How To Draw A Zero Sequence Network

Sequence networks of ability systems are very useful for computing unsymmetrical faults at different points of a power system network. The cognition of positive- sequence network is necessary for load-studies on power systems. If the stability studies involve unsymmetrical faults, so negative- and zero-sequence networks are required also.

A ability arrangement network consists of synchronous machines, transformers and lines. Using these, complete sequence networks of a power arrangement can exist easily fatigued. The positive-sequence network is drawn by examining one line diagram of the power organisation. In fact, the single line reactance diagram, as employed for calculation of symmetrical fault current, is the positive-sequence diagram of the power system.

The negative-sequence network is quite like to positive-sequence network—only generators or rotating machines may take different sequence impedances and the negative-sequence network does not contain whatsoever voltage source. The negative-sequence impedances for transmission lines and transformers are the same equally the positive-sequence impedances.

In many cases only 1 sequence network is drawn for positive- and negative-sequence representation. The reference bus for positive- and negative-sequence networks is the system neutral. Any impedance connected between a neutral and ground is not included in the positive- and negative-sequence networks as neither of these sequence currents tin catamenia through such an impedance.

Zero-sequence sub-networks for different elements of a power system can be easily combined to course complete zero-sequence network. The null-sequence network does not contain whatever voltage source. Whatever impedance included in generator or transformer neutral becomes three times its value in a zero-sequence network. Special intendance needs to be taken in connecting the nil-sequence impedance of transformer.

All the sequence impedances are expressed in per-unit values and referred to the same base of operations MVA and base kV.

The procedure for drawing sequence networks is illustrated through the following case:

Example:

Describe the sequence networks for the power organisation shown in Fig. iii.18:

Solution:

Let base MVA for complete system be 50 MVA, base kV for generator side 11 kV and transmission side 220 kV.

Per unit reactances of generator G₁ –

X₁ = 10_two = j 0.2 pu

and X₀ = j 0.05 pu                        …(Given)

Per unit reactances of generator Chiliad₂ –

X_i = X_ii = [j 0.15 x (l/thirty)] = j 0.25 pu

and X₀ = [j 0.03 x (50/30)] = j 0.05 pu

Per unit of measurement reactances of transformer T₁ –

X_i = X₂ = X₀ = j 0.10 pu

Per unit of measurement reactances of transformer T₂ –

X₁ = X₂ = X₀ = [j 0.075 x (50/30)] = j 0.125 pu

Per unit reactances of transmission line 1 –

Ten₁ = X₂ = 0.1 pu                      …(Given)

Assuming zero-sequence reactance of manual line 3.5 times of its positive-sequence reactance –

X₀ = 0.35 pu

Per unit reactances of manual line 2 –

X₁ = X_two = j 0.12 pu    (Given)

X₀ = j 0.12 x 3.5 = j 0.42 pu

The neural reactance of j 0.024 and j 0.03 comes out to be [j 0.024 x 950/30)] i.e., 0.4 pu and [j 0.03 x (l/30)] i.e., j 0.05 pu.

These neutral reactances appear equally iii 10 j 0.04 i.e., j 0.12 pu and 3 x j 0.05 i.e., j 0.15 pu in the zero sequence networks.

The positive-, negative- and- zero sequence networks are shown in Figs. three.19 (a), three.19 (b) and iii.19 (c) respectively:

Source: https://www.engineeringenotes.com/electrical-engineering/power-system/sequence-networks-of-power-system-electrical-engineering/24556

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