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Topic Title: Understanding Split single phase installations.
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Created On: 10 April 2017 11:41 AM
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 10 April 2017 11:41 AM
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Matt01

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Joined: 25 July 2008

I've had a quote come back from a DNO provider for providing a split single phase supply. Why doing a little research i would just like to confirm my understanding is correct.

Essential it is a supply used mainly in rural areas where the installations are typically pole mounted. The split single phase elements means i have a L1 phase and an L2 phase with a common neutral. The two phase a are 180 degrees apart and are classed at 460V (230+230) not 400V. Meaning i'm missing the L3 Phase and the 120 degree separation. Therefore supplying 3ph equipment is a no go as essentially i have to single phase supplies which i need to try and balance as best i can as i have a common neutral?

Is this correct?
 10 April 2017 12:42 PM
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AJJewsbury

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That's about it. As you've spotted it's 460V between phases rather than 400V so you might find that a lot of 3-phase equipment (DBs) etc aren't rated for that (if you had the idea of using a B-type DB and only using L1 and L2 ways of it) - so often it's simpler just to treat it as two single-phase supplies. I wouldn't worry too much about the common N - it's there to carry any imbalance - just like the N running down the street in an urban 3-phase distribution system with lots of single phase consumers connected to it. Balanced is better of course, but don't kill yourself striving for unnecessary perfection as long as neither line is overloaded.

- Andy.
 10 April 2017 01:19 PM
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Matt01

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Thanks Andy. So basically the MV Primary incomer is single phase and the transformer splits this into two single phase supplies. I note you mention treat it as two separate single phase supplies. But can you supply larger loads equally between L1 and L2 if they are rated for 460V?
 10 April 2017 01:24 PM
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AJJewsbury

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But can you supply larger loads equally between L1 and L2 if they are rated for 460V?

Sure, 460V equipment can be connected between lines (provided it doesn't require an earthed N for any reason), but such equipment is pretty rare.
- Andy.
 10 April 2017 01:32 PM
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alancapon

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The 11kV supply to the DNO's transformer is equivalent to single phase, although they are two phase conductors fed from a three phase source (the neutral is not distributed). As Andy says, you will struggle to get a distribution board rated at 460V +10%, which is the maximum phase to phase voltage you should get.

Regards,

Alan.
 10 April 2017 01:40 PM
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Matt01

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ok thanks so basically its better to treat these as 2 separate single phase supply's i.e each having their own Consumer unit. The DNO has stated it can give me 220kVA supply in this formation but i'm not quite sure if this is 220kVA split between L1 and L2. Therefore by dividing by 460V on a single phase calc would meaning this will be 478A which to mean seems high and i would need a consumer unit rated in-excess of 100A which would be rare. Also in regards to phase angle how if the you only have 1 single phase MV cable coming in do you get on the output of a TX 180 of phase shift between L1 and L2.
 10 April 2017 01:49 PM
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AJJewsbury

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DBs are usually sized according to the loads rather than the available supply (cut-out fuses can be downrated if needed) - what do you have to supply?
- Andy.
 10 April 2017 01:52 PM
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alancapon

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You get the phase shift with a centre tapped secondary winding. It is the same as a 110V tool transformer, where you have two phases of 55V with a 180 degree shift between them, just on a larger scale.

Regards,

Alan.
 10 April 2017 01:58 PM
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Matt01

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multiple temporary loads that could be over the 100A CU limit.

Arh thanks Alan that makes sense now.

So is this just a method to increase the load on the MV circuit by creating this L1 and L2 scenario.

sorry for all the questions just trying to get my head around it.
 10 April 2017 02:08 PM
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OMS

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Originally posted by: Matt01

multiple temporary loads that could be over the 100A CU limit.

Obs - you've asked for a 220kVA supply - to give a bit of context, a single phase 100A domestic supply is 25kVA, a 100A TP&N supply is 70kVA

You have potential here for circa 470A

You need some suitably rated switchgear up front to receive the supply - and then some protection to provide single phase supplies at sub 100A (if that's what you need)


Arh thanks Alan that makes sense now.

So is this just a method to increase the load on the MV circuit by creating this L1 and L2 scenario.

Not really - running overhead supplies is expensive - if you only want a "small" supply in a rural network, it's far cheaper to just run 2 x HV conductors to the primary side of the transformer and 3 conductors from the LV side of the transformer rather than 3 on the HV side and 4 on the LV side to give a "typical" 3 phase output at 120 degree phase separation - the latter supply could well be 220kVA but would cost at least 33% more to provide


sorry for all the questions just trying to get my head around it.



Regards

OMS

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Let the wind blow you, across a big floor.
 10 April 2017 02:29 PM
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OMS

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Just to add if we really counting the cost, it could be done with a single HV line and a big electrode the other side of the winding - so called Single Wire Earth Return (SWER)

You almost certainly won't find that in the UK though - sort of thing you need for a sheep ranch about 50km from the local infrastructure

Regards

OMS

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Let the wind blow you, across a big floor.
 10 April 2017 03:50 PM
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Matt01

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So i almost need two switch panels to bring each phase down from 470A to a distribution level of 100A. Its just seems an expensive way of doing it. unless you can get a suitably rated switch panel that can cope with a split phases supply?

so the 2 x HV connections are single phase supply and a single wire earth return, they terminate into a TX which is centrally tapped creating a Neutral and the because of this the L1 and L2 single phases are 180 degrees from each other @460V. this type of system is used mainly in rural locations at a 2 wire system is the cheapest way of transmitting it. On the secondary side the best way of distributing the LV is class each phase conductor as independent from the other therefore creating two single phase installations.
 10 April 2017 04:25 PM
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alancapon

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No. The HV connections are both live phases. In the U.K., there is no "earth return" wire on the HV side. This allows an HV equivalent of an RCD to detect a wire on the ground. The American system uses a distributed neutral / earth on the HV side (which is what OMS is referring to), and it is not possible to separate earth fault from an unbalanced load.

The advantage the U.K. approach is that an HV wire on the ground is more likely to be dead than in America. That said, it may still be live, so shouldn't be approached.

Regards,

Alan.
 10 April 2017 04:54 PM
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OMS

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Originally posted by: Matt01

So i almost need two switch panels to bring each phase down from 470A to a distribution level of 100A. Its just seems an expensive way of doing it. unless you can get a suitably rated switch panel that can cope with a split phases supply?

You can have one or two (single point of isolation might be worth you considering)

Typically a 600/1000V TP&N switchfuse at 500A (with links rather than fuses) to say a bus bar chamber then allows you to connect any combination of "single" and two phase loads with their own protection.

You could achieve same with a MCCB switchboard (typical COTS TP&N and just ignore the third phase)

I've no idea what you are trying to achieve, but a small switchboard built to BS EN 61439 would be ideal as the incoming/Tier 1 switchgear position - you could then configure that to suit any two phase and any single phase loads



so the 2 x HV connections are single phase supply and a single wire earth return,

No, they are two phases connected to each end of a single HV winding - we don't do SWER in the UK as you can't detect earth faults

they terminate into a TX

see above

which is centrally tapped creating a Neutral and the because of this the L1 and L2 single phases are 180 degrees from each other @460V.

The secondary winding is the equivalent of a centre tapped earth winding - ie each LV phase comes from the winding end and the mid point of the winding is both your neutral and is connected to earth

this type of system is used mainly in rural locations at a 2 wire system is the cheapest way of transmitting it.

Yes

On the secondary side the best way of distributing the LV is class each phase conductor as independent from the other therefore creating two single phase installations.

No - you can have single phase and neutral supplies if you wish or two phase supplies depending on what you want to do - so in a typical agricultural system (just as an example), you may have several single phase DB's doing power lighting etc shared across Phase A and Phase B and a few two phase connections to the heavier kit like slurry pumps or grain blowers

You could even have a two phase motor of suitable rating connected in with a 3 phase idler motor (no connected load) and create your own 3 phase system for 3 phase equipment if required




Regards

OMS

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Let the wind blow you, across a big floor.
 10 April 2017 07:15 PM
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AJJewsbury

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multiple temporary loads that could be over the 100A CU limit.

How much over? 125A type-A distribution boards are fairly readily available, two of those would give you up to 57.6kVA.
- Andy.
 11 April 2017 10:20 AM
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Matt01

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Ok the 2 x HV connections are 2 phases not an earth return, presuming on the same phase angle??

The supply will be required to feed a number of pumps etc which we were potentially going to 3 phase 400V. However i've found a site that sells converters to make a 2 wired system 3 phase (just for these items of kit).
 11 April 2017 12:21 PM
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OMS

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Originally posted by: Matt01

Ok the 2 x HV connections are 2 phases not an earth return, presuming on the same phase angle??

Not sure I understand your question - they will be 120 degrees apart - if they were in phase you would have the same voltage at each end of the winding (and no power)

There will be a phase displacement between the HV and LV side usually



The supply will be required to feed a number of pumps etc which we were potentially going to 3 phase 400V. However i've found a site that sells converters to make a 2 wired system 3 phase (just for these items of kit).

What kind of convertor are you planning on ? - with a number of pumps running I'd be tempted to go for a motor "generator" rather than solid state




Regards

OMS

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Let the wind blow you, across a big floor.
 11 April 2017 12:35 PM
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di515223

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Originally posted by: Matt01

Ok the 2 x HV connections are 2 phases not an earth return, presuming on the same phase angle??



The supply will be required to feed a number of pumps etc which we were potentially going to 3 phase 400V. However i've found a site that sells converters to make a 2 wired system 3 phase (just for these items of kit).


How critical are thes pumps? adding another layer of complexity to convert two phase to 3 phase will reduce reliability and may cost more than getting a 3 phase supply put in. The inrush current of 2 phase motors driving a significant load may make overload protection a challenge.
The loading you are describing is likely to lead to a lot of non standard, and therefore more expensive gear, which may not give any long term benefit.

Dave
 11 April 2017 02:21 PM
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Matt01

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Ok so to OMS's response the 2 x HV connections will have a phase 120 degrees apart at 11kV, however on the outgoing side of the centrally tapped transformer will make this phase angle of 180 at 460V.

The pumps are critical and i'm looking at using a Digital phase converter local to the pump set. The site is in the middle of nowhere and so wouldn't be cheaper to get a DNO 3-phase system. because the site is so big the LV source and the pump set are 600m apart. therefore i don't want ot convert at source as this will affect cable costs. The pumps set are not a permanent fixture they are only required for a 14 month operation.
 11 April 2017 03:39 PM
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OMS

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Originally posted by: Matt01

Ok so to OMS's response the 2 x HV connections will have a phase 120 degrees apart at 11kV, however on the outgoing side of the centrally tapped transformer will make this phase angle of 180 at 460V.

Correct


The pumps are critical and i'm looking at using a Digital phase converter local to the pump set. The site is in the middle of nowhere and so wouldn't be cheaper to get a DNO 3-phase system. because the site is so big the LV source and the pump set are 600m apart. therefore i don't want ot convert at source as this will affect cable costs. The pumps set are not a permanent fixture they are only required for a 14 month operation.

Rotary phase convertor at the pump house ?

Diesel Generator at the pump house? - it is only temporary ?

Semi centralised static convertor at the pump house ?




Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
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