With an increasing number of distributed energy resources (DERs) integrated to the power system, inverters need to take 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 probably 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.

Diversification of control schemes adopted by inverter-interfaced distributed generators (IIDGs) leads to difficulties in fault current estimation in a microgrid, which might make preexisting protection systems invalid and threaten the safety of power electronic devices. It is therefore important to study fault characteristics of IIDGs. This paper investigates characteristics of fault current of IIDGs caused by both symmetrical and asymmetrical faults. Two kinds of widely used control modes, current control (constant current control and PQ control) and voltage control (V/F control and droop control), are under investigation to provide an intuitive comparison on fault current. In particular, a novel algorithm is proposed to calculate fault current of droop-controlled IIDGs. It is found that different limiters have great impacts on fault response of IIDGs and detailed research works are carried out to identify the effects in this paper. Simulation results based on PSCAD/EMTDC and calculation results based on MATLAB/Simulink verify the correctness of the proposed fault models.

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 transient stability of the system, which probably threatens stable operation of the system, especially under fault conditions. In this paper, transient angle stability of a paralleled synchronous and virtual synchronous generators (SG-VSG) system is investigated by compared with that of 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 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.

The increasing growth in the installation of natural-gas fired units has raised necessitates on synergy between the electricity and natural gas networks. This paper discusses how synergistic operation of electricity and natural gas networks can be achieved in a distributed fashion using alternating direction method of multipliers (ADMM). A standard ADMM approach and a consensus-based ADMM approach are developed, respectively, to solve the gas-electric integrated optimal power flow problem with and without a coordination operator. In both cases, the optimization formulation for each operator is modeled, and the solution procedure as well as data exchange among multiple decision-makers are explained. Case studies on both small and large gas-electric integrated systems show that the two ADMM-based distributed approaches have significant efficiencies over the traditional Lagrange relaxation (LR) and augmented LR-based methods.

Cyber-attack is recognized as an emerging risk in smart grids. In this paper, we reveal a potential link between data attack and physical consequences and analyze how the attacker can launch a malicious data attack to trigger sequential outages and thus impose large damages to the grid. In this attack mechanism, the attacker constructs an optimal false data injection (FDI) attack to intentionally trigger a targeted branch outage sequence that trips multiple branches and then leads to subsequent failures. The studied attack mechanism integrates constructing an optimal data attack and identifying critical lines, and imposes a substantial security impact with a high chance of occurring. Simulations on the IEEE 118-bus system verify the introduced attack mechanism and highlight the risk of such attacks in today's smart grids.

With development of power electronic technology, the paralleled current-controlled voltage source converters (CCSs) and virtual synchronous generators (VSGs) system has advantages in providing power and voltage/frequency regulation at the same time in rural area or remote island. However, the paralleled system faces great challenges in safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability, especially during fault periods. This paper focuses on transient stability and stability-oriented control design of the paralleled CCS-VSG system. First, mathematical model of the paralleled system is established and then the effect of two kinds of CCSs’ current injection angle (active current and reactive current) on VSG’s transient stability has been revealed through extended equal area criterion. Based on the theoretical analysis results, transient stability improvement control is put forward by controlling the CCS to track the VSG’s frame. Compared with the conditions that only active-or reactive current is provided by the CCS, the system can achieve the best transient performance when the proposed control is adopted. Moreover, the effect of CCS’s capacity on transient stability of the VSG has also been discussed. Finally, both Lyapunov’s method and simulation/experimental results are provided to validate the correctness of theoretical analysis.