Autotransformer feeds

[Barn]

While staring out of the window this afternoon on the southern part of the MML, one thing was bugging me.

How come the contact cable in the OHLE (which I think is at 25kV) has big ceramic insulators when the autotransformer feeds (which I thought ran at 50kV) were apparently only centimetres away from the metal pillar on weedy litlle insulators?

Have I misunderstood the voltages and how the system works?

 

[Joseph_Locke]

While staring out of the window this afternoon on the southern part of the MML, one thing was bugging me.

How come the contact cable in the OHLE (which I think is at 25kV) has big ceramic insulators when the autotransformer feeds (which I thought ran at 50kV) were apparently only centimetres away from the metal pillar on weedy litlle insulators?

Have I misunderstood the voltages and how the system works?

The southern MML is still on the original BT (booster transformer) system, so the cable on "weedy insulators" is the traction return cable, which shouldn't (normally) be many volts away from ground. BTs are the big things very few km, hung on the OLE structures.

The full AT system has a "negative" 25kV line on virtually full-size insulators, making a 50kVAC difference across contact and return lines.

 

[TGV]

An autotransformer catenary system looks like the attached image.

Note the wires on the outside of the masts - they will look different to what you saw on a normal BR 25kV system.

 

[Barn]

Thanks both; that explains it then!

 

[Bald Rick]

I'm pretty sure that the AT feed has been commissioned south of Borehamwood. Hence all the cash spent on new insulators and protection arrangements.

 

[Edvid]

That's right - during Christmas 2010, an AT system was commissioned between sites at Borehamwood and Kentish Town for about 20km; this includes a new feeder station and neutral section at Borehamwood.

Click here for an article on it.

 

[Joseph_Locke]

That's right - during Christmas 2010, an AT system was commissioned between sites at Borehamwood and Kentish Town for about 20km; this includes a new feeder station and neutral section at Borehamwood.

Click here for an article on it.

My answer was still right on a percentage basis ...