Design of a New DC Power Supply Monitoring System

0 Introduction

       With the development of modern science and technology, the power system is gradually developing towards comprehensive automation and unmanned power stations. The DC power supply monitoring system, as the power source for controlling loads, power loads, and DC emergency lighting loads, is the basis for power system control and protection. Its reliability directly affects the safe operation of the power supply and distribution system [1-2]. Therefore, it is becoming increasingly important to improve the reliability and automation level of the DC power supply monitoring system to meet the needs of power system development.

      This paper combines modern computer technology and automation technology to design an unmanned DC power supply monitoring system. The system adopts a modular centralized management and independent control

Design, with "telemetry, telesignaling, remote control, remote adjustment" functions,

It is easy to realize the comprehensive automation of power system and is a traditional DC

A new generation replacement for source monitoring systems[3].

        Author profile: Wang Bo (1987), male, Master of Engineering, mainly engaged in the research of industrial automation; Bao Linjie (1982), male, Bachelor of Engineering, mainly engaged in the research of industrial automation; Zhu David (1984), male, Bachelor of Engineering, mainly engaged in the research of industrial automation.

1 DC Power Supply Monitoring System

        This DC power supply monitoring system adopts centralized management and independent control. It is mainly suitable for 20~200AH single-battery single-charger systems, and can realize 24-cell battery inspection and 30-way branch insulation monitoring. The system consists of a comprehensive monitoring module, a battery inspection module, an insulation monitoring module, a charging module, and a host computer display control module. The battery inspection and insulation detection are connected to the comprehensive monitoring module through the RS485 interface. The DC power supply monitoring system adopts a centralized integrated plus expansion unit combination structure, which is simple to wire and easy to install. Its structure is shown in Figure 1.

        The integrated monitoring module is the nerve center of the DC monitoring system. It uses a truly industrial-grade 32-bit processor from a well-known company as the main control chip, which can maximize the reliability and operating speed of the system. The integrated monitoring module centrally manages and controls other modules via the RS-485 interface [4]. The battery inspection module and the insulation monitoring module send the monitored single cell voltage, temperature, bus voltage, branch insulation resistance and other signals to the integrated monitoring module via the RS485 interface. The integrated monitoring module compares the internal preset alarm values ​​to generate an alarm signal and records the start and end time of the alarm. In addition, the integrated monitoring module can automatically perform equalization and floating charge management according to the battery pack current, thereby greatly extending the service life of the battery pack.

       In addition, the integrated monitoring module itself can monitor the status of 8-way system switch quantities, three-phase AC input voltage, closing/controlling bus voltage, current and bus insulation status.

3 Battery inspection module

      As a backup power source, the battery is inseparable from the reliability of the entire DC power supply system. Therefore, ensuring the normal operation of the battery is the primary task of the entire DC power supply system [5]. This paper uses the battery inspection module to inspect the terminal voltage, current, and temperature of each battery in the battery pack, and transmits the results to the integrated monitoring module through the RS485 bus. If the voltage of a battery is lower or higher than the specified value, the integrated monitoring module will issue an alarm indication and automatically perform necessary operations; if the battery pack current is too high, the charging module will be instructed to stop charging; if the current is too low, indicating that the performance of the battery has deteriorated or over-discharged, the charging module will be instructed to charge. In this way, the battery can be maintained, the battery life can be extended, and the safe and reliable operation of the system can be ensured. This battery inspection module can detect the voltage of up to 24 single cells, and can detect 2, 6, and 12V single cells respectively, with a measurement accuracy of 0.2%. Its principle is shown in Figure 2.

      When measuring the voltage of a single cell, since most batteries in the system are connected in series and their output voltage is as high as 250V, multiplexing of the input channel is a difficult problem. Currently, the commonly used multiplexing methods are: resistor voltage divider method and relay isolation method. The relay isolation method is simple to operate. Each battery is equipped with a relay. When a battery is to be tested, the relay can be turned on. A decoder should be used to control the relay to ensure that only one relay is turned on at any time [6]. Since the service life of ordinary mechanical relays is limited (no more than 100,000 times), it is far from meeting the requirements of battery inspection devices. Therefore, a photorelay is selected to isolate each battery. Its structure is shown in Figure 3.

       In the battery inspection module, a photorelay is configured for each battery, and its on/off stage is controlled by the CPU. Under normal circumstances, the photorelay is in the off state. When the battery is to be inspected, only one battery is connected to the sampling resistor at a time, and then the sampling signal is sent to the operational amplifier and finally processed by the battery inspection instrument to obtain the battery voltage.

4 Insulation monitoring module

       A common fault in a DC power supply system is single-point grounding. Under normal circumstances, single-point grounding does not affect the operation of the DC system. However, if the grounding fault point cannot be quickly found and repaired, another grounding fault may occur, which may lead to a serious accident. Therefore, it is very necessary to monitor the insulation condition of the DC system in real time and eliminate the grounding fault in time when it occurs [7-9].

This insulation monitoring module has the function of detecting the insulation resistance of 30 branches with a measurement accuracy of ±0.3KΩ. It can also detect the voltage of the busbar (busbar, busbar and busbar negative) to the ground with a measurement error of ±0.4V. The insulation monitoring module sends the monitored voltage value and resistance value to the ground to the integrated monitoring module through the RS485 bus, and the integrated monitoring module makes corresponding processing. The principle is shown in Figure 4.

        There are several methods for detecting insulation resistance at home and abroad, such as "bridge balance method", "low-frequency detection method", "branch leakage current detection method", etc. This paper uses the method of detecting branch leakage current to determine insulation resistance, without injecting small AC signals into the branch, so it does not have any impact on the DC system. The principle is shown in Figure 5.

       In Figure 5, HL1, HL2, and HLn represent Hall current sensors connected to the switches of the DC power supply monitoring system on each power supply branch. If there is no leakage current in the branch, that is, the branch is not grounded, the current flowing through the positive and negative branches of the sensor is equal in magnitude and opposite in direction, and the Hall current sensor on the corresponding branch has no output. When a branch fails, such as a point on the positive pole of branch n in the figure is grounded, the current flows from the positive pole of the DC power supply through the grounding resistor RL to the ground, and then from the ground to the negative pole of the power supply, forming a leakage current IL. IL flows from the ground to the negative pole of the DC power supply. It is a distributed parameter. If there are N branches, the current flowing through each branch is approximately IL. Therefore, the Hall current sensor located in branch N can detect a current of approximately IL. In this way, according to the values ​​of U+, U- and IL, the magnitude of the grounding resistance can be obtained. Then, according to the positive or negative output voltage of the Hall sensor, the polarity of the cable where the grounding fault is located can be determined [10].

5 Conclusion

      The DC power supply monitoring system introduced in this article has the advantages of strong functions, open and flexible structure, good real-time performance and high reliability. Each link adopts the most advanced technology, reflecting the current development trend of DC power supply monitoring systems and has a very broad application prospect.

Source: Electrical Engineering, Issue 5, 2014

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