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Modèles de séquence par unité
Un transformateur Y-Y idéal, mis à la terre par des impédances neutres, présente des réseaux séquentiels par unité semblables à ceux d'un transformate…
Un transformateur Y-Y idéal mis à la terre via des impédances neutres a des réseaux de séquence par unité similaires à un transformateur monophasé lorsque des courants positifs ou négatifs équilibrés circulent, annulant les courants neutres et les chutes de tension.
Les courants séquentiels zéro des trois phases se combinent pour former un courant neutre, provoquant des chutes de tension aux bornes de l’impédance neutre et influençant la tension d’enroulement basse tension.
Les transformateurs Y-Y pratiques incluent des impédances externes dans leurs réseaux de séquence par unité. Chaque phase représente une résistance de perte de cœur en parallèle avec une inductance magnétisante.
Ces transformateurs ont des impédances positives et négatives identiques par unité, tandis que le réseau à séquence zéro dépend des impédances neutres.
Les transformateurs delta-delta ont des réseaux de séquence positive et négative identiques, avec des impédances par unité indépendantes des connexions d’enroulement.
Pour modéliser une séquence unitaire pour les transformateurs triphasés, des transformateurs à trois enroulements sont créés en connectant trois transformateurs monophasés comparables à l’aide d’une base en S commune et de bases de tension proportionnelles.
Dans le réseau à séquence zéro, la connexion haute tension dépend de la configuration des enroulements haute tension.
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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.