Design of an Insulation Fault Location Signal Generator

Design of an Insulation Fault Location Signal Generator

Li Min 1 Yang Shuai 2 Li Ping 3 Sun Lili 4
(1. Li Min, Shandong Provincial Institute of Architectural Design, Jinan, 250001)
(2. Yang Shuai, Ankerui Electric Co., Ltd., Shanghai Jiading 201801)
(2. Li Ping, Ankerui Electric Co., Ltd., Shanghai Jiading 201801)
(2. Wang Changqing, Ankerui Electric Co., Ltd., Shanghai Jiading 201801)

Abstract: Based on the introduction of the working principle of the insulation location signal generator, the hardware and software design of the signal generator are described. The products based on this design have passed the test and can be applied to IT systems to provide safe and reliable power supply solutions for application sites.
Keywords: IT system; single point earth fault; ground insulation monitoring signal generator; fault location; pulse signal; digital filtering; frequency adaptation
0 Introduction Single point earth faults are a common failure in IT systems. Once a single point of earth fault occurs, the IT system becomes a TN-S system. Although it can continue to run with faults, it has lost the advantages of IT systems and increased security risks. Therefore, it is necessary to monitor the insulation status of the system against the ground in real time, and when the insulation fault of the ground is monitored, the fault point branch can be automatically located through the meter. Otherwise, in the event of a failure, the search for the location of insulation faults can only be achieved by manually powering up to tens, hundreds, or even tens of thousands of load branches one by one, which is not only time consuming, but also more serious. Destruction of power supply continuity. This is not allowed in certain special places that require continuous power supply, such as hospital operating rooms [1].
Based on the above situation, a signal generator for locating insulation faults is designed in this paper. It is installed in the IT system and uses the insulation fault locating device to realize the insulation fault locating function. When an IT system has an insulation fault, the signal generator activates and generates a positioning signal that is injected between the IT system and the ground. The insulation fault location device patrols through the sensor one by one. When detecting that the positioning signal flows through a certain branch, it can be determined that the branch is the circuit where the insulation fault is located. At this time, the operator can purposely perform power-off or other protection operations on the faulty branch, eliminating the need to conduct inspections on a branch-by-branch basis, which not only greatly improves work efficiency, but also effectively protects the continuity of the system power supply. . Therefore, it is of great importance to the safety, continuity and reliability of power supply to the power system.
1 Principle of signal generation The working principle of the signal generator is that when a single-point earth fault occurs in a single-phase IT system, a positioning signal is injected between a certain system line and the ground in turn so that the insulation fault locator can detect the positioning on the fault branch. Signal, the principle of its signal as shown in the figure.

Figure 1 The principle of signal generator

In the IT system, the effective value of the injected test signal must be small enough to avoid too much interference with the IT system, causing unnecessary hidden dangers and even causing harm to the system load. In addition, there must be a large enough value for the failure. A sufficiently large current is formed on the branch so that the current transformer of the fault locator can be normally monitored.
Considering the above two conditions, this paper uses pulse signals as test signals. If the amplitude of the pulse signal is sufficiently large and the width is sufficiently narrow, a sufficiently small effective value, a sufficiently large peak, and two desired targets can be achieved. From a simplified design point of view, there is no need to directly generate a high-voltage pulse signal on the signal generator, which can be achieved by intercepting the peak of the AC signal in the IT system.
The power required for common electrical equipment is AC 220V, 50/60Hz. For a specific application, such as the safety lighting of the second type medical place or some meeting places, it can be converted by the isolation transformer. L1, L2 line voltage AC 220V, its peak value is To meet the requirement of a sufficiently large pulse peak. In order to meet the requirement that the effective value is sufficiently small, according to the IEC 61557-9 standard, "the effective value of the positioning signal voltage must not exceed 50V," the voltage threshold is set to 50V [2]. In this way, the pulse width can be calculated (since the pulse width is very small, this peak pulse can be regarded as The rectangular pulse).

When the AC voltage cycle is 50Hz, pulse width

When the AC voltage is 60Hz, pulse width

Using the timer function of the SCM and optocoupler, the 0.4ms peak pulse can be accurately intercepted. Since 0.4ms<0.4304ms<0.5165ms, and because of the actually intercepted pulse signal, the amplitude of the remaining points is smaller than Therefore, its effective value must be less than the set threshold 50V, and it can meet the requirement that the pulse effective value is sufficiently small.
2 hardware design The designed hardware function module mainly includes the power module, the central control module, the monitoring module, the signal generation module, the communication module, the indicator module. The hardware design principle block diagram is shown as in Fig. 2.

Figure 2 Hardware Design Block Diagram

After the signal generator is powered on, the CPU monitors the voltage of the IT system through the monitoring module in real time and measures the AC frequency of the IT system. When the system occurs insulation failure to the ground, the signal generator determines the pulse width and pulse frequency of the test signal according to the measured frequency, intercepts the system peak, generates a test signal, and alternately adds to the L1-PE and L2-PE. As a result of the insulation fault, the faulty branch can be equivalent to a smaller value resistance, connect the fault line of the IT system, and the earth forms a current loop, then the test signal can generate a test current on the faulty branch, the insulation fault locator When the road is touring and monitoring each branch, the test current is monitored on a branch road and the branch can be determined as a failure branch. In this design, the central control module selects 32 bits produced by ST Core SCM STM32F103, the chip processing speed, the maximum operating speed up to 72MHz. The chip has a wealth of on-chip peripheral resources, internal 20KB of on-chip RAM and up to 64KB of FLASH flash memory, multi-channel 12-bit A/D conversion module, and multiple SPIs. Communication interfaces such as CAN can greatly simplify the design of peripheral circuits.
3 software design signal generator control program written in C language, in the program design uses a structured program design method, easy to program code maintenance, transplantation and upgrade. After the system is powered on, it first completes the initialization and self-test of each module to ensure the reliability of the system work. After determining that the hardware circuits of each part of the system are normal, it automatically enters the normal working mode. The flow chart of the main program of the system is shown in Figure 3. .

Figure 3 software flow chart

In order to fully ensure the accuracy and reliability of the signal generator operation, the software design adopts a specific program algorithm for processing, which mainly includes:
(1) Digital filter algorithm. As the power system becomes more and more complex, the harmonic content in the power grid continues to increase, and many places cannot avoid it. The first-hand signal collected by the signal generator naturally also contains a large number of harmonic components, as well as other noise interference. If these interferences are not filtered out, they will have an impact on subsequent calculations. In order to avoid these influences, after the data is collected in the software, a digital filtering algorithm is used to filter out the harmonics, noises, and other interferences in the signal, and only the useful signals are involved in the result calculation, so that the calculation result is further improved. Accurate and reliable.
(2) IT system AC frequency adaptive method. Because of the diversity of working environment, the working voltage is not necessarily 50Hz, and the actual voltage frequency may be higher or lower. Therefore, the monitoring system needs to monitor the IT system's AC frequency in real time. The monitoring module will compare the voltages between the L1 and L2 lines. with The time of each case is counted as with . Since there is a certain threshold voltage when the voltage is compared, it will exist or The phenomenon. in case , that is, the system AC frequency is 50Hz. If there is a system-to-ground insulation failure at this time, you can versus Intercept a pulse with a width of 0.4ms between versus Intercept a pulse with a width of 0.4ms. As shown in Figure 4.

Figure 4 Voltage between L1 and L2 and Intercepted Pulse Voltage

As shown in FIG. 4, during each period of the system voltage, the signal generator intercepts two pulses, respectively at the peak of the positive half-wave of L1-L2 (the second row in FIG. 4), and the negative half-wave of L1-L2. The crest (Figure 3, line 3). If the fault occurs on the L1 line, the pulse waveform intercepted at the peak of the negative half-wave of L1-L2 can appear positive on the faulty branch and can be monitored by the insulation fault locator; if the fault occurs on the L2 line Above, the pulse waveform intercepted at the peak of the positive half-wave of L1-L2 can appear positive on the faulty branch and can be monitored by the insulation fault locator.
in case Considering that the pulse RMS value is less than 50V, two pulses (L1-L2 positive half-waves, L1-L2 negative half-waves) can be intercepted without selecting two pulses per two cycles (L1-L2). Positive half wave, L1-L2 negative half wave). Other frequencies can be analogized in turn.
4 Test results At present, the product prototype based on this design has passed the type test and inspection. The inspection includes the safety performance and electromagnetic compatibility of the product. All the indicators of the product meet the requirements of the national standard.
In the actual test, the pulse waveform generated by the signal generator is shown in Figure 5. As can be seen from the figure, the waveform has a lot of clutter interference, and the peak value is also more theoretical than Some small (the sinusoidal waveform in the figure is the system voltage, as a comparison), but still meet the requirements of insulation fault location, waveforms monitored at the end of the insulation fault locator, after filtering and other preprocessing operations, as shown in Figure 6 .

Figure 5 waveform generated by the signal generator

Figure 6 Waveforms Monitored by Insulation Fault Locator

It can be seen from Fig. 6 that the detected pulse waveform is much higher than the interference waveform, forming a significant drop, and by setting an appropriate threshold and the pulse width and other conditions, it can accurately determine whether this branch has The test signal passes, that is, whether the branch has an insulation fault.
5 Conclusion Insulation fault location signal generator designed in this paper, with adaptive IT system frequency, high-peak injection, low RMS pulse waveform and other functions, and can indicate the current working status through the panel light. Products based on this design meet the requirements of relevant national standards and can provide IT systems with safe and reliable power supply solutions.

The article comes from: "Construction Electric" 2013 the 12th period.

references
[1] China Architecture Northeast Design and Research Institute JGJ 16-2008 Civil Building Electrical Design Code [S]. Beijing: China Building Industry Press, 2008
[2]IEC 61557-9 Electrical safety in low voltage distribution systems up to 1 000 V ac and 1 500 V dc— Equipment for testing, measuring or monitoring of protective measures —
Part 9: Equipment for insulation fault location in IT systems[S].2009
[3] Wang Houyu. Design, installation and inspection of low-voltage electrical devices [M]. 3rd ed. Beijing: China Electric Power Press, 2012.
[4] Wang Houyu. Building electrical installation 600 [M]. 3rd edition. Beijing: China Electric Power Press, 2013.
[5] China United Engineering Corporation. GB50052-2009 Power Supply and Distribution System Design Specification [S]. Beijing: China Planning Press, 2010.
[6] China Electromechanical Design and Research Institute Co., Ltd. GB50054-2011 Design Specification for Low Voltage Distribution [S]. Beijing: China Plan Press, 2012.

About the Author:

Sun Lili, female, undergraduate, engineer of Shenyang Ankerui Power System Integration Co., Ltd., the main research direction is intelligent power monitoring and power management system, technical exchange fax mailbox: