DESCRIPTION OF INCIDENT:
210 MW Generator Short Circuit: Cause/Reason & Analysis
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:
Generator - Transformer Differential
Generator Backup Reactance
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
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.