HVDC Power Converter Control for Negative Sequence Current Limits
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Solution Overview
Problem
In high voltage direct current (HVDC) power transmission networks, power electronic converters face challenges in managing negative phase sequence currents during AC system voltage unbalances, which can lead to overloads and damage due to the unmitigated exchange of currents, especially during faults like single-phase earth faults, as existing control methods lack a flexible and adaptive approach to balance voltage while respecting current limits.
Innovation Solution
A method that determines the amplitude of the negative phase sequence voltage component at the point of connection and regulates it to either zero or the difference between the current amplitude and the limit value, depending on whether the amplitude exceeds a predetermined limit, ensuring the power converter operates within safe current limits and contributes to voltage balancing only when necessary.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If the power converter operates as a synchronous grid forming source to strengthen system stability, then the overall power system stability is improved, but the converter may exchange excessive transient currents that exceed current limits during voltage unbalance events
Solution Approach 1:
The control method dynamically adjusts the negative phase sequence voltage component based on real-time voltage unbalance detection. When voltage unbalance exceeds a threshold, the controller activates negative sequence voltage injection to limit current; when unbalance is within acceptable limits, the controller reduces or disables this injection. This dynamic adaptation allows the converter to maintain current limit compliance while supporting system stability under varying grid conditions.
Solution Approach 2:
The controller modifies the output voltage parameters by injecting a negative phase sequence voltage component when detected voltage unbalance would cause excessive current exchange. This parameter change in the voltage profile allows the converter to regulate current exchange within safe limits while maintaining synchronous grid forming operation and contributing to system stability.
2Reliability
If the converter regulates output voltage to limit negative phase sequence current during asymmetric faults, then current overload is prevented, but voltage balancing capability is reduced
Solution Approach 1:
The controller applies partial voltage regulation by injecting only the necessary negative phase sequence voltage component required to limit current within safe limits, rather than fully regulating all voltage unbalance. This partial action prevents current overload while allowing some natural voltage unbalance to persist, thereby maintaining adequate voltage balancing capability for essential load support.
3Reliability
If the converter allows natural current flow during moderate voltage unbalances, then current limits are respected, but voltage balancing contribution is minimized
Solution Approach 1:
The control system continuously monitors voltage unbalance levels and uses this feedback to determine when to activate negative phase sequence voltage injection. The feedback mechanism compares detected voltage unbalance against predefined thresholds, enabling the converter to naturally follow voltage variations during moderate unbalances (maintaining current compliance) while actively injecting corrective voltage during severe unbalance events (enhancing voltage balancing contribution).
Data Source
AI summary
A method of controlling a power converter in a power transmission network. The method includes determining a first AC voltage at the point of connection and regulating a second AC voltage, output from the power converter for counteracting the first AC voltage. The step of regulating the second AC voltage includes comparing a first amplitude value of a first negative phase sequence voltage component of the first AC voltage to a first limit value; and then outputting a second negative phase sequence voltage component of the second AC voltage to include a second amplitude value that is substantially equal to zero, when the comparing determines the first amplitude value is less than or equal to the first limit value, and, substantially equal to the difference between the first amplitude value and the first limit value, when the comparing determines the first amplitude value is greater than the first limit value.