PV (solar) Systems - Short circuit failure

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Hello Freaks!!! How long? laugh

 

Guys I've been working with On-Grid PV Systems, all kind of range Powers.

After a course last weekend when I saw some bad failures and when the DC electric arc starts  surprise. And this makes me thinks...

 

The general protection used for PV Systems on the DC side (solar modules) is main breaks and fuses for the strings, BUT the problem is that the maximum operation current is only 10% below the short circuit. So when the main breaks or the fuses will trip? It will be hard to trip and the failure will keep until the sun goes down crying

 

There is some other protection for it?

Regards,

Bruno Muswieck

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brunomusw wrote:
BUT the problem is that the maximum operation current is only 10% below the short circuit.

That could be the maximum 'Efficient" operating current and not what the panel is capable of for a surge.  My 12v 7Ah SLA battery can supply 40 amps of current...for a short period, for example.

 

brunomusw wrote:
It will be hard to trip and the failure will keep until the sun goes down

Agreed, and I really have no answer on your observation.

 

JIm

 

 

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Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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brunomusw wrote:
The general protection used for PV Systems on the DC side (solar modules) is main breaks and fuses for the strings, BUT the problem is that the maximum operation current is only 10% below the short circuit. So when the main breaks or the fuses will trip? It will be hard to trip and the failure will keep until the sun goes down

 

Fuses and CB's are used not to protect the equipment, but is there to protect the wiring after the circuit protection device, to prevent over heating leading to fires.

 

A Solar panel is a current source, and can not be harmed by a short circuit, in fact, the spec sheet on the back side of the panel will state Voc (Voltage Open Circuit) and Isc (Current Short Circuit).

It may also state its max power voltage/amp values as well.

Any wiring used on the panel side (DC) should be rated for the max voltage of the string, AND the max current of the string, so no wiring circuit protection is generally needed, unless it is required by local regulations.

As no harm is done to the panel if it is short circuited, and the wiring should be able to handle that current with out over heating!

Any disconnect switch needs to be rated for the DC load and capable of handling a full load disconnect (and arc) when opened.

 

Hope that helps.

Jim

edit:spelling

 

 

 

 

Last Edited: Fri. Jan 25, 2019 - 02:02 PM
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Thanks Jim.

 

East Coast Jim

I would rather attempt something great and fail, than attempt nothing and succeed - Fortune Cookie

 

"The critical shortage here is not stuff, but time." - Johan Ekdahl

 

"Step N is required before you can do step N+1!" - ka7ehk

 

"If you want a career with a known path - become an undertaker. Dead people don't sue!" - Kartman

"Why is there a "Highway to Hell" and only a "Stairway to Heaven"? A prediction of the expected traffic load?"  - Lee "theusch"

 

Speak sweetly. It makes your words easier to digest when at a later date you have to eat them ;-)  - Source Unknown

Please Read: Code-of-Conduct

Atmel Studio6.2/AS7, DipTrace, Quartus, MPLAB, RSLogix user

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Your observation is correct; remember the  fuse generally protects what comes after the fuse.  In your house a 20amp breaker protects the wiring leaving the breakerbox, not the main 200 amp line.  You could still have something plugged in that overheats & causes damage...without tripping the breaker.   The overheating device should have a smaller fuse that protects it.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Consider a magnetic CB instead of a thermal CB.

"Experience is what enables you to recognise a mistake the second time you make it."

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"Fast.  Cheap.  Good.  Pick two."

"We see a lot of arses on handlebars around here." - [J Ekdahl]

 

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likewise are the DC sockets in some data centers (48V sealed lead acid battery stack so up to 60Vdc)

 

"Dare to be naïve." - Buckminster Fuller

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Batteries can deliver high surge currents because there is energy stored in there.

Solar panels can only transform the energy they receive.

brunomusw wrote:
It will be hard to trip and the failure will keep until the sun goes down 

If you're really concerned you can put some kind of hinges on them to rotate the panels out of the sun, or trow blankets over the panels.

 

I can imagine that if the solar converter failed and the solar panels continue pumping a few kW of energy into the converter that can lead to unhealthy situations.

But the fault would have to have just the right impedance near the MPPT point for the panels to deliver much energy.

 

From what I know it is not a good idea to try to open one of the connectors of the solar panels. High DC voltage & current generates long sparks, and you definately do not want to hold each of such a cable in your hands. 

As ki0bk already stated, solar panels do not get damaged by a short. So a thermal fuse in the converter cabinet with some MOSfet's or old fashioned relay that short the solar panels will at least prevent the panels from pumping energy into a faulty circuit.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Any wiring used on the panel side (DC) should be rated for the max voltage of the string, AND the max current of the string, so no wiring circuit protection is generally needed, unless it is required by local regulations.

I aggre.. but some switch equipament is needed..

 

In your house a 20amp breaker protects the wiring leaving the breakerbox, not the main 200 amp line.  You could still have something plugged in that overheats & causes damage...without tripping the breaker. 

  Yes but you are talking about AC circuit where it can delivery all current to trip the main breaker, different situation for the Solar modules, they will delivery only the Isc.

 

likewise are the DC sockets in some data centers (48V sealed lead acid battery stack so up to 60Vdc)

How is that? What is the ideia?

 

From what I know it is not a good idea to try to open one of the connectors of the solar panels. High DC voltage & current generates long sparks, and you definately do not want to hold each of such a cable in your hands. 

  Really bad arc.... :(

 

So a thermal fuse in the converter cabinet with some MOSfet's or old fashioned relay that short the solar panels will at least prevent the panels from pumping energy into a faulty circuit.

  How a thermal fuse will trip? What is the ideia?

Regards,

Bruno Muswieck

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I was curious as to what kind of arcs you can get so I typed in:

https://www.youtube.com/results?search_query=solar+panel+arc

 

Nice sparks from 800V 8A DC. Sunglasses & sun cream recommended :)

 

This also leads to stuff from mitigating risks and safety equipment.

Imagine for example fire fighters which need access to a roof covered with solar panels damaged by the fire.

 

One of the products I saw was an electronic device which can apparently measure if there is an arc anywhere in the loop and open the circuit, but this was for AC circuits.

For 800V DC you need a pretty big magnetic switch.

Another product is some kind of foam to be sprayed over the solar panels themself to passivate them by removing the sunlight.

There are probably more products but I'm not all that interested.

Doing magic with a USD 7 Logic Analyser: https://www.avrfreaks.net/comment/2421756#comment-2421756

Bunch of old projects with AVR's: http://www.hoevendesign.com

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Managing DC circuits is hardly new. PV is just a new application. There are products specifically for this application - https://www.socomec.com.au/range-load-break-switches-photovoltaic-applications_en.html?product=/sirco-mc-pv-iec-60947-3_en.html

As well, in certain markets the inverters are required to have arc detection circuitry that will shut down the inverter, thus removing the load and giving the load break switch an easier time.

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brunomusw wrote:
How is that? What is the ideia?

Development of DC plugs and socket outlets for smart grid and data center : FUJITSU COMPONENT

...

 

Arc extinguish

When unplugging, built-in arc extinguish module, which uses high density magnet field, cut off the arc discharge forcefully by turning off the operating switch provided on socket outlets.

 

...

brunomusw wrote:
Really bad arc.... :(
indeed

A logistics engineer for medium truck vehtronics (24Vdc) said he saw arcs during mating and de-mating a connector.

The solution was to add software such that the operator could de-energize that circuit during maintenance and repair operations.

 

"Dare to be naïve." - Buckminster Fuller

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Automatic panel level safety switches are available from a number of manufacturers. There are several optimizer modules that include safety switches as well. The hazard of high DC voltages is one of the reasons I went with microinverters on my home system.