Structural principle and diagnostic method of DC voltage divider

Author： GOZ Electric Time：2024-06-15 09:24:49 Read：14

With the growth of economy, the electricity consumption continues to increase, and the application of UHV DC transmission technology in power transmission is becoming more and more extensive. In the power grid system, in order to balance the voltage, the UHV converter station is usually equipped with a DC voltage divider. The DC voltage divider is used to measure the DC voltage and maintain the stability of the transmission line. Its function is the same as that of the AC voltage transformer. In the current power grid system, the DC voltage divider measures the DC voltage of the pole bus and the neutral bus. The sampled data is transmitted to the pole protection system of the converter station for accurate measurement, which can control and protect the UHV DC transmission. Compared with the AC voltage transformer, the failure rate of the DC voltage divider is higher. Circuit failures need to be eliminated in time to avoid greater losses. Therefore, diagnosing and analyzing the faults of the DC voltage divider is of great significance to maintaining the stability of the power grid system. In order to analyze the faults of the DC voltage divider in the UHV converter station and solve the fault problems of the voltage divider, the structure and principle of the DC voltage divider are analyzed, and the DC voltage divider capacitive load waveform is analyzed. Through the output waveform and harmonic analysis, the principle of the fault can be better clarified, laying a theoretical foundation for the diagnosis of the fault. For the faults in actual cases, on-site surveys of DC voltage dividers are carried out, the types of faults that occur are diagnosed, and hidden dangers such as faults in voltage divider capacitor components are checked. After analysis and diagnosis, the DC voltage divider faults are eliminated. The analysis and cases have certain engineering application value.

Structure and principle of DC voltage divider

1.1 Structure of DC voltage divider In the power system, high-voltage DC transmission has the characteristics of large transmission power and high stability, and is widely used in long-distance, large-capacity transmission and power system networking. The DC voltage divider is an important device in the transmission system, and the voltage changes rapidly when a fault occurs. The DC voltage divider consists of a primary voltage divider circuit and a secondary voltage divider board, and the secondary voltage divider board is connected to the electronic isolation device and the control and protection system. Together, they form the structure of the UHV DC voltage divider. The structure of the DC voltage divider determines that it may fail during operation. The structure of the DC voltage divider of the UHV DC converter station is shown in Figure 1. As shown in Figure 1, in the DC voltage divider, the primary voltage division circuit consists of two parts. The way to achieve voltage conversion is to use the resistance-capacitance voltage division method to perform voltage conversion. One part is the high-voltage unit, and the other part is the secondary voltage division and control device. The control device can protect the circuit when the voltage system is abnormal to avoid larger faults.

1.2 Principle of DC voltage divider In order to meet the test requirements of the frequency response test of the high-voltage DC voltage transformer, this paper builds a high-voltage broadband power supply consisting of a signal generator, a power amplifier, and an adjustable inductor. The high-voltage broadband power supply structure can adjust the line by collecting signals and amplifying power. The structure is shown in Figure 2. Figure 2 High-voltage broadband power supply structure After the input signal is driven by the low-voltage operational amplifier stage, it drives the power amplifier stage to obtain a high-voltage output that quickly follows the input signal. According to this principle, the waveform of the entire system can be obtained. The DC voltage divider has a protective effect on the circuit and can improve the stability of the circuit.

1.3 DC voltage divider capacitive load waveform In order to ensure that the system can work normally, the entire system is powered by 220V AC input, and the current does not change during the transmission process. The high-voltage output signal forms a closed-loop control through the negative feedback channel, which is conducive to maintaining the stability of the line and acts on the operational amplifier to improve the output stability of the power amplifier. During normal operation, there will be no tripping. When the load current is too large and exceeds the load capacity, the power amplifier will automatically trip to protect its internal circuit. The waveform is shown in Figure 3. Figure 3 Output waveform and harmonic analysis The output waveform and harmonics can reflect the state of the DC voltage divider. Harmonics are related to the current size. The line current and frequency will affect the bandwidth of the DC voltage divider, which is intuitively manifested in the change of the waveform. When the output current is greater than 40mA, the voltage waveform output by the DC voltage divider begins to distort. Changes can easily lead to the normal operation of the system, and the sine wave crest factor becomes smaller and unstable. When the waveform changes, the DC voltage divider can be judged to be faulty by the characteristics of the change. In order to protect the internal circuit, the circuit system will trip to avoid greater damage. Therefore, when a fault occurs in the power grid system and needs to be diagnosed, the DC voltage divider fault can be diagnosed through the output waveform.