Motor generator synchronising and load sharing 1

[Rodmcm]

I am told that you have to match angular shaft position of the incoming generator to allow synchronising. How do you do this?. If two or three identical units are on line how do you get them to share KW? Both motors and generators are synchronous and used for phase changing.

 

[Rodmcm]

That should be frequency changing, not phase changing

 

[waross]

This is a special case of parallel operation. Droop control is not applicable with synchronous motors. The shafts must match angular position with each other.
One solution is to fit a micro adjustable coupling to one of the motor generator shafts.
The units are both run up, and the output phasing of the two sets is compared on a dual trace oscilloscope or a synchroscope. The MG sets are then stopped and the coupling is adjusted. The process is repeated until the units match phases on the 'scope. If a synchroscope is used, the coupling must be adjusted until the pointer is at the 12:00 O-clock position.
With frequency changing you may not yet be out of the woods. Depending on the number of poles on the synchronous motors and the synchronous generators, there may be some possible synchronous positions which are not suitable for paralleling. This will be evident on the dual trace scope, or will be indicated by the synchroscope by the pointer being stable at some position other than 12:00 O-clock.
In this case you must remove excitation from the synchronous motor long enough for the motor to slip a pole.
This may be accomplished by fitting a relay that is ealed in by a set of its own N/O contacts. A set of N/C contacts will remove field excitation. The relay should be powered from the output from the polarized field frequency relay. As the machine starts to slip out of synchronous position, the polarized field frequency relay will attempt to remove excitation. This will de-energize the new relay. As the next set of poles line up, the polarized field frequency relay will re-apply the excitation.
Adjusting the coupling should need only to be done once.
Checking position and possibly slipping poles, if required, may have to be done at every start and synchronization.
Load sharing is a function of the load angle of the synchronous motors. You may be able to affect load sharing by adjusting the excitation level but this will then effect VAR sharing and current sharing. Note, load sharing may not be the same as current sharing.
If you have dis-similar machines and load sharing is an issue, you may be able to set up load sharing at one load level by adjusting the coupling until the load is properly shared at that load level. You will then probably suffer circulating currents at any other load level.
If you are using similar machines they may inherently share the load well.
respectfully

 

[Rodmcm]

Thats great Waross, but can you explain or give a manufacuturers no for a 'polarised field frequency relay'

 

[waross]

The polarized field frequency relay is part of the control circuit for a synchronous motor. Typically a synchronous motor is started with the field de-energised. The polarized field frequency relay is a device that monitors the E.M.F. produced in the field winding. It is sensitive to the frequency, magnitude and polarity of the induced field voltage.
The polarized field frequency relay applies the field when the speed, (as determined by the frequency) the voltage and the relative shaft angle are suitable for the motor to pull into step.
In newer motor starters the electro-mechanical field frequency relay may be replaced with an electronic module that preforms the same function.
In any event be sure that the field discharge resistor is in the circuit while the field supply is disconnected.
With a brushless synchronous motor, the electronic field application circuit will installed on the rotor and will not be accessible.
Hopefully some other posters may suggest a method of inducing a pole slip in brushless synchronous motors.
respectfully

 

[waross]

The Polarized Field Frequency Relay was originally developed and patented by the Electric Machinery Company.

http://www.electricmachinery.com

They do not list their PFFR on their web site. They now use a rotating, micro-proccessor controlled device to apply the field in their synchronous motors.
I suggest doing some homework and looking for synchronous motor starting schemes and PFFRs in old text books before calling EMC.
Also determine how the field is applied on your synchronous motors, and whether they have sliprings and brushes or are a brushless design.
respectfully

 

[Rodmcm]

Best reference I have found

http://www.tecowestinghouse.com/Products/Custom_Engineered/Synchr_Brushless.html

 

[glendale]

The standard way that frequency changers are paralleled is to enstall stator racking gear on the generators. This allows the stator to be rotated by say +/- 30 mechanical degrees or one complete pole pitch. So regardless of how each synchronous motor pulls in to synch with the supply the generator phase relationship can be matched with its supply by advancing or retarding the stator position. Once in synch the breaker can be closed, then by advancing the stator position load can be taken up on the incoming machine.

This is a standard method for frequency changers of say 2MVA and above. We have 8 3MVA machines on our site.

GDF

 

[motorspert]

glendale is correct the only we to load share is to stator racking (or rocking) gear on the generators - doesnt have to be the generator but usually less mechabil hassle havi g it there.
The motor synchronising relay sounds nice, but in reality doesn't add any value, just complexity. certainly it does not help with the load sharing.
If your MG sets are already installed and there is no means of rotating the stator - then you have a real challenge! Normally, single sets are provided like this, or sets that run in parallel with diesel sets.

 

[Rodmcm]

For those interested the supplier does provide an adjustable coupling to set the phase displacement and uses pole slipping by engaging/disengaging the motor excitation to synchronise the incoming with the running unit. They have stated that with two identical units the load sharing will be within 2.5% once both on line.