This function is not the same as the ‘AUTOMATIC START MODE’. The Automatic Start Sequence can be used with any one of the three start modes.
The 505E can be configured to utilize an Automatic Start Sequence to start the turbine. This sequencing logic allows the 505E to perform a complete controlled system startup from zero speed to rated speed. With this function, the turbine’s starting ramp rates and idle speed hold times depend on the length of time the unit was shut down. This sequence logic may be used with any of the three start modes (manual, semiautomatic, automatic), and is initiated by a ‘RUN’
command.
With this function, when a ‘RUN’ command is given, the Automatic Start
Sequence ramps the speed setpoint to a low idle setpoint, holds at this setting for a duration, ramps the speed setpoint to a high idle setpoint, holds at this setting for a duration, then ramps the speed setpoint to a rated speed setting. All ramp rates and hold times are programmable for both hot start and cold start
conditions. The control differentiates between hot and cold starts by using a hours-since-trip timer. This timer starts when a shutdown has been executed and turbine speed has decreased below the low idle speed setting.
Figure 3-11. Automatic Start Sequence
With this sequence, a set of hot-start ramp rates and hold times is programmed for use when a ‘RUN’ command is given and the turbine has been shutdown for less than the programmed ‘HOT START’ time setting. A set of cold-start ramp rates and hold times is also programmed for use when a ‘RUN’ command is given and the turbine has been shutdown for longer than the programmed ‘COLD START’ time setting.
If a turbine ‘RUN’ command is given when the length of time the system has been shutdown is between the ‘HOT START’ and ‘COLD START’ time settings, the control will interpolate between the hot and cold programmed start values to determine starting rates and hold times.
For example, if the unit had the following Automatic Start Sequence settings: COLD START ( > xx HRS ) = 22 HRS
HOT START ( < xx HRS) = 2 HRS
LOW IDLE SETPT = 1000 RPM
LOW IDLE DELAY (COLD) = 30 MIN
LOW IDLE DELAY (HOT) = 10 MIN
HI IDLE SETPT = 2000 RPM
RATE TO HI IDLE (COLD) = 5 RPM/SEC
RATE TO HI IDLE (HOT) = 15 RPM/SEC
HI IDLE DELAY TIME (COLD) = 20 MIN
HI IDLE DELAY TIME (HOT) = 30 MIN
RATE TO RATED (COLD) = 10 RPM/SEC
RATE TO RATED (HOT) = 20 RPM/SEC
RATED SETPT = 3400 RPM
If the unit was tripped for 12 hours, the 505E would interpolate between the Hot and Cold parameters and use the following rates and delays (viewed in the Service Mode):
LOW IDLE DELAY = 20 MIN
RATE TO HI IDLE = 10 RPM/SEC
HI IDLE DELAY = 10 MIN
RATE TO RATED = 15 RPM/SEC
HOURS SINCE TRIP (HRS) = 12 hrs
Based on the example’s configuration and trip time, the speed setpoint would ramp to 1000 rpm at the ‘Rate to Min’ setting and hold for 20 minutes, move to 2000 rpm at 10 rpm/sec and hold there for 10 minutes, and lastly, move to 3400 rpm at 15 rpm/sec. At 3400 rpm, the sequence would be completed.
If the unit was tripped for 2 hours or less and restarted, the 505E would use the Hot Start parameters. If the unit was tripped for 22 hours or longer and restarted, the 505E would use the Cold Start parameters. The ‘Hours since Trip’ timer starts counting when the unit shuts down and the speed has coasted down below the low idle speed setting.
The 505E will automatically set the hours-since-trip timer to its maximum setting of 200 hours to ensure a cold start is selected after a power up or upon exiting the Program mode. The hours-since-trip timer will reset only when the turbine speed has increased above the minimum governor speed setting.
The Auto Start Sequence can be halted at any time from the 505E keypad, contact input or through Modbus communications. The sequence can be halted by a halt command, a raise or lower speed setpoint command, or when a speed setpoint is directly ‘Entered’ from the 505E keypad or through Modbus
communications. When the sequence is halted, the delay timers do not stop if they have already started counting down. The sequence will resume when a Continue command is issued. If there were 15 minutes remaining to hold at an idle speed and the Halt command was issued for 10 minutes before a issuing a Continue command, the sequence would remain at the idle speed for the remainder of the ‘Hold Time’—which in this example is 5 minutes.
If a 505E contact input is programmed to function as a Halt/Continue command, the sequence is halted when the contact is open, and continued when the
contact is closed. The Halt/Continue contact can be either open or closed when a Reset command is given. If the contact is closed, it must be opened to allow the sequence to be halted. If the contact is open, it must be closed and reopened to halt. Alternatively, a relay can be programmed to indicate when the Auto Start Sequence is halted.
An option is available to automatically halt the auto start sequence at the idle setpoints. This feature would result in the unit automatically stopping or halting at the low idle setpoint and at the high idle setpoint. If the unit is started and the speed is above the low idle setpoint, the sequence will initialize as halted. The sequence must be given a ‘Continue’ command once halted. The hold timers are still active with this option. If ‘Continue’ is selected and the hold timer has not expired, the sequence will remain in a timed wait until the hold timer has expired and then continue from that point.
When the ‘Auto Halt at Idle Setpts’ option is programmed, the Auto Start
Sequence Continue contact input only requires a momentary closure to continue the sequence.