A few years back, I was seconded to assist in the commissioning of a cooling tower for a 1000 MW coal thermal power plant. The cooling tower was not a requirement until an expansion of another 250 MW capacity of the power plant. Likewise, it became an environmental issue as the water temperature on the water way supplying cooling water to the power plant has been rising particularly during the summer months. A reduced generating capacity during day time is the only temporary solution and it has been costing the company some lost revenues.
With the target completion date inevitable and a 200K+ penalty per day of delay, the pressure has been on the installation and commissioning team.
The process was not smooth sailing at all, but one issue which has bugged for almost a week is one of the main pump motor rated 1.4MW, 11kV kept on tripping on differential protection. The other motor was completely fine.
We checked and double check all the connections. We disconnected all cables and control wiring then reconnected and checking them every time to ensure that we are doing the right thing but all these effort seems useless. What we are thankful though is that, every time we start the big motor, it trips in just a fraction of a second on differential protection. The differential protection was very fast and reliable. I just can't believe it.
After several brain storming sessions, we came to a redo our process again but this time we need to look at the motor factory connection. And there was the culprit. The motor leads were labeled wrongly on the star connection point of the motor. Instead of having U1-U2 V1-V2 W1-W2, the motor leads was connected U1-V2 V1-U2 W1-W2 causing differential current between phase a & b.
After completing the re-connection, the commissioning team was anxious to test the main pump motor again. The Project manager was still adamant that it may again be another failure. I have actually offered a bet of $100 to prove that I am confident of the result of the latest effort.
The result, of course I won the pot and the rest is history.