1 / 27

Distributed Generation and Power Quality

Distributed Generation and Power Quality. Relaying considerations. DG infeed may reduce the reach of overcurrent relays DG feeds fault, so utility current is fault current minus DG contribution Sympathetic tripping of feeder breakers Defeat fuse saving. DISTRIBUTION. 12.47 kV. Radial Line.

milton
Download Presentation

Distributed Generation and Power Quality

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Distributed Generation and Power Quality

  2. Relaying considerations • DG infeed may reduce the reach of overcurrent relays • DG feeds fault, so utility current is fault current minus DG contribution • Sympathetic tripping of feeder breakers • Defeat fuse saving

  3. DISTRIBUTION 12.47 kV Radial Line 115 kV DG Put recloser here Only one DG: obvious solution to several problems

  4. 12.47 kV Fault 115 kV “Sympathetic” tripping of this circuit breaker (not desired) due to backfeed from DG DG Solution is to use directional overcurrent relays at substation

  5. DG Fault on tap Since DG feeds the fault (backfeed), it will likely defeat fuse saving

  6. Voltage Regulation V DG distance

  7. Feeder trips and recloses DG disconnects V DG Low voltage distance

  8. Feeder trips and recloses DG disconnects VR steps V DG distance

  9. Feeder trips and recloses DG disconnects VR steps DG reconnects V DG distance

  10. Sequence • First we see interruption and reclosure with a voltage sag due to DG disconnect • Then voltage returns to normal (station step regulator or LTC) • Then DG reconnects, and we see a voltage swell • Then voltage returns to normal (station step regulator or LTC) • Obviously needs to have coordinated voltage control • Limits to how much DG one feeder can stand

  11. Power flow reversal in voltage regulators • If excess DG during low load causes power to flow in reverse direction through voltage regulators misoperation is possible • Modern controls recognize this and change to reverse power mode (regulate in opposite direction so that controls operate correctly)

  12. Wind generation case study wind farm 32 step V Reg singly-fed induction gen switched capacitor a b c untransposed line construction (typical)

  13. Detailed 3-phase study shows: • as wind generation increases, one outside phase voltage rises while the other drops • feeder has problems with frequent capacitor switching and/or votlage regulator stepping events • Cases like this may need dedicated feeder or doubly-fed generator to avoid power quality problems for other customers • If DG can generate reactive power, capacitor controls needs coordination with reactive generation

  14. DG Transformer Connections Order used here is HV : LV feeding DG 1. Ygnd : Ygnd interconnects HV and LV grounds 2. D : Ygnd isolates LV ground from HV ground 3. D : D used on existing installations • Y : D used on existing installations • Ygnd : D similar to utility units

  15. DG Transformer Connections • Ygnd : Ygnd interconnects HV and LV grounds. DG may need a neutral reactor to limit 1 phase to ground short circuit currents. DG may need 2/3 pitch winding to avoid large third harmonic votlages (which can cause third harmonic currents on HV and LV sides). - No phase shift between primary and secondary voltage.

  16. Generator voltage harmonics • Windings are distributed and often short-pitched (or chorded): • each coil spans a pitch of less than 2p/p where p is the number of poles, to reduce the time harmonic voltage induced

  17. 3-phase 6-pole 36-slot full-pitch stator winding

  18. Development of stator winding

  19. One coil of the winding: conductors in top of slot 1 return in bottom of slot 7, coil pitch = pole pitch, so a full-pitch winding

  20. One coil of the winding: conductors in top of slot 1 return in bottom of slot 6, coil pitch = 5/6 pole pitch, so a 5/6-pitch winding

  21. What’s so great about a 2/3-pitch coil anyway? Third harmonic flux linking a full-pitch (blue), 5/6-pitch (green), and 2/3-pitch (red) coils. Notice that the flux linkage (net area enclosed) is zero at the 2/3-pitch coil.

  22. Third harmonics • The third harmonic flux will not link a stator winding with a 2/3 pitch • If third harmonic voltages are present, third harmonic circulating currents can be quite large if generator is solidly grounded

  23. DG Transformer Connections • D : Ygnd isolates LV ground from HV ground. DG may need 2/3 pitch winding to avoid third harmonic voltages, but third harmonic currents are contained on LV side. Some utility faults hard to detect due to phase shift.

  24. DG Transformer Connections • D : D used on existing installations. • Y : D used on existing installations. Ungrounded, unless DG provides ground. Some utility faults hard to detect due to phase shift.

  25. DG Transformer Connections • Ygnd : D similar to utility units, isolated grounds, avoids problem with third harmonic currents.

  26. Expected single-phase to ground fault currents on a radial distribution feeder

  27. DG transformer showing possible zero-sequence currents

More Related