How To Draw Zero Sequence Network

Like whatsoever other elements of power system, transformer tin also be represented by its corresponding sequence network. However, the characteristic of transformer is unique which is dependent on how information technology is banked. The transformer representation in its cipher sequence network can exist more than complex compare with its positive and negative components.

Below are the sequence network of a three phase transformer based on information technology's banking connection.

1. Y-Y Transformer with grounding

When a transformer has at least two grounded-wye windings, zero-sequence current can be transformed between the grounded-wye windings. The I0 currents will add up to 3I0 in the neutral and render through ground or the neutral conductor. The I0 currents volition be transformed into the secondary windings and period in the secondary excursion. Any impedance betwixt the transformer neutral points and ground must be represented in the zero-sequence network every bit three times its value to correctly account for the zero-sequence voltage driblet beyond it. Below on the left is a three-stage diagram of a grounded-wye, grounded-wye transformer connection with its cypher-sequence network model on the right. Discover the resistance in the neutral of the secondary winding is modeled by 3R in the zero-sequence network model.

Naught-Sequence model of Y-Y transformer (both grounded)

2. Delta-Wye (wye is grounded)

When a transformer has a grounded-wye winding and a delta winding, zerosequence currents volition be able to flow through the grounded-wye winding of the transformer. The zippo-sequence currents will exist transformed into the delta winding where they will circulate in the delta without leaving the terminals of the transformer. Considering the nada-sequence electric current in each phase of the delta winding is equal and in phase, electric current does not need to enter or exit the delta winding. Below on the left is a 3-phase diagram of a grounded-wye-delta transformer connection with its zero-sequence network model on the correct.

Goose egg Sequence model of Delta-Wye (grounded) transformer

As we can see, all WYE connections are grounded. In case, it is solidly grounded and no impedance exist in the basis conductor and so simply eliminate it and supervene upon information technology with brusque circuit (nada impedance).

All other models can be represented by the figure below,

Full general Representation of Transformer Zero-Sequence Network

Equally shown from the diagram above, there is no null sequence current that will flow to the reference bus of faulted system for delta connection and Wye (ungrounded).

This diagram tin can exist explained past the rules practical on the nothing-sequence networks of transformer.

When magnetizing current is neglected, transformer primary would carry current simply if there is current flow on the secondary side.
Nil sequence currents can menstruum in the legs of wye connectedness simply if information technology is grounded and provide constructive path to the basis.
No zero sequence currents can menses in the lines of delta connexion as no return path is avaiable for these currents.

How to Develop Null- Sequence Network of Transformer? How to Develop Zero- Sequence Network of Transformer? Reviewed by C.A. Cole on Saturday, September 28, 2019 Rating: 5