Inverter grounding and overvoltages

Michael Ropp, Ph.D., P.E.

Northern Plains Power Technologies

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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

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Inverter overvoltage mechanisms

Ground fault overvoltage (GFO)

Inverter controls saturation is wrapped up in this one

Load rejection overvoltage (LRO)

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Need for detailed modeling

Specifics of inverter controls and physics are key to understanding transient behavior

Cant 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

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Key point to remember

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Ground Fault Overvoltage (GFO)

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Pre-fault During an SLG fault

Vcenter Vn 0

GFO and inverterswhy not just require effective grounding?

Normal procedure is to try to ensure X0/X1 < 3 and R0/X1 < 1 (IEEE 142-2007) using a grounding transformer.

But for inverters, R1 and X1 are software-defined and not constant, and theres usually a zero-sequence open circuit. IEEE 1547.8 (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 currentIEEE 367 clause 4.4).

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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

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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.

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GFO simulation results

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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

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Load Rejection Overvoltage (LRO)

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Isource

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

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Example LRO case: 1 PV, partial G

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50% irradiance

Example LRO case: 2 PVs + WT, full power

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100% irradiance

LRO mitigation Because of required speed of response, LRO should

be mitigated in the inverter

DTT isnt fast enough

External meters usually arent fast enough

Software solutions exist

Key factors:

Fast measurements

Stop gating; dont open a breaker

No delay in tripping

Alternative: restrict DG penetration levels

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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.

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Conclusions GFO likely is not a problem with inverter-

based 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

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