solar power inverter issues

[markthespark]

Hi guys I have another interesting situation which has me seriously confused. I was called to a commercial site which has solar power, but the complaint was that some of the socket oulets were dead. I do not have much experience with solar power circuitry but what i found was really interesting! On arrival, after checking for voltage on different circuits I was getting voltages feeding from the inverter that ranged from 100v to 300v which fluctuated like crazy. Almost as though I was testing capacitors. I also had more than 200v between earth and neutral. I subsequently found that the Eskom 40amp double pole supply feeding the inverter had tripped. After resetting this breaker everything tested normal! When I switch off the breaker, again I get these crazy variances. I also find that my plug tester indicates phase and neutral reversed and yet I have thoroughly checked and found no reversal. The LED lighting throughout the building work like normal! But with Eskom feeding the inverter everything tests as normal! Anyone out there with explanations please?

[Dave A]

Sounds like neutral to earth isn't bridged when the installation is isolated from the main supply, so the neutral is floating.

[Justloadit]

Usually with invertors, when there is a power failure, the incoming supply is isolated from the load. The reason for this is that the negative of the battery is the common line in the bridge which creates the AC in the load. The battery is then usually earthed. This makes a certain part of the bridge overload/current unprotected in the case of a Neutral short to earth, and would destroy the electronic switches in the invertor. As regulation calls for an earth to be used, the only way that this can be achieved is to isolate the load from the mains when the invertor is operational.

There is also another reason for both the Neutral and Line to be isolated from the load and mains incoming during the power failure, is that when the mains returns, the switch over from the invertor to the mains is not synchronised, and causes an arc between the No and NC contacts via the common contacts, the isolation of both the Neutral and Live, allow the arc to dissipate into the air so to speak, rather than find its way as feed back to the invertor electronics bridge, and cause catastrophic failure of the bridge due to the high voltage in the reverse direction.

Yes of course with more complex electronics that can overcome all this, in the case of isolation, and isolating transformer can be used, then there would be no issue with the isolated supply, and electronics can be used to synchronise the invertor to the mains before switch over, but at the end of the day, he who has the cheapest unit usually gets the sale, so I have experienced many a time

[Dave A]

Mark, you probably need to take a close look at section 7.12.3 and diagrams S.3 and S.4 of SANS 10124-1.
If this alternative supply is supplying part of a fixed electrical installation, there may be a compliance issue.

[markthespark]

Thanks a ton for the advice guys, I do think their is an earth issue but exactly how should I earth the system? Do i put an earth spike alongside the inverter and connect the negative side of the batteries to this? Or should I run another earth from the batteries to the existing earth spike?

Dave thanks I'll read up on those regs and check the installation again.