Multi-Level Inverter DC Link Voltage Balancing via Periodic Signal Injection
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Solution Overview
Problem
Existing methods for balancing DC link voltages in multi-level inverters, such as three-level inverters, face instability when actual power is low or zero, leading to unbalanced neutral point voltages, especially due to power estimation errors and lack of power for balancing.
Innovation Solution
A method involving the injection of periodic signals with a constant phase shift between the common-mode voltage and power references, allowing energy transfer between the DC link halves through a load, with controlled amplitude and phase adjustments to balance voltages, even at low or zero power conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If common-mode voltage control based on switching pattern selection is used, then NP voltage balancing is achieved under normal power conditions, but voltage balancing stability deteriorates when actual power is low or zero due to power estimation errors
Solution Approach 1:
The patent applies periodic action by injecting a periodic signal into the common-mode voltage reference and a corresponding periodic signal into the active power reference. This periodic injection creates controlled oscillations in the neutral point current that transfer energy between DC link halves, enabling voltage balancing to function reliably even when actual power flow is low or zero, thereby resolving the instability issue at low power conditions
2Reliability
If power flow direction detection is used for NP voltage balancing, then robust balancing is achieved when power is high, but balancing fails when actual power is zero as there is no power available for balancing
Solution Approach 1:
The periodic signal injection method enables the system to generate artificial power oscillations that facilitate energy transfer between DC link halves independent of the actual power flow. This allows the balancing mechanism to function adaptively across the full range of operating conditions, including zero power conditions where traditional power-flow-dependent methods fail
Solution Approach 2:
The patent changes the operating parameters by injecting periodic signals with specific frequencies and amplitudes into the control references. By adjusting these signal parameters, the system can maintain effective voltage balancing across varying power conditions, transforming the balancing capability from being power-dependent to being parameter-controllable
Data Source
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AI summary
The present disclosure discloses a method and an apparatus implementing the method for balancing voltages of a DC link of a multi-level inverter, wherein the DC link is divided into two halves by a neutral point connection. The disclosure further discloses an apparatus implementing the method. The method comprises injecting a periodic common-mode voltage injection signal to a common-mode voltage reference and a periodic power injection signal to a power reference of the inverter. The power injection signal has the same frequency as the common-mode voltage injection signal. A phase shift between the common-mode voltage injection signal and the power injection signal is constant. Amplitude of at least one of the common-mode voltage injection signal and the power injection signal is controlled on the basis of a difference between voltages over the two halves of the DC link.