Risk-Balanced Routing Strategy for Service Function Chains of Cyber-Physical Power System Considering Cross-Space Cascading Failure

Cyber-physical power system(CPPS)has significantly improved the operational efficiency of power systems.However,cross-space cascading failures may occur due to the coupling characteristics,which poses a great threat to the safety and reliability of CPPS,and there is an acute need to reduce the probability of these failures.Towards this end,this paper first proposes a cascading failure index to identify and quantify the importance of different information in the same class of communication services.On this basis,a joint improved risk-balanced service function chain routing strategy(SFC-RS)is proposed,which is modeled as a robust optimization problem and solved by column-and-constraint generation(C-CG)algorithm.Compared with the traditional shortest-path routing algorithm,the superiority of SFC-RS is verified in the IEEE 30-bus system.The results demonstrate that SFC-RS effectively mitigates the risk associated with information transmission in the network,enhances information transmission accessibility,and effectively limits communication disruption from becoming the cause of cross-space cascading failures.

Research on large-scale cascading failure of power systems using synergistic effect

This paper discusses the primary causes from the point of synergistic effects to improve power system vulnerability in the power system planning and safety operation. Based on the vulnerability conception in the complex network theory the vulnerability of the power system can be evaluated by the minimum load loss rate when considering power supply ability.Consequently according to the synergistic effect theory the critical line of the power system is defined by its influence on failure set vulnerability in N-k contingencies.The cascading failure modes are proposed based on the criterion whether the acceptable load curtailment level is below a preset value.Significant conclusions are revealed by results of IEEE 39 case analysis weak points of power networks and heavy load condition are the main causes of large-scale cascading failures damaging synergistic effects can result in partial failure developed into large-scale cascading failures vulnerable lines of power systems can directly lead the partial failure to deteriorate into a large blackout while less vulnerable lines can cause a large-scale cascading failure.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

Reliability-BasedModel for Incomplete Preventive ReplacementMaintenance of Photovoltaic Power Systems

At present,the operation and maintenance of photovoltaic power generation systems mainly comprise regular maintenance,breakdown maintenance,and condition-based maintenance,which is very likely to lead to over-or under-repair of equipment.Therefore,a preventive maintenance and replacement strategy for PV power generation systems based on reliability as a constraint is proposed.First,a hybrid failure function with a decreasing service age factor and an increasing failure rate factor is introduced to describe the deterioration of PV power generation equipment,and the equipment is replaced when its reliability drops to the replacement threshold in the last cycle.Then,based on the reliability as a constraint,the average maintenance cost and availability of the equipment are considered,and the non-periodic incomplete maintenance model of the PV power generation system is established to obtain the optimal number of repairs,each maintenance cycle and the replacement cycle of the PV power generation system components.Next,the inverter of a PV power plant is used as a research object.The model in this paper is compared and analyzed with the equal cycle maintenance model without considering reliability and the maintenance model without considering the equipment replacement threshold,Through model comparison,when the optimal maintenance strategy is(0.80,4),the average maintenance cost of this paper’s model are decreased by 20.3%and 5.54%and the availability is increased by 0.2395% and 0.0337%,respectively,compared with the equal-cycle maintenance model without considering the reliability constraint and the maintenance model without considering the equipment replacement threshold.Therefore,this maintenance model can ensure the high reliability of PV plant operation while increasing the equipment availability to improve the system economy.

Protection Strategies against Cascading Failure for Power Systems of Ring Network

Power grid vulnerability is a key issue with large blackouts, causing power disruption for millions of people. The complexity of power grid, together with excessive number of components, makes it difficult to be modeled. Currently, researchers use complex networks to model and study the performance of power grids. In fact, power grids can be modeled into a complex network by making use of ring network topology, with substations and transmission lines denoted as nodes and edges, respectively. In this paper, three protection schemes are proposed and their effectiveness in protecting the power network under high and low-load attacks is studied. The proposed schemes, namely, Cascaded Load Cut-off (CLC), Cascaded Load Overflow (CLO) and Adaptive-Cascaded Load Overflow (A-CLO), improve the robustness of the power grids, i.e., decrease the value of critical tolerance. Simulation results show that CLC and CLO protection schemes are more effective in improving the robustness of networks than the A-CLO protection scheme. However, the CLC protection scheme is effective only at the expense that certain percentage of the network will have no power supply. Thus, results show that the CLO protection scheme dominates the other protection schemes, CLC and A-CLO, in terms of the robustness of the network, improved with the precise amount of load cut-off determined.

Analysis of Commutation Failure in Multi-Infeed HVDC System under Different Load Models

HVDC technology has been widely used in modern power system. On one hand, HVDC has the advantages of economy, high efficiency and strong controllability. While on the other hand, it makes the dynamic characteristics of the power system becoming more and more complex. That puts forward a new challenge to system stability and raises new questions for power system simulation. This paper focuses on the interaction between AC and DC systems, especially the problem of commutation failure caused by AC system fault. Based on the data of China Southern Power Grid, this paper calculates the fault regions that may cause commutation failure and calculates the system critical clearance time under different load models, analyzes the impacts of different load models on commutation failure and the stability of AC/DC hybrid system.

A review on simulation models of cascading failures in power systems

Among various power system disturbances,cascading failures are considered the most serious and extreme threats to grid operations,potentially leading to significant stability issues or even widespread power blackouts.Simulating power systems’behaviors during cascading failures is of great importance to comprehend how failures originate and propagate,as well as to develop effective preventive and mitigative control strategies.The intricate mechanism of cascading failures,characterized by multi-timescale dynamics,presents exceptional challenges for their simulations.This paper provides a comprehensive review of simulation models for cascading failures,providing a systematic categorization and a comparison of these models.The challenges and potential research directions for the future are also discussed.

Evaluation of commutation failure risk in single-or multi-infeed LCC-HVDC systems based on equivalent-fault method

In single-or multi-infeed line-commutated converter-based high-voltage direct current(LCC-HVDC) systems, commutation failure(CF) induced by alternating current(AC) faults may lead to serious consequences. Considering the randomness of fault occurrences, an accurate evaluation of the CF risk(CFR) from the system point of view becomes necessary in power system planning and operation. This paper first provides a definition of the CF severity(CFS) index corresponding to an AC fault. Then,on the basis of electromagnetic transient(EMT) simulation, an approach to calculate the CFS index considering the randomness of fault-occurrence time is presented. A novel equivalent-fault method is further put forward to make the EMT simulation scalable to calculate the CFS index in terms of a fault occurring in a large-scale receiving-end grid. Thereafter, the CFR index is introduced, which is defined as the sum of the products of the CFS index of each AC fault and the corresponding fault rate.Finally, the proposed method is verified on the modified IEEE 9-bus and modified IEEE 39-bus systems using PSCAD/EMTDC.

Calculation Method for Commutation Failure Fault Level in LCC-HVDC System Under Single-line-to-ground Faults Considering DC Current Variation

Earlier studies have reported some calculation methods for commutation failure fault level(CFFL) in line-commutated-converter based high-voltage direct current(LCCHVDC) system under single-line-to-ground(SLG) faults. The accuracy of earlier methods is limited because they only consider the commutating voltage drop and phase shift, while neglecting the DC current variation. Hence, this paper proposes a CFFL calculation method under SLG faults considering DC current variation, for better planning and designing of LCC-HVDC systems. First, the fault commutating voltage magnitude and phase shift are calculated. Then, the fault DC voltage during different commutation processes is deduced. Based on the commutating voltage magnitude and phase shift, and DC voltage during different commutation processes under SLG faults, the characteristics of CFFL with different fault time are demonstrated and analyzed. Next, the transient time-domain response of the DC current after the fault is obtained based on the DC transmission line model. Discrete commutation processes are constructed based on the commutation voltage-time area rule to solve the extinction angle under different fault levels and fault time. Finally, the CFFL is calculated considering the fault time, commutating voltage drop, phase shift, and DC current variation. The accuracy of the proposed method compared with the traditional method is validated based on the CIGRE benchmark model in PSCAD/EMTDC.

Analysis of the Sending-Side System Instability Caused by Multiple HVDC Commutation Failure

As high-voltage direct current(HVDC)lines with large capacity are being commissioned with higher frequency,the characteristics of“strong”DC and“weak”AC transmission in the power grid are topics of interest.In particular,the coupling and interaction between the sending-side and receivingside AC systems interconnected by large-scale DC links is gaining importance.In this paper,the impact of the multiple HVDC commutation failure on the stability of the sending system under different power flow directions is analyzed based on the threearea AC/DC equivalent model.The main influencing factors and the counter-measures are discussed,and the single HVDC line blocking is taken as a comparison.Finally,the results are verified using the North China-Central China-East China power grid case system.The study provides a basis and reference to ensure security and stability of the ultra-high-voltage(UHV)AC/DC hybrid power grid.

Supplementary Control for Mitigation of Successive Commutation Failures Considering the Influence of PLL Dynamics in LCC-HVDC Systems

Line commutated converter based high voltage direct current(LCC-HVDC)links are widely employed for long distance bulk power transmission and asynchronous alternating current(AC)grid connections.However,LCC-HVDC systems often suffer from commutation failures when AC voltage is distorted,oscillating or reduced by AC faults,which leads to overheating of converter valves and interruptions in transmitted power.All of which can have an adverse impact on the safety and stability of the entire power system.This paper proposes a supplementary control for mitigation of successive commutation failures on the basis of analyzing the influence of phase-locked loop(PLL)dynamics on the commutation process.By analyzing the impact of PLL dynamics on the actual leading angle,it is found that changes in the AC voltage phase remarkably influence commutation.Accordingly,the error between the AC voltage phase and PLL’s output angle is added to the output of the extinction angle or DC voltage control to mitigate the successive commutation failures of LCC-HVDC stations.Simulations conducted on the CIGRE benchmark model in PSCAD/EMTDC validate the performance of the supplementary control,which effectively mitigates successive commutation failures.

Improved Coordinated Control Approach for Evolved CCC-HVDC System to Enhance Mitigation Effect of Commutation Failure

The evolved capacitor commutated converter(ECCC),embedded with anti-parallel thyristors based dual-directional full-bridge modules(APT-DFBMs),can effectively reduce commutation failure(CF)risks of line-commutated converter-based high voltage direct current(HVDC)and improve the dynamic responses of capacitor-commutated converterbased HVDC.This paper proposes an improved coordinated control strategy for ECCC with the following improvements:(1)under normal operation state,series-connected capacitors can accelerate the commutation process,thereby reducing the overlap angle and increasing the successful commutation margin;(2)under AC fault conditions,the ability of ECCC to mitigate the CF issue no longer relies on the fast fault detection,since the capacitors inside the APT-DFBMs can consistently contribute to the commutation process and further reduce the CF probability;(3)the inserted capacitors can output certain amount of reactive power,increase the power factor,and reduce the required reactive power compensation capacity.Firstly,the proposed coordinated control approach is presented in detail,and the extra commutation voltage to mitigate the CFs provided by the proposed control approach and an existing approach is compared.Secondly,the mechanism of the improved control approach to accelerate commutation process and improve the power factor is analyzed theoretically.Finally,the detailed electromagnetic transient(EMT)simulation in PSCAD/EMTDC is conducted to validate the effectiveness of the proposed coordinated control.The results show that the proposed approach can present a further substantial improvement for ECCC,especially enhancing the CF mitigation effect.

A Continuous Fault Ride-through Scheme for DFIGs Under Commutation Failures in LCC-HVDC Transmission Systems

Experimental and theoretical studies have confirmed that,relative to a one-shot voltage fault,a doubly-fed induction generator(DFIG)will suffer a greater transient impact during continuous voltage faults.This paper presents the design and application of an effective scheme for DFIGs when a commutation failure(CF)occurs in a line-commutated converter based high-voltage direct current(LCC-HVDC)transmission system.First,transient demagnetization control without filters is proposed to offset the electromotive force(EMF)induced by the natural flux and other low-frequency flux components.Then,a rotor-side integrated impedance circuit is designed to limit the rotor overcurrent to ensure that the rotor-side converter(RSC)is controllable.Furthermore,coordinated control of the demagnetization and segmented reactive currents is implemented in the RSC.Comparative studies have shown that the proposed scheme can limit rotor fault currents and effectively improve the continuous fault ride-through capability of DFIGs.

Continuous Commutation Failure Suppression Method Based on Self-adaptive Auto-disturbance Rejection Proportional-integral Controller for HVDC Transmission System

Once an asymmetrical fault occurs on the AC side of the receiving-end of a high-voltage direct current(HVDC)transmission system,the current reference will be affected by the control regulation on the DC inverter side and the commutation voltage asymmetry.In this case,the advance firing angle will fluctuate periodically,causing security threats to the system.If the fault cannot be cleared in time,the effect may be even more serious.However,the traditional proportional-integral(PI)controller cannot effectively suppress the periodic components in the input error signal,which is an important cause of continuous commutation failure.Thus,the system requires more time to recover from the fault.Motivated by this,a selfadaptive auto-disturbance rejection PI controller is proposed in this study.The controller has the advantages of fast response speed and strong anti-interference ability of the auto-disturbance rejection controller.On one hand,it can automatically adjust PI,and the parameters can maintain the system’s adaptive ability.On the other hand,the discretization process satisfies the computer simulation requirements.By applying the proposed controller to a system under constant current control and extinction angle control,the dynamic response speed can be improved and the robust performance of the system can be ensured when dealing with a wide range of perturbations.Finally,simulation results show that the proposed algorithm can effectively suppress the continuous commutation failure of DC transmission systems.

First passage failure of dynamical power systems under random perturbations

The first-passage problem of dynamical power system of a single-machine-infinite-bus (SMIB) system under random perturbations is studied.First,the stochastic averaging method for quasi non-integrable generalized Hamiltonian systems is applied to reduce the equations of the SMIB system under random perturbations to a set of averaged It equations.Then,the backward Kolmogorov equation governing the conditional reliability function and the Pontryagin equation governing the conditional mean of first passage time are established and solved numerically,respectively.Finally,the proposed method is verified by using the Monte Carlo simulation of the original system.

Deducing cascading failures caused by cyberattacks based on attack gains and cost principle in cyber-physical power systems

To warn the cascading failures caused by cyberattacks(CFCAs)in real time and reduce their damage on cyber-physical power systems(CPPSs),a novel early warning method based on attack gains and cost principle(AGCP)is proposed.Firstly,according to the CFCA characteristics,the leading role of attackers in the whole evolutionary process is discussed.The breaking out of a CFCA is deduced based on the AGCP from the view of attackers,and the priority order of all CFCAs is then provided.Then,the method to calculate the probability of CFCAs is proposed,and an early warning model for CFCA is designed.Finally,to verify the effectiveness of this method,a variety of CFCAs are simulated in a local CPPS model based on the IEEE 39-bus system.The experimental results demonstrate that this method can be used as a reliable assistant analysis technology to facilitate early warning of CFCAs.

Assessment of commutation failure in HVDC systems considering spatial-temporal discreteness of AC system faults

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.

Modeling cascading failures and mitigation strategies in PMU based cyber-physical power systems

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.

Coordination control method to block cascading failure of a renewable generation power system under line dynamic security

The large application of renewable energy generation(REG)has increased the risk of cascading failures in the power system.At the same time REG also provides the possibility of new approaches for the suppression of such failures.However,the capacity and position of the synchronous generator(SG)involved in regulation limit the power regu-lation speed(PRS)of REG to the overload line which is the main cause of cascading failures,while the PRS of SG is related to the position and shedding power.REG and SGs have difficulty in achieving effective cooperation under constraints of system power balance.Particularly,the dynamic variation of line flow during power regulation causes new problems for the accurate evaluation of line thermal safety under overload.Therefore,a new strategy for quan-titatively coordinating shedding power and power regulation to block cascading failures in the dynamic security domain is proposed in this paper.The control capability and dynamic security domain of the overload line are mod-eled,and the coordination control method based on power regulation is then proposed to minimize shedding power.The algorithm for the optimal control scheme considers the constraints of load capacity,power source capacity and bus PRS.The correctness of the proposed method is verified using case studies.