Interconnection between countries and surrounding countries provides a large power grid. With the improvement of wind power technology, increase of wind power capacity, the impact of wind power on the grid has become an important research topic as well. Large power systems are facing new challenges, such as increasing penetration of renewable energy sources, in particular wind generation, growing demands, limited resources, and competitive electricity markets. Under these conditions, the large power systems have had to confront some major operating problems in voltage regulation, power flow control, protection, and stability. The traditional stand alone (unit) protection schemes are not capable to protect large scale grid system, particularly after large disturbances. As it is known large disturbances might provoke a series of cascading outages, which can lead to the total system blackout and catastrophic consequences to the equipment and society. Current approaches adopted for implementing protection are not significant advancement in such configuration. Advancements in the protection and control have to be brought into the domain of wind farm application to address the specific needs and challenges of this configuration. This paper introduces a complete survey about traditional protection techniques for transmission line connected with renewable resources and the main shortcoming points for such techniques. Also the most recent techniques and new trends of the protection available nowadays for applying and facing such challenges are given. Smart devices deployed on the smart grid and smart based wireless devices can enhance and provide novel ideas for facing such challenges.

Protection techniques with renewable resources and smart grids—A survey

Vulnerability assessment is one of the main tasks in a Self-Healing Grid structure, since it has the function of detecting the necessity of performing global control actions in real time. Due to the short-time requirements of real time applications, the eligible vulnerability assessment methods have to consider the improvement of calculation time. Although there are several methods capable of performing quick assessment, these techniques are not fast enough to analyze real large power systems in real time. Based on the fact that vulnerability begins to develop in specific regions of the system exhibiting coherent dynamics, large interconnected power systems can be reduced through dynamic equivalence in order to reduce the calculation time. A dynamic equivalent should provide simplicity and accuracy sufficient for system dynamic simulation studies. Since the parameters of the dynamic equivalent cannot be easily derived from the mathematical models of generators and their control systems, numerical identification methods are needed. Such an identification task can be tackled as an optimization problem. This paper introduces a novel heuristic optimization algorithm, namely, the Mean-Variance Mapping Optimization (MVMO), which provides excellent performance in terms of convergence behavior and accuracy of the identified parameters. The identification procedure and the level of accuracy that can be reached are demonstrated using the Ecuadorian-Colombian interconnected system in order to obtain a dynamic equivalent representing the Colombian grid.

Identification of dynamic equivalents based on heuristic optimization for smart grid applications

The IEC-60909 is based on assumptions that may not be valid for Wind Turbines (WT) with Doubly-Fed Induction Generator (DFIG-WT), since the DFIG has a different short-circuit response compared to the synchronous generator (SG). This paper presents a comparison of short-circuit behaviors of DFIG and SG based on IEC-60909 definitions, in order to assess the differences between the two generators, as a first step for establishing a mathematical model to predict DFIG-WT short-circuit current and evaluating the suitability of IEC-60909 for DFIG-WT. A nonlinear optimization was used to extract the different components of the fault current by fitting a model based on the IEC-60909 short-circuit definition to the simulated short-circuit current. The results show that the DFIG has a different short-circuit response compared to the SG; also that the IEC-60909 short-circuit definitions are not suitable to the DFIG.

Comparison of short-circuit current contribution of Doubly-Fed induction generator based wind turbines and synchronous generator

This paper presents a new method for detecting the early stages of unstable conditions in large-scale power systems. A reduced order dynamic model is introduced for each control area that includes an aggregated generator and associated controllers. An innovative nonlinear observer proposed that can simultaneously estimate the parameters and states of the model benefiting from available online measurements. A sensitivity analysis is used to improve the efficacy and performance of the proposed estimator. The estimated states of the proposed equivalent generator model are then used to define a Real-Time Transient Stability (RTSA) index. The recommended RTSA index demonstrates to be a clear measure of the distance to instability and prevents economic losses due to blackouts and cascading system trips. The PSS/E simulation results on two case studies using show the excellent performance of the proposed index.

Real-Time Transient Stability Assessment Using Dynamic Equivalents and Nonlinear Observers

Heuristic optimization based approach for identification of power system dynamic equivalents

Dynamic equivalent has been a basic approach for analysis and computation of large scale-increasing interconnected power systems in China. Security defense of power systems and real-time monitoring of system states require more rapid and effective computation of large power grids. This paper introduces a solution for real-time dynamic equivalent of large multi-area power systems. It is based on real-time measurement data from Wide Area Measurement System and parameter identification. A parallel dynamic equivalent model composed of an equivalent generator and an equivalent synthesis load was put forward, in which the equivalent generator uses a 3rd order electromechanical transient model with damping while the equivalent synthesis load uses a static characteristic model considering both voltage and frequency. Identifiability analysis was carried out for this nonlinear dynamic equivalent model. In order to apply Laplace transformation-transfer function method, the model was firstly linearized. Particle Swarm Optimization algorithm was utilized for parameter identification of the dynamic equivalent model. For AC power systems, this model is verified on CEPRI 8-machine 36-bus example system. Load model and the style of disturbance are two remarkable factors that relate to the stability of the whole power system, which are fully considered in this paper. The simulation results show that the dynamic equivalent model can reflect the dynamic responses and the transient stability of the original system, no matter which is transient stable or unstable, angle unstable or voltage stable. When we change the load model in the original system, the static characteristic model with both voltage and frequency factors can throughout represents the dynamic behavior of the original system, which confirms its robustness of this dynamic equivalent model. All the simulation work was performed on PSASP.

A real-time dynamic equivalent solution for large interconnected power systems

When there has a sudden drop of grid voltage, crowbar protection circuit is a widely used method to improve the low voltage ride-through ability of wind turbines with doubly fed induction generator (DFIG), which is commonly used in wind power. This paper studied three-phase symmetrical short circuit current characteristics of DFIG with crowbar protection in depth, and simulations have been made in PSCAD/EMTDC. The analysis and simulation results show that, the output short circuit current concerns with crowbar resistance, dc-link voltage clamp effect and control strategies of the grid side converter (GSC). Short circuit current characteristics of a DFIG wind turbine become complicated because of the responses of GSC, when the dc-link clamp effect occurs. The GSC will also provide a high current to the short circuit point. However, when necessary restrictions have been introduced, short circuit current of DFIG wind turbine can be substituted by stator winding short circuit current of the DFIG whether the dc-link clamp effect occurs or not. Thus, on the basis of previous analysis results, we got short circuit current expressions of a DFIG under no dc-link clamp effect, when short circuit occurred at the normal operation. And an impedance voltage source model was proposed and has been verified at last.

Wang Kui

Papers

A real-time dynamic equivalent solution for large interconnected power systems

Three-phase symmetrical short circuit current characteristic analysis of doubly fed induction generator with crowbar protection

Study of Power System Online Dynamic Equivalent Based on Wide Area Measurement System