Variable Speed Drive - Earth fault path

2011

I want to identify the path of an earth fault for a motor that is fed from a VSD (if a phase to earth fault were to occur at the motor in the field).

I know that if it was a DOL (direct on line) motor (not connected via a VSD) the earth fault loop impedance = Ze + R1 + R2

Where:

Ze = external impedance (from the star point of the transformer to the MCC)

R1 = the resistance of the phase conductor to the motor in the field

R2 = the resistance of the protective conductor back to the MCC

However for a motor connected via a VSD I am not so sure as to what the primary earth fault path is. I feel that it is from the motor back to the VSD (as this is the source), rather than back to the start point of the transformer.
This would mean that the VSD would have some form of star point tied to earth.

Has anyone any thoughts on this?

Thanks in advance!

steifanc

the earth path is still the same , from source to the neutralization point to dissipate the energy, a vsd dosnt create a star point , its process is
uncontrolled rectifier circuit
controlled full wave rectifier circuit
intermediate circuit
the inverter circuit
transistor circuit
pulse amplitude modulation
pulse width modulation

2011

steifanc wrote: »

the earth path is still the same , from source to the neutralization point

I appreciate that the star point of the ESB transformer is tied to earth. My question is whether the secondary side of the VSD has a star point that is tied to earth or not as this will determine the primary earth fault path. Recently I talked directly to the manufacture who were not sure. They felt that after this may be the case, but they need to check. A formal reply is due shorty.

Due to external conditions an earth fault loop impedance tester can not be used.

its process is
uncontrolled rectifier circuit
controlled full wave rectifier circuit
intermediate circuit
the inverter circuit
transistor circuit
pulse amplitude modulation
pulse width modulation

I understand the fundamentals of the operation of a VSD.

As the incoming phases are not directly connected to the outgoing (a continuity test meter would show them to be electrically isolated) I was trying to establish where the fault current would flow to.

steifanc

yes the phz are in theory isolated electrically . but not the earth , you can not break the earth path with any device . you will find that in the etci regs.
if you are familiarly with the process of the vsd then why do you think you would have a star point , if you think about that I'm sure it will come to light .
the earth fault path is no different

2011

steifanc wrote: »

you can not break the earth path with any device . you will find that in the etci regs.

I am not suggesting that any earth is to be broken

if you are familiarly with the process of the vsd then why do you think you would have a star point

Just beacause I understand the principle of operation does not necesserily mean that I am completly sure of the primary earth fault path. As two very experienced electrical engineers have expressed very different views I was looking for another opinion.

the earth fault path is no different

I have thought about this at length and I think you are correct, thanks.

steifanc

Just because I understand the principle of operation does not necessarily mean that I am completely sure of the primary earth fault path. As two very experienced electrical engineers have expressed very different views I was looking for another opinion.

what i meant was understanding the principle would tend to lead you away from the idea that a star point is being created , why , how , and for what reason would you create a star point . i don't think there are any , so it would make scene for the earth path to be directly linked to the existing network

"through a protection wire the earth terminal of the frequency converter is connected to the zero conductor in the supply lead of the installation , if there is no protective multiple earthing in the installation in question one must have the electricity suppliers permission to make one . through a protection wire the earth terminal of the frequency converter is connected to the earth electrode . this method is applied where protective multiple earthing is not allowed . "

facts worth knowing about frequency converters
drives and controls by danfloss

taking that on board would indicate that Zs=Ze+Z1+Ze
where

Ze = external earth impedance
Z1 = phase conductor impedance
Z2 = circuit protective conductor (CPC) impedance