Understanding The Basics of Wye Transformer CalculationsDec 1, 2004 12:00 PM, By Mike Holt, NEC Consultant Find more articles on: Transformers Last month's Code Calculations article covered transformer calculation definitions and some specifics of delta transformer calculations. This month we turn our attention to the differences between delta and wye transformers and to wye transformer calculations. We'll close by looking at why it's so important to know how to perform these calculations, but you'll likely see the reasons as we go. In a wye configuration, three single-phase transformers are connected to a common point (neutral) via a lead from their secondaries. The other lead from each of the single-phase transformers is connected to the line conductors. This configuration is called a “wye,” because in an electrical drawin g it looks like the letter Y. Unlike the delta transformer, it doesn't have a high leg. Fig. 1. Wiring arrangements can have a considerable effect on output voltage. Differences in wye and delta transformers. The ratio of a transformer is the relationship between the number of primary winding turns to the number of secondary winding turns — and thus a comparison between the primary phase voltage and the secondary phase voltage. For typical delta/delta systems, the ratio is 2:1 — but for typical delta/wye systems, the ratio is 4:1 (Fig. 1 above). If the primary phase voltage in a typical delta/delta system is 480V, the secondary phase voltage is 240V. If the primary phase voltage in a typical delta/wye system is 480V, the secondary phase voltage is 120V. Delta and wye transformers also differ with regard to their phase voltage versus line voltage and phase current versus line current. In a delta transformer, EPhase=ELine and ILine=IPhase×√3. In a wye transformer, IPhase=ILine and ELine=EPhase×√3. These differences affect more than just which formulas you use for transformer calculations. By combining delta/delta and delta/wye transformers, you can abate harmonic distortion in an electrical system. We'll look at that strategy in more detail after addressing wye transformer calculations. Fig. 2. As this example shows, the line and phase currents are equal in a wye transformer. Wye current and voltage calculations. In a wye transformer, the 3-phase and single-phase 120V line current equals the phase current (IPhase = ILine) (Fig. 2 on page C20). Let's apply this to an actual problem. What's the secondary phase current for a 150kVA, 480V to 208Y/120V, 3-phase transformer (Fig. 3 on page C20)? ILine=150,000VA÷(208V×1.732)=416A, or IPhase=50,000VA÷120=416A or Phase A and Phase C. 25kVA transformers b) one 3-phase.To find wye 3-phase line and phase voltages. note that the line and phase power and current are the same. In this example. Step 2: Put one-third of the 3-phase load on Phase A. 208V load on Phase A and Phase B. Fig. The following steps will help you balance the transformer: Fig. Before you can properly size a delta/wye transformer. you must make sure that the secondary transformer phases (windings) or the line conductors are balanced. two 208V. the effects of 3-phase loading on the line are the same as on the phase (Fig. Note that balancing the panel (line conductors) is identical to balancing the transformer for wye transformers. 4. 3-phase load has the following effect: Line power=36kVA ILine=VALine÷(ELine×√3) ILine=36. Once you balance the wye transformer. Step 3: Put one-half of the single-phase. or Phase B and Phase C. 75kVA transformer c) a or b d) none of these Phase A=23kVA Phase B=22kVA Phase C=20kVA . since each line conductor from a wye transformer is connected to a different transformer winding. and three 120V. Step 1: Determine the loads' VA ratings. single-phase loads. 3-phase heat strip. 10kVA. and one-third on Phase C. 3kVA single-phase loads? a) three single-phase. 208V. use the following formulas: EPhase=ELine÷√3 ELine=EPhase×√3 Since each line conductor from a wye transformer is connected to a different transformer winding (phase).000VA÷(208V×√3)=100A Phase power=12kVA (any winding) IPhase=VAPhase÷EPhase IPhase=12. 3-phase) would you need for the following loads: 208V. 36kVA. one-third on Phase B. 4 on page C21). Step 4: Place 120V loads (largest to smallest): 100% on any phase. you can size it according to the load on each phase. 3. Now consider the following wye transformer sizing example: What size transformer (480V to 208Y/120V. A 36kVA.000VA÷120V=100A Wye transformer balancing and sizing. Note the four-fold increase in phase current when working with a delta/wye transformer. though. At this point. 22kVA. The clipped peaks typical of saturated transformers cause excess heating in the loads.The Table sums up the kVA for each phase of each load. you can efficiently work with a transformer supplier to develop a good solution. and 20kVA) should add up to the line total (65kVA). Groun ding considerations can make it an undesirable approach. such as electronic ballasts. Keep in mind that this is one of the many ways to mix and match transformers to solve power quality problems. Due to uptime or power quality concerns with complex loads. Always use a “checksum” like this to ensure you have accounted for all items and the math is right. you can see how important they are to being able to do a quality installation any time you're specifying transformers or considering adding loads to existing transformers. . Notice the word “might” in the question of whether to implement this kind of design. Another is to supply them from their own delta/wye and double the neutral. It's usually best to do all the calculations using the nameplate kVA. This issue of transformer loading means you're going to have to perform the transformer calculations just to get basic power quality and reasonable efficiency. For example. But the behavior of the delta/delta transformer itself. The approach you choose will depend on the characteristics of your loads and how well you lay out your power distribution system. depending on the various loads and the design of the overall electrical system. This ability is also important if you're trying to solve a power quality problem or a problem with “unexplained” system trips. When that's done. This would greatly reduce the presence of harmonics in the primary system. You may wish to sharpen this ability by purchasing an electrical calculations workbook or taking on this kind of work in your electrical projects. As a rule of thumb. it goes into core saturation and output consists of distorted waveforms. and motors with varying loads. the maximum unbalanced load can be higher than the nameplate kVA would indicate. Now that you understand delta and wye transformer calculations. identify which loads are high harmonic. you may need to mix and match transformer configurations as in the previous example. Note that the phase totals (23kVA. One approach to such a situation is to supply high-harmonic loads from their own delta/delta transformer. partly due to the absence of a neutral connection. If you overload the transformer. Matching the transformer to the anticipated load then requires a high degree of accuracy if you want to get a reasonable level of either efficiency or power quality. So it's important not to oversimplify your approach to transformer selection. Then. combined with the interaction of delta/delta and delta/wye. 80% loading is a good target. design the distribution system as though all loads are linear. which you would feed from a delta/wye transformer. If you're dealing with high-harmonic loads. And that's something you can't do unless you understand both delta and wye calculations. will also cause a reduction in harmonics. computer power supplies. you might put your computer loads (which have switching power supplies) on a delta/delta transformer. Another issue is proper transformer loading.
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