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DG connection: Model for Steady State Voltage rise in Point of Common Coupling (PCC)

  1. Jun 23, 2017 #1
    Hi!

    According to the book "Renewable Energy on Power Systems" by Freris & Infield, the voltage rise due to injection of power in pcc can be estimated from the Thevenin equivalent representing the network "upstream" pcc (Figure 1). The Thevenin voltage can be taken as the nominal voltage in the pcc, and the Thevenin impedance
    is given by Z= V/√3⋅Isc (1).
    Capture.JPG
    Figure 1: Thevenin equivalent

    I've got some measured fault current values (three phase) for different Pcc's in different LV radial distribution networks, and want to make an estimate of the voltage rise in each Pcc based on the calculated source impedance in formula 1. This is pretty straight forward according to the mentioned literature, but there is something i don't get:

    Is the fault current in the Pcc depending on the numbers of other buses in the LV radial? For me it seems pretty obvious that the impedance seen from the Pcc with DG is lower if your neighbor has a big demand of power, and thus the voltage rise in the Pcc will become smaller (?). I don't get how this Zth calculated from the Isc can be used if this is not the case.
    PS: I probably lack some understanding in source impedance and fault levels.

    << Edit -- clarification of PCC definition >>

    Point of Common Coupling - the point where the the generator is connected to the public grid.
     
    Last edited by a moderator: Jun 23, 2017
  2. jcsd
  3. Jun 23, 2017 #2
    It depends where you are measuring the fault current.

    Your equivalent diagram actually includes a mirror image to the right where the equivalent source impedance from the system is in series with the open circuit voltage from the system.

    If you are measuring the fault current at the PCC, for example, for a single-phase-to-ground fault, the PCC sees the total fault current - from the DG as well as from the system.

    If you are measuring the output of the DG (inverter), they you can use your simplified diagram since the system won't have a significant effect on the results.
     
  4. Jun 26, 2017 #3
    Hi!
    Thanks for replying, magoo. I've been busy this weekend, sorry for the late response.

    I have to make some clarifications, seems like my explanation was confusing:

    The Thevenin equivalent is the network, Snet means produced power minus used power (net) from the DG (PV, wind, CHP, etc...). The fault current is measured in the Pcc, that's the interface between the grid operator and the consumer (and due to DG, manufacturer).

    So my question still stands, but i can try to clarify it with a figure:
    upload_2017-6-26_9-38-26.png


    Is the fault current/level in the Pcc depending on the numbers of other buses in the LV radial (situation A vs. B) ? For me it seems pretty obvious that the total source impedance seen from the Pcc (in a thevenin equivalent) is smaller if your neighbor has a big demand of power (i.e. one or both neighbor buses in situation A have a big power demand), and thus the voltage rise in the Pcc will become smaller (?). Said in another way; How does the topology of the LV distribution network affect the source impedance? Bigger impedance seen from the Pcc = bigger voltage rise (formula 1, post 1) - but if your neighbor in some way uses all your PV-made power, should this impedance then be smaller?.
     
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