Impact of battery energy storage systems on the dynamic behavior of low-inertia power grids

The extensive deployment of non-synchronous generation determines lower level of grid inertia resulting in deteriorated frequency containment performance and abnormal frequency excursions in case of contingency. This calls for identifying assets, controls, and relaying schemes capable to ensure acceptable grid frequency containment and dynamics satisfying the requirements of existing grid codes. A potential way to counterbalance this lack of inertia is to use large-scale battery energy storage systems (BESSs) since they provide large ramping rates and fast power control. As known, there are generally two main approaches to control converter-interfaced BESSs: grid-following and grid-forming controls. As BESSs may provide significant value to system frequency containment, it is of fundamental importance to quantitatively evaluate the dynamics of low-inertia power grids hosting large-scale BESSs. Within this context, it is also of importance to study the behavior of low-inertia power systems subsequent to large contingencies in order to develop appropriate under-frequency load shedding (UFLS) relaying schemes that may take advantage of nowadays distributed sensing technologies enabled by the Phasor Measurement Units (PMUs).Framed within the EU H2020 project "Optimal System-Mix Of flexibility Solutions for European electricity", the Thesis first characterizes the interplay between converter-interfaced BESSs and low-inertia power grids and, then, provides quantitative assessments of system dynamics and quantifies the benefits associated to different control strategies of BESSs. For this purpose, state-of-the-art detailed dynamic simulation models of power grids, BESS, and controls are implemented on a real-time simulator for detailed numerical analyses. At first, contingency tests are conducted. The results verify the substantial influence of inertia reduction on post-contingency dynamics of power systems and quantitatively prove that converter-interfaced BESSs can effectively limit the frequency decreasing and damp the frequency oscillations. Then, the proposed dynamic models are used for one-day-long simulations to assess the impact of converter-interfaced BESSs on the frequency containment of low-inertia power grids in normal operating conditions. For a practical operative context, a day-ahead schedule layer is considered where reserve levels for frequency containment and restoration are allocated considering the current practice required by European transmission system operators. Numerical analyses on suitably defined metrics applied to grid frequency show that the grid-forming control strategy outperforms the grid-following one, achieving better system frequency containment.As large frequency excursions are more likely to occur due to the decrease of kinetic energy stored in rotating synchronous machines, fast adaptive UFLS schemes are necessary to secure low-inertia power systems under contingency. PMUs provide an effective tool to track the network state in any node of interest with reporting rates in tens of frames per second. In this respect, the Thesis proposes and validates two new UFLS schemes suitable for low-inertia power grids. The first scheme is a centralized UFLS that leverages PMU-fed situational awareness systems and is coupled with an Optimal Power Flow (OPF) problem. The OPF problem is formulated to constrain nodal voltages and branch currents in combination with a model capable of predicting the system response.

Real-Time Control Framework for Active Distribution Networks Theoretical Definition and Experimental Validation

Lorenzo Enrique Reyes Chamorro

The great challenge of massively integrating the volatile distributed power-generation into the power system is strongly related to the evolution of their operation and control. The literature of the last decade has suggested two models for such an evolution: (i) the supergrid model, based on enhanced continental/intercontinental network interconnections (mainly DC) for bulk transmission, (ii) the microgrid mode, where small medium/low voltage networks interfacing heterogeneous resources, such as local generation, energy storage and active customers, are intelligently managed so that they are operated as independent cells capable of providing different services from each other and operate in islanded mode. Irrespective of the model that will eventually emerge, the control of heterogeneous distributed resources represents a fundamental challenge for both supergrid and microgrid models. This requires the definition of scalable and composable control methods that guarantee the optimal and feasible operation of distribution grids in order to satisfy local objectives (e.g., distribution grid power balance), as well as the provision of ancillary services to the external bulk transmission (e.g., primary and secondary frequency supports). Several control methodologies have been proposed to achieve these goals, and the majority of them have been inspired by the classic time-layered approach traditionally adopted in power systems that are associated with different time-scales and extension of the controlling area, i.e. primary, secondary and tertiary controls, ranging from sub-seconds to hours, respectively. In the context of microgrids, these three levels of control can be associated with a decision process that can be centralized (i.e., a dedicated central controller decides on the operation of the system resources) and/or decentralized (each element decides based on its own rules). In the current literature, the former is used for long-term, whereas the latter for short-term decisions. In particular, primary controls are typically deployed through fully decentralized schemes mainly relying on the use of droop control. With this in mind, in this thesis we propose, and experimentally validate, a novel control framework called COMMELEC â A Composable Framework for Real-Time Control of Active Distribution Networks, Using Explicit Power Set-Points. It controls a power grid in real-time based on a multi-agent structure, using a simple and low-bandwidth communication protocol. Such a framework enables a controller to easily steer an entire network as an equivalent energy resource, thus making an entire system able to provide grid support by exploiting the flexibility of its components in real-time. The main features of the framework are (i) that it is able to indirectly control the reserve of the storage systems, thus maximizing the autonomy of the islanding operation, (ii) that it keeps the system in feasible operation conditions and better explores, compared to traditional techniques, the various degrees of freedom that characterize the system, and (iii) that it maintains the system power-equilibrium without using the frequency as a global variable, even being able to do so in inertia-less systems. Our framework has been extensively validated, first by simulations but, more importantly, in a real-scale microgrid laboratory specially designed and setup for this goal. This is the first real-scale experiment that proves the applicability of a droop-less explicit power-flow control mechanism in microgrids.

EPFL2016

Control of Battery Storage Systems for the Simultaneous Provision of Multiple Services

Emil Namor, Mario Paolone, Fabrizio Sossan

In this paper, we propose a control framework for a battery energy storage system to provide simultaneously multiple services to the electrical grid. The objective is to maximise the battery exploitation from these services in the presence of uncertainty (load, stochastic distributed generation, grid frequency). The framework is structured in two phases. In a period-ahead phase, we solve an optimization problem that allocates the battery power and energy budgets to the different services. In the subsequent real-time phase the control set-points for the deployment of such services are calculated separately and superimposed. The control framework is first formulated in a general way and then casted in the problem of providing dispatchability of a medium voltage feeder in conjunction to primary frequency control. The performance of the proposed framework are validated by simulations and real-scale experiments, performed with a grid-connected 560 kWh/720 kVA Li-ion battery energy storage system.

2018

A VSC-Based BESS Model for Multi-Objective OPF Using Mixed Integer SOCP

Xiaoying Tang, Tong Wu

This paper presents a new model of voltage source converter (VSC) based battery energy storage systems (BESSs) that interface with power grids. A VSC-based BESS is made up of a series connection of a VSC, its connecting transformer, and a BF-SS. The VSC allows a BF-SS to generate both active and reactive powers in all four quadrants. The proposed model captures the coupling between active power, reactive power, and the voltage of a BESS. In addition, the proposed model explicitly describes the relationship between the control configuration of pulsewidth modulation (PWM) VSC and the power output of the BESS. Therefore, the proposed model possesses unparalleled control capabilities in the operational parameters of both the ac and dc sides of the converter. By incorporating such a model into the active-reactive optimal power flow (A-R-OPF) formulation, we can not only optimize the active and reactive power output of the BESS in power systems, but also understand how the optimal powers are generated by setting the operational parameters of both the BESS and PWM-VSC. To solve the A-R-OPF problem with the proposed model, we propose a sequence of strong relaxations to transform the problem into a mixed-integer second-order cone programming problem. Such a formulation is amendable for efficient solutions using off-the-shelf solvers. Case studies on the IEEE benchmark systems show that more than 17.73% of power losses in transmission lines and more than 0.961% of interface losses can be reduced by using the proposed model in comparison to the traditional BESS model.

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2019

Real-Time Control Framework for Active Distribution Networks Theoretical Definition and Experimental Validation

Lorenzo Enrique Reyes Chamorro

EPFL2016