Power Quality Impacts of Distributed Generation

1. Power Quality Impacts of Distributed Generation Roger C. DuganSr. Consultant Electrotek Concepts, Inc. Tennessee Tech. February 9, 2004

2. Introduction • Renewed interest in Grid-connected DG • Utility Perspective • T & D Capacity Relief • Hedge against high market prices • End User Perspective • CHP and Improved Reliability • Incentives from Utility • Commercial Power Producer • Sell kWh or “ancillary services” Electrotek Concepts, Inc

3. Emphasis of This Presentation • Units connected to distribution (MV) • Typically < 10 MW • Commonly 1-2 MW • Frequently owned by end user • Impact of these units on Power Quality • Operating conflicts • We will look at a few of the many … Electrotek Concepts, Inc

4. OverCurrent Protection Conflicts

5. Typical Overcurrent Protection Electrotek Concepts, Inc

6. Only One Device Has to Operate OperatingDevice Fault Current Electrotek Concepts, Inc

7. Transmission Fault Protection Multiple Sources Two Devices Must Operate to Clear Fault Electrotek Concepts, Inc

8. LV Network Systems Electrotek Concepts, Inc

9. Utility Fault Clearing • Due to cost, distribution systems are designed expecting only one source of fault current • Only one device has to operate • DG must disconnect to allow utility fault clearing process to continue • This is the source of most operating conflicts Electrotek Concepts, Inc

10. Reduction of Reach Utility relays and DG relays may not know anything is wrong. Electrotek Concepts, Inc

11. Reduction of Reach • More of a problem at peak load • When peaking units are likely to be connected !! • More of a problem for high resistance faults • Longer fault detection times Electrotek Concepts, Inc

12. Instantaneous Reclosing

13. Reclosing • Most faults are temporary • Reclosing is prevalent in North America • Reclosing Interval: 0.2 – 5 sec • “Instantaneous” < 30 cycles Electrotek Concepts, Inc

14. Reclosing, cont’d • If DG does not get off, two bad things could happen: • DG is damaged by the reclose • Shafts • Winding damage • Fault is not cleared • Reduced reliability • Damage to utility system Electrotek Concepts, Inc

15. Reclosing DG Must Disconnect Here Electrotek Concepts, Inc

16. Instantaneous Reclosing • Utilities use this for PQ concerns • Increases the probability that DG will not disconnect in time • Makes it almost a certainty that some won’t • If 1-2 sec, most operations will be easily detected by DG protection • Some utilities use 5 sec reclose on feeders with DG Electrotek Concepts, Inc

17. Unintended Consequences ...

18. Voltage Drop After Fault OOPS! Electrotek Concepts, Inc

19. Low Voltage After Interruption • A problem when there is a fault at peak loading • When peaking DG is likely to be on !! • This is often the most limiting condition for how much DG can be accommodated without change • Particularly on rural feeders • 500 – 1000 kW Electrotek Concepts, Inc

20. Ferroresonance Sometimes Conflicting Interconnect Requirements Collide …

21. Ferroresonance DG Required to have separate Transformer and disconnect at first sign of trouble G 2) DG breaker detects negative sequence and trips leaving transformer isolated on cable capacitance with no load/generation 1) Fuse blows or switch opens 3) Ferroresonance develops Electrotek Concepts, Inc

22. Ferroresonance Electrotek Concepts, Inc

23. Ferroresonance Waveform Electrotek Concepts, Inc

24. Reduced Insulation

25. Arrester Duty for SLG Electrotek Concepts, Inc

26. Varying Generation

27. Varying Generation Example 10 000 ft 10 000 ft 20 000 ft 115 kV 12.47 kV Regulator 500 kW 1000 kW 600 kvar Wind Gen 336 MCM ACSR 8 MW (Untransposed) Unbalanced impedances Rapidly-varying “load” Electrotek Concepts, Inc

28. Varying Generation 1-Phase Regulators Regulator Operations Electrotek Concepts, Inc

29. Transformer Connection and Ground Faults

30. Grd Wye-Delta Connection • Some claim this is the best way to interconnect DG • Same way as nearly all central station generation is connected • However, most utilities don’t allow this connection on their system. Electrotek Concepts, Inc

31. Wye-Delta Connection Electrotek Concepts, Inc

32. Problems with Wye-Delta • Increases ground fault current • Interferes with coordination • Faulted Circuit Indicators get fooled • Transformer subject to failure • Special design and/or neutral reactor • Utilities often discourage “Specials” Electrotek Concepts, Inc

33. On Wye-Wye • Most common U.S. connection • Generally well behaved • Does not block Triplen Harmonics • Leads to “Harmonic Surprise” when many machines paralleled with utility system • Also occurs with Delta-Wye • 2/3 pitch machines lessens the impact Electrotek Concepts, Inc

34. Conclusions • Interconnected DG likely to increase • DG must adapt to Distribution protection system • Too much infrastructure already in place to overhaul the system to better accommodate DG • Who would pay for it? Non-participating ratepayers? Electrotek Concepts, Inc

35. Conclusions • DG impact on Power Quality • Often good for individual end users • Largest gainers: Those with high-value product that suffer sustained interruptions • But, should not expect power quality improvement in all areas • Maybe slightly negative for utility • Contributes to operating conflicts • Can we see it in the Average indices? Electrotek Concepts, Inc

36. Conclusions, cont’d • Revisit Overcurrent Protection • Utilities may have to change some practices • Instantaneous reclosing ? • Use of line fuses • Tradeoffs for Transformer Connections • WYE-Delta good for DG but not for ground fault coordination • Special engineering req’d for some DG application. Electrotek Concepts, Inc

37. Despite These Conflicts … • DG can almost always be successfully engineered • Larger conflict may be over who pays • Some long-standing utility practices may have to be changed Electrotek Concepts, Inc

38. How Much DG ? • What is the “No Changes” “Penetration Limit” ? • Typical: 15% of feeder capacity • 5% - 10% if Rural Electrotek Concepts, Inc