DG Connection: Estimating Voltage Rise at Point of Common Coupling (PCC)

[hnes]

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⋅I_sc (1).

Figure 1: Thevenin equivalent

I've got some measured fault current values (three phase) for different P_cc's in different LV radial distribution networks, and want to make an estimate of the voltage rise in each P_cc 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 P_cc depending on the numbers of other buses in the LV radial? For me it seems pretty obvious that the impedance seen from the P_cc with DG is lower if your neighbor has a big demand of power, and thus the voltage rise in the P_cc will become smaller (?). I don't get how this Z_th calculated from the I_sc 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.

 

[magoo]

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.

 

[hnes]

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, S_net means produced power minus used power (net) from the DG (PV, wind, CHP, etc...). The fault current is measured in the P_cc, 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:

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?.