Battery and inverter safety with electric motors (elevator)

OsolemioOsolemio Posts: 9Registered Users ✭✭
Greetings from Spain!

I am considering to install the following PV system for self-consumption. As you see in the following picture, it's grid connected with a PV inverter (5 kW, Fronius) and also has a battery bank with a battery inverter included (Sonnen, 3.3 kW). It's a three-phase system meant to feed a few lights and, most importantly, an 8 floor elevator.

The elevator consists of a traction electric motor of 5 kVA, three phase, no variable frequency drive, star/delta starting, with around 4.5 kVar reactive power, which I would like to compensate by means of a capacitor with contactors (or solid state switches).

These electric motors have quite a high starting current spike (I can upload screenshots of the network analyzer if you want to) and my concern was mostly in regards the safety of the solid state electronics of both inverters, and also the integrity of the actual LFP Sonnen batteries.

In my opinion these inverters should deliver only the amount of power they have been designed to, and leave to the grid the remaining power to be delivered to the loads. Am I right? If you believe this configuration is viable, would there be any considerations worth taking? Is, on the contrary, a bit of a risky configuration, or would this shorten the lifespan of the inverters or the batteries?

Thank you in advance!







Comments

  • mike95490mike95490 Posts: 7,902Solar Expert ✭✭✭✭
    edited January 28 #2
    I would hope that 3 phase inverters would be expected to see large motor loads, and be able to both withstand any spikes they introduce, and be able to supply any reasonable surges they require for starting (big reason for 3 phase = low surges)

    I would worry about adding / switching in Capacitors, without having tested and measured the precise amount of capacitance needed to null out the motor reactance.
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  • OsolemioOsolemio Posts: 9Registered Users ✭✭
    Thanks for your comment @mike95490. Good to know that these surges should not compromise inverter safety.

    I leave a picture of the network analyzer where it is possible to see the amount of reactive power to compensate. I am finding somehow difficult to find a simple solution for this, since apparently the caps should ideally only be connected after the starting phase, only when the elevator is running. I also see that the caps need to be discharged prior to connecting them again...



  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    It was not uncommon to put a 5 watt power resistor across the terminals of capacitors to discharge them between application/use.

    Even if you discharge the capacitors between use--You still have a 50/60 Hz sine wave and a zero volt capacitor--So you will still have some "inrush" current to charge the capacitor (if the contactor is energized at one of the sine wave peaks).

    If the capacitors are connected in parallel with the motor run coils (no starting coils?), you should not need to discharge the capacitors (in parallel with the inductors/motor coils). I am not sure I see a reason for applying the capacitors only after the start phase. Should not hurt during start.

    In theory, if your inverter is large enough, then power power factor does not save you any battery energy (just VA type current through the inverter--Which the inverter, contacts, transformer, etc. do have to be designed to support).

    In general, you do not correct to more than ~0.95 Power Factor. Otherwise, when the motor is turned off and slowing down, the capacitors can cause the motor to self excite and actually start generating voltage/current (and high voltage if there are no "loads" on the motor connections).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OsolemioOsolemio Posts: 9Registered Users ✭✭
    Thanks a lot Bill. I'll consider using the power resistors to discharge them between applications. 

    I thought of compensating reactive power to allow the inverter to charge the batteries and feed the load at the same time. The active power of the elevator is barely 1.6 kW when running. A 5 kW array during sunny hours would have 3.4 kW to feed the batteries (3.3 kW sonnen battery inverter). Otherwise the power would have to be fed by the grid.




  • OsolemioOsolemio Posts: 9Registered Users ✭✭
    Just one more question guys: let's imagine I did not compensate reactive power by including capacitors. For the same load of the graph above (4.9 kVA, 1.6 kW, 4.6 kVar inductive) who would feed what? I mean, would the battery inverter (3.3 kW rated power) feed the 1.6 kW plus part of the reactive part (adding up a total of 3.3 kVA), thus leaving the remaining reactive load to be fed by the grid?

    Or would the inverter feed 3.3 kVA but shaving the same proportion of active and reactive power, leaving some active and some reactive power to be fed by the grid?

    Thanks in advance.
  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    Osolemio,

    First--I want to understand your graphic design of the AC / solar / off grid solar power system... It does not quite make sense to me.

    Grid Tied Inverters are directly tied to the utility power (through your main panel). Nothing special there... If the AC loads are greater than the GT inverter can supply (night, morning, evening, high loads, etc.), then the "excess power" needs is supplied by the utility power (and turns your meter forwards). If the GT inverter+solar is supplying more power than your local loads, then the utility meter turns backwards as the energy is feed back into the grid (think of a giant "AC Battery" system--really does work like that).

    This assumes that GT solar is legal in your country (Spain?). And your meter(s) and utility billing are setup for this.

    Off Grid inverters can/do connect to various AC sources (grid, backup genset)--And use that power to run local AC loads (separate "protected" sub panel). And if the AC power fails, then the battery bank runs the AC inverter and the protected sub panel.

    There are Hybrid AC inverters that are both GT and Battery/Off Grid type of units... They are still wired as above (AC mains, feeding back excess solar energy to grid) and supply power to a protected sub panel (AC mains, backup genset, DC solar+batter power).

    In your diagram... You do not show a "protected sub panel" that supplies AC power to (lights, small loads, emergency elevator power?). You only show the AC mains side.

    Next... The typical GT inverter supplies current that is "in phase" with the AC mains (Power Factor is 1.0). It is "in parallel" with the AC mains. Any current/energy that the local GT inverter cannot supply, the AC mains will (through your utility meter). Just like paralleling "AC Batteries".

    In the US, for residential systems, the utility meters only charge for kWatt*Hours... That is the PF=1.0 current flow. If we have a motor load that is PF=0.60 then the meters only charges 60%*VA=kWH.

    For (some, many) commercial systems... The utility will charge for "reactive power". In many billing schemes, the worst case reactive power (i.e., PF=0.XX) is averaged for 15 minutes, then used as a "correction" to the meter rating (i.e., 1,000 kWH per month * 1/0.60 average PF = "corrected" bill). Notice that in (some) of our billing--You could have a motor that runs for 30 minutes a month, but the whole power bill is penalized.

    Another way is the split the bill in 1/2... 1/2 is based on kWH per month, the second is based on the kVA 15 minute peak... The utility is making "reservation" charges--I.e., they have to size the transformer and utiity drop to supply your worst case kVA loads for the month (or the year). And charge you for actual energy used.

    kWH is fuel for the generator consumed. Reactive Power is the high current that goes "back and forth" but does not do any work (consumes no fuel)--However, the wiring, distribution, gensets, etc. all have to supply that "out of phase" (and non-linear) current (n the US, roughly 1/2 the power costs are fuel/kWH related, the other 1/2 is infrastructure, lines, distribution, transformers).

    So, whether or not you add capacitors to bring your PF to ~0.95 maximum (done a lot in large factories, oil refineries, etc.), really depends on the meter and billing plan of your utility... It may save you nothing, or may save 40% of your utility bill--It Depends.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OsolemioOsolemio Posts: 9Registered Users ✭✭
    Thanks a lot Bill, I appreciate your contribution.

    The diagram is just that: a simple diagram to show how the main components are placed. You are right, there is no main panel, protections, etc. but of course they need to be considered. However too much detail would have been distracting to the readers, in my opinion, and just wanted to ask about reactive power. 

    In Spain, with current laws, there is no net metering or feed in tariff. You are paid by the PV production at the same rate as big utilities are (e.g. nuclear or coal power plants). In the case of installing batteries, they also require a third meter for loads. All this is very bizarre, I agree, and the laws and computations are thought by legislators that do not want renewables to flourish, because they have more interest in keeping fossil fuels alive. This is how it is right now in Spain, unfortunately, so I have to deal with it!

    I have to do the numbers, like you say, with and without compensation of reactive power and see what's more favorable.

    With compensation, I am somehow reluctant to use capacitors with contactors. Like you said, they need to be discharged, and this means they need at least from 1 minute to 5 minutes depending on the solution adopted. Am I right? This means no compensation during this time and therefore no compensation of reactive power of several elevator trips in between. In my opinion there is also the problem of the delta-star starting / stopping. In a normal elevator sequence, the motor starts (star), then runs (delta) and finally, when it arrives to destination, it goes back to star and finally the electro-magnetic brake forces the elevator to stop. Star-delta-Star means two times the contactors change current phase, something that shortens capacitor lifespan. That's why I said that the contactors should "enter" only when the motor is in delta configuration (elevator running). I see that this does not suppose any problem to you, but I believe an elevator with 200 trips per day, 73000 trips per year means that those contactors and capacitors will suffer quite a lot after a while. Very different is a motor in a factory that starts half a dozen times per day?

    I have also checked solid-state contactors for the capacitors (with thyristors). Apparently they have zero crossing detectors that plug the capacitor when voltage crosses zero and they disconnect the capacitor when current crosses zero. I have to double check all this, but if this works as described, this means the capacitor is already discharged and ready for the next use within only ms. These devices are quite expensive though, and I don't have experience with them.

    There is also the option of not compensating reactive power (hence my previous post). Here in Spain meters also average power in 15 minute slots. When you install a PV system, the utility takes advantage of the occasion and upgrades your meter. This means that your old analog meter (which only measures active power) is replaced by a digital one that also measures active and reactive energy consumed. This is, of course, not favorable for the client, because reactive power is penalized in Spain in a three-phase electric bill.

    The inverters I plan to use are Fronius (for the PV array) and Sonnen (battery inverter). If you check the Fronius datasheet you can see that it is mentioned that the inverter can deliver any cos phi. I assume that this means it can adjust to any reactive load. 

    Sonnen inverters do not mention this in their datasheet. I have tried contacting the manufacturer but they don't reply (not even to their distributors here in Spain, which I am also in contact with, and they are surprised of not getting an answer). I cannot assume that this battery cannot deliver reactive power, because the distributor (also PV installer) has confirmed that they have done one standalone (no grid) installation where the battery inverter was, effectively, feeding a load at cos = 0.46.

    So this leaves the question in my last post still unanswered: I meant to query about the ratios of active and reactive power that would be split at a given moment by the battery inverter and the grid (I need this to figure out what the electric bill would be without compensation of reactive power):

    If the load consumes: 4.9 kVA (1.6 kW + 4.6 kVar).

    Then, what would the scenario be:

    A)
    The battery inverter (rated 3.3 kW) delivers 1.6 kW + 2.89 kVar = 3.3 kVA
    The grid delivers 0 kW + 1.86 kVar = 1.86 kVar

    or

    B ) 

    The battery inverter delivers 1.056 kW + 3.135 kVar = 3.3 kVA (these are load values multiplied by 3.3/4.9)
    The grid delivers 0.544 kW + 1.615 kVar = 1.704 kVA 

    I guess this is difficult to know, unless I find a way to test it...


  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    edited January 30 #9
    If the elevator is operated only from your off grid ac inverter. Protected power panel.. it will supply all active and reactive power. Some inverters do better per worse regarding inductive or capacitive loading.

    If operated with gt inverter or hybrid inverter on the mains side, i believe that they will only supply real power. Reactive power will come from utility.

    However, Germany (if i remember correctly) wanted their gt inverters to generate power with both real and reactive power (+/- 0.80 ?? PF). It is programmable as i recall. This was some years ok. Don't know the current status of requirements and what brands may do this.

    The gt obeyed would need to sense utility power for active correction of reactive/poor power factor (use a current sense transformers on utility drop to measure pf, i think).

    A way to correct poor power factor would be converting... ac to dc to ac (off grid ac inverter or vfd or something).

    Electronic zero crossing switching should be relative to line and load voltages (whatever the capacitor voltage and line voltage is at that moment in time).

    There are a lot of "moving parts" here. I am not qualified to give you answers (not a power or elevator engineer).

    I would hope your elevator supplier can give you better answers, these questions are not new.

    I would not suggest on off grid inverter + battery system would be cost effective. A vfd based system may be... but lots of issues there (vfd tends to be square poor modified square wave output. Those harmonics are hard on induction motors).

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    I should add, remember the power correction capacitors are running 50hz ac. They are cycling from zero volts 100x per second. The bleed resistors are there to discharge caps before a service person gets into the electrical (when main power shut off, potentially charged capacitors-zap).

    Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • OsolemioOsolemio Posts: 9Registered Users ✭✭
    Thanks a lot Bill for your comments. I'll go on with my research!
  • BB.BB. Posts: 27,887Super Moderators, Administrators admin
    Let us know what you end up finding out/doing. It is very interesting (at least to me).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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