Some Electrical Engineering Questions

The zero sequence currents are additive - in same phase unlike the positive and negative sequence currents that are destructive at the neutral points due to their phase shifts.

Due to this the only way for the zero sequence current to loop back to source is through the ground connection.

Resistance or reactance in the ground/earth terminal is to limit the fault current.

Power system shunt reactance or the series reactance both do affect the power transfer capability. Only the problem is the amount. The series reactance (capacitor) - the reactive power compensated is proportional to the power. Whereas in case of shant it is inversely proportional to voltage (infact V²) So the decrease in voltage - usually associated with high reactive power- will have the effect of lesser compensation by the shunt (when in fact higher should have been there). But these are any way taken care of by the switched capacitor by controlling the firing angles.

What aload of rubbish.

1- If the current is going to return via the ground, it will end up in the neutral point in any case, at the source! so what makes it decide to take the more resistive earth path????

Your 2nd paragraph is OK

The 3rd paragraph is from another planet I Think. Could not make head nor tail...

Re: 1- If the current is going to return via the ground, it will end up in the neutral point in any case, at the source! so what makes it decide to take the more resistive earth path????

I agree with AP #1 below, but I can see a simpler explanation. Consider the location of the NGR (neutral grounding resistor)--it is between the neutral point of the transformer (or a ZigZag transformer used to create a neutral) and ground. Then consider where the ground / neutral conductor (carried to the power consumer) is connected--it is on the ground side of the NGR.

Thus when a ground fault occurs at the consumer, the fault current has to travel back through that ground / neutral conductor and must go through the NGR to get back to the source (the transformer) to complete the circuit.

Because it must travel through the NGR, the fault current is limited.

to restrict ground fault current. the fault current to ground is decided by the bus capacity.(fault level).

Relation is a mathematical one.

If we have phase a,b,c and a neutral circuit, n,

The relation between neutral current and phase current is given by

In = Ia+Ib+Ic [Vector sum] = 0 under balanced conditions

Vn = Va+Vb+Vc [Vector sum] = 0 under balanced conditions

If a,b,c are balanced , the corresponding symmetrical component transformation , results in 'Zero' value for zero sequence component.

If a,b,c are not balanced, the symmetrical component transformation results in "Non-Zero" component for zero-sequence component.

Since by definition, positive and negative sequence quantities are balanced [with different directions], The unbalanced 0 sequence component having the same phase, has to flow through the neutral.

In otherwords, zero sequence component represents the unbalanced portion in the 3 phase networks and is distinctly available as pure zero sequence component in the neutral.

Thus by including additional resistance or impedance in the neutral circuit, we can minimize the zero sequence component. The objective is to minimize earth fault, short circuit current and resulting damage to equipments and operating personnel.

Your last question:

Power Trasfer Capability is determined by the basic fundamental equation as follows

P = [Vs.Vr/X]Sin(angle 1 - angle 2]

where Vs is sending end voltage magnitude and angle 1 is its phase angle

Vr is the receiving end bus voltage magnitude and angle 2 is its phase angle

X is the series impedance of the transmission lines.

Shunt reactances affects the magnitudes Vs, Vr and hence affects the power flow P.

The dependency of P over magnitude Vs, Vr is lesser compared to angular separation between sending and receiving bus voltages in a pure inductive network. When resistance is dominant in the series circuit, voltage magnitude also affects the value of P significantly

Dear All,

Many thanks for helping me out. Grateful to you all.

Regards

Shazz

Any unbalanced system of voltage / current can be resolved in three balance systems known as Positive Sequence component which rotates in same direction and with same frequency as the original (unbalanced) system, Negative phase sequence which rotates in opposite direction but with the same frequency and zero sequence which does not rotate but varies with same frequency as the original unbalanced system. Each rotating electrical machine offers different reactance to these three phase sequence quantities and are known as positive, negative and zero sequence reactnce respectively. Zero sequence component comes into picture only when system involves ground (as in case of single earth fault or double ground to earth fault). But three phase to ground fault being symmetrical fault does not in unbalance and therefore does not result in either negative sequence or zero sequence component.

Since earth fault current passes through ground, then in system involving grounding of neutral, this current flows through the neutral. This current is then controlled by all the three reactnces. In order to control this fault current to a safe value, resistance is added in the neutral grounding circuit.