I can't help wondering why it needs to be converted to single phase for servers and CNC machines. (I'm assuming small CNC machines <1kW, as larger ones would already be 3-phase)
It seems like a large number of small loads so some could be on each phase to spread the load equally over the three phases.

There is a case to be made about harmonics in datacenters. The sheer volume of non-linear load tends to create situations where a "super-neutral" is required; one that is actually larger than the ungrounded circuit conductors. I don't delve into harmonics all that much - only enough to know when I'm in over my head, so I can't really speak to the math or physics in that area.

Datacenters are always engineering territory anyhow... a situation where every feeder and branch circuit has almost always been sized by a licensed engineer who has then signed off on it.

A single phase system wouldn't have this issue.

Nor a three-phase delta for that matter.

 

As I recall back in the 1970's there was an article in Power and Intelligent Motion magazine that seemed to describe a powerline frequency converter that looked like an AC motor but had no moving parts if I understood it. I did not bother to understand the article in detail because It had nothing to do with my job or career ambitions.

You might come across something like that if you look long enough.

Certainly a motor-generator (also called a dynamotor) can be pretty expensive but also pretty efficient.

Are there others in China with a similar need? What did they do to solve the problem?

 

The third harmonics would add, so the third-harmonic contribution to the neutral current would be the same whether it were single or three phase. The fundamentals would cancel out, if the loads were evenly distributed. Assuming <30% third harmonic, and <10% ninth, the maximum neutral current would be about a third of the total, i.e. the same as the neutral current for one phase of the three.

 

Nor a three-phase delta for that matter.

I wonder what would happen if the datacentre were supplied by a 3P+E supply with a local delta-star transformer. Where would those triplen harmonics go?

 

As I recall back in the 1970's there was an article in Power and Intelligent Motion magazine that seemed to describe a powerline frequency converter that looked like an AC motor but had no moving parts if I understood it. I did not bother to understand the article in detail because It had nothing to do with my job or career ambitions.

You might come across something like that if you look long enough.

Certainly a motor-generator (also called a dynamotor) can be pretty expensive but also pretty efficient.

Are there others in China with a similar need? What did they do to solve the problem?

With similar problem they solved it with three phase rectifier and filtering, also make power source redundant by automatically switching to battery bank nor solar panel (with some module to stabilize current output), and then use DC - DC computer power supply before use three phase rectifier usually they step down AC voltage to about 55v phase-phase; by these way they make very large online UPS system with secondary power from solar panel, also wind turbine (I was see about 100x 24v 250ah battery bank, with about 5kw peak solar panel and 1.5kw peak wind turbine); but in this case my client was have normal AC computer power supply, and they not want to buy DC - DC power supply, also example case is in private company, not government area, we have strict rule to use local component if available, and isn't really reliable for datacenter with 24/7 load, also tier 3 or 4 target, I don't know another example case in government area, I there is no more effective way, maybe I can use same way with three phase DC rectifier but multiply component by three or four independent circuit, so if one is fail, power is supply by another circuit, normally is only circuit A and B, but in this case I think I need circuit A, B, C and D, even may add with circuit E (Because three phase rectifier reliability level is not enough); also by that way maintenance is very complex because need a team with know electronics and low voltage electricity also IT and datacenter power supply system;

 

With similar problem they solved it with three phase rectifier and filtering, also make power source redundant by automatically switching to battery bank nor solar panel (with some module to stabilize current output), and then use DC - DC computer power supply before use three phase rectifier usually they step down AC voltage to about 55v phase-phase; by these way they make very large online UPS system with secondary power from solar panel, also wind turbine (I was see about 100x 24v 250ah battery bank, with about 5kw peak solar panel and 1.5kw peak wind turbine); but in this case my client was have normal AC computer power supply, and they not want to buy DC - DC power supply, also example case is in private company, not government area, we have strict rule to use local component if available, and isn't really reliable for datacenter with 24/7 load, also tier 3 or 4 target, I don't know another example case in government area, I there is no more effective way, maybe I can use same way with three phase DC rectifier but multiply component by three or four independent circuit, so if one is fail, power is supply by another circuit, normally is only circuit A and B, but in this case I think I need circuit A, B, C and D, even may add with circuit E (Because three phase rectifier reliability level is not enough); also by that way maintenance is very complex because need a team with know electronics and low voltage electricity also IT and datacenter power supply system;

Also this benefit is we not face with harmonics problem, because all is in DC system

 

There is a case to be made about harmonics in datacenters. The sheer volume of non-linear load tends to create situations where a "super-neutral" is required; one that is actually larger than the ungrounded circuit conductors. I don't delve into harmonics all that much - only enough to know when I'm in over my head, so I can't really speak to the math or physics in that area.

Datacenters are always engineering territory anyhow... a situation where every feeder and branch circuit has almost always been sized by a licensed engineer who has then signed off on it.

A single phase system wouldn't have this issue.

Nor a three-phase delta for that matter.

Is 2x 3x16mm2 phase wire and 2x 1x25mm2 neutral wire is classified as super-neutral, or I need 2x 1x35mm2 ?