Grounding and Overvoltage Requirements for Distributed Generation (Wind and Solar) Reigh Walling Walling Energy Systems Consulting, LLC

Transcription

1 1 Grounding and Overvoltage Requirements for Distributed Generation (Wind and Solar) Reigh Walling Walling Energy Systems Consulting, LLC

2 2 Scope of Presentation We will cover: Temporary and transient overvoltages on distribution feeders Related to interconnection of distributed wind and solar plants Ways to avoid or mitigate these overvoltages

3 3 Overvoltage Definitions Transient overvoltage impulses and supersynchronous oscillations lasting less than a couple of cycles Examples: lightning, switching, etc. Temporary overvoltage oscillatory overvoltage persisting for many cycles to seconds Examples: load rejection, single-phase faults, ferroresonance, etc.

4 4 Impacts of Temporary Overvoltage Utility equipment surge arresters

5 5 Surge Arresters Surge arresters are designed to limit transient overvoltages, not intended to limit TOV Surge arresters are the likely victims of TOV Failure modes of surge arresters MOV material fails to a low resistance Physical integrity of arrester housing uncertain Ground lead disconnects blow off

6 6 Impacts of TOV on Customers 1.2 p.u. for 3 ms!! TOV withstands of consumer equipment are poorly documented Not defined by any formal standards ITIC (formerly CBEMA) curve is often cited Experience suggests ITIC curve is excessively conservative TOV can result in large claims against utility

7 7 Loss of Ground Scenario Focus of utility concern regarding DG plants Ground fault on feeder Feeder breaker trips; losing normal ground source DG doesn t trip immediately, continues to energize If there is no ground source, high TOV may result C B N Phase A faulted to ground 1 p.u. A 1 p.u p.u.

8 8 Effectively Grounded Systems Definition of effectively grounded system is where the COG < 0.8 (COG = TOV/V L-L ) TOV < 1.39 p.u. in effectively grounded systems C62.91 states X 0 /X 1 <3 and R 0 /X 1 <1 generally results in COG<0.8, but is not the definition Multi-grounded feeders are designed to be effectively grounded Coordinates with arrester TOV capabilities Experience indicates customers are ok

9 9 Ground Sources Desired and Unintended Ground sources are the zero-sequence admittances of the circuit Primary substation transformer breaker open Cable charging and grounded cap banks Grounded-wye loads Grounded-wye delta and zig-zag transformers May be added to feeder to mitigate loss-of-ground Subject to overload from feeder imbalance

10 10 Classic TOV Analysis Sym. component network for single-phase fault Z 1 x V 1 Z 2 x x V 2 I 0 x Z 0 x V 0 x Generators conventionally assumed to be voltage sources behind impedance Loads and shunt capacitance usually ignored

11 11 Wind and Solar Sources Solar (PV) inverters grid interactive Controlled current sources, not voltage source Three-phase inverters are ungrounded source Three-phase inverters may not pass I 2 Wind generation Legacy induction generators (Types 1 and 2) approximately a voltage source until flux collapses Full conversion (Type 4) are inverters just like PV Doubly-fed (Type 3) are current sources until they crowbar, then they are like an induction generator

12 12 Conventional X 0 /X 1 only applies to: Solar (PV) inverters grid interactive Controlled current sources, not voltage source Three-phase inverters are ungrounded source Three-phase inverters may not pass I 2 Wind generation Legacy induction generators (Types 1 and 2) approximately a voltage source until flux collapses Full conversion (Type 4) are inverters just like PV Doubly-fed (Type 3) are current sources until they crowbar, then they are like an induction generator

13 13 Loading Impact on Type 1 and Type 3(crowbarred) WTG TOV These are the only wind and solar DG that can be assumed to behave as a voltage source

14 14 Inverter-Interfaced DG The following behave as current sources: PV Type 4 wind turbines Type 3 (wind turbines when not crowbarred) Conventional X 0 /X 1 criteria do not apply

15 15 Other DG Overvoltage Issues Abrupt isolation of inverter into light load Known as load rejection overvoltage Tests and field events have shown voltage > 2 p.u. Inverter should trip immediately Reclose out of phase of a compensated feeder

16 16 Other DG Overvoltage Issues Self-excitation of induction generator Light load Sufficient capacitive compensation

17 17 Overvoltage Mitigation Provision of ground source where needed Grounded-wye delta or zig-zag grounding banks Grounded-wye delta interconnection transformer Grounded wye-wye does not make a ground source Adequate load relative to DG Load to DG capacity ratio depends on type Coordinated transfer trip DG disconnected before feeder opens Coordinated grounding switch (crowbar)

18 18 Overvoltage Mitigation (cont d) Fast DG overvoltage tripping DG may not see TOV on primary side Coordination with VRT will be needed in future Fast islanding detection Increased challenge with evolving ride-through requirements Sacrificial arresters Difficult to coordinate with utility arresters Coordination with load withstand is uncertain

19 19 Conclusions Overvoltages, particularly TOV, can be a major issue for wind and solar plants interconnected to distribution Conventional X 0 /X 1 criteria are irrelevant for most wind and solar Overvoltage and grounding solutions exist, but must be appropriately analyzed and applied