[Abstract] On the basis of introducing the working principle of the signal generator for insulation positioning, the hardware and software design of the signal generator is expounded. The products based on this design have passed the test and can be used in IT systems to provide safe and reliable power supply solutions for the application sites. ![]()
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Keywords IT system signal generator fault location
0 Preface
Once a single-point ground fault occurs in the IT system, it will become a TN-S system. Although it can continue to operate with a fault, it has lost the advantages of the IT system and increased potential safety hazards. Therefore, it is necessary to monitor the ground insulation status of the system in real time and The instrument automatically locates the fault point branch, otherwise, once a fault occurs, it can only rely on manual to locate the insulation fault point, which not only takes time and effort, but also destroys the continuity of power supply. To this end, this paper designs a signal generator for insulation fault location (hereinafter referred to as signal generator), 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, without having to check the power of each branch to improve the work efficiency and ensure the continuity of the system power supply.
1 Principle of signal generator
The working principle of the signal generator: 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 principle of the signal generator is shown in Figure 1.
Figure 1 Schematic diagram of the signal generator
In the IT system, the effective value of the injected test signal must be small enough so as not to cause too much interference to the IT system or even cause harm to the system load; but it must have a large enough peak to form a large enough on the fault branch Current, so that the current transformer of the fault locator can be monitored normally.
After considering the above two situations, this article uses the pulse signal as the test signal. The pulse signal amplitude is large enough and the width is narrow enough to achieve the two desired goals of sufficiently small effective value and sufficiently large peak value. 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-phase AC IT system, the voltage between the L1 and L2 lines is AC 220V, and its peak value is 220V, which meets 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 in accordance with the standard "the effective value of the positioning signal voltage is not allowed to exceed 50V" in the standard IEC61557-9. According to this, the pulse width can be calculated (since the pulse width is small, for convenience of calculation, this peak pulse can be regarded as a rectangular pulse with an amplitude of 220):
When the AC voltage period is 50 Hz, the pulse width is:
When the AC voltage is 60 Hz, the pulse width is:
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 other points are less than ![]()
V, so its effective value must be less than the set threshold (50V), which meets the requirement that the pulse effective value is sufficiently small.
2 Hardware design
The hardware function modules of the signal generator mainly include a power supply module, a central control module, a monitoring module, a signal generation module, a communication module, and an 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 resistance, connecting the faulty line of the IT system and the ground to form a current loop, so the test signal can generate test current on the fault branch. When the insulation fault locator patrols each branch one by one, the test current is monitored on a certain branch, and this branch can be determined as a fault branch. In this design, the central control module selects ST's 32-bit ARM-M3 core single-chip microcomputer STM32F103. The chip has a fast processing speed and a maximum operating speed of 72MHz; it has a wealth of on-chip peripheral resources, with 20KB of on-chip RAM and up to 64KB of FLASH flash memory, 12-bit A / D conversion module with multiple channels, and Multiple SPI, C, 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 programming 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; then after determining that the hardware circuits in the system are normal, it automatically enters the normal working mode. The main program flow of the system is shown in Figure 3.
Figure 3 System main program flow chart
In order to ensure the accurate and reliable operation of the signal generator, and to ensure that the device will not be misoperation, the software uses a specific program algorithm for processing.
(1) Digital filtering algorithm. The signal generator uses a digital filtering algorithm to filter out harmonics, noise, and other interference in the signal, and only allows useful signals to participate in the result operation, thereby making the calculation results 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 compares the voltage between the L1 and L2 lines, and the times of UL1> UL2 and UL1 <UL2 are recorded as t1 and t2, respectively. Because 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), when there is a system-to-ground insulation failure, you can ![]()
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, at each cycle of the system voltage, the signal generator intercepts two pulses, respectively at the peak of the positive half-wave of L1-L2 (second line in Figure 4), and the negative half-wave of L1-L2 At the peak (third line in Figure 4). If the fault point is on the L1 line, the pulse waveform intercepted at the negative half-wave peaks of L1-L2 can appear positive on the fault branch and can be monitored by the insulation fault locator; if the fault point is on the L2 line, then The pulse waveform intercepted at the positive half-wave peaks of L1-L2 can appear positive on the fault branch and can be monitored by the insulation fault locator.
Figure 4 The voltage between L1 and L2 and the intercepted pulse voltage
If t1 + t2 = 10ms, considering the requirement that the effective value of the pulse is less than 50V, then instead of intercepting 2 pulses per cycle (L1-L2 positive half-wave, L1-L2 negative half-wave), choose to intercept every two cycles Two pulses (positive half-wave L1-L2, negative half-wave L1-L2). Other frequencies can be deduced by analogy.
The actual positioning signal generator is shown in Figure 5. It uses DC 24V power supply, and the panel has LEDs for "Run", "Communication" and "Test" to show the working status.
Figure 5 physical diagram of the signal generator
When the IT power distribution system runs without failure, the signal generator automatically monitors the system frequency. When a single-point ground fault occurs in the IT power distribution system, the signal generator generates a test pulse signal, and cooperates with the insulation monitor and insulation fault locator to locate the fault branch.
The signal generator has passed the type test inspection, and all the indicators have reached the requirements of national standards. At present, it has been successfully applied to the intensive care unit of a hospital, as shown in Figure 6. 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 activated through the communication line to enter the signal generation mode and the fault location mode.
Figure 6 Application diagram of IT system in the intensive care unit of a hospital
In actual engineering applications, the pulse waveform generated by the signal generator is shown in Figure 7.
Figure 7 Waveform generated by the signal generator
It can be seen from Figure 7 that this waveform has a lot of clutter interference, and the peak value is also smaller than the theory (the sinusoidal waveform in the figure is the system voltage, as a comparison), but it still meets the insulation fault location requirements. The waveform monitored at the end of the insulation fault locator, after preprocessing operations such as filtering, is shown in Figure 8.
Figure 8 The waveform monitored by the insulation fault locator
It can be seen from Figure 8 that the monitored pulse waveform is higher than the interference waveform, forming an obvious drop. By setting an appropriate threshold and matching pulse width and other conditions, it can be accurately judged whether a test signal passes through this branch, that is, whether this branch has an insulation failure.
After monitoring the faulty branch, the insulation fault locator displays the number of faulty branches, and at the same time, returns the faulty branch information to the insulation monitor through the communication line. After receiving the information, the insulation monitor immediately alarms, displays the number of fault branches through the interface, and at the same time commands the signal generator and the insulation fault locator to stop sending signals and locate the fault, the signal generator enters the monitoring mode again.
Commissioned the system on site, simulated insulation faults 100 times, and the insulation fault location rate was 100%, which fully proved the feasibility of the signal generator.
4 Conclusion
The signal generator designed in this paper has the functions of adaptive IT system frequency, injection of high peak value and low effective value pulse waveforms, multi-system networking, etc., and can display the current working status through the panel indicator. The signal generator complies with relevant national standards, and together with the insulation monitor and insulation fault locator, can provide a safe and reliable power supply solution for the IT system.
Article Source: "Electrical Technology" 2014 No. 4
references
[1] GB-50054-2011 Low-voltage distribution system design specification [S]
[2] JGJ 16-2008 Civil Building Electrical Design Code [S].
[3] 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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