AC Power System Stability via Synchronous Frame Impedance

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Solution Overview

Problem

Existing methods for analyzing stability in multi-phase AC/DC systems are complex and burdensome, leading to excessively conservative designs and operating parameters, particularly in smaller systems like aircraft, water-borne vessels, and small power plants, where instabilities can occur due to negative incremental impedance characteristics.

Innovation Solution

A simplified methodology using a synchronized frame of reference to model AC power sources and active front-end power converters, applying the standard Nyquist stability criterion to determine stability by approximating eigenvalues with d-d and q-q impedance elements, reducing the complexity of multi-variable systems to a single-input, single-output case.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If traditional complex stability analysis methods are used for multi-phase AC/DC systems, then measurement precision and reliability are improved, but device complexity and ease of operation deteriorate

Engineering Contradiction:

Improvestability assessment accuracyVSAvoidanalysis methodology complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent transforms the complex multi-variable stability analysis into a simplified single-input single-output analysis by changing the reference frame parameters. By applying a synchronous rotating reference frame transformation, the multi-phase AC system parameters are converted into equivalent DC parameters, allowing the use of simple Nyquist stability criterion instead of complex multi-variable analysis methods.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent replaces the complex mathematical analysis system with an equivalent electrical measurement system. By measuring the input impedance of the power converter and comparing it with the source impedance in the synchronous reference frame, the stability assessment is achieved through electrical parameter measurement rather than complex mathematical computation.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Reliability

If traditional complex stability analysis methods are used for multi-phase AC/DC systems, then reliability is improved, but ease of operation deteriorates

Engineering Contradiction:

Improvestability assessment reliabilityVSAvoiddesign and operation simplicity

Core Design Contradiction:

ReliabilityVSEase of operation

Solution Approach 1:

The patent changes the operational parameters by transforming all measurements and calculations into the synchronous rotating reference frame. This parameter transformation converts time-varying AC parameters into constant or slowly varying DC-equivalent parameters, making the stability analysis as straightforward as checking simple impedance values rather than performing complex time-domain or frequency-domain multi-variable analysis.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent segments the complex stability analysis problem into two independent parts: (1) measuring the input impedance of the power converter in the synchronous reference frame, and (2) comparing this impedance with the source impedance using the simple Nyquist criterion. This segmentation breaks down the intractable complex problem into manageable, easily executable steps.

Inventive Principle:

Principle #1Segmentation

3Stability of the object's composition

If conservative design parameters are used to ensure stability, then stability is improved, but productivity and power output deteriorate

Engineering Contradiction:

Improvesystem stabilityVSAvoidpower output capacity

Core Design Contradiction:

Stability of the object's compositionVSProductivity

Solution Approach 1:

The patent implements a feedback mechanism where the measured input impedance of the power converter is continuously compared with the source impedance in the synchronous reference frame. This feedback allows real-time stability assessment and adjustment, enabling the system to operate at optimal power output levels while maintaining stability, rather than relying on conservative fixed design parameters that limit productivity.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

By transforming the stability analysis into the synchronous reference frame, the patent enables dynamic adjustment of operating parameters without compromising stability. The parameter transformation reveals the true stability margins, allowing the system to operate closer to its maximum power output capacity while maintaining stable operation, thus eliminating the need for conservative parameter limits.

Inventive Principle:

Principle #35Parameter changes

Data Source

PatentUS9136773B2Method of evaluating and ensuring stability of AC/DC power systems

Publication Date: 2015.09.15 THE BOEING CO

AI summary

Approximating loci of eigenvalues or characteristic gains of a return ratio matrix of a model of a multi-phase power converter circuit by the loci of the d-d and q-q elements of said synchronous frame of reference applied to said model, allows determination and assessment of stability of the circuit or forbidden operational parameters of the combination of an AC power source and a power converter at an interface thereof by application of a standard Nyquist stability criterion.