The transformer is an S9-30/10. Phase A carries a set of equipment, with a measured long-term operating current of 15-17A. The currents in phases B and C are small, generally not exceeding 2A. The transformer's low-voltage output is made of 25 square millimeter aluminum core wire, with good neutral grounding and a grounding resistance of 3Ω.
1. The three line voltages are basically balanced, all between 385 and 390.
2. Phase A voltage is too low, between 198V and 208V.
3. Phase B and Phase C have high voltages, ranging from 225V to 230V.
4. The voltage difference between phase A and phases B and C is generally no less than 20V, so we dare not use the method of increasing the voltage.
The load on phase A is not adjustable because it's part of a complete equipment system. What's the best solution? Try to increase the voltage of phase A to above 215V.
You haven't provided the specifications for this transformer. I checked and found the table below. It's approximately 30KVA. Theoretically, the current should be around 60 amps at a phase voltage of 220V. Your load current does not exceed the rated current, and the grounding resistance of 3 ohms is less than the required 4 ohms. Your customer requires that only one phase can be connected. Of course, an unbalanced load will cause an unbalanced voltage.
I. General Analysis
Although grounding of the transformer neutral point fixes the neutral point potential, the neutral point voltage of an unbalanced transformer is not "fixed"! There are three reasons for this. First, although the transformer neutral point is grounded, the grounding resistance is not zero. The standard specifies that it should be less than 4 ohms. It is this 4 ohms that will cause a small deviation in the three-phase voltage of the transformer when the load is unbalanced.
Secondly, although the resistance of a three-phase line is very small, the presence of single-phase loads means that the length, cross-section, and load size of the three-phase lines are inconsistent. Therefore, when measured at the user's location, the neutral point will inevitably not be zero, resulting in a deviation in the three-phase voltage. Thirdly, when the transformer's ground wire is disconnected, the neutral point will shift due to the imbalance of the three-phase load, causing inconsistencies in the three-phase voltage. This inconsistency, in turn, further exacerbates the imbalance of the three-phase load.
=(17-2)÷17×100%=88.24%. In a three-phase four-wire transformer, the neutral current caused by unbalanced current must not exceed 25% of the rated current of the low-voltage winding. Yours clearly exceeds this limit, so it needs to be addressed.
