1 Interference sources in the paper machine workshop
(1) The paper machine drive system is the biggest source of interference in the paper machine workshop. The total load of the paper machine transmission system accounts for more than 1/3 of the total load of the papermaking workshop. In the rectification and inverter of the system, high-power power electronic components (IGBT, etc.) are switched on and off at high speed, generating a large number of high-frequency electromagnetic waves , Polluting the entire workshop, and generating a large number of high-order harmonics, polluting the power frequency grid.
(2) Transformer, MCC cabinet, power cable and power equipment. These devices are all industrial frequencies, and the frequency is relatively low. Interference generally occurs in the near field. In the near field, with the characteristics of the interference source, the electric field component and the magnetic field component are very different. Especially when the power equipment is started, the instantaneous current can reach 6 ~ 1 times of the rated current, which will cause transient interference of large current impact.
(3) Interference from power frequency power supply. If the power frequency waveform distortion and higher harmonics are not isolated or filtered, they will enter the control system by supplying power to the paper machine control system, affecting the fieldbus signal.
(4) Sparks and arcs caused by poor wire contact.
(5) Ground current caused by unbalanced three-phase power supply, and ground loop current caused by shield layers not sharing ground.
2 Ways of propagation of interference
(1) Transmission by wires is called conducted interference. In the field bus, the main performance is ground impedance interference and interference from power frequency power supply.
(2) Transmission through space in the form of radiation is called radiated interference.
3 Anti-interference measures of fieldbus
(1) The interference of power equipment and power cables away from the interference source on the field bus is inversely proportional to the square of the distance, that is, as the distance increases, the interference decay is very fast. Therefore, the field bus equipment is far away from the electrical equipment, and the field bus cable and power cable layered bridge arrangement can play a good role in preventing interference. Keeping away from interference sources is an important measure to prevent radiation interference.
(2) Fieldbus equipment and cable shielding The mechanism of fieldbus shielding is that first, external electromagnetic waves generate eddy currents on the metal surface, thereby canceling out the original magnetic field; second, electromagnetic waves generate reflection losses on the metal surface, and the other part of the transmitted waves are in the metal shielding layer During internal propagation, attenuation produces absorption losses. The shield of the field bus is to use the shield made of conductive material and combine with the ground to cut off the source of interference.
(3) The use of UPS power supply or isolation transformer can prevent interference from power frequency power supply.
(4) The transmission speed of the signal is high on the field bus using optical cable! In the case of large transmission distance and large interference, use fiber optic cables as much as possible. After using the optical cable, it has effectively solved many problems of radiated and conducted interference. If it is between two points that do not share a common ground, or when the grounding condition is very bad, the fieldbus signal is transmitted by optical cable, which can effectively prevent interference such as ground circulation. Especially the paper machine control system and paper machine transmission! Between MCCs, it is easy to have a ground potential difference, or ground circulation, and special attention should be paid to the use of optical cables over twisted pairs.
(5) The correct selection and installation of the rectification part of the paper machine transmission system adopts 12-pulse rectification, which can effectively eliminate the 5th and 7th harmonics. The power cable between the transmission cabinet and the motor must be a well-shielded and three-core symmetrical power cable.
4 Grounding of fieldbus
Good grounding is an important measure for fieldbus interference prevention. The so-called / ground 0 is generally defined as the zero-potential reference point of a circuit or system; grounding is to establish a conductive path between two points. Its purpose is to protect the safety of operators and equipment, that is, to protect ground 0; second, to suppress electromagnetic interference, Provide the potential reference in electronic measurement, namely / working place 0. Figure 1 is the grounding method of the working ground:
Single-point grounding means that all electronic devices or components in the system are connected in parallel to a ground reference point. The advantage of this grounding method is that the ground potential of each electronic device or component is only related to the ground impedance of the electronic device or component, and is not affected by other loops; the disadvantage is that multiple ground wires are required, and the length of the ground wire is increased. The interference coupling between the ground wires greatly increases the impedance of the ground wires at high frequencies. Multi-point grounding means that every point in the system that needs to be grounded is directly connected to the grounding point closest to it, so that the length of the grounding wire is the shortest. Because the ground wire is very short, it is suitable for high-frequency situations; the disadvantage is that it is easy to form various ground loops, causing low-frequency ground loop interference. When the operating frequency is below 1MHz, single-point grounding can be used; when the operating frequency is above 10MHz, multi-point grounding can be used; of course, single-point grounding and multi-point grounding can also be used in combination. When the grounding wire is too long, multi-point grounding should also be used to prevent the grounding wire impedance from being too high and the interference noise voltage from being too high. Among the many field buses, the development of Profibu field bus is particularly rapid, and it is widely used in the paper industry. Profibus2DP is now used as an example. The communication rate of Profibus2DP is 9.6, 19.2, 87.5kbit / s and 0.5, 1.5, 3, 6, 12Mbit / s, of which the most commonly used communication rate is 1.5Mbit / s. At this time, the length of the Profibus twisted pair cable can reach 200m (It can be extended by using repeaters). When the Profibus2DP communication rate is 1.5Mbit / s, the communication frequency can be considered as 7.5MHz, the frequency is relatively high, close to 10MHz, multi-point grounding should be used, and the grounding impedance should be minimized as much as possible.
Profibus2DP cable adopts RS248 twisted pair shielded cable. Due to the high communication frequency, the noise current generated by interference only flows outside the shield layer (skin effect). The shield layer is usually grounded at multiple points, generally at both ends of the cable shield layer. , That is, the beginning and the end are grounded at the same time. If one end is grounded (start or end), it is only helpful to eliminate low-frequency interference. However, when the equipotential grounding plate is not placed in the entire workshop, if equipotential grounding cannot be achieved, one end is better than the two ends and can prevent the formation of ground loops.
5 Anti-interference application of fieldbus in papermaking control
Huatai Group PM, 0 paper machine, design speed 1800m / min, roll paper width 635mm, annual production capacity 200,000 tons, put into production on July 20, 200, DCS system is provided by internationally renowned companies, the controller uses AC450, MCC Profibus2D communication connection is used between DCS and MCC in INSUM system, the communication rate is 115Mbit / s. Since INSUM uses LONbus for internal communication, LONbus must be converted to Profibus through the Gateway, and the controller side is connected to the CI541 card. At the beginning of commissioning, Profibus twisted pair shielded cable was used, with a total length of about 150m. The connection used a 9-pin D-type plug. The two ends of the cable were grounded through the D-type plug. Many experiments exclude all other reasons. Finally, when the DC side cable shield is not grounded, all sites can be found, so the MCC side single-ended grounding is used. Although it has been able to run normally, a new situation has occurred. Every once in a while, a certain Gateway suddenly crashes during operation, and it is normal after reset. Analysis reasons Gateway is located in the MCC cabinet, a lot of power equipment exists, and the electromagnetic interference is large. This part of the MCC is close to the transmission, and the interference caused by the paper machine transmission. There is a ground potential difference between DCS and MCC, so that when both ends of the cable shield are grounded, a loop is formed, which interferes with the bus signal. A large number of CI541 card error messages also appear on the controller-side engineer station, suspecting that a large number of error codes exist. In view of the above reasons, it was decided to replace the Profibus shielded cable with an optical cable. After the change, the situation improved and Gateway no longer crashed.
6 Conclusion
In the design of the paper machine control system! installation! During commissioning, special attention should be paid to the anti-interference problem of the field bus. Only when anti-interference measures are taken correctly can the bus run normally and provide a favorable guarantee for production.
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