Inverter grounding and overvoltages. Michael Ropp, Ph.D., P.E. Northern Plains Power Technologies

Transcription

1 1 Inverter grounding and overvoltages Michael Ropp, Ph.D., P.E. Northern Plains Power Technologies

2 2 Scope This presentation will focus on inverter-based DG Includes all DC and variable-frequency primary sources Solar Type IV wind (and Type III in certain cases) Battery storage systems Looking mostly at transient overvoltage

3 3 Inverter overvoltage mechanisms Ground fault overvoltage (GFO) Inverter controls saturation is wrapped up in this one Load rejection overvoltage (LRO)

4 4 Need for detailed modeling Specifics of inverter controls and physics are key to understanding transient behavior Can t model inverters as constant current sources Averaged models must include detailed controls and model the effective gain of the power stage; otherwise GFO and LRO are overpredicted

5 5 Key point to remember

6 6 Ground Fault Overvoltage (GFO) Pre-fault During an SLG fault V center V n 0

7 GFO and inverters why not just require effective grounding? 7 Normal procedure is to try to ensure X 0 /X 1 < 3 and R 0 /X 1 < 1 (IEEE ) using a grounding transformer. But for inverters, R 1 and X 1 are software-defined and not constant, and there s usually a zero-sequence open circuit. IEEE (going to ballot now!) suggests a solution Many grounding transformers may impact utility fault detection and coordination. Ground potential rise at the PV plant must be considered (be sure to use the right fault current IEEE 367 clause 4.4).

8 8 Fundamental mechanisms of GFO Assume a Yg-Yg GSU transformer. Rotating machines support GFO because they act like voltage sources behind impedance. Inverters act as current sources. They do not enforce phase-phase voltage, and thus do not cause GFO. The load impedance (usually neglected from the sequence networks) plays a critical role. Load completes negative and zero sequences Generation-load ratio is critical, as is Yg to delta load ratio

9 9 Conclusions: inverters and GFO Effective grounding of inverters for GFO reasons is probably not necessary. Important caveats: If the GSU transformer has a delta on the MV (feeder) side, GFO is possible because of the transformer, regardless of DG type. If the inverter controls saturate, it may no longer act as a current source, and then it can support GFO.

10 GFO simulation results 10

11 11 GFO and islanding detection It would be best to separate islanding detection and fault detection Islanding detection primarily designed to detect island formation when local generation and loads are very closely matched; has 2 sec to do so No generation-load matching during a fault; many other conditions to detect, and needs to act faster Inverters could easily have the smarts to do fault detection, especially with Yg-Yg GSU transformers

12 12 Load Rejection Overvoltage (LRO) I source

13 13 Load rejection overvoltage Load rejection overvoltage can happen with inverters However, levels of LRO for inverter-based DG are frequently overpredicted NOT linear in generation:load ratio Effective gain of inverter power stage drops as AC voltage rises Cannot neglect power-limited nature of DG Often very short-lived

14 14 Example LRO case: 1 PV, partial G 50% irradiance

15 15 Example LRO case: 2 PVs + WT, full power 100% irradiance

16 16 LRO mitigation Because of required speed of response, LRO should be mitigated in the inverter DTT isn t fast enough External meters usually aren t fast enough Software solutions exist Key factors: Fast measurements Stop gating; don t open a breaker No delay in tripping Alternative: restrict DG penetration levels

17 17 LRO testing and certification Because LRO is a potential issue but solutions already exist, should incorporate LRO testing into UL-1741 or something similar Efforts already under way to do this. Example: the Industry Task Force on Effective Grounding (ITFEG) has been working on such a test for several months.

18 18 Conclusions GFO likely is not a problem with inverterbased DG, but until this is proven many utilities are requiring effective grounding, which brings its own set of challenges LRO can be an issue with inverters, but inverter-based solutions exist; testing and certification will be key