3-Level Inverter DC Bus Balancer for UPS

Solution Overview

Problem

Conventional UPS systems face challenges in balancing DC bus voltages without a neutral line, leading to inefficiencies and increased costs due to the need for additional converter circuits, especially in three-wire mains operations or single-string battery setups.

Innovation Solution

The UPS system employs a 3-level inverter, operating in both 2-level and 3-level modes to balance DC bus voltages by transferring energy between the positive and negative DC buses during specific periods of the AC line cycle, utilizing the inverter as a DC bus balancer to eliminate the need for a dedicated bus balancer circuit.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a dedicated bus balancer circuit is used to balance DC bus voltages, then voltage balance is achieved, but system cost and complexity increase

Engineering Contradiction:

ImproveDC bus voltage balanceVSAvoidsystem complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The inverter is designed to perform multiple functions: it not only converts DC power to AC output power but also actively balances the DC bus voltages by operating in 2-level mode during specific half-cycles. This eliminates the need for a separate dedicated bus balancer circuit, reducing system complexity while maintaining voltage balance functionality.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent combines the voltage balancing function with the existing inverter circuit by enabling it to operate in 2-level mode. This merges two previously separate functions (voltage balancing and AC conversion) into a single integrated operation, thereby reducing the number of components and simplifying the overall system architecture.

Inventive Principle:

Principle #5Merging (Combining)

2Reliability

If a dedicated bus balancer circuit is used to balance DC bus voltages, then voltage balance is achieved, but system cost increases

Engineering Contradiction:

ImproveDC bus voltage balanceVSAvoidsystem cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The inverter is designed to perform multiple functions: it not only converts DC power to AC output power but also actively balances the DC bus voltages by operating in 2-level mode during specific half-cycles. This eliminates the need for a separate dedicated bus balancer circuit, reducing system complexity while maintaining voltage balance functionality.

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent combines the voltage balancing function with the existing inverter circuit by enabling it to operate in 2-level mode. This merges two previously separate functions (voltage balancing and AC conversion) into a single integrated operation, thereby reducing the number of components and simplifying the overall system architecture.

Inventive Principle:

Principle #5Merging (Combining)

3Productivity

If the inverter operates continuously in 3-level mode, then output AC power is provided efficiently, but DC bus voltage imbalance cannot be corrected

Engineering Contradiction:

ImproveAC power output efficiencyVSAvoidDC bus voltage balance

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The inverter alternates between 3-level mode and 2-level mode in a periodic fashion synchronized with the AC line cycle. During positive half-cycles, it operates in 2-level mode to transfer energy and balance voltages; during negative half-cycles, it returns to 3-level mode for efficient AC output. This periodic switching between modes allows the system to achieve both voltage balance and efficient power delivery.

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The inverter dynamically switches its operating mode based on the instantaneous requirements of the system. The controller monitors DC bus voltages and line cycle position, then dynamically adjusts the inverter's operation between 2-level and 3-level modes to simultaneously achieve voltage balancing and efficient AC power output.

Inventive Principle:

Principle #15Dynamics

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

This approach effectively balances DC bus voltages, reducing system costs and footprint by leveraging the inverter's capabilities to manage voltage imbalances, thereby enhancing efficiency and reducing the need for additional hardware.

Implementation Method 1

selectively control, based on the imbalance, the 3-level inverter to operate in a 2-level mode of operation and a 3-level mode of operation to transfer energy between the positive DC bus and the negative DC bus

Methodology Applied

Scientific EffectEnergy transfer:

Data Source

PatentEP3309927B1DC-link bus balancer

Publication Date: 2021.09.29 SCHNEIDER ELECTRIC IT CORP

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AI summary

According to one aspect, embodiments of the invention provide a UPS comprising a plurality of DC busses configured to receive DC power from a converter, the plurality of DC busses including a positive DC bus configured to maintain a positive DC voltage level, a midpoint DC bus, and a negative DC bus configured to maintain a negative DC voltage level, a 3-level inverter coupled to the plurality of DC busses and configured to convert the DC power from the plurality of DC buses into output AC power, and a controller configured to monitor the positive and negative DC voltage levels, identify an imbalance between the positive and negative DC voltage levels, and selectively control, based on the imbalance, the 3-level inverter to operate in a 2-level mode of operation and a 3-level mode of operation to transfer energy between the positive and negative DC busses.