[mschaef]

Synchronous clocks do this too... they've driven by a motor that's designed to work in phase with the electrical grid's frequency. This means they rotate at a fixed speed and can be used as a time base for a clock.

From the perspective of the grid operator, however, 50 or 60Hz is not always 50 or 60Hz. A sudden load or a generator tripping offline (to preserve itself) results in a transient slowdown of the frequency of the entire grid. I spent a summer in high school helping out with the analysis of these kinds of disturbances, and there's a distinct pattern to the fluctuation of grid frequency. There are also slight longer term errors in grid frequency, although operators are held to strict standards.

Getting back to clocks, integrating these transient frequency errors over time results in clocks that shift forward and backward relative to real time. This integrated time error is often displayed in grid control rooms, and it is something they deliberately manage to ensure that the 'grid time' is accurate. In practical terms, this means a period of ever so slightly less than nominal frequency is likely to be followed by a period of deliberately induced slightly higher than normal frequency, so that the overall integrated error tends to zero.

More details on the time control aspect on page 13 here: http://www.nerc.com/docs/oc/rs/NERC%20Balancing%20and%20Freq...

[eriknstr]

I previously worked with Unix sysadmin tasks and software development at Statnett SF [1], the transmission system operator (TSO) for the national level of the Norwegian electrical power distribution network, and am currently studying at the university to become an electrical engineer. We started learning about transformers, generators and motors in three-phase systems this semester, so that document is of interest to me. I looked over the document briefly and intend to read it in full later, for example tomorrow when I will be going on a two hour train trip to Oslo.

The electricity sector in Norway relies predominantly on hydroelectricity. In 2008, hydroelectricity generated 141 terawatt-hours (TWh) and accounted for 98.5% of the national electricity demand. [2]

I have been told that the nature of hydroelectric power generator installations makes the act of balancing the power system in Norway quite different from what most other countries are dealing with but I think the document you linked will be informative to me none-the-less. Besides, understanding power systems balancing in general and not just for Norway would allow me to work in other countries in the future should I want to. Not saying that the university is going to teach me things that apply to Norway only of course but I hope that you understand what I mean.

PS: Statnett has a live view of the Nordic power balance on their website -- http://statnett.no/en/Market-and-operations/

[1]: http://statnett.no/en/About-Statnett/

[2]: https://en.wikipedia.org/wiki/Electricity_sector_in_Norway

[mschaef]

Cool... good luck! (If you ever find an intuitive way to think about reactive power, please let me know... I get it in broad strokes, I've been trying to wrap my head around the details for over 20 years, off and on.)

> I have been told that the nature of hydroelectric power generator installations makes the act of balancing the power system in Norway quite different from what most other countries are dealing with

Hydro has both upsides and downsides. To the upside, they're easily able to respond to changes in demand. It's the difference between opening a wicket gate and adding fuel/air to boil more water, to make more steam, to apply more torque. Hydro can also support stored power.... sometimes you see what are known as pumped storage generators generate 'negative' output. This means they're running as a motor to pump water up a hill. When power is needed they can just drain the water through turbines to generate power.

That said, Hydro also has additional constraints on operation that can be imposed by flood control requirements, reservoir levels [1], environmental regulations, etc.

1] https://wrrc.arizona.edu/drought-diminishes-hydropower