GWML Electrification - getting the electric into the wires above the trains

[whole diary]

[grahame]

http://www2.nationalgrid.com/UK/In-your-area/Projects/melksham-gwm-connections/

Our role
National Grid^s role is to connect the railway with the national electricity network. One of the locations where this needs to be done is Melksham.

The Melksham site
To make the connection in Melksham, we will use 25kV underground cables to carry the electricity. Two lines of cables will run from the existing National Grid substation off Westlands Lane and connect to the railway further north at Thingley Junction.

Building the connection
The connection will take around a year to install and work will be carried out in sections. The cables will be laid down into trenches within a working area about 20m wide. To go under streams along the route, the cables will be drilled horizontally underground as a small tunnel, rather than laid into trenches. When the work is complete, we^ll return the land to how it looked before.

Our commitment to local people
As we build the connection, we^ll work to have as little effect as possible on residents, businesses and road users in the area. We^re also working closely with people whose land is crossed by the cables.

We will:
consider residents, businesses and road usersmanage traffic with traffic lights, keeping one lane open at all times, to keep disruption as low as possible keep footpaths open wherever possible, including rights of way, as we cross farmland. If we do need to close the paths temporarily, we^ll keep people updated and diversions will be in place keep people informed as the project progresses

Considering the local environment
Over the last year, we^ve carried out several environmental and archaeological surveys in the area to ensure our connection is as sensitive as possible to wildlife and the landscape.

Latest update
Work will begin in October 2014, with the connection expected to take around a year to install, finishing in November 2015. Should this change, we^ll keep local residents updated.

The Melksham electricity site is right alongside the TransWilts.

[ChrisB]

So they're already 50% complete?

[Electric train]

So they're already 50% complete?

The info I have ............. yes

National Grid are on program to deliver the 25kV connections as contracted ...............

You can also guess when NR^» (Network Rail - home page) start to pay the connection tariff whether they draw energy or not

[NickF]

So thats what all that work is for in Corsham Road, near the Whitehall Garden Centre, Lacock?

[Electric train]

Grid outages especially ones on the 400kV National Grid are planned in 5 years in advance if a project misses the slot it can be a long wait.

[broadgage]

I thought that the national grid supplied railway traction substations at high voltages, typically either 275KV or 400KV  and that a network rail substation transformed this down to 25KV or to a 25/50KV autotransformer supply for traction purposes.

This report suggests that the national grid are supplying the railway directly at 25KV via buried cables. This seems a bit unlikely, perhaps it is typo ?

[Electric train]

I thought that the national grid supplied railway traction substations at high voltages, typically either 275KV or 400KV  and that a network rail substation transformed this down to 25KV or to a 25/50KV autotransformer supply for traction purposes.

This report suggests that the national grid are supplying the railway directly at 25KV via buried cables. This seems a bit unlikely, perhaps it is typo ?

The supply to the railway from the TNO^▸ (Transmission Network Operator) (Transmission Network Operator) is at 25kV or 25/0/25kV (ie 50kV), the transformer and associated switchgear at the grid site is a "single user asset" and the railways pays for it when new and when it has to be renewed.  The maintenance is paid for through the tariff.

The electricity meter, and yes there is one its in MWh and MVArh, can be located either end of the feed cable depending on how the tariff was negotiated.

[ellendune]

I assume this is a three phase feed. How does that work in the case of 25/0/25 kV?

[Oxonhutch]

Not answering Ellendune's question (over to you ET) but for general information...

25kV-0-25kV is a single phase centre grounded 50kV supply in the same manner as the 60V-0-60V UK^▸ (United Kingdom) building site transformers.  HS1^▸ (High Speed line 1 - St Pancras to Channel Tunnel) uses it and I have an article somewhere from around 10 years ago that describes it. On HS1, the opposite 25kV supply is carried parallel to the energised catenary outside of the pylon structures and the two are connected together at intervals by autotransformers.  The net effect is doubling the power available whilst reducing the voltage drop, and number of TNO^▸ (Transmission Network Operator) supply points.

[Electric train]

I assume this is a three phase feed. How does that work in the case of 25/0/25 kV?

There are several reasons why modern 25kV grid feeds are taken from the 400kV system and not the 132kV system, one it does allow a higher power take,  the unbalanced loading of the railway single phase supply has less effect on the network (negative phase sequence) also the 400kV is strategically more secure.

Not answering Ellendune's question (over to you ET) but for general information...

25kV-0-25kV is a single phase centre grounded 50kV supply in the same manner as the 60V-0-60V UK^▸ (United Kingdom) building site transformers.  HS1^▸ (High Speed line 1 - St Pancras to Channel Tunnel) uses it and I have an article somewhere from around 10 years ago that describes it. On HS1, the opposite 25kV supply is carried parallel to the energised catenary outside of the pylon structures and the two are connected together at intervals by autotransformers.  The net effect is doubling the power available whilst reducing the voltage drop, and number of TNO^▸ (Transmission Network Operator) supply points.

In essence correct, it reduces the I²R losses however the fault level goes for a classic 25kV system it is 6kA but for the 25/0/25kV it is 12kA (although this can be reduced with use of reactors) which increases the I²T.   This means the conversion of a classic 25kV BT system to a 25/0/25kV AT system can mean modifications to the OLE^▸ (Overhead Line Equipment, more often "OHLE") registration arms.  The system designers also have to be conscious of the RoEP (Rise of Earth Potential) so bonding has a big focus currently in the industry.

[ellendune]

Thanks ET I had assumed that a single phase feed would cause too much phase imbalance.  I assume that the Didcot feed will come from a different phase to the Melksham one to at least give the whole grid some better balance. 

[Oxonhutch]

In essence correct

Not bad I thought from a geologist! 
In my industry there is a traditional - and constant - tension between geologists and engineers. I had to look up I²T, but then every day is an education.

To return to your phase supply, am I correct in assuming that with a three-phase grid system (say phases A, B & C) that the supply primaries at (say) Didcot would be A+B, Melksham B+C and 'Somewhere-else' C+A?

[Electric train]

Thanks ET I had assumed that a single phase feed would cause too much phase imbalance.  I assume that the Didcot feed will come from a different phase to the Melksham one to at least give the whole grid some better balance. 

In essence correct

Not bad I thought from a geologist! 
In my industry there is a traditional - and constant - tension between geologists and engineers. I had to look up I²T, but then every day is an education.

To return to your phase supply, am I correct in assuming that with a three-phase grid system (say phases A, B & C) that the supply primaries at (say) Didcot would be A+B, Melksham B+C and 'Somewhere-else' C+A?

There are 3 Grid supply points on the English side of the Seven, Kensel Rise, Didcot and Melksham; all will have 2 Grid transformers the 2 transformers at each Grid site will be split across phase pairs eg one will be R - Y the other will be B - Y.

In the OLE^▸ (Overhead Line Equipment, more often "OHLE") there is an item called a Neutral Section, these have been up to now 2 GRP rods with ceramic beads the 2 rods separated by a short (about 500mm) of earthed contact wire http://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0CAcQjRw&url=http%3A%2F%2Frailsimroutes.net%2Fblog%2F%3Fp%3D912&ei=O_dWVazBJOqw7AaOuYO4Bw&bvm=bv.93564037,d.ZGU&psig=AFQjCNFP5mc14ApAOXDua11_WMf5FrF9yA&ust=1431849145544698
There is a plan to use carrier wire Neutral Sections on GWEP^▸ (Great Western Electrification Program) this is a series of overlapping sections of contact wire where there are 2 floating sections and an earthed section.

The Neutral Section allows the train pan to pass from one electrical supply to the next, there is an Automatic Power Control system which opens the trains circuit breaker.

[ellendune]

There are 3 Grid supply points on the English side of the Seven, Kensel Rise, Didcot and Melksham; all will have 2 Grid transformers the 2 transformers at each Grid site will be split across phase pairs eg one will be R - Y the other will be B - Y.

So the 50kV is between two phases and the 25kV/0/25kV is phase - neutral - phase (e.g. Red - Neutral - Yellow or Blue - Neutral - Yellow)?

[Electric train]

There are 3 Grid supply points on the English side of the Seven, Kensel Rise, Didcot and Melksham; all will have 2 Grid transformers the 2 transformers at each Grid site will be split across phase pairs eg one will be R - Y the other will be B - Y.

So the 50kV is between two phases and the 25kV/0/25kV is phase - neutral - phase (e.g. Red - Neutral - Yellow or Blue - Neutral - Yellow)?

On the 25kV system it is refereed to as +25kV and -25kV and the 0 is traction return.  The + and - from one transformer will be out of phase with its mate at the same Grid site and is likely to be out of phase with the one 50 miles away, even if it were to be in phase the railway would not parallel the 2 Grid sites' all sorts of weird power flows would happen.

Mid way between Grid sites the is a MPATS^▸ (Mid Point Auto Transformer Site - electrification) (Mid Point Auto Transformer Site) Maidenhead is the MPATS between Kensal Rise and Didcot so there will be Neutral Sections at Maidenhead MPATS.  It is also the boundary between 2 different protection schemes (by protection I refer to electrical over current etc) which need to interface with each other. Why different ones well the Crossrail scheme was developed and tendered a few years a head of GWEP^▸ (Great Western Electrification Program), Crossrail will a similar AT protection scheme to that used on the WCML^▸ (West Coast Main Line) and GWEP is "new and novel"