Mike Dolt Enterprises
Mike Dolt Enterprises
User: MechEdetour, Location: NY, USA
Asked on: Friday, December 13, 2024 at 3:29 pm EST
Answered on: Thursday, March 6, 2025 at 6:17 am EST
Question: 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?
Yes, your logic is correct. With a 5A fault current through the neutral grounding resistor (NGR), each of the three single-phase transformers in a zig-zag configuration will carry approximately 1/3 of that fault current, or roughly 1.67A.Here's why: Zig-Zag Operation
The zig-zag transformer's unique winding configuration (each leg is split into two halves, with one half connected to a different phase than the other) allows zero-sequence currents (like ground fault currents) to flow easily. Normal balanced phase currents effectively cancel each other out within the transformer.
Fault Current Path
When a ground fault occurs, the 5A current will flow from the faulted phase, through the system impedance, to ground, then through the NGR, and finally back to the transformer neutral point.
Current Division
At the transformer neutral point, the fault current splits equally into the three zig-zag transformer legs. Since each phase coil is connected in a zig-zag manner, and each phase is 120 degrees apart, the total current flows through each transformer, 1/3 per core.
Impedance Considerations
The impedance of the zig-zag transformer is designed to be low for zero-sequence currents, allowing the fault current to flow relatively unimpeded (aside from the NGR). The impedance is also designed to be high for balanced currents, preventing significant current flow under normal (non-fault) conditions.
Important Considerations
Transformer Sizing
The individual single-phase transformers need to be rated to handle this continuous 1/3 of the fault current. You can't simply pick any small control power transformers. You'll need to consider the RMS value of this continuous current when selecting the transformer VA rating.
Voltage Rating
The voltage rating of the single-phase transformers must match the phase-to-neutral voltage of the system.
NGR Selection
The NGR resistance value and wattage rating are crucial. The resistance determines the fault current magnitude, and the wattage must be sufficient to dissipate the heat generated by the fault current. Your assumption of 5A needs to be verified by calculating the required resistance based on the system line-to-neutral voltage and the desired fault current (Ohm's Law: R = V/I).
Transformer Connection
Make absolutely sure the transformers are wired correctly in the zig-zag configuration. An incorrect connection can lead to overheating and potential failure. Always refer to a wiring diagram and double-check the connections.*
Duty Cycle of Fault Current
While HRG systems aim to limit fault current, the duration of the fault current is still important. The transformer must be able to withstand the current for the expected fault clearing time. Most transformers are designed for short periods of overload current, but continuous operation at the specified fault current is often assumed.
In summary, your initial assumption of 1/3 fault current per transformer in a zig-zag configuration is correct for calculating the necessary transformer current rating. Flag for review