Zero Sequence Circuit diagram fault calculation |
Zero Sequence Network of Transformer Power System
Zero Sequence Circuit diagram🔥Fault calculation Power system🔥
The unbalanced current flows in the circuit during the earth fault is known as the zero sequence current or the DC component of the fault current. The zero phase sequence means the magnitude of three phases has zero phase displacement. The three vector lines represent the zero sequence current and it is detected by adding the vector of three phases current (I).
The impedance offered by the system to the flow of zero sequence current is known as zero sequence impedance.
In previous fault calculations, Z1, Z2, and Z0 are positive, negative, and zero sequence impedance respectively. The sequence impedance varies with the type of power system components under consideration:-
In static and balanced power systems components like transformers and lines, the sequence impedance offered by the system is the same for positive and negative sequence currents. In other words, the positive sequence impedance and negative sequence impedance are the same for transformers and power lines.
But in the case of rotating machines the positive and negative sequence impedance are different.
The assignment of zero sequence impedance values is a more complex one. This is because the three zero sequence currents at any point in an electrical power system, being in phase, do not sum to zero but must return through the neutral and /or earth. In three-phase transformer and machine fluxes due to zero sequence components do not sum to zero in the yoke or field system. The impedance very widely depends upon the physical arrangement of the magnetic circuits and winding.
The reactance of transmission lines of zero sequence currents can be about 3 to 5 times the positive sequence current, the lighter value being for lines without earth wires. This is because the spacing between the go and return(i.e. neutral and/or earth) is so much greater than for positive and negative sequence currents which return (balance) within the three-phase conductor groups.
The zero sequence reactance of a machine is compounded by leakage and winding reactance, and a small component due to winding balance (depends on winding tritch).
The zero sequence reactance of transformers depends both on winding connections and upon the construction of the core.
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