Parallel Voltage Source Converters Droop Control
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Solution Overview
Problem
Existing systems for parallel operation of Voltage Source Converters connected to a filter bus lack independent control mechanisms, leading to conflicts between converters when sharing a common load, especially in scenarios with significant line impedance and no master controller, resulting in inadequate dynamic current control and load sharing.
Innovation Solution
Each control unit for the converters includes a droop coefficient to modify the voltage reference vector, a current limiter to prevent overloading, and a load current feed-forward signal to enhance response speed, allowing independent and coordinated control of converters to share a common load without conflicts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If multiple Voltage Source Converters are connected in parallel to a common filter bus through interface inductors and employ droop schemes for load sharing, then the power rating can be increased and redundancy is obtained, but the converters operate as voltage sources without underlying current control loops, resulting in inadequate dynamic current control
Solution Approach 1:
The patent transforms the control mode of Voltage Source Converters from pure voltage control to a hybrid mode that incorporates current control capabilities. By introducing current reference vectors and current controllers, the converters gain dynamic current control while maintaining their voltage source characteristics for parallel operation. This parameter change enables both high power rating and reliable dynamic current control.
2Reliability
If each converter employs voltage vector control with underlying current controller to regulate filter bus voltage, then dynamic current control is achieved, but the controllers of multiple converters fight with one another when attempting to regulate the same voltage vector, resulting in inability to share common load
Solution Approach 1:
The patent introduces droop control as an intermediary mechanism between multiple converter controllers and the common filter bus. Each converter's controller incorporates droop characteristics that automatically adjust its output based on the common voltage, preventing direct conflict between controllers. This intermediary approach enables stable load sharing while maintaining dynamic current control for each converter.
3Ease of operation
If a master controller is used to set terminal voltages of all converters for load sharing, then load sharing is achieved, but the system becomes dependent on single master controller and requires control communication between geographically dispersed units
Solution Approach 1:
The patent segments the centralized master controller function into distributed individual controllers for each converter. Each converter is equipped with its own controller that incorporates droop control characteristics, eliminating the need for a single master controller and reducing control communication requirements. This segmentation enables autonomous load sharing while maintaining system simplicity.
4Adaptability or versatility
If converters are located far apart with significant line impedance between them, then geographical distribution is achieved, but parallel operation becomes difficult without master controller due to control communication requirements
Solution Approach 1:
The patent enables each converter to serve itself through autonomous droop control, eliminating dependency on centralized master controller signals for geographically dispersed units. Each converter independently regulates its output based on local measurements and droop characteristics, allowing geographical distribution without complex control communication infrastructure.
Data Source
Figure 1~2
Figure 3~4
AI summary
An arrangement for supplying electric power to a load through a filter bus comprises at least two Voltage Source Converters connected in parallel to said filter bus through an inductor each and configured to share said load. Each converter is associated with a control unit (10) configured to regulate the voltage (vf) of the filter bus while maintaining dynamic control of the current from the converter. This control unit involves production of two perpendicularly-intersecting filter bus voltage vectors (νfx, νfy) from the filter bus voltage and the control unit involves production of a current vector (ikx, iky) for each said filter bus vector. The control unit also involves multiplying of each said current vector with a droop coefficient common to all the current vectors. The result of this multiplication is sent to means (13) for subtracting this result from the respective filter bus voltage reference vector ( ).