HSTEd
Veteran Member
- Joined
- 14 Jul 2011
- Messages
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I'm not sure how you would get the current from the insulated conductor rail to the overhead wire?
What insulated conductor rail and what overhead wire?
EDIT:
Let's take an example scheme like Bidston-Wrexham. Something like 50km so at the outside 50 of these 300kW substations. Which is a total of 15000kWe.
Apparently the minimum allowed voltage on the third rail in more modern standards is now 500V - so I will use that as it will simplify the calculations.
That means that the absolute maximum current that a substation can provide is now 600A. Which means that the absolute maximum the scheme can produce is something like 30kA. If the currents in the conductor rail exceed 9kA at any substation [using a hall effect sensor on the conductor rail jumpers] an intertrip will be signalled.
That means the largest fault not detected in that manner is 18kA. [Just under 9kA from each direction].
Hall sensors also have the advantage that they allow quantification of the direction and size of the current quite easily - especially when we have microcontrollers at our disposal.
It is entirely possible to set a limit for the current consumption of any one "segment" of the line between two substations, or for any other arbitrarily determined section of the track bounded by the sensor positions.
If that value is exceeded - and for our 4-car train examples I would set that at something like 5000A total (2 1000kW trains at 500V accelerating plus some margin) then an intertrip will trigger.
So now our maximum fault current is 5000A.
This means that 2500A will flow in from each side of a fault, which implies a maximum (500m to substation) voltage drop of 16.25V.
16.25V means that 5000A must flow through a minimum voltage of ~482.5V to avoid an undervolt intertrip.
Which means the minimum allowable fault resistance must be 0.1ohm. Any lower will cause one of the intertripping systems to go off.
Since 100 milliohm is much greater than the loop resistance to the first substation it appears that the majority of the fault power will be pumped directly into whatever it is. Which would be something approaching 2.5MWe into that resistance.
If its a tree branch or some resistance wire or something it will disintegrate in a couple of seconds. Just like tree branches do when they come into contact with power lines.
You could further increase the minimum fault resistance by chosing values for the maximum allowable current before an overcurrent intertrip to be suitable to the situation - for example there is no reason why 9kA should be flowing in the conductor rail on a Bidston-Wrexham scheme, that is merley the maximum allowed by the third rail standard.
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