Can 1LG fault be higher than 3-phase short circuit level in a VFD? 1

[Dvhez]

I saw a specification from a vendor that established a one-phase short circuit of 60 kA and a three-phase short circuit level of 40 kA. I thought three phase fault was the more severe.

 

[jghrist]

1Ø fault levels will be higher close to the secondary of a delta-grdY transformer because the primary system zero-sequence impedance is short circuited in the zero-sequence network. Z1+Z2+Z0 < 3*Z1

 

[HamburgerHelper]

Fault energywise 3 phase fault is the worst always. The maximum amount of fault current is different than total fault energy.

Take the fault current equations for a 3 phase fault V1/Z1 = 3*V1/(Z1+Z2+Z0), assume Z1=Z2 and look at what the ratio is of Z1 to Z0 when the current values of the two are equal. A single line to ground fault will be greater than three phase like jghrist said, when 3Z1 > (Z1+Z2+Z0) or 3Z1 > (2*Z1 +Z0) if you assume Z1=Z2 or 3/2 > Z0/Z1 You get this when you are close to delta-grdY transformer because thevenin Z0 impedance will be really low when you are next to a ground source.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.

 

[che12345]

Dear Mr Dvhez

Q I saw a specification from a vendor that established a one-phase short circuit of 60 kA and a three-phase short circuit level of 40 kA. I thought three phase fault was the more severe.
A.1 Reference IEC 60909-0
A.1.1 General ... In case of ...conductive part between [one line conductor and local earth], the following two cases must be clearly distinguished with regard to their different physical properties ....
A.1.2 Definition
a)... line-to-line short-circuit (I"k3) is ... conductive path between two or more line conductors with or without earth connection.
b) ...line-to-earth short-circuit (I"k1) is .... conductive path in a solidly earthed neutral system or an impedance earthed neutral ...
A.1.3 calculation of short-circuit currents
A.1.3.1 ... The type of short-circuit which leads to the [highest short-circuit current] depends on the value of the positive-sequence, negative-sequence, and zero-sequence short-circuit impedances of the system. See fig 10 ....illustrates this ...
A.1.3.2 For the common case when ..... the highest initial short-circuit current will occur for the [three-phase short circuit]. However, for ...

Che Kuan Yau (Singapore)

 

[waross]

Are you asking about the output fault current of a VFD or the maximum allowable fault level of the supply system?
100 kA seems a little high for a VFD output.
For a transformer consider a 120/208 Volt transformer.
The fault current at 120 Volts is driven through the winding impedance by 120 Volts x %Z.
At 208 Volts the zero sequence impedance has doubled but the voltage x %Z has only increased by root 3.
Hence a higher fault current line to neutral than line to line.
40 kA x 1.73 = 69.2 kA, rounded down to 60 kA.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr Dvhez 2 Apr 19 11:50

A1. I was expecting for the clarification, but to no avail so far (5/4/2019).

Dear Mr waross
Info: " Are you asking about the output fault current of a VFD or the maximum allowable fault level of the supply system? 100 kA seems a little high for a VFD output."
A2. It is unlikely for any one to install [any] form of short-circuit protective device(SCPD) between the VFD [out put] and the motor. Therefore, the 3-phase or 1-phase kA ratings at the [output] of the VFD may? not be the question asked.
A2.1 However, it is a [must] to have a (SCPD) placed at the [input] i.e. ahead of the VFD, in order to protect it. Therefore, the rating of the (SCPD) must be adequately rated for the system short-circuit levels and shall have the characteristic recommended by the manufacturer, in order to protect the VFD.
FYI: the SCPD recommended by the manufacturer must be respected in order to protect her device; the manufacturer knows better.

Che Kuan Yau (Singapore)

 

[HamburgerHelper]

Fault current contributions from power electronics is current limited. This isn't for a VFD, but type 4 wind farms with AC-DC-AC conversion can at most only put out 1.1 pu. Solar farms it is like 1.05 pu. The fault current on the other side of a VFD is going to be dependent on the VFD and not really what is upstream of it.

------------------------------------------------------------------------------------------
If you can't explain it to a six year old, you don't understand it yourself.