3
\$\begingroup\$

The circuit that describes a single line to ground fault in a system with an ungrounded neutral is: enter image description here

I neglect capacitance between phases and only look at capacitance to ground. The definition of zero sequence current is $$I^{(0)} = \frac{1}{3}(I_1 + I_2 + I_3)$$ But if apllied to the circuit above, Kirchoff current law should state that:$$(I_1 + I_2 + I_3) = 0$$ So how can there be a zero sequence current in this circuit? What am I missing here?

\$\endgroup\$
1
  • \$\begingroup\$ If I take two actual capacitors and connect them between phases A-B and A-C will a zero sequence current flow in this circuit? Spice says that at each moment in time the sum of all phase currents will be 0. \$\endgroup\$ – Cmac c May 10 '20 at 18:40
1
\$\begingroup\$

The parasitic phase-ground capacitance is a load in all three sequence networks. Even though your zero sequence source impedance is infinite (delta), the capacitance provides a path for sequence current to flow. In the figure below i show the 3 sequence networks for your system. The red lines show how they are interconnected for an A-phase to ground fault. This should help.

enter image description here

If you calculate the B & C phase currents in the parasitic capacitance (from the sequence currents you calculate with the above circuit) you will see the phase current path clearer.

enter image description here

UPDATE: Added further explanation on calculating the sequence components and phase currents in the fault, in the ph-ground capacitance, and from the transformer. Note the reference arrows i chose (if you use opposite direction just flip phase angle by 180):

enter image description here

enter image description here

\$\endgroup\$
1
  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – Voltage Spike Jun 12 '20 at 16:39

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.