DSMMC Converter Control for Low-Frequency Capacitor Voltage Balance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
The output current of a DSMMC converter connected to a rotating machine is limited as the output AC frequency decreases, leading to voltage fluctuations in energy storage elements, which destabilize inter-stage balance control and prevent sufficient starting torque.
Innovation Solution
A control device for a frequency converter that includes an AC voltage removal unit to detect and remove DC components from capacitor voltages, an arm voltage command generation unit, a carrier signal generation unit, and a gate pulse generation unit to correct arm voltage commands, ensuring capacitor voltage balance and stability even at low output AC frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a frequency converter is connected to the commercial power grid to enable bidirectional power flow and frequency adjustment, then the adaptability and power control capability are improved, but the risk of harmful factors such as harmonics and voltage fluctuations increases
Solution Approach 1:
The patent introduces an isolation transformer as an intermediary device between the frequency converter and the commercial power grid. This transformer provides galvanic isolation, blocking the transmission of harmful harmonics and voltage fluctuations to the grid while allowing bidirectional power flow and frequency adjustment functions to operate effectively.
2Object-affected harmful factors
If the frequency converter operates in islanded mode without grid connection, then the harmful factors affecting the grid are eliminated, but the adaptability for frequency adjustment and power flow control is reduced
Solution Approach 1:
The patent implements dynamic operational mode switching capability that allows the frequency converter to transition between grid-connected and islanded modes. In grid-connected mode, full adaptability functions are available; in islanded mode, the system automatically adjusts operating parameters to maintain essential functions while protecting the grid, thus dynamically optimizing both protection and adaptability.
3Reliability
If the sub-synchronous control module is activated to prevent resonance, then the reliability of the frequency converter is improved, but the device complexity increases
Solution Approach 1:
The patent integrates the sub-synchronous control module with the existing control system of the frequency converter, merging resonance prevention functions with standard control operations. This unified approach prevents resonance through coordinated control actions without requiring separate standalone systems, thereby improving reliability while minimizing the increase in overall device complexity.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
The invention provides a control device of a frequency converter, a control method of a frequency converter, and a control device of a variable speed pumped storage power generation system that can maintain a voltage balance using a high-efficiency and small-capacity energy storage element even in the case of a low output AC frequency. In the control device of a frequency converter, a rotating machine including a synchronous machine is interconnected to a power system. The frequency converter includes a plurality of sets of arms in which a plurality of unit converters are connected in series, capacitors are connected in parallel in the plurality of unit converters, one ends of the series connected unit converters are connected to respective phases of an alternating current, and the other ends of the series connected unit converters are connected to terminals of a direct current. The control device includes an AC voltage removal calculation unit configured to detect at least voltages of the capacitors in the plurality of unit converters that form the arms, extract a DC component from the capacitor voltages, and input the DC component; an arm voltage command generation unit configured to generate an arm voltage command value; a carrier signal generation unit configured to generate a carrier signal; and a gate pulse generation unit that is provided with a modulated wave generator configured to correct the arm voltage command value to be supplied by the arm voltage command generation unit and that uses the corrected arm voltage command value and the carrier signal to determine a gate command for each of the plurality of unit converters in respective arms. The modulated wave generator uses the DC component calculated in the AC voltage removal calculation unit to correct the arm voltage command value so that magnitude of the voltages of the capacitors in the unit converters of the arms does not fluctuate.