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Say that there are two factories connected to the same 20 kV medium voltage line (see Fig. 1). Each of them has 400 V motors that consume power by turning some machines acting as mechanical loads. One of the factories has a diesel electric generator connected to its 400 V bus and synchronized with it.

Question: Is it possible (without disconnecting the factory with the diesel generator from its transformer) to adjust the power of the generator in such a way as to feed all consumers in the factory without sending energy to the loads in the other industrial facility which does not have a generator or to any other consumer connected to the grid?

For me, it appears that even if the generator is adjusted to deliver exactly the same power as needed by the loads in its factory, nothing stops those loads to draw energy from the grid and nothing stops the power of the generator from going into the medium voltage grid and be consumed by the motors of the other factory or by another industrial facility. But, I can be wrong.

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Fig. 1. Two industrial facilities (surrounded by a dashed blue line) connected to the medium voltage grid through two transformers. One of the factories has a diesel electric generator connected to its 400 V bus.

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    $\begingroup$ you would have to monitor the main power meter ... if it shows extremely low power consumption then lower the generator output $\endgroup$
    – jsotola
    Aug 15, 2022 at 19:21
  • $\begingroup$ It can be noticed in the other plant if the genset is being used for load factor mitigation. $\endgroup$
    – Phil Sweet
    Aug 15, 2022 at 22:28

3 Answers 3

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There should be a connection "box" between the generator and the grid side to prevent "backfeeding" the grid.

All the inverters for the many PV panels connected to the grid will self-disconnect if the grid fails. This is to prevent a phenomenon known as "islanding" which can kill workers.

If there is no such control then it may need checking by the supply company.

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  • $\begingroup$ How can that back-feeding be prevented? I would like more information about that connection box. Once again, the diesel generator is not supposed to start working if the grid fails but it functions continuously even if there is nothing wrong with the grid. My generator is not like the PV panels that are supposed to deliver energy to the grid and disconnect from it if the grid fails. My generator must not provided energy to the grid as long as it works but at the same time the grid should provide energy to the factory. I know I am in an unusual case but I need a solution. $\endgroup$ Aug 15, 2022 at 18:47
  • $\begingroup$ I can disconnect the diesel generator if the grid fails. This issue is already solved. $\endgroup$ Aug 15, 2022 at 18:55
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For me, it appears that even if the generator is adjusted to deliver exactly the same power as needed by the loads in its factory, nothing stops those loads to draw energy from the grid and nothing stops the power of the generator from going into the medium voltage grid and be consumed by the motors of the other factory or by another industrial facility.

That part is easy. You can't have power flow in both directions simultaneously. Provided the generator is in phase with the grid then power is either flowing in or flowing out but not both.

To prevent outbound power flow it would only be necessary to make the generator voltage a little lower than the grid voltage.

Note that the utility company will need to know about your setup and will demand proper automatic disconnect on grid failure to prevent backfeed and potentially killing a lineman working on a supposedly isolated system.

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  • $\begingroup$ If you plug a low resistance inductor of a suitable inductance in the 230 V, 50 Hz outlet, this coil will absorb power from the grid and then deliver it back immediately (most of it except the small resistive losses). So it is possible to have an average bidirectional flow of power. $\endgroup$ Aug 15, 2022 at 23:53
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It is possible, but not 100%, if the breakers to the grid are closed.

I see a comment where you declare that you must not provide power to the grid. If this means never, it will be very difficult to power internal loads without ever feeding the grid.

Think of it like two tanks of water connected by a pipe. The tanks' level/water pressures are equivalent to grid and internal voltages. (Power genertion uses things a concept called house curves to drive power into the grid.) One tank is connected to your loads. You can keep the tank filled (have an internal generator) by matching to your internal load, but if a load ever trips, the tanks will fill and water will flow to the grid tank before you internal generation can react.

As such, if the requirement is to never feed the grid, you need a safety factor of base grid load to ensur that if one of your loads trips suddenly the generator doesn't powr the grid.

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