Abstract:With the deep and pervasive application of information and communication technologies, power grid has become one of the vital cyber physical systems (CPS). However, the traditional analysis or control methods for power grid mainly focus on the physical power system, and treat the cyber and physical systems separately. To fully understand the interdependence and interplay of the cyber and physical systems, CPS should be studied as an integrated system. By studying the CPS, the mutual dependence of cyber and physical parts can be revealed, the risk due to the cyber-physical interaction can be reduced, and the methods for promoting the overall system efficiency can be derived. The special section is dedicated to reflect the latest progress and key technologies in cyber physical power systems (CPPS). This special section includes a total of eleven papers which focus on the false data injection attacks, the general cyber security issue, the cascading failure, the emerging new technologies in CPPS. Among the eleven papers included in the special section, 3 of them discuss the methods for the detection and defense of false data injection attacks, 3 of them deal with the cyber security issue in CPPS, 2 of them present the approaches on modeling and analysis of cascading failures in CPPS, and other 3 papers introduce some new technologies used in CPPS.

Abstract:It has recently been shown that state estimation (SE), which is the most important real-time function in modern energy management systems (EMSs), is vulnerable to false data injection attacks, due to the undetectability of those attacks using standard bad data detection techniques, which are typically based on normalized measurement residuals. Therefore, it is of the utmost importance to develop novel and efficient methods that are capable of detecting such malicious attacks. In this paper, we propose using the unscented Kalman filter (UKF) in conjunction with a weighted least square (WLS) based SE algorithm in real-time, to detect discrepancies between SV estimates and, as a consequence, to identify false data attacks. After an attack is detected and an appropriate alarm is raised, an operator can take actions to prevent or minimize the potential consequences. The proposed algorithm was successfully tested on benchmark IEEE 14-bus and 300-bus test systems, making it suitable for implementation in commercial EMS software.

Abstract:This paper addresses false data injection, which is one of the most significant security challenges in smart grids. Having an accurately estimated state is of great importance for maintaining a stable running condition of smart grids. To preserve the accuracy of the estimated state, bad data detection (BDD) mechanisms are utilized to remove erroneous measurements due to meter failures or outsider attacks. In this paper we use a graph-theoretical formulation for false data injection attacks in smart grids and propose defense mechanisms to mitigating this type of attacks. To this end we discuss characteristics of a typical smart grid graph such as planarity. Then we propose three different approaches for finding optimal protected meters set: a fast and efficient heuristic algorithm that works well in practice, an approximation algorithm that provides guarantee for the quality of the protected set, and an exact algorithm that find the optimal solution. Our extensive simulation results show that our algorithms outperform similar existing solutions in terms of different performance metrics.

Abstract:This paper addresses a set-theoretic method for the detection of data corruption cyber-attacks on the load frequency control loop of a networked power system. The system consists of several interconnected control areas forming a power grid. Based on the overall discrete-time network dynamics, a convex and compact polyhedral robust invariant set is extracted and is used as a set-induced anomaly detector. If the state vector exits the invariant set, then an alarm will be activated, and the potential threat is considered disclosed. The attack scenario used to assess the efficiency of the proposed anomaly detector concerns corrupted frequency sensor measurements transmitted to the automatic generation control unit of a compromised control area. Simulation studies highlight the ability of a set-theoretic approach to disclose persistent and intermittent attack patterns even when they occur at the same time with changes in the power load demand.

Abstract:A key focus recently has been in assessing the risk of a coordinated cyber-physical attack and minimizing the impact of a successful attack. Most of the cyberattackers will have limited system information and conventional power grid N - 1 security analysis cannot be extended to assess the risk. Centrality measures are widely used in the network science and an attacker with incomplete information can use it to identify power system vulnerabilities by defining the system as a complex network but without real-time system measurements. This paper presents a graph theory based centrality indices for vulnerability assessment of the power system due to various bus and branch contingencies using limited system information and provides a preliminary defense mechanism to prevent such an attack. Proposed work answers the fundamental question of possible attack scenarios by balancing risk (limited information with low risk to get caught or high risk attack to access more system information) and impact (identifying contingencies with maximal impact on system operation). Statistical comparisons are made between the graph theory measures compared to the corresponding DC power flow based N - X linear sensitivity measures. A unified N - X centrality based performance index is proposed and validated against the AC power flow based performance index by doing the real-time simulations of an N - 3 attack scenario. Defensive mechanisms using topology-based performance indices are also presented.

Abstract:This paper provides a strategic solution for enhancing the cybersecurity of power distribution system operations when information and operation technologies converge in active distribution network (ADN). The paper first investigates the significance of Internet of Things (IoT) in enabling fine-grained observability and controllability of ADN in networked microgrids. Given severe cybersecurity vulnerabilities embedded in conventionally centralized energy management schemes, the paper then proposes a cyber-secure decentralized energy management framework that applies a distributed decision-making intelligence to networked microgrids while securing their individual mandates for optimal operation. In particular, the proposed framework takes advantage of software-defined networking technologies that can secure communications among IoT devices in individual microgrids, and exploits potentials for introducing blockchain technologies that can preserve the integrity of communications among networked microgrids in ADN. Furthermore, the paper presents the details of application scenarios where the proposed framework is employed to secure peer-to-peer transactive energy management based on a set of interoperable blockchains. It is finally concluded that the proposed framework can play a significant role in enhancing the efficiency, reliability, resilience, and sustainability of electricity services in ADN.

Abstract:The significance of modern power grids is acknowledged every time there is a major threat. This paper proposes the novel approaches to aid power system planner to improve power grid resilience by making appropriate hardening strategies against man-made attack or natural hazards. The vulnerability indices are introduced, which return the most vulnerable component in the system based on a tri-level defender-attacker-operator (DAO) interdiction problem which solves iteratively. The output of DAO is the set of hardening strategies that optimally allocated along the network to mitigate the impact of the worst-case damages. By repeating DAO problem based on the proposed algorithm, the various crafted attack is imposed on the system, and the defender’s behavior demonstrates how an element is vulnerable to threats. The WSCC 9-bus, IEEE 24-bus, and IEEE 118-bus systems are employed to evaluate the model performance. The counter-intuitive results are proven by the proposed robust hardening strategy, which shows how the hardening strategy should be allocated to improve power network resilience against threats.

Abstract:The utilization levels of the transmission network can be enhanced by the use of automated protection schemes that rapidly respond to disturbances. However, such corrective systems may suffer from malfunctions that have the potential to exacerbate the impact of the disturbance. This paper addresses the challenge of jointly optimizing the dispatch of generators and protection settings in this context. This requires a holistic assessment of the cyber (protection logic) and physical (network) systems, considering the failures in each part and their interplay. Special protection schemes are used as a prototypical example of such a system. An iterative optimization method is proposed that relies on power system response simulations in order to perform detailed impact assessments and compare candidate solutions. The candidate solutions are generated on the basis of a security-constrained dispatch that also secures the system against a set of cyber failure modes. A case study is developed for a generation rejection scheme on the IEEE reliability test system (RTS): candidate solutions are produced based on a mixed integer linear programming optimisation model, and loss-of-load costs are computed using a basic cascading outage algorithm. It is shown that the partial security approach is able to identify solutions that provide a good balance of operational costs and loss-of-load risks, both in a fixed dispatch and variable dispatch context.

Abstract:This paper presents a model of cascading failures in cyber-physical power systems (CPPSs) based on an improved percolation theory, and then proposes failure mitigation strategies. In this model, the dynamic development of cascading failures is divided into several iteration stages. The power flow in the power grid, along with the data transmission and delay in the cyber layer, is considered in the improved percolation theory. The interaction mechanism between two layers is interpreted as the observability and controllability analysis and data update analysis influencing the node state transformation and security command execution. The resilience indices of the failures reflect the influence of cascading failures on both topological integrity and operational state. The efficacy of the proposed mitigation strategies is validated, including strategies to convert some cyber layer nodes into autonomous nodes and embed unified power flow controller (UPFC) into the physical layer. The results obtained from simulations of cascading failures in a CPPS with increasing initial failure sizes are compared for various scenarios. Dynamic cascading failures can be separated into rapid and slow processes. The interdependencies and gap between the observable and controllable parts of the physical layer with the actual physical network are two fundamental reasons for first-order transition failures. Due to the complexity of the coupled topological and operational relations between the two layers, mitigation strategies should be simultaneously applied in both layers.

Abstract:Modern power systems are rapidly evolving into complex cyber-physical systems. The increasingly complex interaction among different energy entities calls for a secure, efficient, and robust cyber infrastructure. As an emerging distributed computing technology, Blockchain provides a secure environment to support such interactions. This paper gives a prospective on using Blockchain as a secure, distributed cyber infrastructure for the future grid. Firstly, the basic principles of Blockchain and its state-ofthe-art are introduced. Then, a Blockchain based smart grid cyber-physical infrastructure model is proposed. Afterwards, some promising application domains of Blockchain in future grids are presented. Following this, some potential challenges are discussed.

Abstract:This paper presents a demand dispatch strategy of aggregated electric water heaters (EWHs) for a load aggregation system at demand side, based on the theory of cyber-physical system. The objective is to solve the problem of water heater load control when the cyber-physical load aggregation system participates in demand dispatch of the power grid. First, an implementation framework of the demand dispatch strategy is designed between the cyber space and the physical space, including state awareness, real-time analysis, scientific decision-making and precise execution. Second, a multilevel incentive model, an EWH appliance model and a thermostat setpoint control rule are introduced. Next, based on the models and the rule, the state awareness logic, real-time analysis logic, scientific decision-making logic and precise execution logic of the strategy are designed to implement demand dispatch of aggregated EWHs. Finally, simulation results confirm the effectiveness, the advantage and excellent scalability of the proposed strategy.

Abstract:In the modern power system, both local and centralized reactive power control strategies for photovoltaic (PV) plants, are proposed and compared. While local control improves the network security, it lacks the optimization benefits from centralized control strategies. Therefore, this paper considers the coordination of the two control strategies, depending on external impact from the weather system and consumer behavior, in a low voltage (LV) distribution feeder. Through modeling and simulation in an established real-time cyber-physical simulation platform, the LV network is evaluated with both local and centralized control. A set of boundaries for coordinating between the two strategies are identified, which can help network operators in deciding suitable control in different operating situations. Furthermore, the cyber-physical simulation platform, is used to study the impact of physical perturbations, i.e. changes in irradiance and consumption, and cyber disturbances, in form of communication channel noise, is evaluated for the control strategies. Results show how small and large disturbances in the cyber system affects the centralized control strategy optimizer performance.

Abstract:As one of promising clean and low-emission energy, wind power is being rapidly developed in China. However, it faces serious problem of wind curtailment, particularly in northeast China, where combined heat and power (CHP) units cover a large proportion of the district heat supply. Due to the inherent strong coupling between the power and the heat load, the operational flexibility of CHP units is severely restricted in winter to meet the heat supply demand, which imparts considerable stress on the wind power connection to the grid. To promote the integration of wind power and enhance the flexibility of CHP units, this paper presented a method of heat and power load dispatching by exploring the energy storage ability of electric heating boilers and district heating systems. The optimization results indicate that the proposed method can integrate additional wind power into the grid and reduce the coal consumption of CHP units over the optimized period. Furthermore, the thermal inertia of a district heating system is found to contribute more to the reduction of coal consumption, whereas the electric heating boilers contribute to lower wind curtailment.

Abstract:This paper proposes an economic optimization method with two time scales for a hybrid energy system based on the virtual storage characteristic of a thermostatically controlled load (TCL). The optimization process includes two time scales in order to ensure accuracy and efficiency. Based on the forecast load and energy supply of the system, the first time scale is day-ahead economic operating optimization, carried out to determine the minimum operating cost for the whole day, and to find the period of greatest cost to which the second time scale optimization is applied. Using the virtual storage characteristic, the second time scale is short term detailed optimization carried out for these particular hours. By dispatching thermal load in this period and adjusting energy supply accordingly, we can find the optimal economic performance, and customer requests are taken into account to ensure satisfaction. A case study in Tianjin illustrates the effectiveness of this method and proves that a TCL can make a great contribution to improving the economic performance of a hybrid energy system.

Abstract:This study proposes a novel fully distributed coordination control (DCC) strategy to coordinate charging efficiencies of energy storage systems (ESSs). To realize this fully DCC strategy in an active distribution system (ADS) with high penetration of intermittent renewable generation, a two-layer consensus algorithm is proposed and applied. It collects global information in the first layer and achieves pinning-based DCC in the second layer. Basic objectives of the proposed DCC for ESSs are: to coordinate the ESSs and improve efficiency using associated marginal charging costs (MCCs) in a fully distributed manner; ` to reduce local power mismatch and power transmission losses; ′ to adapt to unintentional communication topology changes. The effectiveness and adaptability of the proposed DCC approach are both validated by simulation results.

Abstract:Demand response (DR) is important to account for behaviors of the demand side to yield an optimal dispatch result. However, it is difficult for energy suppliers to collect customers’ private information unless there is an incentive mechanism for customers to do so. Therefore, this paper proposes a new integrated generation–consumption dispatch based on compensation mechanism considering DR behavior. Firstly, in light of the dayahead load forecast data, we deduce the utility function model of different customers. By subtracting generating units’ operation cost from consumers’ total utility, the dispatch model have a decentralized demand participant structure based on this utility function. The utility function is used to describe consumers’ preferences and energy consumption behaviors. Secondly, an effective compensation mechanism is designed to ensure customers to select the level of compensation appropriate to their willingness to curtail load. Finally, a new dispatch model is proposed that incorporates the DR compensation mechanism into the generation–consumption dispatch model. The new model can improve the interaction of generation and consumption, and benefit both the energy supplier and its customers. The proposed model is piecewise linearized and solved by a mixed-integer linear programming method. It is tested on a six-bus system and the IEEE 118-bus system. Simulation results show that the proposed model can realize both maximum social welfare and Pareto optimal results.

Abstract:Distributed generation including wind turbine (WT) and photovoltaic panel increased very fast in recent years around the world, challenging the conventional way of probabilistic load flow (PLF) calculation. Reliable and efficient PLF method is required to take into account such changing. This paper studies the maximum entropy probabilistic density function reconstruction method based on cumulant arithmetic of linearized load flow formulation, and then develops a maximum entropy based PLF (MEPLF) calculation algorithm for power system integrated with wind power generation (WPG). Comparing to traditional Gram–Charlier expansion based PLF (GC-PLF) calculation method, the proposed ME-PLF calculation algorithm can obtain more reliable and accurate probabilistic density functions (PDFs) of bus voltages and branch flows in various WT parameter scenarios. It can solve the limitation of GC-PLF calculation method that mistakenly gaining negative values in tail regions of PDFs. Linear dependence between active and reactive power injections of WPG can also be effectively considered by the modified cumulant calculation framework. Accuracy and efficiency of the proposed approach are validated with some test systems. Uncertainties yielded by the wind speed variations, WT locations, power factor fluctuations are considered.

Abstract:This paper presents a novel commutation failure (CF) assessment method considering the influences of voltage magnitude drop, phase shift, and spatial-temporal discreteness of AC system faults. The commutating voltage-time area is employed to analyze the spatial-temporal discreteness of AC system faults causing CF in high-voltage direct current systems, and the influences of fault position and fault time on CF are revealed. Based on this, a novel CF criterion is proposed, further considering the influence of voltage phase shift and the spatial-temporal discreteness. Then this research develops a new CF assessment method, which does not rely on electromagnetic transient simulations. A real case from the China Southern Power Grid is used to verify the practicability of the proposed method by comparing with simulation results obtained using PSCAD/EMTDC.

Abstract:The integration of natural gas in electricity network requires a more reliable operating plan for increasing uncertainties in the whole system. In this paper, a threestage robust optimization model is proposed for resilient operation of energy system which integrates electricity and natural gas transmission networks with the objective of minimizing load curtailments caused by attacks. Nonconvex constrains are linearized in order to formulate the dual problem of optimal energy flow. Then, the proposed three-stage problem can be reformulated into a two-stage mixed integer linear program (MILP) and solved by Benders decomposition algorithm. Numerical studies on IEEE 30-bus power system with 7-node natural gas network and IEEE 118-bus power system with 14-node natural gas network validate the feasibility of the proposed model for improving resilience of integrated energy system. Energy storage facilities are also considered for the resiliency analysis.

Abstract:In this paper, a new three-phase grid-connected inverter system is proposed. The proposed system includes two inverters. The main inverter, which operates at a low switching frequency, transfers active power to the grid. The auxiliary inverter processes a very low power to compensate for the grid current ripple. Thus, no active power is processed by the auxiliary inverter. The goal is to produce a grid current with a low total harmonic distortion (THD) and to obtain the highest efficiency from the inverter system. The main inverter is controlled via a space-vector pulse-width modulation owing to its optimum switching pattern, and the auxiliary inverter is controlled via a hysteresis current-control technique owing to the technique’s fast dynamic response. The proposed system is analyzed in terms of different DC-link voltage, switching frequency, and filter inductance values. The optimum system parameters are selected that provide a THD value of less than 5%. A prototype inverter system at a 10-kW output power has been implemented. The main inverter operates at a 3-kHz switching frequency, and the auxiliary inverter compensates for the grid-current ripple. In total, a THD of 4.33% and an efficiency of 97.86% are obtained using the proposed inverter system prototype.

Abstract:The power electronic transformer (PET) has recently emerged as a type of power converter. It features the basic functions of power conversion and isolation as well as additional functions related to power quality control. A novel PET for a distribution grid called a flexible power distribution unit is proposed in this paper, and the energy exchange mechanism between the network and the load is revealed. A 30 kW 600 VAC/220 VAC/110 VDC medium-frequency isolated prototype is developed and demonstrated. This paper also presents key control strategies of the PET for electrical distribution grid applications, especially under grid voltage disturbance conditions. Moreover, stability issues related to the grid-connected three-phase PET are discussed and verified with an impedance-based analysis. The PET prototype is tested, and it passes the voltage-disturbance ride-through function. The experimental results verify the power quality control abilities of the PET.