Design of DC system ground fault detection application based on ARM

0 Introduction
The DC power supply system of power plants and substations is the working power supply of control and signal systems, relay protection and automatic devices. It is very important to ensure the safe operation of power systems, which requires that the DC system and its network must have high reliability. When a point grounding occurs in the DC system, it should be able to be found and processed in time. At present, most of the insulation monitoring devices based on single-chip microcomputers at home and abroad are developed based on traditional balanced bridge method, low-frequency signal injection method, etc., but to solve the problems of large capacitor grounding and ring network influence in DC system, more advanced processing methods are needed, such as detection methods based on wavelet transform, and the limited resources of single-chip microcomputers limit the use of such methods.
This paper designs a DC system grounding fault detection device through an ARM-based embedded system, and implements a detection method based on wavelet transform in the device to solve the problems of large capacitor grounding and ring network influence in DC system, and focuses on the design of application programs based on the system.

1 ARM microprocessor S3C44BOX
Considering the needs of design applications, Samsung's S3C44BOX microprocessor was selected. This is a SOC chip based on ARM7TDMI of the company. On the one hand, it has the advantages of low power consumption and high performance of ARM processors; on the other hand, it has very rich on-chip resources, which is very suitable for the development of embedded products. Its features are as follows:
· Using ARM7TDMI core, I/O voltage 3.3V, core voltage 2.5V;
· Built-in phase-locked loop (PLL), the system frequency is up to 66MHz;
· 4 working modes, can realize power management to reduce system power consumption;
· 8kB system cache (CACHE), greatly improving the system operation speed;
· Support 8 MEMORY BANKs, the maximum external storage space is 256MB, and supports SDRAM;
· Built-in color LCD controller;
· 2 asynchronous serial ports (UART);
· 71 general I/O ports;
· 8-channel analog/digital converter (ADC);
· Real-time clock (RTC) and watchdog circuit (WATCHDOG).

2 Overall structure of the DC system grounding fault detection device
As shown in Figure 1, it can be seen that this detection device is mainly divided into two parts: S3C44BOX main control unit and various interfaces, signal acquisition and preprocessing units.

This device will achieve the following functions:
(1) Normal situation: This device is in the online monitoring state, and it determines whether there is a single grounding in the DC system by collecting field signals and using relevant algorithms;
(2) Single grounding in the DC system: When a single grounding is found in the DC system, the low-frequency signal source is started to inject a low-frequency signal into the power grid;
(3) The low-frequency voltage and current signals of each branch are sampled synchronously in turn; the current signal of each branch is detected by the current transformer on each branch of the DC system. Each branch is selected by a multi-way switch in turn, and the relevant signals are sampled after conditioning, and the low-frequency voltage signal is sampled synchronously, and then the sampled data is processed in turn according to a specific control algorithm based on wavelet transform, so as to determine the faulty branch, and the results are displayed on the LCD or remote terminal so that the user can view and eliminate the grounding fault in time.

3 Overall software design plan
For an open embedded system, there must be system initialization code in its program memory. Initialization is automatically completed after the system is reset. In the system initialization, the following initialization code must be included: setting the entry pointer, setting the interrupt vector table, initializing the stack pointer register, initializing the memory system, initializing the I/O port, and changing the processor's working mode, initializing the application storage space. After that, call and start executing the application.
μC/OS-Ⅱ is a real-time embedded operating system with open source code. Its main features are open source code; portable; curable; customizable; preemptive real-time kernel; multi-task management: execution time can be determined; provides many system services; has interrupt management; stable and reliable. However, μC/OS-Ⅱ only provides a task scheduling kernel, which needs to be expanded into a practical RTOS on its basis. According to the functional requirements of the device, the corresponding user tasks need to be implemented in the system software: a. Monitoring; b. Alarm; c. Start the low-frequency signal source; d. Low-frequency voltage sampling, and calculate its amplitude and initial phase; e. Select each branch and detect its current signal, and then use the wavelet algorithm to make a judgment; f. Display relevant information on the LCD or remote terminal. The specific process of task e is shown in Figure 2.

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Compared with traditional detection methods such as phase-sensitive detection and carrier phase, the above-mentioned processing method based on wavelet transform can give full play to the excellent time-frequency analysis characteristics of wavelet analysis, effectively overcome the influence of factors such as large-capacitance grounding ring network of DC system, and can accurately calculate the branch grounding resistance value, so as to determine the fault branch. Due to the limited resources of 8/16bit microcontroller, it is impossible to implement such a complex algorithm, and the high performance and high reliability of ARM provide a good hardware platform for this algorithm. The overall block diagram of the expanded RTOS is shown in Figure 3.

4 Application Design
Based on the above analysis, the application operation flow chart is designed as shown in 4.

4.1 DC system positive and negative busbar insulation monitoring
After the system is initialized, it enters Task 1. If a ground fault is found, it waits for the keyboard message to set the number of branches to be checked, and then the system calls Task 2; if no ground fault occurs, it delays a specific period of time and calls Task 1 again.
The specific code of Task 1 is as follows:

4.2 Start the low-frequency signal source and synchronously sample the low-frequency voltage
and current branch current After determining that a ground fault has occurred in the DC system, call Task 2, first determine the branch number, then synchronously sample the positive and negative bus low-frequency voltage signals and the current branch current signals. After completing the corresponding sampling, the system calls Task 3. The
specific code of Task 2 is as follows:

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4.3 Filter and extract low-frequency data from sampled data, and calculate ground resistance value
After Task 2 completes the corresponding sampling work, the system calls Task 3. Task 2 and Task 3 form a loop. Task 3 mainly performs 3 B-spline filtering and Morlet wavelet low-frequency component extraction on the data sampled by Task 2, and determines whether the branch is faulty or not. At the same time, it determines whether all branches have been detected based on the current branch number. If there are still branches that have not been detected in the DC system, it returns to Task 2 to start a new sampling. If all have been completed, it returns to Task 1 after a delay of 30 minutes.
The specific code of Task 3 is as follows:

5 Conclusion
Designing and applying related applications based on ARM microprocessors can improve system performance and make the system small, low-cost, and highly reliable. The application design of the ARM-based DC system grounding fault detection system introduced in this paper builds a complete hardware and software platform, which has achieved good results in practical applications.