevolatility weaker.of Low systemvoltages strengthduring generally leads to and disturbance conditions. Low systemduring strength can also increased volatility of network voltages system normal compromise the conditions. correct operation of protection and disturbance Low system strengthsystems, can also and result in PECthe connected generationofsystems disconnecting compromise correct operation protection systems, during disturbances. and result in PEC1 connected generation systems disconnecting during disturbances. Some weak weak systems systems are are easy easy to to identify, identify, for for example, example, an an Some isolated point point in in the the system system with with no no nearby nearby synchronous synchronous isolated generation. In In other other points points in in the the system system where where there there is is generation. multiple concentrated concentrated PEC PEC connected connected generation, generation, weak weak multiple systems can can only only be be identified identified through through complex complex power power systems system studies studies conducted conducted by by engineers engineers using using detailed system models. models. detailed VOLTAGE MANAGEMENT IN IN STRONG STRONG VOLTAGE MANAGEMENT AND AND WEAK WEAK SYSTEMS SYSTEMS Strong Strong power power systems systems exhibit exhibit better better voltage voltage control control in response to to small andand large system disturbances. Weak in response small large system disturbances. systems are more to voltage instability Weak systems aresusceptible more susceptible to voltage instability or or collapse. collapse. INCREASING CONNECTION OF PEC GENERATION Generation that is interfaced to the network using PEC’s requires a minimum system strength to remain stable and maintain continuous uninterrupted operation. Different types of converters use different strategies to match their output to the frequency of the system while maintaining voltage levels and power flows. In a weak AC system, these can fail to operate correctly through even relatively minor disturbances. Power electronic converters include rectifiers and inverters which convert alternating current (ac) to direct current (dc) and dc to ac, respectively. Large scale wind generation is connected to the grid with complex converters whereas rooftop solar is connected to the grid with inverters. 1 Power electronic converters include rectifiers and inverters which converter alternating current (ac) to direct current (dc) and dc to ac, respectively. Large scale wind generation are connected to the grid with complex converters whereas rooftop solar is connected to the grid with inverters. TELEPHONE: 1300 858 724 1 OVERSEAS CALLERS: +61 3 9609 8000 AEMO INFORMATION & SUPPORT HUB: 1300 236 600 TELEPHONE: 1300 858 724 WWW.AEMO.COM OVERSEAS CALLERS: +61 3 9609 8000 AEMO INFORMATION AND SUPPORT HUB: 1300 236 600 FACT SHEET: SYSTEM STRENGTH INCREASING CONNECTION OF OPERATION OF PROTECTION EQUIPMENT FACT PEC GENERATION IN WEAK SYSTEMS SHEET: SYSTEM STRENGTH Generation that is interfaced to the network using PEC’s requires a minimum system strength to remain stable and maintain continuous uninterrupted operation. Different types of converters use different strategies to match their output to the frequency of the system while maintaining voltage levels and power flows. In a weak AC system, these can fail to operate correctly through even relatively minor disturbances. While weak systems are not new to system operators, they are attracting greater attention following the rise of large scale PEC connected generation in the power system. Protection equipment within power systems work to clear OPERATION OF PROTECTION EQUIPMENT IN WEAK faults, prevent damage to SYSTEMS network assets and mitigate risk public Protection equipment may be following triggered the While weak systems are not new to system operators, to they aresafety. attracting greater attention when the current following a fault exceeds the protection rise of large scale PEC connected generation in the power system. activation point, or by the impedance2 calculated from this Protection equipment within power systems work to clear faults, damage network assetstoand current. Weakprevent systems exhibit lowerto fault current relative the be strong networks.when In a weak protective equipment mitigate risk to public safety. Protection equipment may triggered thesystem, current following a fault which are2programmed to activate on measured current exceeds the protection activation point, or by the impedance calculated from this current. Weak systems or the ratio of measured voltage to current, could be exhibit lower fault current relative to the strong networks. In a weak system,operation protective equipment which susceptible to unintended or failure to operate. are programmed to activate on measured current or the ratio of measured voltage to current, could be susceptible to unintended operation or failure to operate. Figure 1 – Current response from synchronous and PEC connected generation Fault occurs Fault occurs Synchronous PEC Current Protection activation point Time 2 The effective resistance in a power system to alternating current (ac). ABOUT AEMO AEMO is responsible for operating Australia’s largest gas and electricity markets and power systems, including the National Electricity Market and interconnected power system in Australia’s eastern and south-eastern seaboard, and the Wholesale Electricity Market and the South West interconnected power system in Western Australia. AEMO also operates the Victorian Declared Wholesale Gas Market and the Victorian gas transmission system; the wholesale gas Short Term Trading Market hubs in Adelaide, Sydney and Brisbane; the Wallumbilla Gas Supply Hub in Queensland; and the Moomba Gas Supply Hub in South Australia. As Australia’s independent energy markets and power systems operator, AEMO provides critical planning, forecasting and power systems security advice and services to deliver energy security for all Australians. FURTHER INFORMATION AEMO Information & Support Hub: 1300 236 600 Media enquiries: 0409 382 121 www.aemo.com.au