Provided that electric utilities are tasked with ensuring that the load demand is always met, how do these utilities determine load demand to be met in the first place in real time rather than using some forecast?
Electronic – How do electric utilities determine the Load Demand on the network
electricitypower-grid
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You're taking this "earth" or "ground" thing too literally.
If I tell you: "There's 100 V DC on this node." Then then you would assume that I mean "100 V DC relative potential to the ground node of the device I'm measuring at that moment.
You'd call me crazy if I said: "There's 100 V DC on this node relative to the metal hull of an oil-tanker which is in the middle of the Atlantic Ocean".
It would be inconvenient to actually measure like that. There's no guarantee that the oil tanker's hull is properly grounded (also: grounded to what ? The bottom of the sea ?)
This silly example demonstrates that "ground" or "earth" is just a local reference point. We define it as having a zero potential voltage. You always need 2 nodes to measure a voltage, a voltage is actually a potential difference. It is extremely convenient to make one of those potentials zero. And we call that one "ground" or "earth".
I am not experienced in this area, but I will tell you what I think I know.
- Do engineers use 'standard lines' (with a fixed capacity per distance) or do we have a variety of lines to choose between (with different capacity)
There are several standard voltage levels and conductor sizes used for transmission lines. In addition, more than one set of three-phase conductors can occupy one overhead support structure or underground conduit. The voltage levels of overhead lines can be identified by the insulators and spacing used. Larger diameter sizes of conductors have higher current capacity for a given operating temperature and lower voltage drop for a given distance, so the current carried by a given size may be less for longer distance lines.
- How is the need for line capacity in general determined? Do we estimate the maximum load and increase capacity to be slightly above this?
The capacity determination is done in the same general way that the capacity for a manufacturing plant is determined. A study is undertaken to determine the current power demand for the region to be served and estimate the expected growth in demand for some period of time into the future. Consideration is given to the possible use of alternative energy sources. Possibilities for future transmission level expansion are considered. Those and other factors are weighed against the cost of building various levels of transmission capacity.
- If two power grids are coupled by a DC power line, do they influence eachother in terms of stability? Why/why not?
When two power grids are coupled, each appears to the other as a large load or supply. The connection would not be made without the two grids being under the control of one operator or under an agreement between two operators in constant communication with each other. The DC interconnection facility would isolate the two from one another from phase and frequency variations, but load variations in one section would influence the other. The interconnection facility would probably reduce the influence between the two in terms of stability compared to an AC tie, but not eliminate it.
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Grossly, the grid frequency is constant when load = generation, rises when generation exceeds load, and falls when there is insufficient generation. If you think about it the whole thing is a torque balance between the turbines and the reaction force on the rotors due to the power being drawn.
Operators leverage this by configuring the various generation facilities to have different set points and slopes on their speed governors. A base line plant for example (Say hydro or nuclear) will typically go to full song if the frequency drops below say 60.25Hz, so it is essentially always running flat out, a gas turbine peaking plant on the other hand might be set to not load up until the frequency drops to 59.75Hz so that most of the time it is idling as spinning reserve. Something like a battery storage system will not load up until the grid drops further then that, whereupon it gets paid mad money to prevent a grid collapse for a few minutes while some more gas turbines spool up or some load is dumped.
That deals with the generation, but there are also serious constraints on transmission, you can only shove so many MVA over a given transmission line before either the insulation fails or the cables overheat and sag.
Power flow is controlled in the first instance be controlling reactive power flow, by switching either inductors or capacitor banks in and out, and the game is to try and always have N+1 redundancy against anticipated load so that a single failure in the transmission network will not turn into a cascading failure that can take out half the state.
If grid operations see a loss of redundancy in either generation or distribution they will (and should) load dump, simply because a lot of generation depends on the grid being up to be able to operate. If the grid goes away then many, many large scale generators will trip off line (And worse their steam production systems will trip offline), so you wind up having to restart things with a lot of your generation down for a few days to a few weeks. Rolling blackouts are MUCH less disruptive than having that happen.
For example a civil nuclear plant that loses grid connectivity will scram the reactor automatically, and a restart is impossible until the core poisons have decayed sufficiently for a controlled delayed criticality to again be possible, maybe a week, but the regulatory types will often extend that. Large scale coal, similar story (Boiler steel thermal behaviour in that case), once you lose the grid you are looking at much time to get it back, especially if the surrounding grids are not in a position to help out.
You really do not want a grid collapse in the transmission system, rolling blackouts (Or even 'just' switching off a city or two) are MUCH less messy than that.
Today we have some very good software that provides simulation support for the dispatchers running this stuff, and grid stability is relatively easy to monitor in realtime, this was not always the case.... Still, dispatcher with the +1 having just tripped off line, and the frequency dropping, with LA on Oscars night being the best choice for a load dump is when you EARN your pay!