1. Background of the renovation
The Zhangjiagang Gangsheng Branch Coal Terminal originally had 16 belt conveyors, 1 stacker-reclaimer, 8 gantry cranes, and 6 silos. The system uses a Schneider Electric 984 PLC for automated control. Signal acquisition and primary power supply for the entire PLC system are distributed across four sub-control stations in different locations. Conveyor belt signals from these four sub-control stations first enter the sub-control stations, then are connected via cables to the PLC in the central control room. Finally, the Schneider Electric 984 PLC controls the conveyor system and interlocks it with the stacker-reclaimer, gantry cranes, and silos. Each conveyor belt in the system has three control modes: local control, sub-control station control, and central control. Central control is operated via buttons on the central control room console and monitored through a simulated display screen in the central control room.
Due to business expansion and production needs, the company will add and upgrade five conveyor belts, one stacker-reclaimer, one ship loader, and one remote control station, and design new workflows based on the added equipment. Because the original system was designed a long time ago, spare parts for the 984 PLC are no longer available, and some process functions are no longer usable. Most importantly, normal production must not be affected during the entire upgrade process, and the switch from the old system to the new system must be completed within three days, making the upgrade extremely challenging. Under these circumstances, we carefully considered the characteristics of various industrial control methods and compared different solutions.
2. Comparison of renovation schemes
Currently available control methods include I/O signal acquisition, fieldbus, and industrial Ethernet.
If the traditional I/O signal acquisition and control method is adopted, the same as the original system, all protection and control signals of the conveyor belt system and related equipment are transmitted to the PLC in the central control room through ordinary cables. The PLC is then connected to the computer, and these signals are acquired on the computer through the special communication protocol provided by the PLC manufacturer. Finally, the computer is configured to control the entire system.
Under this control method, the signal of each conveyor belt must first be collected in the sub-control station via cable, and then transmitted to the PLC via the connection cable between the sub-control station and the main control station. This requires laying a large number of shielded cables, which is costly and time-consuming. It may also affect the stability of the original system because the original cables must be re-examined, and it cannot be guaranteed that production will not be affected during the transformation.
In fieldbus control, each different substation can be connected to the main PLC in the central control room via a single cable or fiber optic cable, significantly reducing field wiring and enabling high-speed data transmission, thus reducing system debugging time. However, its data transmission capability is limited by communication distance. Because the network topology of fieldbus is relatively simple, redundant networks are generally used for PLC systems to ensure network stability; if one node on the network fails, the entire network may collapse.
Furthermore, since fieldbuses are basically proprietary to each PLC manufacturer, and our company's port has a large number of devices of various brands, it is difficult to achieve system networking of the devices, making it difficult to realize the informatization and modernization of the company's equipment management.
With the development of computer and internet technologies, the application of industrial Ethernet control is becoming increasingly widespread. While technically compatible with standard Ethernet (IEEE 802.3), industrial Ethernet, in product design, meets the needs of industrial environments in terms of material selection, product strength, applicability, and especially in terms of real-time performance, interoperability, reliability, anti-interference capabilities, and intrinsic safety. It has the following significant advantages: ① It can be easily integrated with fieldbuses, such as Siemens' ProfiNet network. ② It offers transmission speeds and bandwidths unmatched by other traditional networks, with communication rates increasing from 10Mbps and 100Mbps to 10G. Under the same data throughput, the increased communication rate means reduced network load and latency, significantly lowering the probability of network collisions and ensuring stable and reliable data. ③ It can easily achieve a "one-stop" network for the entire enterprise information system. ④ It has officially become one of the control standards in the industrial control field. Therefore, we adopted industrial Ethernet control in our system upgrade.
3. Renovation Plan
The proposed solution retains the original system's control and management methods. The conveyor belt control system still employs local control, substation control, and central control. The central control system abandons the conventional button control method and replaces it with a computer control system. This eliminates the need for a large console, numerous buttons and indicators, and the PLC portion of the original analog display system, thus saving costs. Figure 1 shows the main network structure of the system.
Figure 1. Main Network Structure of the System <br />The system adopts a distributed control (DCS) approach to control five different I/O stations. These stations are connected via a fiber optic Ethernet ring network, significantly reducing cable laying and saving costs due to the lower cost of fiber optic cables. The system connects directly to the central control room computer via an industrial Ethernet interface card (using a standard network card). The conveyor belt process control is then configured on the computer using AB's RSview32 configuration software. Various control requirements for the conveyor belt can be easily configured according to the actual process needs, and modifications can be made easily when changes occur. The computer network records the operation of these devices over time, and the data can be transmitted to the company's database via a standard office Ethernet connection, facilitating the company's information management. See Figure 2 for the specific configuration screen.
4. Industrial Ethernet <br />Using industrial Ethernet can significantly reduce data collisions during transmission. However, due to the characteristics of fiber optic ring networks, individual network collisions are more severe. Therefore, the waveform distortion caused by a harmonic current is far more serious than the voltage waveform distortion. Especially when the capacitor of a parallel capacitor compensation device forms an oscillating circuit with the inductance of the transmission line and transformer, current resonance may occur. Resonant current will severely overload the capacitor, causing repeated fuse blowns and preventing the capacitor from being switched on.
To prevent resonance, the capacitor in a parallel capacitor compensation device should be connected in series with a reactor. The inductance of the reactor is selected as follows: for the fundamental frequency, the compensation device is capacitive and acts as a reactive power source; for higher harmonics, it is inductive. It should be noted that the selection of the reactor must consider its various functions.
5. Overvoltage of capacitors
Increased operating voltage has adverse effects on capacitors. On the one hand, it raises the temperature, potentially leading to thermal imbalance. On the other hand, it causes the oil-impregnated paper insulation to age under prolonged high electric fields, reducing insulation strength and potentially causing breakdown. High voltage also affects capacitor lifespan; experimental results show that capacitor lifespan is inversely proportional to the 7th to 8th power of the voltage. Therefore, keeping the operating voltage within the allowable range is crucial for ensuring the safe operation of capacitors.
6. Closing the circuit breaker while it is charged will cause the capacitor to explode.
Closing the circuit breaker while the capacitor is charged is not permitted. If the polarity of the voltage at the moment of closing is exactly opposite to the polarity of the residual charge on the capacitor, the sum of the two voltages will generate a large inrush current in the circuit, which can easily cause overheating and explosion. Therefore, each time the capacitor bank is re-closed, it must be done after the circuit breaker has disconnected the capacitor and discharged it for 5 minutes.
The solution to this problem is to install a protection device that automatically trips when the voltage is lost. On-duty personnel should also be careful to avoid such operations. A discharge resistor can also be connected in parallel with the capacitor to enable it to discharge automatically after a power outage.
Click to download: Application of Industrial Ethernet in Conveyor Transformation Systems
