Lec6 impact of embedded generation (1)
- 1. EEE812: ADVANCED POWER SYSTEM PROTECTION Distributed Generation: Impact on Protection Content prepared by Dr Campbell Booth University of Strathclyde
- 2. Overview Conventional distribution networks and protection - summary How distribution network are changing (active distribution networks, DG, potential for islanded operation) Potential issues for future networks – Protection “blinding”, false tripping/coordination problems (several examples) – DG ride-through – Converter-interfaced sources – Use of DC for distribution? – Fault current limitation Protection solutions?
- 4. Distributed Generation Basics Technologies Small and Large scale combined heat and power units Energy from waste units Wind Farms Tidal and wave energy units Stand-by generators (diesel) Types of generating units Self-excited asynchronous generator Mains-excited asynchronous generator Power factor corrected asynchronous generator Doubly fed induction generator (DFIG) Synchronous generator Inverter connected Synchronous Generator (Wind) Inverter connected DC source (fuel cell, PV)
- 5. Distributed Generation Basics Main reasons for Distributed Generation Reduction of gaseous emissions (mainly CO2) Diversification of energy sources Ease of finding sites for smaller generators Short construction times Potentially reduced transmission losses Increased efficiency with combined Heat and Power (CHP) units
- 6. Conventional distribution networks Operated radially Designed for unidirectional power flow 11 kV POWER FLOW DIRECTION Protected with over-current protection relays, reclosers and fuses
- 7. How distribution networks are changing – Increase of distributed generation: – Wind – Hydro – Biomass – Photovoltaic – Wave/Tidal – others – Introduction of network automation – Connection of energy storage
- 8. Active distribution networks Fault current magnitudes and directions becomes unpredictable, potentially causing problems: 11 kV – false tripping of feeders; – lack of coordination between protection devices; – other problems.
- 9. Islanding operation – Reduced fault levels – Changed fault current direction – System control? 11 kV
- 10. Protection of Distribution Networks 132/33kV Distance, differential (some), overcurrent 11kV/415V Overcurrent, reclosers, sectionalisers, fuses, RCDs Remember, majority of faults transient – fuses should only operate if fault is permanent Typically, faults are isolated very quickly by reclosers, multiple reclose attempts are attempted, and if fault is permanent and downstream of fuses, fuses ultimately melt while system is in reclosed state Reclose is then successful If permanent fault between recloser and fuse, then recloser will lock- out after pre-defined number of attempts Automatic sectionalisers/smart links sometimes used
- 11. B A t I t Decreasing Fault Current I Fault 1 Fault 1 tF1 Fault 2 tF2 Fault 2 tF2 Fault 2 Relay 1 Relay 2 Source Protection of Distribution Networks (HV/MV)
- 12. Protection of distribution networks (MV/LV) Distribution network protection is based on overcurrent protection, reclosers and fuses (and sectionalisers) In rural overhead distribution networks, >80% of faults are temporary and auto reclosure automation is adopted. CBT1-11 CBT1-33 CBT2-11 CBT2-33 B33kV B11kV SpurA1 SpurA2 SpurA3 SpurA4 SpurA5 SpurA6 SpurA7 SpurA8 SpurB1 SpurB2 SpurB3 SpurB4 SpurB5 R-A R-B PMAR-A PMAR-B Feeder A Feeder B SpurA9 SpurA10 SpurA1
- 13. Transient fault Recloser will successfully reclose Permanent fault Recloser will reclose multiple times (with variable delays before re-opening) and fuse will melt before max reclosures attempted Sectionalisers/“smart links” may be used instead of, or to “save” fuses Gers and Holmes “Protection of Electricity Distribution Networks”, IEE Power & Energy Series 47 Protection of distribution networks (MV/LV)
- 14. Gers and Holmes “Protection of Electricity Distribution Networks”, IEE Power & Energy Series 47 Protection of distribution networks (MV/LV)
- 15. B A C Permanent Fault PMAR (fast/delayed interruptions and reclosures) Sectionaliser (counts number of overcurrents/interruptions - opens after certain number) Fuse IDMT (with auto-reclose) PMAR Sectionaliser Fuse IDMT Start Open Count 1 1 “shot” Reset Open Count 1 1 “shot” Reset Close Count 1 2 “shots” Start Open Count 2 2 “shots” Reset Open Count 2 2 “shots” Reset Close Count 2 melt Reset Close Reset melted t Fault inception 1 2 0 Protection of distribution networks (MV/LV)
- 16. B A C PMAR Sectionaliser Fuse IDMT PMAR Sectionaliser Fuse IDMT Start Open Reset Open Reset Close Reset Close t Fault inception Protection of distribution networks (MV/LV) Transient Fault
- 17. Protection of distribution networks (MV/LV) Sectionalisers may be used instead of or in conjunction with fuses (“fuse savers”) http://www.hubbellpowersystems.com/catalogs/switching/10D-Elec_Sect.pdf
- 18. B A C Source (Grid) Operate (quickly) Operate (after a delay) Don’t operate V V=IZ 11kV 30MVA Source (Zsource= j4.03W) j1W impedance to fault (ZAB=j0.2W) Fault Behaviour – no DG
- 19. B A C Operate (quickly) Operate (after a delay) Don’t operate V V=IZ With DG at B No DG DG fault contribution 14MVA Fault Behaviour – with DG Source (Grid) 11kV 30MVA Source (Zsource= j4.03W) j1W impedance to fault (ZAB=j0.2W)
- 20. Equivalent circuit – no DG j4.03W j0.8W j0.2W If Z from source to fault = j1W (j0.2W for first feeder + j0.8 W for second feeder): Ifault= Igrid = Vph/Z = 6351/5.03 = 1263A VB = 1263x0.8=1010V VA = 1263x1=1263V Grid V V=I/Z With DG at B No DG B A
- 21. Equivalent circuit – with DG j4.03W 0.8W j0.2W j8.64W Z from sources to fault = j4.23//j8.64 + j0.8 = j3.64W Ifault = Vph/Z = 6351/3.64 = 1745A Igrid = (8.64)/(4.23+8.64) x1745 = 1171A IDG = (4.23)/(4.23+8.64) x1745 = 574A VB = 1745x0.8=1396V VA = VB + (1171x0.2) =1630V Grid DG V V=I/Z With DG at B No DG B A
- 22. Protection issues - “blinding” t If Fault current as measured at upstream relay with no downstream DG
- 23. Protection issues - “blinding” Under very high DG penetrations and very low grid infeed, infeed from grid could be markedly reduced, therefore increasing risk of feeder protection “blinding” (slow or non-operation of relay at A for backup scenario in this case). Problem? Probably not significant in interconnected system – but in islanded mode? t If Fault current as measured at upstream relay with no downstream DG t If Fault current as measured at upstream relay with significant downstream DG
- 24. B A C Source (Grid) V=0 Infeed=5000A (4500+500)A Infeed= 4500A Infeed=500A 0.6W 0.8W D E Protection issues – generator tripping on undervoltage
- 25. G59 – DG interface protection settings
- 26. B A C Source (Grid) V=0 Fault current=5000A (4500+500)A Infeed= 4500A Infeed=500A Z=0.6W Z=0.8W D E VA=VD=5000x0.6=3000V (less than 50% nominal) VED=500x0.8=400V, therefore VE=3400V So if protection at A does not operate within 0.4-0.5s, chance of DG at E tripping, unnecessarily, on undervoltage – also slight chance of relay at D operating if threshold violated Protection issues – generator tripping on undervoltage
- 27. B A C Source (Grid) V=0 D E If protection at D is non-directional overcurrent, then if contribution from DG at E exceeds setting on D, potential for mal-operation In this case, if Ithreshold<500A at D, potential for D to trip unnecessarily Fault current=5000A (4500+500)A Infeed=500A Z=0.6W Z=0.8W Infeed= 4500A Protection issues – feeder protection maloperation
- 28. Protection issues - DG impact on instantaneous (“high set”) protection
- 29. 2 1 3 Operate (time delay) Operate? Operate (instant) If Protection issues - DG impact on instantaneous (“high set”) protection
- 30. 2 1 3 DG If(at 2 and 3) Protection issues - DG impact on instantaneous (“high set”) protection
- 31. Use of directional relays to provide correct protection operation on parallel feeders From NPAG: - chapter 9 Sensitive and fast acting directional protection here (Ithreshold=10% of rated line current) Looks “up” into line – prevents R2 operating for this fault Protection issues - DG impact on directional protection
- 32. DG on load side – R’2 (and possibly R2) might maloperate? Protection issues - DG impact on directional protection
- 33. DG on load side – R’1 R’2 (and possibly R1 and R2) might maloperate? Even under load conditions – back-feed if DG>local load? Protection issues - DG impact on directional protection
- 34. Use of overcurrent relays for protection of ring mains From NPAG: - chapter 9 Fault as shown: R5’ operates after 1.7 (R6’ after 2.1) R2 after 0.5 (R3 after 0.9) Protection issues - DG impact on directional protection
- 35. Additional DG contribution may result in coordination problems? R1’ will operate for this scenario? Protection issues - DG impact on directional protection Use of overcurrent relays for protection of ring mains From NPAG: - chapter 9
- 36. Impact on section switches/fuses/…? Blinding/maloperation/impact on automation/auto- reclose/sectionaliser logic? Possible problems if DG penetration/fault contribution is high? Performance in islanded mode – if permitted? B A C PMAR Sectionaliser Fuse IDMT