Voltage Source Converter Zero-Sequence Control During AC Ground Faults
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Solution Overview
Problem
In flexible DC transmission systems, zero sequence voltage fluctuations caused by a single-phase ground fault in an AC system lead to overmodulation and instability, affecting the secure operation of converter stations connected to the DC power grid, as existing methods fail to simultaneously suppress both AC side negative sequence and DC side frequency doubling, and do not address zero sequence voltage suppression in non-faulty stations.
Innovation Solution
Detecting a zero sequence voltage greater than a set value at the AC or DC side of a voltage source converter, increasing the DC side voltage to a set value that is greater than twice the non-faulty phase voltage peak, and generating an appropriate reference wave to control overmodulation, which can be achieved by increasing capacitor voltage or adding redundant sub-modules.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If DC voltage is kept unchanged during AC system ground fault, then converter structure remains simple, but overmodulation occurs and DC voltage fluctuates causing system instability
Solution Approach 1:
The patent implements dynamic DC voltage adjustment by detecting zero sequence voltage and automatically increasing DC voltage when ground faults occur. The control system dynamically modifies the DC voltage reference value based on fault conditions, allowing the converter to adapt to changing system states without structural modifications.
Solution Approach 2:
The patent changes the DC voltage parameter from a fixed value to a dynamically adjustable value. By modifying the DC voltage reference in response to zero sequence voltage detection, the system prevents overmodulation and maintains stable operation during AC ground faults without requiring structural changes to the converter.
2Device complexity
If zero sequence voltage is not controlled during ground fault, then control system remains simple, but overmodulation occurs and affects non-faulty converter stations
Solution Approach 1:
The patent implements feedback control by continuously detecting zero sequence voltage at the converter station and using this information to adjust the DC voltage reference. This feedback mechanism ensures that each converter station can independently respond to ground faults and prevent zero sequence voltage propagation to non-faulty stations.
Solution Approach 2:
The patent takes preliminary action by increasing DC voltage before overmodulation can occur. By detecting zero sequence voltage early and proactively adjusting the DC voltage reference upward, the system prevents the development of overmodulation conditions that would otherwise propagate to other converter stations.
Data Source
Figure 1~2
AI summary
Provided is a method for controlling a zero sequence voltage of a voltage source converter. When an alternating current system ground fault occurs, a zero sequence voltage at an alternating current (AC) side or a zero sequence voltage at a direct current (DC) side of a converter is detected. If the detected zero sequence voltage is greater than a set value V0ref, a DC side voltage of the converter can be increased to a set value Udc0, thereby generating an appropriate reference wave at the AC side of the converter to match an AC voltage, and accordingly controlling overmodulation of a reference wave at a faulty station to suppress fluctuation of a DC voltage.