Cross Compound Generators' - Why is only one grounded? 1

[Soloten]

First, as per the attached picture in the link, a cross-compound unit with two generators is shown, and one is shown to rotate at 3600 rpm, while the other is at 1800 rpm.

1. Does the number of poles vary in each generator so as to produce the same frequency?

2. Why is it recommended (from various sources I read) that only one of the two Low-pressure or High-pressure generator units be grounded and not both?

 

[crshears]

Basic synchronous machine theory: when connected to a nominal 60 Hz grid, a 60 Hz machine with one set of stator poles per phase will rotate @ ~ 3600 rpm, & a 60 Hz machine with two sets of stator poles per phase will rotate @ ~ 1800 rpm.

To my understanding, only one machine is grounded so as to preclude issues with circulating current, although others will undoubtedly be able to explain it in much greater detail...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[Soloten]

Why is this an issue with Cross-compound units specifically, since I didn't see the recommendation of grounding only one unit in other type of parallel generating units?

 

[crshears]

Cross-compound steam-turbine-driven units have the stator windings of the higher RPM and lower RPM alternators galvanically connected together onto the LV winding of their main output transformer, and therefore electrically act as one. A fault detected in one of the machines would drop the appropriate relay flags for that machine but the entire unit would trip off due to its being one electrical zone.

The only other place I have seen this sort of thing done is in a small hydro-electric generating station where three units of 600 kW each were connected to a common bus before being tied to the LV winding of the step-up transformer, except that here it was possible to shut down the plant, open a disconnect for one unit, and re-synch the remaining two.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[MatthewDB]

If the generators are wound with a different pole pitch*, you do not want to 4 wire parallel them. Different pole pitches result in different harmonic spectra of line-neutral voltage distortion. This will cause a circulating harmonic current if the generators are paralleled including the neutral. The distortion (mostly) cancels out on the line-line voltage, so the effect of paralleling them in three wire results in much lower circulating current.

Typically when generators are paralleled in 4 wire, they are identical generators so they produce the same harmonic spectra. IF the 1800 RPM and 3600 RPM generators have a different pole pitch this may be why they only run one of the two machine's neutral to the grounding resistor. Unlike paralleling multiple units, a cross compound is run as one machine. Both generators are always interconnected, so grounding one meets the need of establishing a neutral reference.

*pole pitch is how many stator slots there are per magnetic pole.