Modular Multilevel Converter Black Start via Terminal Voltage Charging
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Solution Overview
Problem
Multilevel voltage source converters face challenges in performing a black start without external power supply units, which are often costly and unreliable, especially in remote locations like offshore windfarms, and cannot rely on cell capacitors for power.
Innovation Solution
A multilevel voltage source converter with a black start control unit that uses DC voltages applied to the converter terminals to energize the power supply units of the cells, allowing for sequential or simultaneous pulsing of switches to charge the gate control units and enable switch operation without external power or cell capacitor reliance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If external power supply units are used for black start, then gate control units can be powered, but cost increases and reliability decreases in remote locations
Solution Approach 1:
The converter enables self-powered black start by using its own DC terminal voltage to charge power supply units through controlled switch operation. The system serves itself by generating the necessary gate control power from internal sources rather than requiring external power supply infrastructure.
Solution Approach 2:
The invention extracts and utilizes the DC voltage already present at the converter terminals as a power source for black start operations. By taking out this existing voltage source and applying it to charge power supply units, the system eliminates the need for separate external power supply units.
2Reliability
If cell capacitors are used to power gate control units, then black start is possible, but the system cannot operate without relying on stored energy
Solution Approach 1:
The system continuously charges power supply units from the DC terminal voltage during normal operation, ensuring they are ready for black start without relying solely on cell capacitor stored energy. This self-charging mechanism provides continuous operational capability.
Solution Approach 2:
Power supply units are charged in advance from the DC terminal voltage before black start is needed. This preliminary charging ensures that when black start is required, the power supply units already contain sufficient energy to operate gate control units immediately.
3Productivity
If only some gate control units receive power during black start, then partial operation is possible, but complete converter restart is limited
Solution Approach 1:
The black start control unit operates switches in periodic sequences, first charging power supply units for initial gate control units, then subsequently charging power supply units for other gate control units. This periodic operation ensures complete converter restart capability.
Solution Approach 2:
The black start process is divided into sequential phases where different groups of gate control units are powered at different times. The system segments the power-up sequence to manage limited power availability while ensuring complete system recovery.
Data Source
Figure 1~3
Figure 4
AI summary
A voltage source converter comprises a black start control unit and cells (14), where each cell comprises a first switch (S1) between a first end of a cell capacitor (C1) and a first cell terminal (CT1), a second switch (S2) between a second end of the cell capacitor and the first cell terminal, where a second cell terminal (CT2) is provided via the first or the second end of the cell capacitor, a first power supply unit (PS1) and a second power supply unit (PS2), where the black start control unit is configured to perform, after the initial application of a DC voltage to a cell terminal (CT2) of a cell and the subsequent initial charging of the chargeable power supply unit (PS1) of one of the switches (S1), charging of an uncharged power supply unit (PS2) of the other switch (S2) through repeatedly turning on and off the switch (S1) with initially charged power supply unit.