Three-Phase UPS Control for Zero Wire Loss Bus Voltage Balance
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Solution Overview
Problem
In three-phase UPS systems with a three-level topology, the loss of a zero wire leads to unbalanced positive and negative bus voltages due to unequal currents through the transverse bridge in alternating-current positive and negative half cycles, causing voltage imbalances and potential power-off issues.
Innovation Solution
A control method that uses an alternating current with a harmonic frequency of the mains as a 0-axis reference current to generate a 0-axis modulated wave, ensuring equal currents through the transverse bridge in both half cycles by adjusting the 0-axis sampling current to be one third of the sum of A-phase, B-phase, and C-phase currents, thereby balancing the positive and negative bus voltages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a zero wire is lost in a three-phase UPS system with three-level topology, then the system continues to operate, but the positive and negative bus voltages become unbalanced due to unequal currents through the transverse bridge in alternating-current positive and negative half cycles
Solution Approach 1:
The patent changes the control parameter from a conventional DC 0-axis modulated wave to an AC 0-axis modulated wave with harmonic frequency. This parameter change transforms the current characteristics through the transverse bridge, ensuring equal current magnitudes in both positive and negative half cycles, thereby maintaining bus voltage balance even after zero wire loss
Solution Approach 2:
The patent implements a feedback mechanism where the 0-axis modulated wave is generated based on the difference between the 0-axis reference current (harmonic frequency AC) and the 0-axis sampling current. This feedback control dynamically adjusts the rectifier output to compensate for zero wire loss and maintain current balance through the transverse bridge
2Ease of operation
If conventional DC 0-axis modulated wave control is used after zero wire loss, then the control method is simple, but the currents through the transverse bridge in positive and negative half cycles are unequal, causing bus voltage imbalance
Solution Approach 1:
The patent changes the fundamental parameter of the 0-axis modulated wave from DC to AC with harmonic frequency. This single parameter change fundamentally alters the current waveform characteristics, making the currents through the transverse bridge equal in magnitude for both positive and negative half cycles, thereby resolving the voltage imbalance issue while maintaining control feasibility
Data Source
AI summary
A three-phase UPS control method and apparatus, and a three-phase uninterruptible power supply resolve a problem that after a zero wire is lost, when a current control method is applied to a three-phase UPS using a three-level topology, a 0-axis direct-current modulated wave results in that positive and negative buses are unbalanced. The method includes: determining that a zero wire of a three-phase uninterruptible power supply is lost; using an alternating current whose frequency is a harmonic frequency of mains as a 0-axis reference current; and generating a 0-axis modulated wave according to a difference between the 0-axis reference current and a 0-axis sampling current, to control a rectifier in the three-phase uninterruptible power supply to convert a received alternating current to a direct current.