Multilevel Converter Active Power Balancing Control
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Solution Overview
Problem
Existing multilevel converter systems struggle to distribute active power evenly across all phases of a three-phase network, leading to technical complexity and inefficiencies in filtering out harmonics and reactive power components.
Innovation Solution
An operating method that determines active and asymmetry components from phase voltage and current values, filters these components, and calculates a zero-sequence current to ensure even active power distribution, using a control device to manage the converter strands and compensate for flicker and reactive power imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a multilevel converter is used for reactive power compensation, then harmonics and reactive power components can be filtered, but the active power is not distributed evenly to all phases of the three-phase network
Solution Approach 1:
The control method segments the compensation current into three distinct components: active component (iPa), reactive component (iPr), and asymmetry component (iPas). Each component is calculated and controlled separately through specific control loops, allowing independent optimization of filtering performance and active power distribution. This segmentation enables the converter to simultaneously achieve harmonic filtering and balanced active power distribution across all phases.
Solution Approach 2:
The invention changes the control parameters by introducing asymmetry components (iPas) that are specifically designed to balance active power distribution. By adjusting the asymmetry component based on the difference between actual and desired active power in each phase, the system dynamically modifies the compensation current to achieve even active power distribution while maintaining effective harmonic filtering.
2Reliability
If the converter strands are heavily loaded to compensate for all current components, then compensation performance improves, but the converter strands may become overloaded
Solution Approach 1:
The control method applies partial action by selectively compensating for different current components based on system conditions. The active component compensation is adjusted to maintain converter strand loading within safe limits, while the reactive and asymmetry components provide the necessary compensation performance. This partial compensation approach prevents overloading while maintaining adequate compensation effectiveness.
Solution Approach 2:
The invention implements feedback control by continuously monitoring the actual active power in each phase and comparing it with the desired active power. The asymmetry component is dynamically adjusted based on this feedback to balance active power distribution. Additionally, the converter strand currents are monitored to prevent overloading, creating a feedback mechanism that protects the converter strands while maintaining compensation performance.
3Speed
If fast response is implemented to compensate for rapidly changing power components, then flicker compensation improves, but the control complexity increases
Solution Approach 1:
The control system segments flicker compensation into dedicated asymmetry component control (iPas) that responds rapidly to changing power conditions. By separating flicker compensation from other compensation functions, the system achieves fast response for flicker mitigation without requiring the entire control system to operate at maximum speed, thereby managing complexity effectively.
Solution Approach 2:
The control method calculates and prepares the asymmetry component in advance based on predicted active power imbalances. By proactively adjusting the asymmetry component before significant flicker occurs, the system achieves fast effective response without requiring complex real-time computation during rapid power changes, thus reducing control complexity while maintaining high-speed flicker compensation performance.
Data Source
AI summary
A multilevel converter (7) comprises a plurality of converter strands (8-10), which are connected to the phases (2-4) of a three-phase-network (1) in a star or delta connection. From phase voltage values (U2-U4) and phase current values (IL2-IL4), an active component (w) and at least two asymmetrical components (w', b') are determined. The phase voltage values (U2-U4) are characteristic of the phase voltages present at the phases (2-4), the phase current values (IL2-IL4) are characteristic of the phase currents flowing in the phases (2-4). The active component (w) is characteristic of the overall active current flowing in the three-phase network (1). The at least two asymmetrical components (w', b') are characteristic of a distribution of the overall flowing active and reactive currents to the phases (2-4). The active component (w) and the at least two asymmetrical components (w', b') are each filtered with a filter characteristic. The phase voltage values (I2-U4) are multiplied with the filtered active component (w) and the at least two filtered asymmetrical components (w', b'). The multiplied phase voltage values (U2-U4) are multiplied with a particular weighting factor (ga-gc) and are then added to the phase current values (IL2-IL4). From the asymmetrical components (w', b') and the phase voltage values (U2-U4), a zero current (10) is determined and is also added to the phase current values (12-14). The zero current (10) is determined such that it symmetrizes a potential asymmetrical active current flow of the multilevel converter (7) that would occur without the zero current (10). From the phase current values (IL2-IL4) modified in said manner, a control state (A) for the converter strands (8-10) is determined. The converter strands (8-10) are actuated accordingly.