Neutral Point Earthing Modes of Earthing Transformers: Resistor Earthing versus Reactor Earthing

Earthing Transformers

Field: Manufacturing

10Year<

China

Key Operating Principles

An earthing transformer (commonly Z-connected / zig-zag transformer) merely provides an artificial neutral point for the system. The actual system performance characteristics are governed by the device connected to its neutral terminal.

Neutral Grounding Resistor (NGR) vs Neutral Grounding Reactor (NER)

Item	Neutral Grounding Resistor (NGR)	Neutral Grounding Reactor (NER)
Grounding Component	Resistor R connected in series at neutral point	Inductor L connected in series at neutral point
Single-phase Earth Fault Current	Limited to tens to hundreds of amperes (value can be precisely set) Controllable	Determined by system capacitive current and reactor compensation degree Difficult to control accurately
Overvoltage Suppression	Resistive component dissipates energy, arcs are easy to extinguish Excellent performance in suppressing arc overvoltage	Pure inductive characteristic, compensates capacitive current; resonance overvoltage will occur under detuning condition Risk of resonance
Fault Feeder Selection & Protection	Large zero-sequence current, sensitive protection and reliable fault feeder detection Easy to implement	Small residual current, difficult fault feeder identification; dedicated fault detection equipment is required High implementation difficulty
Operation with Persistent Fault	Relatively large earth fault current, generally requires rapid fault clearance Short allowable fault duration	Low residual current; short-term earth fault operation is theoretically permitted (arc suppression coil mode) Short-time fault operation supported
Equipment Damage	Resistor heats up during faults, design must comply with short-time withstand current Resistor thermal rise needs verification	Low fault current leads to mild thermal stress on equipment Low thermal load
Adjustment Difficulty	Fixed resistor type is simple and reliable; additional mechanical structure is required for variable resistors	Arc suppression coil needs tap switching according to variation of system capacitive current (automatic tuning) Dynamic tuning is mandatory
Typical Application Scenarios	Industrial power distribution (6~35 kV), offshore platforms, mines, data centers, and scenarios requiring fast protection response	Urban medium-voltage power grids (large capacitive current), rural overhead lines (continuous power supply required under faults)
National & Industry Standards	GB/T 14285, DL/T 620	GB/T 14285, DL/T 620 (for arc suppression coil grounding)

Rockwill 15kV 20kV 22kV 30kV 33kV 35kV oil-immersed neutral grounding resistance to transformer (earthing transfoemer)

Neutral Point Earthing Modes of Earthing Transformers: Resistor Earthing versus Reactor Earthing

Rockwill Three Phase Cast Resin Dry Type Zigzag (Z-Type) Grounding Transformer 35kV 36kV 44kV 66kV

Resistor Grounding (NGR — Neutral Grounding Resistor)

Upon occurrence of a single-phase earth fault, the zero-sequence loop is jointly determined by the system capacitance and the neutral grounding resistor, with the resistor contributing the resistive current component.
Arcing ground overvoltage suppression: The resistive current hinders the restriking of the fault arc after current passes through zero crossing, thereby restraining arc restrike.
Resistance value selection: Calculated by the formula \(I_R = \dfrac{U_{ph}}{R}\). Generally, \(I_R \ge I_C\) (system capacitive current) is adopted to guarantee sufficient sensitivity of zero-sequence protection.

Reactor Grounding (Arc Suppression Coil Grounding)

The inductive current is opposite in phase to the capacitive current; these two currents compensate each other at the fault point, making the residual current close to zero and enabling spontaneous extinction of the fault arc.
Compensation degree is defined as \(\delta=\dfrac{I_L-I_C}{I_C}\). Over-compensation (\(\delta>0\)) is the prevailing operation mode to prevent system resonance risks.
The residual current still contains active power components caused by line losses as well as higher-order harmonics, so the residual current cannot be reduced to absolute zero.

Selection & Decision-Making Logic

Summary

	Neutral Grounding Resistor (NGR)	Reactor Grounding (Arc Suppression Coil)
Core Trade-offs	Higher fault current → reliable protection performance, requiring fast fault clearance	Low residual fault current → allows continuous operation under faults, yet poses difficulties in faulty feeder identification
Applicable Scenarios	Industrial facilities, mines, offshore platforms with safety as the top priority	Urban cable power grids & rural overhead line networks with power supply continuity as the primary requirement

Vziman earthing transformers are specially engineered to address the absence of natural neutral points within medium-voltage distribution networks. For systems configured with delta connections or with inaccessible neutral terminals, Vziman products adopt a zig-zag winding topology to artificially establish a stable neutral point. This structure lays the foundational condition for the deployment of arc suppression coil grounding or low-resistance grounding schemes, fundamentally improving the operational safety and power supply reliability of the power grid.

Edited From：Leon

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Topics:

Earthing Transformer

Resistor Earthing Versus Reactor Earthing

Neutral Point Earthing Modes

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