An advanced voltage droop control concept for grid-tied and autonomous DC microgrids

Abstract

Droop Control has been a well-established control technique both in AC and DC power distribution grids for many years, because it provides a simple way to equally distribute the load current between remote power sources. With the increasing demand for low voltage DC microgrids supplying high-reliability equipment, like servers in data centers, to work both grid-tied and autonomously without a connection to the AC mains and fueled only by local renewable and conventional power sources, voltage droop control is facing new challenges. With power equipment being delivered from several manufacturers the demand for a communication less control scheme that only uses the voltage at the terminal point or the converter output current as an indicator how the control set point should be changed in order to satisfy the energy demand of the loads arises. In consequence, the commissioning time of the DC microgrid is greatly reduced since all components can be simply plugged together without the need for adaptions. An outline for such an inherently autonomous voltage droop control scheme to keep the system voltage within a narrow band of ± 10 % of its 380 VDC nominal value is given in the following paper by describing voltage droop control modelling basics and the selection of characteristic droop curves for different kinds of power sources as well as by giving simulative results from a small-scale DC microgrid

Cost-Optimized Control of DC Microgrids based on Characteristic Diagrams

“…Therefore decentralized approaches are often used [3]. Popular options are adaptive droop voltage [4] or fuzzy control [5]. By using e.g.…”

Investigation on an AC Grid Failure Handling of Industrial DC Microgrids with an Energy Storage

“…Furthermore, the computational effort of the central unit and the mandatory real-time communication, which is necessary for this architecture, limits the number of participants in DC microgrids. The decentralized droop curve control is a commonly used approach in many DC microgrid applications, like DC-powered ships or server data centers [4], [5]. The droop curve control is a method to control the grid participants output impedance virtually [6].…”

“…Each source has a local voltage measurement to adjust the current output with respect to the droop curve. The voltage measurement must be low-pass filtered to prevent an oversensitive reaction of the droop curve control [5]. Although droop curve control has advantages like low costs and complexity, it is not flexible enough in many cases such as on changing constraints, like the volatile supply of the PV system or the State of Charge (SoC) of an ESS [3].…”

Modelling and measuring complex impedances of power electronic converters for stability assessment of low-voltage DC-grids

“…In this frequency range, the output impedance can actively be shaped by the designer by modifying the compensator or the power stage with its inner current loop. Advanced control strategies, like voltage droop control as described in [4] and [5] aim to shape the output impedance of Bode plot of the simulated buck converter output impedance source converters in this frequency region to optimize the load current distribution to the different sources and to compensate voltage drops over long cables. For higher frequencies, the impedance Z C,out of the output capacitor dominates the curve progression.…”