In power plant DCS (Distributed Control System) applications, performance issues with controllers are often due to three critical factors: the controller’s processing cycle, controller specifications, and controller application functions. Therefore, during the DCS equipment selection phase, it is essential to ensure that the controller meets or exceeds the following parameters:
1. Controller Scanning Cycle
Analog Control Loop Scanning Cycle:
Conventional loops: ≤250ms
Fast response loops: ≤125ms
Slow-changing processes (e.g., temperature variation): ≤500ms
Digital Input/Output (DI/DO) Control Cycle:
Conventional DI/DO: ≤100ms (including MFT controllers)
Turbine protection systems (ETS): ≤50ms (when integrated with DCS)
OPC/OPT logic processing: ≤20ms (to meet overspeed protection requirements)
2. Controller Specifications
Redundancy and Fault Tolerance:
Controller switch-over response time: ≤4ms
Memory Safety Margin:
Effective memory margin for each controller: ≥40%
Processing Capacity Limits:
Maximum peak load rate: ≤60% (to support scanning cycle)
Average load rate: ≤40% (typical operating conditions)
3. Controller Application Function Requirements
Offline and Online Download Functions:
The download process should be error-free and reliable.
Fault Diagnosis and Alarming Functions:
Fault diagnosis and alarms must be accurate and error-free.
Online Parameter Modification:
Parameters should be modifiable online with a secure and reliable mechanism. Downloading modified configurations should not reset other online tuning parameters.
Online Tuning Parameter Safety Storage:
Controller settings should be securely stored so that power loss, network disconnection, or operational mode switching does not result in the loss of online tuning parameters.
Offline/Online Parameter Comparison and Synchronization:
The system should be able to compare and retrieve differences between offline and online parameters at any time, and support bulk synchronization of online tuning parameters to offline application software.
These aspects are critical to ensuring that DCS controllers meet design requirements. During the selection process, special attention should be given to the controller’s scanning cycle, redundancy, memory safety margin, and functional integrity to ensure stable and reliable system operation.