Multiphase Power Converter Control for Seamless Mode Transition
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Three-phase PWM converters face challenges in transitioning between current control and voltage control modes, especially when operating in island mode or with unbalanced loads, due to the non-linear nature of current limiters and variable load conditions, and the need to avoid transformer saturation.
Innovation Solution
A control system with a supervisory controller and voltage control module that uses d-q domain transformations, current and voltage feedback signals, and a positive-negative sequence voltage regulator to manage active and reactive power, ensuring smooth transitions and balanced voltage generation across different operational modes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If current limiters are designed with enough bandwidth to generate balanced voltages under unbalanced load conditions, then voltage balance is improved, but the design and tuning becomes cumbersome due to non-linear nature and variable loads
Solution Approach 1:
The patent transforms the control approach by changing from direct voltage control with non-linear limiters to current control in the synchronous reference frame. By transforming voltages and currents into the d-q reference frame, the controller can independently regulate active and reactive power components, achieving voltage balance under unbalanced loads without complex limiter tuning. The parameter transformation linearizes the control problem and enables decoupled control of different power components.
Solution Approach 2:
The patent replaces the mechanical/non-linear current limiter hardware with a software-based control algorithm in the synchronous reference frame. Instead of using physical limiters with non-linear characteristics that require cumbersome tuning, the invention uses mathematical transformations and digital signal processing to achieve the same current limiting function with linear, easily tunable controllers.
2Measurement precision
If controllers use PLL circuits and grid voltages/currents to obtain control signals for current control mode, then current control precision is improved, but the system cannot operate when the grid is not available
Solution Approach 1:
The patent designs a universal control system that can operate in multiple modes: grid-connected current control mode and islanded voltage control mode. The synchronous reference frame controller serves both purposes by switching reference frames - using grid-synchronized d-q frame for current control when connected, and independent voltage-oriented d-q frame for voltage control when islanded. This multi-functional design eliminates the need for separate control systems for different operational modes.
Solution Approach 2:
The patent implements dynamic switching between control modes based on grid availability. The controller can dynamically transition from grid-synchronized current control to independent voltage control and back again. The synchronous reference frame methodology allows smooth mode transitions by maintaining the same mathematical framework while changing the reference signal source, enabling the system to adapt to changing operational conditions without loss of control precision.
3Reliability
If controllers are designed to avoid transformer core saturation, then transformer reliability is improved, but the control design becomes more complex with additional constraints
Solution Approach 1:
The patent incorporates transformer saturation prevention as a preliminary constraint in the control design. By establishing voltage and current limits in the synchronous reference frame before execution, the controller proactively prevents transformer core saturation. The control algorithm includes built-in saturation checks and adaptive current limiting that act beforehand to avoid harmful operating conditions, rather than reacting after saturation occurs.
4Reliability
If smooth transition between current control and voltage control is implemented, then operational reliability is improved, but the control algorithm becomes more complex
Solution Approach 1:
The patent uses the synchronous reference frame as an intermediary mathematical framework that bridges current control and voltage control modes. By transforming all control signals into the d-q reference frame and using the same mathematical structures for both modes, the transition between them becomes a simple matter of switching reference signal sources rather than fundamentally changing control algorithms. This intermediary framework enables smooth transitions while maintaining algorithmic simplicity.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A control system (34) for controlling a multiphase power converter (12) includes a current control module (40), a voltage control module (100) and a current command selector (33). The current control module (40) generates grid voltage command signals for the multiphase power converter (12) and the voltage control module (100) generates reference converter current command signals for the current control module (40). The current command selector (33) supplies active and reactive current command signals from a supervisory controller (27) to the current control module (40) when the multiphase power converter (12) is connected to the grid and supplies the reference converter current command signals to the current control module (40) when the multiphase power converter (12) is unconnected to the grid.