Introduction to the function of DC voltage divider isolation amplifier in battery swap station

Author： GOZ Electric Time：2024-07-09 09:13:05 Read：15

Since the DC project, in order to achieve the isolation between the secondary circuit of the DC voltage divider and the control protection measurement board, the DC voltage divider with the secondary circuit for electrical signal transmission has adopted an isolation amplifier to avoid the impact of the measurement board failure on the secondary circuit of the entire DC voltage divider. During the commissioning of a converter station, it was found that the pole bus DC voltage divider isolation amplifier would output a reverse high voltage after power failure, causing DC system disturbance. If the same problem occurs on the station (rectifier side), it will lead to DC lockout, and there is a hidden danger of DC lockout caused by a single component failure. This paper simulates and studies the waveform of this abnormal phenomenon and finds that when a DC voltage divider isolation amplifier with a rated voltage of ±25V is used, there is a problem of outputting high voltage after power failure. When a DC voltage divider isolation amplifier with a rated voltage of ±10V is used, this problem does not exist. It is recommended that the converter station using the DC voltage divider isolation amplifier with a rated voltage of ±25V take advantage of the opportunity of power outage for maintenance to replace it, eliminate hidden dangers, and ensure the safe and stable operation of the DC system.

Principle of DC voltage divider

1 Function of DC voltage divider DC voltage divider is an important measuring device in DC transmission system. Its function is to convert the DC high voltage on the primary side into the low voltage required by the secondary side control, protection, measurement, recording and other devices in proportion under the premise of meeting the corresponding accuracy, so as to ensure the safe and stable operation of DC transmission system.

1 Principle of DC voltage divider As a receiving station, the high and low-end valve groups of each pole are connected to 500kV and 1000kV AC power grids respectively. The high and low-end valve groups correspond to different parameters of converter transformers, such as the number of converter transformer tap gears and tap adjustment steps, which are not easy to adjust completely synchronously. Unlike conventional UHV DC projects, two converters need to be independently and accurately controlled to achieve their respective control targets. Therefore, a DC voltage divider is added between the high and low-end valve groups of a converter station to measure the voltage at the connection point of the high and low-end valve groups for valve group voltage control and valve group voltage balance control, and its control strategy is also adjusted accordingly. As a hierarchically connected UHV converter station, a certain station has DC voltage dividers mainly distributed in the pole bus area, between the high-end and low-end valve groups, and the neutral bus area. All SF6 gas-insulated DC voltage dividers should ensure that six independent analog signal outputs are provided for each voltage signal. Each channel has an independent low-pass filter, a high-stability resistor secondary voltage divider, and an isolation amplifier. The failure of any output channel should not cause abnormal signals in other output channels. The measurement circuit should have a complete self-test function. When the measurement circuit or power supply is abnormal, a signal to prevent mis-export should be provided to the control or protection device. There should be an overvoltage protection device between the secondary output terminal and the ground, and the discharge voltage of the overvoltage protection device should not be greater than 0.5kV.

1 Under normal operation of the DC control system, the rectifier side quickly adjusts the trigger angle α through the current controller to keep the DC current constant; the inverter side controls the DC voltage by quickly adjusting the shutdown angle γ. Correspondingly, the rectifier side converter transformer tap control maintains a constant trigger angle, and the inverter side converter transformer tap control maintains a constant DC voltage. The external characteristic curve is shown in the figure. As the AC system voltage drops, the current cannot be controlled at the command value even if the trigger angle on the rectifier side is adjusted to the minimum angle. The minimum trigger angle control curve on the rectifier side will intersect with the positive slope control curve (AB) on the inverter side. After the AC voltage continues to drop, the current regulator on the inverter side will work and control the voltage by controlling the minimum trigger angle on the rectifier side.