Using y-y generator transformer for step up and step down

[Bugman1400]

Bugman,


I disagree! There will always be a circulating-current even if grounding-connections are separated! Even if the generators are "identical!" Circulating-current magnitude is based on the relative differences in generator parameters and operating conditions!


Regards, Phil Corso

If the grounding is impedance grounding, the circulating current between the generators are minimal. What sequence current are you referring to for circulating current?

 

[Phil Corso]

Bugman,


Ignoring obvious physical parameters (eg, winding-pitch) circulating-current, Ic, magnitude is related to the difference between the airgap-voltages, Egp1 and Egp2, divided by 3x(Rn1-Rn2), ignoring seq impedances! Thus, even a slight difference in AVR action will produce a circulating-current!


For one generator, Ic is added, vectorially, to the gen's stator-current, while for the other it's subtracted! However, I do accept that it's impact is nil!


Regards, Phil

 

[Bugman1400]

What kind of circulating current would you get if the two units were attached to the same bus but, only one is grounded and the other unit is not?

 

[denvar]

8.) The 30 deg shift is not a problem if you are just supplying plant load unless there is an alternate source to the same load. In this case you need to verify the phase shift is the same or use break-before-make ATS for the transfer.

Two different generators should never share a common ground! This would cause I2 circulating current and result in overheating of the generator windings.

thank you Bugman and Phil This is what i have garnered from the discussion:

• The best option is to use a 1:1 d-yn(load side) isolating transformer for the local load and a D-Y(HV side) step up transformer.
• Due to the high current rating ~800A for the first generator i believe its fault current would be high hence need for low impedance grounding to minimize stator current in case of a fault.
• The bigger generator neutral should be earthed permanently while the small generator will only be earthed (grounded) when not synchronized to the busbar or when the bigger generator is not synchronized to the busbar hence i will use a neutral earthing contractor for smaller generator.
• Am not sure how the AVR should be configured to minimize reactive power sinking from generator to another as they share a bus bar close to its terminals since the AVR's might not be from the sane manufacturer.

Kindly correct me if am wrong.
Also kindly advise would using a generator with a lower power factor eg 0.8 instead of 0.85 help boost feeder voltages during peak when voltages fall slightly bellow under voltage limits?

 

[denvar]

Bugman,


I disagree! There will always be a circulating-current even if grounding-connections are separated! Even if the generators are "identical!" Circulating-current magnitude is based on the relative differences in generator parameters and operating conditions!


Regards, Phil Corso

thank you Bugman and Phil This is what i have garnered from the discussion:

• The best option is to use a 1:1 d-yn(load side) isolating transformer for the local load and a D-Y(HV side) step up transformer.
• Due to the high current rating ~800A for the first generator i believe its fault current would be high hence need for low impedance grounding to minimize stator current in case of a fault.
• The bigger generator neutral should be earthed permanently while the small generator will only be earthed (grounded) when not synchronized to the busbar or when the bigger generator is not synchronized to the busbar hence i will use a neutral earthing contractor for smaller generator.
• Am not sure how the AVR should be configured to minimize reactive power sinking from generator to another as they share a bus bar close to its terminals since the AVR's might not be from the sane manufacturer.

Kindly correct me if am wrong.
Also kindly advise would using a generator with a lower power factor eg 0.8 instead of 0.85 help boost feeder voltages during peak when voltages fall slightly bellow under voltage limits?

 

[Bugman1400]

I contend that you can ground both generators if impedance ground is used on both. However, you can also run un-grounded too. If you use impedance grounding or no-grounding, you'll need OV protection on the generator bus to detect ground faults.
If both generators are synchronous then, you should be able to set both AVRs to the same bus voltage. You'll need to this to avoid excess I2 current.
If you are having voltage issues at the load then you need to re-visit your voltage schedule. Perhaps running a higher bus voltage during peak loading would help. Obviously, a lot depends on the load. If its motors then you need caps. If its mostly resistive then, the higher bus voltage should work.

 

[denvar]

I contend that you can ground both generators if impedance ground is used on both. However, you can also run un-grounded too. If you use impedance grounding or no-grounding, you'll need OV protection on the generator bus to detect ground faults.
If both generators are synchronous then, you should be able to set both AVRs to the same bus voltage. You'll need to this to avoid excess I2 current.
If you are having voltage issues at the load then you need to re-visit your voltage schedule. Perhaps running a higher bus voltage during peak loading would help. Obviously, a lot depends on the load. If its motors then you need caps. If its mostly resistive then, the higher bus voltage should work.

Thank you bugman. i meant boosting the utility feeder voltages (11KV side) not the local loads

 

[Bugman1400]

Thank you bugman. i meant boosting the utility feeder voltages (11KV side) not the local loads

Same principle applies. You can increase the excitation on the generators to produce more VARs and raise the voltage but, you'll need to ensure its not above 1.05pu. However, it may be better to change the NLTC on the GSU if you have one. Additionally, if the drag on the 11kV voltage is due to reactive losses then you can add distribution caps fairly cheaply since caps at that voltage level are common. If the drag on the 11kV voltage is due to mostly resistive losses then there isn't much you can do other than add some type of additional distribution regulator.

 

[denvar]

Same principle applies. You can increase the excitation on the generators to produce more VARs and raise the voltage but, you'll need to ensure its not above 1.05pu. However, it may be better to change the NLTC on the GSU if you have one. Additionally, if the drag on the 11kV voltage is due to reactive losses then you can add distribution caps fairly cheaply since caps at that voltage level are common. If the drag on the 11kV voltage is due to mostly resistive losses then there isn't much you can do other than add some type of additional distribution regulator.

Thank you so i guess its better to use 0.8pf as it supplies more KVAR as the voltage variation is not so bad to warrant another regulator eg OLTC.

Would it be a better idea permanently ground (solid ground) the bigger generator and only ground the small generator when not synchronized to the busbar or when the bigger generator is not synchronized to the busbar hence i will use a neutral earthing contractor for smaller generator?

 

[Bugman1400]

Thank you so i guess its better to use 0.8pf as it supplies more KVAR as the voltage variation is not so bad to warrant another regulator eg OLTC.

Would it be a better idea permanently ground (solid ground) the bigger generator and only ground the small generator when not synchronized to the busbar or when the bigger generator is not synchronized to the busbar hence i will use a neutral earthing contractor for smaller generator?

I keep forgetting these are low voltage and small generators. I don't think the grounding method will matter much. I would start with a solid ground on the larger unit. This will allow sufficient ground fault current to trip the relays should anything occur. However, you will not have that current if the unit is offline. Eventually, when you get both units online, I would check the I2 current circulating between the units. If its higher than expected then, you may want to go with a different grounding method. At this voltage level, the cost will be minimal.

If you have a large daily or seasonal swings of voltage on the 11kV side then you may be better off with an OLTC instead of switching caps in/out all the time. This will also allow your generator bus voltage to be more even. The amount of kvar that a synchronous generator produces or consumes is dictated by the capability curve. If you have an induction generator(s) then, the pf is the indication of how much the unit consumes only.