Zero-Sequence Impedance (Z0) in Transformers

When a ground fault occurs, the currents flowing in the three phases have a component that is equal and in phase in all of them: the zero sequence. The path that current finds depends mostly on the transformers, on how their windings are connected and on where the neutral is grounded. The zero-sequence impedance (Z0) of a transformer sums up that path in one number, and ground fault currents, as calculated in studies and seen by the protections, depend on that number. For the zero-sequence diagram of a transformer, see symmetrical components and the zero-sequence diagram of a transformer.

What zero-sequence impedance is

It is the impedance a winding presents to three equal, in-phase currents, like those of a ground fault. The general concept is in zero-sequence current. There is a path for those currents only if the winding has an accessible, grounded neutral, that is, if it is wye with neutral (YN) or zigzag with neutral (ZN). A delta winding, or a wye without a grounded neutral, does not let zero sequence in from the network.

That is why Z0 is not a single value per transformer: it is defined for each winding with an accessible neutral and depends on what happens in the other windings.

What it depends on

• Delta windings: a delta, whether the low-voltage winding or a tertiary, lets zero sequence circulate inside it. With a delta, the Z0 seen from the wye winding is close to the short-circuit impedance between that winding and the delta.
• Core type: in a three-limb core without a delta, the zero-sequence flux has no return path through the iron and closes through the air and the tank, which gives a relatively low, nonlinear Z0. In a five-limb core, or in a bank of single-phase units without a delta, Z0 can be very high.
• Neutral grounding: a neutral resistor or reactor adds to the transformer Z0 in the fault calculation.
• Tap changer position, which changes the windings in the circuit.

Why it is measured

For studies

The short-circuit study needs the Z0 of each transformer to calculate the single-phase fault current and how it splits between sources. If the value used is theoretical or from a similar transformer and not the actual one, the calculated ground fault currents can be far from the real ones.

For protection

• Ground overcurrent 51N/50N: its pickups and times are coordinated with ground fault currents, which depend on Z0. We explain this in phase and ground overcurrent protection.
• Directional ground 67N: a transformer with a grounded neutral is a zero-sequence source; its Z0 defines how much current it contributes and, in some schemes, the neutral current is used for polarization. We cover it in directional ground overcurrent.
• Distance 21: the ground compensation factor of distance elements is calculated with the Z0 of the line, not that of the transformer. But the Z0 of the transformers at each end changes how the fault current splits and, with it, what the ground elements see.
• Restricted earth fault and grounding transformers: in a zigzag or wye-delta transformer used to ground a system, Z0 directly sets the ground fault current available.

What the measurement reveals

Which equipment and levels it applies to

It applies to power transformers and autotransformers at high and extra-high voltage with an accessible neutral, to YN medium-voltage transformers in substations and industry, to generating unit and solar plant transformers, and to grounding transformers. It does not apply to windings without an accessible neutral.

Standards and what they require

• IEC 60076-1 describes the measurement of zero-sequence impedance as a special test, performed when agreed, and requires limiting the current to avoid overheating the metallic parts.
• IEEE C57.12.90 describes the test method for liquid-immersed transformers.
• Neither sets an acceptance value: Z0 depends on the design. The criterion is consistency with the factory value and with the value used in studies and settings.

How to interpret it, in general

In the test the line terminals of the winding are tied together and voltage is applied against the neutral, with the other windings in a defined state. The Z0 per phase comes from three times the ratio of voltage to total current, and is expressed in ohms or as a percentage of the base impedance of the winding. There are three typical interpretation errors. The first is confusing the total current with the phase current, which changes the result by a factor of three. The second is comparing the factory value with one measured in a different winding configuration. The third is measuring with the neutral still connected to its resistor or reactor.

For faults between phases the equivalent figure is the short-circuit impedance.

Testing and studies with Tiza Group

Our team measures zero-sequence impedance and the other power transformer tests in substations and industry at medium, high and extra-high voltage, in Colombia or elsewhere on request. We can take the measured value to the short-circuit study and to the review of the ground settings, adapted to the local code and the utility’s requirements. See our power transformer testing and the protection coordination study, or write to us.

Related guides

• Zero-sequence current
• Short-circuit impedance (%Z)
• Directional ground overcurrent (67N)
• Restricted earth fault protection 87N
• Short-circuit study
• Transformer marshalling box
• Transformer Symmetrical Components: Zero-Sequence Diagram

References

• IEC 60076-1, Power transformers – Part 1: General.
• IEEE Std C57.12.90, Standard Test Code for Liquid-Immersed Distribution, Power, and Regulating Transformers.
• IEEE Std C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.

Frequently asked questions