Microgrid Power Converter Virtual Impedance Control

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Solution Overview

Problem

Conventional microgrid systems face challenges in controlling reactive power in AC-coupled batteries, leading to reduced energy harvest and potential converter damage due to lack of direct current control, especially when transitioning between islanded and grid-connected states.

Innovation Solution

Implementing a virtual voltage-virtual impedance control technique that controls power converters to appear as a virtual AC voltage source in series with virtual impedance, allowing for seamless transitions and limiting reactive current during charging to prioritize active power flow, thereby maximizing energy harvest.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of operation

If droop control is used to operate energy storage resources in a microgrid, then autonomous load sharing among parallel AC generators is achieved, but direct control of real and reactive currents is lost making it difficult to impose current limits

Engineering Contradiction:

Improveautonomous load sharingVSAvoidcurrent limit control

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The control system is segmented into two distinct modes: droop control mode for autonomous operation and voltage control mode for precise current limiting. The system can switch between these modes based on operational requirements, allowing both autonomous load sharing and direct current control to be achieved at different times without compromise

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The control system dynamically switches between droop control and voltage control modes based on real-time operational conditions. This dynamic adaptability allows the system to maintain autonomous load sharing when needed while imposing direct current limits when safety or performance requires it

Inventive Principle:

Principle #15Dynamics

2Reliability

If energy storage resources provide reactive power to loads, then reactive power support is achieved, but the maximum charging rate is reduced thereby reducing economic value

Engineering Contradiction:

Improvereactive power supportVSAvoidcharging rate

Core Design Contradiction:

ReliabilityVSProductivity

Solution Approach 1:

The control system dynamically adjusts the balance between reactive power provision and active power charging based on real-time conditions. When charging opportunities arise, the system can reduce or suspend reactive power support to maximize charging rate, and vice versa, optimizing the trade-off between reliability and productivity

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system periodically evaluates the operational state and switches between providing reactive power and maximizing charging. This periodic decision-making allows the system to capture charging opportunities when they arise while maintaining reactive power support during steady-state operation

Inventive Principle:

Principle #19Periodic action

3Ease of operation

If virtual source voltage phasor is commanded during droop control, then autonomous operation is maintained, but real or reactive current phasor may exceed maximum converter capability causing damage or shutdown

Engineering Contradiction:

Improveautonomous operationVSAvoidconverter damage risk

Core Design Contradiction:

Ease of operationVSObject-affected harmful factors

Solution Approach 1:

Before commanding the virtual source voltage phasor during droop control, the system preemptively checks whether the resulting current phasor would exceed converter capabilities. If it would, the system takes preventive action by switching to voltage control mode or adjusting the voltage command, thereby avoiding converter damage before it can occur

Inventive Principle:

Principle #9Preliminary anti-action

Solution Approach 2:

The control system continuously monitors the commanded voltage phasor and the resulting current phasor, providing feedback to detect when current limits are approaching. This feedback mechanism allows the system to adjust the voltage command or switch modes to prevent converter damage while maintaining autonomous operation when safe

Inventive Principle:

Principle #23Feedback

4Speed

If phase shift virtual impedance droop control is used, then dynamic response and harmonic compensation are improved, but direct phasor control of real and reactive currents is lost

Engineering Contradiction:

Improvedynamic responseVSAvoidcurrent control capability

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The control system segments functionality by using phase shift virtual impedance droop control for dynamic response and harmonic compensation, while separately implementing voltage control mode for direct phasor control of currents. This segmentation allows each control mode to excel at its specialized function without compromise

Inventive Principle:

Principle #1Segmentation

Data Source

PatentUS11190016B2Method and apparatus for increased energy harvest in a microgrid

Publication Date: 2021.11.30 ENPHASE ENERGY INC

AI summary

A method and apparatus for power converter current control. In one embodiment, the method comprises controlling an instantaneous current generated by a power converter that is part of an AC battery such that the power converter appears, from the perspective of an AC line coupled to the power converter, as a virtual AC voltage source in series with a virtual impedance, wherein real and reactive phasor currents for the power converter are indirectly controlled by modifying amplitude and phase of a virtual AC voltage waveform that defines the virtual AC voltage source.