Application of microcomputer insulation monitoring device in DC system and discussion on grounding fault point detection

1 Introduction
　　 The insulation of the DC system to the ground is not very good due to the humid plant environment of the hydropower station. The DC loop cables are widely distributed, there are many components, and the fault point is difficult to find. When the insulation of the DC system decreases, it will affect the safe operation of the unit control system. In this regard, we conducted an application study on the DC system microcomputer insulation monitoring device, with the aim of quickly finding the fault point, eliminating hidden dangers in time, and improving the operating reliability of the DC system.
　　Chencun Hydropower Station is currently the largest hydropower station in Anhui Province. Chencun Station has 3×50 MW units and Jicun Station has 2×17 MW units. In 1998 and 1999, a WZJ-1 microcomputer DC insulation monitoring device was installed at Jicun Station and Chencun Station respectively. The two devices were put into operation after artificial simulation grounding tests and various technical parameters were measured correctly. The device can correctly respond and detect the fault point when a DC insulation fault occurs, playing an important role in the production site.
　　The main reasons for the grounding of the DC system are: insulation aging, moisture, damage of cables or equipment components, and damage to the power board of the device. The hazards of multi-point grounding in DC system: it may cause relay protection and automatic devices to malfunction, refuse to operate, burn out relay contacts and blow fuses.
　　The traditional method of finding DC grounding is to pull the line to find the segmented processing method and manually untie the line to find the fault point. When the circuit power is pulled for a short time, the protection device or automatic device may trip due to the anti-interference performance or fault judgment problem due to the loss of DC power. The harm caused by the DC branch method to detect the fault point is serious. Therefore, the State Power Corporation issued a comprehensive letter on power transmission [2001] No. 238 "Several Regulations on Unmanned Hydropower Plants" in Article 7.6.1, which pointed out: It is strictly forbidden to use the method of cutting DC load during equipment operation to find and handle DC grounding faults. When using a 500V megger to test insulation, all weak current circuits must be withdrawn to prevent damage. This method cannot accurately locate the fault point, making on-site personnel feel helpless and difficult when finding DC grounding. In addition, the disadvantage of the old insulation monitoring device using the bridge balance principle is that it cannot truly reflect the insulation of the DC bus, but can only reflect the imbalance of the insulation resistance of the positive and negative busbars. The grounding monitoring relay in the grounding detection loop limits the insulation resistance of the busbar to 30kΩ. The use of the WZJ-1 device can overcome the above shortcomings.
　　Through research on the advantages and disadvantages of the current domestic microcomputer DC insulation monitoring devices, the detection principle and accuracy are affected by the characteristics of the DC system itself and can only be measured by indirect methods. To improve the detection accuracy will lead to the complexity of the device structure, which will have an adverse impact on the DC system and make installation and maintenance difficult (such as adding a large number of branch current sensors and secondary wiring). Therefore, it is particularly important to meet the practical requirements of the on-site detection accuracy of the microcomputer DC insulation monitoring device and the comprehensive analysis and judgment of the fault point. This article makes some analysis and discussion on this issue.

2. Device features and field applications
2.1. Introduction to device principles and features
　　The WZJ-1 device host realizes online detection of DC system voltage and positive and negative bus insulation resistance and fault judgment; when a point grounding occurs in the DC system, a low-frequency small signal oscillator is put into operation, and a portable DC grounding point detection clamp meter is used to find the specific location of the grounding fault point without disconnecting the DC power supply.
　　The device uses a single-chip microcomputer to automatically put a detection resistor into the positive and negative poles of the DC system, calculates the insulation resistance of the DC system to the ground in real time and displays it, and alarms when abnormal; it can improve the detection sensitivity and overcome the detection dead zone of the insulation monitoring device. In order to avoid the influence of the large distributed capacitance of the DC system cable to the ground on the measurement accuracy of the detection grounding resistance, a comprehensive judgment criterion is used in principle to correct the influence of the distributed capacitance, so that the accuracy rate of judging the grounding fault reaches 100%, solving the problem of misjudgment of grounding. The signal injection method (via capacitor coupling) can accurately find the location of the fault point. The real-time display of the device facilitates timely understanding of the insulation status of the DC system, which provides great convenience for on-site operators.
　　The main technical indicators of the WZJ-1 device: the internal resistance of the device is greater than 200kΩ, the sensitivity of grounding resistance detection can reach 20kΩ, and the device does not affect the reliable operation of the DC system.
2.2 Field application technical parameter measurement of the device
　　After simulating the DC negative pole to be grounded through a 4kΩ resistor behind the 110kV bypass mother #410 protection screen, the WZJ-1 device displays: R+＝ 72kΩ (R+＝100kΩ in normal conditions); R-=3kΩ (R-= 98kΩ in normal conditions). At this time, the grounding signal light on the device is on, and a DC insulation drop GP signal is issued. The low-frequency small signal switch on the device is turned on, the positive pole light is on, and the negative pole light is off, indicating that there is a grounding fault in the DC negative pole. The grounding branch and fault point are accurately found using the caliper meter on the portable detection receiver. By artificially simulating the grounding fault of the DC system, the device responds correctly, and the caliper meter can accurately find the location of the fault point.
　　In actual operation, if a DC grounding point occurs (≦20 kΩ), close the low-frequency small signal switch and check the positive and negative self-test lights. If the lights are off, it means the bus is faulty. The on-site personnel can use a caliper meter to find the fault point of the grounding branch.

3. Detection and analysis of DC grounding fault points
3.1 Statistics of faults detected by WZJ-1 device on site
　　 The statistics of faults detected by WZJ-1 device in the five years since its on-site operation are as follows:

3.2 Example of fault detection by WZJ-1 device on site
　　The schematic diagram of insulation drop detection of 110kV#402 switch control circuit at Jicun Station on June 5, 2000 is shown in Figure 1. The switch was in the open state. The caliper meter was used to accurately locate the ground fault point. It was found that the insulation of one end of capacitor C was damaged. After replacing C, it was normal. It can be seen that the device is convenient and quick to find the fault point. [page]

3.3 WZJ-1 device provides real-time information to timely discover major equipment hidden dangers. On
　　April 5, 2002, the operator of Chencun Station found that the light sign GP of "110kV#487 line switch is abnormal" was slightly lit, and WZJ-1 showed R+ 24kΩ. After inspection, it was found that the density meter of the SF6 switch of the #487 line had caused the insulation between the contacts and the DC system to decrease due to the infiltration of rainwater. According to the comprehensive analysis of the fault phenomenon, the fault point was quickly found. The slight light of GP indicated that the insulation of the contact of the density meter had been short-circuited by water. If it was not discovered in time, it would be very harmful. During the heavy rain on April 16, 2002, WZJ-1 showed R+＝25kΩ; R-＝100kΩ. After a few hours after the rain stopped, R+＝50kΩ. Comprehensive analysis showed that it was caused by the rainy weather causing the equipment to be damp. It can observe and reflect the operating status of the equipment more clearly than before, and preventive measures can be taken in time to prevent the expansion of hidden dangers.
　　Subsequent inspections revealed that the densitometers of the nine 110kV SF6 switches (Model FXT-11) at Chencun Station all had hidden dangers of the cable connection parts of the contacts being easily invaded by rainwater. There are two pairs of threshold contacts when the SF6 gas pressure drops. The first threshold alarms, and the second threshold prohibits the switch from tripping and closing. Because there is a gap in the rain cover just above the connector where the densitometer is connected to the cable, water easily enters the two pairs of contacts, causing insulation degradation. The first threshold causes the central control room to mistakenly send an alarm signal, and the second threshold will lock the switch operation circuit. When a line fails in thunderstorm weather, it will cause the switch to refuse to operate, expanding the accident. For this major equipment hidden danger, improvement measures were taken in time to prevent rainwater from intruding outdoor equipment.
3.4 Use comprehensive analysis methods to quickly detect, locate and isolate fault points
　　. Statistics show that the failure rate of insulation degradation in DC systems is currently lower than before. In the past, only the cable circuits, main controllers, terminal boxes and other places with weak insulation that were damp at the turbine layer had frequent insulation degradation failures, making it difficult to handle the fault points. According to the previous handling of on-site faults, there are many problems with wiring process quality and cable and equipment aging. The previous cable insulation material was rubber, which aged seriously after long-term operation and the insulation performance decreased, especially the cable of the accident lighting circuit of the hydropower station. The old plant should speed up the pace of technical transformation to improve the overall insulation level of the DC system.
　　At present, the automation components used in the technical transformation of the unit equipment are of good quality (joint venture or imported products), the reliability of the components has been greatly improved, the sealing is good, and the situation of moisture and insulation degradation rarely occurs, which was the weakest link before. The new switchgear attaches importance to the moisture-proof design of the outdoor terminal box (such as the moisture-proof performance of ABB switch and ALSTHOM switch mechanism box), which greatly reduces the moisture of the insulation of outdoor electrical equipment. At the same time, after replacing the bakelite terminals of the outdoor terminal box with terminals of new materials, the electrical insulation performance in the outdoor environment is improved. Especially after the control cable is replaced, the insulation level of the DC system has been greatly improved.
　　From the current on-site operation situation: equipment failures in the unit excitation circuit and speed regulation circuit; switch operation circuit tripping, closing coil burnout; device switch power board failure, etc. are easy to cause DC grounding faults. The DC grounding faults that have occurred in recent years are mainly caused by design problems or wiring problems in the control circuits of outdoor high-voltage switches, emergency lighting circuits, and new equipment when they are put into operation. For this reason, it is necessary to focus on on-site technical management and take preventive measures for weak links.
　　Through the analysis of the technical transformation practices of our station in the past 5-6 years, the application of new technologies and new materials has greatly improved the insulation level of the DC system itself. After the control cables of the entire plant were replaced with shielded cables with good anti-interference performance and grounded at both ends, the distributed capacitance of the DC system has also been greatly reduced. Therefore, the insulation drop fault characteristics of the DC system are very different from before, mainly manifested as sudden insulation drop faults, and the situation where the insulation of the DC system is often bad or dropped in many places in the past rarely occurs now. Therefore, the monitoring technology principle of the WZJ-1 device still has high accuracy and practical value under the new situation, and it will not make the detection circuit too complicated, and the fault finding is simple and fast, the physical concept is clear, and it is easy for on-site personnel to master. Through the real-time display of the WZJ-1 device, some changes in the insulation resistance of the on-site equipment can be seen. If it is introduced to the host computer for comprehensive analysis, it is conducive to mastering the insulation condition of the on-site equipment.

4 Discussion on relevant issues in device application In
　　2000 and 2003, our station carried out technical renovation of the DC system and battery of Chencun Station and Jicun Station respectively, but the WZJ-1 device was still retained in the DC insulation monitoring device, which met the on-site operation requirements and saved equipment investment. However, the following issues should be noted in the equipment interface processing.
　　Chencun Station has a DC insulation monitoring circuit on the new battery screen, which is connected in parallel with the monitoring circuit of WZJ-1, resulting in WZJ-1 displaying an insulation resistance to ground of 5kΩ and sending an alarm signal. The improvement method is to remove the ground resistance circuit of the DC insulation monitoring device on the new battery screen and only put the monitoring circuit of the WZJ-1 device into operation. The new GZDW intelligent DC cabinet of Jicun Station comes with a balanced bridge relay insulation monitoring alarm device. The monitoring circuit is removed and not used, and only the WZJ-1 device is used.
　　Aspects of the device that need to be improved: The device uses an AC220V power supply. After switching to factory power, the device must be manually reset to resume operation. It is better to use DC220V power supply on site. It is recommended to use a high-reliability electronic switch to replace the imported micro-relay switching resistor solution to measure the insulation resistance of the DC system to ground. The LED display uses an active liquid crystal display to display more information. The input of the set value is stored in EEPROM, and the device fault diagnosis function is added. It has the function of fault report storage and communication with the host computer. According to the regulations: when the insulation of the DC system drops to 15-20kΩ, the device should be able to operate and alarm, and it is more appropriate to increase the set value from 5kΩ to 15-20kΩ. Because the switching resistor solution is used to measure the insulation resistance of the DC system to ground, R+ and R- are connected in parallel with 100kΩ, and the actual insulation resistance can only be displayed as 100kΩ when it is greater than 100kΩ. Since the low-
　　frequency oscillator is not put into operation under normal circumstances, and the number of ground faults occurring each year is generally no more than 5 times, each detection time is counted as 1 hour. It can be seen that the low-frequency oscillator with a small signal amplitude has a negligible impact on the DC system.
　　At the site, we used a large number of LFP-900 and RCS-9000 microcomputer protection devices of Nanrui Protection Company, which have very small anti-interference capacitance in hardware structure. According to the IEEE document in October 1994: 0.05μf is enough for microcomputer protection anti-interference capacitance, which can achieve the purpose of anti-interference and will not make the DC system capacitance to ground too large. At the same time, the grounding of the two ends of the control cable shield also reduces the capacitance of the DC system to ground. This can reduce the detection error for the microcomputer DC insulation monitoring device running on site. Since the
promotion and application of microcomputer protection, the on-site personnel are worried about the damage of electronic components by the high voltage series protection device during the test of the insulation detection of the secondary circuit. Therefore, it is imperative to study and test the online insulation monitoring of the secondary circuit. Long-term operation shows that the real-time detection function of WZJ-1 is more practical on site.
　　On-site operation shows that the WZJ-1 device can meet the operation requirements of the DC system of medium-sized hydropower stations in terms of detection accuracy, anti-interference ability, and impact on the DC system.

5 Application Experience
　　Two WZJ-1 microcomputer DC insulation monitoring devices have been put into operation in our station. The devices have been running continuously for five years without any failure. They are stable and respond correctly when a ground fault occurs in the DC system, and can accurately locate the fault point. The device is installed on the control panel in the central control room, which makes it easy for operators to understand the insulation of the DC system to the ground at any time. It has been welcomed by on-site personnel since it was put into operation. It
　　is not appropriate to use the detection method of adding auxiliary magnetic rings to each branch to improve the insulation monitoring of the DC system of the old power station. It is more appropriate to use the WZJ-1 device for detection. The site is quite satisfied with the results achieved. In power plants where the DC system capacitance to the ground is not very large, the detection accuracy of this solution is high and can meet the needs of the site.
　　It is necessary to point out that due to the particularity of the DC system itself, the function of the insulation monitoring device must first be able to correctly detect the system insulation drop fault, but the technical realization of the automatic and accurate location identification and alarm function of the fault point or branch is far from reaching the technical performance of the correct action of the relay protection in the AC system to remove the fault, and further breakthroughs are still needed in the technical principles and technical implementation methods. The effective method to ensure the safe operation of the DC system on site is to carry out technical renovation of the cables with aging insulation in the unit control circuit, old components, terminal boxes with poor sealing performance, SF6 density meters, and accident lighting circuits; at the same time, the insulation inspection work should be strengthened during equipment maintenance, especially the insulation of the tripping and closing coils and their circuits in the switch mechanism box. Since they are movable parts, their failure rate is relatively high. After the insulation level of the DC system is improved through technical transformation, it is easier to find the fault point when the insulation drop fault occurs, and the sensitivity of signal injection and caliper meter detection will also be greatly improved. In the past, the insulation level of the whole plant was very low, and it was difficult to find the insulation drop fault, and the sensitivity of signal injection and caliper meter detection was also low. By summarizing the on-site operation experience and combining the use of advanced microcomputer monitoring devices, the safe operation level of the DC system will be further improved.
　　The detection results of the fault point by WZJ-1 on site were all correct, and there was no false alarm. However, it is understood that the magnetic ring branch type has the problem of false alarm, and there may still be deficiencies in the principle judgment, which needs to be further improved. For the public-XM audio bus signal circuit, if the insulation of the signal circuit decreases, each branch and circuit must be checked one by one, and even the use of magnetic ring branch alarm cannot solve the problem. Therefore, the microcomputer DC insulation monitoring device combined with the portable detection caliper meter is currently the most effective detection equipment for finding the grounding fault point of the DC system on site.