With the development of substation integrated automation, DC systems are increasingly using microcomputers for control. The WD CX-2 microcomputer online insulation monitoring device, as a component of the microcomputer DC system, is used to automatically find the grounding line and display and print it when the insulation of the DC system fails. This greatly saves the time and effort required for manual line finding and provides a guarantee for quick and accurate troubleshooting. However, if the device itself has a problem, such as incorrect response, then troubleshooting will go in the wrong direction and cause unnecessary trouble. Therefore, when debugging the device on site, it is necessary to ensure that the device operates correctly. The following is an introduction to some problems that occurred during debugging and operation. [b]1 Case Introduction[/b] (1) One day, the DC system of substation M experienced a grounding. The DC system of the substation is shown in Figure 1. The WDCX-2 device (hereinafter referred to as the device) responded that the 6th line was grounded. However, after repeated searches, it was found that the grounded line was the 3rd line, and the device made a mistake. After the DC system of the substation was upgraded, grounding tests were conducted on each branch, and the device responded correctly. [img=303,246]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/40-1.jpg[/img] (2) On a certain day, the DC transformation of substation T was completed, and a grounding test was conducted on the device. The device responded correctly. After the test was completed, each DC feeder was introduced into the transformed microcomputer DC system. Then, a grounding test was conducted on the spare 6th feeder. After the device operated, it responded that the 2nd feeder was grounded, and the device made a wrong selection. The DC system of the substation is shown in Figure 2. [img=267,158]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/40-2.jpg[/img][align=left][b]2 Cause Analysis[/b] Why did the wrong selection occur? Here, we will first introduce the working principle of the device: When the insulation of the DC system is intact, the positive and negative poles are balanced with respect to ground. The device collects and displays the positive and negative pole voltages to ground as +110 V and -110 V, respectively. At this time, no signal is emitted. When the insulation of the DC system is damaged, the voltage value of the pole that is grounded will decrease, while the voltage value of the other pole will increase. At this time, the device automatically alarms and simultaneously generates a low-frequency AC signal, which is capacitively coupled to the DC system. The transmitting and receiving lines of this low-frequency signal (i.e., DC feeders) both pass through a current transformer, as shown in Figure 3. [img=300,283]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/jsdjgc/2000-2/40-3.jpg[/img] During normal operation, only DC current flows through the primary side of this current transformer. When the low-frequency signal emitted by the device enters a healthy branch, its receiving port can receive the signal. That is, the low-frequency AC signals flowing through the primary side of the current transformer, when superimposed, result in zero. Therefore, there is no current on its secondary side, and this branch will not be selected. The device continues to emit low-frequency signals to the next branch. When the emitted signal enters a branch where a ground fault has occurred, due to the presence of the grounding point, the signal directly enters the ground. There is no low-frequency AC signal in the receiving line, but there is an AC signal on the primary side of the current transformer, generating a current on its secondary side. After sensing this current, the device performs calculations, comparing the calculated ground resistance with the resistance to ground of the line under normal conditions stored in the device's memory. If there is a significant change, the device considers the line to be grounded; if there is no significant change, the device judges the line to be normal. If it is a DC bus ground fault, the resistance to ground of each branch will change significantly, in which case the device judges it as a bus ground fault. Whether it is a negative or positive ground fault is determined by the device based on the collected positive and negative bus voltages to ground. Therefore, the WDCX-2 device determines the branch that experienced a grounding fault based on the change in grounding resistance of each branch after a grounding fault occurs in the DC system. Under normal circumstances, the ground resistance of the DC system is considered to be in the megaohm range; therefore, the value stored in the input device's memory is also at its maximum value, i.e., infinity. Having gained a general understanding of the device's principle, the following analysis addresses the cause of the misselection: Previously, secondary equipment in substations was primarily electromagnetic or transistor-based, and the DC power supply to these devices was completely insulated from ground. However, most modern equipment is microcomputer-based or digital. To prevent interference or damage to the components within these devices, anti-interference capacitors coupled to ground are typically installed at the DC power input terminals to prevent AC interference. If the input power contains AC components, these components pass through the capacitor to the ground, while the DC power supply is input normally (load 1 in Figure 3). Therefore, when the DC power supply to these devices contains a certain AC component, it is not completely insulated from ground. If a ground fault occurs in the DC system, the device emits a low-frequency AC signal. When this signal enters a branch with a ground coupling capacitor (but not grounded), the emitted signal directly enters the ground due to the capacitor's presence. The device receives no signal, and the current induced on the secondary side of the current transformer in that branch is collected and calculated. The device then compares the calculated ground resistance value with its stored value to the change in ground resistance. If the calculated value is the change in ground resistance, the device will determine that the branch is grounded, and the actual grounded branch will not be selected. Sometimes, because there are many branches with ground coupling capacitors, a ground fault in one branch of the DC system may be mistaken for a bus ground fault. In the two substations mentioned earlier, before commissioning the DC system, grounding tests were conducted on each branch. Before simulating a ground fault on a branch, the other branches were disconnected. This resulted in the ground resistance of the remaining branches being infinite. Therefore, regardless of whether these branches contain ground coupling capacitors, they would not be selected, while the branch undergoing the test would have a significantly different ground resistance and would be selected accordingly. Therefore, the device's response is correct. After the DC system is put into operation, the parameters of each branch change. If a ground fault occurs in a normally operating DC system, the effect of the ground coupling capacitance becomes apparent. [b]3 Solution[/b] From the above analysis, we know that the ground resistance value in the device's memory is key to solving the problem of device misselection. The specific solution is: After the test is completed and the load is connected, the device should scan the entire DC system, that is, input the parameters of each branch in the DC system, so that the ground resistance reference value in the device's memory matches the actual value. In this way, when a ground fault occurs in the DC system, when the low-frequency AC signal emitted by the device enters a branch containing ground coupling capacitance, if the ground fault occurs in that branch, the ground resistance will change significantly, and that line will be selected; if it is not that branch that is grounded, the AC signal enters the ground, and there is a signal on the secondary side of the current transformer on the branch. After the device collects this signal, it calculates it and compares it with the reference value. Since the reference value at this time is the ground resistance value calculated by the device, the line will not be selected. This method can solve the problem of device misselection. It is particularly important to note that when a new branch is connected to the DC system, the DC system needs to be re-scanned; otherwise, incorrect selection may occur. Sometimes, when disconnecting the other branches to perform a grounding test on a certain branch, the device may report a bus grounding. This is generally due to the polarity of some current transformers installed on each branch being reversed or broken.