Engineering Design usually has a very definitive approach and one has to work through a given input conditions for an intended output utilising calculated/predicted performance/behaviour of individual equipment and systems. But commissioning is always a challenging and thrilling task as we try to match the behaviour of actual equipment and systems at the site with predicted design values under different environmental conditions as well as uncertainties in the process of equipment manufacture.
The story narrates an incident that occurred during commissioning of an open cycle gas turbine power station in Oman more than a decade back. Sultanate of Oman decided to build 3 x 92 MW Gas Turbine Open Cycle* Power Plant at Al-Kamil, about 200 km away from the capital city of Muscat for the supply of power to the surrounding areas and the coastal cities of Sur and Al Ashkharah.
The EPC (Engineering Procurement & Construction) contract was awarded to Al Kamil Power Co. (AKPC) for installation and commissioning of three (3) GE Frame 9E** machines, capable of firing both natural gas and distillate as the backup fuel, under the supervision of a Consultant Architect from Germany. We were assigned as technical experts of Consultant’s commissioning team for review and certification of the commissioning protocol, trial operation and final performance test of the Plant in line with the EPC contract.
It was a hot and sunny noon in August 2002. The scorching sun was piercing our clothes as we walked down to the Gas Turbine local control room after lunch to observe the Full Load Throw-off Test.
[Full Load Throw-off Test is a standard test done on Gas turbine power plants. The test allows disconnecting the gas turbine generator from the high voltage electrical grid during unit full load operation by force opening of generator transformer (GTX) HV side circuit breaker. The test is successful if the gas turbine continues to operate feeding its auxiliary load without getting tripped. This avoids tripping and restart of the individual units in case of tripping of HV side breaker due to HV transformer (GTX) or transmission line faults. This allows quick restoration of power supply.] Refer the figure below.
Typically, as seen in any plant site in middle-east countries, there were no green patches and open areas between the facility buildings and access roads were covered with stone chips / gravels, which made the ground condition worse. Sign of relief from hot blast appeared on our faces as we entered the control room. All the concerned engineers were already present at the site except Owner’s representative. The Gas Turbine was running at full load; we noted the parameters from the control room monitors - everything appeared to be normal. Oh my God! The inlet air temperature to GT is showing 54 degC. We made a quick inspection around the Gas Turbine enclosure to check if there are any leakages, vibrations or abnormal sounds – apparently no issues. In the meantime, Owner’s representative arrived. The required checks before the test were jointly carried out and test protocols signed off. Communication between the GT local control room, 132 kV switchyard control room and 132 kV grid receiving substation were established. HV side breaker (HVCB) of the gas turbine generator transformer (GTX) was put in “manual” mode.
Three – Two – One……Boom!
Breaker opening sound was heard from the 132 kV Switchyard. There was an immediate jump in the gas turbine speed and the unit tripped. The disappointment was visible on all faces – has the GT tripped on over speed ?
Everybody jumped on the Alarm List and Sequence of Event (SOE) print outs to identify the “first out” or the “root cause of tripping”. To the surprise of one and all, the GT has not tripped on “Overspeed”. Maximum speed recorded is 107% of rated speed.
Can you guess what could be the “root cause” of tripping?
Overspeed Trip – All Gas Turbine Generators are protected from mechanical failure through an Overspeed trip safety device to avoid excessive centrifugal force/stress in gas turbine and compressor blades in the event of over speeding. Generally, the machine is electrically and/or mechanically tripped in case the operating speed exceeds to between 108 of 110% of the rated speed as per OEM recommendation. Fuel is automatically cut-off and the turbine is shut down.
Since this test causes a lot of mechanical stress in the Gas Turbine Generator equipment as well as the electrical disturbance in HV grid, it was agreed that further trial would be conducted only after thorough study and rectification of observed problems. We left the control room noting that GTG is running in turning gear mode.
We returned to our site porta-cabin office with printouts of the process parameters, SOE data and Alarm List. Our Commissioning Chief was already waiting in the cabin. He looked quite serious - “Guys, we will have an internal meeting tomorrow at 8-30 AM sharp. Please go through the test data and design documents. We must be able to identify the root cause and provide a workable solution tomorrow to continue with testing activities and maintain the commissioning schedule”.
It was already 5:00 PM. The Guest House was far off from project site. We sent a message to the Guest House Manager that we may be late for dinner and concentrated on the test data.
Look - The Gas Turbine Generator load dropped to < 1 MW immediately after the tripping of the HV breaker and there was a jump in GT speed - quite normal. The highest speed recorded was 107%, which is lower than the first electrical Overspeed trip value set at 108%; definitely, the Gas Turbine did not trip on ‘Overspeed’.
So what could be the reason behind GTG trip ?
We started reading between the lines of SOE data and Alarm List to check if we overlooked anything in hurry. Suddenly, our eyes got stuck on a line in Alarm List ‘loss of Auxiliary power’. We checked the corresponding test parameters in the same timeline – yes, the data shows ‘low voltage’ in Auxiliary Power Supply Panel. The SOE print out corroborated the data; ‘LVCB tripped’. We felt little relieved – at least some cause is identified. The Electrical and Instrument Commissioning Engineer stood up and brought the design documents. Exercise continued to identify the cause……
What prompted the tripping of LVCB!!
It was about 9-00 PM, the site was quiet except humming sound from HV transformers. Silence broke with ringing sound from our mobile phone – Is it our Boss? I picked up the phone - “Guest House Manager speaking…. Please reach Guest House latest by 10-00 PM beyond which it will be difficult to serve the dinner”. Our Electrical Engineer AMB looked up “let’s spend half an hour more” and got lost in logic diagrams again. I looked at the Instrument engineer – “if nothing is found today, we have to reach site office early tomorrow”. Next fifteen 15 minutes was perfect silence except for the intermittent noise of turning drawing sheets. Suddenly AMB looked up – “I think I found the cause”.
Look - the protection logic indicates that “the power supply to UAT and LV panel should be disconnected if the supply frequency reaches 53 Hz”. I think this is as per provision of Oman Grid Code.
Oman Grid Code specifies - “Power Producers shall be responsible for protecting their equipment against damage should frequency excursions outside the range of 51.5 Hz to 48.0 Hz ever occur. It is up to the Power Producers to decide whether to disconnect his plant for reasons of safety of Plant and/or Persons”
Highest GTG speed recorded is 107%, which corresponds to a frequency of the generated voltage from GTG of 53.5 Hz and this must be the cause of tripping the LVCB. I think we are on right track, I said. Let’s take a break and think overnight for a solution.
Next morning we were about an hour early in the breakfast table. AMB said one possible solution is to “increase the trip setting of LV side breaker to 54 Hz”. I said, it’s workable, but we need to get design approval as all the equipment fed from LV supply must be validated for over frequency operation from the respective OEM.
The Instrument Engineer interrupted, hay guys – “I think none of GTG auxiliaries will be unsafe if they are subjected to transient over frequency operation up to 55Hz as the highest Overspeed setting of GTG is 110%. How about adding a time delay (say around 5 – 10 seconds) in the frequency signal in the present protection circuit which will allow the governor to bring down the GTG speed to 105% or below and avoid LVCB tripping”? Good suggestion.
We quickly drove to the site to validate our finding. The recorded test data showed that 10 seconds delay would be sufficient. AMB made a quick write-up for explaining his idea to Commissioning Chief.
The next part of the story was full of events and anxiety. The proposal was discussed with Commissioning Chief followed by EPC and GE commissioning representatives, who agreed and made required modification in the LVCB electrical protection circuit.
The Retest was scheduled at 3-00 PM.
Anxiety and seriousness prevailed in the control room as everybody assembled at 2-45 PM. We kept our fingers crossed as HV breaker trip switch was forced open.
Boom! Breaker opening sound was heard from Switchyard; GTG speed shoot up and returned to normal and unit continued to operate at house load. Everybody cheered up. Commissioning Chief threw a dinner party at Guest House to celebrate the occasion.
There is a proverb that “All well that ends well”. The design of a plant is considered to be full-proof if it performs well at the site. The task of a commissioning team is always thrilling as they are instrumental in establishing a complete synchronisation of all equipment/systems and demonstrate design performance of the complete plant at the site. Any discrepancy or gaps in design need to be instantly analysed and rectified through the innovative approach.
[*Open Cycle GTG Plant – Under this scheme, the hot exhaust gas from the gas turbine is released to the atmosphere without recovery of exhaust heat energy through Heat Recovery Steam Generator(HRSG). The plant has lower electrical efficiency but is capable of quick starting and power generation. These types of plants are common in the middle-east region where fuel is cheap and in abundance. The plant can be installed on a compact schedule and requires few auxiliaries and operating staff.] Refer to the power generation scheme below:
[** GE Frame 9E Gas Turbine – Manufactured by GE, Frame 9E (PG 9171 model) gas turbine Generator is one of the most widely used power generating units. It has an ISO rating of 123 MW at 50 Hz and has a gross electrical efficiency of ~ 34.6%. The unit can operate with either liquid fuel or natural gas and is fitted with multiple concentric can type DLN burners. The unit can reach its full load within 30 minutes from start command.]