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Transformer Recommendations
General Recommendations
• Transformers should have multiple taps (e.g. ±2.5%, ±5%) so the operating voltage can be
adjusted to compensate for local voltage conditions.
• All transformers should include electrostatic shielding to minimize the effects of high-energy
transients from the utility.
• All conductors, sub panels, transformers, and other equipment between the array and the
interconnect must be capable of carrying the full array current under all conditions. This includes
the extreme case where all of the other loads being supplied by the common equipment (if any)
are off.
Site-Specific Considerations
The ever-increasing prominence of nonlinear loads can drive voltage harmonics onto utility
distribution lines. As a result, current harmonics may be present in inverter output and in the
transformer core. These harmonics decrease the capacity of the transformer to handle 60 Hz
power, increasing the risk a transformer will be operating in saturation – a situation that should be
avoided. It is important to select a transformer that will handle the harmonics load appropriately and
in accordance with the methods specified in IEEE Standard C57.110, “IEEE Recommended Practice
for Establishing Transformer Capability When Supplying Nonsinusoidal Load Currents.” Ten K
recommends the use of transformers with a minimum K-factor of 4. The presence of harmonics
may require a higher factor. In some environments, neutral current due to excessive harmonics may
be significant, and extra engineering consideration may be required if there is a large third or ninth
harmonic component.
• For arrays using single-phase inverters, the transformer must be capable of operating in an
unbalanced condition indefinitely.
• If the transformer will be subject to reverse energy flow, make sure it is suitable for back feeding.
There are several cases where back feeding is not recommended, including some transformers
with compensated windings and some network transformers. Check with the transformer
manufacturer and the utility.
• Open-delta three phase services require special attention. Do not connect any inverters on the
phase pair with the missing transformer. All inverters should be connected to the phase pair
with the split winding, essentially making the array a single-phase source. Subject to the utility’s
permission.
• If the array design favors it, use a transformer with dual windings on the array side, and feed half
the array into each winding.
Ten K Solar, Inc. | 9231 Penn Ave. South, Minneapolis, MN 55431 | 877-432-1010 | www.tenksolar.com
SIMPLY MORE ENERGY
Transformer winding styles may be governed by local codes and ordinances, and by the utility.
Always follow these local codes and ordinances when installing a Ten K array. The following chart
shows the technically acceptable winding styles of the array side of the transformer.
Inverters
1Φ 208 V
1Φ 240 V
1Φ 277 V
3Φ 480 V
Winding Style
120/208 wye
120/240 high-leg Δ;
240 corner ground Δ;
240/416 wye
277/480 wye
277/480 wye
Always follow the transformer manufacturer’s guidelines for installation, including wire sizing.
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©Ten K Solar, Inc., 2016