Out-of-step Protection Using Energy Equilibrium Criterion in the Time Domain

Disturbances in power systems are common and they result in electromechanical oscillations called power swing. The power swings could be severe and it may lead to loss of synchronism among the interconnected generators. This is referred to as out-of-step condition. The voltage and current swings during an out-of-step condition damage power system equipments and also cause unwanted operations of various protective devices. The protection systems require an effective algorithm for fast and accurate detection of out-of-step condition. This research is focused on the development of a simple and effective out-of-step relay capable of detecting out-of-step condition in a complex power system. To achieve this, the research has gone through four distinct stages: development of an algorithm, simulation, hardware implementation and its testing. An out-of-step algorithm is proposed based on equal area criterion in time domain. The equal area criterion in time domain is obtained by modifying the traditional equal area criterion in power angle domain. A single machine infinite bus system, a two machine infinite bus system and a three machine infinite bus system and a 17-bus multiple machines system are used as case studies and are modeled using simulation tool(PSCAD™). To test the effectiveness of the proposed algorithm, various out-of-step conditions are simulated by applying disturbances at various locations in the above chosen power system configurations. For hardware implementation and testing of the algorithm, a digital signal processing board (ADSP-BF533 from Analog Devices ™) is used. To test the performance of the developed digital relay in a closed loop, real time power system signals are necessary and therefore for this purpose, a Real Time Digital Simulator (RTDS™) available in the power research laboratory is used. The RTDS™ simulator mimics the actual power systems in real time. The signals required by the relays can be tapped from the RTDS™ and the signals coming from relay can be fed back into the RTDS™, which makes the closed loop testing of the digital relay possible. This research has yielded a simple out-of-step algorithm and unlike the other out-of-step detection techniques proposed in the literature does not need offline system studies to arrive at a solution.The developed digital out-of-step relay is capable of making decisions based only on the information available from its point of installation, thus it avoids the communication devices which is advantageous for the out-of-step protection of a complex power system. Finally, the simulation results show that the proposed algorithm can be applied to any power configurations and is faster compared to the conventional concentric rectangle schemes used in the literature

Optimal Energy Management of Distribution Systems and Industrial Energy Hubs in Smart Grids

Electric power distribution systems are gradually adopting new advancements in communication, control, measurement, and metering technologies to help realize the evolving concept of Smart Grids. Future distribution systems will facilitate increased and active participation of customers in Demand Side Management activities, with customer load profiles being primarily governed by real-time information such as energy price, emission, and incentive signals from utilities. In such an environment, new mathematical modeling approaches would allow Local Distribution Companies (LDCs) and customers the optimal operation of distribution systems and customer's loads, considering various relevant objectives and constraints. This thesis presents a mathematical model for optimal and real-time operation of distribution systems. Thus, a three-phase Distribution Optimal Power Flow (DOPF) model is proposed, which incorporates comprehensive and realistic models of relevant distribution system components. A novel optimization objective, which minimizes the energy purchased from the external grid while limiting the number of switching operations of control equipment, is considered. A heuristic method is proposed to solve the DOPF model, which is based on a quadratic penalty approach to reduce the computational burden so as to make the solution process suitable for real-time applications. A Genetic Algorithm based solution method is also implemented to compare and benchmark the performance of the proposed heuristic solution method. The results of applying the DOPF model and the solution methods to two distribution systems, i.e., the IEEE 13-node test feeder and a Hydro One distribution feeder, are discussed. The results demonstrate that the proposed three-phase DOPF model and the heuristic solution method may yield some benefits to the LDCs in real-time optimal operation of distribution systems in the context of Smart Grids. This work also presents a mathematical model for optimal and real-time control of customer electricity usage, which can be readily integrated by industrial customers into their Energy Hub Management Systems (EHMSs). An Optimal Industrial Load Management (OILM) model is proposed, which minimizes energy costs and/or demand charges, considering comprehensive models of industrial processes, process interdependencies, storage units, process operating constraints, production requirements, and other relevant constraints. The OILM is integrated with the DOPF model to incorporate operating constraints required by the LDC system operator, thus combining voltage optimization with load control for additional benefits. The OILM model is applied to two industrial customers, i.e., a flour mill and a water pumping facility, and the results demonstrate the benefits to the industrial customers and LDCs that can be obtained by deploying the proposed OILM and three-phase DOPF models in EHMSs, in conjunction with Smart Grid technologies.1 yea

Multi-time scale control of demand flexibility in smart distribution networks

This paper presents a multi-timescale control strategy to deploy electric vehicle (EV) demand flexibility for simultaneously providing power balancing, grid congestion management, and economic benefits to participating actors. First, an EV charging problem is investigated from consumer, aggregator, and distribution system operator’s perspectives. A hierarchical control architecture (HCA) comprising scheduling, coordinative, and adaptive layers is then designed to realize their coordinative goal. This is realized by integrating multi-time scale controls that work from a day-ahead scheduling up to real-time adaptive control. The performance of the developed method is investigated with high EV penetration in a typical residential distribution grid. The simulation results demonstrate that HCA efficiently utilizes demand flexibility stemming from EVs to solve grid unbalancing and congestions with simultaneous maximization of economic benefits to the participating actors. This is ensured by enabling EV participation in day-ahead, balancing, and regulation markets. For the given network configuration and pricing structure, HCA ensures the EV owners to get paid up to five times the cost they were paying without control

Grid-Forming Inverter-based Wind Turbine Generators: Comprehensive Review, Comparative Analysis, and Recommendations

High penetration of wind power with conventional grid following controls for inverter-based wind turbine generators (WTGs) weakens the power grid, challenging the power system stability. Grid-forming (GFM) controls are emerging technologies that can address such stability issues. Numerous methodologies of GFM inverters have been developed in the literature; however, their applications for WTGs have not been thoroughly explored. As WTGs need to incorporate multiple control functions to operate reliably in different operational regions, the GFM control should be appropriately developed for the WTGs. This paper presents a review of GFM controls for WTGs, which covers the latest developments in GFM controls and includes multi-loop and single-loop GFM, virtual synchronous machine-based GFM, and virtual inertia control-based GFM. A comparison study for these GFM-based WTGs regarding normal and abnormal operating conditions together with black-start capability is then performed. The control parameters of these GFM types are properly designed and optimized to enable a fair comparison. In addition, the challenges of applying these GFM controls to wind turbines are discussed, which include the impact of DC-link voltage control strategy and the current saturation algorithm on the GFM control performance, black-start capability, and autonomous operation capability. Finally, recommendations and future developments of GFM-based wind turbines to increase the power system reliability are presented

Volt-VAr optimization of a low voltage

Volt-VAr optimization (VVO) is important in a distribution system as the performance of the entire network depends on the voltage profile to a very large extent. The deployment of renewable energy sources, particularly photovoltaic (PV) systems, is usually achieved at the medium voltage (MV) and low voltage (LV) levels of distribution feeders and may exacerbate the challenges associated with maintaining the voltage profile within the pre-defined limits. For such a PV-rich system, VVO can be carried out such that the inverter-based PV systems can actively participate in voltage regulation and optimization by providing flexible reactive power support. This paper addresses the voltage concerns associated with high PV penetration by implementing a distribution grid optimal power flow (DOPF) problem on a realistic LV distribution network in Kano, Nigeria. The current injection-based I-V DOPF formulation is used to model the grid and the VVO utilizes the reactive power of the PV inverter to minimize the active power losses in the network. The results demonstrate the ability of the VVO to perform voltage regulation and can serve as a viable technique for mitigating voltage violation issues in the Nigerian grid

Hierarchical approach for optimal operation of distribution grid and electric vehicles

A centralized approach to solve optimal operation of a practical sized distribution grid with a large number of electric vehicles (EVs) is a computationally challenging task. This work proposes a bi-level hierarchical optimization framework to solve the problem. In the hierarchy, optimal operation of the distribution grid is considered in one level, while the optimal operation of EVs is carried out in another level. The hierarchical optimization framework is based on information exchange among distribution grid control center, EV aggregators, individual EVs, and market operator. The proposed framework consists of detailed mathematical modelling of distribution system components, EVs, and operational constraints. The proposed framework is applied to coordinate charging of hundreds of EVs in the IEEE 34-node three-phase unbalanced distribution system. Case studies demonstrate that the proposed hierarchical optimization framework provides benefits to the distribution grid operations as well as to the individual EV customers

Analysis and reduction of total harmonic distortions in distribution system with electric vehicles and wind generators

Harmonic distortion on voltages and currents increases with the increased penetration of Plug-in Electric Vehicle (PEV) loads in distribution systems. Wind Generators (WGs), which are source of harmonic currents, have some common harmonic profiles with PEVs. Thus, WGs can be utilized to subside the effect of PEVs on harmonic distortion. This paper studies the impact of PEVs on harmonic distortions and integration of WGs to reduce it. A harmonic decoupled power flow model is developed, where PEVs and WGs are represented by harmonic current loads and sources, respectively. The developed model is first used to solve harmonic power flow on IEEE 34-node distribution test feeder with low, moderate, and high penetrations of PEVs, then its impact on total harmonic distortions (THDs) is studied. Next, optimal size of WGs are calculated using Genetic Algorithm in the test feeder to reduce voltage and current THDs below the IEEE recommended values

Out-of-step protection for multi-machine power systems using local measurements

The classical equal area criterion in the power-angle domain, which is typically used for power systems planning/stability studies, is modified to the time domain for out-of-step protection. Wide-area measurements are necessary to obtain the power-angle information if the traditional equal area criterion is applied for out-of-step detection. However, its time-domain modification requires only the information local to the out-of-step relay. The proposed method evaluates the transient energy, which is the area under the power-time curve, and by comparing the areas, differentiation between stable and out-of-step swings is made. The proposed method can be directly applied to multimachine power systems without network reduction, unlike the conventional equal area criterion in which the equivalent two area of the multimachine system needs to be obtained. The proposed out-of-step detection method has been tested on a Single Machine Infinite Bus System and a 17-Bus Multimachine system