Multiplus II Integration to UPS-Backed Off-Grid Three-Phase Generator

Q&A and troubleshooting
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1

Hi!

I’m an electrical engineer and I’d like to verify if the solution that I have in mind is fine. You’ll find the details below.

I have a customer that has a mountain lodge. They are currently generating electricity using a 10kVA three phase generator driven by a turbine. The voltages in the output are a mess. I ran some measurements and found out that in one day, the 380V-rated system had over 1000 events of over voltages (up to 450V) and under voltages (up tu 330V). Because of this, their appliances are constantly dying. The frequency is not an issue, it was stable. The loads are fairly constant, 5kW 6,5kVA.

I first considered using a voltage stabilizer to stabilize the voltages (duh) but given that the transients are quite quick and the price difference is not that high, I ended up selecting an online UPS to do this. The UPS has a wide input range which is suited to the measurements I did and can provide stable three-phase power to the loads. I already selected a 10kVA three phase pure sinewave UPS which is more than enough to power all loads (Enersafe’s UPSESOLTRIPT10CB model). The online-mode efficiency is around 94%, which is acceptable. In case the demand grows, more UPSs can be connected in parallel to increase the output. This will of course require an overhaul to the turbine+generator stack.

The customer already has three MultiPlus II 5000/48V Inverters and six 48V 4800Wh Lithium-Ion batteries that are currently not properly installed. This means, the inverters never supply the loads, they just mantain the batteries charged. My idea is to connect the UPS output to the inverter’s AC input in order to achieve two things. Firstly, provide backup power in case the UPS fails, needs maintenance or their internal batteries run out, or if the turbine+generator stack has issues (clogged pipes, broken belt). Secondly, use the PowerAssist function to provide power in case the demand exceeds 9.5kVA. This never happened during the measurements, but it’s something useful to have in order never to overload the UPS. For this I’ll need the optional CTs.

Here are some additional considerations:

  • The AVR and frequency controller will still be connected to the generator output and not the UPS output, so that the generator runs fine.
  • The UPS has internal batteries that can only sustain the loads for about 5 minutes. Therefore, the UPS will only be used to stabilize the voltages. The existing batteries are not compatible with the UPS’s DC voltages.
  • I don’t want to use the inverters to stabilize the generator’s voltages. Connecting the generator’s output directly to the inverters and setting up input voltage limits in the inverters to have clean voltages in the AC OUT terminals would make the internal contactors operate over 1000 times a day. I’m not sure how long they would last, but I don’t want to have to change the inverters because some small contactor failed.

I need to know if the inverters can run into issues when connected to the UPS’s output. I imagine this shouldn’t be an issue, but I was unable to find application notes or any other documents showing such a topology. Please let me know if this is something that can be done.

Kind regards,

Cristián

2

Hi @CristianLadisla

What type of loads does the customer have?
Some loads seem to be constant but in realty are PWM like induction stoves switching on/off every second.

Maybe this switching causes fast load changes, generator RPM changes and in finally voltage fluctuations of the turbine based generator.

Does the generator have a flywheel to stabilize frequency? Did you measure frequency changes in 1 second periods?

Yes, theoretically you can put the UPS in between generator and MP2 AC_in. Then you need to configure no grid code and maybe deactivate LOM detection. With LOM the MP2 will try to change frequency to detect LOM.

I would try to simplify the system.

Option 1 direct AC-coupling:
If MP2 stabilize the voltage, this is the easiest setup.
Build a hybrid generator with MP2 and turbine at AC_in. Start turbine to recharge the battery. Cycle batteries between 30-100% SOC.
Cycle efficiency will be around 80% but generator might be of 25% of time.

Option 2 DC-coupled:
If MP2 do not stabilize or do not accept generator.
Serve the loads from MP2 / battery.
Get some high efficient telco PSUs to convert generator AC to 48V-DC.
This will work in any case. Huawei and Eltek FlatPack 2 are suitable single phase PSUs. You get them used around 150 EUR. Maybe have a spare and plugs for self replacement.

Additionally:
Add PV to lower generator usage.
Increase battery size to get longer cycle times.