High Voltage, high frequency?!

[dknguyen]

There is a type of induction motor that can be wired up as a transformer. At equal frequencies on primary and secondary, it appears as a transformer. If one is slightly off the induction motor will slowly spin and allows for two different frequencied grids to be connected together. I think it's used somewhere between the Canadian and American electrical grids.

 

[Boncuk]

Could the higher 'skin effect' (higher line resistance) of using 400hz on very long power lines Vs 50/60 hz be a factor in selecting the lower frequency for power distrubution?

Lefty

Hi Lefty,

I think the skin effect does not influence a 400Hz line very much if at all. I also don't know why 50/60Hz is used by electric suppliers. The german railroad however uses 16 1/3Hz for the electrically driven trains.

In the beginning of supply of electric power to households just DC was used. Since DC tends to fry organisms like human bodies it was changed to AC later. Never touch a high tension DC power line. You'll not be able to separate from the cable and BBQ'ed right away!

Regards

Hans

 

[Leftyretro]

Hi Lefty,

I think the skin effect does not influence a 400Hz line very much if at all. I also don't know why 50/60Hz is used by electric suppliers. The german railroad however uses 16 1/3Hz for the electrically driven trains.

In the beginning of supply of electric power to households just DC was used. Since DC tends to fry organisms like human bodies it was changed to AC later. Never touch a high tension DC power line. You'll not be able to separate from the cable and BBQ'ed right away!

Regards

Hans

"Never touch a high tension DC power line. You'll not be able to separate from the cable and BBQ'ed right away! "

Good advice, but I think I will apply that same rule to high tension AC power lines also

 

[Leftyretro]

And from wikipedia:

"Electric power transmission over long lines favors lower frequencies. The effects of the distributed capacitance and inductance of the line are less at low frequency."

Lefty

 

[Boncuk]

"Never touch a high tension DC power line. You'll not be able to separate from the cable and BBQ'ed right away! "

Good advice, but I think I will apply that same rule to high tension AC power lines also

just with a slight difference: Touching a high tension AC power line you'll be dead instantaneously. Touching a high tension DC power line you'll be electrolytically shrunk and need a small box only for the funeral.

 

[Pommie]

Apart from the historical and a legacy issue, there are tecnical reasons:

>>>> The inductive reactance of the long power lines (and capacitive reactance in the underground cables):

With 400 Hz the series inductive reactance is higher (reactance = inductance x 2 x Pi x frecuency), giving more voltage drop.

And the paralel capacitive reactance is lower, which gives more leakage current.

>>>> 400 Hz is well into the audio spectrum, so a 400 Hz grid (and its harmonics) should cause more interference in "plain old analog" telephone circuits.

Capacitive and Inductive losses don't exist as they are 90° out of phase with each other. There maybe increased I²R losses due to the out of phase currents however this is the same whatever frequency. The impedances are higher/lower due to the higher frequency but can be corrected with smaller inductors/capacitors.

Mike.

 

[ecerfoglio]

Capacitive and Inductive losses don't exist as they are 90° out of phase with each other. There maybe increased I²R losses due to the out of phase currents however this is the same whatever frequency. The impedances are higher/lower due to the higher frequency but can be corrected with smaller inductors/capacitors.

Mike.

Mike: I didn't speak about losses (I^2R) as they don't deppend on frecuency.

I was speaking about voltage drop and leakage currents (in both cases 90º out of phase, so they don't cause extra losses)

 

[RODALCO]

For very long runs sometimes even DC is used for transmission voltages.
In New Zealand between the North and South Island a 620 kV DC link is in used between Benmore and Haywards (Wellington).

400 Hz is used in the airforce and navy, main reason is weight and efficieny.

I worked in the airforce in The Netherlands and we used diesel gensets running at 1500 RPM with 32 pole alternators to get the required frequency.

When on the 50 Hz mains rotary converters were used running on 3Ø 380Volts 50 Hz converting it in 3Ø 416 Volts 400 Hz.

The physical size of the alternators wasn't that much bigger than the 50 Hz ones i recall of memory.

The American 60 Hz system is more efficient than the 50 Hz system which was pushed by AEG and the European SI system which prefers numbers 1,2 and 5. The 6 didn't fir in that series hence they settled on 50 Hz.

Idealy 230 Volts 60 Hz would be the better option than 110 Volts 60 Hz and 230 Volts 50 Hz IMO.

In the 50's railways used 16 2/3 Hz and 25 Hz (USA) and some countries still do. Germany and Switzerland use 15 kV 1Ø at 16 2/3 Hz.

Mainly to reduce sparking at the brushes at the traction motors at low frequency and have the advantage of AC with autotransfomers along the line and thinner OH catenary as opposed to the 1500 V and 3 kV systems in The Netherlands and Belgium. with many rectifier stations and heavy OH wiring.

 

[RODALCO]

Nigel, I didn't read the second page of this topic, hence this reply.

Electric master clocks as you probably aware from of my other posts, i collect them and know a bit about them too.

The power station masterclock had a differential mechanism in it which was driven via a mains operated clock and a pendulum precision clock,
( now these days the pendulum clock is a precision quartz clock which may even get corrected occasionaly via a satelite time signal when the drift is out more than 1 second or so. )

If the difference was say more than one or a couple of seconds slowly the governor is adjusted by a minute amount. This has to be done slow otherwise the risk exists that one or more power station alternators grab all the load and trips on overload. Reactors are fitted in the lines to the step up 11 kV / 220 kV transformers to dampen excess currents, also the transmission lines will absorb minor frequency differences in extra losses.
A thing you want to avoid in a grid is hunting of alternators which can cause instability and stations to trip out on over or underfrequency. Time delays are also applied to the controlgear to minimise it going back and forth.

Alternators, when synchronised will keep in synch with each other. If the prime mover loses power the alternator will run as a motor and will remain locked in untill taken off line by the protection settings.

 

[RODALCO]

http://www.med.govt.nz/templates/MultipageDocumentPage____4335.aspx

Link explains some of the terminology used in power systems.

Hope it works

Cheers, Raymond

 

[Nigel Goodwin]

Nigel, I didn't read the second page of this topic, hence this reply.

Electric master clocks as you probably aware from of my other posts, i collect them and know a bit about them too.

The power station masterclock had a differential mechanism in it which was driven via a mains operated clock and a pendulum precision clock,
( now these days the pendulum clock is a precision quartz clock which may even get corrected occasionaly via a satelite time signal when the drift is out more than 1 second or so. )

If the difference was say more than one or a couple of seconds slowly the governor is adjusted by a minute amount. This has to be done slow otherwise the risk exists that one or more power station alternators grab all the load and trips on overload. Reactors are fitted in the lines to the step up 11 kV / 220 kV transformers to dampen excess currents, also the transmission lines will absorb minor frequency differences in extra losses.
A thing you want to avoid in a grid is hunting of alternators which can cause instability and stations to trip out on over or underfrequency. Time delays are also applied to the controlgear to minimise it going back and forth.

Alternators, when synchronised will keep in synch with each other. If the prime mover loses power the alternator will run as a motor and will remain locked in untill taken off line by the protection settings.

Thanks for that information!.

 

[Hero999]

In the beginning of supply of electric power to households just DC was used. Since DC tends to fry organisms like human bodies it was changed to AC later. Never touch a high tension DC power line. You'll not be able to separate from the cable and BBQ'ed right away!
Hans

It isn't that simple.

Low frequency AC power is actually more dangerous at low voltages because the peak voltage is higher and it takes less AC current to kill you than DC.

AC is said to be four to five times more dangerous than DC. For one thing, AC causes more severe muscular contractions. For another, it stimulates sweating that lowers the skin resistance. Along that line, it is important to note that resistance goes down rapidly with continued contact. The sweating and the burning away of the skin oils and even the skin itself account for this. That is why it's extremely important to free the victim from contact with the current as quickly as possible before the climbing current reaches the fibrillation-inducing level.

**broken link removed**

Ventricular fibrillation

A low-voltage (110 to 220 V), 50 or 60-Hz AC current travelling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60mA. With DC, 300 to 500 mA is required.

http://en.wikipedia.org/wiki/Electric_shock

 

[Nigel Goodwin]

It isn't that simple.

Low frequency AC power is actually more dangerous at low voltages because the peak voltage is higher and it takes less AC current to kill you than DC.

This is so old it's not worth arguing about, check the original fights between Edison and Westinghouse, which resulted in the creation of the electric chair.

Regardless of safety or not though, AC has too many advantages.

 

[Hero999]

I totally agree, although they were more prominent back then then they are now. A DC distribution system is far more feasible than it used to be. I often think it'd be good to have a separate DC supply in the house for lighting and electronics items. You'd still need switch mode power supplies to convert them to different voltages but it'd be safer, more efficient and there wouldn't be all the problems associated with harmonics and power factor. I'd advocate a +/- 50V supply, which should be enough for most things.

I'm not talking about going to DC 100%, there will still be AC for appliances over 1kW. I suppose I know it isn't that practical as it'd mean changing the infrastructure but it's an interesting idea.

 

[Nigel Goodwin]

I would disagree with 'safer', most of the electronic and electrical trade consider AC far safer than DC (as do I). An AC shock will usually throw you clear, a DC shock tends to freeze you on the wire - and is much more likely to kill you.

I would also disagree with 'feasible distribution', it's far too inefficient, and the original DC premise was based on a small power station on every street corner.

It's also a very bad idea to have multiple mains supply sockets, that's been done, and is long over now (although you still do see it very occasionally) - different mains sockets upstairs and downstairs was never a good idea.

DC mains was also done years back, that was never a good idea either!.

 

[Pommie]

Here in Aus they are phasing out incandescent lights by 2010. It would make a lot of sense to have a 48V DC lighting system with compact fluorescents once this happens.

Mike.

 

[Nigel Goodwin]

Here in Aus they are phasing out incandescent lights by 2010. It would make a lot of sense to have a 48V DC lighting system with compact fluorescents once this happens.

Why would it?.

 

[Pommie]

I believe this is the best voltage to run them at.

Mike.

 

[Nigel Goodwin]

I believe this is the best voltage to run them at.

Buy why?, it sounds a really BAD voltage to run them off - presumably you must have some reason otherwise?.

 

[Hero999]

I would disagree with 'safer', most of the electronic and electrical trade consider AC far safer than DC (as do I).

The IEE consider DC to be safer than DC (the maximum touch voltage in dry conditions is 60VDC or 25VAC) and I trust them more than I trust you or the service trade.

At least when you've had a shock from a DC source your heart can start beating again, AC tends to mess up the pace maker so it doesn't restart. Not to mention the fact that AC will kill you with one fith of the current of DC.

An AC shock will usually throw you clear, a DC shock tends to freeze you on the wire - and is much more likely to kill you.

Don't count on being thrown free, AC can make your mussels freeze up too.

I would also disagree with 'feasible distribution', it's far too inefficient, and the original DC premise was based on a small power station on every street corner.

Maybe in the 1980s, but today quasi resonant switch mode power supplies can be as efficient as any distribution transformer and there are no skin effect losses in the distribution system.

It's also a very bad idea to have multiple mains supply sockets, that's been done, and is long over now (although you still do see it very occasionally) - different mains sockets upstairs and downstairs was never a good idea.

The US have 240V and 120V sockets and they don't have a problem.

We have 110V, 230V and 400V sockets at work and we've never had a problem. They are colour coded and keyed to prevent mistakes from being made.

**broken link removed**

Don't get me wrong, I'm not suggesting we should switch over to DC, I can just see the benifits of a DC system, especially for smaller appliances.