Voltage Source Converter Negative-Sequence Current Generation
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Solution Overview
Problem
Conventional three-phase electric power networks struggle to generate negative-sequence currents necessary for balancing power flow and reducing disturbances, as existing arrangements with wye-connected phase legs can only generate reactive power, leading to instability and energy variations across capacitors.
Innovation Solution
A Voltage Source Converter with a M2LC topology, featuring semiconductor assemblies connected in series with energy storing capacitors and a control unit that calculates and adds zero-sequence voltage or current to maintain constant energy storage, allowing for the generation of negative-sequence currents without charging or discharging capacitors, thus stabilizing the network.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a wye-connected Voltage Source Converter is used to generate reactive power, then reactive power compensation is achieved, but negative-sequence current generation is prevented and capacitor energy varies
Solution Approach 1:
The converter is divided into three independent phase legs, each with its own semiconductor assemblies and energy storing capacitors. This segmentation allows independent control of each phase, enabling negative-sequence current generation while maintaining overall network stability through coordinated control of individual leg energies
Solution Approach 2:
The control system dynamically adjusts the switching states of semiconductor assemblies in each phase leg to generate negative-sequence currents when needed, while simultaneously dynamically balancing the energy in capacitors to maintain constant energy storage. This dynamic control enables the converter to adapt between reactive power generation and negative-sequence current generation modes
2Device complexity
If the number of switching cells is reduced to lower costs, then device complexity and cost decrease, but the ability to maintain constant capacitor energy while generating negative-sequence current becomes more difficult
Solution Approach 1:
Each switching cell is designed to perform multiple functions: generating reactive power, generating negative-sequence currents, and participating in energy balancing across phases. This multi-functionality allows the converter to achieve stable capacitor energy and negative-sequence current generation with fewer switching cells, as each cell contributes to multiple objectives simultaneously
Solution Approach 2:
The control system changes operating parameters (switching states, pulse widths, frequencies) of the semiconductor assemblies to maintain constant capacitor energy while generating negative-sequence currents. By dynamically adjusting these parameters, the system achieves reliable capacitor energy stability without requiring additional switching cells
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 enables efficient generation of negative-sequence currents to balance power flow and reduce disturbances in the network, maintaining constant energy storage in capacitors and reducing the need for additional switching cells, resulting in cost-effective and stable power management.
Implementation Method 1
each switching cell has on one hand at least two semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a rectifying element connected in anti-parallel therewith and on the other at least one energy storing capacitor
Data Source
Figure 1
Figure 2
AI summary
An arrangement for exchanging power, in shunt connection, with a three-phase electric power network (1) comprises a Voltage Source Converter (5) having at least three phase legs (6-11) with each a series connection of switching cells (15). Each switching cell has at least two semiconductor assemblies (16, 17) connected in series and having each a semiconductor device (18) of turn-off- type and a rectifying element (19) connected in anti-parallel therewith and at least one energy storing capacitor (20). A control unit (41) is configured to control the semiconductor devices of each switching cell and to deliver a voltage across the terminals thereof being zero or U, in which U is the voltage across the capacitor. The control unit is also configured to calculate a value for amplitude and phase position for a second negative sequence-current or a zero-sequence voltage or a value of a dc current for which, when added to said three phase legs upon generation of a negative-sequence current, the resulting energy stored in the energy storing capacitors in each said phase leg will be constant and to control the semiconductor devices of said switching cells of the phase legs to add such a current or voltage to the currents and voltages, respectively, of each phase leg of the converter.