Brief Analysis of Frequency Response Characteristics of Resistor-Capacitor DC Voltage Divider

Author： GOZ Electric Time：2024-07-05 09:27:56 Read：18

The frequency response characteristics of the RC DC voltage divider are crucial to the control and protection system of the HVDC transmission system. During the operation of the RC DC voltage divider, insulation flashover, insulation breakdown and other faults may occur inside it, causing changes in its equivalent capacitance value, which in turn causes changes in its frequency response. Since the RC parameters of the RC DC voltage divider in operation are difficult to measure directly, it is difficult to quantify the compensation RC parameters. Based on the current research status, this paper proposes an indirect measurement method for the RC parameters of the RC DC voltage divider in operation and a frequency response compensation measure. The RC parameters of the RC DC voltage divider in operation are indirectly measured through the frequency response test results under different standard capacitance loads on the secondary side. Then, the capacitance parameters of the low-voltage arm are adjusted according to the parameter balance formula of the RC voltage divider to compensate for the frequency response characteristics of the RC voltage divider. The simulation results show that the method proposed in this paper can realize the frequency response compensation of the RC DC voltage divider in operation.

Due to its advantages such as large-capacity and long-distance power transmission, HVDC transmission is currently widely used in power transmission projects at home and abroad]. High-voltage DC voltage measuring devices are important links in DC metering systems and DC control and protection systems, and are closely related to the reliability of DC transmission system operation. With the development of high-voltage flexible DC transmission technology, the control and protection system of high-voltage DC transmission systems has put forward higher frequency response characteristics requirements for high-voltage DC voltage measuring devices.

According to their measurement principles, high-voltage DC voltage measuring devices can be divided into two types: resistance voltage divider type and resistance-capacitance voltage divider type. The resistance voltage divider can realize accurate measurement of high-voltage DC voltage. Due to the existence of spatial stray capacitance, the resistance voltage divider is prone to local uneven voltage distribution under lightning impulse voltage, resulting in component breakdown. Therefore, the resistance voltage divider is mainly used as a standard instrument in the laboratory, and the DC voltage measuring device used on site usually adopts the resistance-capacitance voltage divider. In order to ensure that the high-voltage DC voltage measuring device has a linear frequency response characteristic, each voltage divider branch of the voltage divider should have the same resistance-capacitance time constant. Related literature analyzes the amplitude-frequency response characteristics of the resistance-capacitance DC voltage divider that uses cable to transmit secondary voltage, and finds that stray inductance and stray capacitance have little effect on the frequency response of the resistance-capacitance voltage measuring device with parallel large capacitance, and the error value is within 1%. The relevant literature tested five different types of RC DC voltage dividers under 40-300 Hz conditions and found that some types of measuring devices had an error of 2.38%, but the cause of the error was not analyzed. The relevant literature proposed a RC voltage divider for power quality measurement with good amplitude-frequency characteristics. The relevant literature studied the influence of different resistor and capacitor topologies of the secondary voltage divider on the transmission characteristics of the high-voltage DC measurement device, but none of them considered the influence of its component deviation on the measurement results. During the field operation of the DC voltage divider, the capacitance of its high and low voltage arms may change due to various reasons. When the capacitive element of the high voltage arm breaks down under overvoltage due to insulation defects and other reasons, the equivalent capacitance value of its high voltage arm will increase, thereby affecting the normal operation of the DC control and protection system. Therefore, it is necessary to carry out research on the frequency response characteristics and compensation measures of the RC voltage divider in operation, so as to realize the field optimization adjustment of the frequency response characteristics of the DC voltage divider in operation.

1. Frequency response analysis model of RC DC voltage divider

The RC DC voltage divider is mainly composed of the voltage divider body and the secondary voltage divider of the low voltage arm. The primary body uses a precision resistor-capacitor voltage divider to collect DC voltage signals, which is composed of multiple modular resistor-capacitor unit modules connected in series. The secondary voltage divider consists of 12 parallel resistor-capacitor voltage divider branches, which divide the low-voltage signal output by the low-voltage arm of the voltage divider for secondary voltage division and convert it into 12 independent signal outputs. Each voltage divider branch has the same resistor-capacitor time constant as the high- and low-voltage arms. Each output signal can be connected to a remote module, and the digital signal output by the remote module is transmitted to the control room through optical fiber for use by the secondary equipment.

2. Analysis of the influence of capacitor elements on the frequency response of DC voltage divider

When the high-voltage arm capacitor element breaks down, the equivalent capacitance value of the high-voltage arm will increase. At this time, the DC voltage measurement accuracy of the voltage divider is almost unaffected and there is no significant fault feature. Therefore, this fault is usually difficult to find. At the same time, due to the increase in the equivalent capacitance of the high-voltage arm, the ideal resistor-capacitor time constant no longer meets the requirements, resulting in a large error in the frequency response result of the DC voltage divider, affecting the normal operation of the DC control and protection system.

3. Conclusion

The frequency response characteristics of the RC DC voltage divider are crucial to the control and protection system of the HVDC transmission system. Therefore, this paper recommends that in actual practice, the frequency response test of the RC voltage divider should be carried out regularly at the power frequency. If the ratio difference at the power frequency exceeds 3% and does not meet the requirements of the national standard [21], it is necessary to use the method proposed in this paper to measure the RC parameters of each circuit of its low-voltage arm and the secondary voltage divider, and adopt corresponding compensation measures to adjust it, so as to achieve the compensation of the frequency characteristics of the RC DC voltage divider in operation.