A energy saving device for street lights being tested in suburbs here in B.C. Also check out the video clip top right.

i don't understand...

The device can be programmed to dim streetlights during off peak periods (early morning when there's little traffic for example) saving energy.

is Dimming a HID lamp,done by reducing current to the ballast?

Yes HID lamps CAN be dimmed.......in fact I think some lights in Frederick has some dimming feature....although how those HPS dimmed was kinda weird....it was like a SUDDEN dim with a slight flicker......I was driving down the road with shoe box fixtures and around 11:00  PM or so....I was driving on that road and it just made a weird SUDDEN dimming which somehow gave me a headache!

yes they can indeed be dimmed,but how exactly is it done?

My theory is the following:

First, you must know that: Amps X Volts = Watts. So I think that the dimmer reduces the current but raises the volts to keep the same watt output.

I can't believe the HPS would not extingish as they seem to sensitive to power flux. Unless they are controlled by a electronic ballast?

When I need to dim my lamps I only decrease incoming voltage. The current decreases itself proportional to decreased voltage. There is some more difficult but that is what I would do.

But why dim street light? Aren't they lit up the way? I think that it is not good. What will happen if you fall down on 'dimmed' street and would be killed cause the driver can't see you? There must be other ways for that problem.

In Czech Republic the dimming of HPS is done on many places, but dimmers are set to reduce the voltage in <5% steps, one per ~5 minutes (down to ~70..80%), so lamps do not extinguish, you even do not notice it happen, unless you have a luxmeter with you... But moreover European lamps in European circuits do not suffer as much, as described with US installations... As (EU)HPS/MH 70W dimmable ballast i use two F36T8 and one PL9..15W coils in parallel for 100% setting (the impedance is equal to 70W SON/MH coil) and disconnect one F36T8 coil for reduced setting without any intermediate step, the HPS simply reduces output (even no visible color change), the CMH flicker ~1minute and then light smoothly, with a bit green tint of course.

I can't believe the HPS would not extingish as they seem to sensitive to power flux. Unless they are controlled by a electronic ballast?

SON is very sensitive to OCV drops, not as much to the current drop. Because the OCV is determined mainly by the mains voltage, the SON does not like mains dips. So when the dimming is made by increasing the serial ballast impedance (connect additional choke in series), the OCV stay unchanged, so the lamp does not extinguish.
When dimmed by voltage change, the dimming should be splitted into multiple steps with max 5% each and leave the time (few minutes) for the lamp to cool down before next step.

That is a very interesting system, at the same time I don't know that I like it.

But why dim street light? Aren't they lit up the way? I think that it is not good. What will happen if you fall down on 'dimmed' street and would be killed cause the driver can't see you? There must be other ways for that problem.

You need high flux streetlighting only, when there are lot of cars going in the opposite direction, because their headlights cause light glare, so you have to "overpower" them by light coming from the top in order to keep visibility. But when there are no cars to cause the glare, you have nothing to overpower, so the visibility stay good even with much lower illumination level. Of course, when some part of that road is not dimmed, there should be "transition" section to allow the eye to adopt, so a section is dimmed only partially (e.g. when transitioning from 100% around crossings to 50% on straight stretch, sections dimmed to 80% and 63% are kept for transition).

I think that's also a very interesting system, I wonder how they manage to dim the HPS lights. I read on the Advance website that you can change the capacitance of the capacitor on a CWA ballasts the dim the light.

I recall reading that some places in Europe had streetlight with two sets of lamps and ballasts where one set was switched after aorund midnight.

I think that's also a very interesting system, I wonder how they manage to dim the HPS lights. I read on the Advance website that you can change the capacitance of the capacitor on a CWA ballasts the dim the light.

Each ballast set has the dimmer block, what read the remote command (or has a timer for it's own), what switch these capacitors. But i think newer installations would use HX, as this gives the HPS longer life before cycling start (gives more thermally stable system, however more sensitive to mains dips), what in turn might be dimmed by the voltage reduction (as the EU systems).

I recall reading that some places in Europe had streetlight with two sets of lamps and ballasts where one set was switched after around midnight.

This was an old system used for MV, it had another advantage: Te same lamp type (125W) was used everywhere, on main streets, squares, residential streets, only number of lamps differ.

Today (new installations) larger posts most often use HPS 150W with group dimming (by voltage reduction), in residential areas 70WHPS are used at most, somewhere are ~56W CFL's.
Decorative lighting and city centers use often MH lamps instead of HPS (as the gear is compatible in Europe)