Name: transformer sequence networks in per unit
Uploaded: 2024-11-21T14:54:31.000Z
Duration: 1 min 26 s
Channel: JoVE

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.

modèles de séquence par unité

Per-Unit Sequence Models

Un transformateur Y-Y idéal, mis à la terre par des impédances neutres, présente des réseaux séquentiels par unité semblables à ceux d'un transformateur idéal monophasé lorsqu'il est soumis à des courants équilibrés de séquence positive ou négative. Ces courants ne produisent pas de courants neutres et les chutes de tension qui leur sont associées.

Les courants homopolaires, identiques en amplitude et en phase, génèrent un courant neutre, ce qui entraîne des chutes de tension au niveau de l'impédance neutre et de l'enroulement basse tension. Si le neutre du transformateur n'est pas relié à la terre, les courants homopolaires ne peuvent pas circuler vers la terre, mais peuvent toujours circuler dans les enroulements.

Les réseaux séquentiels des transformateurs Y-Y pratiques intègrent des impédances externes. Leurs branches de dérivation représentent une charge d'impédance Y équilibrée, chaque phase correspondant à une résistance de perte de noyau parallèle à l'inductance magnétisante. Ces transformateurs possèdent des impédances de séquence positive et négative par unité identiques, tandis que les impédances neutres influencent le réseau homopolaire.

Les transformateurs delta-delta sont des réseaux à séquence unitaire avec des impédances identiques en séquence directe et en séquence inverse. Cependant, des connexions d'enroulement différentes peuvent influencer ces impédances dans les transformateurs pratiques. Des modèles de séquence unitaire de transformateurs triphasés à trois enroulements peuvent être construits en connectant trois transformateurs monophasés identiques, en utilisant une base S commune pour les bornes et des bases de tension proportionnelles.

Dans le réseau homopolaire général, la configuration des enroulements haute tension détermine la connexion haute tension. Les impédances du réseau homopolaire par unité correspondent toujours aux impédances du réseau homopolaire.

Modèles de séquence par unité

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 lorsqu’il est équilibré, positif ou ...

Education

Enseignement

JoVE Core

Electrical Engineering

Génie électrique

Unit 1

Unité 1

Symmetrical and Unsymmetrical Faults

Défauts symétriques et asymétriques

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

transformer sequence networks in per unit

Transformer sequence networks in per unit show how three-phase transformers respond to positive-, negative-, and zero-sequence currents. For an ideal Y-Y transformer grounded through neutral impedances, the per-unit sequence networks act much like those of a single-phase ideal transformer when the current is balanced. Balanced positive-sequence and negative-sequence currents do not create neutral current, so they do not cause voltage drops across the neutral path.

Zero-sequence current behaves differently because all three phase currents have the same magnitude and phase. This creates a neutral current and produces voltage drops across both the neutral impedance and the low-voltage winding. If the neutral is not grounded, zero-sequence current cannot flow to ground, but it can still circulate inside the windings.

Practical Y-Y transformers add external impedances to the per-unit sequence network. Their shunt branches represent a balanced-Y impedance load, with each phase modeled by a core loss resistor in parallel with the magnetizing inductance. In these transformers, the per-unit positive-sequence and negative-sequence impedances are the same, while the neutral impedances affect the zero-sequence network.

Delta-delta transformers also have identical per-unit positive-sequence and negative-sequence impedances. In practical transformers, however, the winding connection can change these impedances. Three-phase, three-winding transformer models can be built by connecting three identical single-phase transformers with a common S-base for the terminals and proportional voltage bases. In the general zero-sequence network, the high-voltage winding configuration sets the high-voltage connection, and the negative-sequence network impedance always matches the positive-sequence network impedance.

Transformer Sequence Networks in Per Unit

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per-unit sequence models

Transformer sequence networks in per unit show how three-phase transformers respond to positive-, negative-, and zero-sequence currents. For an ideal Y-Y transformer grounded through neutral impedances, the per-unit sequence networks act much like those of a single-phase ideal transformer when the current is balanced. Balanced positive-sequence and negative-sequence currents do not create neutral current, so they do not cause voltage drops across the neutral path. Zero-sequence current behaves...

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key terms and concepts

Learn how Y-Y, delta-delta, and three-winding transformers use per-unit sequence networks to model positive, negative, and zero-sequence currents.

Transformer Sequence Networks in Per Unit | JoVE K12

JoVE Core Electrical Engineering offers easy-to-understand animated videos that cover fundamental concepts of electric circuits along with circuit analysis techniques. It incorporates first-order and second-order circuits using operational amplifiers and circuit transformation concepts. The inclusion of practical design examples and real-world applications through scientist-in-action videos adds significant educational value, making complex concepts accessible.

electrical engineering

Transformer Sequence Networks in Per Unit

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