ECML Class 91 + Mk4 stock to GWML?

[Ken.W]

Ron, hard to tell really. Although the most complicated train we've had, conversely they're the one's we've learned the least about technically, and as I said, first impressions, my experience of them so far's reaching the required hours in training.

So, whether it's the control software, outright power, or just the way they're geared, couldn't really say. Someone did comment on the 800 thread that the series of Hitachi train the 800s are based on's a commuter train, if so that could explain a lot, away like a rat out a trap, even on diesel, but lacking in high end power, even on electric.

 

The 91 on the other hand's basically a bespoke design for the ECML. The way they're geared up, they're slow away starting off a heavy (for passenger) 500 tonne load with just 80 tonne adhesion, but get them up to about 60 and they get their act into gear, they're geared up for high power at speed. As you get up to 125, they'd just keep on going.

An example I've used before so apologies if you've already read this. The 185s are fast setting away, and at York you'll sometimes see a simultaneous departure of a 91 and 185 southbound. The 185 may literally leave the 91 standing in the station, but is being passed at Colton!

Edited June 26, 2019 by Ken.W

[Coryton]

18 hours ago, Ken.W said:

Rather than the electric bill it's likely it's the electric supply that's an issue, It's quite common when you get two 91s under full load in the same electrical section at once for the OHL supply to drop to 20kv or even less at times.

 

 

Roger Ford mentioned voltage drops on the ECML in a recent Modern Railways.

 

I didn't follow his reasoning though, which was along the lines that Power=Volts * Current, and there is a constant power available so if you draw more current the volts go down.

 

Is that really how it works?

[phil-b259]

1 hour ago, Coryton said:

 

Roger Ford mentioned voltage drops on the ECML in a recent Modern Railways.

 

I didn't follow his reasoning though, which was along the lines that Power=Volts * Current, and there is a constant power available so if you draw more current the volts go down.

 

Is that really how it works?

 

At a crude level - Yes*.

 

ALL power supplies will have a maximum power rating be they supplying a 25KV railway or your Smart Phone.

 

Given electrical power is a function of V x A then the current drawn goes beyond the design limits of the power supply then it will result in the voltage output falling in proportion thus keeping the equation intact.

 

This is why you sometimes get limits on the number of trains which can be within the area fed by one particular substation - one legacy of BRs 'low cost electrification schemes' was to install substations with lower power supply capabilities further apart. This tactic gave rise to only a single 442 allowed west of Poole, InterCity trains from Bradford FS needing to be HSTs (even though WYPTE metro services were 25KV EMUs), the East Grinstead line only being able to support 8 car trains and perhaps most surprisingly restrictions on the number of electrically powered trains which could be in section north of Newcastle to Edinburgh at any given time.

 

Post privatisation many of these schemes have had to be upgraded to support new / lengthened trains with all heir mod-cons like air con but even so it still remains a fact that the maximum traction power available is a fixed value and too many trains drawing large currents can case the voltage to drop.

 

* I had an extensive set of arguments over electrical theory in a thread about motorhomes / RVs and that thread may assist those wanting to know more about the current / voltage / power relationship.

Edited June 26, 2019 by phil-b259

[Nimbus]

20 hours ago, Ken.W said:

I've heard it said that the reason some Mk4 sets have been taken out of service already (4 so far), even though HSTs 'need' to go by the end of the year, is to avoid having too many 91s operating among the 800s.

 

Where have the 'resting' 91s been tucked away?

 

The Nim.

[phil-b259]

28 minutes ago, Nimbus said:

 

Where have the 'resting' 91s been tucked away?

 

The Nim.

 

Given the relatively poor reliability of the 91s you may find that in fact the locos are being retained at depots (just not being used as often/ being plundered for spares) while some Mk4s  rakes (which are reliable) are the things being stored.

[Coryton]

25 minutes ago, phil-b259 said:

 

At a crude level - Yes*.

 

ALL power supplies will have a maximum power rating be they supplying a 25KV railway or your Smart Phone.

 

Given electrical power is a function of V x A then the current drawn goes beyond the design limits of the power supply then it will result in the voltage output falling in proportion thus keeping the equation intact.

 

 

It depends on the power supply surely?

 

A true constant voltage power supply will just provide whatever current is demanded, until that gets too high and a circuit breaker trips or something melts.

 

It doesn't have to be designed to drop the voltage to keep the current from exceeding some limit.

 

I would have thought something fed by a transformer from the grid would be a good approximation to a fixed voltage power supply.

 

How is the voltage dropped to permit higher current? And how does that help? You're still getting the same power but less efficiently because the voltage is lower.

 

Doesn't the transmission grid adjust for changing loads by altering frequency, not changing the voltage?

 

 

 

 

 

 

 

 

 

 

[Ken.W]

2 hours ago, Coryton said:

 

Roger Ford mentioned voltage drops on the ECML in a recent Modern Railways.

 

I didn't follow his reasoning though, which was along the lines that Power=Volts * Current, and there is a constant power available so if you draw more current the volts go down.

 

Is that really how it works?

 

Basically, yes

 

23 minutes ago, Coryton said:

 

How is the voltage dropped to permit higher current? And how does that help? You're still getting the same power but less efficiently because the voltage is lower.

 

Doesn't the transmission grid adjust for changing loads by altering frequency, not changing the voltage?

 

The voltage drop limits the power rather than increasing the current.

When the voltage drop occurs,there's a marked drop in train performance, plus the 91 cab has an OHL voltmeter.

It generally occurs when too many trains are under heavy load  at once, or another train enters the same OHL section. It could in places be caused by two trains, while other sections may be able to take more power draw.

The voltage drop's alleviated by either or both drivers reducing  the power controller position, or one of the trains passing through an OH Neutral Section into the next section,  and the volts will then rise.

If the line voltage drops further, below about 20k, you then notice as the cab air-con shuts up. Drivers then do need to be careful what they're doing with the power controller, as dropping much further will trip the overheads out altogether.

I've known instances where, due to some incident or failure happening, several trains have been stopped in the same OHL section, and when things have been able to start moving again the signaler's had to stagger the trains starting off, rather than letting them follow each other off as the signals clear, or instruct drivers to only use a certain controller position, say maybe a 1/3rd, when starting off.

[Ken.W]

2 hours ago, Nimbus said:

 

Where have the 'resting' 91s been tucked away?

 

The Nim.

 

Don't think there's been any 91s 'rested' yet, except maybe in the repair shop, while, AFAIK, there's now four Mk4 sets out of service.

The class 90s, of which there was usually a pair hired in, do seem to have disappeared from the Mk4 workings though, which would leave an extra couple of 91s as spare.

[david.hill64]

6 hours ago, Coryton said:

 

It depends on the power supply surely?

 

A true constant voltage power supply will just provide whatever current is demanded, until that gets too high and a circuit breaker trips or something melts.

 

It doesn't have to be designed to drop the voltage to keep the current from exceeding some limit.

 

I would have thought something fed by a transformer from the grid would be a good approximation to a fixed voltage power supply.

 

How is the voltage dropped to permit higher current? And how does that help? You're still getting the same power but less efficiently because the voltage is lower.

 

Doesn't the transmission grid adjust for changing loads by altering frequency, not changing the voltage?

 

 The nationwide transmission grid will approximate to a fixed voltage supply taking rail requirements in isolation but the interfacing transformer between the incoming national grid voltage supply and the outgoing 25kV traction supply doesn't have infinite capacity to take as much current as demanded. The power that can be carried is limited, so as the current draw goes up, eventually the voltage must come down. Traction transformers have a marvellous ability to take overloads for short periods - but do have a maximum power rating.

[caradoc]

Conversely, can the OLE voltage rise considerably above 25kV at times ? The reason I ask is that in May 1991 the Motor Coach of set 303038 exploded at Shields Depot, and IIRC the reason (or part of it) was that the line voltage was well above 25kV, as it was a quiet period in the evening.

[Zomboid]

2 hours ago, david.hill64 said:

 The nationwide transmission grid will approximate to a fixed voltage supply taking rail requirements in isolation but the interfacing transformer between the incoming national grid voltage supply and the outgoing 25kV traction supply doesn't have infinite capacity to take as much current as demanded. The power that can be carried is limited, so as the current draw goes up, eventually the voltage must come down. Traction transformers have a marvellous ability to take overloads for short periods - but do have a maximum power rating.

The grid is pretty good as a fixed voltage supply, though it does fluctuate with demand to a certain extent.

The railway supply is fed through fixed impedance transformers (very few automatic tap changers around), but they're 4-6 Ohm impedances in general, so will drop a fair few volts before you get anywhere near the overhead lines at times of high demand. And then the OLE itself has an impedance that varies between 0.5 to 1 ohm/km per track (many variables in that), so that can be quite significant on some of the long sections on the ECML. Hence 20kV at the pantograph.

 

The voltage can rise too, usually driven by the grid, but the capacitance of the OLE adds a bit. Off load voltages of 27.5kV are normal, and it should trip off if the supply voltage is over 29kV for any prolonged time. Transients can be much higher, but only last milliseconds.

[Fat Controller]

1 hour ago, caradoc said:

Conversely, can the OLE voltage rise considerably above 25kV at times ? The reason I ask is that in May 1991 the Motor Coach of set 303038 exploded at Shields Depot, and IIRC the reason (or part of it) was that the line voltage was well above 25kV, as it was a quiet period in the evening.

Working in a control centre, I get to overhear drivers calling in low and high voltages; for 25kV, Lows in 'normal' operation can be around 22.5kV, and highs around 27.5kV; I have heard calls for voltages below the lower figure and above the upper one.

[Coryton]

2 hours ago, david.hill64 said:

 The nationwide transmission grid will approximate to a fixed voltage supply taking rail requirements in isolation but the interfacing transformer between the incoming national grid voltage supply and the outgoing 25kV traction supply doesn't have infinite capacity to take as much current as demanded. The power that can be carried is limited, so as the current draw goes up, eventually the voltage must come down. Traction transformers have a marvellous ability to take overloads for short periods - but do have a maximum power rating.

 

I still don't understand.

 

A transformer is in principle a simple affair - the voltage ratio is defined by the ratio of turns in the primary and secondary windings, and the current on each side matches using P=VI (less losses, of course).

 

Of course there's a limit on how much power it can carry without melting or catching fire, but unless a cut-out is tripped it will just keep on trying to draw whatever current it needs and the output voltage is only going to change if the current draw makes the input voltage drop. And I think the 132 kV grid is too stiff for a few trains to bring it down to 105 kV (or equivalent for whatever input voltage is used).

 

You could use a transformer with multiple tap-offs and drop the voltage as current increases and I suppose that would help if the trains are trying to draw a fixed current regardless of voltage. But I don't think that's how the system works.

 

What am I missing here with my simplistic model?

[Coryton]

41 minutes ago, Zomboid said:

The grid is pretty good as a fixed voltage supply, though it does fluctuate with demand to a certain extent.

The railway supply is fed through fixed impedance transformers (very few automatic tap changers around), but they're 4-6 Ohm impedances in general, so will drop a fair few volts before you get anywhere near the overhead lines at times of high demand. And then the OLE itself has an impedance that varies between 0.5 to 1 ohm/km per track (many variables in that), so that can be quite significant on some of the long sections on the ECML. Hence 20kV at the pantograph.

 

The voltage can rise too, usually driven by the grid, but the capacitance of the OLE adds a bit. Off load voltages of 27.5kV are normal, and it should trip off if the supply voltage is over 29kV for any prolonged time. Transients can be much higher, but only last milliseconds.

 

Our posts overlapped.

 

Right, so it's Ohmic losses in the transformer and the wires.

 

That's what I thought.

 

Thanks.

 

Edited June 27, 2019 by Coryton

[david.hill64]

1 hour ago, Coryton said:

 

Our posts overlapped.

 

Right, so it's Ohmic losses in the transformer and the wires.

 

That's what I thought.

 

Thanks.

 

I think more correctly impedance losses (which will include resistance of the windings).

 

Remember that a transformer works by passing the magnetic field generated by one set of windings via a soft magnetic core to another set of windings, which convert the magnetic field into electricity. The capacity of any given transformer is limited by the ability of the core to carry the magnetic field. Once saturated it can do no more.