Hybrid system linked to main grid

Alberto91Alberto91 Registered Users Posts: 2
Hello everyone, 

I'm new to this website and I hope you'll be able to help me.
I'm trying to understand a hybrid system located in Uganda. It is characterized by PV technology and diesel generators, used to supply a hospital.
Due to frequent outages and the necessity to supply surgery rooms and refrigerators 24h a day, the system is characterized by a main line and an internal emergency line. 
My question is about the solar generation. The hospital direction has installed a power capacity of 200 kW, much larger than the load requirements, and does not want to increase the actual battery storage (bad experiences in the past, i don't know what happened). The excess of power produced (in case the hospital is connected to the main grid) goes to the main line (even if the government does not allow that).
On the contrary, if it is disconnected due to a national outage, this surplus power is dissipated by inverters, transformers and diesel generators creating big problems  (disconnection of the inverters and damages to diesel generators and transformers).
I'd like to know if you have any suggestion in order to help me to solve this problem of excess of power production.

Comments

  • EstragonEstragon Registered Users Posts: 3,030 ✭✭✭✭✭
    Some more info would be helpful:
    1. A list of equipment (DC charge controllers, and AC inverters) with models numbers.
    2. Type, size, and configuration of battery banks.
    3. Approximate running loads for normal operation with grid power, and for emergency operation.

    It would also help to understand the grid connection better. When you say the gov't doesn't allow connection, does this mean you just run a meter backwards during the day?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 28,172 admin
    In general, solar electric power is either used or "lost". Batteries are the best, but imperfect, energy storage--And usually only good for 2-3 days of energy storage (and roughly 4-6x larger than your daily load--Expensive, especially if poor maintenance or other mistakes kill a battery bank early--Easy to do).

    Or you have a small battery bank that can store a few hours of energy. Basically just long enough start the diesel genset. Or variations (i.e., enough storage to take you through a typical afternoon power cut, and only run the genset if you have a longer outage).

    Feeding excess energy back to the grid (and turning your meter backwards) is good for the customer--But can cause issues with the if there are a lot of systems feeding power into the grid (more or less, if you have 10% or more of your customers feeding power back to the grid, it becomes very difficult for the utility to keep the voltage and frequency stable). Also, just turning the meter backwards, you are buying and selling power at retail--Sort of like buying food from the store in the morning and selling it back to them in the afternoon at the same price. The store cannot make money to keep the infrastructure going (transformers, distribution wiring, utility generators, etc.). This has been a huge problem for some regions in the US, Europe, and Australia. Lots of "solar and wind power", that has caused problems with over heated power lines and massive outages. In parts of the US, the regulators have basically made Grid Connected (and feeding power back to the grid) not worth the costs (i.e., meters that do not turn backwards, high monthly connection charges, etc.).

    About the only thing you can do with "excess" PV power (if you cannot feed back to grid, charge batteries, etc.) is to find uses for the excess power. Such as water pumping/irrigation/pumping water to storage, electric hot water heating, etc. But even that has limitations (tanks/ponds are full, water tanks are hot).

    You have to do some math and accounting to figure out what is the break even point between solar+batteries (plus risks of battery failure, maintenance) and running a genset (maintenance, fuel prices, etc.). So far, no magic solutions yet.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 3,030 ✭✭✭✭✭
    Bill - my reading was that the issue isn't how to use the extra pv power, it's that inverters and generator are disconnecting when the grid goes down. Sounds more like a configuration problem to me.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • EstragonEstragon Registered Users Posts: 3,030 ✭✭✭✭✭
    @Alberto91

    Just to clarify, excess solar capacity can't (normally) be "dissipated" by generators, etc.

    PV will only produce enough to satisfy loads. With no grid, you only have your local loads. There is no extra power to dissipate. If you have 1kw of loads running and no faults, your 200kw of panels will only produce 1kw(+/-) no matter how sunny it is.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • jonrjonr Solar Expert Posts: 1,122 ✭✭✭✭
    edited March 2017 #6
    You don't need to dissipate or store excess solar power - done right, it can be "left in the solar panel", causing no harm to anything.

    Sounds like you may have grid-tie inverters that don't do things right and supply excessive voltage to the local "grid".   You could disconnect them while on generator power (easy solution) or possibly replace them with more adjustable units (ie, flexible active power curtailment).
  • BB.BB. Super Moderators, Administrators Posts: 28,172 admin
    If I understand correctly, the issue Alberto is seeing with internal power network problems (disconnecting inverters and generators) is just the small version of the problems the utility has with with Grid Tied Solar/Diesel genset connected customers.

    Some sort of energy storage element is pretty much needed when mixing solar panels (which cannot supply surge power, and output varies with sun position+weather) and other energy sources (genset, battery storage, unreliable grid).

    Designing the right size battery bank with the mix of energy sources is the solution... But limitations of Lead Acid battery banks, such as needing a pretty large battery bank to supply surge power and accept very high (short term) charging currents vs other battery technologies such as LiFePO4 which can be better in these applications vs much higher costs (and unknowns about long term reliability--Both batteries and battery suppliers/manufacturers).

    Otherwise, conservation of energy usage (LED lighting, picking loads/devices/equipment to use the minimum amount of power needed to do "its job", load shedding of "optional loads" that don't need to run during grid failures, etc.) at least keeps the power plant smaller and cheaper.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Alberto91Alberto91 Registered Users Posts: 2
    @Estragon @BB. @jonr
    Thank you all for answering me.
    I will try to give a better view of the problem, I am studying the plant from my office in Italy and I will move to Uganda in April. However there is a big lack of information. 
    The Main Line is the principal electric grid, developed with the hospital since its birth. It connects all the hospital buildings, including medical wards, residences, schools and technical rooms. It is fed by the Ugandan distribution utility Umeme, by a diesel generator and photovoltaic plants. 
    The hospital is connected to the external distribution grid (UNEME) in middle voltage (11 kV) through a 1 MW transformer. The energy consumed (by national grid) is around 655000 kwh/year, and 100000 kWh/year are supplied by genset Perkins 3008-TAG4 (400 kW).
    The Emergency Line is dedicated to serve some sensible loads of the hospital that need continuity of service, like computers, medical equipment and lighting of critical wards. Continuity is guaranteed by an Uninterruptible Power Supply system (UPS Riello Master Hp of 160 kVA) that connects the line with the sources and the storage of batteries (model=10 OPzV solar power 1250, string of 240 batteries in series and tight to the two supply systems: the Main Line and the Emergency generator of 175 kVA).
    The whole photovoltaic power system of the Hospital is composed of four different solar plants working correctly, which are directly connected to the grid by dedicated inverters, plus two others plants used only to charge battery for backup.
    Currently I have partial informations of six solar photovoltaic plants installed on the roofs of some wards.  
    Theaters: PV Solar Modules Lorentz LA 75 Wp, 69 panels = 5175 Wp 
    grid inverter: 3x 2kVA Victron Energy 2000/230 V
    Inverter: 3x Victron Quattro 24V/5000VA/120-100/100-230V
    Batteries: 24x Victron OPzV solar battery 1500Ah, 2V (12in series x2) 
    Surgery: Photovoltaic modules 130 Wp 12V Victron, 120 panels = 15.6 kWp 
    Grid inverter: Grid interactive three phases solar inverter AROS Riello Sirios K15 
    Surgery II: 3x photovoltaic modules 245 Wp 12V Victron, 66 panels each module = 16.17 kWp; 48.5 kWp total 
    Inverter: 3x Grid inverter 3 phase Sma Sunny tripower 15KW 
    Maternity: 3x photovoltaic modules 245 Wp 12V Victron, 66 panels each module = 16.17 kWp; 48.5 kWp total 
    Inverter: 3x Grid inverter 3 phase Sma Sunny tripower 15KW

    The two following solar plants are not directly grid connected and they feed battery banks using as backup. 
    Intensive Care Unit ward: PV Lorentz LA 75 Wp, 69 panels = 5175 Wp total 
    Inverter: 3x Outback 3 kVA VFX3024E + power control
    batteries: 24x Victron OPzV solar battery 1500Ah, 2V (12in series x2) 
    Laboratory: PV Lorentz LA 75 W_p, 69 panels =5175 W_p;  
    inverter: 3x Victron Quattro 24V/5000VA/120-100/100-230V, 
    batteries: 24x Victron OPzV solar battery 1500Ah, 2V (12in series x2) 

    RESUME
    Average load of the hospital is of 2708.6 kWh/day,
    The daily average power is about 110 kW, with a peak of 245 kW.  
    Reliability (ratio between the energy supplied by the national distributor and the total one) lower than 0.75.

    It would also help to understand the grid connection better. When you say the gov't doesn't allow connection, does this mean you just run a meter backwards during the day?
    The gov't does not buy the electricity so the extrapower is however injected in national grid and dissipated by transformes, inverters, genset and so on.

    Bill - my reading was that the issue isn't how to use the extra pv power, it's that inverters and generator are disconnecting when the grid goes down. Sounds more like a configuration problem to me.
    There is a configuration problem 'cause the hospital main office receives components from no profit organizations without knowing exactly how to connect them to the electricity system. Their attitude is to install whatever they receive without understanding
    future problems associated. Moreover, this components works well with european regulations, which are different from Uganda. 
    The whole of these factors can explane why inverters fall (high voltage and frequency fluctuations).

    Many thanks for your help. 
    Alberto




  • EstragonEstragon Registered Users Posts: 3,030 ✭✭✭✭✭
    Assuming the various labs, wards, etc. aren't independently wired for emergency power with some sort of transfer switch system, I don't know how all those different brands of inverter are going to sync. Ideally an integrated solution would be engineered to your application, but I understand the need to work with what you get.

    It will be hard for you to deal with this remotely. Even if there are electrical as-builts to go from, there are probably lots of undocumented modifications.

    FWIW, my suggestion is to simplify as much as possible and focus on the main goals. If grid-tie is uncompensated and illegal this should not be done, both for the reasons Bill noted, and because it's probably a complication you can live without at the moment.

    That said, paging @BB. - should this perhaps be moved to grid-tie? Even though grid tie should be severed, this seems to me to be a sort of local mini-grid situation. Some of the inverters could maybe "sell" into the local grid? Grid-tied folks may be more familiar with capabilities of the various inverters.
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • westbranchwestbranch Solar Expert Posts: 5,114 ✭✭✭✭
    Found this PDF and it looks like, to me, that the gwn should be running full time for best fuel consumption..  If I read the table correctly... http://www.michelecaroli.com/pdf_perkins/3008tag4.pdf
    and not as a standby unit...

    Intuition tells me you should be getting information about the 2 surgeries and the maternity war, that there are some batteries to store the excess power or do those PV arrays supply power to the other buli8ldings..

    A sketch of the building layout with distances from each building will be a great help in visualizing how it all fits together...
    hth
    We await your additional information.
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • BB.BB. Super Moderators, Administrators Posts: 28,172 admin
    It sounds like a real difficult system to figure out and put right/make efficient.

    Just to give you an idea of what can happen with a smaller, 3 phase generator+solar system:

    http://forum.solar-electric.com/discussion/17986/is-the-system-set-up-right/p1

    Having multiple sources of power and loads--And mix+match hardware--This will probably be a difficult task to set right--If not near impossible without abandoning some of the hardware.

    More or less, "things" that are less than 1/10th of the source (i.e., if you have solar panels+GT inverter that is less than 1/10th of the local grid, loads, and/or generator output--You can probably ignore the smaller GT system. If the mix+match components (grid, genset, solar) are >1/2 between the "large and the small"--Then you you definitely have to take it all into account.

    And you may find that some things are simply not compatible (i.e., a backup genset with Grid Tied Solar that is equal to, or more than the loads) and be more trouble than they are worth.

    Out of curiosity--What is the goal of the various systems? Is it to save money (electrical/fuel costs) with solar, to run for many hours to days without utility power, save money on utility power with GT Solar, or what?

    -Bill

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • EstragonEstragon Registered Users Posts: 3,030 ✭✭✭✭✭
    BB. said:
    It sounds like a real difficult system to figure out and put right/make efficient.

    Just to give you an idea of what can happen with a smaller, 3 phase generator+solar system:

    http://forum.solar-electric.com/discussion/17986/is-the-system-set-up-right/p1

    Having multiple sources of power and loads--And mix+match hardware--This will probably be a difficult task to set right--If not near impossible without abandoning some of the hardware.


    Interesting thread.  Lots to go wrong.

    Would it make any sense to just turn off/bypass the grid tied inverters?  They don't appear to be doing anything for backup power. 

    It looks like the theatre, ICU, and lab have their own battery backup systems.  I assume that when grid power is lost, the UPS continues to power the "emergency" line with its own batteries and generator.  Are the battery-based inverters in the ICU etc. powered by the main or emergency line?
    Off-grid.  
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • BB.BB. Super Moderators, Administrators Posts: 28,172 admin
    In the US, there are regulations about medical devices (i.e., you cannot just go buy a cheap inverter and battery bank and throw it on the protected circuit for an operating room).

    There are (at least) two ways to design a backup power system. Just large enough to run the loads for 15-30 minutes--And in the meantime, the genset is started, brought up to speed, and takes over the loads.

    The other is to expect the batteries+inverter to run the loads for 1-3 days via a large battery bank. And the genset can either (or both) recharge the battery bank and support your loads (and/or solar charging too).

    There are variations too... A Hybrid AC inverter--That has both a Grid Tie function (feeds energy back through the mains to turn the meter backwards), and can go "off grid" and support the loads with battery bank+solar+genset power.

    There is even "generator support" in some of these hybrid inverters... Basically, you can run a (made up numbers) a 4 kWatt AC genset to power your loads, but the inverter will take AC power from the genset to recharge the battery bank (if the loads are less than 4kWatt), or if you have high loads (starting/short term loads) that exceed the Genset's 4kWatt output, the inverter will run "in parallel" with the genset and supply the extra power needed (i.e., 8 kWatt load, 4kW from the genset and 4 kW from the AC inverter+battery bank+solar array).

    GT inverters, in general, are there to "save money" by slowing down the billing meter, and even turn the billing meter backwards (i.e., during the day, the solar array supplies 10 kWatts, with a 2 kW load--At night the grid supplies the 2 kWatt load). GT inverters are really a result of regulatory "green energy" or "green washing" by governments and environmentalists. What utility in the their right mind would want to sell you power at $0.20 per kWH and buy it back during the day at $0.20 per kW when it only costs them $0.05 to $0.10 to "buy power" from their generators (plus, how they pay for distribution wiring, transformers, and people to fix your power lines without "markup" of power costs).

    If you are in a region that does not allow you to "sell power" back to the utility--It is very difficult to see how a GT system saves much money--And if you are running your own backup gensets--Management of the local AC backup grid becomes difficult (if GT solar output is >> than the AC loads, the line voltage / frequency goes out of regulation and knocks the GT inverter offline for ~5+ minutes--And or the genset gets power pushed backwards through it--Causing electrical problems, and/or unloading the diesel motor (diesels usually "like" around 40-60% minimum loading or they start having problems with "coking", oil pushed up past the piston rings, etc.).

    When you have your own gensets (emergency backup power--GT solar inverters really can become a headache if the GT Power is >~10% of the total load/genset capabilities. There are GT inverters that can be "throttled"--But that just adds more complexity and may not be present in the current hospital power system.

    And if you can save 10% of your daytime fuel (you still have to run the gensets for stable power, pure GT inverters do not set voltage/frequency of AC mains)--Is it worth the hassles and complexity?

    It is an interesting set of issues--And could be a challenging project. However "challenging power project" + "main hospital power/backup system" usually do not go well together.

    Since much of these issues revolve around the hardware (gensets, inverters, etc.)--Talking with the other stakeholders in the project (i.e., the guys that sold and maintain the equipment) to understand what is happening now and get some ideas about how to move forward--I would start there.

    And, take everything with a grain of salt until proven otherwise--As the example of the link from Netherlands, even engineers get over their heads sometimes.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jonrjonr Solar Expert Posts: 1,122 ✭✭✭✭
    edited March 2017 #14
    You could try having the generator frequency adjusted to say +5% so that it automatically disconnects the grid-tie inverters.  Otherwise, disconnect them before starting the generator.  Too inconvenient - permanently disconnect ones that can't be adjusted to curtail power well enough.
Sign In or Register to comment.