Petros Aristidou

Dynamic Simulation of Large-Scale Power Systems Using a Parallel Schur-Complement-Based Decomposition Method

Power system dynamic simulations are crucial for the operation of electric power systems as they provide important information on the dynamic evolution of the system after an occurring disturbance. This paper proposes a robust, accurate and efficient parallel algorithm based on the Schur complement domain decomposition method. The algorithm provides numerical and computational acceleration of the procedure. Based on the shared-memory parallel programming model, a parallel implementation of the proposed algorithm is presented. The implementation is general, portable and scalable on inexpensive, shared-memory, multi-core machines. Two realistic test systems, a medium-scale and a large-scale, are used for performance evaluation of the proposed method.

Active Management of Low-Voltage Networks for Mitigating Overvoltages Due to Photovoltaic Units

In this paper, the overvoltage problems that might arise from the integration of photovoltaic (PV) panels into low-voltage (LV) distribution networks is addressed. A distributed scheme is proposed that adjusts the reactive and active power output of inverters to prevent or alleviate such problems. The proposed scheme is model-free and makes use of limited communication between the controllers in the form of a distress signal only during emergency conditions. It prioritizes the use of reactive power, while active power curtailment is performed only as a last resort. The behavior of the scheme is studied using dynamic simulations on a single LV feeder and on a larger network composed of 14 LV feeders. Its performance is compared with a centralized scheme based on the solution of an optimal power flow (OPF) problem, whose objective function is to minimize the active power curtailment. The proposed scheme successfully mitigates overvoltage situations due to high PV penetration and performs almost as well as the OPF-based solution with significantly less information and communication requirements.

Contribution of Distribution Network Control to Voltage Stability: A Case Study

A case study dealing with long-term voltage instability in systems hosting active distribution networks (DN) is reported in this paper. It anticipates future situations with high penetration of dispersed generation (DG), where the latter are used to keep distribution voltages within desired limits, in complement to load tap changers. The interactions between transmission and active DN are investigated on a 3108-bus test system. It involves transmission grid, large generators, and 40 DN, each with DG steered by a controller inspired by model predictive control. The reported simulations show the impact of distribution network voltage restoration, as well as the benefit of load voltage reduction actuated by the dispersed generators.

Microgrid Modelling and Analysis Using Game Theory Methods

Game theory is a branch of applied mathematics that is, most notably, used in economics as well as in engineering and other disciplines. Game theory attempts to mathematically capture behaviour in strategic situations, in which an individual’s success in making choices depends on the choices of others. The microgrid encompasses a portion of an electric power distribution system that is located downstream of the distribution substation, and it includes a variety of DER units and different types of end users of electricity and/or heat. Microgrids promote the use of new technologies, under the general Smart Grids’ umbrella, in order to achieve more efficient use of electric energy, better protection, improved control and provide services to the users. For the materialization of the infrastructure needed to implement this model, engineers have nominated technologies like smart agents, distributed computing, smart sensors and others, as well as, a solid and fast communication infrastructure. In this decentralized environment, multiple decision making participants interact, each striving to optimize its own objectives. Thus, a game theoretic approach is attempted to model and analyse the strategic situations arising from the interactions.

A relaxation scheme to combine phasor-mode and electromagnetic transients simulations

This paper deals with a new scheme for coupling phasor-mode and electromagnetic transients simulations. In each simulation, an iteratively updated linear equivalent is used to represent the effect of the subsystem treated by the other simulation. Time interpolation and phasor extraction methods adapted to this scheme are presented and compared to existing methods. Finally, simulation results obtained with a 74-bus test system are reported.

Optimal planning of distribution grids considering active power curtailment and reactive power control

In this paper, a new planning methodology is proposed for existing distribution grids, considering both passive and active network measures. The method is designed to be tractable for large grids of any type, e.g., meshed or radial. It can be used as a decision-making tool by distribution system operators which need to decide whether to invest in new hardware, such as new lines and transformers, or to initiate control measures influencing the operational costs. In this paper, active power curtailment and reactive power control are taken into account as measures to prevent unacceptable voltage rises as well as element overloads, as these allow postponing network investments. A low-voltage, meshed grid with 27 nodes is used to demonstrate the proposed scheme. In this particular case, the results show that by using control measures, an active distribution system operator can defer investments and operate the existing infrastructure more efficiently. The methodology is able to account for variations in operational and investment costs coming from regulatory influences to provide an insight to the most cost-efficient decision.

Co-Simulation of Electromagnetic Transients and Phasor Models: A Relaxation Approach

Co-simulation opens new opportunities to combine mature electromagnetic transients (EMT) and phasor-mode (PM) solvers, and takes advantage of their respective high accuracy and execution speed. In this paper, a relaxation approach is presented, iterating between an EMT and a PM solver. This entails interpolating over time the phasors of the PM simulation, extracting phasors from the time evolutions of the EMT simulation, and representing each subsystem with a proper multiport equivalent when simulating the other subsystem. Various equivalents are reviewed and compared in terms of convergence of the PM-EMT iterations. The paper also considers the update with frequency of the Thévenin impedances involved in the EMT simulation, the possibility to compute the EMT solution only once per time step, and the acceleration of convergence through a prediction over time of the boundary variables. Results are presented on a 74-bus, 23-machine test system, split into one EMT and one PM subsystem with several interface buses.

Exploiting localization for faster power system dynamic simulations

This paper proposes an algorithm for exploiting the localized response of power system components to accelerate dynamic simulations. During the simulation, components marginally participating to the system dynamics are characterized as latent and their dynamic models are replaced by much simpler equivalents. At the same time, components with significant dynamic activity are characterized as active and their original dynamic models are used. Based on the criterion proposed, components switch status between active and latent to increase performance while retaining accuracy. Two realistic test systems, a medium-scale and a large-scale, are used for the performance evaluation of the proposed method.

Hybrid approach for planning and operating active distribution grids

This paper investigates the planning and operational processes of modern distribution networks hosting distributed energy resources (DERs). While in the past the two aspects have been distinct, a methodology is proposed in this paper to co-optimise the two phases by considering the operational flexibility offered by DERs already in the planning phase. By employing AC optimal power flow (OPF) to analyse the worst-case scenarios for the load and distributed generator injection, the optimal set-points for the DERs are determined such that the networks security is ensured. From these results, the optimised individual characteristic curves are then extracted for each DER which are used in the operational phase for the local control of the devices. The optimised controls use only local measurements to address system-wide issues and emulate the OPF solution without any communication. Finally, the proposed methodology is tested on the Cigre LV benchmark grid confirming that it is successful in mitigating with acceptable violations over- and under-voltage problems, as well as congestion issues. Its performance is compared against the OPF-based approach and currently employed local control schemes.

Power System Dynamic Simulations Using a Parallel Two-Level Schur-Complement Decomposition

As the need for faster power system dynamic simulations increases, it is essential to develop new algorithms that exploit parallel computing to accelerate those simulations. This paper proposes a parallel algorithm based on a two-level, Schur-complement-based, domain decomposition method. The two-level partitioning provides high parallelization potential (coarse- and fine-grained). In addition, due to the Schur-complement approach used to update the sub-domain interface variables, the algorithm exhibits high global convergence rate. Finally, it provides significant numerical and computational acceleration. The algorithm is implemented using the shared-memory parallel programming model, targeting inexpensive multi-core machines. Its performance is reported on a real system as well as on a large test system combining transmission and distribution networks.