Star-Point Reactor for DC Voltage Balancing in HVDC Converters
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Solution Overview
Problem
Multilevel converters in high-voltage direct-current transmission systems face challenges in balancing voltages in the DC voltage circuit with respect to ground potential, leading to poles with different voltage magnitudes relative to ground potential.
Innovation Solution
A star-point reactor is connected between the system connection unit and the converter, featuring inductor coils forming a grounded star point with high impedance to AC fundamental frequency and low impedance to DC, allowing for effective voltage balancing by preventing AC flow while allowing DC flow, and optionally connected to ground via a nonreactive resistor to prevent resonances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If multilevel converters are used to generate multiple voltage levels, then the voltage control flexibility is improved, but the voltage balancing between poles with respect to ground potential deteriorates
Solution Approach 1:
A star-point reactor is introduced as an intermediary component between the DC voltage intermediate circuit and ground. This reactor provides a controlled coupling path that enables voltage balancing by allowing small DC leakage currents to flow while blocking AC fundamental frequency currents, thereby stabilizing the pole voltages with respect to ground potential without affecting the multilevel converter's voltage control capability
Solution Approach 2:
The impedance characteristics of the coupling path are changed by using a star-point reactor with specific inductance values. The reactor is designed to have high impedance to AC fundamental frequency (blocking AC) and low impedance to DC (allowing DC leakage), creating a frequency-dependent parameter change that resolves the voltage balancing issue while maintaining voltage control flexibility
2Reliability
If the DC voltage intermediate circuit is isolated from ground, then the safety is improved, but the voltage balancing capability deteriorates
Solution Approach 1:
The star-point reactor serves as a controlled intermediary that provides a weak coupling to ground. This coupling is sufficient to enable voltage balancing through DC leakage current paths while maintaining electrical isolation that preserves safety. The reactor's high AC impedance ensures that normal AC operation is not affected and safety is maintained
Solution Approach 2:
Instead of fully grounding the DC voltage intermediate circuit (excessive action), a partial coupling is implemented through the star-point reactor. This partial coupling provides just enough DC path for voltage balancing while maintaining sufficient isolation for safety, avoiding the extremes of complete isolation or complete grounding
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 configuration balances the DC voltage intermediate circuit, ensuring poles have approximately equal voltages with respect to ground potential, reduces energy losses, and maintains system stability even under extreme excitation, while being cost-effective and reducing circulating currents.
Implementation Method 1
the inductor coils being configured in such a way that they represent a current path with a high impedance to ground potential, for alternating current with the fundamental frequency of the AC voltage system, and a current path with a low impedance to ground potential, for a direct current
Implementation Method 2
The inductor coils of the star-point reactor are configured in such a way that they represent a current path with a high impedance for the fundamental component of the alternating current, which is generally 50 or 60 Hz, with the result that the alternating currents cannot flow away via the grounded star point
Data Source
AI summary
A device for inverting an electric parameter in the field of power transmission and distribution has a converter that can be connected between an alternating current grid and a DC circuit and having power semiconductor valves extending between an alternating current connection and a DC voltage connection. Each power semiconductor valve has a series circuit of bipolar submodules, each with an energy storage device and a power semiconductor circuit in parallel with the energy storage device, and a grid connection unit connected to the alternating current connection for connecting to the alternating current grid. Simple, effective, and inexpensive symmetrization of the voltages in the DC voltage circuit relative to ground potential is brought about. A star point reactor is connected between the grid connection unit and the inverter with a potential node, including throttle coils connected to a grounded star point. The throttle coils are implemented so that said coils implement a current path with a high impedance to ground potential for alternating current at the base frequency of the alternating current grid, and with low impedance to ground potential for DC current.