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Modeling Secondary in Mapping and Engineering

Modeling Secondary in Mapping and Engineering. 2010 IEEE REPC Orlando, FL May 17, 2010 Greg Shirek, PE Milsoft Utility Solutions Abilene, TX. OUTLINE. Reasons For Modeling Modeling Requirements Center-Tapped transformers Secondary Triplex Conductor

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Modeling Secondary in Mapping and Engineering

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  1. Modeling Secondary in Mapping and Engineering 2010 IEEE REPC Orlando, FL May 17, 2010 Greg Shirek, PE Milsoft Utility Solutions Abilene, TX

  2. OUTLINE • Reasons For Modeling • Modeling Requirements • Center-Tapped transformers • Secondary Triplex Conductor • System Level Loss analyses with and without secondary • Calculating X/R for Transformers • Two Winding • Three Winding Center Tap • Sensitivity Analysis • Vary Transformer X/R and %Z • Load Balance • Summarize

  3. WHY NOW? • Advances in computer processor speed and memory can now handle detailed models • Smart-Grid, Real-Time Analysis & State Estimation • Mapping-Engineering Analysis (GIS-EA) seamless integrations • Perform loss studies at all system component levels; aid in transformer and secondary conductor sizing practices • Arc Flash – Highest energy occurs at transformer low-side and service entrance

  4. Transformer X/R Calculations • Transformer losses are a function of core (no-load) and copper (load) PLoss total = PCU + Pcore (1) • Need at least 2 of the 3 from (1) to get PCU • Cooper losses are a function of resistance PCU = I2R = I2Rp + I2Rs

  5. Transformer X/R Calculations 3ph. 300 kVA, 208/120V, 4% Z, Load Losses = 4,000 W

  6. Center-Tapped Transformer Impedance ZTWO Two-Winding non-Center-Tap Connection Three-Winding Connection (Center-Tapped)

  7. Center-Tapped Transformer Impedance ZTWO Two-Winding non-Center-Tap Connection Three-Winding Connection (Center-Tapped) [ZTWO] = [RA + jXA]

  8. Modeling Requirements • Center-Tapped Transformers • kVA • %Z • X/R • Nominal/Rated Input/Output Voltages • No-load losses • Diversity of 120V loads on each ½ winding • Diversity of 240V loads

  9. Triplex Impedance Equations • Self Impedance i = 1, 2, n 1 2 j = 1, 2, n n • Mutual Impedance

  10. Modeling Requirements • Triplex Secondary • Resistance at Operating Temperature • Secondary Service Conductor Length • Construction Spacing Between Phases and neutral • Geometric Mean Radius (GMR) • Conductor Diameter • Earth Resistivity

  11. System Level Loss Analyses • How will models with varying levels of detail affect the allocation of system load • Primary Line Based • Primary Line & Distribution Transformers • Primary Line & Distribution Transformers and Secondary Conductor

  12. Distribution System Losses • 10,000 kW - feeder peak • 22,000 kVA - installed transformer kVA

  13. Sensitivity Analyses • Large variance on system transformer specs, how much detail do I need to maintain in the equipment database? • X/R • %Z • 1/0 AAC secondary conductor specs and conductor spacing were held constant

  14. Sensitivity Analyses • 25 kVA 7200V - 120/240V center-tapped transformer • 100 feet of 1/0 AAC Triplex • Loads • 120V half-winding 1: 5 kW @ 95% P.F. • 120V half-winding 2: 5 kW @ 95% P.F. • 240V full-winding: 15 kW @ 90% P.F. • Total: 25 kW @ 92% P.F. • All loads constant kVA load mix type

  15. Sensitivity Analyses • Transformer %Z varied from 1.0 to 4.0 percent • Transformer X/R varied from 1.0 to 3.0 • Delivered voltage held constant at 7,200V • 3 cases…..

  16. Sensitivity Analyses Case 1 : Transformer only, no secondary

  17. Sensitivity Analyses Case 2: Transformer with secondary

  18. Sensitivity Analyses Case 3: Transformer and secondary with unbalanced 120V loads Transformer was set with Z=2% and X/R = 1.5 (mid-point values), while 120V load was varied

  19. Sensitivity Analyses • Case 1: Transformer only Worst Case

  20. Voltage Drop

  21. Simplified Transformer Voltage Drop • Test Case 1 had 25 kW @ 92% P.F. => 113 Amps • Ir = 103.5 real amps • Ix = 44 reactive amps • For 25 kVA transformer with X/R of 1.0 and 4%Z • Ib = 25 kVA/240V = 104 Amps • Zb = Vb/Ib =240V/104A = 2.30 Ω • Z = 2.30 Ω * 4% = 0.092 ohms => R = X = 0.065 Ω Worst Case Test Point

  22. Sensitivity Analyses • Case 2: Transformer and Secondary Same profiles as Case 1, with additional 2 Volt Drop on secondary conductor.

  23. Sensitivity Analyses • Case 3: Transformer and Secondary with unbalanced 120V loads • Overall, a 40% unbalance with 7.5 and 17.5 kW on each ½ winding 0 kW 5 kW 5 kW 10 kW

  24. Summary • No excuses for not modeling secondary since computing power and software capabilities now exist • Load allocation: Level of detail modeled will affect allocation of losses • Can calculate X/R for any transformer if given load losses and %Z • Sensitivity analysis proved X/R and %Z is extremely important when modeling distribution transformers • No right or wrong answer to how many transformers with unique X/R and %Z values to store in the equipment database. • Options: • Maintain low, high, or average %Z? • Maintain low, high, or average X/R? • Maintain separate databases for analysis type (Fault Current, Voltage Drop, Arc Flash, etc.)

  25. _ Questions?

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