With the spread of wind generation and the increasing number of wind power plants connected to the grid, short circuit calculation models of wind turbine generators and their protection system have become relevant topics. This case develops the short circuit analysis of a wind offshore power plant, consisting of 75 wind turbines, connected to the 220 kV transmission grid. Two typical design options are analysed: Permanent Magnet Synchronous Generators and Doubly Fed Induction Generators. The circuit models adopted and the fault locations considered have been chosen in order to cover the full range of short circuit current values, while optimizing the number of simulations. Each fault has been simulated taking worst case phase and voltage into account. Sensitivity analysis has then been carried out, taking into account different possible short circuit power of the transmission grid. Software used for this study is EMTP ATP. For Permanent Magnet Synchronous Generators, results show the maximum prospective short circuit current to be interrupted by Generator Circuit-breakers (approximately 25 kA) has a degree of asymmetry as high as 97 % in case of a fault close to the rotating machine and when machine is delivering full power prior to fault. It has been proved that in case of different pre-fault conditions (e.g. machine delivering partial power prior to fault) the degree of asymmetry of the fault current can even exceed 100 % thus leading to delayed current zeros. Such values may not be within the breaking capacity of standard switching devices, requiring the use of specifically designed circuit-breakers. Results for Doubly Fed Induction Generators also show high values of maximum short circuit current, approximately 20 kA. In this case, however, the degree of asymmetry can reach extremely high values: after few milliseconds, the fault current can be essentially described as fully asymmetrical. This is due to the activation of crowbar resistors, which do not affect the asymmetrical component of the fault current, while being fast and effective in limiting its symmetrical component. This effect must not be overlooked when designing fault protection and interruption systems for wind farms, and may require the deployment of circuit-breakers specifically designed for the purpose.
Con la grande diffusione e il crescente numero di impianti eolici connessi alla rete, i modelli di calcolo corto circuito dei generatori eolici e i loro sistemi di protezione hanno assunto una grande importanza. In questo studio abbiamo effettuato l'analisi in condizioni di corto circuito di un impianto eolico offshore, composto da 75 turbine e connesso alla rete di trasmissione a 220kV. Abbiamo considerato due casi tipici, quello di generatori sincroni a magneti permanenti e quello di macchine asincrone a doppia alimentazione (doubly fed induction generator, DFIG). La scelta dei modelli da utilizzare per il corto circuito e dei possibili guasti è stata effettuata per coprire un ampio intervallo di correnti di corto circuito, ottimizzando il numero di simulazioni. Ciascun guasto è stato simulato nelle condizioni peggiori di tensione e di fase. E' stata poi effettuata un'analisi di sensitività, considerando diversi possibili valori per la potenza di corto circuito della rete di trasmissione. Il software utilizzato per lo studio è EMTP ATP. Per i generatori a magneti permanenti, i risultati mostrano che la massima corrente di corto circuito presunta che l'interruttore di macchina dovrà interrompere (circa 25kA) può raggiungere valori di asimmetria pari al 97% in caso di guasto vicino ai generatori e quando la macchina fornisce piena potenza prima del guasto. E’ stato provato che in caso di diverse condizioni di pre-guasto (ad es. il generatore che invia potenza parziale prima del guasto) il grado d’asimmetria della corrente di guasto può eccedere il 100%, portando ad un passaggio per lo zero ritardato. Questi valori possono essere troppo elevati per un interruttore standard, e possono richiedere l'utilizzo di apparecchi speciali. Anche i risultati per i DFIG mostrano valori assai elevati di corrente di corto circuito massima, circa 20kA. In questo caso, però, il grado di asimmetria può raggiungere valori elevatissimi a causa dell'attivazione delle resistenze di crowbar, che in pochi secondi limitano la componente simmetrica della corrente, riducendola in pratica alla sola componente asimmetrica. Questo effetto deve essere tenuto in considerazione nella progettazione dei sistemi di protezione e interruzione dei guasti per impianti eolici, e potrebbe richiedere l'uso di interruttori progettati specificamente per questa applicazione.
Application of generator circuit-breakers in large wind power generation plants
LUGHIGNANI, CHIARA ISABELLA
2016/2017
Abstract
With the spread of wind generation and the increasing number of wind power plants connected to the grid, short circuit calculation models of wind turbine generators and their protection system have become relevant topics. This case develops the short circuit analysis of a wind offshore power plant, consisting of 75 wind turbines, connected to the 220 kV transmission grid. Two typical design options are analysed: Permanent Magnet Synchronous Generators and Doubly Fed Induction Generators. The circuit models adopted and the fault locations considered have been chosen in order to cover the full range of short circuit current values, while optimizing the number of simulations. Each fault has been simulated taking worst case phase and voltage into account. Sensitivity analysis has then been carried out, taking into account different possible short circuit power of the transmission grid. Software used for this study is EMTP ATP. For Permanent Magnet Synchronous Generators, results show the maximum prospective short circuit current to be interrupted by Generator Circuit-breakers (approximately 25 kA) has a degree of asymmetry as high as 97 % in case of a fault close to the rotating machine and when machine is delivering full power prior to fault. It has been proved that in case of different pre-fault conditions (e.g. machine delivering partial power prior to fault) the degree of asymmetry of the fault current can even exceed 100 % thus leading to delayed current zeros. Such values may not be within the breaking capacity of standard switching devices, requiring the use of specifically designed circuit-breakers. Results for Doubly Fed Induction Generators also show high values of maximum short circuit current, approximately 20 kA. In this case, however, the degree of asymmetry can reach extremely high values: after few milliseconds, the fault current can be essentially described as fully asymmetrical. This is due to the activation of crowbar resistors, which do not affect the asymmetrical component of the fault current, while being fast and effective in limiting its symmetrical component. This effect must not be overlooked when designing fault protection and interruption systems for wind farms, and may require the deployment of circuit-breakers specifically designed for the purpose.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/138730