April 1, 2009

Fast-Tracking a Control System Retrofit

Kevin Geraghty, PT Inco Indonesia

Surviving Start-Up Challenges

Start-up of the unit began with dry tests. We stroked the wicket gates and bypass valve to set the timing nuts on the distributor valves. The settings determine the maximum speed of the servo motors and were adjusted to meet the existing speeds. During this time, the vendor provided hands-on training for O&M personnel.

The next stage was to simulate the generator circuit breaker interlocks. We then began filling the penstock to begin the no-load tests. This allowed us to check the penstock intake gate interlocks. With the penstock full, our project team started the auxiliary systems manually, and we began to ramp up the generator speed. Soon after the unit started rotating, there was an unexpected sound. Workers on a previous project had installed vibration-monitoring probes but had failed to check the clearances. As a result, a series of bolts on the shaft sheared the probes and sent them airborne. Once we were satisfied that all the pieces had been found, we once again began to ramp up the generator speed, stepping uneventfully through 25%, 50%, 75%, and 100% of rated speed.

With the units up to speed, we performed the overspeed trip test. When the speed exceeded the limit, the wicket gates began to close and the bypass valve began opening. The bypass valve had been configured to open at a rate of 5% per second when the penstock pressure exceeded 16.3 bar (236 psi) and close at a rate of 2.5% per second when the pressure dropped below this limit. This caused oscillations in the bypass valve, which resulted in pressure spikes over 20.5 bar, causing one of the rupture discs to let go. We now faced the job of draining the penstock, replacing the ruptured disc, and refilling the penstock — which is typically a 12-hour process. It was decided that the governor logic had to be revised to control the bypass valve based upon wicket gate closing rate, which was how the previous governor had functioned.

We then moved on to test the canal and headpond blackout protection. We were only able to perform a simulation, because no one would give us permission to actually black out the power station.

When the unit was finally ready to be brought online, we placed the unit control PLC in automatic and started the system. The PLC brought up all the auxiliary systems while the governor brought the generator up to rated speed. We then manually synchronized the generator and held it at minimum load for 30 minutes. There were no indications of heat or vibration problems, so we gradually stepped up the load to 60 MW. Our team then took the unit offline to test the auto synchronizer. Eventually, the unit auto was synchronized. At this point the unit was handed over to operations staff and the replacement of the Balambano unit control PLCs began.

There were other start-up challenges:

• As-built drawings. Our project team checked to ensure that all the construction and commissioning changes were recorded in the drawing package. We issued a copy of the red line drawings to the maintenance staff (a task that usually takes weeks).

• Software fine-tuning. We had to add several changes to the governor tuning to make the unit share the load under different generator configurations. Also, we had to modify several alarm and trip setting delays to suit operating conditions.

• Online operation. During the rough furnace operation, high imbalance currents caused the MW transducer to output less than 4 mA. This caused the governor to fail the transducer and to transfer to "speed mode." On several occasions, this caused the unit to trip. We remedied this problem by installing a digital transducer that can be limited to 4 mA.

• Controlling water flow. In order to limit water flow through the penstock, we programmed the governor to shut the unit down if the position of the wicket gates plus the bypass valve exceeded 105%. The rough nature of the furnace load caused this problem to occur on several occasions. We added a time delay to give the governor a chance to close the bypass valve if such conditions occurred.

Meeting the Project’s Main Goals

Of the many requirements for this project, we managed to meet the important ones:

• We returned the unit to operations in 12 days and on time.

• The new controls have reduced unit restoration time after a trip.

• The new HMI provides greatly advanced monitoring.

• We provided protection against penstock overpressure.

• We implemented canal and head pond blackout protection.

• The controls related to bypassing water to the Balambano Reservoir are functioning well.

• The SOE recorder is now functioning.

• The advanced load and generation rejection schemes system has been successfully commissioned.

• As far as achieving optimum reactive power sharing, we have the hooks in place but need new exciters before this can be realized.

• In regard to integrating water dispatch, we have the hooks in place but need to upgrade Larona 1 and Larona 2 control systems before this can be achieved.

--Kevin Geraghty (kevin.geraghty@valeinco.com) is a senior maintenance specialist with PT International Nickel Indonesia. He has 25 years of industrial experience. Starting in 2002, he worked as a control system mentor in the process plant engineering group for PT International. In 2005, he transferred to the company's utilities engineering group.