MechEdetour
Member
- Location
- NY, USA
- Occupation
- Design Engineer
Hello folks.
I am trying to make sense of a pretty common industry practice for HRGs... the use of a zig-zag transformer to produce an artificial neutral.
For some context, existing ungrounded systems that would like to upgrade to high resistance grounded but lack a system neutral need an additional means to measure the fault current. One way to go about this is by deploying a zig-zag transformer that provides the required neutral, and therefore a means for the fault current to be measured through the grounding resistor.
Typically this is done with an individual 3-phase zig-zag transformer with no secondary winding (err, autotransformer?). With that being said, three single-phase transformers can also be wired to yield the same zig-zag setup, also resulting in an artificial neutral... some HRG vendors go about it this way. I am trying to better understand the setup and the individual transformers sizing + fault current magnitude at different parts of the circuit.
Please forgive the crude sketch, but assuming a fault current of 5A, is my logic correct in that each CPT only carries 1/3 the fault current?
Each CPT is 480V Primary/240V Secondary, so doing a transformer sizing calc, my 1.67A "load" @ 240V would require at a minimum a 240V*1.67A = 400VA transformer. Since pulsing current is usually another 5A (for a total of 10A), let's say 800VA. Am I going about this the right way?
My questions:
1) Even though these are single-phase CPTs, they are wired in a 3-ph setup. Do I need to treat my calcs that same as a 3-ph setup and consider the 1.73 multiplier in my calcs?
2) Is the primary current on my CPTs actually 1.67A per primary? I do have a 2:1 transformer ratio between the primary and secondary of the CPTs so it could be that it's only .83A on the primary? I don't think so since this isn't a conventional use of a transformer, but I am having my reservations.
I am trying to make sense of a pretty common industry practice for HRGs... the use of a zig-zag transformer to produce an artificial neutral.
For some context, existing ungrounded systems that would like to upgrade to high resistance grounded but lack a system neutral need an additional means to measure the fault current. One way to go about this is by deploying a zig-zag transformer that provides the required neutral, and therefore a means for the fault current to be measured through the grounding resistor.
Typically this is done with an individual 3-phase zig-zag transformer with no secondary winding (err, autotransformer?). With that being said, three single-phase transformers can also be wired to yield the same zig-zag setup, also resulting in an artificial neutral... some HRG vendors go about it this way. I am trying to better understand the setup and the individual transformers sizing + fault current magnitude at different parts of the circuit.
Please forgive the crude sketch, but assuming a fault current of 5A, is my logic correct in that each CPT only carries 1/3 the fault current?
Each CPT is 480V Primary/240V Secondary, so doing a transformer sizing calc, my 1.67A "load" @ 240V would require at a minimum a 240V*1.67A = 400VA transformer. Since pulsing current is usually another 5A (for a total of 10A), let's say 800VA. Am I going about this the right way?
My questions:
1) Even though these are single-phase CPTs, they are wired in a 3-ph setup. Do I need to treat my calcs that same as a 3-ph setup and consider the 1.73 multiplier in my calcs?
2) Is the primary current on my CPTs actually 1.67A per primary? I do have a 2:1 transformer ratio between the primary and secondary of the CPTs so it could be that it's only .83A on the primary? I don't think so since this isn't a conventional use of a transformer, but I am having my reservations.