An insulation monitor (IMD) for industrial IT systems is introduced, and the hardware and software design principles of the insulation monitor are detailed. At present, the insulation monitor has been verified by tests and sold in the market, providing reliable insulation monitoring for industrial IT distribution systems.
Zhou Zanqiang 1 Shen Biao 2 Li Ping 3
(1. China United Engineering Corporation, Hangzhou 310052, China)
(2. Ankerui Electric Co., Ltd., Jiading, Shanghai 201801, China)
(3. Ankerui Electric Co., Ltd., Jiading, Shanghai 201801, China)
introduction
In some important industrial sites (such as mines, glass plants and certain gathering places for safety lighting, some electric furnace testing equipment, metallurgical plants and chemical plants, etc.), accidental power outages can cause casualties and significant property damage. Therefore, it is necessary to supply power with IT systems with high security and reliability. In the IT system, the degree of insulation of the system to the ground decreases with the passage of time. When the first point of ground fault occurs, the IT system can still operate normally, but at this time, the IT system has potential safety hazards, if different phases appear again. The second point of the ground fault will generate a large short-circuit current, causing the front-end circuit breaker to trip, causing the system to have a power failure. According to Article 7.2.3 of (JGJ 16-2008) "Code for Design of Civil Building Electrical Appliances", the IT power distribution system must be equipped with an insulation monitor. When the first point of ground fault occurs in the system, the device generates warning or alarm information, promptly reminds the maintenance personnel to troubleshoot the system, and does not need to trip in a short time, thus ensuring the reliability and continuity of the power supply of the IT system.
Foreign research on power system monitoring and fault diagnosis technology began in the 1960s, and all developed countries paid much attention to it, but in the 1970s and 1980s, along with sensor technology, signal acquisition technology, digital analysis technology and computer technology. Development and application, online diagnostic technology has been rapidly developed. Traditional measurement methods include balanced bridge method, differential current detection method, and 555 timer measurement resistance method. These measurement methods have their own advantages, but due to the different application sites and the influence of the on-site environment, the above measurement methods still have the disadvantages of insufficient reliability, narrow measurement range and low measurement accuracy. In response to these problems, this paper proposes an AC-based insulation monitoring device design: the hardware uses STM32 built-in 12-bit AD sampling, fourth-order low-pass filter circuit and 128x32 liquid crystal display, software software filtering and least squares method Slope and offset. Maximize measurement accuracy (3%), measurement range (0-999K), and meet the needs of precision monitoring in different environments.
Insulation monitor works
The working principle of the insulation monitor is shown in Figure 1:
Figure 1: How the insulation monitor works
In the figure, R1 is a voltage dividing resistor, and Rf is the object monitored by the insulation monitor—the system ground resistance, the live conductor at the power supply end is not grounded, and only serves as the protective ground for the equipment casing. Under normal circumstances, the system is insulated from the ground. At this time, Rf is equivalent to infinity. When the system has insulation failure, if the system wire is in direct contact with the outer casing, the system is directly connected to the ground. At this time, Rf is equivalent to 0. . The insulation monitor injects a DC signal into the system and enters the insulation monitor through Rf to form a closed loop. The Rf can be calculated by a simple Ohm's law. The measurement principle is simple and reliable, and is suitable for an IT system without a DC component, and the DC signal can effectively avoid the influence of the system capacitance, so that the measured impedance has high accuracy and can well reflect the system. Insulation performance.
hardware design
In this design, the central processing module selects the 32-bit ARM cortex-M3 core chip (STM32F103RBT6) produced by ST company. The chip has fast processing speed, the main frequency can reach 72MHz, and has abundant on-chip peripheral resources. The internal has 20KB. On-chip SRAM and up to 64KB of FLASH flash memory, multi-channel 12-bit AD conversion module, and multiple SPI, IIC, CAN and other communication interfaces, greatly simplifying the design of peripheral circuits.
In addition to the most basic measurement system ground resistance, the instrument has two relay outputs, using 128x32 liquid crystal module as human-machine interface, with RS 485 communication, following Modbus-RTU protocol, with early warning alarm function, each parameter can be self-contained. set up.
The hardware function module of the device mainly comprises a power module, a signal injection module, a signal measurement module, a human machine interface, a ferroelectric storage module, a communication module and a switch output module. The hardware block diagram is shown in Figure 2:
Figure 2: Insulation monitor hardware module design
1 signal measurement circuit
In AC IT systems, there are different voltage levels, such as 400V and 760V (higher voltage levels are required for use with high voltage couplers). Therefore, the insulation monitor needs to have a step-down circuit that satisfies these different voltage levels. After the insulation monitor is powered on, the signal injection module continuously injects a specific DC voltage into the monitored system. The system measures the sum of R1, R2, and Rf. Since the values ​​of R1 and R2 are known, they are reduced. R1 can be obtained by going to R1 and R2. The measurement circuit is shown in Figure 3:
Figure 3. Signal Measurement Circuit
2 filter amplifier circuit
In the actual power system, due to the presence of high-frequency signals, it may cause interference to the signal sampling. Therefore, the sampling signal should be filtered. The design uses a fourth-order low-pass filter circuit with good circuit cut-off characteristics and curve attenuation rate. Steep, while improving measurement accuracy, the filter circuit shown in Figure 4:
Figure 4: Fourth-order low-pass filter circuit
Since this circuit consists of two identical second-order circuits, only one analysis is required. For the first second-order circuit: when the frequency f=0, both C1 and C6 are open, the passband amplification
.jpg)
(1) Let R6, C6 and R7 intersect at M, the input voltage signal is Ui, and the output voltage signal is Uo. According to the virtual short and short break of the amplifier, the current equation for the M point is listed:
.jpg)
(2) among them
.jpg)
(3) Solving the above two equations can be obtained:
.jpg)
(4) Comparing the voltage-controlled voltage source second-order low-pass filter circuit model can be obtained:
.jpg)
(5) In the formula, f0 represents the cutoff frequency, and the data is f0 ≈ 2.567 Hz. The filter allows the waveform with a frequency lower than f0 to pass, and the waveform larger than the frequency will attenuate to different degrees.
The simulation is performed for this circuit below. The input is a clutter whose input contains a DC signal, a high frequency signal. Its waveform is shown in Figure 5:
Figure 5: Input Waveform
As can be seen from Figure 5, in addition to the DC waveform we injected, there are some high-frequency clutter signals. After the filter circuit, the waveform is shown in Figure 6:
Figure 6: Waveform after filtering
Comparing Fig. 5 and Fig. 6, the high frequency clutter signal is filtered out, and the filtering effect is good to meet the expected requirements of the test.
2.3 Self-test circuit
According to IEC 61557-8 "Testing, measuring and monitoring equipment for electrical safety protection measures in low-voltage distribution systems of 1000 volts and 1500 volts or less." Part 8: Insulation monitoring devices in IT systems, regulation 4.2, insulation monitoring devices shall include a test The device or the test device connector is installed to test whether the insulation monitoring device can perform its function.
Figure 7: Linear fitting of the least squares method
In response to this requirement, a self-test circuit is designed inside the meter, and a high-precision resistor R2 is built in. As shown in Figure 7. When the self-test is started, the relay operates to switch between the sampling signal Sample and self-inspection in the test circuit. The purpose of the self-test is to simulate a normal signal, whether the test device can measure the built-in resistance value, and issue a self-test normal information.
software design
The insulation monitor adopts the structural programming idea and is written in C language. The device initializes the internal clock and the required peripherals at power-on, and then begins to read the calibration parameters stored in the ferroelectric factory for commissioning. The calibration coefficients are stored in the ferroelectric memory without worrying about data loss due to power loss. When the device self-tests all circuits, it begins to enter the normal monitoring mode. The program flow chart is shown in Figure 8:
Figure 8: Software Processing Flowchart
1. Linear fitting with least squares method
Ideally, the insulation monitoring device should be linear throughout the measurement range. However, due to the difference in internal circuit components, the resistance measurement may be a curve. In this case, the least squares method is needed to find the most within a certain range. A line close to the calibration point. A schematic diagram of the linear fitting of the least squares method is shown in Figure 9:
Figure 9: Linear fitting of the least squares method
If known: y = ax + b, then the equation is
Substituting the coordinate values, obtaining the coefficients a and b, and saving the coefficients, when looking for another point of ordinate, simply substituting the parameters. For this monitor, each point in the figure represents each calibration point, and the slope and 0 offset can be obtained by substituting data. The traditional way is to find the slope and offset values ​​of the two points, so the measurement accuracy is relatively low. The specific comparison is shown in Figure 10:
Figure 10: Schematic
1-Ideal meter curve 2-This paper introduces the instrument linear curve 3-a commercially available instrument linear curve
It can be seen from Fig. 9 that the linear curve obtained by the least squares method of the meter is closer to the ideal curve.
2. Digital filtering algorithm
In industrial IT power distribution systems, most electrical equipment generates a lot of interference signals, so the device needs to filter out the noise interference in the signal and let the required signals participate in the result operation. After the data is collected by the insulation monitor, the internal digital filtering algorithm is used to filter out the noise interference, and then the insulation resistance is calculated. Here, the median-average filtering method is adopted. The basic process is: first sort the data from large to small (bubble method), remove the minimum and maximum values, and retain the intermediate values ​​(median value filtering method) ). Do this several times and take the average of these times. (average filtering method)
test results
Insulation monitors have passed relevant type tests, including electrical performance tests and electromagnetic compatibility (EMC) tests. Performance parameters exceed the requirements of international standards. At 60 ° C, the measured data of the insulation monitor is compared with the standard resistance and a commercially available instrument as shown in the table below.
Table 60 ° C (air humidity 95%) comparison
According to IEC 61557-8 "Testing, measuring and monitoring equipment for electrical safety protection measures in low-voltage distribution systems of 1000 V and DC 1500 V or less", Part 8: Table 1 of Table 4.6 of insulation monitoring devices in IT systems, the relative uncertainty must be Within ±15%. From the above table, the 555 timer method has a relatively large fluctuation range of measurement error, and the display error of the high-accuracy meter is kept within 3%, and the measurement accuracy is significantly higher than that of the meter, so the use effect in different environments is more stable and reliable. .
Conclusion
Due to the security of IT systems and the continuity of power supply, there is a good development prospect in China, and its security and continuity are based on real-time monitoring. However, there are few types of commercially available insulation monitoring instruments, and the measurement range is narrow, and the measurement accuracy in different environments is inconsistent. In response to this situation, a high-precision insulation monitor was designed. The instrument adopts software and hardware measurement and processing methods with high comprehensive performance, wide measuring range (0-999K), high measurement accuracy (accuracy can be controlled within 3% under the condition of -20-65 °C air humidity 95%) This is not available in traditional instruments.
Article source: "Electrical Applications", 2015, issue 8.
references
[1] Wang Houyu. On the application of it system. China Aviation Industry Planning and Design Institute (Beijing).
[2] JGJ 16-2008 Civil Building Electrical Design Specification [S].
[3] Liu Guoping. Ship Electrical and Communication. First Edition. Beijing: Ocean Press, 2004.
[4] Huang Shengjie, Yao Wenjie, et al. On-line monitoring and state maintenance of electrical equipment insulation. First edition. Beijing: China Water Resources and Hydropower Press, 2004.
[5] Hua Chengying, Tong Shibai. Analog Electronic Foundation. Fourth Edition. Higher Education Press, 2006.
[6] He Jing et al. Design of resistance measurement circuit based on single chip microcomputer and 555 timer. Electronic Engineer, 2008 (2).
[7] IEC 615578 Electrical safety in low voltage distribution systems up to 1000Va.c.and1500V dc
Equipment for testing, measuring or monitoring of protective measures – Part8: Insulation monitoring devices for IT systems
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
Zirconium Target ,Zirconium Bars,Zirconium Tubes
Baoji Yongshengtai Titanium Industry Co., Ltd. , https://www.bjystititanium.com