3 phase imbalances within system

[gazzamikes]

hi all,

a local factory has had a new machine installed which meant their system couldnt handle the load so i installed a generator on the warehouse to run this separatley.

upon monitoring their current usage i noticed phase 2 to always be high, in some case peaked at 80 amps difference between that and the other 2 phases. This obvioulsy creates and imbalance within the 3 phase system and machinery, this results in 'wasted' current. is this difference big enough to cause a lot of potential 'wasted' current? as their supply is only 500amp and they are running close, just trying to delve in to as why they seem to be running so high.

hope this all makes sense!

 

[Knobhead]

It doesn’t create wasted power. More current will flow in the neutral than necessary. This can some times cause problems where 3½ core supply cables have been used. The calculation for neutral current is N = √ ((A ²+B ²+C ²) - (AxB+AxC+BxC))

If possible look to transferring single-phase feeds from the heavily loaded phase to the others and try to balance the load.

I know it’s an expensive option but an upgrade to the supply would seem in order, but for an 800A supply the DNO would insist on a dedicated transformer.

 

[Knobhead]

One other thing to look at is power factor corection. If non is fitted the loading could be artificialy high.

 

[gazzamikes]

thanks for your reply, sorry yes thats correct causes neutral current. they have been quoted for a new transformer approx £100,000 which taking into account the generator rental costs etc and the fact that a power failiure would cost excessive amounts of downtime its not a great deal money.

i am in the process of explaining about swapping circuits about, unfortuantely the 'engineer' here as wired a room full of single phase boards one the same phase, so trying to get them to run extra phases may not be easy.

thanks for your advice and reply.

 

[gazzamikes]

there is power correction fitted into the main switch room onto the main panel

 

[Electro-tech]

Obviously you can get a power factor figure from your Electricity bill, online Graphs with usage and peaks.
Check Power factor is correct, and I would be looking at voltage drop on circuits.As mentioned neutral currents and size of conductor important.

 

[Knobhead]

From what you say the “management” aren’t blind to the problem. Installing the new cables to balance the load is another matter, they will regard it as a small issue compared to the transformer, and to be honest 80A imbalance on a 500KVA transformer is nothing to worry about. 500KVA = 666.7A per phase. BUT once you throw the new machines load on to the system it will rear it’s ugly head again. What is the loading of the new machine?
They will be nailed for exceeding maximum demand on even just one phase. And if as you say the 500A supply is near full load they’ll be sailing close to the wind when the new machine goes on line.

 

[Knobhead]

Just thinking about it 500KVA just about gets their ar*e out of the fire. 750KVA is more realistic, 1000A gives more room for expansion. But the DNO start getting funny at this kind of loading depending on the location.

One plant I worked on originally had 2 x 250KVA transformers. These were changed to 2 x 500KVA, a few years later changed to 2 x 750KVA. I made brass tags to fit to each transformer pen at this point “Lassie” and “Rover”.
5 years latter “Lassie” and “Rover” were called out of their pens and changed for 1000KVA units, the name tags stuck to their pens!