Sudden disconnection from the grid of a photovoltaic system: effects on inverters

[Sempronio1960bis]

Good evening, a silly question, I don't know the sector.
What happens to the inverters in the event of a sudden disconnection from the grid of a photovoltaic system, for example due to an intervention of a RCD, or fuse, or circuit breaker between the inverter and the grid?

 

[itsmoked]

Inverters can suspend output in less than a line cycle unlike a generator. It shouldn't bother them at all. Further, if the current drops drastically there's no bunch of inductance like with a generator to cause a large overshoot spike.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fonefon]

Some grid codes require delayed inverter disconnection, up to 2 seconds in the most extreme cases (e.g. Hawaii). Inverter manufacturers have a certain degree of freedom to decide what to do during this disconnection, whether to momentarily cease power injection, or to inject capacitive current to try to hold the voltage in the case of microgrids.

 

[cranky108]

They don't delay disconnection, they are required to ride through a voltage excursion.

 

[bacon4life]

It is not at all a silly question; many folks have spent countless hours trying to figure out how inverters should respond to problems on the power grid.

In the early days of photovoltaic inverters, inverters were designed to set the output to zero (i.e. trip) as soon at they detected a disturbance on the grid. While this worked fine when PV was very rare, this approach was terrible for the power grid once PV exceeded a few percent of connected generation capacity.

The ideal control algorithm for modern grid connected inverters has two somewhat conflicting goals:
1) Stay online during short circuits on grid. This keeps the grid stable and allows the rest of the grid to correctly identify and isolation the equipment with the short circuit.
2)If the PV is isolated from the rest of the grid, turn off the output as soon as possible to avoid forming an electrical island.

The standards that govern inverter behavior continue to evolve, and manufactures have significant freedom within the standards to choose how they implement logic to detect and react to events on the power grid. An example of the alorithms working very poorly is the Blue Cut Fire PV Interruption report where enough solar PV to power about a million homes tripped offline.

 

[Sempronio1960bis]

This is my case: an inverter is connected to the grid via an RCD. The RCD often opens, so the entire photovoltaic system, from the panels to the inverter output, is powered by the panels but without load. The inverter has failed multiple times for no apparent reason, with the VDRs failing. Is it possible that this is due to overvoltages inside the inverter due to oscillations between inductances and capacitors inside the inverter?

 

[cranky108]

From what I have herd, it is also possible it failed because of poor grounding. And the installations I have seen lately, they even install a grounding bank on the high side to be sure it does not float.

 

[bacon4life]

The inverter should be able to handle an upstream protective device opening without having the inverter fail.

Is this connected at les than 600 volts or at medium voltage (4 kV-35 kV)? Medium voltage vacuum circuit breakers can cause voltage spikes when opening, so sometimes surge arrestors are placed on the load side of vacuum breakers.

 

[electricuwe]

First check, if the VDRs failing are phase-phase or phase-ground.

If the VDRs failing are phase-ground, most likely an overvoltage phase-ground (e.g. lightning surge) on the grid is the root cause.

If this doesn't solve the issue continue as follows: Check if the RCD is suitable for the purpose and check the isolation of the PV system and inverter.

 

[crshears]

RCD: ReClosing Device? Another TLA [three-letter acronym] not defined . . . and what's a VDR?

 

[Sempronio1960bis]

VDR voltage-dependent resistor Link
RCD device Residual-current device Link

 

[crshears]

Thanks, Sempronio!

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Sempronio1960bis]

The inverter is connected at LV 400/230 Three-phase + N (Italy)