A New Approach to the On-line Estimation of the Loss of Generation Size in Power Systems

Following an unintended disconnection of a synchronous generator (SG) from the power system, what is also known as a loss of generation (LoG), it is not trivial to precisely estimate the post-event power system's inertia and the LoG size. One of reasons for that is that both of them are a function of the unknown inertia reduction. To solve this challenging problem, this paper presents an analytical method based on the rate-of-change-of-frequency (RoCoF). The method relies on a modified swing equation, allowing a simultaneous estimation of both unknowns. To this end, the values of mechanical starting time, apparent power and loading of lost generator are formulated for the power system under study. In a practical application, the method can use RoCoF measured by phasor measurement units (PMUs). The paper discusses the impact of various frequency estimation approaches to the proposed LoG estimation. Furthermore, a new method for LoG size estimation, based on the interpolated estimated inertial response, is proposed. The efficiency of the proposed approach is validated through extensive simulations with Matlab/Simulink using a simple power system and the IEEE 39-bus test network.

Performance assessment of grid-forming and grid-following converter-interfaced battery energy storage systems on frequency regulation in low-inertia power grids

Mario Paolone, Fabrizio Sossan, Zhao Yuan, Antonio Zecchino, Yihui Zuo

Power systems with large shares of converter-interfaced renewables may be characterised by low grid inertia due to the lack of frequency containment provided by synchronous generators. Battery energy storage systems (BESSs), which can adjust their power output at much steeper ramping than conventional generation, are promising assets to restore suitable frequency regulation capacity levels. BESSs are typically connected to the grid with a power converter, which can be operated in either grid-forming or grid-following modes. This paper quantitatively assesses the impact of large-scale BESSs on the frequency containment of low inertia power grid and compares the performance of grid-forming and grid-following control modes. Numerical results are provided considering a detailed dynamic model of the IEEE 39-bus system where fully characterized models of stochastic demand and generation are taken into account. In order to assess the performance of the BESS control modes in a practical operative context, daily long simulations are considered where reserve levels for frequency containment and restoration are allocated considering the current practice of a transmission system operator in Europe. Numerical analyses on various metrics applied to grid frequency show that grid-forming outperforms grid-following converter control mode.

2021

Power-System State Estimation based on PMUs

Lorenzo Zanni

An increasing number of phasor measurement units (PMUs) are being deployed in power systems in order to enhance the situational awareness and, in the near future, we expect that many networks will be extensively equipped with PMUs. These devices provide accurate and synchronized voltage/current phasors (called synchrophasors) at a reporting rate up to 60 measurements-per-second, which is a significantly different type of information with respect to the commonly-used voltage/current magnitude and power measurements of remote terminal units (RTUs). PMUs are commonly associated with transmission systems, but are gaining consideration also in the context of distribution networks in order to implement fast control schemes due to the presence of highly-volatile distributed generation and for fault location purposes. Power-system state estimation (SE) is a functionality that might largely benefit from the use of synchrophasor measurements. Best current practice consists in estimating the state every few tens of seconds (or even minutes) by using asynchronous measurements of RTUs. A measurement infrastructure exclusively composed of PMUs allows SE to become a linear and not iterative process characterized by a refresh-rate of tens of estimates-per-second and sub-second time-latency. This is what we call real-time SE. Even if SE is a well-established power-system function, it still deserves research in view of the proliferation of PMUs. Improvements in terms of accuracy, computational time and time-latency are required in order to make SE suitable for a wide range of applications, from control to fault management. In this dissertation, we first describe in detail the advantages of using exclusively synchrophasor measurements for the most common SE algorithms, i.e., weighted least squares (WLS), least absolute value (LAV) and Kalman filter (KF). Then, we propose two methods for the on-line estimation of the process-model uncertainties used by the KF, because power-system operating conditions are continuously varying. Our goal is to improve the estimation accuracy by effectively filtering the measurement noise. We designed a heuristic method for quasi-static conditions and a rigorous method that is also able to deal with step changes of the system state. Zero-injections represent equality constraints in the SE problem. We propose a method based on LQ-decomposition for linear WLS-SE that strictly satisfies the equality constraints while reducing the state-vector dimension by the number of constraints. Therefore, the computational time is significantly reduced and the problem becomes less ill-conditioned. An important contribution of this dissertation consists in the validation of the theoretical findings via real-scale experiments. We deployed PMUs at every bus in two real power-systems located in Switzerland. First, we demonstrate the practical feasibility of running SE at high refresh-rate (50 estimates-per-second) and low time-latency (below 70 ms). Second, we compare and discuss the results of WLS, LAV and KF by using real synchrophasor measurements. Finally, we intend to prove that PMU-based real-time SE exhibits unique accuracy, refresh rate and time-latency, which satisfy the requirements of fault location and, potentially, protective relaying. We propose a fault detection and faulted-line identification method based on WLS-SE, which works for any network and fault type as well as in presence of large amount of distributed generation.

EPFL2017

Analog microelectronic emulation for dynamic power system computation

Ira Nagel

Power system dynamic simulators can be classified according to multiple criteria, including speed, precision, cost and modularity (topology, characteristics and model). Existing simulators are based on time-consuming numeric algorithms, which provide very precise results. But the evolution of the power grid constantly changes the requirements for simulators. In fact, power consumption is steadily increasing; therefore, the power system is always operating closer to its limits. Moreover, focus is put on decentralized and stochastic green energy sources, leading to a much more complex and less predictable power system. In order to guarantee security of supply under these conditions, real-time control and online security assessment are of the utmost importance. The main requirement for power system simulators in this context thus becomes the simulation time. The simulator has to be able to reproduce power system phenomena much faster than their real-time duration. An effective way to accelerate computation time of power system stability simulators is based on analog emulation of the power system grid. The idea is to avoid the heavy, time-consuming numerical matrix calculations of the grid by using an instantaneous analog Kirchhoff grid, with which computation becomes intrinsically parallel and the simulation time independent of the power system topology size. An overview of the power system computation history and the evolution of microelectronics highlights that the renaissance of dedicated analog computation is justified. Modern VLSI technologies can overcome the drawbacks which caused the disappearance of analog computation units in the 1960s. This work addresses therefore the development of a power system emulation approach from its theoretical principles to the behavioral design and the microelectronic implementation of a first demonstrator. The approach used in this research is called AC emulation approach and is based on a one-to-one mapping of components of the real power system (generator, load and transmission line) by emulating their behavior on a CMOS microelectronic integrated circuit (ASIC). The signals propagating on the emulated grid are the shrunk and downscaled current and voltage waves of the real power system. The uniqueness of this emulation approach is that frequency dependence of the signals is preserved. Therefore, the range of phenomena that can be emulated with an AC emulator depends only on the implemented models. Within the framework of this thesis, we restrict our developments to transient stability analysis, as our main focus is put on emulation speed. v We provide behavioral AC emulation models for the three main power system components. Thereby, special attention is paid to the generator model, which is shown to introduce a systematic error. This error is analyzed and reduced by model adaptation. Behavioral simulation results validate the developed models. Moreover, we suggest custom programmable analog building blocks for the implementation of the proposed behavioral models. During their design, application specific requirements, as well as imperfections, calibration, mismatch and process-variation aspects, are taken into account. In particular, the design of the tunable floating inductance used in all three AC emulation models is discussed in detail. In fact, major design challenges have to be addressed in order to achieve an inductance suitable for our application. Finally, a first AC emulation demonstrator is presented. A benchmark using a fixed two- machine topology has been implemented using a 0.35μm 3.3V CMOS technology. The characteristics of the emulated components (i.e. generators and transmission lines) are reprogrammable, allowing short circuits to be emulated at different distances from the generator. The emulated phenomena are shown to be 10′000 times faster than real time, therefore proving the high-speed capabilities of AC emulation.