HST power supply

[trebor79]

Why did BR specify a non-standard 3 phase electrical supply for HST coaches?
What advantage does it provide as from a passenger experience POV there seems to be very little difference from a standard LH MK3.

 

[43096]

Why did BR specify a non-standard 3 phase electrical supply for HST coaches?
What advantage does it provide as from a passenger experience POV there seems to be very little difference from a standard LH MK3.

The improvement is under the floor and comes from no longer needing the motor-alternator set to generate 415V three-phase power - all Mark 3 auxiliaries run off three phase supply, the difference is how it is generated. This means the coaches are cheaper to build, lighter weight (which reduces fuel consumption) and more reliable (fewer components).

In retrospect it would have been daft to generate three phase supply from the alternator, rectify it, send to the coaches, then put it through a motor-alternator to turn it back into three phase on coaches that were part of fixed-formation trains.

 

[dgl]

Plus as most industrial/commercial equipment is designed for 415v 3 phase it means that there are a lot more options for off the shelf equipment which again reduces costs, plus iirc deriving ~240v from 415v 3 phase for non 3 phase
devices is as simple as using just 1 phase to neutral (taking into account, of course, equal loading on all 3 phases), no static convertors or motor alternator sets required (assuming the voltage swing from directly deriving the supply from the ETS is acceptable to the equipment needing to be powered).

Plus as the HST coaches had dedicated locomotives and would not be required to be hauled by anything else (having two power cars generally means that a total failure/complete loss of traction requiring a standard loco to assist is less likely) so could be designed to use the most efficient/cost effective kit without the requirement of backwards capability as is required with conventional locomotive hauled stock.

 

[Journeyman]

The improvement is under the floor and comes from no longer needing the motor-alternator set to generate 415V three-phase power - all Mark 3 auxiliaries run off three phase supply, the difference is how it is generated. This means the coaches are cheaper to build, lighter weight (which reduces fuel consumption) and more reliable (fewer components).

The reliability of HST vehicles is FAR better than loco hauled Mark 3s. I speak from bitter experience when I tell that just about every fault on the loco-hauled ones is down to kit that isn't fitted to the HST ones, because a simpler arrangement was devised.

 

[apk55]

In the days when HSTs were conceived power electronics was in infancy and the only practical way of using standard ETH supply to supply lighting and air conditioning was to use a motor alternator set to to convert the high voltage ETH supply to a low voltage supply. Motor alternator sets are a big, high maintenance and costly item (remember they are running all the time the unit is in service). Therefore supplying standard 3 phase AC down the length of the train resulted in cost and weight savings.

Nowadays with power electronics would could make a relatively small and light box that could replace a MA set.

 

[trebor79]

Makes sense. Thanks for the responses.

 

[Pigeon]

no static convertors or motor alternator sets required (assuming the voltage swing from directly deriving the supply from the ETS is acceptable to the equipment needing to be powered).

And the frequency swing... the supply frequency is proportional to the engine revs, and therefore changes wildly according to how much throttle is being used.

This caused problems for BR when they wanted to use ordinary commercial microwave ovens in the buffet cars and they wouldn't work with the raw supply. So they needed an inverter which would take the variable-voltage variable-frequency train supply as input and produce a steady 240V 50Hz output. These days there are probably hundreds of suitable units available off the shelf on alibaba, but at the time high-power MOSFETs, although definitely the most suitable things for the purpose available, were still pretty exotic, expensive and somewhat fragile devices, and designing such an inverter was not the everyday trouble-free procedure that it now is. So BR put out a tender to various outfits that it thought might be capable of putting one together.

Now as it happened our lab at GEC was already doing that, only in a slightly more complex version with variable-frequency output for driving lift motors, and the project wasn't that far short of completion. Its input side was already capable of handling the range of variation of the HST supply, and its load-handling capabilities were also entirely adequate for the microwave oven load. It would have been trivial to meet BR's requirement just by setting the output frequency to a fixed 50Hz and leaving the rest of the design unchanged. Of course I was dead keen to work on this and get to visit depots commissioning it etc. But the bosses barely even looked at the proposal before deciding that they couldn't be arsed, even though it was such an easy project; well, that was basically GEC all over, ie. bloody useless, couldn't even handle a full-on snouts-in-the-trough defence contract let alone something as radical as building an existing design minus one of its functions.

I kept a copy of the specification, and am thoroughly infuriated to find that I can't locate it now no matter how I try.

 

[GrimShady]

If a failed HST set is being hauled, how long can one PC be allowed to run in idle to provide ETS before it becomes a fire risk?

This was a concern when the surrogate PCs were in use during Class 91 testing.

 

[43096]

If a failed HST set is being hauled, how long can one PC be allowed to run in idle to provide ETS before it becomes a fire risk?

This was a concern when the surrogate PCs were in use during Class 91 testing.

More than a day! The concern was around use day in, day out.