Voltage Source Converter Control for DC Voltage Stability
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Solution Overview
Problem
Conventional voltage source converters (VSCs) in HVDC transmission systems face challenges in maintaining stable DC voltage during transients, especially in multi-terminal DC networks, leading to potential global network instability due to sudden changes in local DC voltage.
Innovation Solution
A control apparatus for VSCs that includes a reference voltage generator for maintaining DC voltage stability and an independent overcurrent controller to prevent overcurrents, allowing the VSC to behave as a current-limited voltage source by using embedded energy storage for voltage regulation and current limiting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional voltage source converters are used in HVDC transmission systems, then AC to DC conversion can be achieved, but DC voltage stability deteriorates during transients in multi-terminal networks
Solution Approach 1:
The control system is segmented into two independent control loops: a voltage control loop that generates the reference voltage signal, and a current control loop that independently limits the current. This segmentation allows each loop to optimize its function without interfering with the other, enabling the VSC to maintain voltage stability while preventing overcurrents during transients.
Solution Approach 2:
The control apparatus prepares the VSC to respond to transients by having the voltage control loop continuously generate reference voltage signals and the current control loop standing ready to independently limit current if needed. This preliminary arrangement ensures that when transients occur in multi-terminal networks, the VSC can immediately respond with stable voltage regulation and overcurrent protection without delay.
2Device complexity
If the current control is coupled with the voltage control loop, then the control system is simpler, but the VSC cannot act as a stable voltage source during transients
Solution Approach 1:
The control system is divided into separate voltage control and current control loops that operate independently. The voltage control loop generates reference voltage signals without being constrained by current limitations, allowing the VSC to maintain stable voltage source behavior during transients while the current control loop provides independent overcurrent protection.
3Adaptability or versatility
If VSCs are used instead of LCCs, then self-commutation capability is improved, but DC voltage stability during transients deteriorates
Solution Approach 1:
The control apparatus is configured in advance to handle transient conditions by having the voltage control loop continuously prepare reference voltage signals and the current control loop ready to independently limit current. This preliminary setup enables VSCs to maintain their self-commutation advantage while providing stable DC voltage during transients in multi-terminal networks.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances DC voltage stability during transients and prevents overcurrents, ensuring a stable DC network even in multi-terminal HVDC systems by decoupling the current control from the voltage control loop, allowing the VSC to act as a voltage source and absorb or provide energy as needed.
Implementation Method 1
each cell comprising an energy storage element, such as a capacitor, and a switch arrangement that can be controlled so as to either connect the energy storage element between the terminals of the cell or bypass the energy storage element
Data Source
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AI summary
This application relates to methods and apparatus for control of voltage source converters (VSCs). The control apparatus has a reference voltage generator (402) configured to generate a voltage control signal as part of a voltage control loop. The reference voltage generator may be a DC voltage/power controller (402) operated to regulate DC voltage or power and may generate the voltage control signal, such as a voltage reference, based on a feedback DC voltage/power signal and a defined set-point. An overcurrent controller (403) is configured to generate a current control signal for modulating the voltage control signal to prevent an overcurrent. The overcurrent controller generates the current control signal as part of a control path that is independent from the voltage control loop. The DC voltage/power controller (402) may thus directly generate a voltage reference and may act to keep the DC voltage substantially constant over the time scale of short term transients, with the overcurrent controller (403) providing current limiting only when required.