29.7
Per-Unit Sequence Models
An ideal Y-Y transformer grounded via neutral impedances has per-unit sequence networks similar to a single-phase transformer when balanced positive or negative-sequence currents flow, negating neutral currents and voltage drops.
Zero-sequence currents from all three phases combine to form a neutral current, causing voltage drops across the neutral impedance and influencing the low-voltage winding voltage.
Practical Y-Y transformers include external impedances in their per-unit sequence networks. Each phase represents a core loss resistor in parallel with a magnetizing inductance.
These transformers have identical per-unit positive and negative-sequence impedances, while the zero-sequence network depends on neutral impedances.
Delta-delta transformers have identical positive and negative-sequence networks, with per-unit impedances independent of winding connections.
To model per-unit sequence for three-phase, three-winding transformers are created by connecting three comparable single-phase transformers using a common S-base and proportional voltage bases.
In the zero-sequence network, the high-voltage connection depends on the high-voltage windings' configuration.
An ideal Y-Y transformer, grounded through neutral impedances, displays per-unit sequence networks akin to those of a single-phase ideal transformer when subjected to balanced positive- or negative-sequence currents. These currents do not produce neutral currents, and their associated voltage drops.
Zero-sequence currents, which are identical in magnitude and phase, generate a neutral current, resulting in voltage drops across the neutral impedance and the low-voltage winding. If the transformer's neutral is ungrounded, zero-sequence currents cannot flow to the ground but can still circulate within the windings.
Practical Y-Y transformers' per-unit sequence networks incorporate external impedances. Their shunt branches represent a balanced-Y impedance load, with each phase corresponding to a core loss resistor parallel to the magnetizing inductance. These transformers possess identical per-unit positive- and negative-sequence impedances, whereas the neutral impedances influence the zero-sequence network.
Delta-delta transformers feature per-unit sequence networks with identical positive- and negative-sequence impedances. However, different winding connections can influence these impedances in practical transformers. Per-unit sequence models of three-phase, three-winding transformers can be constructed by connecting three identical single-phase transformers, using a common S-base for terminals and proportional voltage bases.
In the general zero-sequence network, the configuration of the high-voltage windings dictates the high-voltage connection. Per-unit negative-sequence network impedances always match the positive-sequence network impedances.
An ideal Y-Y transformer grounded via neutral impedances has per-unit sequence networks similar to a single-phase transformer when balanced positive or negative-sequence currents flow, negating neutral currents and voltage drops.
Zero-sequence currents from all three phases combine to form a neutral current, causing voltage drops across the neutral impedance and influencing the low-voltage winding voltage.
Practical Y-Y transformers include external impedances in their per-unit sequence networks. Each phase represents a core loss resistor in parallel with a magnetizing inductance.
These transformers have identical per-unit positive and negative-sequence impedances, while the zero-sequence network depends on neutral impedances.
Delta-delta transformers have identical positive and negative-sequence networks, with per-unit impedances independent of winding connections.
To model per-unit sequence for three-phase, three-winding transformers are created by connecting three comparable single-phase transformers using a common S-base and proportional voltage bases.
In the zero-sequence network, the high-voltage connection depends on the high-voltage windings' configuration.
- Zero Sequence Network of Transformer - Pertains to the neutral path in a three-phase electrical system.
- Unit Sequence - Reference to positive, negative, or zero sequence components in power system engineering.
- Per Unit - A unitless system for system analysis where system quantities are expressed as fractions of a selected base unit.
- Sequence Networks - Networks representing the behavior of power systems under balanced or unbalanced conditions.
- Y-Y Transformer - A kind of three-phase transformer where both primary and secondary windings are star-connected.
- Define Zero Sequence Network of Transformer – Understand its role within a three-phase electrical system (e.g., zero sequence network of transformer).
- Contrast Positive vs Negative Unit Sequences –Explore the differences in power system engineering (e.g., unit sequence).
- Explore Per Unit system – Gain understanding of this method used in power systems analysis (e.g., per unit).
- Explain Sequence Networks – Understand the distinct behavior under balanced conditions and unbalanced conditions.
- Apply Y-Y Transformer in Context – Illustrate the functionality and application of a Y-Y transformer in a power system.
- What is the zero sequence network of a transformer and how does it function in a three-phase system?
- How is a unit sequence differentiated into positive and negative sequences?
- What role does a 'per unit' system play in power systems analysis?
From Chapter 29:
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