Greetings! I'm a regular over at Ed Yourdon's forum.

First, I'ld like to thank Rick for the valuable information at his site.

Now my question(s). The grid runs at 60Hz. How do all the generators stay synchronized? If one generator is out of sync by a half cycle, wouldn't this cause problems? What type of cummunications is used to pass any synchronization info back and forth? Thanks in advance for your help.

-- Anonymous, March 04, 1999

Answers

Rick,

I'm gonna wait anxiously for the answer to this one!!!! (big grin) I guess this is why you make the big bucks!

(My answer would have to be "physics")

Good Luck!

-- Anonymous, March 04, 1999

There are two sides of this issue - maintaining synchronicity of the machines (the actual generators) with the outgoing transmission systems, and maintaining synchronicity of the interconnected transmission systems.

Here's some information from the Energy Information Agency (a part of the U.S. Department of Energy) on how the first part occurs (machine to system):

System Stability

Power systems stability problems represent other system operating constraints. Generally they are grouped into two types:

• Maintaining synchronization among the generators of the system
• Preventing the collapse of voltages.

In a synchronous, interconnected operating system, all generators rotate in unison at a speed that produces a consistent frequency. In the United States, this frequency is 60 cycles per second. When a disturbance (fault) occurs in the transmission system, the power requirements from the generators change. The fault may reduce the power requirements from the generator; however, the mechanical power driving the turbine stays constant, causing the generator to accelerate. Removing the fault alters the power flow and the turbine slows down. This results in oscillations in the speed at which the generator rotates and in the frequency of the power flows in the system. Unless natural conditions or control systems damp out the oscillations, the system is unstable. This is referred to as transient instability and may lead to a complete collapse of the system. To avoid transient instability, power transfers between areas are limited to levels determined by system contingency studies.(10) Steady-state instability can occur if too much power is transferred over a transmission line or part of a system to the point that the synchronizing forces are no longer effective. Steady-state instability is an unusual occurrence because it is easily preventable; however, it acts as a constraint on transmission power transfers.(11) Small-signal instability, also called dynamic instability, usually occurs when normal variations in generation or consumption are too small to be considered disturbances, but initiate oscillations at low frequencies. These conditions can lead to large voltage and frequency fluctuations, resulting in loss of overall system stability.(12)

Voltage instability occurs when the transmission system is not adequately designed to handle reactive power flows. Large amounts of reactive power flows on long transmission lines result in severe drops in voltage at the consumption end, causing the consuming entities to draw increasing currents. The increased currents cause additional reactive power flows and voltage losses in the system, leading to still lower voltages at the consumption end. As the process continues, the voltages collapse further, requiring users to be disconnected to prevent serious damage. Finally, the system partially or fully collapses.(13)

I'll post more later on interconneted transmission system synchronization, but the essential elements are the same.

-- Anonymous, March 04, 1999

Rick,

Pretty solid answer. My I take a whack at it from a different angle? I took a poll in the office on how to answer this complex subject for lay people. The following is the best answer I got.

An analogy for stability/synchronization. It is by no means precise, and is certainly an imperfect example but it get the point across.

Picture two automobiles travelling in parallel lanes down an interstate. A tow bar is welded to each car. As the power to each car engine is regulated, they can come side by side to allow a single bolt to be attached to connect the two cars' tow bars together. They are now interconnected.

The cars will now tend to travel at the same speed and direction. Either car can add or decrease power, and they will still tend to match speed and direction. If one driver takes his foot off the gas, the other car will have to increase power to maintain speed.

What will cause the two to come apart, or lose synchronism? A sudden and drastic change in one of the machines. Say one car blows a tire, locks up a wheel bearing. Mechanical stresses on the bolt could cause the connection to break and the cars to drift apart.

Now back to electrical generators. The cars are generators and the gas pedals are steam, excitation voltage etc. The "bolt" that holds them together is a magnetic field that tends to keep the rotating portions of the machine "rolling" at the same "speed" and "direction", similar to the car example.

I know this has many flaws, but it seems there may be some that perhaps didn't want to wade through EE theory. Please take no offense, this was intended to be helpful and I hope no one is insulted.

-- Anonymous, March 04, 1999

Thanks very much Rick and Art. But I still don't understand what the "bolt" really is. How does a generator is New York keep in exact synchronization with one in Pittsburgh for example. I understand that they both would run at 3600 rpm, but how do they both put out the 60 cycle pulse at the exact same time, if you know what I mean? Or don't they? Does something happen at the "interconnect" do get the timing right? Sorry if I'm being a pain.

-- Anonymous, March 04, 1999

I also like this simple example of generator synchronization. Dick Mill's has a good article related to y2k and synchronization - readAnother Myth, We Need Computers to Synchronize".

Regards, FactFinder

-- Anonymous, March 04, 1999

Ok Factfinder, now I understand. Thanks for your help guys. :)

-- Anonymous, March 04, 1999

Fact Finder,

Those were excellent links that illustrate why you are one of the most valuable and respected voices in this forum - far better than my feeble attempt.

The guys I asked in my office all scratched their heads at the prospect of defining power flow topics in a general forum. One of them does volunteer work at a science museum on weekends - I will pass these links along and he we be very greatful.

-- Anonymous, March 04, 1999

Rick:

It's my understanding of the grid distribution system that the master 60 Hz "clock" is located in Indiana (?) .. and like a metronome, it sends out a synchronizing signal via telephone, satellite, and microwave to the various generating sites nationwide.

Yes/no?

Dan

-- Anonymous, March 05, 1999

Art, I sure do appreciate the compliment, but the "fact" is, I though you and Rick both gave very good explanations on a pretty short notice. I have worked with EDG's and familiar with synchronization, but I must admit I didn't want to delve into this one myself. I took the easy way out and found these links. Many times someone somewhere has already taken a lot of time to provide information on any given subject). So, what I am really good at is using the search engines (do this a lot in my Y2K work)...

Regards, FactFinder

-- Anonymous, March 06, 1999

Don't sell yourself short FactFinder. Knowing how to find an answer is just as good as knowing the answer! Thanks for your help. <:)=

-- Anonymous, March 08, 1999

I read the link on Dick Mills' Synchronization article and he says something very interesting:

"If we do it wrong in a real power grid, the result is truly violent. So-called missynchronization can result in explosions, fires and many millions of dollars of damage."

Remember the past thread calling attention to some recent utility explosions (i.e. Ford plant) and how the writer was roundly criticized for being "off topic" and having "red truck" syndrome.

What do we make of this?

-- Anonymous, March 10, 1999

Hi Joseph. This was not the reason I asked this question, but boy did I sure notice that line! We've had several threads of discussion about all the explosions over on Ed Yourdon's forum. <:)=

-- Anonymous, March 10, 1999