3 Phase UPS Bus Balancer Neutral Current Reduction
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Solution Overview
Problem
Existing UPS systems operating in a 3:1 configuration face limitations due to significant 50Hz/60Hz ripple current in DC-bus capacitors and increased current in the neutral wire, which reduce capacitor lifetime and require additional capacitance or de-rated output power.
Innovation Solution
A UPS topology is developed where two output phases are coupled in parallel, and the third phase acts as a return line/bus balancer connected to the neutral line, reducing ripple current and neutral wire current by selectively controlling switches to manage load current distribution between the neutral line and the third phase.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If a UPS operates in a 3:1 configuration with two output phases coupled in parallel, then the output power capability is improved, but significant 50Hz/60Hz ripple current appears in DC-bus capacitors reducing their lifetime
Solution Approach 1:
The third phase line acts as an intermediary bus balancer between the positive and negative DC buses. By connecting this intermediate phase to the neutral line and controlling its switches, the system balances the DC bus voltages and redirects ripple current away from the capacitors, thereby extending their lifetime while maintaining the 3:1 power configuration capability
2Power
If a UPS operates in a 3:1 configuration with two output phases coupled in parallel, then the output power capability is improved, but current in the neutral wire increases requiring additional capacitance or de-rated output power
Solution Approach 1:
The third phase line serves as a mediator to balance the neutral current. By controlling the switches on this intermediate phase, the system distributes the return current more evenly across all three phases, reducing the burden on the neutral wire and eliminating the need for additional capacitance or power derating
Solution Approach 2:
The system dynamically changes the operating parameters of the third phase by selectively opening and closing its switches based on the instantaneous voltage conditions of the positive and negative DC buses. This parameter modulation enables the third phase to absorb or supply current as needed, balancing the neutral current and optimizing the overall current distribution
Data Source
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AI summary
According to one aspect, embodiments of the invention provide a UPS (100) comprising a plurality of inputs (102a-c), a PFC converter (104) configured to convert 3-phase input power into DC power, an inverter (108) coupled to a positive DC bus (106) and a negative DC bus (107) and configured to convert the DC power received from the positive DC bus (106) and the negative DC bus (107) into output AC power, a first output (110a) configured to provide a first portion of the output AC power from the inverter (108) to a load (111), a second output (110b) configured to provide a second portion of the output AC power from the inverter (108) to the load (111), a third output (110c) configured to be selectively coupled to a neutral line (112) via the inverter (108), and a controller (114) configured to operate the inverter (108) to generate current between the load (111) and the neutral line (112) via the third output (110c) and the inverter (108).