| In the situation that I mentioned, a previous step down delta-delta
| transformer was changed to a delta-wye transformer. Then single phase loads
| referenced to ground could be take from each phase. Increased capacity.
| Certainly there will be neutral current in the case of unbalanced loads. Is
| this a major factor for safety? Not really, provided that the neutral is
| properly grounded and of adequate size. Note that the heaviest currents in
| the neutral (about 97-100%) were those due to the 120/240 Edison system
| customer loads -these would not change but account for 95-100% of the total
| neutral current. In fact, any primary neutral current will actually reduce
| the current in the neutrals (admittedly not by much). In your home you are
| dealing with 120V/240V loads which are rarely balanced so the neutral
| carries current- does this bother you? Yes, equipment is tied to a seperate
| ground for good reasons. Note that the current carrying neutrals of a MV LV
| or HV-MV system are also very well grounded and the fact that they may carry
| current in the case of unbalanced loads is recognised and accounted for.
Having the neutral carry current does NOT bother me when there is a
separate grounding conductor. Even in cases where that is not quite
true, such as poor connetions, a fraction of LV is not nearly the
same level of issue as a fraction of MV. There is no separate ground
in MV distribution circuits. That, combined with connection between
the current carrying MV neutral and the customer service drop neutral,
are what I have issue with. Add the 4th (for L-L transformers) or 5th
(for L-N transformers) wire and use it correctly, then I do not have
an issue with a solid metallic path from customer to distribution.
| Now consider the delta with a neutral tap on one side. Will this mean that
| the neutral is not carrying current- ideally so but ??? Suppose also that
| it was 12.5KV line to line. That means that 2 legs are at 6.25KV with
| respect to ground and the other is at 14KV with respect to ground. Is this
| better than having all 3 legs at 7.2KV to ground? Zig- zag grounding
| transformers were often used to get a neutral point which was equidistant
| electrically from all phases. The center tapped leg of a delta is a cheap,
| but poorer alternative to this.
I'm not suggesting a center tapped delta.
| Seeing that the user with a single phase supply sees no difference from the
| situation where the distribution transformer is connected l-l vs l-n on the
| primary- your last question is meaningless. Run a separate ground wire if
| you want.
Please clarify what you mean by "Run a separate ground wire if you want."
There are a number of different ways to accomplish that. But given that
power company practice is to connect the secondary of the transformer to
the primary current carrying conductors, then the first step to running a
separate ground wire is to have another transformer with its primary wired
L-L (240 volts) and its secondary not connected to the primary at all ...
not even to the service drop neutral (which also must not be grounded to
earth anywhere near the points the new separate ground is earthed).
| If you are looking at an industrial system taking 3 phase from a delta with
| a neutral on one side and single phase to neutral loads on the tapped side-
| where while neutral current can't flow, unbalanced voltages can result -then
| I would prefer a Grounded wye system. A separate safety ground wire to the
| frames of equipment is just as feasible there as with the household single
| phase system.
| Note also that ground fault protection is a hell of a lot easier with a Y.
I still think you are misunderstanding me. It seems you are assuming that
when the loads (transformers at customer taps) are L-L or L-L-L, then the
source of the circuit they connect to must have a delta secondary. I do
realize you described the case where a town switched from delta secondary
(for example at 7200 volts) feeding L-L and L-L-L loads, to a wye secondary
(for example at 12470 volts) feedling L-N and L-N*3 loads, to achieve a
73% boost in system capacity.
But this was all in repsonse to my description of how things should be from
the beginning, which would have precluded that down from having the starting
point they had in the first place.
The substation transformer secondary should be WYE. The center point is
earthed at the substation. Now there are two different ways to run that
1. Run 5 wires, identified as A,B,C for the phases, N for the neutral,
and G for the grounding wire. Loads can then be connected to any
combination of A,B,C,N as needed.
2. Run 4 wires, identified as A,B,C for the phases and G for the grounding
wire. Loads can then be connected to any combination of A,B,C as needed.
With design #1 you can have L-N taps for customer service transformers.
With design #2 you are limited to L-L taps. In all cases the secondary
would be WYE.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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