Abstract: The SCS (System-Controlled System) of the 30MW unit at Wujing Thermal Power Plant originally used the Modicon 984 series programmable controller from the United States, configured in a dual-machine hot standby mode, with MODSOFT software as the programming tool and communication with the upper-level DCS (Distributed Control System) via MB (Mean Interchange Controller). This SCS upgrade adopted Modicon's QUANTUM series cards and used CONCEPT on the Windows platform as the programming software, adding redundant power supply modules and changing the original communication method between the SS (System-Controlled Controller) and DS (Distributed Control System) upper-level systems. By adding upper-level monitoring software, the stability of the original SCS was increased, and the functionality of the original programmable control system was improved.
Keywords: PLC; redundant power supply module; MB+ communication method; supervisory control and control software
1. Problems with the original SCS system
The two 300MW thermal power units at Wujing Thermal Power Plant, commissioned in the early 1990s, have been using the original SCS control system ever since. This system primarily controls the interlocking and protection functions of six main fans, air preheaters, feedwater pumps, condensate pumps, vacuum pumps, and the condensate drainage system. The original system used MODICON-984 series PLC cards, with a dual-machine hot standby CPU configuration, and consisted of 9 remote stations, 35 racks, and over 2700 I/O points. After more than a decade of operation, the following main problems have emerged:
(1) The equipment is aging and the performance of the control cards is unstable. This series of control cards has been discontinued for a long time and spare parts cannot be purchased.
(2) The system configuration is unreasonable. The original control system uses a single power supply card with a remote station and connects 2 to 3 I/O racks in series under the remote station. If the power supply or communication of the remote station fails, all the I/O signals of the remote station and its downstream units will be lost, which has very serious consequences.
(3) The original SCS-PLC control system communicated with the DCS via MB communication. One MB communication line was drawn from the main communication card and one from the backup communication card to the upper-level DCS communication card. However, when the PLC-side CPU switched to hot standby, the MB communication also switched, but the upper-level DCS communication card only switched when the DCS communication card failed, not due to the lower-level system's communication switching. Therefore, in actual operation, when the PLC-side switched to hot standby, the upper-level DCS system could not follow the PLC-side MB communication switching, causing a communication interruption.
(4) The programming software is MODSOFT software under the WIN98 platform, which can no longer meet the requirements of commonly used operating software, and backup is very inconvenient.
2 Hardware upgrade and modification of the SCS system
The original SCS system used MODICON 984 series CPUs as local stations, and constructed 9 remote stations, 35 racks, and over 2700 I/O points using P890, P810 power and communication modules and coaxial cables. The I/O modules were all standard AC 220V input modules, DC 24V input/output modules, and relay output modules. Therefore, the module upgrade to the QUANTUM series was essentially a one-to-one replacement. The local stations used a 140CPU534 configuration, and the I/O modules were the corresponding 140 series modules. Since the QUANTUM series motherboards have a maximum of 16 slots, while the original MODICOM 984 series motherboards had a maximum of 10 slots, by merging some racks with fewer original cards, the number of racks was reduced from 35 to 29, saving considerable installation space.
3 SCS System Configuration Optimization and Power Redundancy Configuration
The original SCS system used a single power supply card with a remote station, and connected 2-3 I/O racks in series with that remote station. This configuration had poor security and stability. If the power supply card failed, it could cause power loss to that rack and all 2-3 I/O racks in the same series circuit, resulting in the loss of all I/O points on the I/O cards in those racks, causing serious consequences. Our factory has previously experienced critical auxiliary equipment tripping and even a Master Time Off (MFT) event due to a power supply card failure in the SCS system. Considering the crucial role of the SCS system in the main control system, this upgrade will optimize the SCS system configuration and configure redundant power supplies.
The configuration optimization transforms the original 2-3 cascaded I/O racks into independent remote stations. A power or communication failure in one rack will not affect the normal operation of other racks. Furthermore, the original single-power-supply configuration of the remote stations is upgraded to a QUENTUM series redundant power supply configuration. If one power supply card fails, the redundant power supply will independently power the rack and display a fault indicator light, prompting maintenance personnel to replace the power supply card.
4. Resolve the communication switching issue between the SCS system and the host DCS system.
Originally, both the SCS local master station and the hot standby station had one MB communication line connected to the DCS master/standby MB communication card. If the SCS underwent a master/standby switchover (i.e., the original master station MB communication line switched to the hot standby station MB communication line), but the upper-level DCS MB communication card did not switch accordingly, causing a communication interruption between the SCS and DCS. This upgrade changed the original MB communication method to MB+ communication. The master station and hot standby station MB+ communication lines are combined into one line via an MB+ splitter and sent to the DCS. The MB+ splitter then splits the connection into two lines, connecting to the DCS master/standby MB+ communication cards. Because the MB+ communication cards use the same address during SCS master/standby switchover, the DCS can configure this address in the MB+ communication protocol. Regardless of the SCS master/standby switchover, the DCS can locate and communicate with the active SCS local PLC through the MB+ splitter. Similarly, if the DCS experiences a master/standby switchover, communication with the SS is maintained through the MB+ splitter.
5. Upgrading SCS programming software and installing host software
The original SCS programming software was MODSOFT, a product from the 1980s. This product was a 16-bit DOS application and could not be installed on current 32-bit operating systems (such as Windows XP). Backups still used 3-inch hard drives, which was neither convenient nor reliable. Furthermore, MODSOFT could only be used as a programming tool for the LL984 language, its functionality was relatively limited, and the communication between the programmer and the PLC was a somewhat unstable MB communication method, which resulted in slow speeds when dealing with large amounts of data.
The upgrade uses CONCEPT on the Windows XP platform. Modsock can be directly converted to CONCEPT using a conversion tool without reprogramming, although some function block parameters need to be reset. Furthermore, CONCEPT supports TCP/IP communication, replacing the original MB communication between the programmer and the PLC, thus improving both communication speed and stability. During the software upgrade, a host software suite, INTOUCH, was also installed. This added screens for relevant devices, making system maintenance more intuitive, and also included a reporting function to sequentially record status signals from the PLC for easy analysis.
6 Conclusion
Modern control systems are diverse and upgraded rapidly. MODICON, a widely used programmable control system in power plants, has seen numerous upgrades in recent years to QUANTUM, UNITY, or other series products. This SCS system upgrade not only updated the corresponding hardware and software but also added redundant power supplies, resolved communication switching issues with the higher-level DCS, and installed monitoring software to enhance the reliability and monitoring capabilities of the original system.
For details, please click: Upgrade and transformation of SCS-PLC programmable control system
