Three-Level EV Drive Inverter With Bus Midpoint Balancing
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Solution Overview
Problem
Conventional three-level electric drive systems face limitations in balancing the electric potential of the bus midpoint, particularly when the offset is large, due to limited adjustment capabilities in software control modes like SVPWM, leading to difficulties in voltage regulation and increased switching losses in power devices.
Innovation Solution
The electric drive system incorporates a three-level inverter circuit with two bus capacitors and an electric excitation circuit that obtains power from these capacitors, allowing the controller to adjust power consumption and switching frequencies to balance the bus midpoint potential, reducing the required withstand voltages of power devices and enabling efficient voltage balancing even with large offsets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If software control mode (SVPWM) is used to adjust bus midpoint potential, then the control is simple to implement, but the adjustment capability is limited and cannot balance potential when offset is large
Solution Approach 1:
The patent introduces an electric excitation circuit as an intermediary component to balance the bus midpoint potential. This circuit includes switching elements and reactive components that can actively transfer charge between the positive and negative bus capacitors, providing enhanced adjustment capability beyond software control while maintaining system modularity
Solution Approach 2:
The electric excitation circuit serves multiple functions: it balances the bus midpoint potential, provides excitation current to the motor, and can operate in different modes (first working mode for balancing, second working mode for motor excitation). This multi-functionality resolves the contradiction by adding balancing capability without requiring a separate dedicated circuit
2Device complexity
If conventional electric excitation circuit directly obtains power from power battery pack, then the circuit design is simple, but the withstand voltage requirement for power devices is high leading to high costs
Solution Approach 1:
The patent segments the power supply path by introducing a DC link with positive and negative bus capacitors between the power battery pack and the electric excitation circuit. This segmentation divides the high voltage from the battery pack into lower voltage levels at the bus capacitors, reducing the withstand voltage requirements for power devices while maintaining circuit functionality
Solution Approach 2:
The DC link with bus capacitors acts as an intermediary between the power battery pack and the electric excitation circuit. This intermediary component buffers the high voltage from the battery pack, allowing the excitation circuit to operate at lower voltage levels with reduced device stress and lower costs
3Productivity
If three-level inverter circuit is used instead of two-level system, then the NEDC efficiency is improved and output voltage harmonics are reduced, but the bus midpoint potential balancing becomes more difficult
Solution Approach 1:
The patent implements feedback control by monitoring the voltages of the positive and negative bus capacitors and using this information to control the switching states of the electric excitation circuit. The controller adjusts the excitation circuit operation based on the detected voltage imbalance, automatically balancing the bus midpoint potential while maintaining the benefits of the three-level inverter system
Data Source
AI summary
An electric drive system is connected to a power battery pack to drive a motor, the motor includes an exciting winding, and the electric drive system includes a bus, a three-level inverter circuit, an electric excitation circuit, and a controller. The bus includes a positive bus and a negative bus. The three-level inverter circuit includes a first bus capacitor and a second bus capacitor. The first bus capacitor is connected between the positive bus and a bus midpoint, and the second bus capacitor is connected between the negative bus and the bus midpoint. A first input terminal of the electric excitation circuit is connected in parallel to the first bus capacitor, a second input terminal of the electric excitation circuit is connected in parallel to the second bus capacitor, and an output terminal of the electric excitation circuit is connected to the exciting winding of the motor.