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.
