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Ground in a multiphase system

Ground in a multiphase system

A 3-phase system may be connected in star (Y-) connection or in delta connection. Whereas both schemes are physically possible and each one can be transformed mathematically into the other, there are some obstacles for simulation.

Preferably, a 3-phase voltage source should be modeled in Y-connection, connecting the neutral point to GND (unless the circuit absolutely requires the GND connection elsewhere - remember: if you place more than one GND symbol, those nets are electrically connected).

The rest of the circuit, apart from the voltage sources, may be connected either in Y-connection or delta connection, as required by the circuit to be simulated (see figure 1).
If the simulation runs into problems, see "How to deal with SPICE error messages".

figure 1.png

Figure 1 3-phase system, sources in Y connection

The obvious way of drawing the voltage sources in delta connection raises the question of where to put the necessary GND connection. Since physically there is no GND connection in a delta circuit, we are free to place GND anywhere (see figure 2.4-2). This spares us from the SPICE error message "no conduction path to ground!".

figure 2.png
Figure 2 3-phase system, sources in Delta connection

However, you will now get an error message stating that the voltage sources are in a loop. This can be remedied by a small series resistance (1 Ohm in figure 2), but this may not be what you want to simulate.

If you want/need to simulate a delta connected 3-phase source anyway, take care and observe the orientation ("polarity") of the sources. It is best to check with a short simulation the correct behavior before building the rest of your circuit. Wrong orientation ("polarity") of the voltage sources gives unexpected results since the line voltages are not what they should be.

SPICE help topics to look at: Voltage Source

Harald Kapp, 2014-05-13
Harald Kapp
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