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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. Page 2 ©Ten K Solar, Inc., 2016