Valve-side bushings in converter transformers are critical components that are subjected to various transient over voltages and complex high-order harmonic voltages during normal operation. The insulation state of these bushings faces significant challenges. Existing offline inspection methods have limitations in terms of real-time performance and applicability, and they cannot fully simulate real operating conditions. Therefore, this study investigates the amplitude and phase distribution patterns of the direct voltage, power frequency, and harmonic voltages in valve-side bushings. It establishes the mapping relationship between the time series correlation of leakage current and the insulation state. Based on this, an on-line insulation monitoring method based on time series correlation of leakage current is proposed, which ensures real-time performance through data-driven approaches and applicability through mutual supervision, while maintaining the realism of operating conditions through online implementation. The results of simulation experiments demonstrate that this method could accurately identify early-stage degradation of insulation in valve-side bushings. It exhibits higher sensitivity compared to existing methods and holds significance for ensuring the safety and stability of direct current transmission systems.
To realize the online monitoring and condition assessment of transformers in a substation, synchronous acquisition of the secondary signals from multiple interval transformers across the entire station is required. A signal acquisition system has been designed, consisting of a time synchronization unit, a signal conditioning unit, a data acquisition unit, and a host computer. The time synchronization unit synchronizes the secondary signals from multiple interval transformers. The signal conditioning unit converts the output signals from the transformers into low-voltage signals. The data acquisition unit controls the sampling timing of the analog-to-digital converter (ADC) using FPGA and reduces phase errors through a time-digital-converter-based ADC sampling delay compensation method. The host computer employs the FFT algorithm with a fourth-order Blackman-Harris window to calculate the amplitude and initial phase of the secondary signals. The test results demonstrate that the system's error meets the requirement of a 0.05-class accuracy. The measurement errors between different channels and different data acquisition units are all less than 0.05%
When the bipolar HVDC transmission line works in the operation mode where the ground is used as the loop line, the DC current will be generated on the power transmission line, which will result in the DC magnetic bias of the current transformer (CT). The DC magnetic bias may bring many disadvantages to a CT, such as the magnetic saturation and the change of its transfer characteristic. Then the accuracy of the CT will experience a degeneration, which affects the fairness of power trading and the reliability of the relay protection. Therefore, a method of compensating the DC magnetic bias is proposed in this paper and it is comprised of 4 steps: recognizing the second harmonic, identifying the optimal compensation, V-I transformation and creating the opposite magnetic field. Moreover, tests are carried out to verify the validity and the results indicate that the proposed method makes much sense in power trading, relay protection and the stable operation of the power system.
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