using voltage drop too your advantage

[gavin.sibley]

I have an install to complete where I have to take a 100amps per phase three phase supply over 330m too power a new installation. The power at source is obviously 415v (give or take) however the rating on the machine at the other end is 400v (build in Europe somewhere). I'm currently having a 'conversation' with another engineer regarding whether we could use a smaller cable too feed the machine too

A. Save on costs of cable

B. Use the voltage drop of the cable too get the power down too the required voltage.

What would your thoughts be on this?

Many thanks

 

[davesparks]

It sounds like a recipe for disaster if you ask me.
The more volts you drop on a cable run the more power is dissipated in the cable as heats

 

[ImpededLoop]

As a designer, you can deviate from BS7671 as long as you are prepared to bear any consequences of doing so should anything go wrong.

You will also need to consider whether your OCPD will operate as a smaller cable will have a higher impedance / lower current carrying capacity.

In my mind, I wouldn't do it as the volts that you are losing will be lost as heat, so you'll be wasting energy unnecessarily.

 

[gavin.sibley]

The cable carrying capacity even at the lower sized cable is still well above what the OCPD rating will be, in the scale of things it's not a huge amount of power we need too transfer it's simply the distance which is making this somewhat more complicated....

 

[Darkwood]

The 400v and respectively 230v is the standard rating now of anything sold to the UK as oppose to old standards 415v and 240v, you should look at the supply as been 400v too regardless of its actual measured voltage been 415v so when calculating the VD you need to use 400v as it may be the case the supply is upgraded or the tappings changed on the TX to bring it into line with nominal voltages we now use.

At that distance it may be cheaper to have the DNO put an independent supply to the unit depending on the lay and availability of their network, otherwise you will be looking at parallel cables to reduce costs.

 

[gavin.sibley]

Unfortunately I don't think the DNO option is a go-er. Looking at the local network map which I have been given (friends in the DNO) the network is know where near where my site is, however we have got an underground duct run from the nearest sub direct too where my connections need too be. Which is why I was hoping too pull in cables from here

 

[jeremy]

I apologise for my first reply; it was less than helpful. I believe that if the machine was built in Europe for the European market, then it should work fine with as little voltage drop as possible. UK source voltage can vary by +-10% P/E. Again, apologies for my first response.

 

[Marvo]

Yeah, as pointed out it's not very efficient, if you're dropping 15volts per phase at 100A load then you're losing 1.5kW per phase as heat dissipation in the wire which is 4.5kW in total for the supply. Maybe look at asymmetrical transformers and step up to just under 1kV then back down at the load end to the exact optimum voltage, that way your losses will be considerably less, cable sizes should be smaller and you can achieve a more accurate load voltage..

 

[gavin.sibley]

Ok, i can understand why some people would be against the voltage drop through cable selection idea. Is there anyone that would please be kind (and bored) enough to do a quick calculation for me? I don't tend to carry my books around hotels with me and I wasn't expecting too have to design as well as install on this project! As previously mentioned I need too get a supply 100amps per phase from the local sub to our site 330m away. Obviously in SWA and it will be 60% in underground duct and the remaining on cable tray. Would anyone be able too indicate too me what might be a possible cable size selection?

Many thanks

 

[Leesparkykent]

300mm with a quick calculation.

 

[amlu]

The cable carrying capacity even at the lower sized cable is still well above what the OCPD rating will be, in the scale of things it's not a huge amount of power we need too transfer it's simply the distance which is making this somewhat more complicated....

100Amps per phase and 3 phases its quite a lot of power. Unless you work on substations. But then you should know what you are doing anyway.
What sort of machine you will be powering? Ask their European manufacturer if it will work on UK voltages.

 

[Darkwood]

100Amps per phase and 3 phases its quite a lot of power. Unless you work on substations. But then you should know what you are doing anyway.
What sort of machine you will be powering? Ask their European manufacturer if it will work on UK voltages.

A point I made earlier amlu - to the EU our voltages are 400v and 230v and thats what it says on the machine, tbh I don't think its an issue but no harm in asking the manufacturers.

 

[marconi]

You quote a requirement for 100 Amps per phase, I assume an rms value. However, you need to take into account as well:

a. the currents which are drawn as transients for example when the machine is turned on. These current may be higher than 100 Amps and cause even higher voltage drop per line conductor.

b. the duty cycle ( - the cycle of operation of the machine or does it operate intermittently rather than continuously) ie: how often is the machine in use and for how long compared with its off state?

c. the load factor ( - average load divided by peak load in a given time frame while the machine is running). This tells you the proportion of time the machine is working hard and for how long once it is in operation. Crudely, a low load factor indicates the machine is idling much of the time whereas as it approaches unity the machine is working to rated capacity much of the time. Thus, a low load factor could allow investment in a lower csa cable because for much of the time it is carrying less current and ohmic heat loss is I2R.

d. what is the rating factor of the machine? Is it continuously rated at full load or at some lower loading?

These all have a bearing on the design of supply cable. I think you need to open up the books on the machine and then do some calculations on cable selection and cost estimates.