Converter Pulse Pattern Control for Filter Cell Voltage Balancing

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Solution Overview

Problem

Existing control schemes for 3L(A)NPC+AF converters face challenges in balancing active filter cell voltages across the entire range of operating points, leading to oscillations and inefficiencies, particularly at low fundamental frequency ratios, which affect power quality and harmonic distortion.

Innovation Solution

A method that adjusts offline-computed optimized pulse patterns by shifting switching instants to balance filter cell capacitor voltages, using a time-domain based average adjust algorithm, and integrates with model predictive control to minimize flux errors, ensuring independent control of each phase and reduced computational demand.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If common mode injection balancing scheme is used, then filter cell voltage balancing is achieved, but low frequency oscillations occur and the scheme fails at certain operating points

Engineering Contradiction:

Improvefilter cell voltage balancingVSAvoidfilter cell voltage stability

Core Design Contradiction:

ReliabilityVSStability of the object's composition

Solution Approach 1:

The patent segments the filter cell balancing control into phase-independent individual control loops, where each phase's filter cell voltage is balanced separately through independent switching instant adjustments, avoiding the oscillatory behavior of common mode injection

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent dynamically adjusts switching instants of the main stage based on filter cell voltage deviations, implementing a dynamic balancing mechanism that adapts to different operating conditions without causing oscillations

Inventive Principle:

Principle #15Dynamics

2Reliability

If conventional real-time carrier-based PWM is used, then filter cell balancing is attempted, but the modulator is driven into overmodulation range when voltage ratio is below 40%, leading to balancing difficulties

Engineering Contradiction:

Improvefilter cell balancing capabilityVSAvoidoperating range coverage

Core Design Contradiction:

ReliabilityVSAdaptability or versatility

Solution Approach 1:

The patent changes the control parameter from modulation index to switching instant adjustment, allowing effective filter cell balancing across the entire operating range including low voltage ratio conditions where conventional PWM fails

Inventive Principle:

Principle #35Parameter changes

3Object-generated harmful factors

If optimized pulse patterns are computed offline for all modulation indices and switching frequencies, then current distortion is minimized, but computational storage requirements increase

Engineering Contradiction:

Improvecurrent distortionVSAvoidstorage memory capacity

Core Design Contradiction:

Object-generated harmful factorsVSQuantity of substance

Solution Approach 1:

The patent pre-computes and stores optimized pulse patterns only for discrete modulation indices and switching frequencies offline, then uses real-time switching instant adjustments to adapt these pre-computed patterns to actual operating conditions, reducing storage requirements while maintaining low current distortion

Inventive Principle:

Principle #10Preliminary action

4Reliability

If filter cell voltages are adjusted to reference together, then voltage balancing is achieved, but independent control of each phase is lost

Engineering Contradiction:

Improvefilter cell voltage balancingVSAvoidindependent phase control

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The patent segments the balancing control into independent phase-specific loops, allowing each phase's filter cell voltage to be controlled independently through its own switching instant adjustments, enabling both balancing and independent phase control

Inventive Principle:

Principle #1Segmentation

Data Source

PatentEP4304073A1Model predictive control for converter with filter cells based on offline-computed pulse patterns and online cell balancing

Publication Date: 2024.01.10 ABB (SCHWEIZ) AG

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Figure 3a~3b

AI summary

An electrical converter (10) comprises a main stage (12) adapted for converting a DC voltage into an intermediate voltage comprising at least two voltage levels and a filter cell stage (14) with a filter cell (30) for each phase of the main stage (12), each filter cell (30) being adapted for adding or subtracting a cell voltage of the filter cell (30) to the intermediate voltage. A method for controlling an electrical converter (10) comprises a pattern determination part (44) comprising the steps of: selecting and reading an offline-computed optimized pulse pattern (OPPbase) from a database, wherein the selected optimized pulse pattern (OPPbase) comprises switching instants for the main stage (12) and the filter cells (30) over a next computation window (αwin) of a predetermined width (αw); adjusting the optimized pulse pattern (OPPbase) by moving its switching instants such that average output voltages generated in the filter cells (30) are shifted towards an average output voltage reference (Ubal,abc) of the filter cells (30), which is determined from measurements in the filter cells (30), and compensating these adjustments by corresponding modifications of switching instants of the main stage (12). The method comprises further a model predictive control part (46) comprising the following steps, which are performed several times during the next computation window (αwin): modifying the adjusted optimized pulse patterns (66) by moving at least one transition time of a switching instant, such that a flux error determined from a difference between an estimated flux (74) of the electrical converter (10) and a reference flux trajectory (Ψref) is minimized; and applying at least a next switching instant from the modified adjusted pulse pattern to the electrical converter (10).