Zakir Hussain Rather

A Systematic Approach for Dynamic Security Assessment and the Corresponding Preventive Control Scheme Based on Decision Trees

This paper proposes a decision tree (DT)-based systematic approach for cooperative online power system dynamic security assessment (DSA) and preventive control. This approach adopts a new methodology that trains two contingency-oriented DTs on a daily basis by the databases generated from power system simulations. Fed with real-time wide-area measurements, one DT of measurable variables is employed for online DSA to identify potential security issues, and the other DT of controllable variables provides online decision support on preventive control strategies against those issues. A cost-effective algorithm is adopted in this proposed approach to optimize the trajectory of preventive control. The paper also proposes an importance sampling algorithm on database preparation for efficient DT training for power systems with high penetration of wind power and distributed generation. The performance of the approach is demonstrated on a 400-bus, 200-line operational model of western Danish power system.

Dynamic Reactive Power Compensation of Large-Scale Wind Integrated Power System

Due to progressive displacement of conventional power plants by wind turbines, dynamic security of large-scale wind integrated power systems is significantly compromised. In this paper we first highlight the importance of dynamic reactive power support/voltage security in large-scale wind integrated power systems with least presence of conventional power plants. Then we propose a mixed integer dynamic optimization based method for optimal dynamic reactive power allocation in large-scale wind integrated power systems. One of the important aspect of the proposed methodology is that unlike static optimal power flow based approaches, the proposed method considers detailed system dynamics and wind turbine grid code compliance while optimizing the allocation of dynamic reactive power sources. We also advocate that in large-scale wind integrated power systems, 1) better utilization of existing wind turbines especially wind farms with additional grid support functionalities like dynamic support (e.g., dynamic reactive power support, etc.) and 2) refurbishment of existing conventional central power plants to synchronous condensers could be one of the efficient, reliable and cost-effective option to address not only the issue of dynamic voltage security but also to strengthen other dynamic capabilities of the system including system inertia, etc. that are also significant challenges in large-scale wind penetrated power system. The proposed methodology is applied to the detailed model of the western Danish power system which is characterized by large-scale wind integration and least presence of central power plants.

Realistic Approach for Phasor Measurement Unit Placement: Consideration of Practical Hidden Costs

This paper presents a realistic cost-effective model for optimal placement of phasor measurement units (PMUs) for complete observability of a power system considering practical cost implications. The proposed model considers hidden or otherwise unaccounted practical costs involved in PMU installation. Consideration of these hidden but significant and integral part of total PMU installation costs was inspired from practical experience on a real-life project. The proposed model focuses on the minimization of total realistic costs instead of a widely used theoretical concept of a minimal number of PMUs. The proposed model has been applied to IEEE 14-bus, IEEE 24-bus, IEEE 30-bus, New England 39-bus, and large power system of 300 buses and real life Danish grid. A comparison of the presented results with those reported by traditional methods has also been shown to justify the effectiveness of the proposed model with regard to its realistic and practical nature.

Dynamic security assessment of Danish power system based on decision trees: Today and tomorrow

The research work presented in this paper analyzes the impact of wind energy, phasing out central power plants and cross border power exchange on dynamic security of Danish Power System. Contingency based decision tree (DT) approach is used to assess the dynamic security of present and future Danish Power System. Results from offline time domain simulation for large number of possible operating conditions (OC) and critical contingencies are organized to build up the database, which is then used to predict the security of present and future power system. The proposed approach is implemented in DIgSILENT PowerFactory environment and applied to western Danish Power System which is experiencing a phase of major transformation. The results have shown that phasing out central power plants coupled with large scale wind energy integration and more dependence on international ties can have significant impact on dynamic security of Danish power system in future, if alternative measures are not considered seriously.

Coordinated Voltage Control in Offshore HVDC Connected Cluster of Wind Power Plants

This paper presents a coordinated voltage control scheme (CVCS) for a cluster of offshore wind power plants connected to a voltage-source converter-based high-voltage direct current system. The primary control point of the proposed voltage control scheme is the introduced Pilot bus, which is having the highest short-circuit capacity in the offshore AC grid. The developed CVCS comprehends an optimization algorithm, aiming for minimum active power losses in the offshore grid, to generate voltage reference to the Pilot bus. During the steady-state operation, the Pilot bus voltage is controlled by dispatching reactive power references to each wind turbine (WT) in the wind power plant cluster based on their available reactive power margin and network sensitivity-based participation factors, which are derived from the dV/dQ sensitivity of a WT bus w.r.t. the Pilot bus. This method leads to the minimization of the risk of undesired effects, particularly overvoltage at the terminals of the WT located far away from the AC collector substation, by dispatching lower reactive power references compared with the ones nearer to the substation. In addition, this paper proposes a control strategy for improved voltage ride through capability of WTs for faults in the offshore grid, thus leading to improved dynamic voltage profile in the offshore AC grid.

Importance sampling based decision trees for security assessment and the corresponding preventive control schemes: The Danish case study

Decision Trees (DT) based security assessment helps Power System Operators (PSO) by providing them with the most significant system attributes and guiding them in implementing the corresponding emergency control actions to prevent system insecurity and blackouts. DT is obtained offline from time-domain simulation and the process of data mining, which is then implemented online as guidelines for preventive control schemes. An algorithm named Classification and Regression Trees (CART) is used to train the DT and key to this approach lies on the accuracy of DT. This paper proposes contingency oriented DT and adopts a methodology of importance sampling to maximize the information contained in the database so as to increase the accuracy of DT. Further, this paper also studies the effectiveness of DT by implementing its corresponding preventive control schemes. These approaches are tested on the detailed model of western Danish power system which is characterized by its large scale wind energy penetration and high proportion of distributed generation (DG). DIgSILENT/PowerFactory is adopted for the power system simulation and Salford Predictive Modeler (SPM) is used for data mining.

Impact of electric vehicle load response variation on frequency stability

Electric vehicle (EV) load in future power systems is likely to increase due to decarbonisation of the transport fleet. EV load can be utilised to provide primary frequency reserve (PFR), provided its variability with respect to system requirements is taken into consideration. EV based PFR can in certain cases negatively impact system frequency stability. Better estimation of EV reserve and consideration of its volume and responsiveness, relative to system PFR requirements, therefore becomes important. This paper presents a model for EV load estimation, and proposes control mechanisms which cater for available EV reserve variability and system requirements. The results show that the proposed control mechanisms are robust and ensure frequency stability, with negligible impact on the speed of response and hence the frequency nadirs.

Automatic voltage control (AVC) system under uncertainty from wind power

An automatic voltage control (AVC) system maintains the voltage profile of a power system in an acceptable range and minimizes the operational cost by coordinating the regulation of controllable components. Typically, all of the parameters in the optimization problem are assumed to be certain and constant in the decision making process. However, for high shares of wind power, uncertainty in the decision process due to wind power variability may result in an infeasible AVC solution. This paper proposes a voltage control approach which considers the voltage uncertainty from wind power productions. The proposed method improves the performance and the robustness of a scenario based approach by estimating the potential voltage variations due to fluctuating wind power production, and introduces a voltage margin to protect the decision against uncertainty for each scenario. The effectiveness of the proposed approach is demonstrated on IEEE 39-bus model. Further, Monte Carlo simulation is used to verify the results.

Coordinated control scheme for ancillary services from offshore wind power plants to AC and DC grids

This paper proposes a new approach of providing ancillary services to AC and DC grids from offshore wind power plants (OWPPs), connected through multi-terminal HVDC network. A coordinated control scheme where OWPPs AC grid frequency modulated according to DC grid voltage variations is used to detect and provide the ancillary service requirements of both AC and DC grids, is proposed in this paper. In particular, control strategies for onshore frequency control, fault ride-through support in the onshore grid, and DC grid voltage control are considered. The proposed control scheme involves only local measurements and therefore avoids the need of communication infrastructure otherwise required for communication based control, and thus increases the reliability of the control system. The effectiveness of the proposed control scheme is demonstrated on a MTDC connected wind power system developed in DIgSILIENT PowerFactory.

Economic valuation of reserves on cross border interconnections; A Danish case study

Denmark avails the highest penetration of wind power in the world. Danish power system is subdivided into two asynchronous parts; western and eastern Danish power systems. With more than 35% wind penetration, western Danish power system has the merits to be considered as the pilot grid for many regions that plan for high penetration of intermittent renewables. Extreme intermittency in the nature of wind power imposes elevated risk levels to power system operation. This every day challenge of wind dominant power systems necessitate the crucial role of operating reserves. In this paper, we propose an optimization framework that minimizes the cost of reserve procurement. The framework decouples the share of upward and downward primary, secondary, and tertiary reserve services within DK1 (western Danish power system) and neighboring cross border resources (Norway and Germany). Results indicate the economic benefit of reserve provision provided by cross border interconnections. The focus here will be on reserve services from abundant hydropower resource in Norway, taking advantage of fast VSC-based HVDC interconnection that is expected to be commissioned in immediate coming years.