Zigzag Transformer
A zigzag transformer is a three-phase autotransformer with interconnected star windings that provides a low-impedance path for zero-sequence (ground-fault) currents while drawing negligible current under normal conditions. It is used to create an artificial neutral on delta or ungrounded systems, enabling system grounding, ground-fault protection, and harmonic mitigation.
On the shop floor, zigzag transformers are installed in power distribution areas, such as near main breakers or transformer secondaries, to ground delta-connected or ungrounded systems. They are paired with a Neutral Grounding Resistor (NGR) to control ground-fault current, allowing protective relays to detect and clear faults without damaging equipment. They also mitigate triplen harmonics from non-linear loads like VFDs, improving power quality and reducing overheating. These transformers are typically short-time rated (e.g., 10 seconds) for fault duty and require proper sizing, labeling, and inspection per IEEE-32 and OSHA/WorkSafeBC standards.
- Improper sizing and misapplication with NGR, leading to ground-fault current too high (overheating) or too low (relay failure to detect faults).
- Incorrect connection point or missing isolation, such as installing downstream of main breakers or bonding multiple buses to one unit, altering grounding topology and violating coordination.
- Lack of inspection, labeling, and arc-flash assessment, including degraded insulation, loose terminations, or bypassed NGR, creating unrecognized hazards and non-compliance with OSHA/WorkSafeBC.
How does the zigzag winding create a low-impedance path for ground-fault current but high impedance for normal currents?
The zigzag transformer has two identical windings per phase on different core legs, connected in a wye pattern. Under balanced conditions, opposing fluxes cancel, drawing only magnetizing current. For zero-sequence (ground-fault) currents, all phases are in phase, and the winding presents low impedance, allowing fault current to flow through the neutral and NGR.
How is a zigzag grounding transformer sized and coordinated with protective relays?
Sizing is based on system voltage and desired maximum ground-fault current via NGR selection. The transformer is short-time kVA rated (e.g., 10 seconds) per IEEE-32. Relays are set to operate at currents slightly below the maximum, with time-current coordination matching transformer/NGR thermal limits and breaker curves.
How does a zigzag transformer affect electrical safety documentation under WorkSafeBC?
It impacts single-line diagrams (showing connection point and NGR rating), arc-flash assessments (reflecting ground-fault contribution), safe work procedures (requiring specific lockout/tagout for testing), and maintenance testing (verifying continuity and insulation). WorkSafeBC inspectors look for clear labeling and current schematics.