Neutral Grounding Transformer
A neutral grounding transformer (NGT) is a specialized grounding transformer, typically a three-phase dry-type zigzag auto-transformer, used to create a system neutral on ungrounded or delta-connected three-phase systems. It connects this neutral to ground through a neutral grounding resistor (NGR) to control ground-fault current and voltage, enabling detection and limiting damage. Designed per IEEE-32 and CSA C22.2 No. 295, it provides high-impedance grounding for industrial power systems.
On a manufacturing shop floor, an NGT is installed on medium-voltage systems (e.g., 2.4–13.8 kV) to create a neutral point for high-resistance grounding via an NGR. This limits ground-fault current to 5–10 A, reducing arc-flash energy and allowing continued operation during a single line-to-ground fault for controlled shutdown. It supports protective relaying and fault detection on delta systems, and is critical in mining and heavy industry where continuous monitoring of NGR integrity is required per CEC. The NGT is typically located in electrical rooms near main switchgear, integrated with ground-fault relays and alarms to meet code requirements for automatic de-energization if neutral continuity is lost.
Why use a neutral grounding transformer instead of solidly grounding the system neutral?
Solidly grounded systems allow high ground-fault currents, increasing arc-flash energy and damage. An NGT with an NGR provides high-resistance grounding, limiting fault current to manageable levels (e.g., 5–10 A) while maintaining phase-to-ground voltages within limits. This is preferred in industrial and mining applications per CEC where continuity of service during a single ground fault is important.
How does the zigzag connection in a neutral grounding transformer work electrically?
Each phase winding has two sections connected in zigzag: one section to its own line, the other to a different line, so vector sums cancel under balanced conditions. Under a single line-to-ground fault, zero-sequence currents flow through the zigzag windings into the neutral, passing through the NGR to ground, providing a controlled, measurable fault current path.
What are the code conditions for using a neutral grounding transformer on a system serving line-to-neutral loads?
Per CEC Section 10, neutral grounding devices are permitted only when all neutral conductors are insulated to nominal system voltage. For line-to-neutral loads, the system must automatically de-energize upon a ground fault, a grounded neutral on the load side, or loss of neutral continuity. Continuous monitoring of neutral continuity and NGR status is mandatory for compliance.