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Realization of remote monitoring system for frequency conversion equipment based on CAN bus

Publisher:BlossomBeautyLatest update time:2011-06-08 Reading articles on mobile phones Scan QR code
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Overview :
In recent years, with the proposal of building a conservation-oriented society, the importance of energy conservation and power saving has become increasingly high, especially the energy-saving transformation of large-scale electrical equipment in state-owned enterprises has been strengthened. The number of inverters used in large state-owned enterprises such as oil fields, coal mines, power plants, and steel mills has increased. Especially in oil fields, power electronic equipment such as pumping unit inverters, submersible electric pump inverters, water injection pump inverters and heating power supplies have been widely promoted and used in oil fields. According to the particularity of field operations in oil field production , the distance between each device is relatively far, but relatively concentrated, so centralized control based on various field buses is becoming more and more necessary. The CAN bus has shown outstanding advantages in the centralized management and control of frequency conversion equipment. Its ultra-long-distance transmission and super anti-interference performance are unmatched by other buses. Another important point is that in the entire control system, there is no master-slave networking topology, which is convenient for adding and reducing communication nodes.
The medium and low voltage inverters and heating power supplies of Shandong Xinfengguang Electronic Technology Development Co., Ltd. have stable performance, complete protection functions, and RS232 and RS485 communication interfaces. As of June 2007 , the total number of units in use in Daqing Oilfield and Shengli Oilfield exceeded 700 , of which Shengli Oilfield has about 500 units, and the number is increasing rapidly. In addition, Xinxiang Pharmaceutical Group also has hundreds of fermentation tank inverter equipment. This is a very high market share for inverters of the same brand in the same field. This provides a great market condition for the remote centralized monitoring system based on the CAN bus, and it is necessary to promote it. In addition, the system can be flexibly applied to production sites with multiple inverter equipment at the same time.
CAN bus introduction:

The communication protocol used by CAN is CSMA / CD protocol ( Carrier Sense Multiple Access/ Collision Detection ). Each node on the network always monitors whether the bus is idle when sending data to the bus. If no conflict is detected, that is, the bus is in an idle state, then each node sends data to the bus and each node has an equal opportunity, that is, carrier sense multiple access. If two nodes send data to the bus at the same time, the node detects a conflict and makes corresponding lossless arbitration processing. That is, after the conflict, the data can remain unchanged , continue to monitor the bus, and wait for the next transmission.
The CAN protocol is a protocol based on message format rather than a protocol based entirely on node ID transmission, which abolishes the traditional station address encoding. Based on this protocol: the transmission of messages is not only transmitted from one node to another according to the address, but also multicast and broadcast can be realized. During broadcasting, each node in the system can receive the data transmitted on the bus and confirm whether each message can be received correctly. At the same time, each node can determine whether the received data should be saved or discarded immediately. The error detection in the CAN communication process mainly includes: response error, format error, bit error, and fill error. The main error states include error activation, error recognition, bus shutdown, etc.
The CAN bus has the following characteristics:

(1) CAN can be a peer-to-peer structure, that is, a multi-host working mode.
Figure 1 System topology
Inverter node information collection:
Node information mainly refers to the various operating parameters and operating status of the inverter. The commonly used parameter settings of wind and solar inverters are: operating frequency, maximum frequency, starting frequency, acceleration time, deceleration time, rated current, V/F curve selection, open loop / closed loop setting, multi-speed setting, actual input current and voltage, actual output current and voltage, low frequency compensation; in addition, there are a variety of fault protection monitoring: short circuit protection (instantaneous current exceeds 1.8 times of the rated current), overcurrent protection (current exceeds 1.5 times of the rated current and lasts for 1 minute ), overvoltage protection (input voltage exceeds 1.2 times of the rated voltage), undervoltage protection (input voltage is lower than 0.8 times of the rated voltage ), temperature rise protection (inverter internal temperature is higher than 7 5 degrees Celsius ), phase loss protection, external abnormality protection, etc.
There are currently two acquisition methods: For the original model, the main control chip is N 87C 196MC , the external communication mode of the main control board is RS485 , so an RS485 - CAN conversion circuit must be added when communicating with the CAN bus.
Figure 2 Schematic diagram of node information collection
With the upgrading of products, the application of DSP makes communication easier . The 2407A chip integrates a CAN communication interface, so the above circuit can be omitted. The upgraded wind and solar inverter will make CAN bus communication much simpler.
Data transmission and conversion:
After the data is sent to the bus, it must be transmitted through the medium. Since the CAN protocol itself optimizes the error control algorithm, it does not have high requirements for the medium. An ordinary twisted pair can achieve this. The transmission distance is 10km . However, considering the absolute reliability of inverter operation and equipment control, we Use twisted pair cables within a 5km transmission range. At 5km, optical fiber is used as the intermediate transmission medium to improve anti-interference capabilities.
When CAN is transmitted to the control computer, an external level converter must be added to complete the data exchange with the main control computer. The conversion principle is shown in Figure 2. In addition, there are relatively mature products of this type of conversion device in the current market. As shown in Figure 3 , the conversion interface launched by Zhou Ligong is basically a transparent device for users and is relatively convenient to use.
Figure 3 CAN-232 conversion interface card
Human Machine Interface:

The monitoring program of the monitoring computer is configured in two types: a specific system and a general system. The following is a general-purpose centralized monitoring human-machine interface for 20 sets of equipment. The entire human-machine interface system consists of two parts: the equipment monitoring part and the equipment control part.
If a device is added to the system, a device number can be set for the device manually and stored in the system, that is, a device number is enabled. After a device is removed from the system, the device number can be deleted and reused. The enabled device number is the operable number, otherwise the number is inoperable. When the device is running normally, the green light is on when it is normal, and the red light is on when an abnormality occurs.
For the convenience of operation, multiple devices in the whole system can be divided into several control groups for management. When operating by group, all devices in the group can be operated at the same time: start, stop, emergency stop, parameter setting, frequency adjustment, etc. The common parameter settings in the group can be viewed at the same time. Double-clicking the device number can also view the operating status and operating curve by device, and can also conveniently perform various operations on a single device. All devices in the system can also be operated at the same time.
Figure 4 Schematic diagram of human-machine interface
Conclusion:
In the whole system, the design is based on the principle of flexibility and convenience, and is designed to adapt to multiple work sites. After a hardware node fails, the data can be uploaded in the shortest time. The host computer can also observe and control each device in real time, achieving the purpose of remote monitoring, facilitating operation, enhancing system reliability, and saving production costs.
Reference address：Realization of remote monitoring system for frequency conversion equipment based on CAN bus

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