DESCRIPTION OF INCIDENT:
On dated: 13-06-2014 at about 09:53 Hrs. when the unit # 4 (210 MW Steam Turbine Generator) was generating 155 MW load, a sudden complaint was reported regarding badly leaking of Hydrogen Gas from inlet flange of Hydrogen Line, situated at the bottom of Generator body. At once, the unit was stopped in emergency at 09:59 Hrs. & started to evacuate H2 from Generator to carryout maintenance work at required location. After attending the said job by M.E(Turbine) section, the same Unit #4 was again started to get it on load on dated: 14-06-2014. During the preparation of unit, it was revealed that the H2 gas pressure was not being increased above 1.65 Kg / cm2, even that the H2 pressure at Hydrogen station was recorded around 8 Kg / cm2 at that time.
In that situation, after performing several steps the local management decided to continue the start-up procedure of the Unit #4. In this connection, the T/B was rolled up to 3000 rpm & then proceeded for synchronizing the Machine by building the Generator terminal voltage.
While increasing the voltage up to 15.7 KV, at about 21:16 Hrs. a heavy sound was heard & smoke was seen from the Generator’s outgoing C.Ts Duct. At the same time, following protections were found operated and Indication were appeared:
Protections:
 Generator Differential
 Generator - Transformer Differential
 Generator Backup Reactance

Indications:
 Signal Relay not reset
 Generator loss of Cooling Water
 MK Breaker in open position
Due to all above, the process of unit synchronization was stopped & all the activities were reversed back to their position of Shutdown.
Following readings were recorded in ION Energy meterat the time of incident .
Ia= 18,182.59 Ib= 32,527.29 Ic=14,446.37
Va=15.71 KV Vb=15.73 KV Vc=15.75 KV
P=15 MW, Q=7.5 MVAR , S=16.5 MVA
where as following is
GENERATOR DATA:
Type: Q F S N 210-2
Q(Turbine) F(Generator) S(Water Cooling) N(Internal H2 Cooling) 210 (Capacity in MW) 2No. of Poles
Rated Capacity: 247,000 KVA
Active Power: 210,000 Kw
Rated Voltage: 15.75 Kv
Rated Current: 9,056 A
Rated Speed: 3,000 Rpm
Conn: of Stator Winding: 2-Y
Rated H2 Press: 3 Kg / Cm2
Total Wt: of Generator: 275 Tonn.

c) THE COOLING & SEALING SYSTEM OF GENERATOR

• The QFNS 210-2 type of Turbo-Generator is designed with the Water-Hydrogen Cooling System. In this system, stator winding is water cooled, whereas Rotor Winding & Stator core are H2 cooled. The exterior gas control system ensures H2 filling, its replacement and maintaining of pressure. In order to seal the H2 inside the Generator, seal rings are introduced. The seal oil control system ensures the oil flow, oil temperature & oil pressure.

• The normal Hydrogen working pressure of this type of Generator is 3Kg / cm2. The H2 comes from H2 storage station. In order to monitor H2 pressure, different pressure gauges are equipped with the piping of H2 circuit.

Oh, that's sad accident, differential protection normally comes in operation on trouble somewhere around the generator. assuming that's the sequence of protection signals appeared on the data logger of generator, I would go for preliminary testing of generator stator windings. Also, it was a bad step taken to start generator under low hydrogen gas pressure, the thermal balance of rotor in excitation mode is very critical. The vibration behaviour of rotor during all this process should be checked against the normal start. Temperature of rotor should be calculated based on cold gas temperature.. Hope nothing serious would have happened, we are in electricity crises already :-(

Please keep us updated as you progress through this very expensive repair operation. In particular it could be interesting to learn if you can get your stator winding repairs carried out on-site, or if you must ship the whole stator to the manufacturer for repairs.

Did you relay happen to record the angles of the currents and voltages? Also, were you able to view the operate/restraint currents of the differential relay? I am not familiar with the events that an ION meter records, but if you have a SEL, Beckwith, or GE relay the events will be much more useful than the ION meter information. The event data from the protective relay will make it much easier to figure out where the problem may have originated from. -Jeff

This appears to me to be a case of winding overheating. Hydrogen pressure was not sufficient and there was no enough cooling so the windings got overheated and burnt. Management should have done a risk analysis to understand why hydrogen pressure did not build up after repair work instead of giving instruction to continue with synchronistion. I think the repair work must have caused a blockage or valve misalignment that prevented hydrogen circulation.

Hello Members.

I don't believe overheating could be a cause over such a short time in the re-commissioning synchronisation process, but I most certainly agree about the questionable decision to restart with less than 2 kg/m^3 of H2 - a definite indication of a leak that was STILL present, despite the attempted repair.

The currents, smoke, and 'bang' almost inconclusively indicate a three-phase stator short-circuit inside the generator. The low H2 gas pressure ( with three times higher at the supply side) clearly indicates a hydrogen leak there.

So a three-phase fault has to be initiated by an insulation breakdown, and there is a clear indication of an H2 gas leak.

My assumption based on this is that the leak originated from a fault at the main lead bushings - explaining the low H2 pressure. This leak was not repaired, and the generator was started with a faulty (cracked) main lead bushing insulator. This failed when the generator stator reached full voltage.

It's an assumption but it will be very interesting to hear what you find.

About this - I suspect you will have to repair bushings / insulators, but there will not be serious damage to the stator winding itself, either in the slot or the overhang.

Whatever it is - please keep us updated about what you find!

Take care.
Robert.

Robert and everyone else;
It seems to me that the problem may have occurred in the rotor, because the only cooling for the rotor is H2 (while the stator has some water cooling). I would not expect the stator to experience problems due to cooling under such a low load, even with low H2 pressure.

I am not sure if thermal expansion would have caused the rotor to expand into the stator, but it certainly seems likely that a loss of cooling in the rotor could have caused the integrity of the insulation to suffer, and perhaps that is where the fault originated.

Please comment your thoughts
-Jeff

I think it is due to insulation breakdown H2 leak cause pressure change happen in winding section.. .
Because of this happen moisture build up and getting insulation problem.
So all 3 Phase Voltage go to common.. .