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The Whys of Distribution System Analysis

The Whys of Distribution System Analysis. W. H. Kersting Milsoft Utility Solutions. Why???????. Planning studies Unbalanced studies Non-transposed lines Phasing important Power losses important Power factor important Exact transformer connections No symmetrical components

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The Whys of Distribution System Analysis

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  1. The Whys of Distribution System Analysis W. H. Kersting Milsoft Utility Solutions

  2. Why??????? • Planning studies • Unbalanced studies • Non-transposed lines • Phasing important • Power losses important • Power factor important • Exact transformer connections • No symmetrical components • Exact induction motor model • R and X settings on voltage regulators

  3. Why Not Use Transmission System Power Flow Programs? • Distribution lines are not transposed • Distribution lines can be • Three-phase • Two-phase • Single-phase • Loading is not balanced

  4. What is the goal? • Under all operating conditions the ANSI voltage standards must be met • Nominal Utilization voltage = 115 volts • Max. Service voltage = 126 volts • Min. Service voltage = 114 volts • Min. Utilization voltage = 110 volts • Voltage Unbalanced less than 3%

  5. Secondary Voltage Drop Assumption

  6. What Actually Happens

  7. Why Unbalanced Studies? • Confirm that voltage unbalance is within limit • Determine how to balance loading • Reduce losses

  8. Why is Phasing of Lines Important? • Mutual coupling between phases is different because of different spacings between phases • Need to know phasing for load balancing

  9. Genera Feeder Component Model

  10. IEEE 13 Node Test Feeder

  11. Line 632 - 671

  12. Untransposed vs. Transposed Line Z

  13. Voltage Unbalance Error

  14. Power Losses

  15. Power Loss by the Numbers

  16. Neutral and Dirt Power Losses

  17. Finally

  18. IEEE 13 Node Test Feeder

  19. Why Voltage Regulators? • Voltage at Node 671 • Need to balance voltages and increase magnitude

  20. R and X Settings • CT primary rating = 500 • Pt = 2400/120 = 20

  21. With Regulator • Voltage Level = 120 volts • Bandwidth = 2 volts

  22. Why Power Factor Correction? • With regulator the complex power out of the substation is: • Can add 300 kvar/phase

  23. 900 kvar at node 671

  24. IEEE 13 Node Test Feeder

  25. Induction Motor PQ Model? • Modeled as constant 21 kW + j9 kvar

  26. Induction Motor Exact Model: • 25 Hp, 240 Volt • Zs = 0.0774 + j0.1843 Ohms • Zr = 0.0908 + j0.1843 Ohms • Zm = j4.835 Ohms

  27. Exact Motor Model Results

  28. Why is it necessary to model the exact transformer connection? • Transformer bank at Node 633 • Ungrounded wye-delta • Phase A: 25 kVA • Phase B: 15 kVA • Phase C: 15 kVA • Loads • Single phase on phases a-b (15 + j10) • Three-phase induction motor (25 Hp)

  29. IEEE 13 Node Test Feeder

  30. Case 1: Lighting transformer on phase A

  31. Case 2: Lighting transformer on phase B

  32. Case 3: Lighting transformer on phase C

  33. Case 4: Grounded Wye – Delta Lighting Xfm on Phase A

  34. Case 5: Open Wye – Delta Lighting Xfm on Phase A

  35. Conclusions • Engineers performing power-flow studies need to know the answers to the “why” questions presented in this paper. • Of primary importance an example system demonstrated • Why un-transposed lines must be modeled • How to computed power loss • Why voltage regulators are necessary • The importance of power factor correction • Why an exact model of induction motors must be used • The importance of the choice of three-phase transformer connections for given load conditions

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