VFD DC Link Support Using Precharged Energy Storage
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Solution Overview
Problem
Existing variable frequency drives (VFDs) face issues with grid disturbances causing voltage dips, leading to insufficient power delivery and potential turbomachine trips due to oversizing energy storage means or auxiliary power converters, which increase size, weight, and cost.
Innovation Solution
A power supply device with a switch and energy storage device, including an additional impedance, is used to manage DC link voltage by loading energy storage means when the switch is open and closing it during voltage dips to supply energy, avoiding the need for oversized components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If energy storage means are directly connected to the DC link without an auxiliary power converter, then the VFD size and complexity are reduced, but the energy storage means must be oversized to maintain DC link voltage during voltage dips
Solution Approach 1:
The energy storage means are pre-charged to a voltage higher than the nominal DC link voltage (specifically, to a voltage corresponding to the maximum DC link voltage plus a threshold). This preliminary charging action allows the energy storage means to immediately supply power during voltage dips without requiring an auxiliary converter, while the switch enables controlled connection only when needed, avoiding the need for oversized capacity.
Solution Approach 2:
A switch is introduced to dynamically control the connection between the energy storage means and the DC link. The switch is opened during normal operation and closed during voltage dips, allowing the system to adapt its configuration based on grid conditions. This dynamic control enables the use of smaller energy storage capacity compared to direct connection approaches.
2Reliability
If the VFD and electric machine are oversized to deliver minimal required torque during voltage dips, then turbomachine trips are avoided, but the VFD and electric machine size, weight, and cost increase
Solution Approach 1:
The energy storage means act as an intermediary component between the grid and the electric machine during voltage dips. When the grid voltage drops, the pre-charged energy storage means immediately supply the required power through the switch, maintaining DC link voltage and enabling the electric machine to continue delivering torque without tripping. This intermediary approach allows the use of correctly-sized (not oversized) VFD and electric machine components.
3Power
If an auxiliary power converter is implemented to supply the DC link during voltage dips, then power delivery is maintained, but the VFD size, weight, and cost increase
Solution Approach 1:
The invention extracts and removes the auxiliary power converter from the system, replacing it with a simpler configuration consisting of the energy storage means directly connected to the DC link through a switch. The energy storage means are pre-charged during normal operation and then directly supply power during voltage dips without requiring conversion electronics, thereby maintaining power delivery capability while significantly reducing device complexity, size, and cost.
Solution Approach 2:
The energy storage means are loaded during normal operation by the DC link itself, storing energy that will be used to support the DC link during voltage dips. This self-service approach eliminates the need for an external auxiliary power converter, as the energy storage means autonomously provide the required support function when grid disturbances occur.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach maintains consistent power delivery during voltage dips without oversizing the VFD or electric machine, reducing complexity, weight, and cost while preventing turbomachine trips.
Implementation Method 1
energy storage means configured to be connected in parallel with the rectifier and comprising a capacitor
Data Source
Figure 1
Figure 2~3
Figure 4~6
AI summary
A power supply device (23) for variable frequency drive (2) is proposed. The power supply device (23) comprises: - a switch (24) and an energy storage device (25), the energy storage device including an additional impedance (27) and energy storage means (26), - loading means (28) configured to load the energy storage means (26) when the switch (24) is open so that the voltage of the energy storage means is equal to a loading voltage value equal to the sum of a DC maximal value and a voltage threshold, and maintain the load of the energy storage means when the switch (24) is open so that the voltage of the energy storage means is equal to the loading voltage value, and - control means (29) configured to close the switch (24) when a DC link voltage of the variable frequency drive (2) is equal or smaller than a DC minimal value minus a predetermined voltage value.