Doubly-Fed Generator Control for Unbalanced Grid Stability

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Solution Overview

Problem

Doubly-fed electric generators face challenges in controlling grid real and reactive power when connected to an unbalanced grid, leading to oscillations, reduced torque, mechanical damage, and thermal issues due to prior art's assumption of a balanced grid.

Innovation Solution

The method involves dividing the grid voltage into symmetrical positive and negative sequence components, using the positive sequence to control the generator, allowing for stable operation even in unbalanced conditions by calculating and utilizing the stator flux positive sequence component to determine rotor flux and torque references.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If prior art control methods assume a balanced grid, then the control system is simpler, but the generator experiences large oscillations in currents and reduced reliability when connected to an unbalanced grid

Engineering Contradiction:

Improvecontrol system complexityVSAvoidgenerator operation reliability

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The invention segments the grid voltage into symmetrical positive and negative sequence components. By separating the balanced positive sequence from the unbalanced negative sequence, the controller can independently handle each component, enabling stable control under unbalanced conditions while maintaining reasonable system complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention introduces symmetrical component transformation as an intermediary step between the unbalanced grid voltage and the control system. This intermediary transformation converts the problematic unbalanced three-phase system into two separate balanced systems (positive and negative sequences), allowing standard control methods to work effectively

Inventive Principle:

Principle #24Intermediary (Mediator)

2Productivity

If the generator operates at nominal speed with unbalanced grid, then the slip frequency reaches 100 Hz causing voltage oscillations, but reducing slip frequency limits the operational range

Engineering Contradiction:

Improveoperational rangeVSAvoidvoltage oscillations and current spikes

Core Design Contradiction:

ProductivityVSObject-affected harmful factors

Solution Approach 1:

The invention converts the harmful effect of unbalanced grid voltage into beneficial information by extracting the positive sequence component. The negative sequence (unbalance) is transformed into a useful indicator that allows the controller to compensate for unbalance effects, enabling stable operation even at high slip frequencies where voltage oscillations would normally occur

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

Data Source

PatentEP2456064B1Control method for doubly-fed electric generator

Publication Date: 2015.09.30 ABB TECH OY

Figure 1~2

AI summary

An object of the present invention is thus to provide a method of controlling a doubly-fed electric generator with a frequency converter comprising a torque and flux controller, wherein the frequency converter is connected to a rotor of the generator. The method comprises calculating a stator flux positive sequence component from a stator flux. A rotor flux amplitude reference and an estimated torque is then calculated by using the stator flux positive sequence component. The rotor flux amplitude reference and the estimated torque are used in selecting a voltage vector to steer the rotor flux.