This article takes the tripping and shutdown fault of a desulfurization system in a power plant as an example to analyze the causes of DCS signal interference and propose methods to suppress interference, which has achieved good results and is for reference only.
Preface
Phase I project of a certain power plant (5 × The boiler, steam turbine, and generator set of the 600MW unit are respectively manufactured using imported technology by Shanghai Boiler Co., Ltd., Shanghai Steam Turbine Co., Ltd., and Shanghai Steam Turbine Generator Co., Ltd. The boiler is manufactured by Shanghai Boiler Co., Ltd. with the model number SG-2028/17.5-M908. It is a subcritical, primary intermediate reheat, solid slag removal, balanced ventilation, four corner tangential combustion, and controlled circulation drum boiler. The steam turbine is an N600-16.7/538/538 imported intermediate reheat condensing steam turbine unit produced by Shanghai Steam Turbine Co., Ltd., with a rated power of 600.243MW. The generator is a QSFN 600-2 water hydrogen cooled steam turbine generator set manufactured by Shanghai Steam Turbine Generator Co., Ltd.
The vibration signals of the six major fans and desulfurization booster fans in the power plant are input into the Bentley 3500 card using a frequency probe. After the collection and calculation of the 3500 card, a 4-20mA signal is output to the AI card in the DCS. During the operation of Unit 5, due to interference, the vibration signals of the booster fan motor and fan all jumped and exceeded the set value of the protective shutdown fan. The booster fan stopped, causing desulfurization shutdown. When checking the fan vibration signals, it was found that the FUM280 card cabinet in the electronic room vibration signal acquisition card was not grounded. According to the design requirements, all control cabinets in the electronic room are first grounded to a grounding cabinet in the DCS control cabinet, and then grounded uniformly by the DCS cabinet. After grounding treatment, the signal is normal and there is no signal jump.
2 Reasons for interference
To understand the purpose of grounding, we will first introduce several interferences faced in applications. Interference, also known as noise, is an electrical signal that enters or superimposes on power or signal lines and is independent of the signal. Interference can cause measurement errors and serious interference (such as lightning strikes, large serial mode interference can cause equipment damage).
2.1 Interference introduced by resistance coupling
When several signal lines are transmitted together, interference is introduced into other signals due to aging insulation materials and leakage, which affects other signals. In some control systems that use electrical energy as a means of execution, such as electric actuators, signal sensors may experience leakage and come into contact with charged objects, which can also introduce significant interference. Due to improper grounding, such as grounding at both ends of the signal line, a significant interference may be added due to the ground potential difference. The two ends of the signal line are grounded simultaneously, so if the distance between points A and B is far, there may be a significant potential difference, which may generate a large circulating current on the signal line between the two ends.
2.2 Interference introduced by capacitance inductance coupling
Because there are often many signals connected to the control cabinet at the controlled site, and these signal lines either run through cable trays or cable conduits, but they are definitely many signals running together. There are distributed capacitors between these signals, which will add interference to other signal lines. At the same time, an alternating magnetic flux will be generated around the alternating signal line, and these alternating magnetic fluxes will generate electromotive force between parallel conductors, which will also cause interference on the line.
2.3 Interference introduced by lightning strikes
Lightning strikes may cause significant electromagnetic interference around the signal or introduce interference through various grounding wires.
3 Interference suppression
The above lists several reasons for interference. If these interferences are not well suppressed and prevented, they can affect the accuracy of signal measurement technology, making normal control impossible, and even cause equipment damage. Many methods for interference suppression have been summarized in long-term engineering practice.
3.1 Isolation
(1) Insulate all signal lines well to prevent leakage, thus preventing interference caused by contact;
(2) By isolating and laying different types of signal lines (in different cable trays or separated by partitions), we can classify them according to their ability to resist noise interference. The anti-interference ability of analog signals (input and output, especially low-level input signals such as thermocouple signals, thermal resistance signals, etc.) to high-frequency pulse signals is very poor. It is recommended to use shielded twisted pair cables for connection, and these signal wires must occupy separate wire conduits or cable trays, and cannot be routed in the same cable conduit (or tray) as other signals. The above explanation indicates that the same type of signal may be placed in a cable duct or trough, while similar types of signals must be separated by metal partitions if they must be routed in the same cable trough.
(3) Another type of isolation is to electrically isolate the signal source from the control cabinet, which greatly reduces the harm of common mode interference to the control cabinet card components. Use an isolation amplifier to completely isolate the input terminals of the signal from the control cabinet (in some systems, isolation transformers or relays are used to isolate, while optoelectronic devices or relays can be used to isolate switch quantities). In this way, the interference signal generated by the different ground potentials between C and D cannot form a loop, which suppresses the harm of interference.
(4) The fourth type of isolation is the isolation of the power supply system. In order to prevent the introduction of common mode high-frequency interference signals on the power supply line, an isolation transformer can be installed on the power supply line for interference isolation. To achieve good interference suppression effect, two points must be noted: the shielding layer of the transformer must be well grounded; The secondary coil of the transformer must be twisted pair.