T&D Question: Grid - Voltage & Frequency Regulation

[rjmaurer]

New to forum; M.E. migrating to T&D role in ESCO. Having difficulty understanding Frequency and Voltage relationships in grid and ultimately for qualifying and registering assets for Reserves and Regulation programs. From M.E. perspective - Voltage = Pressure; Current = Flow; Resistance = Pipe Losses.....so, what is Mechanical Equivalent of Frequency? Why does Grid need frequency stabilization when demand fluctuates -- Isn't all power generated at 60Hz continuously as a function of basic 3-phase relationships and rotor/stator construction? Why won't electric generators with governors regulate frequency of grid correctly and keep frequency stable, then you would only have voltage (pressure) fluctuations when demand inc or decr, no? So, isn't grid stability ultimately one of voltage stabilization and not frequency stabilization? Help me understand the difference and / or relationship. Any good explanations / articles / diagrams would be helpful and appreciated.

 

[jghrist]

I don't know if there is a useful mechanical equivalent of frequency. Most mechanical equivalents are really dealing with direct current, not alternating current because you don't often deliberately vibrate water back and forth in a pipe.

Frequency instability can occur when the load suddenly increases or decreases. The prime mover for a generator cannot respond instantly and the frequency will increase if the load suddenly drops and vice versa. Think of pushing in the clutch on your car while climbing a hill with your foot on the gas. The load on the engine disappears and until you can reduce the throttle, the rpms will go up.

 

[waross]

Grid frequency excursions have been discussed here before. Spend a little time searching this site.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Think of frequency as rotations/sec or rpm and phase angle as spatial relationship. In fact, electrical frequency (cycles/second) in is another expression of rpm.

Now you can think of many mechanical analogies to electrical synchronization. One of them could be what if the two drives driving a common shaft starts rotating at different speed?

Or saw the movie Speed? If the two bus speed and direction are not exactly the same and physically next to each other, it becomes difficult to trasfer people from one another while still in motion.



Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

DC system can be likened to static mechanics, AC systems can be likened to dynamic mechanics.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

I like the Bus analogy Rafiq. Don't forget the 180 degree out of phase closure when they crash head on.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rjmaurer]

Spent all afternoon looking thru posts....didn't find relevant answer. Please direct me to helpful comments if you can.

 

[rmw]

The prime movers aren't infinitely powerful (or instantaneously able to respond). Taking the car on the hill/clutch analogy the other direction, if your car was trying to take off while climbing a hill, and you let off on the clutch suddenly, the engine would bog until you could get the fuel regulated sufficiently to provide the torque necessary to overcome the load. Even if you get the engine all good and revved up, it will bog somewhat until the fuel gets set just right.

The grid acts the same. Load swings tend to try to slow down the drivers that furnish the frequency to the grid on load increases (or speed them up on load loss).

You dump a big enough load on a generator (or a grid full of generators) and it will bog it down just like your engine would if you let out the clutch too quickly. Frequency suffers in the process.

It is a hard thing for ME's (like me) to get our minds around because there is no mechanical analogy to apply to it.

rmw

 

[rbulsara]

Bill, thanks. you encouraged me (grin) to extend my analogy a little further. Feel free to embellish it.

rjmaurer:
Try this, in response to your second half of the question:

The frequency of a generator is directly related to rpm of the prime mover. n=2f/p where n is in rps and p in no. of poles, which is fixed for a given machine. The change in electrical load, directly affect the rpm of a prime mover which directly affect the frequency of the generator. So is the voltage affected by change in loads.

To take the mechanical analogy further, a simplified grid is analogous to a long rotating shaft driven by several prime movers connected via gear and pinion arrangement along its length. Similarly the load are also connected using other gear and pinions along the shaft. Imagine the challenge of keeping the rpm of the shaft precisely at a fixed number while there are constantly varying loads and quite an array of types and sizes of prime movers and even the fuels. Even the quality of a bad fuel batch could affect the speed of a prime mover.

A weak prime mover will start getting driven by the massive shaft, same as a weak generator end up being driven as a motor (reverse powering). If one resists, something will break. This again simultaneously making sure that shaft does not fall over or sag or sway at any location, this is “loosely” akin to maintaining voltage at the same time.

The above analogy is not a make believe thing, that is how initial workshops used to run, with a steam engine driving a shaft and a number of belt and pulleys off the shaft running different machines.

If you can extend your imagination to visualize the a mesh of flexible rotating shafts, prime movers and loads transferring power over a wide area network, that is what a grid of three phase ac power lines is like. In fact the grid is another expression of a mesh.





Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

I understand your frustration but it is little tough to explain the collective knowledge of generations gathered over a century, for which volumes of books are written and PH. D's are offered, in a few paragraphs. Even then many aspect of electricity's behavior are not fully understood today. We all do the best with what we know today.

This forum can't be a substitute for a formal education and experience.

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

To make it more fun there are generating plants that sync with the grid and supply a fixed amount of power, based on the amount of steam, fuel, water or whatever fuel is fed to the prime mover. These units have governors but often the governor only comes into play to prevent a runaway if the grid connection is lost. Some generating units are controlled to control the frequency of both themselves and the constant load units.
Have you ever seen a flyball governor? They were used to control the steam feed to early generating plants. A couple of phrases that originated with the old flyballs are still in use although the origin is probably mostly forgotten.
A flyball governor had a construction similar to an umbrella with only two sets of ribs left. There was a ball on the end of each rib. As the shaft was turned centrifugal force caused the balls to arc out and up. Coarse frequency control was done with the flyball governors. Fine control was by a chronograph and an electric clock. If the clock was getting ahead of the chronograph the flyball governor was set a little slower until the clocks were even. If the clock got far behind, the governor was set faster and the balls rose higher. With the generator running at the maximum allowable overspeed the ribs would be flat out and the balls at the greatest radius of rotation. Hence the phrases, "Balls to the Walls" and "Flat Out".
And by the way, I have set the frequency/speed of a couple of generators by comparing an old synchronous type electric clock and a digital wristwatch. Just a couple of my adventures in the Moskito Coast.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Try this link for bedtime reading for a few days.

http://www.google.com/custom?hl=en&...m&oe=ISO-8859-1&q=grid+frequency&start=0&sa=N

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Bill:
No, I haven't. But thanks for rectifying my distorted image of balls to the walls!

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

The thanks belongs to one of our UK friends who described the operation of old time power plants.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[rjmaurer]

Thanks for all the comments / references. Gaining clearer understanding now. Fly-ball Governor - check, have seem these on top of elevators. Also appreciate references to the adopted "vernacular" on the evolution of phrases stemming from FBGs. Keep it clean, boys!
Sure I'll have many more inquiries of this forum as I am involved in a mid-size CHP project at the moment installed on the Distr side that is expected to participate in Transmission-side programs.....interesting spin on my first project. At the point of understanding just what I don't understand.

 

[JimCook]

Check out Scribd for an upload of Olson's Dynamical Analogies - 1943 which describes several different mechanical systems compared to electrical systems.

http://www.scribd.com/doc/12914354/Olson-1943-Dynamical-Analogies