Keywords: IT system signal generator fault location
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. If there is no automatic positioning function, once a fault occurs, you can only rely on manual search for up to tens, hundreds, or even tens of thousands of load branches one by one, which not only takes time and effort, but also seriously damages 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 carry out power-off or other protection operations for the faulty branch, and it is not necessary to check the power of each branch, which not only improves work efficiency, but also effectively guarantees the continuity of 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 IT system, the positioning signal is alternately injected between a certain line of the system and the ground, so that the insulation fault locator can monitor the positioning signal on the fault branch. The generating principle shown in Figure 1 is often used.
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 or damage to the system load; and there must be a large enough peak to form a large enough current on the fault branch, The current transformer of the fault locator can be monitored normally.
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 effective value small enough and peak value sufficient 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.
For a single AC IT system, the voltage between the two lines L1 and L2 is AC220V, and its 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 the actually intercepted pulse signal, except for the peak point, the amplitude of the remaining points are less than 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 modules of this design mainly include power supply module, central control module, monitoring module, signal generation module, communication module and indicator 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. It connects the faulty line and the ground of the IT system to form a current loop. The test signal can generate test current on the fault branch. When monitoring each branch, this test current is monitored on a certain branch, and this branch can be determined as a faulty branch. In this design, the central control module selects the 32-bit ARM CortexTM-M3 core single-chip microcomputer STM32F103 produced by ST Company. The chip has a fast processing speed and the maximum operating speed can reach 72MHz. The chip has abundant on-chip and peripheral resources, on-chip RAM 20KB and FLASH flash memory 64KB, with multi-channel 12-bit A / D conversion module, and multiple communication interfaces such as SPI, I2C, CAN, etc. design.
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 uses specific program algorithms for processing, mainly including:
(1) Digital filtering algorithm. As the power system becomes more complex, the harmonic content in the power grid continues to increase. 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, the software uses digital filtering algorithm to process after collecting the data, filtering out the harmonic and noise interference parts of the signal, and only allowing useful signals to participate in the result operation, thereby making the calculation result more accurate 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 compares the voltage between the two lines L1 and L2, and counts the conditions of UL1> UL2 and UL1 <UL2 respectively, and records them as t1 and t2. Since there is a certain threshold voltage during voltage comparison, there will be a phenomenon of t1> t2 or t2> t1. If t1 + t2 = 20ms, that is, the system AC frequency is 50Hz, if a system-to-earth insulation fault occurs at this time, a width between (t1 / 2-0.2) ms and (t1 / 2 + 0.2) ms can be intercepted For a 0.4 ms pulse, a pulse with a width of 0.4 ms is intercepted between (t2 / 2-0.2) ms and (t2 / 2 + 0.2) ms.
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.
If t1 + t2 = 10ms, considering that the effective value of the pulse is less than 50V, you can not intercept two pulses per cycle (L1-L2 positive half wave, L1-L2 negative half wave), but choose to intercept two every two cycles Subpulse (L1-L2 positive half wave, L1-L2 negative half wave). Other frequencies can be deduced by analogy.
4 Application of signal generator in medical IT insulation monitoring and fault location system
The signal generator designed based on this article has been successfully applied to the intensive care unit of a hospital. The system application is shown in Figure 5. Through the communication line, the insulation monitor, insulation fault locator and signal generator form a local area network. After the signal generator is powered on, it automatically enters the monitoring mode to monitor the frequency of the IT system. When the insulation monitor detects an insulation fault to the ground in the IT system, the signal generator and the insulation fault locator are started through the communication line to enter the signal generation mode and the fault location mode.
Figure 5 Application diagram of IT system in the intensive care unit of a hospital
In practical engineering applications, the pulse waveform generated by the signal generator is shown in Figure 6, as can be seen from the figure, the waveform has a lot of clutter interference, and the peak value is also more theoretical It is too small (the sinusoidal waveform in Figure 6 is the system voltage, as a comparison), but it still meets the requirements for insulation fault location. The waveform monitored at the end of the insulation fault locator, after preprocessing operations such as filtering, is shown in Figure 7. .
Figure 6 The waveform generated by the signal generator
Figure 7 The waveform monitored by the insulation fault locator
It can be seen from Figure 7 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 is tested. The signal passes, that is, whether there is an insulation fault on this branch.
After monitoring the fault branch, the insulation fault locator displays the number of fault branches, and at the same time, returns the fault branch information to the insulation monitor through the communication line. The insulation monitor immediately alarms and displays the number of fault branches through the interface. At the same time, the signal generator and the insulation fault locator are commanded to stop the signal generation and fault location through the communication line, and the signal generator enters the monitoring mode again.
After the completion of the construction of the project, the system was debugged on site to simulate 100 insulation faults and the insulation fault location rate was 100%. Fully prove that this design is feasible in engineering applications.
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. At the end of this article, we also made a preliminary discussion on the insulation fault location of the IT system in the intensive care unit of the hospital to provide a reference for the hospital building electrical designers. In application, the actual situation of different projects is very complicated, and many new problems will be encountered, and colleagues hope to discuss further.
Article source: "Intelligent Building Electrical Technology", Issue 1, 2014
references:
[1] JGJ 16-2008 Civil Building Electrical Design Code [S].
[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
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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