Grid tie inverter connection to VFD AC input without grid voltage

jemsol

I have 250kw solar panels and two 160KW VFD to drive 2*120kw water pumps with a 300kw generator connected to a VFD via AC input while solar is via DC input. But currently we bought 5*50kw GTI. No grid at all!!!!!!
Could I connect some solar strings to a GTI then to a VFD AC input and some strings to a VFD via DC input as before?
Using VFD as battery charger to fool GTI.
Please please please help me...

BB.

Jemsol,

Welcome to the forum. Lets keep your discussion to this thread... My reply to another of your posts:

I am guessing you are somewhere in/around Ethopia?

I am certainly no expert in solar+large pumping projects... Guessing you are presently using 3 phase 160 kWatt VFDs (variable frequency drives). These are basically Variable frequency AC inverters. This drives a (large) 3 phase pump. The VFD provides both "soft start" (the inverter starts at "low frequency" and slowly brings the motor up to speed). And VFDs also let you run the pumps at less than maximum RPM (reduce water flow/reduced pressure).

In the last decade or so, VFDs have been interfaced with solar panels. Solar panels are connected to the power input of the VFD, and the VFD is programmed to run the pump as normal, plus monitor the solar input voltage/power--And can run the pump slower in the morning and faster around noontime in full sun.

Since I am not in the business... I really cannot give you suppliers' names and available systems. Where you purchased your VFDs would be a good place to start.

The difference between a standard AC input VFD like you have (AC 3 phase power in, VFD power out) vs solar (high voltage DC power in, VFD power out) is not that much different--And there are many places that market these devices (India, China, Europe) and I am sure they sell to Africa/Middle East, etc.

That would probably be your first step in researching the issue.

There could be a "helpful" option of connecting your GTI to the Diesel Genset AC output. The big question there is for your Genset Company/Designers/Engineers. In theory, GTI to Diesel AC output--The solar will "assist" the genset's AC output and can reduce fuel usage (always a good thing).

The problem is integrating the Solar GTI inverters with the Genset. It can be pretty easy to damage/smoke the genset alternator if the GTI output exceeds the Pump loading--Alternators don't really like power being fed back to them (GTI 150 kWatt, and pump 120 kWatt, 30 kWatt going back "into genset").

While almost anything can be done (from an engineering point of view)--It may not be cheap (lots of engineering, lots of control equipment, safety equipment, and such).

Anyway--From my perspective, I would look for Solar input VFDs and see if you can find what you need (for the right price). It could be your lowest cost/most efficient solution. However, you would have to have a pumping system that can manage variable speed pumps (i.e., lower RPM in morning/evening, full RPM middle of day, and impaired operations during cloudy weather).

Guessing in the region of Addis Ababa, a fixed solar array will see (hours of sun per day):

http://www.solarelectricityhandbook.com/solar-irradiance.html

Addis Ababa
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 81° angle from vertical:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
7.18	7.11	6.43	6.08	6.32	5.87
Jul	Aug	Sep	Oct	Nov	Dec
5.08	4.95	5.56	6.68	7.19	7.27

That is an impressive amount of sun year round... Typically >3 hours of sun per day is "not bad". And >5 hours of sun per day is very nice to have.

-Bill

jemsol

Respect from my heart 💔.
Thanks too much yes am from Ethiopia SNNP regional state Omorate Town.
I accept and respect everything you have said!!!!!
Let me express my pumping stations power.
I have two Siemens sinamics VFD(for 2*120kw pumps),
it's working currently as follow.
It has two inputs, 
1. Solar (DC, 510-720Vdc) input directly connected to a DC link of the VFD with 160KW solar power. Both VFD DC links are paralleled and interconnected share solar 160kw power.
2. Diesel generator (300kw) AC input shared for both VFD and rectified to DC - DC link.
3. One 3phase power output.
As you said as sunshine increase the generator power production decrease I,e fuel saving...
But recently my company bought 90kw solar panels and 5*50kw grid tie inverter for all site, by the way we have three pumping stations with the same power as I expressed above. So that my company bought 10*50kw GTI for two sites. Let me do correct thing for one site.
Can I do this?
Make two solar power, 120kw each which is equal to pump power then separately connect to the VFD one for each, one 120kw to one VFD then to one pump . Also subdivide this 120kw in to two power 60kw each then 60kw DC power directly to VFD DC input without GTI and the Left 60kw to a GTI then to VFD AC input line. Doing this for both VFD.
Does this activate my GTI?
                      OR
 Can I use PV genset device for what I have done?

BB.

That is great, your VFDs both have DC inputs you can correct directly to the solar arrays.

I am probably well beyond my knowledge here.... What are the GTI's? In general if these are Grid Tied/Interactive inverters--They are generally designed to connect to a stable 480 VAC 50 Hz (or whatever your local utility power is for your power levels). They monitor the input power from the grid (AC genset, etc.) and when they see stable voltage and frequency, they will then take whatever power is available from the attached solar array (this is usually MPPT--Or maximum power point tracking--Where the controller figures out the "equation" of Pmp=Vmp*Imp. every few seconds or every few minutes or even a fraction of second).

Because of the MPPT operation of the GTI, generally, each GTI needs its "own" solar array. If you put 2 or more GTI with MPPT on the same solar array , they can confuse each other (the GTI will adjust the current drawn and monitor the working voltage of the array to figure use Vmp at that moment in time). If you have two or more on one (electrically connected array) the two (or more) GTI's will confuse each other as one does the MPPT algorithm, the array voltage varies, and the other does its own current variation also causing array voltage to vary--And the Pmp=Vmp*Imp can break down as you have multiple GTI's "playing with the arra".

On the other hand, I know nothing about large GTI systems. It is very possible that they are synced together and "share" the MPPT function together--So the multiple GTI's can work together on one array. One small "home size" solar power systems, nothing is done like this (as far as I know).

On the AC output of the GTI's, in general they only track the AC Sine Wave voltage and output current based on the voltage. No current as the sine wave crosses zero volts, and maximum available current/power at the Peak (plus and minus peaks) of the AC power. So they need a "source" to sync too. That is how home/residential sized GTI (Grid Tied Inverters) work. No stable AC power, no output power to the national grid.

But--Again, there is are other inverter types that can take solar power in, and output stable AC power at xx Hz. The issue with AC power is there is little information in the 480 VAC at 50 Hz for the loads.

With the national grid (or a large generator), all the AC loads "assume" that they can pull as much or as little 480 VAC current as they need and the grid power is "unlimited" (up to the service rating).

With solar power, the amount of energy available from the solar array varies with time of day, weather, dirt on panels, etc. So a typical AC load does not have the ability to figure out how to vary is power draw (not much power draw morning/evening, and maximum power draw middle of day, etc.). You plug a 60 kWatt motor on an AC source with > 100 kWatt of power--No problem. As the sun goes down and the power available drops below 60 kWatts, then the AC load "collapses" the AC power supply and everything stops.

There are ways around this... For example vary the AC frequency. at 50.0 Hz, there is full array power available If there is less array power available, start to "dither" the AC frequency. For example, 50.0 Hz +/- 0.5 Hz (48.5/50.5 Hz) means only 50% power capacity available.. 50.0 Hz +/- 1.0 Hz, means that there is no more capacity available and the AC load(s) should shut down. That is done today on some GTI for some off grid solar power systems.

Again, you are working with some relatively large power systems and dedicated hardware designed specifically for your pumping system needs. So what is done in your installation--I can only guess.

DC voltage from solar arrays have the "available power" signal "naturally". I.e., If the DC bus is over 510 VDC--The VFD(s) can pull full energy. (if they have MPPT for solar, they would figure out that the array Vmp is 601 VDC and use that for 100% available power--If array voltage falls below 601 VDC, then the VFD would cut pump RPM and draw a bit less power to bring back to 601 VDC. If the voltage starts to rise above 601 VDC, then the VFD can increase pump RPM and draw more power and bring back to 601 VDC Vmp).

As you can guess--When you have one or more sources and one or more large power users--They can have system resonance issues.... I.e., while one VFD wih MPPT is cutting back on power draw, the other sees Vmp-array rise and thinks it can draw more power--Then the second VFD sees the Vmp-array fall, and cuts back while the first VFD decides to draw more power... And the systems can end up "hunting" a varying pump RPMs trying to get an optimum operating point....

It sounds like you have much more knowledge than I about the various hardware devices in your pumping system (your job and education, and no knowledge on my part--just guesses).

I am really out of my depth here... You understand how your system(s) work. Doing the interfacing on the DC voltage side of the VFDs sounds like a better place to start than using (standard) GTI solar inverters and working on the AC side--At least from a control theory point of view. Large system need complex controllers to monitor power sources (and power sinks like your VFD pump controllers).

For example, two large diesel generators. Each can run independently (i.e., only one genset needed for over night power) and the can both run together (say during the day with high power demands). And the controller(s) for the gensets vary Alternator Phase, Voltage, Current and diesel fuel flow to support two power sources to be synced together and operate as "one".

Without the specialized genset load monitors and controllers, you cannot just take to 480 VAC 50 Hz generators and run them in parallel. They would most likely destroy each other. Such a controller may bring Genset A up and get a stable 480 VAC 50Hz output, then start/bring Genset B up to speed and get 480 VAC and 50 Hz too--Then bring them into "phase" (sine wave peak voltage in phase with each other) and then connect. And vary alternator excitor current, fuel flow, etc. so that the two gensets share the load equally and do things like move load between the two gensets so that (for example) genset B can shut down at night while A carries the load. This is how national level grids work all the time (integrating multiple AC sources into one unified national grid.

Guessing that you are are Power Engineer--You probably know all of this much better than I.

Hopefully, I have been helpful in your bouncing of ideas for possible solutions for your needs. But I am way over my head at this point.

From your knowledge about the systems involved... What do you think your solution would be? What are the possible implications/downsides of your solution? Have you talked with the companies/engineers that supply and support your systems about what they would suggest?

Sorry I cannot be more helpful, but I know my limits. And when you say you need to connect two VFDs together, and then subdivide the output power---Too many variables/options (AC or DC input power. VFDs with "subdivided outputs"?). I don't follow/know enough to make  possible suggestions.

You certainly have a very interesting power/pumping system. And I love to learn. But being much help to you--Probably not.

Sincerely,
-Bill B,.