Re: Question re Live Rail fatalities

Robert Wilson wrote:
R.C. Payne wrote:
Robert Wilson wrote:
amogles@xxxxxxxxx wrote:
On 2 Jul., 15:28, "R.C. Payne" <rc...@xxxxxxxxxxxx> wrote:

Though the fact that ac electrification is single phase can cause
trouble with balancing the load across the three phases, as an express
train is a very large unbalanced load. On dc systems, the substation
load is balanced across all three phases.

That is, if the railway is fed from a single phase. Of course if you
use a rotary converter or the static-state equivalent then you load
all three phases equally. This is done for example on 16.7Hz systems
such as in Germany and Switzerland, but could theoretically also be
done for 50Hz systems.
There was an earlier discussion about load balancing on OHLE systems and is appears that mostly they distribute over different supply phases like domestic final distribution at street level. Ie, every third house being on the red phase etc.

Though what is the load provided by a single house? It is unlikely that you will have all the "red" houses drawing lots of current while the yellow and blue ones sit there dormant. On the other hand, with a single train drawing as much as 3 MW routinely, and every possibility of no balancing trains on the other phases, the degree of imbalance is much greater for a railway than for a domestic situaion. Clearly (by demonstration) it is possible to deal with this problem, but it is a problm that DC systems to not exhibit.

Robin
No actually even domestic supplies tend to balances out. A house is restricted to 22kW based on the supply being 220V and the Consumer Unit current being 100A or thereabouts.

Even on a Railway, there will be sufficient traffic to balance out the load. The network is designed with these requirements in mind. There are special transformers around that help in this regard.

The people who were involved in the original correspondence can help here...if only I could remember their names.

Sincerely,
Rob.

The UK 25kV ac OLE network does *not* have any specific measures to load-balance, other than the previously mentioned alternating of the phase connections at consecutive feeder stations along the route. The imbalance is limited to something like 0.5% by the national grid companies, although I believe the railway exceeds these limits on occasion at specific high load locations. Phase balancers are *available*, but are expensive and not used in the UK. I believe they are on other railways.

As others have stated, the DC network is load-balanced by the use of a three phase ac supply rectified to DC.
.

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