Energy Storage Control and Requirements For Inverter-Based Microgrids

The intermittent nature of distributed renewable sources such as wind or solar requires integration of energy storage systems. In this dissertation a distributed form of the Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) method is used to determine the energy storage requirements of three-phase inverter-based microgrids. The overall control is appropriate to be integrated into a hierarchical control system. As the primary control, a novel dq droop control sets the local references and is supported by a level-zero Hamiltonian controller which includes energy storage feed-forward and feedback, and an inverter feed-forward controls. Here, the energy storage element performs as the sole actuator of the system and enforces the references that are set by the droop control while the inverter feed-forward matches the voltage levels of the inverter to the local bus. The control method as well as the power flow and energy transfer model of the microgrid system enables the capacity and bandwidth of the storage system to be determined. The Hamiltonian control is further derived for parallel operation of hybrid, band-limited and reduced-order battery and flywheel storage systems. Moreover, a control scheme is proposed to enable sharing of power between parallel battery and flywheel storage systems according to their bandwidth support capabilities. Here, battery storage systems are considered as the primary storage elements while flywheel systems are controlled to complement the deficit for higher power fluctuations. Power and energy sizing guidelines are presented and relevant trade-offs are addressed in illustrative examples. Energy storage baseline requirements for pulsed power loads are also presented in this work. Here, the energy storage system combination with the pulsed load is controlled to mimic a constant power load that can further be integrated into power buffer systems. Examples of control and requirements for ideal, band-limited and reduced-order battery and flywheel storage systems are given. By considering these requirements, a system designer can derive the specifications for source-side or load-side energy storage elements and their control systems

Droop control in DQ coordinates for fixed frequency inverter-based AC microgrids

This paper presents a proof-of-concept for a novel dq droop control technique that applies DC droop control methods to fixed frequency inverter-based AC microgrids using the dq0 transformation. Microgrids are usually composed of distributed generation units (DGUs) that are electronically coupled to each other through power converters. An inherent property of inverter-based microgrids is that, unlike microgrids with spinning machines, the frequency of the parallel-connected DGUs is a global variable independent from the output power since the inverters can control the output waveform frequency with a high level of precision. Therefore, conventional droop control methods that distort the system frequency are not suitable for microgrids operating at a fixed frequency. It is shown that the proposed distributed droop control allows accurate sharing of the active and reactive power without altering the microgrid frequency. The simulation and hardware-in-the-loop (HIL) results are presented to demonstrate the efficacy of the proposed droop control. Indeed, following a load change, the dq droop controller was able to share both active and reactive power between the DGUs, whereas maintaining the microgrid frequency deviation at 0% and the bus voltage deviations below 6% of their respective nominal values

Energy storage requirements for inverter-based microgrids under droop control in d-q coordinates

This work proposes a novel distributed control approach for Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) method to determine Energy Storage (ES) requirements for a three-phase, inverter-based Microgrids (MGs). Here, local system references are obtained through a primary d-q droop control which is supported by a level-zero Hamiltonian controller. ES devices are the actuators of the system to enforce reference points. The control approach as well as power flow and energy transfer model of the MG enables the ES capacity and bandwidth to be obtained. As a result, a zero-output ES element analysis is defined which can further be used to study storage requirements versus additional constraints. Communication network update-rate can affect Energy Storage Systems (ESSs) and filtering requirements. The developed analysis is demonstrated in parallel and looped nine-bus WSCC reduced-order MG system examples to obtain ES requirements versus communication network update-rate and (bandwidth)

Effects of DQ droop settings on energy storage systems of inverter-based microgrids

The distributed nature of micro-sources with intermittent behavior requires development of appropriate microgrid integration schemes. In a system comprising of solely renewable sources that have no inertia, the operation at a fixed frequency with effective power sharing capability is a viable solution. This paper presents the effects of dc droop settings on the energy storage systems of three-phase microgrids that utilize a dq droop control for power sharing at a fixed frequency. The microgrid control is modeled according to the Hamiltonian Surface Shaping and Power Flow Control (HSSPFC) scheme. The energy storage control law is developed so that the required storage is in a minimal form however, the choice of grid-side local droop control settings may impose significant burden on source-side dc energy storage systems. Two simulation examples of energy storage requirements versus the dq droop settings are presented in this paper

Energy storage baseline requirements for pulsed power loads

Pulsed power loads (PPLs) are highly non-linear and can cause significant stability and power quality issues in a microgrid. One way to mitigate many of these issues is by designing an Energy Storage System (ESS) to offset the PPL. This paper provides a baseline for ESS control and specifications to mitigate the effects of PPL\u27s. ESS will maintain a constant bus voltage and decouple the generation sources from the PPL. The ESS specifications are realized with ideal, band-limited hybrid battery and flywheels models and simulated to demonstrate the efficacy of the control system