Converter Current Limiting for Flexible DC Islanding Faults
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Solution Overview
Problem
In flexible direct current transmission systems operating in an islanding state, there is a lack of current limiting and protection coordination between AC and DC systems during faults, leading to potential converter overloads and delayed fault removal.
Innovation Solution
A fault current limiting control method that sets the converter output current command limit Imax based on AC line protection sections, allowing for partial short-circuit current provision and coordination with AC protection to remove faults without affecting converter operation, using pre-set two-dimensional numerical values for voltage-dependent interpolation to adjust current limits dynamically.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If no current limiting measure is implemented in flexible DC islanding operation, then the converter can maintain normal operation, but the converter may experience instantaneous overload and delayed fault removal when AC system faults occur
Solution Approach 1:
The patent pre-configures multiple current limit thresholds (Ilim1, Ilim2, Ilim3) corresponding to different AC protection sections before faults occur. When islanding operation is detected, the system proactively sets the appropriate current limit based on the selected protection coordination mode, preparing the converter to immediately respond to AC faults without delay or overload.
2Reliability
If the converter output current is limited to coordinate with AC protection sections, then fault removal is accelerated and converter overload is prevented, but the converter's current limiting capability must be dynamically adjusted based on voltage conditions
Solution Approach 1:
The patent implements dynamic current limiting by continuously monitoring the DC side voltage (UDC) and comparing it against reference voltages (UDC1_ref, UDC2_ref). Based on the voltage level, the system automatically switches between different current limit thresholds, enabling the converter to adapt its current limiting capability to real-time operating conditions while maintaining protection coordination.
Solution Approach 2:
The system employs feedback control by detecting the DC side voltage and using it to determine the appropriate current limit. The voltage detection result feeds back to the current limit setting mechanism, which adjusts the limit accordingly. This closed-loop feedback ensures the converter operates within safe current boundaries while maximizing power transfer capability under varying voltage conditions.
Data Source
Figure 1~2
AI summary
A fault current limiting control and protection coordination method for a converter of a flexible direct current transmission system operating in an islanding state, wherein the current output command limit Imax is used to limit the current commands ILdq* of inner loop dq currents, and in the event of the fault of AC system, Imax is set equal to or slightly larger than a value for an AC line overcurrent protection section III. The value for the II section of the converter bridge arm overcurrent protection is smaller than the value for the II section of the AC line overcurrent protection or the value is equal; the set value for AC line overcurrent protection section I is smaller than the set value for converter bridge arm overcurrent protection section I; the value for proximal AC line overcurrent protection section II is less than or equal to the value for converter bridge arm overcurrent protection section II, and under such condition, the delay is longer than the AC line overcurrent protection section II. At the time of detecting a fault on the connected line side of the converter, the value of current output command limit Imax of the converter is changed from the allowable multiple of output current of the converter under normal condition to the value for the AC line protection section III in the event of fault. And, partial short circuit current is provided by limiting the output current of the converter, the AC fault is removed by the value for AC protection section III without affecting the normal operation of the converter. After the AC side fault is removed, Imax is restored to an allowable multiple of output current of the converter under normal condition. The method of switching Imax is that: pre-setting a series or curve of n groups of two-dimensional numerical values (ULj, Imaxj) (j = 1, 2, ... n), the limit current command value Imax is obtained by using interpolation or lookup table method, to limit the output current value of the converter operating in passive state.