New 20 Kw installation

Mswakini · September 2013

I am planning to install a 20 Kw turbine or pv panels and feed back power into the main grid.
My small hotel is located in Zanzibar Tanzania and we are directly connected through a 320 Kva transformer which we own.
The local electric company has never done this before but has allowed us to do so if we buy our own reversible meter.
I have no idea of what kind of equipment I need in order to feed back the power directly into the HT main grid.
Thanks
Adriano

BB. · September 2013

Re: New 20 Kw installation

A true utility power meter will fequently turn backwards if the power flow is reversed. Newer meters may be designed to not turn backwards, always turn forward (to keep people from installing meter backwards to steal power), or with new digital meters--They can have multiple "registers" that record forward and reverse power (by time of day, etc.).

Usually, if your power flow into the grid is not that large (relative to the rest of the grid in your area--Usually, less than 1% of utility generation in your region is ; 10% can be problems for the utility).

In general, how cost effective distrbuted genration is depends on how your utility will charge you.

In the US, for example, the "best for the customer" is a "net metering" plan. The utility will read your meter once a month and it will "go forward" if you are consuming power, and turn backwards if your are generating power. If you used power, you will pay for the power used. If you are a net generator of power, they will credit your account... In general, I generate extra power in summer and use more utility power in winter--My utility will charge me at the end of one year if I owe money. Or, if they "owe me money", the account is set to zero.

The worst case plans will charge you retail for power you consume, and pay you wholesale for power you generate (with two separate meters).

Until you get an agreement with the utility, it is risky to build an RE grid tied system. In some places (US and Canada), utility companies are starting to refuse connections for completed systems--Changing rules that have been in place for years (utilities do not like distributed generation).

Solar panels + a Grid Tied inverter is pretty straight forward. Find an inverter that meets your rated power, voltage, line frequency and install a field of solar panels to support the GT inverter(s). Obviously, if you already have a supplier in your region, I would talk with them first.

Wind turbines can work, but the require a pretty windy site and very high towers (minimum of 20-30 meters). And the turbines require a fair amount of mechanical maintenance. Unless you are next to a wind farm with a long term maintenance contract (larger turbines), or are prepared to do the maintenance yourself (say less than 5-10kW turbines), I would suggest that you do solar panels first.

-Bill

Mswakini · September 2013

Re: New 20 Kw installation

Hi Bill, from the other side of the globe thanks! My main concern is that i am the only user after the HT transformer so the excess power i generate will have to flow back to the main utility HT lines, through my transformer or directly into the HT lines. There is nobody else connected to the transformer so i am the only user. Can i still feed back the power into the HT national grid. Legally my utility company says yes, but they have no idea if technically it is feasible. As for wind vs sun i agree with you, we have a huge portion of unused land and we could have a good size solar farm. If in theory we can put back the power in the main grid than of course we would engage a professional company to do it. It's just that my knowledge of this is somehow limited.
I appreciate your help.
Adriano

Cariboocoot · September 2013

Re: New 20 Kw installation

Back-feeding through the transformer should not be a problem (although we can't speak to the quality of the grid there).

Some of the technical info needed: Voltage and frequency (I think it's 230 VAC 50 Hz), and size of main distribution panel at your place. You should check whatever electrical code you have there, but since this seems to be a first-off there may be no code covering issues like back-feed loading. In that case we'll have to work wit adapting the NEC over here to apply. For the most part it's a good set of rules.

The amount of power in kW hours you typically use will be important too, as it will give some idea how much back-feeding will actually go on and how much self-produced power will be used on site.

Otherwise the basic design is pretty straightforward: PV's feed a Grid-tie inverter which then connects to the standard wiring. The size of the system will be limited by the size of your existing electrical feed, specifically the bus bars in the breaker box. An alternative to this is to do a line-side tap where the connection is made on the meter side of the service box. Some installations have two separate meters: one measuring power going in to the building, the other measuring power coming out from the solar.

For a standard household install over here, 20kW would be too large. But for line-side tap or possibly a commercial breaker box it may not be. 20kW on 230 Volts is about 87 Amps which would require a service of 400 Amps (2X normal household here) or a line-side tap.

BB. · September 2013

Re: New 20 Kw installation

Adriano,

In general, yes, the AC grid is bi-directional. For all the world, the utility grid is like a giant AC battery bank. As seen from your side.

One problem for people at the end of a long power run is--Say your average load is ~4kWatts and your voltage varies from 230 VAC to 220 VAC. A lot of that voltage variation would be from voltage drop due to wiring resistance and transformer losses.

With your 20 kWatt of solar array (you "consume" 4kW*24hours per day=96 kWH; and you generate 20kW*5 hours a day of sun=100kWH), you are generating 5x your average power used, and you could end up with a 50 volt swing in your local voltage until the utility installed heavier cable/better regulation.

Not saying that you will see that big of swing--Your local AC consumption is probably more variable (water pumps, air conditioning, refrigeration, etc.). But local voltage regulation has been a problem with large solar farms on small grids (was/is a big issue on Puerto Rico, a US protectorate in the Caribbean Sea. There is a US Military Base with a large solar farm that has been causing line regulation problems.

For to break even on power used vs power sold, you will have 5-6 hours to generate when the sun is up and 24 hours per day to consume--Poor line regulation can be a problem because of the higher current and reversed flow (i.e., you have 0-10 volt drop due to consumption and a 0-20 volt rise due to peak solar production).

If your line regulation is pretty solid (normally less than 5 volt variation during the day), you might be OK with a "break even" solar farm. If your line variation is already >>10 volts in a normal day, then adding a large solar power system may cause you (and your utility) some real headaches with local grid voltages (in the US, we see ~216-264 VAC as the maximum variation for a 240 VAC nominal line voltage). +/- 10 volts with 240 VAC is probably "normal". "Slow voltage variations" are not usually a problem. "Fast" variations can cause noticeable/irritating flickering in your lights.

In some cases, customers at the end of a long line have large swings in line voltage... For example say your measured high line is near 245 VAC (assuming ~253 VAC is your maximum)--You only have ~8 volts of "voltage rise" if you are generating power. Your GT Inverter may be set for ~250-253 VAC for high line--And will cutoff (stop generating power) for 5 minutes if it sees too high of utility voltage. So, in this case with already high line voltage, your system will have very little headroom for "voltage rise".

The rest of the issues will be around utility management of generation/network regulation. If this will be a problem... I don't know. My guess (as I said before), if your 20 kW system is less than 1% of their online generation (power plants), they probably will not notice. If it will be >10%--Then they will need to account for your production in their overall operations.

You could also have variable AC line frequency... Some smaller/older power systems could have frequency control/stability issues. Normally a GT Inverter expects. Normally, a GT inverter is programed for 50/60 Hz +/- ~0.5 Hz (outside of that range, the GT inverter will shut down). Many can be programmed for a wider frequency tolerance--But that is another possible issue between your system and the utility.

Beyond this--I probably cannot help you. You will need a boots on the ground power engineer to evaluate your local installation/utility to see if it is compatible with GT power.

-Bill

Mswakini · September 2013

Re: New 20 Kw installation

Our small hotel project is 20 villas 2 restaurants 2 kitchen and 1 bar.
The average power consumption in 24 h is 55kwh per villa. We have not yet determined the consumption for the public areas and infrastructures. Cold-rooms (2) and electric ovens are major consumers as well.
The tricky bit for us is that the major power request is at night when all air-conditioners are switched on.
Since we have been given approval to start with only 5 villas we want to explore environmentally sound and cost effective ways to reduce the cost of power. Wind power would make more sense since we are located on a cliff by the sea and wind is always between 10 and 30 m/s. The down side is that there is no possibility of professional maintenance and that is why we are opting for solar.
Wiring is not an issue since all the electrics must be provided by us including the HT transformer (we have acquired a 315 kva)
Being right on the equator we have 10-13 hours of sun and cloudy days are scarce. Given the size of the project is not possible to use a battery bank or at least it would be enormously expensive. Our 20 kw pv system is test run which will be expanded to cover the total power request once all 20 villas are ready. You were saying to increase the size of the cables to avoid voltage fluctuations and spikes. Could you elaborate on this, since we have not yet bought our cables.

Cariboocoot · September 2013

Re: New 20 Kw installation

The Good: You have 10 to 13 hours of sun per day, which means "equivalent good sun" hours on the panels will be in the 6 hour neighborhood. You could even place arrays facing Easterly and Westerly and gain a lot of solar "harvest".

The Bad: You have 10 to 13 hours of sun per day right on the Equator, which means the panels will be operating at pretty temperatures resulting in a drop in Voltage and thus loss of power potential.

Oh well, it will still work.

Are you planning one GT install per villa? If so then you could size the arrays quickly by dividing 55 kW hours by 6 hours of sun and coming up with 10 kW arrays. Easy. Pretty much the standard size for North American homes.

Your heavy power use at night is another issue, as panels do not produce at night. Thus you have a situation where production during daylight gets "stored" by the utility grid and returned at night. Is this going to be a problem for supply? If everyone's power use on the grid is in the same pattern, then the demand at night will be maximum and that's when you could experience brown outs.

That is when you'd consider batteries, but they are a big expense and you'd need very large banks. This would be on the magnitude of 2,000 Amp hours @ 48 Volts, depending on how much power you need to store for night use. The system can be divided to have both types of inverter so that some of the power produced is direct GT and some "time shifted" via the batteries.

westbranch · September 2013

Re: New 20 Kw installation

with that much daily (nightly) consumption, it appears that there is opportunity to implement a conservation plan, ie lights to LED, more insulation to reduce daytime internal heat rise in the villas, high efficiency air conditioners, etc.

putting solar panels on the roof can also lower the internal temps.

Have you taken on any of these measures?

Cariboocoot · September 2013

Re: New 20 Kw installation

westbranch wrote: »

with that much daily (nightly) consumption, it appears that there is opportunity to implement a conservation plan, ie lights to LED, more insulation to reduce daytime internal heat rise in the villas, high efficiency air conditioners, etc.

putting solar panels on the roof can also lower the internal temps.

Have you taken on any of these measures?

Sounds like the place hasn't been built yet, so maybe there's nothing that can be measured at this point.

zoneblue · September 2013

Re: New 20 Kw installation

This sounds like a great project, but its serious installation requiring professional assistance. Thats probably your next step, finding a suitable consultant. Agree, demand managment is the first place to look for savings. A cluster of buildings offers some interesting efficiencys too. A limited battery bank can be used as peak load support to avoid blowing transformer fuses. Power is usually priced by peak load, so big potential for savings there.

New 20 Kw installation

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