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Modelli di sequenza per unità
Un trasformatore stella-stella (Y-Y) ideale, collegato a terra tramite impedenze neutre, mostra reti di sequenza per unità simili a quelle di un trasf…
Un trasformatore Y-Y ideale collegato a terra tramite impedenze neutre ha reti di sequenza per unità simili a quelle di un trasformatore monofase quando scorrono correnti di sequenza positiva o negativa bilanciate, annullando le correnti neutre e le cadute di tensione.
Le correnti di sequenza zero provenienti da tutte e tre le fasi si combinano per formare una corrente neutra, causando cadute di tensione attraverso l'impedenza neutra e influenzando la tensione dell'avvolgimento a bassa tensione.
I pratici trasformatori Y-Y includono impedenze esterne nelle loro reti di sequenza per unità. Ogni fase rappresenta un resistore di perdita del nucleo in parallelo con un'induttanza magnetizzante.
Questi trasformatori hanno impedenze di sequenza positiva e negativa identiche per unità, mentre la rete di sequenza zero dipende dalle impedenze neutre.
I trasformatori delta-triangolo hanno reti di sequenza positiva e negativa identiche, con impedenze per unità indipendenti dai collegamenti degli avvolgimenti.
Per modellare la sequenza per unità per i trasformatori trifase, i trasformatori a tre avvolgimenti vengono creati collegando tre trasformatori monofase comparabili utilizzando una base S comune e basi di tensione proporzionali.
Nella rete a sequenza zero, il collegamento ad alta tensione dipende dalla configurazione degli avvolgimenti ad alta tensione.
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Q1: How do positive and negative-sequence currents behave in an ideal Y-Y transformer with grounded neutral?
In an ideal Y-Y transformer grounded via neutral impedances, balanced positive or negative-sequence currents flow without producing neutral currents or voltage drops. The per-unit sequence networks resemble those of a single-phase transformer. This symmetry simplifies analysis because positive and negative-sequence impedances are identical, allowing straightforward modeling of balanced fault conditions.
Q2: What happens to zero-sequence currents in a Y-Y transformer with an ungrounded neutral?
Zero-sequence currents cannot flow to ground through an ungrounded neutral but can still circulate within the windings. These identical-magnitude, in-phase currents combine to form a neutral current, causing voltage drops across the neutral impedance and affecting the low-voltage winding voltage. This circulation path allows zero-sequence analysis even without a ground connection.
Q3: How do practical Y-Y transformers differ from ideal transformers in per-unit sequence modeling?
Practical Y-Y transformers incorporate external impedances in their per-unit sequence networks. Shunt branches represent balanced-Y impedance loads, with each phase containing a core loss resistor in parallel with magnetizing inductance. Despite these additions, positive and negative-sequence impedances remain identical, while zero-sequence networks depend on neutral impedance configuration.
Q4: What is the relationship between positive and negative-sequence impedances in delta-delta transformers?
Delta-delta transformers have identical positive and negative-sequence networks with per-unit impedances independent of winding connections. This consistency simplifies fault analysis since both sequence types experience the same impedance. However, practical transformers may show variations depending on actual winding configuration and core design.
Q5: How are three-phase, three-winding transformers modeled using per-unit sequence networks?
Three-phase, three-winding transformers are modeled by connecting three identical single-phase transformers using a common S-base for all terminals and proportional voltage bases for each winding. This approach ensures consistent per-unit representation across all three phases and windings, enabling accurate analysis of sequence currents and voltages throughout the transformer.
Q6: Why does the high-voltage winding configuration matter in zero-sequence network analysis?
In the general zero-sequence network, the high-voltage winding configuration dictates the high-voltage connection and determines how zero-sequence currents flow through the transformer. Different configurations—such as grounded or ungrounded Y, or delta—create different zero-sequence paths and impedances, directly affecting fault current distribution and voltage response during unsymmetrical faults.
Q7: How do per-unit sequence models support analysis of unsymmetrical faults in power systems?
Per-unit sequence models decompose unsymmetrical faults into positive, negative, and zero-sequence components, each with defined impedances. By analyzing how sequence networks of rotating machines and transformers interact during faults, engineers can calculate fault currents and voltages. This approach enables proper circuit breaker and fuse selection for system protection.