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Sequenzmodelle pro Einheit
Ein idealer Y-Y-Transformator, der über Neutralimpedanzen geerdet ist, weist bei symmetrischen positiven oder negativen Strömen Sequenznetzwerke pro E…
Ein idealer Y-Y-Transformator, der über Neutralleiter geerdet ist, verfügt über Sequenznetzwerke pro Einheit, ähnlich wie ein einphasiger Transformator, wenn ausgeglichene positive oder negative Sequenzströme fließen, wodurch Neutralströme und Spannungsabfälle negiert werden.
Nullfolgeströme aus allen drei Phasen verbinden sich zu einem Neutralstrom, der Spannungsabfälle über die Neutralleiter verursacht und die Niederspannungswicklungsspannung beeinflusst.
Praktische Y-Y-Transformatoren integrieren externe Impedanzen in ihre Sequenznetzwerke pro Einheit. Jede Phase stellt einen Kernverlustwiderstand dar, der parallel zu einer magnetisierenden Induktivität steht.
Diese Transformatoren haben identische positive und negative Impedanzen pro Einheit, während das Nullsequenznetzwerk von Neutralleiterimpedanzen abhängt.
Delta-Delta-Transformatoren verfügen über identische Netzwerke mit positiver und negativer Sequenz mit Impedanzen pro Einheit, die unabhängig von den Wicklungsanschlüssen sind.
Um die Sequenz pro Einheit für dreiphasige Transformatoren mit drei Wicklungen zu modellieren, werden drei Transformatoren erstellt, indem drei vergleichbare einphasige Transformatoren unter Verwendung einer gemeinsamen S-Basis und proportionaler Spannungsbasen verbunden werden.
Im Nullsequenznetz hängt der Hochspannungsanschluss von der Konfiguration der Hochspannungswicklungen ab.
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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.