Understanding the Basics of Wye Transformer Calculations

12/29/2010

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Understanding The Basics of Wye Transformer Calculations Dec 1, 2004 12:00 PM, By Mike Holt, NEC Consultant

0 Add this information to what you know about delta transformers and you may be ready to solve power quality problems Find more articles on: Transformers Las t month's Code Calculations article covered trans former calculation definitions and some specifics of delta trans former 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 thes e calculations, but you'll likely see the reasons as we go. In a wye configuration, three s ingle-phase trans formers are connected to a common point (neutral) via a lead from their secondaries . The other lead from each of the single-phase trans formers is connected to the line conductors. This configuration is called a “wye,” because in an electrical drawing it looks like the letter Y. Unlike the delta transformer, it doesn't have a high leg. 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 s econdary winding turns — and thus a comparis on between the primary phase voltage and the s econdary phas e 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 s ys tem 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 trans formers als o 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 trans former, IPhase =ILine and ELine =EPhase ×√3.

Fig. 1. Wiring arrangements can have a considerable effect on output voltage. These differences affect more than just which formulas you use for trans former calculations . By combining delta/delta and delta/wye transformers, you can abate harmonic distortion in an electrical system. We'll look at that s trategy in more detail after addres sing wye transformer calculations . Wye current and voltage calculations. In a wye transformer, the 3-phase and single-phas e 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 www.printthis.clickability.com/pt/cpt?acti…

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Generated Foxit PDF Creator © Foxit Software 12/29/2010 Understanding The Basics of Wye by Transf… http://www.foxitsoftware.com For evaluation only. phase current for a 150kVA, 480V to 208Y/120V, 3-phas e transformer (Fig. 3 on page C20)? ILine =150,000VA÷ (208V×1.732)=416A, or IPhase =50,000VA÷120=416A To find wye 3-phase line and phase voltages , 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), the effects of 3-phase loading on the line are the s ame as on the Fig. 2. As this example shows, the line and phase currents are phase (Fig. 4 on page C21). A 36kVA, 208V, 3-phase load equal in a wye trans former. has the following effect: Line power=36kVA ILine =VALine ÷(ELine ×√3) ILine =36,000VA÷(208V×√3)=100A Phase power=12kVA (any winding) IPhase =VAPhase ÷EPhase IPhase =12,000VA÷120V=100A Wye transformer balancing and sizing. Before you can properly size a delta/wye transformer, you must make s ure that the secondary transformer phases (windings ) or the line conductors are balanced. Note that balancing the panel (line conductors) is identical to balancing the trans former for wye transformers. Once you balance the wye transformer, you can size it according to the load on each phas e. The following steps will help you balance the transformer: Step 1: Determine the loads ' VA ratings. Step 2: Put one-third of the 3-phase load on Phase A, onethird on Phase B, and one-third on Phase C. Step 3: Put one-half of the s ingle-phase, 208V load on Phase A and Phas e B, or Phase B and Phase C, or Phase A and Phase C. Step 4: Place 120V loads (largest to s mallest): 100% on any phase. Now consider the following wye transformer sizing example: What size trans former (480V to 208Y/120V, 3-phase) would you need for the following loads: 208V, 36kVA, 3-phas e heat strip; two 208V, 10kVA, single-phase loads; and three 120V, 3kVA single-phas e loads?

Fig. 3. Note the four-fold increase in phase current when working with a delta/wye trans former.

a) three single-phas e, 25kVA transformers b) one 3-phase, 75kVA transformer c) a or b d) none of these Phase A=23kVA Phase B=22kVA Phase C=20kVA The Table sums up the kVA for each phase of each load. Note that the phase totals (23kVA, 22kVA, and 20kVA) should add up to the line total (65kVA). Always use a “checksum” like this to ens ure you have accounted for all items and the math is right. If you're dealing with high-harmonic loads, the maximum unbalanced load can be higher than the nameplate kVA would indicate. 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. One approach to such a s ituation is to s upply high-harmonic loads from their own delta/delta transformer. Another is to supply www.printthis.clickability.com/pt/cpt?acti…

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Generated Foxit PDF Creator © Foxit Software 12/29/2010 Understanding The Basics of Wye by Transf… http://www.foxitsoftware.com For evaluation only. them from their own delta/wye and double the neutral. The approach you choos e will depend on the characteristics of your loads and how well you lay out your power distribution system. For example, you might put your computer loads (which have switching power s upplies) on a delta/delta trans former, which you would feed from a delta/wye transformer. This would greatly reduce the presence of harmonics in the primary s ys tem, partly due to the absence of a neutral connection. But the behavior of the delta/delta transformer itself, combined with the interaction of delta/delta and delta/wye, will also caus e a reduction in harmonics. Notice the word “might” in the ques tion of whether to implement this kind of des ign. Grounding cons iderations can make it an undesirable approach, depending on the various loads and the design of the overall electrical system. Keep in mind that this is one of the many ways to mix and match transformers Fig. 4. In this example, note that the line and phase power and current are the same, s ince each line conductor from a wye to solve power quality problems. transformer is connected to a different trans former winding. Due to uptime or power quality concerns with complex loads , you may need to mix and match transformer configurations as in the previous example. And that's s omething you can't do unless you understand both delta and wye calculations. Another issue is proper trans former loading. As a rule of thumb, 80% loading is a good target. If you overload the transformer, though, it goes into core s aturation and output cons ists of distorted waveforms . The clipped peaks typical of saturated transformers cause excess heating in the loads. This is sue of transformer loading means you're going to have to perform the transformer calculations jus t to get basic power quality and reas onable efficiency. So it's important not to oversimplify your approach to transformer selection. It's usually best to do all the calculations using the nameplate kVA. Then, design the distribution system as though all loads are linear. When that's done, identify which loads are high harmonic, such as electronic ballasts , computer power s upplies, and motors with varying loads. At this point, you can efficiently work with a transformer supplier to develop a good solution. Now that you understand delta and wye transformer calculations, you can see how important they are to being able to do a quality installation any time you're specifying trans formers or cons idering adding loads to exis ting trans formers. This ability is also important if you're trying to solve a power quality problem or a problem with “unexplained” system trips. You may wish to sharpen this ability by purchasing an electrical calculations workbook or taking on this kind of work in your electrical projects.

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