Technical question on power factor correction in electrification systems

[rodent279]

Technical questions for the electric traction & electrification engineers out there.

 

Is power factor correction equipment installed in railway feeder stations?

 

And what sort of power factor does something like an AC electric loco work at when running on part load?

Edited November 7, 2024 by rodent279

[Nearholmer]

So far as I’m aware, and open to correction, the answers are:

 

1) on some systems, yes, but I don’t think that it is provided on either the ‘grid’ or ‘railway’ side of the intake on either ac or dc electrified railways in the UK.

 

2) 0.8 lagging, to unity. There is a table in the document used to define tariff arrangements between NR and TOCs that sets out assumed PF for trains of different types, and most are assumed to be close to or at unity.

 

One thing to bear in mind is that on dc electrified railways, there is usually a railway-owned HV cable distribution network linking to the substations, and that presents a leading power factor due to cable capacitance, so the overall power factor presented to the grid can in some cases vary from lagging to leading as load via the rectifier transformers (lagging) varies.

[rodent279]

As high as 0.8 at part load, even for a big ac electric like a class 87?

 

What lead me to the question was considering why we did not have something like the lightweight low to medium power locos designed for light passenger/freight in the UK, like the Br 140/141 in Germany. Instead we chose to have a fleet of 200 relatively similar in power output, (classes 81-86), with nothing in the 2000hp 75mph range. Is that because we chose EMUs for secondary passenger services? 

 

A 4000hp 100mph class 86 can't have been very efficient at part load on a 5 coach stopper, or a lightly loaded van train.

[Nearholmer]

The VAr that you pay for, and which create additional capital cost for infrastructure, are a function of the actual loads presented to the system, not the rated capacities of the traction units. Added to which, very significant extra costs are created by having a fleet containing a multiplicity of different types of traction unit. Germany had I think nearly 900 locos in the BR140 family, and even the “small” fleet of BR103 was nearly 150 locos, which makes UK fleet sizes look paltry.

PS: if you change the title of this to something like “power factor correction of railway traction loads” others who know this topic better than I may chip-in.

 

 

Edited October 19, 2024 by Nearholmer

[rodent279]

Thanks that makes sense. 

 

I can't change the topic title, I've never been able to on any topic that I've started

[Edwin_m]

The cost difference between a low-powered Bo-Bo electric and a high-powered one probably isn't that much, considering it has most of the same components but some of them are a bit more beefy.  When working on light duties, a large electric will probably use about the same power as a small one.  

 

A Co-Co electric would be significantly higher cost.  I assume the WCML planners didn't feel there was enough heavy freight or severe gradients on the WCML to justify one, and went for the highest power they could pack into a Bo-Bo with the technology of the time.  

 

Diesels are likely to be a bit different because a large engine will use more fuel than a small one, even if put on a relatively un-demanding duty.  Combined with diesels out-numbering electrics in the BR of the 1960s, it would make more sense to have a range of diesel locos for different duties.  Though probably not as many types as they actually had...

[ChewChoo]

On 19/10/2024 at 12:34, rodent279 said:

I can't change the topic title, I've never been able to on any topic that I've started

The topic title is edited from the initial post in the topic.

 

Go to post 1 in the thread

Click on   edit

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Make your change to the title text as required

Click the Edit Topic button at the bottom of the page

 

 

[96701]

Wouldn't the fact there are long transmission cables increase capacitance anyway? If that is is the case, one cannot just use the power draw of the loco / emu, one has to look at the overall system.

[Nearholmer]

In a typical 750V dc electrification, where energy gets to within a few km of the train by railway-owned 3-phase HV cables, as I mentioned above, the HV cable capacitance can become significant in power factor terms, but SFAIU the capacitance presented by 25kV OLE is so low as to be much less significant in this regard (it is in some other regards). There are usually no long, railway-owned cable links in a 25kV OLE system.

But yes, one analyses the system by looking in from the point of connection to the grid, so as a whole system, not by looking at individual trains.

 

 

 

Edited November 8, 2024 by Nearholmer