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P.K . Sen, PhD, PE , Fellow IEEE Professor of Electrical Engineering Colorado School of Mines

Power System Grounding and Safety- Transformer Connections & Grounding for Distributed Generation (DG): An Update on IEEE Std. 1547.2. P.K . Sen, PhD, PE , Fellow IEEE Professor of Electrical Engineering Colorado School of Mines psen@mines.edu. Outline. Motivation and System Grounding

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P.K . Sen, PhD, PE , Fellow IEEE Professor of Electrical Engineering Colorado School of Mines

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  1. Power System Grounding and Safety- Transformer Connections & Grounding for Distributed Generation (DG): An Update on IEEE Std. 1547.2 P.K. Sen, PhD, PE, Fellow IEEE Professor of Electrical Engineering Colorado School of Mines psen@mines.edu DOE / EFCOG, 2014 Electrical Safety Workshop National Renewable Energy Laboratory (NREL), Golden, CO July 15, 2014

  2. Outline • Motivation and System Grounding • Distribution System and Distributed Generation (DG): Grounding • DG Grounding: Utility Practices • IEEE Std. 1547.2: Snapshot • Distribution Transformer Connections • Conclusions 2014, EFCOG - DOE (Dr. PK Sen)

  3. Electrical Hazards • Hazards of Electricity identified by NFPA 70E-2009: Standard for Electrical Safety in the Workplace. • Electrical Shock • Electrical Arc-Flash • Electrical Arc-Blast 2014, EFCOG - DOE (Dr. PK Sen)

  4. Electrical Hazards • Electrocution accounts for a greater proportion of work-related injury deaths in adolescents than in adults. Contact with an energized power line caused more than 50% of the electrocutions. (Source: NIOSH Publication No. 2009-113, Bureau of Labor Statistics) 2014, EFCOG - DOE (Dr. PK Sen)

  5. Grounding and Personnel Safety 2014, EFCOG - DOE (Dr. PK Sen)

  6. Introduction • The Type of Grounding Design can Impact: • System Operation • Equipment Damage • Personnel Safety • Grounding of Transformer and/or Generator Neutrals Provides: • A “Zero” Potential Reference for the Power System Operation and Maintenance • An Electrical “Path” for Ground Current 2014, EFCOG - DOE (Dr. PK Sen)

  7. Electric Shock Basis 1. Current, Amount of Current flowing through the body 2. Path, Current path through the body 3. Time, Duration of contact. 2014, EFCOG - DOE (Dr. PK Sen)

  8. Current Magnitude and Human Reaction 2014, EFCOG - DOE (Dr. PK Sen)

  9. System Grounding: Recap • Different Degrees of Grounding: • “Grounding Design” Depends on the (a) System Design & Operation Principle, (b) Protection Requirements, and (c) System Voltage • Ungrounded (aka Capacitive Grounding): Small Current, Typically (1-3)A • High Resistance/Reactance, Typically (10)A • Low Resistance/Reactance, Typically (100-1,200)A • Solidly Grounded: Large Current, Typically (10-20+)kA 2014, EFCOG - DOE (Dr. PK Sen)

  10. Arcing: Arc Energy = 2014, EFCOG - DOE (Dr. PK Sen)

  11. Ungrounded System:Persistent Ground Fault • If the ground fault is not isolated, the steady state condition called “Neutral Voltage Displacement (NVD)” can occur • This can lead to significant damage on a system not designed for steady state Line-to-Line Voltages BEFORE FAULT DURING FAULT Vc CϕFaulted to Ground Neutral Point at Ground Potential Vc GROUND POTENTIAL Vb Va Neutral Point Shifted Away from Ground Bϕ&AϕVoltages now L-L values to ground Vb Va 2014, EFCOG - DOE (Dr. PK Sen)

  12. Ungrounded System • Transient Overvoltage caused by interactions with line capacitance and other system reactances initiated by switching or fault events can be much greater in magnitude and more damaging on an ungrounded system • Transient overvoltage conditions of 2.0pu or much higher (≈ 5.0+ pu) can occur until the equipment insulation breakdown • Sometimes an ungrounded system is appropriate when reliability and/or very little ground fault current is desired, but it is rarely used and has to be designed properly 2014, EFCOG - DOE (Dr. PK Sen)

  13. Impedance Grounding • Impedance grounding is used to provide a path for zero sequence current to flow, but limits the magnitude of the ground fault current. Also limit the transient overvoltage to an acceptable limit. • High Impedance is used to detect a ground fault, reduce ungrounded system hazards, but still provide for increased reliability (~10A) • Low Impedance is used to limit damage to equipment caused by ground faults, but provide enough current for selectivity and coordination (100-1,200A) Z 2014, EFCOG - DOE (Dr. PK Sen)

  14. Solidly Grounded System • Greatly limits transient overvoltage and steady state NVD is not an issue • Provides highest zero sequence ground current and allows for greatest selectivity for coordination Very high fault current BϕGround Fault ~ IF = 3I0 ~ ~ 2014, EFCOG - DOE (Dr. PK Sen)

  15. Effectively Grounded System • IEEE Std. 142 (Green Book) Defines an Effectively Grounded System as one that Meets the Following Criteria: 2014, EFCOG - DOE (Dr. PK Sen)

  16. Grounding Options:For an Ungrounded System • If a system is ungrounded a neutral deriving transformer is installed to ground the system • There are two common design practices: • Zig-Zag transformers are designed with no secondary winding • Delta-Wye Grounding transformers connect to the system with a grounded-wye and have a delta secondary • Both designs utilizes small kVA sizes, so they allow source current for a limited amount of time (typically seconds) 2014, EFCOG - DOE (Dr. PK Sen)

  17. Distribution System&Distributed Generation 2014, EFCOG - DOE (Dr. PK Sen)

  18. Distribution System Operation • The Traditional Mode on the Distribution System is Configured for “Radial” Operation • Power (Current) Flows from the Source (or Utility) to the Loads • The Radial Mode of Operation is Well Understood by Industry & Utilities & have been Practiced for many Decades • The Design Benefits are: • Simple • Reliable • Relatively Easy to Design • Minimized Engineering Analysis • Allows for Selectivity for Coordination 2014, EFCOG - DOE (Dr. PK Sen)

  19. Distribution System Operation G Generation Transmission Network Transmission Distribution 2014, EFCOG - DOE (Dr. PK Sen)

  20. Distribution System Operation:An Overview • Faults on a Radially Designed System are also Easy to Handle: • 3-phase: Current Flows from the Source to the Fault • 1-phase: Ground Fault Current Flows from the Neutral Connection to Ground 1-phase Fault 3-phase Fault Fault Current 2014, EFCOG - DOE (Dr. PK Sen)

  21. Distribution Generation (DG) & Grounding • The Number (and the Output Rating) of DG Installations is Increasing • Existing Distribution System was not Designed for Loop (or Back) Feed • Large Penetrations (30+ %) of Renewable DG System could Cause Operational, Safety and Other Related Problems 2014, EFCOG - DOE (Dr. PK Sen)

  22. Distribution Generation (DG) & Grounding: • DG Includes Renewables such as Wind & Solar but can also take Many Other Forms from Non-RenewablesEnergy Source • For DG Applications, it is Important to Specify how the Power is Generated: • Conventional Rotating: Generally Greater (60Hz) Fault Current Capability • Non-Rotating & Inverter Based: Very Low (60Hz) Fault Current Contribution 2014, EFCOG - DOE (Dr. PK Sen)

  23. Distribution Generation (DG) & Grounding • Renewable Sources: • Rotating: Wind, Hydro, Geo-Thermal • Non-Rotating: Solar (PV and CSP) • Non-Renewable Sources: • Rotating (CHP): Micro-Turbine and Fuel Oil • Non-Rotating: Battery Storage System DG 2014, EFCOG - DOE (Dr. PK Sen)

  24. Distribution Generation (DG) & Grounding • Rotating Generation: • Fault contribution is generally much higher (≈ 6 x FLA) • Fault current decays to (2-4 x FLA) after several cycles • Rotating DG can cause ratings for switchgear to be exceeded FLA: Full-Load Ampere 2014, EFCOG - DOE (Dr. PK Sen)

  25. Distribution Generation (DG) & Grounding: • Non-Rotating Generation: • Fault contribution is generally lower • Programmed (typically) to shut off within cycles • Fault contribution is so short (and transient nature) causing a false trip due 2014, EFCOG - DOE (Dr. PK Sen)

  26. Distribution Generation (DG) & Grounding G Generation • The focus is on larger (1.0MVA and higher) 3-phase connected DG, not smaller single phase systems • The area which DG grounding effects are: • Local circuit (green) • Distribution circuit (blue) • Effect on Transmission system is insignificant for low penetration Transmission Transmission Network Distribution ~ DG 2014, EFCOG - DOE (Dr. PK Sen)

  27. DG Grounding Issues • Four grounding issues which need to be considered when connecting a larger 3-phase DG system: • Protection Coordination Issues • Equipment Rating Issues • Islanding & Overvoltage Issues • Personnel Safety 2014, EFCOG - DOE (Dr. PK Sen)

  28. IEEE Std. 1547.2 Distributed Generation: Grounding Recommendations “The grounding scheme of the DR interconnection shall not cause over-voltages that exceed the rating of the equipment connected to the Area EPS and shall not disrupt the coordination of the ground fault protection on the Area EPS.” [IEEE 1547.2-2008]. • Two Issues: overvoltage and mis-coordination issues, in reality it is a trade-off in the design for larger DG. • Recommendations in IEEE Std. 1547.2, grounding is one issue that does not have a ‘one-size fits all’ requirement or approach 2014, EFCOG - DOE (Dr. PK Sen)

  29. Utility Practice for Grounding DG: Concern for Damage to their System • The increased usage of DG resources has led to most utilities requiring specific transformer connections, protective relaying schemes or other upgrades before a customer can install and interconnect a DG resource. • Three-phase connected DG resources are the primary concern, although requirements change depending on the size and type of the DG resource being installed. 2014, EFCOG - DOE (Dr. PK Sen)

  30. Utility Practice for Grounding DG: Concern for Damage to their System • PG&E, San Francisco, California • 3-wire “Delta” primary (Utility Side) connections recommend a PT with (59N) Examples 2014, EFCOG - DOE (Dr. PK Sen)

  31. Utility Practice for Grounding DG: Concern for Damage to their System • PG&E, San Francisco, California • 3-wire “Wye-Delta” connections recommend (59N) – high resistance grounding 2014, EFCOG - DOE (Dr. PK Sen)

  32. Utility Practice for Grounding DG: Concern for Damage to their System • PG&E, San Francisco, California • 4-wire (grounded) “Wye-Delta” recommend (51G) in transformer neutral 2014, EFCOG - DOE (Dr. PK Sen)

  33. Utility Practice for Grounding DG: Concern for Damage to their System • PG&E, San Francisco, California • “Delta-Wye” connections recommend grounding transformer (grounded wye-delta) with (51G) 2014, EFCOG - DOE (Dr. PK Sen)

  34. Utility Practice for Grounding DG: Concern for Damage to their System • OPPD in Omaha, Nebraska • Typically only allow (grounded) “Wye – (grounded) Wye” transformers • May require DG owner to install or upgrade utility transformer insulation levels and increase lightning arrestor levels Examples 2014, EFCOG - DOE (Dr. PK Sen)

  35. Utility Practice for Grounding DG: Concern for Damage to their System • Utility interconnection practices do not always address whether DG can be from a rotating machine or a non-rotating machine. • These differences change how the grounding of the DG source and distribution transformer cause issues on the utility’s distribution network • This can also affect the design and cost of a DG interconnection 2014, EFCOG - DOE (Dr. PK Sen)

  36. DG Transformer Connection:Advantages & Disadvantages 2014, EFCOG - DOE (Dr. PK Sen)

  37. Conclusions • DG is Increasingly becoming more and more Common and is Increasing in Size. • Most Utilities have DG Interconnection Guidelines, Regarding Transformer Connections and/or DG Grounding Requirements. • Most Utilities Require Interconnections to be “Effectively Grounded” During Islanding. • There are Trade-Offs with DG Applications, Transformer Connections, and Grounding: • Damaging Transient Over-Voltages • Equipment and Personnel Safety • Coordination Issues • Over-Voltages & Coordination Issues. 2014, EFCOG - DOE (Dr. PK Sen)

  38. Questions? 2014, EFCOG - DOE (Dr. PK Sen)

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