PV/RCD problem

[Spherenergy]

Hello,
This one is a real head scratcher and I would appreciate any help.

We have installed a 4kw PV system in a field. We ran a 10mm 3 core SWA from the inverter in the field down to a DB in an outbuilding. The outbuilding DB is a 10 way Wylex with a 100 amp main switch. The cable supplying this DB is protected by an 80amp 30 milli amp double pole RCD located in the main building.

The outbuilding DB supplies lighting, sockets, outside lights, swimming pool pump, swimming pool heater, garage and garden lights. As well as our new PV installation. Before we started everything was working ok and the RCD was holding in.

The PV system has been tested and commissioned.

With all of the other breakers in the outbuilding DB off the PV array works fine and feeds back thought the double pole RCD in the house with out any problems.
If the lights, sockets and garden lights are on as well as the PV it all works fine.
If all the breakers are on with the PV generating after a minute or two the RCD trips.
With all the breakers on but with the PV not generating (at night) everything is fine.
As soon as the PV starts generating with all the breakers on the RCD trips.
Everything on and PV off everything is fine.
It's only when the garage, swimming pool pump or swimming pool heater are turned on and the PV is generating does the RCD trip.

I understand that a buildup of multiple earth to neutral faults over a few circuits will eventually hit the 30 milli amp limit but surely a neutral to earth fault would be present with the breakers on or off? So why is it only when the PV is generating and the garage, swimming pool heater and swimming pool pump are on does the RCD trip off????

The only thing I can think of doing is replacing the RCD with a new one but this method rarely works as new RCDs are normally more sensitive.

There is no room in the board to fit RCBOs.

Thanks for any help or advice.

Regards
Tom

 

[BruceB]

what sort of inverter are you using?

 

[Q.S.]

Hi Tom, Check to see if the RCD is D.C. sensitive - however the more likely cause is earth leakage from having so much on one RCD, you need to have local multi RCD protection in the outbuilding to prevent this. Often you will test a circuit and it is clear but when you turn on an electronic load it generates a natural earth leakage, even heating controls etc. The best way to see whats going on is to use a low reading (mA) clamp meter round both the phase and neutral. This will show you what the RCD is seeing. Try this with different loads on and off to see if the is one thing that is causing a real problem.

 

[Spherenergy]

Bruce- we installed an SMA sunny boy 4000TL

QS- Thanks, that makes sense. I'll try the clip on meter tomorrow. This doesn't explain why the RCD holds in at night when all breakers are on but the PV isn't generating?

 

[MorganPVI]

Maybe you should read the manual that comes with the inverter?

 

[Graeme Harrold]

When reading, take note of cable size and max length and RCD protection. With the field mount, is the array frame adequately earthed?

 

[Spherenergy]

Morgan- I don't see how the problem can be anything to do with the inverter alone as the PV system generates and feeds back through the RCD without any problem if the other breakers are off. I think as QS said it is an earth leakage fault.

 

[Sparky198421]

Did you carry out an rcd test out to confirm rcd is ok? that does sound like a head bake! the only thing i can think of is installing an earth rod to the outbuilding and not use the earth in the cable! Installing a rcd in the outbuilding as above!

 

[MorganPVI]

I really do recommend reading the installation manual that conveniently comes with the inverter in the box.

 

[Spherenergy]

Morgan- I have read the installation manual numerous times, is there something that im missing???

 

[Spherenergy]

Sparky198421- the RCD seems to be working fine when tested.

Graeme Harrold- the cable length is 85m, 10mm 3 core. The ground mounted system is bonded to the supply cable.

 

[MorganPVI]

Morgan- I have read the installation manual numerous times, is there something that im missing???

5.3 Connection to the Power Distribution Grid (AC)
5.3.1 Conditions for the AC Connection
• Comply with the connection requirements of your utility operator.
Residual current protective device
The inverter is equipped with an integrated all-pole sensitive failure current monitoring unit. The
inverter can automatically distinguish between real fault currents and "normal" capacitive leakage
currents.
If an external RCD or residual current breaker is strictly required, you must use a switch that triggers
at a failure current of 100 mA or higher.
Cable sizing
The grid impedance of the AC cable must not exceed 1 ohm. Otherwise, the inverter will disconnect
at full feed capacity due to excessive voltage at the feed-in point.
The conductor cross-section should be dimensioned such that cable losses do not exceed 1 % at
nominal power. Use "Sunny Design" (SUNNY DESIGN. SMA Solar Technology AG) for this.
The maximum cable lengths relative to the conductor cross-section are shown in the following table.
The conductor cross-section required in individual cases depends on the following factors, among
others:
• Ambient temperature,
• Routing method,
• Cable losses,
• Valid installation requirements of the respective country (installation location).

Page 26 http://files.sma.de/dl/5692/SB30TL_40TL_50TL-IEN103630.pdf

Your max run on a 10mm cable is 38.8m

 

[edexlab]

page 26 5.3.1 of the manual refers to selection of Rcd size

 

[Spherenergy]

Morgan- thanks for that, looks like I'll have to change the DB for a split load and put the PV on the non RCD.

 

[edexlab]

MorganPVI obviously quicker at typing than me!

Why change the Db for a split load, most installers will fit a one way cu just for the Pv