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Thread: Voltage drop & rise

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    Voltage drop & rise

    Hi,

    I usually use a voltage drop calculator to establish my conductor sizing over greater distances. One of the inputs is PF and conductor temperature.

    1. I wanted to verify with inverters PF would be 1. Is that correct?

    2. What is the conductor temperature input asking? Is it the ambient temperature around the conductor? Or the operating temperature of the conductor? If the latter, where do I get that number from?

    Thanks for your help!

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    This is a good article that covers both items.

    Resistance changes with temperature. Load power factor will affect the voltage drop calculation.

    http://www.ecmweb.com/content/calcul...bution-systems
    Ron

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    Quote Originally Posted by electro7 View Post
    Hi,

    1. I wanted to verify with inverters PF would be 1. Is that correct?
    Check your inverter data sheets and utility interconnection requirements. With small systems (residential) you're probably okay assuming 1. With larger systems it could be different. Sometimes utilities for engineering reasons request an inverter to operate at lower PF, which is a capability of most inverters nowadays, to at least like .95 or .9 usually.

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    It used to be you could assume PF for inverters is always 1 but with new smart inverter standards the PF could be variable depending on what the requirements of the utility are at any particular time. It's probably not going to be that much less than 1 for much time, that would cost the owner production, so I would use 1 unless there was some lower fixed value for a specific installation.

    Worst case for the temperature is a higher temperature, and it's the conductor temperature not the ambient. So for instance, if you used 90°C conductor you could go worst case and assume the conductor could be up to 90°C, or you can try to figure out what the actual high temperature might be. Ambient plus some factor maybe, never tried to work it out myself.

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    Quote Originally Posted by pv_n00b View Post
    It used to be you could assume PF for inverters is always 1 but with new smart inverter standards the PF could be variable depending on what the requirements of the utility are at any particular time. It's probably not going to be that much less than 1 for much time, that would cost the owner production, so I would use 1 unless there was some lower fixed value for a specific installation.

    Worst case for the temperature is a higher temperature, and it's the conductor temperature not the ambient. So for instance, if you used 90°C conductor you could go worst case and assume the conductor could be up to 90°C, or you can try to figure out what the actual high temperature might be. Ambient plus some factor maybe, never tried to work it out myself.
    I think that there is a pretty good chance that an inverter with settable power factor will produce more VA to match the requested PF instead of drawing less power from the PV array. All it would require would be some regeneration back to the DC bus and a slightly higher current rating for the pass elements.

    Sent from my XT1585 using Tapatalk

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    What am I missing when it comes to PF and inverters?

    Input is DC - shouldn't be power factor there. Output - I suppose there can be a distortion power factor but displacement power factor should be load dependent.
    I live for today, I'm just a day behind.

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    Quote Originally Posted by kwired View Post
    What am I missing when it comes to PF and inverters?

    Input is DC - shouldn't be power factor there. Output - I suppose there can be a distortion power factor but displacement power factor should be load dependent.
    Output PF can be deliberately lowered. I believe the reasons are to have the inverter supply some vars so that not all of the vars have to come from the utility if the loads cause displacement. I'm not an expert on this but I've been told it can be required by utilities to mitigate engineering demands on their distribution network.

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    Quote Originally Posted by jaggedben View Post
    Output PF can be deliberately lowered. I believe the reasons are to have the inverter supply some vars so that not all of the vars have to come from the utility if the loads cause displacement. I'm not an expert on this but I've been told it can be required by utilities to mitigate engineering demands on their distribution network.
    Guess I'm not seeing how that can be done, but makes sense if it can be done.

    Does such an inverter put out a leading power factor if there is little or no load on it or does it adjust to loading?

    PFC at individual loads still seems to make the most sense for most applications.
    I live for today, I'm just a day behind.

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    What you can do with a strictly utility interactive PV inverter and no local loads is deliver PFC current as if the GTI included a PF correction capacitor bank.
    Because the incoming grid voltage gives no clue as to what the utility current looks like anywhere in the system, there must be a protocol by which POCO can command a particular power factor from inverters with that capability.
    This allows POCO to fine tune their own local and ovaerall PF taking the place of switched capacitor banks (an ongoing maintenance issue in addition to their initial cost) and avoidinghhh low PF loading on their prime mover generators.

    Basically the active electronics of the inverter can deliver current with a waveform offset from the sensed voltage, emulating a capacitor.
    In theory, it would just be a matter of providing additional information to allow the GTI to correct for distortion PF just as well as displacement PF. (The same sort of thing that active power factor correction front ends do for VFCs and other DC rectifying products.)

    Sent from my XT1585 using Tapatalk

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    A PV inverter is also, with a little additional programming, be a static VAR compensator. Static VAR compensators can source or sink VARs as needed to change the PF. But it is constrained by the rating on the inverter, a 100kW inverter cannot put out 100kW and produce VARs at the same time, the real power output has to be reduced which means the owner loses that income when producing VARs. So owners get concerned when utilities talk about requirements to produce VARs with no compensation.

    Fun fact, PV inverters can produce or consume VARs at night. No PV array input required because it's all imaginary power anyway.

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