Three-phase transformer connections, banks and testing

Lesson 8 introduced transformers and listed the common three-phase connections. In the field, electricians build three-phase banks from single-phase units, connect and phase-check new transformers, parallel transformers, keep a system running on an open delta when one unit fails, provide a ground reference on an ungrounded system, and test transformers before energizing them. Each task depends on understanding how voltages and currents relate in each winding connection. This lesson works through those relationships, the 30° phase shift, building banks with correct polarity, open-delta operation, paralleling conditions and load sharing, grounding transformers, sizing, and the acceptance tests performed before energization.

Voltages and currents in each connection

In a wye winding, each coil is connected between a line and the neutral, so the coil voltage is the line-to-neutral voltage, \(V_{\text{line}}/\sqrt3\), and the coil current equals the line current. In a delta winding, each coil is connected between two lines, so the coil voltage equals the line voltage, and the coil current is \(I_{\text{line}}/\sqrt3\) for balanced loads (Lesson 6).

When single-phase transformers form a three-phase bank, each unit's rating and voltages must suit its position:

The 30° phase shift

Delta-wye and wye-delta connections shift the secondary voltages by 30° relative to the primary, because a delta coil voltage is a line-to-line voltage while a wye coil voltage is a line-to-neutral voltage, and these are 30° apart. North American standards (ANSI) specify that for delta-wye and wye-delta transformers, the high-voltage side leads the low-voltage side by 30°. Delta-delta and wye-wye connections produce no shift.

The shift does not matter to the loads, which see a normal three-phase supply, but it matters whenever two sources are connected together: two transformers can be paralleled, or a load transferred between two sources without interruption, only if their secondaries have the same phase shift relative to a common primary. A delta-wye transformer cannot be paralleled with a delta-delta transformer, even with identical voltages, because their secondaries are 30° apart: connecting them would create a large circulating current, effectively a short circuit through a voltage difference of about \(2\sin 15° = 52\%\) of rated voltage.

Building a bank: polarity and connections

Single-phase transformer terminals are marked H1, H2 (high voltage) and X1, X2 (low voltage). H1 and X1 have the same instantaneous polarity. In a subtractive polarity transformer (standard for larger units and higher voltages), H1 and X1 are physically opposite each other; in an additive polarity transformer (common for small distribution units), they are diagonally opposite. Polarity can be checked with a simple test: connect H1 to the adjacent low-voltage terminal, apply a low voltage to the high-voltage winding, and measure between H2 and the other low-voltage terminal; a reading less than the applied voltage indicates subtractive polarity, more indicates additive.

Connecting a bank with one unit reversed produces wrong voltages. In a wye secondary, a reversed coil puts its voltage 180° out of position, so two of the line-to-line voltages read only about 120 V instead of 208 V. In a delta secondary, a reversed coil makes the voltage around the closed delta twice the coil voltage instead of zero, causing a large circulating current when the delta is closed. The standard practice before closing a delta is to leave one corner open and measure across the opening: it should read zero (or near zero); a reading of twice the coil voltage means one coil is reversed.

Open-delta operation

A delta-delta bank can keep supplying three-phase power with only two transformers, connected in open delta (V-V). This is used to keep a load running when one unit fails, and for new installations expecting future growth, where the third unit is added later. Because the two remaining transformers carry line current rather than coil current (now \(\sqrt3\) times larger relative to their rating), the bank capacity is reduced:

Paralleling transformers

Paralleling transformers, connecting their primaries to the same supply and their secondaries to the same bus, increases capacity or provides redundancy. Conditions for satisfactory parallel operation:

• The same voltage ratios (and tap settings): any difference drives a circulating current between them even at no load.
• The same phase shift and polarity: as explained above, transformers of different phase displacement cannot be paralleled.
• The same phase sequence on the secondary.
• Similar percent impedances, ideally within about 7.5% of each other, so they share load in proportion to their ratings, and similar X/R ratios.

Load divides between paralleled transformers in inverse proportion to their impedances expressed in ohms. When the per-unit impedances on their own ratings are equal, they share in proportion to their kVA ratings. When they differ, the unit with lower impedance takes more than its share and may be overloaded before the other reaches its rating.

Paralleling also raises the available fault current on the secondary bus (both transformers contribute), which may exceed the ratings of existing equipment (Lesson 11). Many installations therefore use a normally open tie breaker between two transformers rather than permanent paralleling.

Grounding transformers

An ungrounded delta system has no neutral to ground (Lesson 7). A grounding transformer creates one. The zigzag (interconnected star) transformer has each phase's winding split between two legs of the core, connected so that normal balanced currents see a high impedance and the transformer draws only magnetizing current, while zero-sequence (ground-fault) currents flow freely through the neutral, which can be grounded solidly or through a resistor. A wye-delta transformer with the wye neutral grounded and the delta closed but unloaded does the same. Grounding transformers are used to convert ungrounded systems to high-resistance grounding, and on the collector systems of wind and solar plants.

Sizing a transformer

Transformer sizing combines the load calculation with practical allowances:

• The calculated demand load, using code demand factors, or measured demand (Lesson 5).
• Future growth, often 20% to 25% spare for commercial buildings.
• Harmonic content: derating or a K-rated unit for heavy nonlinear loads (Lesson 3).
• Ambient temperature and altitude: ratings assume an average ambient of 30 °C and maximum of 40 °C at altitudes up to 1,000 m; hotter or higher locations need derating.
• Motor starting: large motors cause voltage dips proportional to their starting kVA and the transformer impedance; a transformer that carries the running load may be too small to start a large motor without unacceptable dip.
• Efficiency: transformers are often lightly loaded most of the time, so low no-load (core) losses matter; energy efficiency regulations set minimum efficiencies for distribution transformers.

Testing before energization

New and repaired transformers are tested before being placed in service. Common field tests include:

• Visual and mechanical inspection: nameplate data match the drawings, connections are tight, grounding and bonding complete, taps set as specified, no shipping damage, clearances and ventilation adequate.
• Insulation resistance: a megohmmeter test between each winding and ground and between windings, typically at 1,000 V DC for low-voltage windings and higher for medium-voltage, with readings compared against manufacturer data and recorded with temperature. A polarization index (the ratio of 10-minute to 1-minute readings) indicates insulation condition on larger units.
• Turns ratio test (TTR): applying a low voltage and measuring the ratio on each phase and tap, which should be within 0.5% of nameplate; it also confirms polarity and phase relationship.
• Winding resistance: detects loose connections, broken strands and tap changer problems; the three phases should match closely.
• For liquid-filled units, oil tests (dielectric strength, moisture, dissolved gases) at commissioning and periodically thereafter.

After energization, the electrician measures secondary voltages line to line and line to neutral, checks phase rotation, and confirms that the voltages are within the expected range, adjusting taps if necessary with the transformer de-energized.