Generator Excitation Power

[PowerOut]

Excitation power (kW) on some generators can be calculated as the product of field voltage and current divided by an assumed factor for AC to DC conversion element losses and 1000 W/kW, and sometimes amounts to several hundred kW.

When the generator in question contains a brushless excitation system however, the calculation of excitation power is negligible.

What are the phyiscal features of the excitation components on the first machine that produce the appreciable excitation power, and what are the features of the excitation components on the second machine (with brushless exciters) that produces negligible excitation power?

 

[waross]

Similar generators will require similar excitation power regardless of how the power is developed.
At one time, telephone takeout gensets were making their way to Central America where many were being used in fishing boats and lobster boats to power refrigeration. These sets were typically excited by DC generators mounted on the outboard end of the shaft. There would be a commutator and brushes to pull out the DC. Then there was a set of brushes and slip rings to feed the DC back in to power the field.
As the exciter was shaft mounted, the energy was supplied directly by the prime mover.
All those brushes were a headache in a marine environment.
What came to be a common upgrade was to tap the windings of the DC exciter in three or more places. This would be an AC source. Rotating diodes would be installed and the resulting DC fed directly to the main field. The next development came when the rewind shops replaced the exciter windings with a winding optimized for three phase power and removed the commutator and slip rings completely.
The point is, the main field remained the same and the energy was still provided by the prime mover.
These machines ranged from about 20 kW to several hundred kW.
In the larger machines that you are concerned with, the main field losses will be about the same, even though they may be supplied by a static exciter, by a DC exciter or by a brushless exciter.
The excitation demands may be more difficult to measure or calculate when they are supplied by the prime mover. As ScottyUK pointed out in a previous thread, in a large machine you must still burn fuel to supply the excitation losses.

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

 

[dpc]

Homework problem?

 

[edison123]

Powerout

The excitation power, (as you probably took from the nameplate) in a brushless system, is actually the small DC field power for the AC exciter, which produces the real DC field power for the generator rotor. This power indeed comes from the prime mover.

As waross says, all generators require a fixed amount of excitation power, regardless of what mode of excitation is chosen - DC generator, Brushless ac exciter or static excitation.

 

[wolf39]

Bill is right.

If a generator requires a certain field current for generating a specific output it doesn't matter whether this current is coming from an old fashioned DC exciter machine, a modern static excitation system or a brushless excitation equipment. Brushless exciters are mainly used for large turbo generators and occasionally for high speed hydro generators. A certain disadvantage of brushless excitation systems is the slow response time of the field in case fast load changes are specified.

Regards

Wolf

http://www.hydropower-consult.com

 

[PowerOut]

Thanks for the insight gents, 3 out of 4 ain't bad.

GE Frame 7E's and smaller use brushless excitation, whereas larger F technologies and above don't, at a 9FA (~270 MW) site in Abu Dhabi, over 900 kW parasitic load was pulled off upstream of the guarantee test power output meter location for excitation.

Now I want to know where the manufacturer draws the line regarding what is economically feasible for brushless versus other excitation systems as a function of generator size.

 

[edison123]

It is a designer's privilege. However, brushless exciters will be restricted by the availability of diodes carrying high currents and by the space for fixing multiple parallel diodes in a rotor.

No such restrictions for DC generator or static excitation.

 

[waross]

Designing an excitation system is an exercise in compromises.
I don't know why GE changes from brushless to static at a particular size. If the machine you mentioned was coverted to brushless, you would no longer see the 900kW on the meters but you would still burn the same amount of fuel. On diesel (because I happen to know a typical figure for diesel and don't have to look it up) your 900kW will cost you about 1500 US gallons per day of diesel.

Now I want to know where the manufacturer draws the line regarding what is economically feasible for brushless versus other excitation systems as a function of generator size.

Design compromises, but the line seems to be moving slowly upwards over the years.
Faster response with static, less maintenance with brushless.

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

 

[rmw]

All brushless exciters aren't created equal either. Some AVR's only have the capability to drive the excitation current in one direction-they can only drive it up.

rmw

 

[PowerOut]

Great answers:
-No restrictions (availability of diodes carrying high currents... or ...space for fixing multiple parallel diodes in a rotor) for DC generator or static excitation,
and
-Faster response with static, less maintenance with brushless

The comment: "you would no longer see the 900kW on the meters but you would still burn the same amount of fuel", raises another question:

Two identical generators, one with and one without brushless excitation, which machine has greater output at identical test conditions? Is one type of excitation system advantageous to the OEM purely from the perspective of output?

 

[edison123]

Both brushless and brushed, dc-generator-excited generators have the same output since the excitation power comes from the prime mover and not from the generator per se.

In the case of static excitation fed from the generator terminals, the excitation power comes from the generator (which in turn comes from the prime mover) and so it will have a lesser usable output.

Ultimately, all exciter power has to come from the prime mover.

 

[wolf39]

In case the utility is specifying brushless excitation in order to save maintenance costs, such equipment can be built, of course, for low-speed hydro generators as well. I've found in the internet an application for a 90 Megawatt hydro generator with a rated speed of 125 rpm only.

The main application for brushless excitation is in the field of large turbo-generators (up to 1,700 Megawatts). The AC exciter machine has to deliver excitation currents of 10,000 Amps, and even more. As edison123 pointed out already, the excitation power required for exciting the AC exciter machine is quite small. This propably led PowerOut to believe that the generator excitation power is negligible but he mixed up both figures. The prime mover (turbine, for instance) has to supply the full output of the AC exciter machine.

Brushless excitation is also applied in the lower output range of generators installed in industrial plants, where atmospheric conditions may prohibit the use of collector rings and carbon brushes. The same can be the case for hydro generators installed in air conditioned underground caverns (low air humidity).

Brushless excitation can be provided for the complete generator power output range. Whether this is economical for all applications, that's another question.

Regards

Wolf

http://www.hydropower-consult.com

 

[edison123]

wolf

About the brushless system for hydro machines, how do you get field suppresion/field forcing during faults ? Especially, in hydro machines, where over-speeding on fault trips is always there.

One thing, I don't like about brushless excitation is that one cannot directly measure/read the acutal main field current.

 

[wolf39]

edison:

Of course you can de-excite (field suppress) a hydro unit equipped with a brushless excitation system in case of a load rejection. Admittedly it takes time until a response is noticible and a voltage rise to about 120% rated voltage may be the result. However, load rejections are quite rare (at least in the countries where I worked) and 20% overvoltage for a few seconds duration should pose no problem for a well designed generator.

Excitation currents can be determined by use of a permanently installed shunt resistor. The voltage drop then is measured with the help of temporarily installed slip rings (collector rings) during commissioning. This is especially important when excitation losses have to be determined for efficiency calculation at various load points. At a later stage these slip rings will be removed.

If you don't like to use slip rings, you can temporarily attach an electronic milli voltmeter to the shaft and record the voltage drop from the instruments scale by use of a stroboscope or by using a digital camera. It's not that important to know the exact excitation current at any given time and load. What could be done during commissioning, however, is to determine the characteristic of the hydro generator DC excitation current over the excitation current of the AC exciter machine with one of the methods described above.

Regards

Wolf

http://www.hydropower-consult.com

 

[rcw retired EE]

A GE generator engineer told me years ago that the usual brushless exciter created bearing problems on the larger machines. The rotating AC field and diodes were overhung on the end of the generator shaft, extending out past the main generator bearing with no support bearing on the outside end of the exciter.

A large generator required a large (heavy) field winding and diode assembly that created mechanical design problems. Adding another bearing support point raised some issues also.

Switching to a static exciter and collector rings avoided the mechanical issues.

Also, utilities required most large generators to have fast acting voltage regulation. The inherent delay of the brushless system and the inability to reverse the field voltage was not acceptable to many utilities. New, large generators had to have static exciters, brushes and collector rings.

 

[edison123]

Thank you wolf and wilson.

I agree static excitation is the way today given many drawbacks to the brushless system.