clean power generator questions

[Soundguy]

since you already sounded like you knew what you were talking baout in the questions you asked, I kinda thought the 'how' was redundant. but yeah.. pretty much what you said..

I did loded and unloaded tests too, using both inductive and restive loads.

grid power had as much as 1/hz deviation, though for very short periods. Perhaps a power factor issue.. seeings as how I'm sharing a feed line with other houses on my block, vs my genny output that is a closed circuit with the load attached.

( and yes, I used the same load in those tests on grid and genny, just to make sure I wasn't skewing the data.. perhaps not completely scientific.. but the results were surprising.. I had expected the grid power to respond more favorably to loads liek a well pump kicking on.. etc.. )

Didn't own a real welder at the time, but that would have been a neat test if I had a helper to do the test. I understand some people have arc stability problems with gennies.... though my genny has always run my stick welder just fine.

soundguy

Since I asked "how" not "what you used", I'll try to determine how you gathered the data by reading between the lines above...

You...
1) plugged the probes of the "graphing oscope or meter" into the generator output, and took an instantaneous frequency reading along with some periodic data points which you graphed versus time, then
2) repeated the above using the utility power as input to the "graphing oscope or meter".

Is this correct?

Subsequently you...
1) computed the mean value and/or deviation from the meter's idea of 60Hz for both the generator and the utility power over a given time interval, then
2) compared the two and determined that the generator was either more accurate to 60Hz, more stable around 60Hz, or both.

Is this correct?

What were the utility and generator deviations, and over what period of time?

Wrooster

 

[tsteahr]

I'm struggling to understand how and why the frequency of the utility grid would vary. There is tremendous inertia in the grid. All generation stations are synchronized together when connected to their grid. Every generation station, regardless of how large (even the largest nuclear plants >1500Mw) must match generator speed with the grid prior to closing the breaker connecting the generator to the grid. If the generator (and the turbine that drives the generator) are not matched at exactly 1800 rpm for 60 hz, the grid will instantly match the turbine speed for you. If this happens, millions of dollars of equipment are damaged. There is no room for error when syncing to the grid. The inertia of the grid is essentially infinite relative to any one generation station.

Since there are many, many large generation stations all connected together, and they are all spinning at exactly the same speed. A "quick" frequency variation of 1 Hz would mean every generation station on that grid would have to change speed by about 30rpm (1.6%) instantly and then go back. There is WAY to much mass in a turbine to change speed quickly. The turbine/generator shaft does have some elasticity, but I find it had to believe a 1.6% change in speed, even for just a second, can be absorbed in turbine/generator shaft windup.

Having said all that, I'm not a electrical guy. But my first reaction when someone says the grid frequency varies is there's a problem with the measuring equipment or method.

 

[Soundguy]

I'm going to -guess- it has something to do with harmonics and reactive or non linear loads ( think power factor ), and the test equipment itself. at least some of it anyway.. I'm sure there is more to it than that though.

I must admit I'm not up on -all- the intracacies of power generation and distribution.. but did study electronics in college, just switched from EE to CE at about 3/4's the way thru

soundguy

 

[Chilly807]

Most affordable inverter generators aren't even in the ball park for enough power to run a welder, nor are most UPS units.

I think a Honda 3000 watt inverter is about $1500 these days, and that won't run much of a welder, maybe a 120 v MIG at best.

My experience has been that my 5000 watt Honda EZ generator (basic model, not inverter) will run my Miller 180 MIG, but not very well. It trips the breaker at higher settings, and the weld quality is iffy at best.

Sean

 

[fishman]

Not the same thing but my generac 4000 watt generator runs my Hobart ac/dc stick welder just fine on dc. Used it to build a nice outbuilding for a friend and some other projects for myself. Probably 30 hours use total with no issues. When I posted a question about it here before doing it, maybe 6 years ago, most said it wouldn't work but a couple folks who had tried it said it would. I listened to the folks with hands on experience.

 

[wrooster]

I'm struggling to understand how and why the frequency of the utility grid would vary.

The frequency of the grid varies, very very very slightly, as a function of load. And when I say slightly, I mean less than 0.25%, or about 0.15Hz on a nominal 60Hz generation/distribution system, and that is over a course of a day -- not instantaneously. There is no possible way for a modern interconnected power grid to wander 1Hz off the nominal (60Hz here in the USA and most countries with 120Vac mains, 50Hz in most other countries with 240Vac mains, and interestingly both 60Hz *and* 50Hz in Japan).

Here is 2 months of data, taken at 1 second intervals, by a private individual:

Note the excursion over that period is within 0.05Hz of nominal 60Hz, or about 0.08% variance.

There is tremendous inertia in the grid. All generation stations are synchronized together when connected to their grid. Every generation station, regardless of how large (even the largest nuclear plants >1500Mw) must match generator speed with the grid prior to closing the breaker connecting the generator to the grid. If the generator (and the turbine that drives the generator) are not matched at exactly 1800 rpm for 60 hz, the grid will instantly match the turbine speed for you. If this happens, millions of dollars of equipment are damaged. There is no room for error when syncing to the grid. The inertia of the grid is essentially infinite relative to any one generation station.

^^^ This!

There is limited "official" reported data on the USA's distribution system. In contrast, you can see the UK's power transimssion system frequency shown graphically here,
Dynamic Demand

As you can see, 0.1Hz in either direction (0.2%) is the normal condition.

There is a historical graph of the same data here,
National Grid: Real Time Frequency Data - Last 60 Minutes

Again, the excursion is within 0.1Hz.

I find it had to believe a 1.6% change in speed, even for just a second, can be absorbed in turbine/generator shaft windup.

You are right -- it can't -- the power imbalance in this case would result in a power transmission fault and the affected generator will automatically be pulled offline. As you can imagine there is a potential for a cascade problem -- a few generators trip, the rest slow down due to undercapacity, more trip, more slow down, ... this is why load shedding and fault isolation measures have to happen extremely rapidly with the power transmission control system.

Having said all that, I'm not a electrical guy. But my first reaction when someone says the grid frequency varies is there's a problem with the measuring equipment or method.

The typical handheld frequency/voltage/distortion measurement device lacks sufficient measurement accuracy to make an valid assessment of grid frequency. For example, a (not inexpensive) Fluke 43B Single Phase Power Quality Analyzer (Power Quality Analyzer, Power Quality Analyzers, Single Phase Power Analyzer specifies "0.5% + 2 counts" as the base frequency measurement accuracy. Using this type of device to measure power line frequency will not give you a good indication of the actual line frequency -- short term measurement repeatability (precision) will be good, but the accuracy is not sufficient to make a statement of merit.

In the lab at work we use a 10MHz/1pps feed from a cesium (atomic) reference clock (it's about 1 part in 10e14 accurate) as the timebase for a TIE (time interval error) analyzer, which produces Allan deviation plots similar to the one pictured below -- which incidentally is an Allan deviation plot for the 60Hz mains. As you can see, for "medium term" durations, say 2 minutes, the power line frequency is held to better than 10e-4.

A small (Pout <= 20KW) , mechanically-governed, gasoline- (or diesel-) engine powered portable generator can not match the above noted utility line frequency performance to any degree; there is simply not enough rotational inertia nor a PLL-like external control mechanism which self-governs the generator speed and therefore the frequency. This effect is compounded with the introduction of dynamic loads -- a step-function load of ~50% on a small generator will typically cause an easily detectable (>0.25Hz) phase angle to nominal 60Hz.

Wrooster

 

[955Lincoln]

But my first reaction when someone says the grid frequency varies is there's a problem with the measuring equipment or method.

An accurate statement. Why is it that most of the threads on this forum end up bursting with TMI

 

[tsteahr]

Wrooster,

Thanks for taking the time to write that up. Very interesting.

 

[Haywire]

Now the real question is this... how stable does the frequency and voltage have to be to run the welder satisfactorily and without risk of damage to the components? It doesn't matter if the power is stable to +/- .0000001% if the welder only requires stability to +/- 1.0%.

Ian

 

[Redneck in training]

In fact there is only one generator in the whole American power system that controls the frequency. All the other run in "droop". In other words their contribution to grid is proportional to the grid frequency. If the grid is slower they contribute more power and vice verse. Therefore the grid might run slightly slower at high load and slightly faster at low load. The number of periods/per day is maintained exactly the same every day.