Abstract: In isolated power supply systems, to prevent serious consequences caused by multiple grounding points, it is necessary to monitor the system's insulation to ground in real time and locate the fault when an insulation fault is detected. This paper introduces the working principle of an insulation location signal generator and elaborates on its hardware and software design. The product designed in this paper has been tested and verified, and can be applied to IT systems, providing a safe and reliable power supply solution for application environments.
Keywords: IT system, signal generator, fault location
0 Introduction
In IT systems, single-point grounding faults are a common type of fault. Once a single-point grounding 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 safety hazards. Therefore, it is necessary to monitor the system's insulation to ground in real time and to automatically locate the fault branch through instruments when an insulation fault to ground is detected. Without automatic location function, once a fault occurs, it is necessary to manually disconnect and search for dozens, hundreds, or even thousands of load branches one by one, which is not only time-consuming and laborious, but also seriously disrupts the continuity of power supply. This is not allowed in some special places that require continuous power supply (such as hospital operating rooms) [1].
Based on the above, this paper designs a signal generator for insulation fault location, which is installed in the IT system to work with the insulation fault location device to achieve the insulation fault location function. When an insulation fault occurs in the IT system, the signal generator starts and generates a location signal, which is injected between the IT system and ground. The insulation fault location device uses sensors to inspect each branch one by one. When the location signal is detected flowing through a certain branch, the branch where the insulation fault is located can be determined. At this time, the operator can perform targeted power outages or other protective operations on the faulty branch, without having to disconnect power to each branch one by one to check. This not only improves work efficiency but also effectively ensures the continuity of power supply to the system. Therefore, it is of extremely important significance for the safety, continuity, and reliability of power supply in the power system.
1. Signal Generation Principle
The signal generator works by injecting location signals between a specific line in the system and the ground in turn when a single-point grounding fault occurs in the IT system. This allows the insulation fault locator to detect the location signal on the faulty branch. The principle shown in Figure 1 is commonly used.
Figure 1. Signal generation principle
In IT systems, the effective value of the injected test signal must be small enough to avoid causing too much interference to the IT system or harming the system load; it must also have a large enough peak value to generate a large enough current on the faulty branch so that the current transformer of the fault locator can monitor it normally.
Considering both scenarios above, this paper uses a pulse signal as the test signal. If the pulse signal amplitude is large enough and the width is narrow enough, it is possible to achieve both a sufficiently small effective value and a sufficiently large peak value, meeting the two desired objectives. From a design simplification perspective, it is unnecessary to directly generate a high-voltage pulse signal on the signal generator; this can be achieved by extracting the peak value of the AC signal in the IT system.
For a single-phase AC IT system, the voltage between the two lines L1 and L2 is AC220V, and its peak value is , which meets the requirement that the pulse peak value is large enough. In order to meet the requirement that the effective value is small enough, this paper sets the voltage threshold to 50V in accordance with the standard IEC61557-9 "the effective value of the positioning signal voltage shall not exceed 50V"[2]. Based on this, the pulse width can be calculated (since the pulse width is very small, for the convenience of calculation, this peak pulse can be regarded as a rectangular pulse with an amplitude of ].
When the AC voltage period is 50Hz, the pulse width
When the AC voltage is 60Hz, the pulse width
By utilizing the timer function of the microcontroller in conjunction with an optocoupler, a peak pulse of 0.4ms can be accurately captured. Since 0.4ms < 0.4304ms < 0.5165ms, and the amplitude of the captured pulse signal is smaller than 0.4ms except for the peak, its effective value will definitely be less than the set threshold of 50V, which satisfies the requirement that the effective value of the pulse is sufficiently small.
2 Hardware Design
The hardware functional modules of this design 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 hardware design principle block diagram is shown in Figure 2.
Figure 2 Hardware Design Principle Block Diagram
After the signal generator is powered on, the CPU monitors the voltage of the IT system in real time through the monitoring module, measuring the AC frequency of the IT system. When a ground insulation fault occurs in the system, the signal generator determines the pulse width and pulse frequency of the test signal based on the measured frequency, extracts the system peak, generates a test signal, and applies it alternately between L1-PE and L2-PE. Due to the insulation fault, the faulty branch can be equivalent to a small-value resistor, connecting the faulty line of the IT system to the ground, forming a current loop. The test signal can generate a test current on the faulty branch. When the insulation fault locator circulates and monitors each branch, it can determine that the branch is the faulty branch if it detects this test current on a certain branch. In this design, the central control module uses the STMicroelectronics STM32F103 32-bit ARM CortexTM-M3 core microcontroller, which has a fast processing speed and a maximum operating speed of 72MHz. The chip has abundant on-chip and peripheral resources, including 20KB of on-chip RAM and 64KB of FLASH memory, a multi-channel 12-bit A/D conversion module, and multiple communication interfaces such as SPI, I2C, and CAN, which greatly simplifies the design of peripheral circuits.
3 Software Design
The control program for the signal generator was written in C language, employing a structured programming approach to facilitate code maintenance, portability, and upgrades. After power-on, the system first initializes and performs self-tests on each module to ensure system reliability. Then, once all hardware circuits are confirmed to be functioning correctly, it automatically enters normal operating mode. The main program flowchart is shown in Figure 3.
Figure 3 Software Flowchart
To ensure the accuracy and reliability of the signal generator's operation, specific software algorithms are employed, including:
(1) Digital Filtering Algorithm. As power systems become increasingly complex, the harmonic content in the power grid continues to increase. The first-hand signals acquired by the signal generator naturally contain a large number of harmonic components, as well as other noise interference. If these interferences are not filtered out, they will affect subsequent calculations. To avoid these effects, the software uses a digital filtering algorithm to process the acquired data, filtering out the harmonics, noise, and other interferences in the signal, allowing only the useful signals to participate in the result calculation, thereby making the calculation results more accurate and reliable.
(2) Adaptive AC Frequency Method for IT Systems. Due to the diversity of working environments, the working voltage is not necessarily 50Hz. The actual voltage frequency may be higher or lower. Therefore, the AC frequency of the IT system needs to be monitored in real time through a monitoring module. The monitoring module will compare the voltage between the two lines L1 and L2, and time the cases UL1>UL2 and UL1<UL2 respectively, denoted as t1 and t2. Since there is a certain threshold voltage when comparing voltages, there will be a phenomenon where t1>t2 or t2>t1. If t1+t2=20ms, that is, the AC frequency of the system is 50Hz, if a system-to-ground insulation fault occurs at this time, a pulse with a width of 0.4ms can be intercepted between (t1/2-0.2)ms and (t1/2+0.2)ms, and a pulse with a width of 0.4ms can be intercepted between (t2/2-0.2)ms and (t2/2+0.2)ms.
Figure 4. Voltage between L1 and L2 and the extracted pulse voltage.
As shown in Figure 4, in each cycle of the system voltage, the signal generator captures two pulses: one at the peak of the positive half-wave of L1-L2 (as shown in the second row of Figure 4), and the other at the peak of the negative half-wave of L1-L2 (as shown in the third row of Figure 4). If the fault point occurs on line L1, the pulse waveform captured at the peak of the negative half-wave of L1-L2 will appear positive on the faulty branch and can be detected by the insulation fault locator; if the fault point occurs on line L2, the pulse waveform captured at the peak of the positive half-wave of L1-L2 will appear positive on the faulty branch and can be detected by the insulation fault locator.
If t1+t2=10ms, considering the requirement that the effective value of the pulse is less than 50V, instead of taking two pulses per cycle (L1-L2 positive half-wave, L1-L2 negative half-wave), we can choose to take two pulses every two cycles (L1-L2 positive half-wave, L1-L2 negative half-wave). The same logic applies to other frequencies.
4. Application of Signal Generator in Medical IT Insulation Monitoring and Fault Location Systems
The signal generator designed in this paper has been successfully applied in the intensive care unit of a hospital, and the system application is shown in Figure 5. An insulation monitor, insulation fault locator, and signal generator are connected to form a local area network via a communication line. After power-on, the signal generator automatically enters monitoring mode to monitor the frequency of the IT system. When the insulation monitor detects a ground insulation fault in the IT system, it activates the signal generator and insulation fault locator via the communication line, entering signal generation mode and fault location mode respectively.
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 noise interference and the peak value is smaller than theoretical (the sine waveform in Figure 6 is the system voltage for comparison), but it still meets the requirements for insulation fault location. The waveform monitored at the insulation fault location instrument, after filtering and other preprocessing operations, is shown in Figure 7.
Figure 6. Waveform generated by the signal generator
Figure 7. Waveforms monitored by the insulation fault locator
As can be seen from Figure 7, the monitored pulse waveform is much higher than the interference waveform, forming a significant drop. By setting an appropriate threshold and combining it with conditions such as pulse width, it is possible to accurately determine whether a test signal passes through this branch, i.e., whether this branch has an insulation fault.
Upon detecting a faulty branch, the insulation fault locator displays the number of faulty branches and simultaneously transmits the faulty branch information back to the insulation monitor via the communication line. The insulation monitor immediately alarms, displays the number of faulty branches on its interface, and simultaneously commands the signal generator and insulation fault locator to stop generating signals and locating faults via the communication line. The signal generator then re-enters monitoring mode.
After the completion of this project, the system was debugged on-site, simulating 100 insulation faults, with a 100% insulation fault location rate. This fully demonstrates the feasibility of this design in engineering applications.
5 Conclusion
This paper designs an insulation fault location signal generator with functions such as adaptive IT system frequency, injection of high peak value and low RMS pulse waveforms, and indication of current operating status via panel indicator lights. The product based on this design meets the requirements of relevant national standards and can provide a safe and reliable power supply solution for IT systems. Finally, this paper also provides a preliminary discussion on insulation fault location in the IT system of a hospital intensive care unit, offering some reference for hospital building electrical designers. In actual application, the actual conditions of different projects are very complex, and many new problems will be encountered. Further discussion among colleagues is encouraged.
Source: *Intelligent Building Electrical Technology*, 2014, Issue 1
References:
[1] JGJ 16-2008 Code for Electrical Design of Civil Buildings [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
Email: [email protected] Mobile: 15000539641 Tel: 021-33510314
