Transient Angle Stability Research Articles

Transient Angle Stability of Paralleled Synchronous and Virtual Synchronous Generators in Islanded Microgrids

With the development of virtual synchronous generator (VSG) techniques, parallel operations of synchronous generators (SGs) and VSGs become increasingly common in a microgrid. The differences between paralleled systems will affect the transient stability of the system, which probably threatens stable operation of the system, especially under fault conditions. In this article, the transient angle stability of a paralleled synchronous and virtual synchronous generators (SG-VSG) system is investigated by comparing it with that of the paralleled VSGs system. It is observed that the paralleled SG-VSG system is more prone to transient instability due to the differences between their speed governors. Then, a control method is proposed to improve the transient stability of the paralleled SG-VSG system. Furthermore, a Lyapunov method is employed to establish the nonlinear model of islanded microgrid, by which the attraction domain of paralleled system is quantified. The hardware-in-loop experiment is performed to validate the theoretical analysis.

Transient angle stability of inverters equipped with robust droop control

Transient angle stability of inverters equipped with the robust droop controller is investigated in this work. At first, the conditions on the control references to guarantee the existence of a feasible post-disturbance operating point are derived. Then, the post-disturbance equilibrium points are found and their stability properties are characterized. Furthermore, the attraction regions of the stable equilibrium points are accurately depicted by calculating the stable and unstable manifolds of the surrounding unstable equilibrium points, which presents an explanation to system transient stability. Finally, the transient control considerations are provided to help the inverter ride-through the disturbance and maintain its stability characteristics. With these, it is shown that the transient angle stability is not a serious problem for droop controlled inverters with proper control settings.

A hybrid data-driven method for fast approximation of practical dynamic security region boundary of power systems

In the context of high-level integration of stochastic renewable energy resources such as wind and solar power, the practical dynamic security region (PDSR) with hyper-plane expression is a powerful tool for power system situational awareness and stability-constrained operation. In this paper, a hybrid data-driven method is proposed to fast approximating the PDSR boundary with hyper-plane expression. Firstly, the operating point data in the high-dimension nodal injection space is analyzed by the RELIEF algorithm to identify the key generators, active output of which has relatively great effect on transient angle stability. Then, based on the orthogonal point selection method and key generators, the equivalent search space and critical points inside it are determined with high accuracy and high speed. The hyper-plane expression of the PDSR boundary, which matches the critical points best, is finally fitted by the least square approach. Test results obtained using the IEEE 39-bus test system show the efficiency of the proposed method.

Cyber-Physical Attacks Targeting Communication-Assisted Protection Schemes

The dependence of modern societies on electric energy is ever increasing by the emergence of smart cities and electric vehicles. This is while unprecedented number of cyber-physical hazards are threatening the integrity and availability of the power grid on a daily basis. On one hand, physical integrity of power systems is under threat by more frequent natural disasters and intentional attacks. On the other hand, the cyber vulnerability of power grids is on the rise by the emergence of smart grid technologies. This underlines an imminent need for the modeling and examination of power grid vulnerabilities to cyber-physical attacks. This paper examines the vulnerability of the communication-assisted protection schemes like permissive overreaching transfer trip to cyberattacks using a co-simulation platform. The simulation results show that the transient angle stability of power systems can be jeopardized by cyberattacks on the communication-assisted protection schemes. To address this vulnerability, two physical solutions including the deployment of communication channel redundancy, and a more advanced communicated-assisted protection scheme, i.e., directional comparison unblocking scheme (DCUB), are considered and tested. The proposed solutions address the vulnerability of the communication-assisted protection schemes to distributed denial of service attack to some extent. Yet, the simulation results show the vulnerability of the proposed solutions to sophisticated cyberattacks like false data injection attacks. This highlights the need for the development of cyber-based solutions for communication channel monitoring.

Transient Angle Stability of Virtual Synchronous Generators Using Lyapunov’s Direct Method

With an increasing number of distributed energy resources integrated into the power system, inverters need to take on the corresponding responsibility for the security and stability of the system. Virtual synchronous generators (VSGs) are proposed to mimic dynamic characteristics of traditional rotational synchronous generators (RSGs) to compensate for the loss of inertia and reserve capacity. Similar to RSGs, VSGs will experience transient angle instability under certain conditions, which likely threatens the system security. In this paper, transient angle stability of a VSG is investigated by Lyapunov’s direct method. The deteriorative effect of reactive power control loop on transient angle stability is first analyzed and then voltage variation is incorporated into an approximate Lyapunov’s direct method. In this method, the inverter internal voltage is treated as a parameter rather than a state variable. Moreover, the influence of different parameters on transient angle stability is studied. Finally, an enhanced control strategy is presented to improve the transient angle stability by adjusting the reference power. Numerical simulation results are presented to validate the effectiveness of the proposed method and the enhanced control.

An analysis of power system transient angle stability using finite element technique

An analysis of power system transient angle stability using finite element technique

Research on Optimal Wind Power Penetration Ratio and the Effects of a Wind-Thermal-Bundled System under the Constraint of Rotor Angle Transient Stability

Large-scale wind-thermal-bundled power that is transmitted by mixed ultra-high voltage direct current (UHVDC)/ultra-high voltage alternating current (UHVAC) systems has become crucial for large-scale wind farms in China. Equations describing the electromagnetic power characteristics under short circuits for UHVAC lines and UHVDC blocks are derived based on an analysis of the external characteristics of a doubly fed wind farm and UHVDC systems. The effect of wind power penetration ratio on rotor angle transient stability is analysed, and the optimal wind power penetration ratio under the constraint of rotor angle transient stability is determined. The effects of system parameters, such as the UHVDC transmission capacity and the reactance of UHVAC lines on the optimal wind power penetration ratio are discussed. The trend of rotor angle stability varies from a monotonic deterioration to concave, and the optimal wind power penetration ratio increases from 0 to 30% under an UHVDC block when the reactance of UHVAC lines increases from 0.005 to 0.02. The optimal wind power penetration ratio under a short circuit increases from 40% to 60% when the reactance of UHVAC lines decreases from 0.02 to 0.006 and decreases from 40% to 30% when the capacity of UHVDC decreases from 3200 MW to 1600 MW. The analysis is verified by simulating an actual system in China’s Northwest Power Grid.

Pole‐to‐ground fault ride through strategy for half‐/full‐bridge hybrid MMC‐based radial multi‐terminal HVDC systems with low‐impedance grounded

Pole-to-ground (PTG) fault is one of the most common DC-side faults in multi-terminal high-voltage direct-current (HVDC) systems. During the PTG fault, the loss of system transmission power is a challenging issue. This study proposes a PTG fault ride through strategy for low-impedance grounded radial multi-terminal HVDC systems. It is realised by the coordinated operation between half-/full-bridge hybrid modular multilevel converters and mechanical DC switches. The mechanical DC switch is utilised to isolate the fault and a ground return path is formed to transmit the active power during PTG fault. If the fault is located at the sending terminal, half of the rated transmission power can be kept at the entire fault duration. If the fault is located at the receiving terminal, the successfully restarted active power regulator stations can further return to normal operation after the isolation of the PTG fault. With the proposed strategy, the loss of transmission power during PTG fault is minimised, which can improve the transient angle stability of the AC grid. PSCAD/EMTDC simulation is performed to verify the proposed strategy.

Optimization of the Event-Driven Emergency Load-Shedding Considering Transient Security and Stability Constraints

Emergency control is an important measure for prevention of power system blackouts. This paper presents a new event-driven emergency load-shedding scheme that minimizes load shedding while meeting constraints on transient angle stability, voltage/frequency deviation security, and availability of load to be shed at any bus. The evaluation of these constraints is based on time domain trajectories provided by numerical simulations. The optimal event-driven load-shedding (OEDLS) model was created to minimize the total load-shedding cost. By analyzing the characteristics of the optimal solution of this model, a trajectory sensitivity based solution approach is presented. It linearizes the nonlinear OEDLS model through trajectory sensitivity and iteratively converges to the optimal solution. To accelerate the problem solving process, a parallel computation scheme is implemented for obtaining the trajectory sensitivities. The validity of the proposed algorithm and parallel computation scheme was tested and verified using two different power systems.

A two stage model for rotor angle transient stability constrained optimal power flow

Abstract Transient stability constrained optimal power flow (TSCOPF) is a nonlinear optimization problem with both algebraic and differential equations. This paper utilizes the Imperialist Competitive Algorithm (ICA) as an evolutionary optimization algorithm and Artificial Neural Network (ANN) to develop a robust and efficient two stages scheme to solve TSCOPF problem. In the first stage an Artificial Neural Network is constructed to predict the rotor-angle transient stability margin, and is then incorporated in the TSC-OPF as the transient stability estimator. To solve the proposed TSC-OPF problem the ICA is used as the optimizer. The performance of the proposed method is verified over the WSCC three-machine, nine-bus system under different loading conditions and fault scenarios.

Damping of Power System Oscillations by using ANFIS based UPFC supplementary controller

The main purpose of this paper is to investigate an Adaptive Neuron- Fuzzy Inference System (ANFIS) based supplementary Unified Power Flow Controller (UPFC) in superimposing a damping function on the control signal of UPFC who’s incorporated in multi-machines power system. By using a hybrid learning procedure, the proposed ANFIS construct an input –output mapping based on stipulated input-output data pairs.This control scheme designed to produce supplementary signal for damping oscillations in the interconnected power systems. Oscillations in power systems have to be taken a serious note when the fault takes place in any part of the system; else this might lead to the instability mode and shutting down of the power system. The UPFC device is modelled using dynamic phasor method and interfaced with an interconnected power system. The improved damping performance was studied and it is validated on a benchmark power system example taken from literature. The systems model is evaluated via simulations during unbalanced faults. The control actions are formulated as a set of local controllers, namely the generator excitation controller, power system stabilizer (PSS) and unified power flow controller, which are coordinated to maintain transient angle stability, transient voltage stability and damping of inter-area and local oscillations following a disturbance.

A Control Strategy for Transient Stability Improvement of Doubly-Fed Wind Power Generation System

A novel inverter control strategy to enhance the transient stability of grid-connected wind farm based on doubly-fed induction generator (DFIG) is presented. Adding transient angle control strategy in the rotor side converter active control loop, this can dissipate the system unbalancing energy and restrain the system oscillations by the variation of wind turbine speed. Adding transient voltage control strategy in reactive control loop, this can provide fast reactive power compensation and support the restoration and reconstruction of the grid voltage when fault occurred. The control strategy which can improve the transient Angle stability and transient voltage stability at the same time is put forward. Finally, a testing system including a DFIG-based wind farm is realized using DigSILENT/Power Factory, the strategy validation and the contribution to power system stability enhancement are verified by simulation.

Impact on Transient Angle Stability with DFIG-Based Wind Generation Connected to Power System

This paper investigates the impact of wind generation consisting of doubly-fed induction generators (DFIG) on the system angle stability. 3 different factors are considered: the penetration level of wind generation, the connection location of wind generation and the reactive power control strategy that the DFIG utilizes. A modified IEEE-3 machine and 10 bus system has been used as the test network and a number of simulations have been conducted. It shows that the higher the penetration level of the DFIG-based wind generation, the better the system angle stability. Compared with unity power factor control strategy, the system stability is better when the DFIG utilizes constant voltage control strategy. Beside, different connection locations of wind generation also have effects on system stability.

Assessment of dynamic available transfer capability for deregulated power industry

This paper describes the assessment of dynamic Available Transfer Capability (ATC) by considering transient rotor angle stability. Since the loads and wheeling transactions vary dynamically in the deregulated power market, the power producers must respond quickly to those changes without affecting the system security and stability. This paper presents an application of the Evolutionary Programming (EP) algorithm to determine the optimal generation dispatch of power producers with dynamic security constraints. The proposed algorithm is demonstrated to determine dynamic ATC for practical IEEE-30 and Indian utility-62 bus systems with changing load conditions.

Global power system control using generator excitation, PSS, FACTS devices and capacitor switching

A novel power system control scheme is developed in this paper using a framework for coordinating dissimilar controllers at different locations. The control actions are formulated in the so-called global controller as a set of local controllers, namely the generator excitation controller, power system stabilizer (PSS), unified power flow controller (UPFC) and capacitor switching, which are coordinated to maintain transient angle stability, transient voltage stability and damping of long-term inter-area oscillations following a disturbance. The proposed coordinated control scheme is tested on a nine-bus example system.

Quantitative assessments for transient voltage security

Transient voltage security means that the power system is both transient voltage stable and transient voltage dips acceptable. These definitions are clarified in this paper. A self-start technique for computing transient voltage stability related limits is proposed. Based on a margin for transient voltage dip acceptability, a criterion for early termination of the simulation is put forward. Moreover, the procedure of calculating critical clearing time for transient voltage security is extended to that of calculating the critical load level, the minimum reactive power resources and minimum load shedding for keeping transient voltage security. It is also suggested that transient angle stability and transient voltage stability and transient voltage dip acceptability be quantitatively assessed along the same simulation trajectory. The most critical one among the three kinds of limits is regarded as the global transient security limit. The effectiveness and robustness of the proposed approach are demonstrated in this paper with statistic information resulting from 47,987 test cases on two practical power systems.