frequency tolerance

[don_resqcapt19]

Physics,

That's something that's always interested me Don, not enough to actually try to find out I suppose, but what in the world is the method used to keep all these individual generators, or alternators, in synch?

My understanding is that everything in the interconnect is at the same frequency. The generators that are connected together act as one very large machine. Once they are tied to the grid their frequency is locked to that of the grid. Large increases in load serve to drive the frequency down a bit. At this point the frequency controls (governors) will increase the mechanical power input to the generators to drive the frequency back up. Since the grid has no real method of storing electricity the generation must always be matched exactly to the load. When it is not, there will be small changes in frequency to bring everything back into balance. That is when the load exceeds the generation the frequency drops and when the generator exceeds the load the frequency increases.
Don

 

[coulter]

edited cause I fat fingered the send button

 

[coulter]

Every alternator (generator) has the same two control knobs, DC to the rotating field, and the throttle. How they make the gen react depends on the operating connection, island or parallel.

For island mode, the DC field current controls the voltage, and the throttle controls the frequency. The load is what it is (my brilliant statement of the day. If the load is heavily inductive, the voltage drops and the regulator increases the field current, which picks up the voltage. If the load increases, as Don said, the freq drops and the governer opens the throttle, which picks up the freq.

Now for parallel operation:
First, practically there is nothing one gen can do to affect the grid. There is a lot of stored energy in the rotating mass out there that will pick up most small disturbances.

The buss voltage is stuck. The buss freq is stuck.

This is true enough that the same assumption works well enough for small off-grid parallel generation systems.

But we still have the same two knobs. So, (paraphrasing winnie and dbuckley) to sync, one adjusts the throttle to match the frequency and phase angle of the gen to the grid, and adjusts the regulator to match the voltage. When they are close enough, close the main CB. No sweat, works well.

Now the two knobs have a different effects:
The throttle controls load sharing. The field current controls var sharing.

Open the throttle and the gen puts more power out to the grid. Increase the field current, and the gen puts out vars.

Now the governer is set to provide constant power. The voltage regulator is set to provide a constant power factor.

So, how do all the gens stay in sync? Well, after they are on-line, you damn near can't get them out of sync.

carl

 

[zbang]

Carl- that's the best explanation yet I've see for the controls. Now if I can find a good explanation of the governer...

(A while ago, I took a hard-hat tour of Hoover Dam, and we were allowed to look into a turbine's governer. We could watch some needles on the front twitch to show the wicket gate control and the RPM. Inside there was some really nice 3-coil motors, linkages, etc. Amazing what/how things were done in the '30s.)

 

[coulter]

Carl- that's the best explanation yet I've see for the controls. Now if I can find a good explanation of the governer...

Thank you. It's a simplistic explanation, but workable. I'm by no means a whiz, but I have been around them a lot (and I don't like Holiday Inn)

As for the governer internals, I don't have much help. I treat them as a black box. They have an input (mechanical or electrical - doesn't matter). It could be based on frequency or power output. It has an output (again mechanical or electrical) that is connected to the throttle (yet again mech or elect). There also has to be a feedback link to close the error gap.

I've never looked inside one. Check the inputs, check the outputs. If it isn't doing the right thing, beat it with a hammer until your attitude improves (Well, maybe send it out to a specialist shop.)

The whole thing has to operate slow enough the gen does not hunt, and fast enough to pick up the load with out sagging or overshooting (at least not beyond spec)

The most common problem I've seen (other than a broken BB) is when the gen/driver is not matched to the load. Some problems are big gens with small loads, high leading power factor with low real power loads, gen barely big enough for motor starting loading.

I've now told you more than I know - time to stop.

carl

 

[Physis 3]

Pretty much all of the responses since my last post on this thread have been rather impressive! I wish I had the time to address more of the things that have really caught my interest. Maybe later I'll get to some of the others but I'll bring this one up at the moment.

By Carl:

There is a lot of stored energy in the rotating mass out there that will pick up most small disturbances.

The stored inertial mass of a motor or generator has always intrigued me. It's similar to a capacitor but it's electrical quality is magnetic rather than static. With a motor, first you inflate the magnetic fields, then you begin storing mechanical energy in the rotation of the armeture. Plus, the energy stored can be far greater than what could be acheived with a capacitor. Of course that stored energy wasn't free, that's why the lights dimmed when the motor started, for instance, non the less, a very interesting energy storage phenomenon. And in all my studies of electrical engineering I don't recall going over a motor's capacity to store energy as inertial mass.

By the way, how have you been Carl?

 

[mpross]

EE/Power System Dynamics

EE/Power System Dynamics

Physis, are you an EE student too?

Here is my take on the subject of a generator and the storage of rotating energy, but keep in mind that I am also a student and not an industry expert.

In the special area of power system dynamics, it is very important to understand the amount of rotational kinetic energy that can be stored in all of the spinning masses connected to the shaft of a generator. This mass is generally large, and thus there is a lot of energy stored up here.

On one hand, a capacitor stores energy by means of an electric field, and the inductor stores energy by means of magnetic field. On the other hand, a generator and the shaft-common masses have an associated energy stored by means of rotation.

This rotational kinetic energy helps to correct small scale frequency deviation, in a very small initial time frame. Once the control system has a chance to react to the disturbance, more thermal energy is input (in the case of a freq. decrease) to the system to compensate.

Power system dynamics is a great area to study! There is a lot of great text and literature to read, and there are still many problems that need to be solved in this area.

Later,

-Matt