Parelling using wye and open delta PT

[PDI2011]

Hi I need help from the expert. We are paralleling two genset in island mode with the same ratings,2kW 2400V, 3P, 60Hz. One genset (GA) uses open delta PT and
the other genset (GB) is using wye. If the common bus uses wye connected PT, what will be the phase shift if we sync GA
to the Bus? I learned that there is a 30 deg shift. Is it positive or negative 30 deg?

 

[davidbeach]

Yes, it is either a 30 degree + or a 30 degree - phase shift. You need to draw out the vectors of each and see. No way of telling without the drawings. Could also be some other odd multiple of 30 degrees. It all just depends.

 

[PDI2011]

Hi David thanks, I will ask someone to check on site. Another thing, should I got the connection drawing which will be the reference voltage. The genset or the bus?

 

[davidbeach]

Yes, one will be the reference bus. Depends on how reference is defined. Could be either. Could be some conditions when one is the reference and other conditions for which the other is the reference. No mind readers here.

 

[PDI2011]

Hi David, sorry for the confusion. I was referring to the reference in getting the phase displacement. I was thinking since the we will sync with the bus, it should be the reference in getting the phase shift. I attached herewith the wiring diagram of the later. The supplier was planning to test the system by using a + 30 deg phase compensation . What would be the danger in paralleling it to the bus if the needed phase shift is a negative 30 deg.

 

[waross]

I ran across a situation where five generators were paralleled as the load increased. Three used line to line PTs and two used line to neutral PTs. The breaker on the smaller set often tripped several times before a successful connection was made. About once a year a coupling would shear the key stock.
Once a successful connection was made the synchro-scope would jump to 2:00 o-clock.
The plant had been running that way for years before I became involved.
This was an error of 30 degrees.
I would suggest identical PTs, all connected either in delta or in star, as long as all the connections are the same. Use the bus as the reference voltage.
If the phase shift is in the wrong direction you will have an error of 60 degrees. Expect severe electrical and mechanical damage.
I assume that you mean 2 MW. 2kW is on the small side for a 2400 Volt set.


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

 

[rcw retired EE]

There is no phase shift on the phase-to-phase voltages. The voltage at terminals V11-V12 on your synchronizer is in phase with the voltage on V21-V22, even though the one VT is wye and the other is open delta. Phase-phase voltages are the same.

But you need to verify that the synchscope, the synch check relay and the synchronizer all can work with different phase-to-ground voltages on the secondary. If the open delta VT has B phase grounded as is normally done, the A & C inputs will be 120V phase to ground (assuming the VT ratio gives 120V phase-phase). The wye-wye connected VT's will have 120 V phase-phase but 67V phase to ground.

The synch equipment has to monitor and control to phase-phase voltages, not phase-neutral.

As long as your relays and wiring isolate the inputs and don't try to use a common "neutral" or ground reference point it will work. If the wiring has one side of the synchscope and synchroniser common to both VT inputs, you will blow fuses on B phase. Insert a 120:120V isolating VT if that is the case.

If you do put an intentional 30 degree phase shift in the VT circuit, you will synch 30 degrees out of phase and possibly damage your generator or worse.

We have successfully used the wye-wye VT's on the bus and open delta VT's on the generator many times. It is a standard design for the GE 7F gas turbines.

 

[PDI2011]

Thank you guys for your response:

Waross, "I assume that you mean 2 MW. 2kW is on the small side for a 2400 Volt set."
That is correct. It is a 2MW genset, I agree the final solution is to replace the open delta PT with Wye-wye
however the supplier insist that the controller is capable of handling this kind of PT configuration. I attached
herewith the setup they use as example.

rcwilson. It was also my opinion that there is no phase shift since the bus is
supplied by identical generating set with only difference of having a wye-wye PT.
In my opinion introducing a 30deg phase shift is not necessary. As you suggest we will check it with an oscilloscope.

I will revert soon we have tried your recommendation. again many thanks Guys...