MV generator earthed at switchboard 1

[ag2508]

Hi

We have 2 9MVA 11kV generators connected to an MV switchboard. The system has a high resistance earthing at the switchboard. Main concern is that the generator will be running unearthed before it is connected to the switchboard. Would that be acceptable (IEC), and could that affect generators synchronization.

Thanks

 

[waross]

I am not familiar with IEC rules but it should not affect synchronization. (As long as the sync circuits are properly wired and do not rely on a ground connection. That would be bad practice in any event.) The sync PTs should always be connected line to line or line to neutral, never line to ground. Solidly grounded generators may have a switch to open the ground connection. Sync PTs should be connected on the neutral side of the switch, never on the ground side.

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

 

[dpc]

I agree with Bill - synchronizing should not be an issue. You could provide ground fault detection (three voltage transformers connected broken delta to a voltage relay) to detect generator ground fault prior to synchronizing. I have no idea regarding any specific IEC requirements.

 

[odlanor]

waross
"Sync PTs should be connected on the neutral side of the switch, never on the ground side."



Normally neutral side of each monophase PT are connected direct to grounding grid of power house;- this is the neutral of the star connexion of PTs. So, PTs will be connected to neutral independent of switch position.

Where is ground ? The ground is the remote ground of grounding grid.

 

[waross]

These are synchronizing PTs. If the system is running with a ground on the phase that the sync PT is connected to and the PT is connected to ground instead of to the neutral then the PT will not produce enough voltage for the synchroscope to operate.

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

 

[odlanor]

waross,
That 's my opinion:

1- ... generator will be running unearthed before it is connected to the switchboard...
It means during sinchronization at generator side voltage line-neutral is floating!!
It is very hard to synchronize in this condition; a connectiion line-to-line is a must!

2- Usually a power plant has a grounding grid extended from power house up to substation, or two grounding grids interconnected.
Neutral side of each monophase PT are connected direct to grounding grid ; There is no drop voltage between these connexions ;- this is the neutral of the star connexion of PTs.
Where is ground ? The ground is the remote ground of grounding grid.

 

[squeeky]

Not having an earth or earthed star point during start up means that you won't have any earth fault protection until you are synchronised. Analyse you risk and how long it takes. If you do develop a fault, then you will switch into the utility with a bang!

 

[waross]

Synchronizing PTs must be connected the same way at the buss and at the generator. If the bus PTs are line to line then the generator PTs must be line to line. If the bus PTs are connected line to neutral the generator PTs must be connected line to neutral. On an impedance grounded system a PT connection to ground may not reflect the correct phase angle.
System protection PTs may be connected line to ground but if there is impedance grounding then the synchronization PTs may not use a ground connection.

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

 

[odlanor]

squeeky,
- We're not talking about protection, which is another issue;
- I know perfectly the meaning of bad synchronization;- it is not uncommon in Brazil, mainly during commissioning.

waross,
- Synchronizing PTs must be connected the same way at the buss and at the generator.....
-Yes . For synchronization both side must be connected PT line to line.

- System protection PTs may be connected line to ground but if there is impedance grounding then the synchronization PTs may not use a ground connection....
-You did not understand! There is no influence of impedance grounding during synchronization with generator circuit breaker open. In that condition generator is ungrounded.

Do you know that?
I decided to use my old annnotations and analyze this problem in detail, as follows:

1- Out-of-phase synchronization comparison

generator three-phase short-circuit
Isc3g = Eg / ( Xg)

out-of-phase synchronizaton current at local of generator circuit breaker closing
Ioop = (Es/0 – Eg/180 ) / ( Xg + Xs)

Assume Es=Eg=1 e Xs = 0 (infinite bus) all in pu same MVA- kV bases we get:
Ioop = 2/(Xg) or Ioop= 2* Isc3g

.Highest torque current:
Ioop(60º) 1,0 * Isc3g
Ioop(120º) 1,732 * Isc3g
Ioop(180º). 2,0 * Isc3g


2- Out-of-phase synchronization damage
The three-phase fault current can be applied as the maximum withstand limit when assessing generator winding durability for an out-of-phase event. This is because windings are subject to deformation that is directly related to the current magnitude alone. Unfortunately, the fault current criteria is not applicable to the mechanical system. There is no direct relationship between the magnitude of stator current and torque imposed on the shaft by electrical system transients.

However we can say:
-Ioop(120º). Magnetic forces created by high synchronizing current can deforms the laminate and are transmitted to the housing legs of hydrogenerator or turbogenerator;

-Ioop(180º). Magnetic forces created by high synchronizing current can loosen windings of hydrogenerator or turbogenerator;

Ioop(120-180) for turbogenerators(mainly)
Torsional stress imposed on the mechanical system can cause slippage of couplings, bearing misalignment, and fatigue damage to the shaft and turbine blades. Increased shaft vibration following and out-of-phase closure or any other torsional event is a symptom of coupling or bearing movement.

Reference: Protective Relaying for Power Generation Systems - Donald Reimert

3- Synchronizing limits
I introduced this item only complement my reasoning. IEEE C50.12 – 2005 define synchronizing limits:
4.2.2 Synchronizing
Generators shall be designed to be fit for service without inspection or repair after synchronizing that is within the limits listed in Table 2.
—Synchronizing limits
Breaker closing angle ±10º
Generator side voltage relative to system 0% to +5%
Frequency difference ±0.067 Hz.
Additional information on synchronizing practices can be found in IEEE Std C37.102™-1995 .

4- Conclusion
There is no influence of impedance grounding during synchronization because system is assumed balanced and bad synchronization results in three-phase short-circuits.

Assume protection and check synchronism did not work; try paralleling generator and system that are out of phase.
Generator and system will get themselves in phase very quickly. The stress on the prime mover/drive train is something else;- damage of item 2 can occur months after;

You can choose synchronization with PT voltage line-to-neutral at both side, although at generator side PT voltage is floating;
Perhaps automatic synchronization will take 2 hours; manual synchronization will take 20min (without check synchronism).

Faster synchronization you'll get with PT voltage line-to-line at both side.

 

[squeeky]

Odlanor, I would never drive a car without break lights. I would appricate it if someone told me they were not working. But understood.

 

[waross]

The basic synchroscope uses only one PT per generator and one PT for the bus.

You can choose synchronization with PT voltage line-to-neutral at both side, although at generator side PT voltage is floating;
Perhaps automatic synchronization will take 2 hours; manual synchronization will take 20min (without check synchronism).

I don't understand this. I have never had a problem sync'ing sets when the PTs were connected line to neutral.

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

 

[odlanor]

waross ,
I think this is associated with the imperfections of the generator. The more MVA more imbalance and leakage currents. My experience is with hydrogenerators of 80 MVA and above.

 

[waross]

Are you getting a distorted wave form from the unloaded GSU transformers? I have seen badly distorted wave forms on unloaded wye:wye transformer banks. A similar issue could cause issues when trying to sync a genset.

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

 

[odlanor]

No. System side(Bus running) phase sequence is almost static ; generator side(Bus incoming) phase sequence is always running but can be controlled easily if generator neutral is grounded and hardly if generator neutral is full ungrounded.
I confess I've never done a survey on this subject: - why does generator side voltage PT keep floating in ungrounded generator?