Power Grid

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mikeames

mikeames

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Teacher - Master Electrician - 2017 NEC
How does the power grid work? I always wondered. I assume its key is the huge reactive properties, but how do multiple power plants all feed into the same lines. Is there an illustration someone has or is it simply the size and the reactance that keeps everything balance.
 
With careful metering and careful prime mover control.

In a nutshell, everything is in parallel with everything.
 
its very simple, really. just a bunch of wires.

think of a system with thousands of batteries, tens of thousands of interconnections, and 100's of thousands of loads where every load is conceivably receiving some part of its power from every battery.
 
090203-1906 EST

mikeames:

A power plant generator (alternator) can be treated as a voltage source with some internal impedance. From the power plant there is a distribution system that also contains internal impedance. Much of the distribution system is a distributed impedance.

Now greatly simplify this to two variable voltage DC sources each with an internal resistance of 10 ohm and feeding a single load of 50 ohms load.

The equations for this circuit are:
Iload = I1 + I2
Vload = Iload * Rload
Vload = V1 - I1*R1
Vload = V2 - I2*R2
If Vload is to remain constant, then a reduction in V2 requires V1 to be adjusted higher. This causes more of the load current to come from V1, and thus more energy is being supplied by V1 both from increased internal source voltage and the resultant increase in I1 current.

Obviously V1 can not be allowed to supply too much energy or the generator would be overheated. This is aside from the issue if there is enough steam energy to drive the generator.

If V2 is lowered to the point where V2 = Vload, then V1 is supplying all the energy. If Vload is 10 V and the resistances are as above, and V2 is 10 V, then V1 is is 12 V. If V1 is changed to 11 V, then what is the required V2 voltage to maintain Vload at 10 V?

With any such system there are stability problems.

Does this help?

.
 
All this makes sence but none of the generators are in perfect sync. Like putting two 5,000 watt generators together, you cant do it unless there are electronics there to sync them like the hondas have. Are the power plants treally synced that well. I am aware they they do monitor power befor comming online so they can match grid power, but it all seems too simple. I guess maybe it is.
 
power flow

power flow

i may be way off base on this but i believe you can also control the power flow by mataining the phase angle but varing the voltage?
 
http://forums.mikeholt.com/showthread.php?t=109759

mikeames said:
Are the power plants treally synced that well. I am aware they they do monitor power befor comming online so they can match grid power, but it all seems too simple. I guess maybe it is.
Click to expand...

(A rough approximation:) The torque required by a generator depends upon its relationship to synchronism. If a generator were to spin slightly too fast, then the torque that it requires would increase, causing it to slow down. Similarly a generator running too slowly will see a reduction in torque, causing it to speed up. The steady state speed of a generator is when the torque that it consumes matches that produced by its prime mover.

So the grid itself actually ties the generators into synchronism.

-Jon
 
Once the generator is tied to the grid, it has to follow the frequency of the grid. For all practical purposes the complete grid is at the same frequency. Yes, there are disturbances that start local, but they work their way across the complete grid being dampened as the move away from the source. Both load changes and generation changes result in frequency deviation. I think that the grid operators (the automatic control systems) start taking action if there is a deviation greater than 0.05 hertz.
 
Mike01 said:
i may be way off base on this but i believe you can also control the power flow by mataining the phase angle but varing the voltage?
Click to expand...
Power flow is mostly controlled by the power to the prime mover. Take an engine generator for example. If you increase the throttle, you make the generator spin a little faster. The generator can't spin continually spin faster than the power system frequency, but the angle between the generator and the system increases a little and more power flows out of the generator.

Increasing the voltage will increase the reactive power into the system.

In the grid, "base load" generators are run at their full capacity. These are plants like nuclear and large coal fired plants that are expensive to start up and shut down and generate at low cost/kWh. System operators have to vary the power output from smaller generators to match the load on the system. The amount of load is uncontrollable for the most part and generation has to match the load or the frequency goes out of whack. Which generators are run at what output depends on economics and power generation contracts.
 
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