Panayiotis Demetriou

Dynamic IEEE Test Systems for Transient Analysis

Transient stability analysis is performed to assess the power systems condition after a severe contingency and is carried out using simulations. To adequately assess the systems transient stability, the correct dynamic models for the machines (i.e., generators, condensers, and motors) along with their dynamic parameters must be defined. The IEEE test systems contain the data required for steady-state studies. However, neither the dynamic model of the machines nor their specific parameters have been established for transient studies. As a result, there is a demand for test bed systems suitable for transient analysis. This paper defines dynamic machine models along with their parameters for each IEEE test bed system, thus producing full dynamic models for all test systems. It is important to mention that the parameters of the proposed dynamic models are based on typical data. The test systems are subjected to large disturbances, and a case study for each test system, which examines the frequency, angle, and voltage stability, is presented. Furthermore, the proposed dynamic IEEE test systems, implemented in PowerWorld, are available online.

Investigation of different Fault Ride Through strategies for renewable energy sources

The Fault Ride Through (FRT) capability of renewable energy sources (RES) for providing support to the power grid under several disturbances allows us to consider them not only as passive elements in the power system network, but also as ancillary services for the mitigation of evolving contingencies. In this paper, an investigation of the FRT operation of RES according to current grid regulations highlights some impacts on the system response that were not considered previously and require to be addressed. Slight modifications of the current grid codes are therefore necessary to solve these problems. A critical dead band zone modification for smother fault recovery is suggested. Moreover, this paper proposes the use of an adjustable parameter into the FRT strategy for a fair compromise of voltage and frequency support. The proposed FRT strategies are applied to a RES that is interconnected with the IEEE 14-bus test system and dynamic electromagnetic transient simulation results are provided.

On implementing a spectral clustering controlled islanding algorithm in real power systems

Wide area blackouts can be caused by unexpected fault scenarios, in particular protection maloperation, or simultaneous low probability events, which as such might lead to e.g., un-damped electromechanical oscillations. A Spectral Clustering Controlled Islanding (SCCI) method to find a suitable islanding solution for preventing such events was previously proposed and tested using different IEEE test networks. The sole constraint applied to this solution was related to generator coherency. The method demonstrated promising results when it was implemented on these IEEE test networks. The SCCI method was later tested using a simplified Cypriot Network and the Polish Network. The results achieved when implementing the SCCI method on actual power systems highlight practical issues that were not previously considered and require to be addressed when using it. Therefore, this paper presents a robust SCCI method. An outlier problem which affects the quality of clustering and a problem with the computational efficiency of the algorithm (when dealing with systems larger than 500 nodes) are presented and analyzed in this paper. To improve the clustering quality, a more robust clustering algorithm, k-medoids, is used to cluster all nodes in the solution subspace and to find the islanding solution.

Dynamic modeling of IEEE test systems including renewable energy sources

A Renewable Energy Source (RES), enhanced with Fault Ride Through (FRT) capability, can provide proper voltage and frequency support to the power grid under several disturbances. This paper investigates how a realistic dynamic power system with high penetration of RES reacts under different operating conditions. More specifically, the power system operation when RES operates according to the current grid regulations of the Cyprus TSO is demonstrated. For the purposes of this investigation, a RES is interconnected with the IEEE 9-bus test system, reinforced with an appropriate directional over-current protection scheme, and dynamic electromagnetic transient simulation results are provided.

Enhancing power system voltage stability through a centralized control of renewable energy sources

The integration of renewable energy sources in power systems has steadily increased in the recent years in the vision of significantly decreasing the carbon emissions produced by the conventional power plants. Although the incorporation of the renewable energy sources in the power system network is environmentally beneficial, at the same time it poses some challenges regarding their smooth operation and control. In this sense, the renewable energy sources should be considered not only as passive elements in the power system network, but, if possible, as ancillary services for the mitigation of evolving contingencies. In this paper, a methodology to preserve the voltage stability of the power system after the occurrence of a contingency utilizing the renewable energy sources is presented. The proposed methodology was applied to the IEEE 14-bus system and simulation results are provided.

Controlled Islanding Solution for Large-Scale Power Systems

Intentional Controlled Islanding (ICI) has been proposed as a corrective measure of last resort to split the power system into several sustainable islands and prevent cascading outages. This paper proposes a novel ICI algorithm based on a linear programming (LP) formulation that directly determines an islanding solution with minimal power-flow disruption for any given number of islands, while ensuring that each island contains only coherent generators. In addition, the proposed algorithm enables operators to constrain any transmission line to be excluded from the solution, allows the control of the size of islands and ensures their connectivity. The basis of the proposed LP formulation is an exact mixed integer linear programming (MILP) formulation. A search space reduction procedure that generates additional constraints for reducing the search space of the MILP is also proposed. In most cases, these additional constraints are enough for the relaxed MILP formulation (LP formulation) to generate optimal solutions. Nonetheless, the proposed LP formulation is executed as a part of a recursive linearization procedure, which ensures that optimal solutions are always obtained. Multiple simulation results demonstrate the ability of the proposed LP ICI algorithm to meet the requirement of real-time controlled islanding in large-scale power systems.

Applying exact MILP formulation for controlled islanding of power systems

Power systems are prone to cascading outages leading to large-scale blackouts. Intentional controlled islanding (ICI) is an effective corrective control action that limits the consequences of these catastrophic events by splitting the system into smaller sustainable islands. This paper proposes an ICI algorithm based on an exact Mixed Integer Linear Programming Formulation (MILP) that directly determines an islanding solution with minimal power-flow disruption for any given number of islands, while ensuring that each island contains only coherent generators. In addition, the proposed algorithm enables operators to constrain any transmission line to be excluded from the solution, allows the control of the size of islands and ensures their connectivity. To reduce the search space of the MILP and the overall complexity of the problem, a preprocessing procedure that finds the trees which connect the generators of each coherent group with the minimum number of nodes is also presented. The exact ICI algorithm is tested using dynamic models of the IEEE 39- and IEEE 118-Bus test systems. Multiple case studies are developed to demonstrate the effectiveness of the algorithm to different system conditions.

Real-time identification of coherent generator groups

This paper presents a two-step approach for defining coherent generators in disturbed power systems based on the similarity among their inter-area oscillations and swing curves. In the first step, an online visualization of the generator swing curves is performed by extracting time domain solutions of the swing equation using PMU measurements. In the second step, the similarity between each pair of the generator swing curves is examined based on an intraclass correlation analysis. The pairwise similarity coefficients obtained are served as edges weights to a fully connected graph. The grouping of the coherent generators is completed through a graph minimization algorithm. The proposed methodology is applied to the 39-Bus New England System and coherent generators are obtained for different operating points to provide a more accurate and realistic grouping.

System splitting strategy considering power system restoration

An Intentional Controlled Islanding (ICI) algorithm based on an exact Mixed Integer Programming Formulation (MILP) was previously proposed and tested using IEEE test systems. The proposed algorithm directly determines an islanding solution with minimal power-flow disruption for any given number of islands, while ensuring that each island contains only coherent generators. However, since one or more of the created islands might reach a local blackout after the splitting strategy is carried out, the aforementioned algorithm is extended to consider power system restoration constraints. Considering that data collection is essential to properly run a restoration process and assuming a completely observable power system at normal operating conditions, the extended ICI algorithm creates islands that are also completely observable, includes at least one blackstart unit within each island, and guarantees sufficient generation capacity to match the load consumption within each island. These new constraints can be viewed as a power system restoration planning stage.

Intentional Controlled Islanding and Risk Assessment: A Unified Framework

Power systems are prone to cascading outages leading to large-area blackouts, and intentional controlled islanding (ICI) can mitigate these catastrophic events by splitting the system into sustainable islands. ICI schemes are used as the last resort to prevent cascading events; thus, it is critical to evaluate the corresponding system risks to ensure their correct operation. This paper proposes a unified framework to assess the risk of ICI schemes. First, a novel ICI method to create islands with minimum power imbalance is presented. Further, a risk assessment methodology is used to assess the probability and impact of the main operational modes of the ICI scheme. The unified framework provides insights on the benefits of implementing ICI, considering the uncertainties related to its reliability. The ICI scheme is demonstrated using the IEEE 9-bus system. The proposed unified framework is then fully deployed on the actual power system of Cyprus. Multiple case studies on the real network are created to demonstrate the adaptability and robustness of the proposed scheme to different system conditions. The adoption of the unified framework highlights that the system risk significantly reduces with the ICI in service, even when the reliability uncertainties associated with the scheme are considered.