Protection for grid-tie systems

lazza · September 2012

HI

I was wondering what the forum considers the most adequate protection between the PV array and the grid-tie inverter for say, a 400V grid-tie PV series of around 5A Imp.

I have seen normal DINrail 10A fuses 10x38mm being used. The problem is these tend to arc on extraction.... begging the question of whether they are suitable at all for this purpose ??

ggunn · September 2012

Re: Protection for grid-tie systems

lazza wrote: »

HI

I was wondering what the forum considers the most adequate protection between the PV array and the grid-tie inverter for say, a 400V grid-tie PV series of around 5A Imp.

I have seen normal DINrail 10A fuses 10x38mm being used. The problem is these tend to arc on extraction.... begging the question of whether they are suitable at all for this purpose ??

You should NEVER pull fuses under load unless the fuse holder is load break rated. I don't know if any of them are. Your system should have a DC disconnect; many inverters have integrated discos. You should open the disco before opening the fuse blocks.

inetdog · September 2012

Re: Protection for grid-tie systems

ggunn wrote: »

You should NEVER pull fuses under load unless the fuse holder is load break rated. I don't know if any of them are. Your system should have a DC disconnect; many inverters have integrated discos. You should open the disco before opening the fuse blocks.

Same with disconnecting MC4 connectors under load, and to a lesser degree connecting them under load.

If you don't have a disconnect with safe interrupting capability, cover the panels.

BB. · September 2012

Re: Protection for grid-tie systems

A touch safe fuse holder used for solar panel combiners will actually catch fire when the holder is opened under load (one brand model at least).

-Bill

jaggedben · September 2012

Re: Protection for grid-tie systems

lazza wrote: »

HI

I was wondering what the forum considers the most adequate protection between the PV array and the grid-tie inverter for say, a 400V grid-tie PV series of around 5A Imp.

I have seen normal DINrail 10A fuses 10x38mm being used. The problem is these tend to arc on extraction.... begging the question of whether they are suitable at all for this purpose ??

Are you asking about 'overcurrent protection' or 'disconnecting means'? They are two different things. Overcurrent protection is generally only required if you have more than two strings. Disconnecting means are always required, and as inetdog said you need a disconnect with a safe interrupting rating.

inetdog · September 2012

Re: Protection for grid-tie systems

jaggedben wrote: »

Are you asking about 'overcurrent protection' or 'disconnecting means'? They are two different things. Overcurrent protection is generally only required if you have more than two strings. Disconnecting means are always required, and as inetdog said you need a disconnect with a safe interrupting rating.

One thing which is also often overlooked is that the fuseholders and the fuses within them have to be capable of interrupting a high DC voltage (particularly in the case of GTI strings or strings for a 48 volt battery system. Automotive type fuses (and fuseholders) can fail miserably under those conditions, and a fuseholder which can catch fire when you pull the fuse can also catch fire when the fuse fails to interrupt the current safely. So the first step is to confirm that the fuseholder and fuse each have a DC rating for current and voltage which matches your system. Back in the days when nominal 12 volt panels were used to charge 12 volt batteries using a PWM controller, the interrupting capability of the fuses was not such a big issue.

lazza · September 2012

Re: Protection for grid-tie systems

Thanks for your replies.

The system I was looking at had a failing inverter. Yet, I figure that the fuses would only arc if there was current flowing (under load).. so what this means is that current was flowing, even if the inverter was not recognising it.. does this sound correct?

ggunn · September 2012

Re: Protection for grid-tie systems

lazza wrote: »

Thanks for your replies.

The system I was looking at had a failing inverter. Yet, I figure that the fuses would only arc if there was current flowing (under load).. so what this means is that current was flowing, even if the inverter was not recognising it.. does this sound correct?

DC circuits will only arc if current is flowing when you break the circuit; that much is correct.

lazza · September 2012

Re: Protection for grid-tie systems

Well that's a lesson learnt! Will check with clamp meter in future, even if inverter says there is no current...

BB. · September 2012

Re: Protection for grid-tie systems

DC current clamp meter. The AC current/voltage detection devices won't work with DC.

-Bill

lazza · September 2012

Re: Protection for grid-tie systems

No worries, my clamp meter does DC.. i'd normally always check first, but as the inverter seemed to be registering zero.. I wrongly assumed no current was present... I dont like to make mistakes with 400 Volts , so it wont be happening again!

The truth is it's made me doubt the use of MMPT systems in general for off-grid (for grid-tie there isnt a choice). In our off-grid systems, we only put in normal PWM type charge controllers, basically due to cost, although it's a shame to lose on the efficiency side. However after this episode, i'm thinking that maybe simply for saftey's sake.. it's best to leave off-grid systems at 24V. People have a tendency to do silly things on their installations, and we could at least avoid risk on the DC side of the system. Food for thought

Larry

westbranch · September 2012

Re: Protection for grid-tie systems

lazza wrote: »

, i'm thinking that maybe simply for saftey's sake.. it's best to leave off-grid systems at 24V. ..... Food for thought

Larry

yup, that's why I opted for 24 vs 48V on our new system.

Protection for grid-tie systems

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