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

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

A SuperCapacitor Agent for Providing Real-Time Power Services to the Grid

Supercapacitors-based storage systems are expected to play a key role in microgrids in view of their capability to compensate high-power imbalances. We define an agent for the control of supercapacitor arrays within the context of the novel control framework Commelec, proposed by the Authors as a composable method for real-time control of active distribution networks with explicit power setpoints. An important function of such an agent is to advertise the real-time power capabilities and operational preferences of the supercapacitor array based on local information. Given the small energy capacity of such a device, its internal state can largely vary from one setpoint implementation to the next one. For this reason, the use of an accurate model is crucial in the agent definition. We show that it is possible to infer the real-time power capabilities of the device by using simple measurements on the supercapacitor array suitably coupled with an accurate representation of the cells composing the array. Results show that the agent is able to speak for the resource, thus allowing its use from an external controlle

A composable method for real-time control of active distribution networks with explicit power set points. Part II: Implementation and validation

In this second part, we evaluate the performances of our control framework by applying it to a casestudy that contains a minimum set of elements allowing to show its applicability and potentials. Weshow how the computation of the PQt profiles, belief functions, and virtual costs can be synthesized forgeneric resources (i.e., dispatchable and stochastic generation systems, storage units, loads). The metricsof interest are: quality-of-service of the network represented by voltages magnitudes and lines currentmagnitudes in comparison with their operational boundaries; state-of-charge of electric and thermalstorage devices; proportion of curtailed renewables; and propensity of microgrid collapse in the case ofrenewables overproduction. We compare our method to two classic ones relying on droop control: thefirst one with only primary control on both frequency and voltage and the second one with an additionalsecondary frequency control operated by the slack device. We find that our method is able to indirectlycontrol the reserve of the storage systems connected to the microgrid, thus maximizing the autonomy inthe islanded operation and, at the same time, reducing renewables curtailment. Moreover, the proposedcontrol framework keeps the system in feasible operation conditions, better explores the various degreesof freedom of the whole system and connected devices, and prevents its collapse in case of extremeoperation of stochastic resources. All of these properties are obtained with a simple and generic controlframework that supports aggregation and composability

Handling Large Power Steps In Real-time Microgrid Control Via Explicit Power Set-points

We consider a microgrid with real-time control using explicit power-setpoints. Sudden power-steps, such as load disconnections or load in-rushes, directly affect the decisions of the microgrid controller that aims at avoiding voltage or line-ampacity violations. When trying to completely avoid these violations, the grid operation may be too restricted, which may lead to large suboptimality. However, temporary violations of the steady-state bounds are allowed by grid standards and could enable the exploitation of the flexibility of other resources to better control the system's state. In this paper, we propose a method by which such temporary violations are controlled so that they remain within the limits imposed by grid standards and safe operation. The method is experimentally tested and validated on a real microgrid

Robust Real-Time Control of Power Grids in the Presence of Communication Network Non-Idealities

Deploying a power grid controller in the field makes it susceptible to message losses caused by the inherent uncertainties and non-idealities of communication networks, especially when the control action is taken at a sub-second time-scale. We consider a centralized power grid controller that monitors and controls resources in real-time. The resources send advertisements that contain information about their state, and an estimation of their behavior in the time horizon when the control action is expected to be implemented. The controller uses this information to compute and issue setpoints that are thus only valid for this time horizon. An occasional loss of one or more advertisements might render the controller incapable of issuing valid setpoints. We introduce advertisements with a longer-term prediction interval, which are constantly sent along with the short-term ones, and can be used by the controller when it is missing information from some or all resources. We show the advantages of using such an approach on a controller that, by exploiting local resources flexibilities, performs frequency support on the CIGRE benchmark low-voltage microgrid

Aggregation of Power Capabilities of Heterogeneous Resources for Real-Time Control of Power Grids

Aggregation of electric resources is a fundamental function for the operation of power grids at different time scales. In the context of a recently proposed framework for the real-time control of microgrids with explicit power setpoints, we define and formally specify an aggregation method that explicitly accounts for delays and message asynchronism. The method allows to abstract the details of resources using high-level concepts that are device and grid-independent. We demonstrate the application of the method to a Cigre benchmark with heterogenous and lowinertia resources

Dispatching Stochastic Heterogeneous Resources Accounting for Grid and Battery Losses

We compute an optimal day-ahead dispatch plan for distribution networks with stochastic resources and batteries, while accounting for grid and battery losses. We formulate and solve a scenario-based AC Optimal Power Flow (OPF), which is by construction non-convex. We explain why the existing relaxation methods do not apply and we propose a novel iterative scheme, Corrected DistFlow (CoDistFlow), to solve the scenario-based AC OPF problem in radial networks. It uses a modified branch flow model for radial networks with angle relaxation that accounts for line shunt capacitances. At each step, it solves a convex problem based on a modified DistFlow OPF with correction terms for line losses and node voltages. Then, it updates the correction terms using the results of a full load flow. We prove that under a mild condition, a fixed point of CoDistFlow provides an exact solution to the full AC power flow equations. We propose treating battery losses similarly to grid losses by using a single-port electrical equivalent instead of battery efficiencies. We evaluate the performance of the proposed scheme in a simple and real electrical networks. We conclude that grid and battery losses affect the feasibility of the day-ahead dispatch plan and show how CoDistFlow can handle them correctly

Aggregation of Power Capabilities of Heterogeneous Resources for Real-Time Control of Power Grids

Aggregation of electric resources is a fundamental function for the operation of power grids at different time scales. In the context of a recently proposed framework for the real-time control of microgrids with explicit power setpoints, we define and formally specify an aggregation method that explicitly accounts for delays and message asynchronism. The method allows to abstract the details of resources using high-level concepts that are device and grid-independent. We demonstrate the application of the method to a Cigre benchmark with heterogenous and lowinertia resources

Slack Selection for Unintentional Islanding: Practical Validation in a Benchmark Microgrid

Upon an intentional or emergency disconnection from the main grid, a microgrid is expected to continue working in islanded mode. Thus, (at least) one resource needs to act as slack and compensate for power variations to keep the power balance, and ensure the security of supply. Although several resources might be eligible to become slack, some are more suitable than others (energy storage systems in particular) depending on the state of both the resources and the grid before the islanding transition. In this paper, we validate a recently proposed method to select in real-time the best slack-candidate using an abstract representation of the internal state of the available resources. The same method can be used to actively switch the slack during islanded operation to accommodate the intrinsic stochastic nature of the microgrid’s resources. Our main contribution is the validation of the method in a real-scale microgrid, including a discussion of implementation and deployment aspects. To support our findings, we present extensive experimental results in different operating conditions