ohm
Senior Member
- Location
- Birmingham, AL
You would think that the drop in frquency at the generator(s) would be indicative of total load but you still would not know how much and from what direction.
wind power
- Thread starter petersonra
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You would think that the drop in frquency at the generator(s) would be indicative of total load but you still would not know how much and from what direction.
- Location
- Illinois
- Occupation
- retired electrician
ohm
Senior Member
- Location
- Birmingham, AL
Don, you obviously know more about the Grid than I do. Maybe you can bring us up to speed.
Is it true that if an entire city goes down the grid is not capable of supplying it?
What's the % loss/mile on the grid?
I've heard there is a long wait (3 yrs) for someone doing co-generation to get tied-in to the grid.
Thanks
- Location
- Illinois
- Occupation
- retired electrician
I don't know a lot about it. If a city or area is dropped out that is a result of the loss of transmission lines that feed the the area. This could be physical damage or the result of the protective relays doing their job...protecting the grid. In theory the outage should be localized but in some cases the combination of the loads and other conditions on the grid result in a wide spread blackout.
When a large amount of load drops off line (a city or area) the frequency on the rest of the grid will rise and if it rises too much the generators will trip off on over frequency and sometimes these are the conditions that cascade the outage to a large blackout. The same can happen the other way when a very large load is placed on the system and the frequency drops too much the generators will trip off on under frequency.
I don' know what the grid loss is.
There is a wait in some areas. There is a lot of engineering that is required when generation of any size is tied into the grid, but sometimes it is just the local utility that doesn't really want the cogen connected to their distribution system.
When a large amount of load drops off line (a city or area) the frequency on the rest of the grid will rise and if it rises too much the generators will trip off on over frequency and sometimes these are the conditions that cascade the outage to a large blackout. The same can happen the other way when a very large load is placed on the system and the frequency drops too much the generators will trip off on under frequency.
I don' know what the grid loss is.
There is a wait in some areas. There is a lot of engineering that is required when generation of any size is tied into the grid, but sometimes it is just the local utility that doesn't really want the cogen connected to their distribution system.
ohm
Senior Member
- Location
- Birmingham, AL
The reason I ask about transmission between power plants is I heard because of very high transmission losses one plant can't really take over for another, even if they had the capacity.
Since the bulk of of power generation is rotational sources I guess frequency actually is a good indicator of load. I wonder if Solar PV becomes more common if frequency will still be a good indicator of load, since inverters will be the prime movers?
Also, how great is the storage effect of parallel transmission lines? I heard the POCO's have to discharge them even after the disconnects are opened, before doing maintenance.
Utilities are another world to most of us, thanks.
Ohm i can answer some of your questions as I use to be a POCO Sub Station Relay Controll engineer years ago. I know Charlie E another Mod can if he picks up on this.
POCO's all work in parallel through the grid. Loosing just one generator is usually not a problem since there is more than one in parallel. Not to say it cannot be a problem, but generally it is not.
Well if you are talking residential grid-tied solar PV, the inverters used in that application are synced to the grid and the frequency tracks the POCO. In the event of a power failure the grid-tied inverter disconnects as designed and required.
You are talking two seperate issues. One is static electricity, and the other is protection from accidental reclosrue. Lineman do ground the lines when working to protect themselves. Since the lines are overhead and wind blowing the ground discharges the static build up on the lines. Also in the event there is a accidental reclosure on the line, the ground will trip the realys back at the sub station and de-energize the lines.
But surely they all still need to be the same frequency?
ohm
Senior Member
- Location
- Birmingham, AL
Thanks Don. Looks like we'll be seeing a lot about infrastructure in the future.
- Location
- Illinois
- Occupation
- retired electrician
The frequency is the "same" through out the interconnect, but there are load and generation changes within the interconnect that create small frequency disturbances. These propagate across the complete interconnect much like the small waves created when you toss an object into water. The magnitude of the frequency disturbance is dampened as you move away from the source. I have been told that the propagation speed of the frequency disturbance movement across the interconnect is about 1/3 the speed of light, but don't have a document to back that up.
If you are asking about the frequency of the 3 interconnects, yes the base frequency is 60 hertz, but they are not in sync with each other. There are no direct connections between the interconnects for this reason.
If there are no direct connections, how are the three grids interconnected?
- Location
- Illinois
- Occupation
- retired electrician
There are some connections using inverters to sync one interconnect with another. I thing there may be other methods that are also used to make connections between the grids. The 3 interconnect grids in the US are east of the Rockies, west of the Rockies and Texas. Everything within an interconnect grid is connected together, but there are very few connections between the grids.
Texas is still doing the whole lone star thing, even with energy.
You mean a DC link, perhaps?
- Location
- Illinois
- Occupation
- retired electrician
Yes, that has been done, but sometimes it is just one piece of equipment, much like a VFD where you bring the power into it from one grid and you take the output to the other grid. There are others where there are DC transmissing lines between two interconnects with converters at each end.
That's what I meant by a DC link. (Not transmission.)
A converter to get from AC to DC then an inverter, possibly mains commutated, to get it back to AC.
Not quite sure I see the point for the sort powers at grid interconnector level though, unless you are going to take advantage HVDC transmission.
Most of the alternators I have dealt with are automatically synchronised to grid frequency, closed on to the bars and subsequently controlled by a static exciter, generally on voltage, sometimes with a bit of VAr compensation.
You can't get them out of sync. You can't close up until you have synchronisation in both phase and frequency.
Once closed, if there was any tendency towards getting out of sync, they would trip on some parameter, maybe overcurrent, excessive VArs, reverse power etc. before that happened.
Having said that, my direct experience is on alternators up to 30 MVA which, for grid-connected units, is relatively small but I have seen similar systems used on machines
- Location
- Illinois
- Occupation
- retired electrician
That is true. To tie a source to the grid it must be synced with the grid and once it is tied to the grid it will remain in sync with the grid. The issue is that there are 3 interconnect grids in the US and they are not in sync with each other. Any transfer of power between the grids requires some device to transfer power between the out of sync grids. Sometimes there is excess power available or power at a cheaper rate available in the ajoining grid and such a device lets you transfer this power to where it is needed.
Do you have any specific details of those devices and their power ratings?
Fulthrotl
~Autocorrect is My Worst Enema.~
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- E
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- Location
- Illinois
- Occupation
- retired electrician
I haven't looked at that too much, but here is one with a 150 MW capacity. There is also something called a VFT (variable frequency transformer) that can be used for this purpose.
ohm
Senior Member
- Location
- Birmingham, AL
Cool, we've come a long way from simple generation & distribution. I for one am starting to form a new appreciation for our POCO's.
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