Keywords: AC IT system insulation monitoring device high voltage IT distribution system
introduction
In some important industrial places (such as mines, glass factories and certain assembly venues for safe lighting, certain electric furnace test equipment, metallurgical plants and chemical plants, etc.), accidental power cuts will result in casualties and major property losses. Therefore, it needs to be powered by IT systems with high security and reliability. In the IT system, with the passage of time, the degree of insulation of the system to the ground decreases, when the first point of the ground fault occurs, the IT system can still operate normally, but at this time, the IT system already has security risks, if there are different phases The second point of the ground fault will cause a large short-circuit current, causing the front-end circuit breaker to trip, resulting in a power failure. According to Article 7.2.3 of the "JGJ 16-2008" "Design Code for Civil Building Electrical Appliances", the IT distribution system must be equipped with an insulation monitor. When the first ground fault occurs in the system, the device generates warning or alarm information, prompts 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.
The research on power system monitoring and fault diagnosis technology in foreign countries began in the 1960s. Various developed countries attach great importance, but in the 1970s and 1980s, with the development of sensor technology, signal acquisition technology, digital analysis technology, and computer technology. Development and application, online diagnostic technology has only developed rapidly. The traditional measurement methods include balanced bridge method, differential current detection method, and 555 timer measurement resistance method. These measurement methods all have their own advantages, but due to the different applications and the impact of the site environment, the above measurement methods also have the disadvantages of insufficient reliability, narrow measurement range, and low measurement accuracy. To solve these problems, this paper proposes a design of insulation monitoring device based on AC IT. The 12-bit AD sampling, 4th-order low-pass filtering circuit and 128x32 liquid crystal display built in STM32 are used on hardware. The software uses software filtering and least square method to find Slope and offset. The measurement accuracy (3%) and measurement range (0-999K) are maximized, and the precision monitoring needs can be met in different environments.
Insulation monitor working principle
The working principle of the insulation monitor is shown in Figure 1:
Figure 1: How the insulation monitor works
In the figure, R1 is the voltage divider resistor. Rf is the object monitored by the insulation monitor—the resistance of the system to the ground. The live conductor of the power supply end is not grounded, and it is only the protective grounding of the device housing. Under normal circumstances, the system and the ground are insulated, at this time Rf is equivalent to infinity; when the system has an insulation fault, such as the system wire and the shell directly contact, then lead to direct connection between the system and the ground, at this time Rf equivalent to 0 . The insulation monitor injects a DC signal into the system and enters the insulation monitor via Rf to form a closed loop. The Rf can be calculated by simple Ohm's law. The measurement principle is simple and reliable. It is suitable for IT systems that do not contain DC components, and because the use of DC signals can effectively avoid the effects of system capacitance and make the impedance measured with high accuracy, which can reflect the system's accuracy. Insulation performance.
hardware design
In this design, the central processing module uses a 32-bit ARM cortex-M3 core chip (STM32F103RBT6) produced by ST. The chip has a high processing speed, a clock frequency of up to 72 MHz, and has rich on-chip peripheral resources, with an internal 20 KB. On-chip SRAM and up to 64KB of FLASH flash memory, with multi-channel 12-bit AD conversion module, and multiple SPI, IIC, CAN and other communication interfaces, greatly simplifies the design of the peripheral circuit.
In addition to the most basic measurement system resistance to earth, the instrument comes with two relay outputs, adopts 128x32 LCD module as man-machine interface, with RS 485 communication, follows Modbus-RTU protocol, and has an early warning alarm function, each parameter can be self set up.
The hardware function module of the device mainly includes a power module, a signal injection module, a signal measurement module, a human-machine interface, a ferroelectric memory 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 the AC IT system, there are different voltage levels, such as 400V and 760V (higher voltage levels need to be used with high voltage couplers). Therefore, an 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 will continuously inject 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, the Go to R1 and R2 to find Rf. 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 existence of high-frequency signals, it may interfere with signal sampling. Therefore, the sampled signal must be filtered. The design uses a fourth-order low-pass filter circuit, which has good circuit cut-off characteristics and the attenuation rate of the curve. Steep, while improving the 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
(1)Let M6 be the point where R6, C6, and R7 intersect, the input voltage signal is Ui, and the output voltage signal is Uo. According to the imaginary short and virtual imaginary fault of the amplifier, the current equation for the M point is:
(2)among them
(3)The solution to the above two equations is:
(4)Comparison of the second-order low-pass filter circuit model of voltage-controlled voltage source can be obtained:
(5)In the formula, f0 represents the cut-off frequency, substitutes the data to obtain f0 ≈ 2.567Hz, this filter allows the frequency to pass the waveform below f0 to pass, the waveform that exceeds this frequency will decay to varying degrees.
The circuit is simulated below. The input is a clutter and its 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 waveforms are shown in Figure 6.
Figure 6: Filtered waveform
Comparing Fig. 5 and Fig. 6, the high-frequency clutter signal is filtered out, and the filtering effect is good to meet the test expectations.
2.3 Self-test circuit
According to IEC 61557-8 "Testing, measuring and monitoring equipment for protective measures against electrical safety in low voltage distribution systems up to 1000 V ac and 1500 V dc" Part 8: Insulation monitoring devices in IT systems Regulation 4.2, Insulation monitoring devices shall include a test The device is equipped with a test device connector to test whether the insulation monitoring device can perform its function.
Figure 7: Schematic representation of the least-squares method for linear fitting
To meet this requirement, a self-test circuit was designed inside the meter and a high-precision resistor R2 was built in. As shown in Figure 7. When the self-test is started, the relay operates, switching between the sampling signal Sample and self-inspection in the test circuit. The purpose of the self-test is to simulate a normal signal. The test device can measure the resistance of the built-in resistor and issue a self-test normal message.
software design
The insulation monitor adopts the idea of ​​structured programming and is written in C language. The device initializes the internal clock and required peripherals upon power-up, and then begins to read the calibration parameters stored in the ferroelectric factory. The calibration coefficients are stored in the ferroelectric memory, and there is no need to worry about power loss and data loss. When the device self-checks all the circuits, it enters the normal monitoring mode. Program flow chart shown in Figure 8:
Figure 8: Software Processing Flowchart
1. Least square method for linear fitting
Ideally, the insulation monitoring device should be linear over the entire measurement range. However, due to the differences in the parameters of the internal components of the circuit, the resistance measurement value may be distributed as a curve. At this time, the least squares method must be used to find out the most within a certain range. Straight line near the calibration point. The least-squares linear fitting diagram is shown in Figure 9.
Figure 9: Linear Fitting with Least Squares
If you know: y= ax + b, the equation is
Substituting the coordinate values, the coefficients a and b are obtained, and the coefficients are saved. When finding the other ordinate, only the parameters can be substituted. For this monitor, each point in the figure represents each calibration point, and the slope and offset can be obtained by substituting the data. The traditional way is to find the slope and offset values ​​about two points, so the measurement accuracy is relatively low. The specific comparison is shown in Figure 10.
Figure 10: Schematic
1 - Ideal Instrument Curve 2 - This article describes the instrument linear curve 3 - a commercially available instrument linear curve
It can be seen from Fig. 9 that the linearity curve obtained by the least-squares method of the instrument is closer to the ideal curve.
2. Digital filter algorithm
In an industrial IT distribution system, most electrical devices generate a lot of interference signals, so the device needs to filter out the noise in the signal and let the required signals participate in the result calculation. After the insulation monitor has collected the data, it filters out the noise interference through the internal digital filtering algorithm, and then calculates the size of the insulation resistance. The median filtering method is adopted here. The basic process is:
First sort the data in descending order (bubble method), remove the minimum and maximum values, and retain the intermediate values ​​(median filter). To do this several times, take the average of these times. (average filter method)
test results
Insulation monitors have passed relevant type tests, including electrical performance tests and electromagnetic compatibility (EMC) tests. Performance parameters exceed international standards. At 60°C, the data measured by the Insulation Monitor compared to the standard resistance and a commercially available meter are shown in the table below.
Table 60 °C (air humidity 95%) contrast
According to IEC 61557-8 "Testing, measuring and monitoring equipment for protective measures against electrical safety in low voltage distribution systems up to AC 1000V and 1500V dc" Part 8: Insulation monitoring devices in IT systems Table 4.6 of table 1 states that relative uncertainty must be Within ±15%. From the above table, however, the 555 timer method has a large measurement error fluctuation range, the high accuracy meter display error is kept within 3%, and the measurement accuracy is significantly higher than that of the meter, so the use effect is more stable and reliable in different environments. .
Conclusion
Due to the security of IT systems and the continuity of power supply, there are good prospects for development in the country, and their security and continuity are based on real-time monitoring. However, there are few kinds of insulation monitoring instruments available on the market, and the measurement range is narrow, and the measurement accuracy in different environments is inconsistent. For this situation, a high-precision insulation monitor was designed. The instrument's hardware and software measurement and processing methods have high overall performance, wide measurement range (0-999K), and high measurement accuracy (accuracy can be controlled within 3% at -20-65°C and 95% air humidity) This is something traditional instruments do not have.
Article Source: "Electrical Applications" 2015 No. 8.
references
[1] Wang Houyu. On the application of IT system. China ## Industrial Planning and Design Institute (Beijing).
[2] JGJ 16-2008 Civil Building Electrical Design Specification [S].
[3] Liu Guoping. Marine Electrical and Communication. First Edition. Beijing: Ocean Publishing House, 2004.
[4] Huang Shengjie, Yao Wenjie, et al. On-line monitoring and status maintenance of electrical equipment insulation. First Edition. Beijing: China Water Power Press, 2004.
[5] Hua Chengying, Tong Shibai. Analog Electronic Foundations. Fourth Edition. Higher Education Press, 2006.
[6] He Jing et al. Design of resistance measurement circuit based on MCU and 555 timer. Electronic Engineer, 2008(2).
[7] IEC 61557-8Electrical 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
About the author: Li Ping, Bachelor, Jiangsu Ankerui Electric Manufacturing Co., Ltd., technical engineer, the main research direction for intelligent power monitoring and power management system. Technical Communication Fax Mobile QQ Homepage: http://eim.acrel.cn
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