Balancing Intermediate Circuit Voltages in Multi-Level Compensators
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Solution Overview
Problem
Self-commutated multilevel reactive current compensators face instability due to unbalanced intermediate circuit voltages, which can damage semiconductor switches and limit the adjustment range of multi-quadrant controllers.
Innovation Solution
A method that balances intermediate circuit voltages by applying pulse-width-modulated voltages composed of compensator voltage and mains-frequency balancing voltage, ensuring the sum of balancing voltages is zero and each is in phase or antiphase with the reactive current, using a reference variable generated from the arithmetic mean or derived intermediate circuit voltages, and suppressing twice the mains frequency components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If intermediate circuit voltages are not balanced, then the compensator can operate with simple control, but the voltages drift causing damage to semiconductor switches and limiting controller adjustment range
Solution Approach 1:
The patent implements a feedback mechanism where the control unit continuously monitors intermediate circuit voltages and adjusts the balancing voltage accordingly. The control unit generates a balancing voltage that is added to the compensator voltage based on the detected voltage levels, creating a closed-loop system that automatically maintains voltage balance without requiring complex external intervention.
Solution Approach 2:
The patent changes the voltage parameter by introducing a mains-frequency balancing voltage with selectable amplitude that is superimposed on the compensator voltage. This parameter modification allows the system to actively control and balance intermediate circuit voltages, preventing drift while maintaining reliable operation of the semiconductor switches.
2Power
If high intermediate circuit voltages are applied, then the compensator voltage can be increased, but the semiconductor switches are endangered
Solution Approach 1:
The patent applies a counterbalancing voltage that opposes voltage imbalances in the intermediate circuit. By generating a mains-frequency balancing voltage that is in phase or antiphase with the fundamental component of the reactive current, the system creates a counteracting force that prevents excessive voltage buildup, thereby protecting semiconductor switches from overvoltage damage while maintaining adequate compensator voltage levels.
3Object-affected harmful factors
If low intermediate circuit voltages are applied, then the semiconductor switches are protected, but the adjustment range of multi-quadrant controllers is limited
Solution Approach 1:
The patent applies a preliminary balancing voltage to the intermediate circuit before the compensator operates. This mains-frequency balancing voltage is generated in advance based on the detected intermediate circuit voltage levels and is added to the compensator voltage, ensuring that voltages are pre-balanced to optimal levels. This preliminary action expands the effective adjustment range of the multi-quadrant controllers while maintaining semiconductor switch protection.
Data Source
Figure 1~1A
Figure 2~3
Figure 4~4B
AI summary
The invention relates to a method for balancing the intermediate circuit voltages (u d l to u dn ) in a self-guided, multi-level blind current compensator (1) with a plurality of multi-quadrant rectifiers (2a to 2n) connected in series, wherein each multi-quadrant rectifier (2a to 2n) applies a pulse width modulated voltage that is made up of an equal portion (u v l to u vn ) of compensator voltage (u k ) imprinting the blind current (i k ) and an additional mains-frequency balancing voltage (u b ) having selectable amplitude. The sum of all balancing voltages (u b ) is thus zero and every balancing voltage (u b ) lies in phase or counter-phase to the fundamental oscillation of the blind current (i k ), dependent on whether the measured intermediate circuit voltage (u dl to u dn ) of the multi-quadrant rectifier (2a to 2n) is smaller or larger than a reference value (formula (I)) formed from the intermediate circuit voltages (u dl to u dn ). The invention also relates to a multi-level blind current compensator.