Grounding Transformer: Zigzag and YNd, Protection, Tests

Short answer

A grounding transformer creates an artificial neutral in a system that does not have one, for example the delta side of a transformer. Under normal conditions it carries almost no current; during a ground fault it offers a low-impedance zero-sequence path, which lets the ground protection detect the fault and limits the overvoltages on the healthy phases.

What a grounding transformer is

Many medium-voltage systems start out without an accessible neutral: the delta secondary of a transformer, the tertiary of an autotransformer, or the collector network of a solar or wind plant with delta-connected transformers on the medium-voltage side. Without a neutral, a ground fault produces very little current (only the capacitive one) and the healthy phases rise toward the phase-to-phase voltage. The ground protection barely sees the fault and the insulation is subjected to sustained overvoltages.

The grounding transformer solves this by creating an artificial neutral that is connected to ground solidly or through a resistor or a reactor. Its job is not to transfer power, but to offer a low and known zero-sequence impedance.

Main parts

Three-phase, three-leg core.
Windings, in one of two connections:
- Zigzag (ZN): each phase is split into two half-windings located on different legs and connected in opposite directions. It is the most common connection.
- Grounded wye with delta (YNd): the wye provides the neutral and the delta lets the zero-sequence current circulate. The delta can be closed or can feed station service.
Phase and neutral bushings, with a current transformer in the neutral for the protection.
Neutral grounding resistor or reactor, when the system is impedance grounded to limit the fault current.
Transformer accessories: tank, oil or dry insulation, Buchholz relay, pressure relief valve, thermometers and control cabinet.

How it works

In normal operation the three phases are balanced and the transformer only draws its magnetizing current. Its positive- and negative-sequence impedance is very high.

During a ground fault, zero-sequence current appears. In the zigzag, the two half-windings of each leg carry zero-sequence currents in opposite directions, their fluxes cancel and the impedance seen by that current is low, almost only the leakage impedance. The fault current 3I0 rises through the neutral and is shared equally among the three phases of the transformer (I0 in each). The zero-sequence current is therefore the variable that defines its design and its protection.

The transformer impedance and the neutral resistor together set the maximum ground fault current of the system and, with it, the sensitivity of the protection and the overvoltage on the healthy phases.

Types

High voltage: uncommon, because in high and extra-high voltage the transformers usually have a grounded wye neutral. It is seen on delta tertiaries and in special design cases.
Medium voltage in substations: the most common use, on the delta side of power transformers, with solid or resistance grounding.
Solar and wind plants: in collector networks, to limit temporary overvoltages during ground faults and to give a reference to the ground protection, according to the utility requirements; the network is described in solar plant collector network.
Industry: 4.16 kV, 6.6 kV or 13.8 kV systems grounded through a low- or high-resistance resistor, where the aim is to limit damage in motors and, with high resistance, to keep operating until the fault is located.

Associated protection

Neutral overcurrent (51N) with the neutral CT. It acts as backup for the ground faults of all circuits in the system and protects the transformer and the resistor against heating from uncleared faults. It must coordinate with the feeder ground protection. See phase and ground overcurrent.
Phase overcurrent (50/51) on the system connection side, for internal faults.
Restricted earth fault, which compares the sum of the phase currents with the neutral current and detects internal ground faults with good sensitivity. See restricted earth fault 87N/64REF.
Mechanical protection: Buchholz, pressure and temperature.
Neutral resistor supervision, in high-resistance grounded systems, to detect an open circuit that would leave the system ungrounded.

A trip of the grounding transformer must be handled in the scheme: the associated source is often tripped or blocked so that the system does not operate ungrounded.

On generators the neutral is often grounded with a distribution transformer and a resistor on its secondary. There, the stator ground fault is protected with 59N and, for the neutral end, with the 100 % stator ground fault 64S.

Commissioning and maintenance tests

Winding insulation resistance.
Power factor or tan delta on oil-filled units.
Winding resistance per phase.
Turns ratio and vector group, on YNd units with a secondary winding.
Zero-sequence impedance, the test that confirms the equipment does its job.
Excitation current.
Neutral CT tests (ratio, polarity and excitation) and neutral resistor tests (ohmic value and insulation).
Review of the control cabinet, alarms and trips, as described in transformer control cabinet verification, and protection tests with secondary injection.

Common failures

Overheating from ground faults that are not cleared in time, because the unit is rated for short duration.
Permanent neutral current from system unbalance, which heats the unit and can cause 51N trips.
Open or deteriorated neutral resistor, which leaves the system without a reference and nobody notices if it is not supervised.
Wrong neutral CT polarity, which makes the restricted earth fault protection trip for external faults.
Uncoordinated settings between the neutral 51N and the feeder ground protection.

We support you with your grounding transformer

At Tiza Group we test and commission grounding transformers, neutral resistors and their protection in substations, industry and solar plants, in Colombia or elsewhere on request. For protection settings and trip analysis we work remotely, worldwide. If you need testing or a review of ground trips, contact us.

References

IEEE C57.32: requirements, terminology and test procedures for neutral grounding devices.
IEC 60076-6: power transformers, reactors (includes neutral grounding transformers).
IEEE C62.92 series: guide for the application of neutral grounding in electrical utility systems.
IEEE 142: grounding of industrial and commercial power systems.
IEEE C37.91: guide for protective relay applications to power transformers.
J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions

Why does the grounding transformer carry neutral current when there is no fault?

Because any voltage or load unbalance to ground in the system produces a small zero-sequence current that returns through its neutral. If that current grows, look for single-phase loads to ground, unbalanced capacitances or a high-impedance fault before raising the 51N setting.

What happens if the grounding transformer goes out of service?

The system is left ungrounded: a ground fault no longer produces enough current for the protection and the healthy phases rise toward line voltage. That is why many schemes trip or block the associated source when the grounding transformer trips.

Can the grounding transformer withstand a ground fault indefinitely?

Normally not. It is usually specified with a short-time neutral current, of the order of seconds, and a much lower continuous current. The protection must clear the fault within that time, according to the nameplate and the specification.