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

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.

modelli di sequenza per unità

Per-Unit Sequence Models

Un trasformatore stella-stella (Y-Y) ideale, collegato a terra tramite impedenze neutre, mostra reti di sequenza per unità simili a quelle di un trasformatore ideale monofase quando sottoposto a correnti bilanciate di sequenza positiva o negativa. Queste correnti non producono correnti neutre e le relative cadute di tensione.

Le correnti di sequenza zero, che sono identiche in ampiezza e fase, generano una corrente neutra, con conseguenti cadute di tensione attraverso l'impedenza neutra e l'avvolgimento a bassa tensione. Se il neutro del trasformatore non è collegato a terra, le correnti di sequenza zero non possono fluire verso terra ma possono comunque circolare all'interno degli avvolgimenti.

Le reti di sequenza per unità dei trasformatori stella-stella pratici incorporano impedenze esterne. I loro rami paralleli rappresentano un carico di impedenza a stella bilanciato, con ogni fase corrispondente a una resistenza di perdita del nucleo parallela all'induttanza magnetizzante. Questi trasformatori possiedono impedenze di sequenza positiva e negativa per unità identiche, mentre le impedenze neutre influenzano la rete di sequenza zero.

I trasformatori delta-delta sono caratterizzati da reti di sequenza per unità con impedenze di sequenza positiva e negativa identiche. Tuttavia, diverse connessioni degli avvolgimenti possono influenzare queste impedenze nei trasformatori pratici. I modelli di sequenza per unità di trasformatori trifase a tre avvolgimenti possono essere costruiti collegando tre trasformatori monofase identici, utilizzando una base S comune per i terminali e basi di tensione proporzionali.

Nella rete generale a sequenza zero, la configurazione degli avvolgimenti ad alta tensione determina la connessione ad alta tensione. Le impedenze di rete a sequenza negativa per unità corrispondono sempre alle impedenze di rete a sequenza positiva.

Modelli di sequenza per unità

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 bilanciato ...

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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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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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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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