The S7-300 PLC inherits the well-established design features of Siemens' S series PLCs, offering a compact size, flexible configuration, and robust functionality. This makes it highly suitable for thermal protection systems that require high accuracy, reliability, and completeness. Its advanced capabilities ensure efficient and stable operation in complex industrial environments.
The OP5 Operator Panel is a user-friendly man-machine interface developed by Siemens. It serves as a key tool for displaying and monitoring internal data from the programmable logic controller (PLC) and allows users to adjust internal parameters via function keys. The panel primarily shows event and alarm information, with alarms taking priority over regular events. When an alarm condition is met, any ongoing event display is replaced by a flashing alarm message, ensuring immediate operator attention.
The SIMATIC PLC system offers a wide range of convenient interfaces for the OP5 panel. It can be directly connected to the S7 series PLC for real-time human-machine interaction or configured offline. The current OP series includes text display devices capable of showing numerical values or related symbols, with functions and interfaces defined using the COROSProtool/Lite software in the Windows environment. For the o-shaped display device, functions are also set up using COROSProtool. Designers can use familiar Windows-based tools like Designer, CorelDraw, or Paintbrush to create graphics, which are then imported into the process display in bitmap format, such as bar graphs or flow diagrams.
Based on the requirements of the unit control system at Shentou No. 1 Power Plant, the OP5 operator panel was designed with several essential functions: automatic tour scroll display, manual single-screen display, alarm display, protection activation and deactivation, system settings, system reset, and password protection. These features ensure comprehensive control and monitoring of the plant’s operations.
The protection system within the S7-300 PLC monitors critical thermal parameters of the furnace and issues either a warning or a protective action based on the over-limit condition. To ensure reliable operation and prevent false signals, the system follows the signal processing characteristics of the Siemens S series PLC, requiring confirmation of measurement signals before making an over-limit judgment.
To achieve this, the signal measurement loop for each thermal parameter is redundantly configured. Two signals are used: one from the program control system (left input), and another from the instrument (right input). Both signals undergo validity checks, and only valid signals are used for over-limit decisions. In case of a fault, the system sends a fault status signal to alert operators, who can then quickly address the issue.
The working principle and input/output logic of the protection system are as follows:
a. Input signals are first diagnosed individually. The diagnostics include checking if the signal exceeds the maximum possible range (range fault), if it changes too rapidly (rate fault), or if either of these occurs, indicating a channel fault. Once any of these faults occur, their status is displayed and flashed on the OP5 panel, while a channel fault signal is also sent to the central control console. The alarm stops flashing only after being acknowledged on the OP5, but the fault details and time of occurrence are stored in the buffer for future reference.
b. When a channel failure occurs, the protection output is blocked. The system will not resume normal protection output until the fault is resolved and the reset button is pressed, at which point the protection output is re-enabled.
c. After confirming the signal's validity, the system checks for over-limit conditions. If the over-limit threshold is exceeded, a forecast signal is issued after a 2-second delay (or left-right phase/zero delay). If the protection limit is exceeded and no lockout is active, a protection signal is issued after a 2-second delay (or left-right phase/2-second delay).
d. During the 2-second delay, if the limit returns to normal, the delay is reset. Similarly, if the protection is closed or blocked during the delay, the timer is also reset.
In summary, the S7-300 PLC continues the legacy of Siemens’ S series PLCs with its compact design, flexible configuration, and powerful performance. It is ideally suited for thermal protection systems where precision, reliability, and completeness are crucial. The protection system built around the S7-300 PLC, with its OB35-based programming, enhances both fault diagnosis and protection logic. Notably, the rate of misoperations has been significantly reduced, far surpassing the previous relay-based hard-wired protection system.
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