Generators in a utility system

Discussion in 'General Electronics Chat' started by Cerkit, Oct 21, 2015.

  1. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Hi.

    I am trying to get my head around a few things.

    If I have an inverter connected to a distribution network, if the power factor of the network is 0.98 for example and the inverter has a power factor of 0.95, can the inverter still export to the grid?

    The way I understand it is that the inverter in this scenarion could still export real power if the voltage at the inverter busbar is higher than the distribution network voltage but it will draw Vars from the network and hence decreas the power factor of the network at the point of connection.

    Is all this correct?

    Thanks
     
  2. crutschow

    Expert

    Mar 14, 2008
    13,006
    3,232
    What inverter "power factor" are you referring to? :confused:
     
  3. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    The power factor of a load is what it is. The generator is defined by the power factor it can put up with. Most electro-mechanical generators are defined in VA and can take pretty low power factors. Electronic inverters, on the other hand, can be ,ore finicky about this. Some are, in fact, very finicky.
     
  4. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Ok. Let me see if I understand. If the network load is .95 power factor lagging.

    Then I connect a PV system for example with an inverter to feed into the grid.

    If the inverter is hypothetically producing power at unity power factor (and at a voltage slightly higher than the grid in orderto export) then it will raise the voltage of the network as well as improve the power factor of the network? Is this correct?
     
  5. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    This is not complicated but we get into some semantic questions. An unloaded generator has no "power factor" because there is no current to factor.

    An inverter [generally] outputs a rectangular wave, not sine, so it's not as simple as you may think.

    We need to know much more about what you are trying to do but if you are thinking of just plugging the output of an inverter to the mains that is a terrible idea for many reasons. The first and foremost is that they are not synchronized nor in phase so you would end up blowing up something. Probably the inverter.
     
  6. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    Three phase electro-mechanical generators can be connected in parallel because, once they are connected, they stay in sync. If one of them wants to get ahead it will be pulling more load and it will slow down while the ones pulling less load will speed up and catch up.

    To connect a generator to the grid it is brought up to speed and frequency and when it is in phase the switch is thrown. Connecting when out of phase would result in disaster. The old way of doing this was with three lights connected between each of the generator phases and the grid phases. As the frequency were not totally the same the lights would blink, bright when out of phase, no light when in phase. As the speed and frequency got closer the blinking slowed down. When it was below a certain number of blinks per minute then the switch could be thrown and the generator was put on line.

    A basic home inverter has no idea of the mains phase and will not synchronize anyway as there is no feedback of any kind like there is in a mechanical generator. Connecting an inverter to the mains requires special equipment and authorization. The power companies take a dim view of people putting power into lines which are down for repair and, by doing so, putting in danger the lives of repairmen.

    Even though the waveform put out by an inverter is not sinus, this is not a major problem and any power put into the grid is... well, power put into the grid. The problems this presents are not major and are mainly to do with harmonics, just like non-linear loads, etc.

    Half of Japan is on 50 Hz and the other half on 60 Hz and they need to transfer energy between the two. Instead of using a motor driving a generator they use a purely electronic system where they rectify current and charge huge capacitor banks and then put pulses of energy out which are square, not sine. The thyristors used are humongous.
     
  7. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Why does exporting reactive power into the grid raise the voltage of a network?
     
  8. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    This question does not make sense. Can you explain?
     
  9. Cerkit

    Thread Starter Active Member

    Jan 4, 2009
    275
    3
    Exporting reactive power means operating with the current leading the voltage.

    Which is the equivalent of having capacitors on a network, these can be used to raise the voltage.
     
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