Hello everyone,

First let me tell you I am not an engineer so please forgive me if my questions seem dumb to you...

Here's my problem.

I am considering building a minigrid for an isolated community. This off-grid system would be powered by two diesel generators of different sizes, in order to reduce diesel consumption.

One genset is 3kW, the other is 1kW. Both gensets cannot operate under 40% of their rated power to prevent excessive wear.

The isolated community requires very little electric power.
At night it will be at 0,3 kW
During the day, the load will be around 2 kW.
At peak hours, the load is 3,3 kW.

I have considered operating the gensets as follows:

- At night, the 1 kW gen runs alone at its lower limit : 0,4 kW (some energy is lost but it can't operate under this limit)

- During the day, the 3 kW genset will run alone at 2 kW

- At peak hours, both gensets will run and supply the load proportionally to their size:
The 1 kW genset will provide 1/(1+3)*3,3kW = 0,825 kW
The 2 kW genset will provide 3/(1+3)*3,3kW = 2,475 kW

This way, energy loss due to the 40% limit constraint is kept to a minimum.

Now the question is: is there a control system that can do this? Or am I completely fooling myself?

Thank you very much for you help!

Rob

 

Roberto Benedetti,

This could be automated with a control system, but there is nothing OTS ("off the shelf") to do this, with the possible exception of something the diesel generator manufacturer might make or sell. I venture that anyone selling you something that would allegedly do this doesn't know what they're talking about and couldn't--and wouldn't--support the product after the sale. Power plants and process plants pay hundreds of thousands of US Dollars, even more, for control systems that either don't work very well and/or are not operated properly (because of incorrect perceptions, poor implementation, and poor operator training).

To do this would require an inexpensive PLC (Programmable Logic Controller) with the ability to monitor load (at least three analog inputs) and several discrete inputs and outputs to be able to start and stop the diesels as well as to raise and lower the load on the diesels. Unless the diesel governors have both Droop and Isochronous capability it would also be necessary for the PLC to monitor frequency

Unless you are blessed with the ability to program PLCs and select and configure the necessary inputs and outputs (which could be costly and require some intermediate electrical skills) this is not simply automated. And it would likely be costly to purchase such a system, have it installed, and maintained.

A solar system or wind turbine charging a battery bank (used two motor (forklift) batteries work well for this) and a suitably-sized inverter would be great for this and not too difficult to assemble and maintain.

There are also micro turbines and fuel cells available which be more costly to purchase but probably cheaper to operate and maintain.

In my personal opinion the solar/wind/battery option is the least costly to install, operate, maintain and troubleshoot. No fuel or oil to buy; no diesel maintenance, fumes, tank(s), noise.

 

Thank you for your reply CSA.

I might hybridize the system with PV and batteries, but diesel back up will be mandatory to ensure 24/7 reliability.

If I understand correctly, programming a PLC to do this would be possible and the device itself wouldn't be too costly.

However you say the automated system would be expensive to purchase, operate and maintain. Why is that? And do you have an approximate price range in mind?

Maybe the savings on diesel fuel could balance this price...

Regarding the programming part, I might have a shot myself - or ask a professional if it's too complicated.

Thanks again for your comments on this, this is really helpful.

 

Roberto Benedetti,

An OTS (Off-the-shelf) multiple engine-generator controller system--just for the controller--would probably cost about USD10,000.00. They are pretty specialized and highly user-unfriendly.

For any kind of automated system (PLC-based or PMS (Power Management System) you're going to need to monitor the total load on the system and that's going to require at least one CT (Current Transformer). I would also suspect that to properly control each diesel they would have to have individual CTs as well. Certainly, PTs (Potential Transformers) would also be required as to accurately measure power one needs to know the voltage (which the PTs would provide) and the current (which the CTs would provide).

Synchronizing two generator-sets together also requires some special equipment: at least a manual synchroscope or some means of knowing when the two since waves are "in phase" with each other. Most systems have at least one synchronizing "check" relay to ensure that when breaker(s) is(are) being manually closed to synchronize one generator to another that it can't be closed out of phase and damage the generators, the load couplings, and/or the prime movers (in this case, the diesel engines).

An automated system such as you are describing would need to be able to start and stop each of the diesel generators, to know when each generator was at rated speed and/or was synchronized (or not), and would need to be able to send RAISE- and LOWER signals to each diesel's governor in order to control load and frequency.

And, that's probably yet another analog input to the automated system that would be required: a frequency measurement signal of some sort (I suppose a discrete input with some hysteresis could be used for this).

To be as automated as possible, each of the diesel governors would have to have Isochronous Speed Control capability, and each would also require the ability to be switched in and out of Isochronous and Droop Speed Control mode. This is necessary to maintain your rated system frequency.

We haven't really discussed the need for protective relays--devices that sense faults on the system and open circuit breakers to protect the load and the generators, and the prime movers. Faults include ground faults, excessive current flows (such as when a device shorts out internally or grounds (such as through contact with water). I suppose you could ask people to use GFCIs (Ground Fault Circuit Interrupter-type outlets/receptacles), but I think you're going to need some kind of over-current relay(s) to protect the generators and prime movers. There are single, multi-purpose relays available for this, but they're not inexpensive, either. Probably a few thousand USD, also.

The synchronizer/synch-check system may be available somewhat inexpensively off the shelf, but it would still probably set you back a couple thousand dollars, also.

We don't know much about the weather (sun; wind) at your site, and, certainly, if you want to have the highest reliability a diesel generator set would be good. But, you're still going to need to synchronize the two--the diesel generator-set and whatever else you're using. And, that takes some specialized equipment (synchronizer; synch-check relay; synchroscope; etc.).

AC power systems seem very simple, but, they aren't so simple. It's a great credit to the original system designers and to present-day designers that flipping a switch on a wall is so "easy." Building an AC power system, one that's reliable and safe, isn't as easy. At least it's not very inexpensive--especially when one is trying to operate two or more generator-sets in parallel (and that includes wind turbine-generators and PV systems). (Parallel and synchronized are two words for, essentially, the same thing: operating multiple generators sets together to supply an electric load which may exceed the rating of any single generator operating by itself.)

Hope this helps! This kind of equipment is pretty expensive. I did some checking on portable gas engine-powered generators sets in the USA and a couple of manufacturers have "parallel" operation equipment that is only a couple of hundred USD, but they only work with specific generator-sets (probably digital governors). I also noticed some expensive generators with inverters--which would produce some very "clean" AC. The nice things about these units is that they can run at low speeds when the load is low, thus conserving fuel.

This sounds like an interesting project, though my interest is no more than as a curious observer.

 

Hello CSA,

Thanks again for your detailed reply.

Your explanations are great because you don't make it sound too complex - although there are obviously a lot of parameters to take into account (voltage, current and frequency of both generators and the load).

I didn't think it was going to be that expensive, though.

There are for example some islanding inverters that seem to be able to operate PV, batteries and a back-up diesel generator, and they are not so costly.

I figured adding a second diesel generator was worth the shot, but then maybe not if it implies adding $10k worth of equipment to make in run...

I also didn't think this was going to be very complex because the simulation software I used to test several system designs suggested to add a second generator quite naturally. It didn't emphasize the fact that I would need a specific control system to make it work in real conditions.

Maybe I will rerun a few simulations and add a $10k automated control system to see if it makes sense money-wise.

Again, thanks for you help!

PS: I didn't mention the names of the inverters and software I am referring to because I don't know if I can do this here.

<b>Moderator's Note:</b> as long as you are not libeling the company you can post the names here.

 

Roberto Benedetti,

Most of my experience is with industrial applications (MUCH larger industrial applications producing AC power). So, most of my experience is with larger, industrial (commercial) equipment (think megawatts; tens to hundreds of megawatts).

What I did not understand from your posts is that you seem to be going to use a power inverter to convert DC power from batteries to AC power, and to use the diesel engine-driven generators to charge the batteries. If that's the case, then you don't need the synchronization and power monitoring equipment I described, which would only be necessary if the diesel engines were driving AC generators and needed to be operated in parallel. (I presumed you were talking about AC generators, not DC generators.)

It would seem that some good-sized power inverters are available with built-in displays for current and voltage and load. The key here is to use "deep-cycle" batteries (examples are "marine batteries" or "golf-cart" batteries; larger deep-cycle batteries are used on electric fork-lifts).

Diesels driving DC generators could be cycled as necessary to keep the batteries charged and, depending on the size (capacity) of the battery bank would not have to run continuously (that may be what you were thinking of; again, I didn't get that from your posts). PV cells and/or wind turbines could be added to the installation over time to reduce the need for the diesels to charge the batteries, but they would be available in the absence of "renewable" energy.

The issue I see with this DC/inverter scheme is a simple one of spare parts. A spare power inverter would be an easy swap-out. Batteries also require maintenance, and periodic replacement. Gassing of batteries during charging should also be considered when building the enclosure or when placing the batteries in an existing building. But, this DC/inverter scheme is probably much less expensive to build and maintain than an AC-only system with multiple AC generators.

It would be good for many people to know these sites you're using for reference. I did a little searching with "islanding inverters" and found some useful information. I even found a couple of power inverters that had discrete outputs for starting engine-driven generators when the battery voltage drops below a certain level.

If you're using diesel engine-drive AC generators to power battery chargers to convert the AC to DC, you're still going to need some method of synchronizing two AC generators together....

Let us know how you proceed!

 

CSA,

Actually you were right to consider an AC system.
The generators will be AC, because they will have to be able to supply the load (AC), and recharge the batteries when necessary.

I found a video that explains quite well what I am talking about (its a video from a manufacturer of islanding inverters). In my case, the grid will most likely be single-phased, and the system will be PV-Diesel-Batteries.

<

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As you said, it seems that if I want to add a second diesel generator to this system, I will need the kind of special equipment you talked about.

If I find this type of equipment of the shelf for small applications like this, I'll let you know!

Or maybe it would be possible to operate two DC generators without the synchronization and power monitoring equipment you described, and convert their output to AC to supply the load when needed, through the central DC to AC inverter (to which the batteries and the PV would also be connected).

I'll keep you posted if I find some answers!

 

Roberto Benedetti,

It's not necessary to have all of the synchronization equipment for operating multiple DC generators together--only for AC generators.

And measuring DC is somewhat simpler than measuring AC (no PTs/CTs required).

You seem intent on having multiple diesel generators for the high-load times. I have seen several gas engine-powered generators which offer a "paralleling harness" and associated controls to allow to of the same units to be operated together (in parallel; synchronized) to supply a load that is larger than either of the two gas engine-powered generators can supply. If you could find something similar for diesel engine-powered AC generators that would suit your application very well.

When I was in university I worked for a tug-and-barge company, and they used small Lister or Petter diesel engine-powered generators for power on the tugs and barges and they were very reliable. www.lister-petter.com, and I saw a couple on eBay, too. Perhaps you need to find a marine-centered supplier and work with them to find what you're looking for.

I would suggest you re-consider using two diesel engine-powered generator sets. Since you will have a battery bank supplying the load through a power inverter if you could purchase a diesel generator set that is a little larger than your maximum anticipated load you could run the diesel generator set to both provide power to the load and supply a little excess power to charge the batteries.

Alternatively, if you could find a power inverter that would charge the batteries from the diesel engine generator, you could have two diesel generator sets, each supplying the battery through its own power inverter. Or, just simply use two battery chargers, one powered by each diesel generator set.

Operating two (or more) AC generators to supply the same load will require some kind of "parallel harness/controls" or synchronization controls. You're likely going to need some kind of battery charger or inverter between the AC outputs of the two diesel generator sets to avoid that necessity; I don't think it's going to be reliable to operate two diesel generator sets producing AC directly coupled together without some necessary equipment, including some kind of "parallel harness" between the generators.

Thanks for the feedback! Please let us know how you proceed! And your thought process and considerations are useful, as well, as you make your way through these decisions and the design of your microgrid power system.

 

Thanks CSA,

I will look into both solutions now that I know a little more about AC and DC systems.

Maybe Lister/Petter diesel generators or other marine suppliers have the built-in paralleling harness functionality I need. If not, I will try to find the right synchronizing equipment for the AC generators because they need to supply either the AC load or the battery bank when needed.

DC generators may also be interesting to look into, even if it seems that they cost more than AC generators. But since they don't need the special equipment AC generators need to be synchronized, maybe the total investment will be the same (or even lower). Connecting them to a central islanding inverter that will make them work the way I want doesn't seem to be too complicated.

Regarding the need of two generators, it is only a matter of final cost of electricity, really. If an higher initial investment means that the generation system will work with lower OPEX, hence reducing its total lifecycle cost, then it's all good to me.

HOMER, the simulation software I mentioned earlier, lead me to think that adding a second generator to my system would do just that: the CAPEX would be a little higher, but it would make sense because it would increase the system's global efficiency, thus reducing the OPEX (the diesel fuel represents most of the OPEX in an hybrid PV-Battery-Diesel system).

This software seems great, however I wonder how the systems it designs perform in real life.

 

Roberto... I suggest you consider 2 automobile-alternators, each driven by a Diesel-engine!

1) Since their outputs are DC, parallel them to the same load-bus via blocking-diodes. The diodes will permit you to monitor the output voltage of each generator!

2) Connect the various loads, if AC, via 12Vdc/120Vac inverters!

Regards,
Phil Corso.

 

Hello Phil,

Thanks for your input!

I might consider that option, it sounds original and clever. Are you suggesting this because you think the equipment would be cheaper? I did a little searching and it seems that it may be. But installing such a system could also require more time and supporting equipment (and more money).

But I will seriously look into it and see if it a better and cheaper option than DC generators or AC generators with built-in parallel harness.

This solution should also be automated and be able to run as I described earlier. Do you think it would be feasible?
Would the diodes only help me control the output voltage of the alternators, or would they also have an effect on the diesel engine, and on its diesel consumption?

Also, what would be the efficiency of these generators? This will be the most important feature I guess, since OPEX are way higher than CAPEX in diesel systems.

By the way, I found a quite detailed document that talks about control methods for hybrid and diesel minigrids. I need to read it carefully because it seems a bit technical (maybe too technical for me), but I'm sure it holds some interesting findings.

Here's the link to the pdf, if you're interested:
<http://www.iea-pvps.org/index.php?i...1[showUid]=1127&tx_damfrontend_pi1[backPid]=3>

Thanks again!

 

Roberto... addition to my earlier post:

1) ... The blocking-diodes will then permit you to monitor output Voltage, Amperes, and Power, from each generator!

Regards,
Phil

 

Roberto... can you provide us with some idea of the mini-grid load?

Phil

 

The link doesn't point to the document, but you can search it with its title:

"PV Hybrid Mini-Grids: Applicable Control Methods for Various Situations" from the IEA PVPS Task 11.

 

Phil,

It is an AC microgrid designed to serve a currently unelectrified community.

As described in my first post on this thread, the load will then be very low. But in several years, the microgrid will have to be upgraded as the electricity demand from the community will grow (as people will buy more electrical appliances, etc.).

For now though, in average:
At night: 0,3 kW
During the day, the load will be around 2 kW.
At peak hours, the load is 3,3 kW.

As a reminder, one generator is 3kW, the other is 1kW. Both generators cannot operate under 40% of their rated power to prevent excessive wear.

I would like to control the generators so they run as follow (when their is no PV or battery production):

- At night, the 1 kW gen runs alone at its lower limit : 0,4 kW (some energy is lost but it can't operate under this limit)

- During the day, the 3 kW genset will run alone at 2 kW

- At peak hours, both gensets will run and supply the load proportionally to their size:
The 1 kW genset will provide 1/(1+3)*3,3kW = 0,825 kW
The 2 kW genset will provide 3/(1+3)*3,3kW = 2,475 kW

This way, energy loss due to the 40% limit constraint is kept to a minimum.

I hope this is what you were asking for?

 

Roberto... I am deeply involved in Alternative and Renewable Energy Management and I can assure you the IEA Methodology is intended for those with LOTs of money (as alluded to by CSA.) If you want specific detail contact me off-forum!

For the loads you cited following is a list of basic materials:

o Two vehicle-alternators.

o Two DC Voltmeters.

o Two Shunt-connected DC Ammeters.

o Two DC Powermeters if you want more than simplying multiplying V x A.

o Two Blocking-Diodes

o A Fuse-panel.

o Several Dc to Ac Inverters if the Loads are AC powered!

Phil

 

Roberto... I wanted to know the nature of the community load, i.e., lights, refrigeration, pumps, etc!

Phil

 

The nature of the community loads would be lighting for a large part.

For productive use during the day, there would be a combination of small grinding mills, a few sewing machines, refrigeration, small water pumps, a battery and phone charge center, a TV, maybe a small welding machine...

According to my calculations and based on similar projects, the total investment for an off-grid system like this would be around $100k to $150k...

How can I contact you off forum?

 

Roberto...

[email protected]