Neutral Point Balancing for Back-to-Back Voltage Source Converters
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Solution Overview
Problem
Three-level back-to-back neutral point clamped (NPC) converters in wind turbine systems face challenges with neutral point voltage balancing due to DC and AC unbalances, which can lead to capacitor voltage drift and oscillations, especially in doubly-fed induction generator (DFIG) systems where frequency mismatch between rotor-side and line-side converters exacerbates the issue.
Innovation Solution
A method and system for coordinated neutral point balancing in three-level back-to-back voltage source power conversion assemblies, utilizing adaptive controller gains based on slip, modulation index, power factor, and active power to determine voltage compensation and coordinate common mode voltage injection from both rotor-side and line-side converters, minimizing DC and AC unbalances through proportional integral (PI) regulators and selective switching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If conventional common mode voltage injection with carrier-based PWM techniques is used, then voltage balance can be provided in single converter systems, but the solution is restricted to systems with close frequency match and cannot handle frequency mismatch in back-to-back converters
Solution Approach 1:
The patent implements dynamic adaptive controller gains that adjust in real-time based on operating conditions including frequency variations. The controller gains are modified according to slip frequency, modulation index, power factor, and active power to maintain effective neutral point voltage balancing across varying frequency conditions in back-to-back NPC converters
Solution Approach 2:
The patent changes control parameters by implementing adaptive controller gains that vary with operating conditions. The controller dynamically adjusts gain values based on slip, modulation index, power factor, and active power to maintain voltage balance effectiveness across different frequency mismatches and operating states
2Device complexity
If no PWM technique or external hardware circuitry is used for balancing, then device complexity is reduced, but DC unbalance causes capacitor voltages to drift resulting in one capacitor charging to total DC link voltage and the other to zero
Solution Approach 1:
The patent replaces external hardware balancing circuitry with a control-based PWM technique. The neutral point voltage balancing is achieved through coordinated common mode voltage injection control using pulse width modulation, eliminating the need for additional external hardware circuits while maintaining capacitor voltage stability
Solution Approach 2:
The patent enables the power conversion system to self-balance its neutral point voltages through integrated control. The controller automatically detects and corrects DC and AC unbalances through coordinated common mode voltage injection from both converters, making the system self-regulating without external intervention
3Reliability
If DC unbalance is solved through PWM techniques, then capacitor voltage drift is prevented, but triplen frequency neutral point voltage oscillation (AC unbalance) persists and requires additional large capacitors or control attention
Solution Approach 1:
The patent implements a unified control strategy that simultaneously addresses both DC unbalance and AC unbalance (triplen frequency oscillations) through coordinated common mode voltage injection. The same control mechanism handles multiple types of voltage imbalance, eliminating the need for separate solutions for DC and AC components
Solution Approach 2:
The patent merges the correction of DC unbalance and AC unbalance into a single coordinated control framework. By combining common mode voltage injection from both rotor-side and line-side converters with adaptive controller gains, the system simultaneously suppresses both DC capacitor voltage drift and triplen frequency neutral point oscillations
4Adaptability or versatility
If adaptive controller gains are implemented for both converters, then coordinated neutral point balancing is achieved under varying operating conditions, but control algorithm complexity increases
Solution Approach 1:
The patent modifies control parameters by implementing adaptive controller gains that dynamically adjust based on operating conditions. The gains are modified according to slip frequency, modulation index, power factor, and active power to maintain optimal balancing performance across varying operating states without requiring complex reconfiguration
Data Source
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AI summary
A method for controlling a three-level back-to-back voltage source power conversion assembly includes receiving an indication of a DC or AC unbalance occurring in voltage of a DC link. The power conversion assembly has a first power converter coupled to a second power converter via the DC link. In response to receiving the indication, the method includes activating a balancing algorithm that includes determining a deviation of a midpoint voltage of the DC link as a function of a total voltage of the DC link, calculating a voltage compensation needed for pulse-width modulation signals of the power conversion assembly based on the deviation, and coordinating common mode voltage injection from each of the power converters independently at a neutral point of the power conversion assembly based on the voltage compensation, thereby minimizing the at least one of the DC unbalance or the AC unbalance at any given operating condition