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AC Sources in Parallel

Discussion in 'Electronic Basics' started by Peace Maker, Apr 14, 2004.

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  1. Peace Maker

    Peace Maker Guest

    Can AC voltage sources (generators) be connected in parallel like DC
    voltage sources?

    One electrician told me it was impossible to connect two or more
    generators in parallel for the purpose of powering appliances in a
    house. He claimed the coils of both generators would be burnt in the

    Is that true?

    I doubt his claim because DC sources can be connected in parallel.
    And, since DC network analysis is similar to AC network analysis
    (except in complex numbers arithmetic), I see no reason why AC sources
    (generators) can't be connected in parallel.

    I used Multisim Demo 2001 to simulate the connection of AC voltage
    sources in parallel. The simulation failed to run because of an error,
    which I haven't understood.

    Kind regards
  2. Pure DC sources cant be connected in parallel either. Its usually
    equivalent to placing a S/C across the sources. Often DC sources have
    isolating diodes to prevent one feeding the other. However, only one
    source will actually provide the current.

    This is the deal. If one source voltage is only *slightly* larger than
    the other there will be I=delta_V/Rsource = very large current. That is,
    one source will attempt to sink all of the current of the other source.
    Essentially, yes.

    If you use a control system, you may be able to overcome this issue.
    That's because you haven't done I=V/R

    However, you can usually connect transformer windings in parallel. Why
    do you think this might be so?
    The error is because ideal voltage sources have zero internal
    resistance. This means infinite amps. Spice will not let you connect
    sources in parallel or inductors in parallel. Again, what current will
    flow if you apply 1mv DC to an ideal inductor?

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  3. It is certainly possible. however. the power grid is fed by many
    generators fed in parallel, but they are precisely frequency and phase
    matched before being paralleled. Once connected together, the
    generators interact with each other to help distribute the load
    between them. For instance, if one tries to go a bit faster (gets a
    fraction of a cycle ahead of the others) than the others, that one
    sees a big increase in load that slows it down and the others see a
    decrease in load that lets them speed up a bit . If you just connect
    two generators without having the instrumentation and controls to let
    you make sure they are at the same frequency and phase, them you can
    expect bad things to happen.
    Not if they are not very well matched in voltage and have a little
    resistance between them to limit the current that passes out of the
    highest and into the lowest.
    Try putting a low resistance (say an ohm or two) between them. And
    make the voltage of one slightly different (say a percent or so) than
    the other and also the frequency (say, a percent or so).
  4. Peace Maker

    Peace Maker Guest

    LOL! Am I under some sought of examination by you? Your questions
    (though insightful and constructive) give me that feeling. Is drilling
    your potential customers part of your marketing strategy? :)

    I think the reasons why it's possible to connect, in parallel, the
    secondary windings of two or more transformers are:

    1. The AC sources connected to the primary windings are electrically
    isolated from the secondary windings. So any dangerously high current
    that may develop at the secondary windings is isolated from the AC

    The additonal magnetic fluxes that such high current may develop at
    the secondary sides of each transformer is counteracted by an equal
    and opposite additonal magnetic flux developed in each primary coil.

    The vectorial sum of the output voltages of the transformers must not
    be zero for current to flow.

    2. In actual fact, the combined inductive reactances offered by the
    secondary coils prevent dangerously high current from circulating
    along the secondary coils of the transformers.
    Your second question...

    In DC network analysis, an inductor is always seen as a short circuit.
    Since f = 0 (always for pure DC)
    I=1mv/0 = oo A

    I would gladly love more light to be thrown on my answers to either
    correct or improve them.

    Mr. Aylward, I appreciate your posting of the insightful and
    constructive questions. Please continue posting such at every
    available opportunity.

    Kind regards
  5. Don Kelly

    Don Kelly Guest

    .. They must be in synchronism(same frequency) and it is a great benefit to
    be able to control the "speed" which controls the real power, and also to
    control the excitation or internal voltage to control reactive flow. It is
    not a difficult problem provided you have the necessary controls. First of
    all the incoming generator must be matched for voltage phase and frequency
    with the on-line unit and then connected (at essentially 0 voltage across
    the switch terminals)(this can easily be done with a set of lights ) and
    then "speed" and voltage adjustments as needed. Having good metering helps.
    If your generators do not have such control it is a problem but there are
    devices available now which will do this for you- how well they work is
    something that I don't know.

    Please note that all AC grids have parallel generators- this is normal
  6. Don Kelly

    Don Kelly Guest

    It's been done for over 100 years. For both AC and DC. How do you think a
    multigenerator power system operates? The control needed is not complex and
    preferrably consists of speed and excitation control. You are correct in
    that internal impedance is important -in the AC case it does affect reactive
    rather than power sharing. The basic idea is to match the voltage, frequency
    and phase of the incoming machine to the on-line unit. The rest is a matter
    of tweaking to get the desired distribution of load. A typical lab
    experiment for students is to run a couple of AC machines in parallel with a
    load and see what the effect of manual changes in load, prime mover speed
    settings and excitation have on the performance.
  7. What part of "Pure" did you have trouble understanding?

    What part of "If you use a control system, you may be able to overcome
    this issue." did you have trouble understanding?
    Pretentious crap. Get real dude. You should learn to read the post, and
    understand the point of the post.

    The control needed is

    You completely missed the point. Its clear that the original poster is
    not aware of even the most basic of electrical properties concerning
    sources. That is, ideal current course in series don't work (infinite
    volts generated), and ideal voltage sources in ¦¦ don't work (infinite
    currents). This is the basic information required to know what the
    essentials of the issues are. Sure, one can get around this issues with
    a control system, but such systems were not relevant to the content of

    orginal text:

    The question was whether or not one can simple put (of the store shelf)
    generators directly in ¦¦. The answer to this question is most
    definitely no.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  8. Bill Vajk

    Bill Vajk Guest

    Actually, that's not quite true.

    To make such a connection one has to monitor the waveforms coming
    off the generators (they will never be 100% synchronized while
    running free) and when they match (waveforms line up) one can
    parallel them.

    The tandem set will, from that point onwards, self-synchronize.
  9. Peace Maker

    Peace Maker Guest

    I am aware of basic electronics, which you implied I wasn't. I posted
    the original message out of curiosity in knowing how several power
    plants (hydro, thermal) in my country are fed to the national control
    center (NCC) and then re-distributed to other parts of the country.
    That's why I emphasized the use of "generators". When the electrician
    told me that it was impossible to connect two similar generators we
    had to power appliances in our house, I then wondered: "So, how come
    the national power grid works?" This probably got me confused in
    erroneously mentioning that DC sources can be connected in parallel.

    Your posts and others have informed me more on the processes involved.

    Anyway, thanks for making certain things more obvious to me concerning
    DC sources and their internal resistance.

    I was expecting you to comment on the answers I returned to your

    Kind Regards
  10. Well, with all due respect... My point here is that basic electronics
    surely includes V=IR. You can't get much more simpler. An application of
    this formula immediately shows what the issue is. A delta V between
    generators of only 1V, AC or DC, theoretically, could result in huge
    currents from one generator feeding the other because the internal
    resistance is so low.
    If you get say, two petrol generators from the hardware store, and
    connect them in ¦¦, its pretty much guaranteed to blow a fuse, so the
    electrician is correct. Voltage, frequency and phase will not be

    The national grid is a different issue. Steps are taken such that the
    generators can be connected together.
    I have been a bit busy. Still am..

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  11. Yes it is, assuming that the generators have been *specifically*
    designed to be || capable.
    You are moving the goalposts. What part of "directly connected, off the
    store shelf generaters" are you having trouble with? Direct connection
    means || the power plug sockets only.

    Sure, we have *already* pointed out that *if* you synchronise voltage,
    frequency and phase, than you can || such generators. Synchronising
    voltage requires a control circuit to makethem exactly equal.

    Kevin Aylward
    SuperSpice, a very affordable Mixed-Mode
    Windows Simulator with Schematic Capture,
    Waveform Display, FFT's and Filter Design.
  12. Don Kelly

    Don Kelly Guest

    Don Kelly

    remove the urine to answer
    This control circuit could be nothing more complex than a voltmeter and or a
    light across the open circuit- exactly equal is desired but not absolutely
    necessary. Look for and adjust for slow voltage swings and refine until
    voltage is nearly 0 and holding. Such a setup was in use for many years for
    manual synchronising of alternators-( lights alone, for a 3 phase machine
    can give close enough indication of relative speed phase and voltage). A
    difference in voltage will cause a reactive flow between the generators and
    as long as it is not excessive- no problem. However it would be *very*
    desirable to have voltage control and speed control of at least one of the
    machines in order to balance real and reactive loads.
    However, for the small home generators, it really is simpler to buy an
    electronic device designed for this. As far as I can tell it is basically a
    rectifier/inverter setup dessigned for ease of use.

    Don Kelly
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