Keywords: IT system; single-point ground fault; ground-insulation monitoring signal generator; fault location; pulse signal; digital filtering; frequency adaptation
0 Preface
In IT systems, a single-point ground fault is a very common fault. Once a single-point ground fault occurs, the IT system will become a TN-S system. Although it can continue to operate with the fault, it has lost the advantages of the IT system and increased potential safety hazards. Therefore, it is necessary to monitor the insulation status of the system in real time, and when the insulation fault is detected, the branch of the fault point can be automatically located by the instrument. Otherwise, once a fault occurs, the location of the insulation fault point can only be achieved by manually powering down as many as tens, hundreds, or even thousands of load branches one by one, which is not only time-consuming and labor-intensive, but also more serious Destroy the continuity of power supply. This is not allowed in some special places that require continuous power supply, such as hospital operating rooms, etc. [1].
Based on the above situation, this paper designs a signal generator for insulation fault location, which is installed in the IT system and cooperates with the insulation fault location device to realize the insulation fault location function. When an insulation fault occurs in the IT system, the signal generator starts and generates a positioning signal, which is injected between the IT system and ground. The insulation fault location device patrols the roads by sensors. When it detects that the positioning signal flows through a branch, it can be determined that the branch is the loop where the insulation fault is located. At this time, the operator can purposely perform power failure or other protection operations for the faulty branch, without having to check the power of each branch, not only greatly improving the work efficiency, but also effectively guaranteeing the continuity of the system power supply . Therefore, it is extremely important to the safety, continuity and reliability of power supply in the power system.
1 Principle of signal generation
The working principle of the signal generator is that when a single-point ground fault occurs in the single-phase IT system, the positioning signal is injected between a certain line of the system and the ground in turn, so that the insulation fault locator can detect the positioning signal on the fault branch, and its signal The principle of occurrence is shown in the figure.
Figure 1 The generation principle of the signal generator
In the IT system, the effective value of the injected test signal must be small enough to avoid too much interference to the IT system, bring unnecessary hidden dangers, and even cause harm to the system load; and there must be a large enough peak to facilitate failure A sufficiently large current is formed on the branch circuit, so that the current transformer of the fault locator can be normally monitored.
Considering the above two situations, this article uses pulse signals as test signals. If the amplitude of the pulse signal is large enough and the width is narrow enough, the two desired goals of sufficiently small effective value and sufficiently large peak value can be achieved. From the perspective of simplifying the design, 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 supply required for ordinary electrical equipment is AC 220V, 50 / 60Hz. For a specific application, such as the safety lighting of a second-class medical place or some assembly places, it can be obtained by converting the isolation transformer. The voltage between L1 and L2 line is AC 220V, the peak value is To meet the requirement that the pulse peak value is large enough. In order to meet the requirement that the effective value is sufficiently small, this article sets the voltage threshold to 50V according to the standard "the effective value of the positioning signal voltage is not allowed to exceed 50V" in the standard IEC61557-9 [2]. According to this, the pulse width can be calculated (because the pulse width is small, for convenience of calculation, the peak pulse can be regarded as the amplitude of
Rectangular pulse).
When the AC voltage period is 50Hz, the pulse width
When the AC voltage is 60Hz, the pulse width
Using the timer function of the single-chip microcomputer and the optocoupler, it can accurately intercept the peak pulse of 0.4ms. Since 0.4ms <0.4304ms <0.5165ms, and because of the actual intercepted pulse signal, the amplitude of the other points is less than the peak point Therefore, its effective value must be less than the set threshold of 50V, which can meet the requirement that the pulse effective value is sufficiently small.
2 Hardware design
The hardware function module of this design mainly includes power supply module, central control module, monitoring module, signal generation module, communication module and indicator light module. The block diagram of the hardware design is shown in Figure 2.
Figure 2 Block diagram of hardware design
After the signal generator is powered on, the CPU monitors the voltage of the IT system in real time through the monitoring module and measures the AC frequency of the IT system. When the insulation fault to the ground occurs in the system, the signal generator determines the pulse width and pulse frequency of the test signal according to the measured frequency, intercepts the system peak, generates the test signal, and adds it between L1-PE and L2-PE in turn. Due to an insulation fault, the fault branch can be equivalent to a smaller value resistance, connecting the fault line of the IT system and the ground to form a current loop, then the test signal can generate test current on the fault branch, and the insulation fault locator When the road patrol monitors each branch, the test current is detected on a certain branch, and this branch can be determined as a faulty branch. In this design, the central control module uses 32 bits produced by ST The core single chip microcomputer STM32F103, the chip has a fast processing speed and the highest running speed can reach 72MHz. The chip has abundant on-chip peripheral resources. There are 20KB of on-chip RAM and up to 64KB of FLASH flash memory, a 12-bit A / D conversion module with multiple channels, and multiple SPI,
, CAN and other communication interfaces greatly simplify the design of peripheral circuits.
3 Software design
The control program of the signal generator is written in C language, and a structured program design method is adopted in the program design to facilitate the maintenance, transplantation and upgrade of the program code. After the system is powered on, first complete the initialization and self-test of each module to ensure the reliability of the system's work, and then determine that each part of the hardware circuit in the system is normal, and automatically enter the normal working mode. .
Figure 3 Software flow chart
In order to fully ensure the accuracy and reliability of the signal generator operation, the software design uses a specific program algorithm for processing, mainly including:
(1) Digital filtering algorithm. With the increasing complexity of the power system, the harmonic content in the power grid continues to increase, and many places cannot be avoided. The first-hand signal collected by the signal generator naturally also contains a lot of harmonic components, as well as some other noise interference. If these interferences are not filtered out, it will affect subsequent calculations. In order to avoid these effects, after the data is collected on the software, a digital filtering algorithm is used to process, filter out the harmonics, noise and other interference parts of the signal, and only allow useful signals to participate in the result operation, thereby making the calculation result more Accurate and reliable.
(2) The adaptive frequency method of AC frequency in IT system. Because of the diversity of the working environment, the working voltage is not necessarily 50 Hz, and the actual voltage frequency may be higher or lower, so the AC frequency of the IT system should be monitored in real time through the monitoring module. The monitoring module will compare the voltage between the two lines L1 and L2. with
The situation is separately timed, recorded as
with
. Since there is a certain threshold voltage during voltage comparison, there will be
or
The phenomenon. in case
, That is, the AC frequency of the system is 50Hz.
versus
Intercept a pulse with a width of 0.4ms, in
versus
Intercept a pulse with a width of 0.4ms. As shown in Figure 4.
Figure 4 The voltage between L1 and L2 and the intercepted pulse voltage
As shown in Figure 4, each cycle of the system voltage, the signal generator intercepts two pulses, respectively at the peak of the positive half-wave of L1-L2 (as shown in the second line of Figure 4), and the negative half-wave of L1-L2 At the peak (see the third row of Figure 4). If the fault point 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 fault branch and can be monitored by the insulation fault locator; if the fault point occurs on the L2 line On the above, the pulse waveform intercepted at the peak of the positive half-wave of L1-L2 can appear positive on the fault branch and can be monitored by the insulation fault locator.
in case ， Considering the requirement that the effective value of the pulse is less than 50V, instead of intercepting two pulses per cycle (L1-L2 positive half-wave, L1-L2 negative half-wave), choose to intercept two pulses every two cycles (L1-L2 Positive half-wave, L1-L2 negative half-wave). Other frequencies can be deduced by analogy.
4 Test results
At present, the product prototype based on this design has passed the type test inspection, and the inspection content includes the safety performance and electromagnetic compatibility performance of the product, and the various indicators of the product have reached the requirements of the national standard.
In the actual test, the pulse waveform generated by the signal generator is shown in Figure 5. From the figure, we can see that there is a lot of clutter interference in this waveform, and the peak value is also more theoretical Some are too small (the sinusoidal waveform in the figure is the system voltage, as a comparison), but it still meets the requirements of insulation fault location. The waveform monitored at the end of the insulation fault locator, after preprocessing operations such as filtering, is shown in Figure 6. .
Figure 5 The waveform generated by the signal generator
Figure 6 Waveforms monitored by the insulation fault locator
It can be seen from Figure 6 that the monitored pulse waveform is much higher than the interference waveform, forming an obvious drop. By setting an appropriate threshold and matching the pulse width and other conditions, you can accurately determine whether this branch has The test signal passes, that is, whether there is an insulation fault on this branch.
5 Conclusion
The signal generator for insulation fault location designed in this paper has the functions of adaptive IT system frequency, injection of high peak value and low effective value pulse waveform, etc., and can indicate the current working state through the panel indicator. Products based on this design meet the requirements of relevant national standards and can provide safe and reliable power supply solutions for IT systems.
The article comes from: "Building Electricity" No. 12 of 2013.
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 installations [M]. 3rd edition. Beijing: China Electric Power Press, 2012.
[4] Wang Houyu. 600 Questions of Building Electrical Installations [M]. 3rd Edition. Beijing: China Electric Power Press, 2013.
[5] China United Engineering Corporation. GB50052-2009 power supply and distribution system design specifications [S]. Beijing: China Planning Press, 2010.
[6] China Electromechanical Design and Research Institute Co., Ltd. GB50054-2011 Low-voltage power distribution design specifications [S]. Beijing: China Planning Press, 2012.
About the Author:
Yu Jing, female, undergraduate, engineer of Wuhan Ankerui Electric Co., Ltd., the main research direction is intelligent power monitoring and power management system
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