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## DC microgrid control principles - hierarchical control diagram

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In this chapter, the hierarchical control of DC microgrids (MGs) is introduced. The definitions for each control level have been discussed. Primary control is responsible for distributed generator (DG) load sharing and is predominately implemented using the droop control. The droop control can be perceived as a virtual resistance, and its value can affect system stability and maximum DC bus voltage deviation. Two inherent issues with conventional droop control are discussed. Both terminal DC voltage control accuracy and cable resistance have impacts on the power sharing among DGs. Droop control can be mainly divided into two groups: current-based droop and voltage-based droop. The difference is the implication for system stability. In addition, another nonlinear droop curve is also mentioned with adaptive droop gain. It can reduce power sharing error in heavy load. Furthermore, dynamic power sharing during load transition is also discussed. Different load dynamic spectrums are assigned to different DGs according to their dynamic response. Finally, to receive commands from upper control levels, interfaces are pointed out without changing the basic primary control loop structure. The purpose of the secondary control is to restore the DC bus voltage deviation caused by conventional droop control from the primary control level. It can be implemented remotely in an MG central controller (Centralized approach) or locally inside each DG (distributed approach). Both dedicated analogue and digital communication links can be used in a decentralized secondary control to transmit current signals. To enhance the reliability of the control, a communication link based on power lines is also possible. The tertiary control level is responsible for the connecting process of MGs to the upper grid. A basic mode structure based on DBS is introduced to accommodate energy-management algorithms.

Chapter Contents:

• 1.1 Introduction
• 1.2 The hierarchical control for DC MGs
• 1.3 Primary control
• 1.3.1 Basics of droop control
• 1.3.2 Power sharing errors
• 1.3.3 Droop strategies
• 1.3.4 Dynamic power sharing
• 1.3.5 Interfaces to upper levels
• 1.4 Secondary control
• 1.4.1 Centralized approach
• 1.4.2 Distributed approach
• 1.4.2.1 Communication through dedicated communication channels
• 1.4.2.2 Communication through power lines
• 1.5 Tertiary control
• 1.6 Summary
• References

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