Three-Limb vs Five-Limb Transformer Core: What Changes

What sets one core apart from another

A three-phase transformer can have a three-limb, a five-limb or a shell-form magnetic circuit. There is also a seven-leg shell-form variant. A bank of three single-phase transformers has three separate magnetic circuits.

The guide to transformer parts describes how each core is built. This guide explains the electrical consequence, according to IEC 60076-8, clauses 2.3 and 4.4. The difference appears when the transformer receives a set of unbalanced three-phase voltages whose sum is not zero, that is, with a zero-sequence component.

That situation occurs in a ground fault on the network. It can also be described the other way around: a zero-sequence current with no ampere-turns balancing it in any other winding. That current acts as magnetizing current and is governed by a zero-sequence magnetizing impedance, across which a zero-sequence voltage drop develops.

Three-limb core

The positive- and negative-sequence flux components in the limbs with windings add up to zero at every instant. They cancel each other through the yokes. The zero-sequence residual flux does not have that path and must return from yoke to yoke outside the excited winding (IEC 60076-8, clause 2.3.1). For more detail on the zero-sequence diagram of a transformer, read symmetrical components and the zero-sequence diagram of a transformer.

That external flux meets a high reluctance. For a given flux, that is, a given applied zero-sequence voltage, a considerable magnetomotive force is needed. A high magnetizing current is therefore required. In the electrical circuit, this shows up as a relatively low zero-sequence magnetizing impedance. That impedance varies nonlinearly with the magnitude of the zero-sequence component.

The inverse effect also counts. An uncompensated zero-sequence current acts as magnetizing current. The result is a superimposed asymmetry in the phase-to-neutral voltages, which is the zero-sequence voltage component. More on neutral current and geomagnetically induced DC in neutral current and geomagnetic DC.

Stray-flux heating

The zero-sequence flux of the yokes induces eddy currents and circulating currents in the clamping structure and in the tank. This produces additional losses in those parts. It can also increase the eddy-current losses in the windings.

For this reason there are limits on long-duration neutral current in service. Clause 4.8 develops them.

Reference values

In three-limb transformers with a flat-plate tank, the per-unit zero-sequence impedance is on the order of 0.25 to 1.0. That value corresponds to a neutral current 3 × I0 equal to the rated current of the winding (clause 4.3). A thin corrugated-sheet tank behaves differently, because the parasitic currents circulate throughout the tank.

The same clause explains that this impedance is nonlinear with current. It varies from one design to another. That is why the manufacturer measures the zero-sequence impedance when it is requested (IEC 60076-1, clauses 10.1.3 and 10.7).

In a three-limb YNyn transformer with no additional delta winding, the magnetizing impedance is not negligible. It lowers the effective through impedance to about 90 % to 95 % of the positive-sequence short-circuit impedance. With five limbs or with a shell-form core there is no such reduction (clause 4.3.1).

Five-limb core, shell-form core or single-phase bank

In these designs the zero-sequence flux has return paths through the parts of the magnetic circuit that carry no winding (IEC 60076-8, clause 2.3.2):

• the outer limbs with no winding, in the five-limb core;
• the outer parts of the frame, in the shell-form core;
• the two legs without winding between the limbs with winding, in the seven-leg shell-form core.

The zero-sequence flux meets a low reluctance. That is equivalent to a very high magnetizing impedance, similar to what the normal positive-sequence voltage sees. A three-phase bank of single-phase transformers reacts the same way, because its magnetic circuits are independent for any service voltage.

This very high impedance has a limit. When the parts without winding reach saturation, the impedance drops. The magnetizing current becomes distorted, with peaks.

Because of these phenomena it is common to equip these transformers, or the banks, with delta stabilizing windings. This is stated in clause 2.3.2.

Saturation under abnormal voltage

Clause 4.9 describes what happens when the applied voltage is abnormally high. The core material saturates during part of each half cycle. The reluctance increases considerably and the source delivers a magnetizing current with peaks and greatly increased.

With saturation, a significant flux also appears outside the core steel, between the core and the winding. That flux can induce high eddy currents in metal parts outside the windings. The result is local heating and discharges through unintended contacts.

There can also be local saturation from excessive leakage flux during a heavy overload. Part of that flux returns through the magnetic circuit. Under those conditions the service voltage is also likely to be abnormal, and the combined result can be unplanned saturation in certain areas of the core.

What it means for your installation

• When buying or specifying: ask for the type of magnetic circuit and the winding connection. Both together give the zero-sequence impedance.
• When studying ground faults: do not use the same zero-sequence impedance for a three-limb and a five-limb transformer. If the value decides a setting, request the measurement as a special test.
• When loading the neutral: a three-limb distribution transformer with a Yyn connection is generally not suitable for feeding phase-to-neutral loads. If the neutral carries more than about 10 % of the rated winding current, the voltage asymmetry becomes objectionable (clause 4.8). That is why Dyn or Yzn connections are preferred.
• When diagnosing noise or heating: zero-sequence flux and saturation are possible causes. The guide on abnormal noise and vibration places them among the other causes.

We can help with your transformer

At Tiza Group we carry out power transformer testing and commissioning in Colombia, or elsewhere on request, including zero-sequence impedance measurement. With the transformer data we also prepare protection coordination studies, remotely and worldwide. If you need to define or verify this data, we will quote the work.

Related guides

• Transformer core and windings: parts
• Zero-sequence impedance test on power transformers
• No-load excitation test of power transformers
• Transformer with abnormal noise or vibration
• Zero-sequence current
• Transformer Neutral Current and Geomagnetic DC (GIC)
• Transformer Symmetrical Components: Zero-Sequence Diagram

References

• IEC 60076-8, power transformers: application guide, clauses 2.3, 4.3, 4.4, 4.8 and 4.9.
• IEC 60076-1, power transformers: general requirements, clauses 10.1.3 and 10.7.

Frequently asked questions