Inverter Frequency Control for Distributed Energy Resources
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Solution Overview
Problem
Distributed energy resources (DER) systems face challenges in seamlessly transitioning between island and grid modes due to differences in frequency control and communication coordination compared to centralized energy resources, requiring methods to control microsources locally without communication between them.
Innovation Solution
Implementing a unit power controller that reduces the operating frequency of inverters to increase output power, using a voltage vs. reactive current droop, and extending this method to control multiple microsources with varying power set points, allowing for 'plug and play' operation without modifying existing control systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If distributed energy resources use inverter-based microsources with internal clock frequency control, then the system can operate independently without communication coordination, but the frequency cannot be dynamically adjusted to match grid frequency during transitions
Solution Approach 1:
The patent applies dynamics by making the inverter's operating frequency variable rather than fixed. The controller dynamically adjusts the instantaneous frequency of the inverter output based on the operating mode (grid-connected or islanded). In grid-connected mode, the inverter frequency is adjusted to match the utility grid frequency, while in islanded mode, it operates at a different frequency. This dynamic frequency adjustment resolves the contradiction between independent operation and adaptability to grid conditions.
2Reliability
If distributed energy resources transition rapidly between island and grid modes, then system reliability improves, but control coordination and frequency matching become more difficult
Solution Approach 1:
The patent applies segmentation by dividing the control function into independent local controllers for each microsource. Each controller operates autonomously based on local voltage and frequency measurements, without requiring communication with other microsources or central coordination. This segmentation enables rapid mode transitions because each unit can independently detect grid presence/absence and switch modes simultaneously, eliminating coordination delays while maintaining reliability.
Solution Approach 2:
The patent applies self-service through autonomous operation of each microsource controller. Each controller independently monitors local electrical parameters (voltage, frequency) and automatically adjusts its operation based on the detected grid status. This self-service capability eliminates the need for external communication coordination, enabling rapid and reliable mode transitions while reducing control complexity.
3Stability of the object's composition
If multiple microsources are controlled with fixed frequency ratios, then system stability is maintained, but power output cannot be optimized for varying load conditions
Solution Approach 1:
The patent applies parameter changes by making the frequency ratio between multiple microsources variable rather than fixed. Each microsource controller independently adjusts its output frequency based on local load conditions and power setpoints. The frequency ratio between microsources changes dynamically to optimize power distribution while maintaining overall system stability. This allows each microsource to contribute optimally to varying load conditions while the system remains stable through coordinated frequency adjustments.
Data Source
AI summary
A method of controlling the output inverter of a microsource in a distributed energy resource system is disclosed. Embodiments of the invention include using unit or zone power controllers that reduce the operating frequency of the inverter to increase its unit output power. Preferred embodiments includes methods wherein the inverter reaches maximum output power and minimum operating frequency at the same time, and further comprising using a voltage controller implementing a voltage vs. reactive current droop. Other aspects of this embodiment relate to an inverter that implements such methods, and a microsource containing such an inverter. These methods can be extended to control inverters in a plurality of microsources, organized in a single zone or in a plurality of zones.