The grid-forming and grid-following controls are adopted in three-phase voltage source converters according to different grid conditions. However, the basic operation principle of the two kinds of control are different and will leads to the instability of the grid-connected system through various paths. In this paper, the energy function model of the grid-forming and grid-following controlled converters are established and compared in detail. The influence of system parameters is studied, and the large-signal stability boundaries are derived through a general method. Moreover, compared to the grid-forming controlled converter, the grid-following converter will lose stability by exhibiting a varying damping coefficient transient. The equivalent damping coefficient can be used as a criterion of whether the grid-forming/following control is suitable for the weak/strong grid condition. The simulation and experiment results show the difference of the transient process and validate the control criterion for different grid strength under large-signal disturbance.

While many radio technologies are available for mobile devices, none of them are designed to deal with asymmetric available energy. Battery capacities of mobile devices vary by up to three orders of magnitude between laptops and wearables, and our inability to deal with such asymmetry has limited the lifetime of constrained portable devices. This paper presents a radically new design for low-power radios --- one that is capable of dynamically splitting the power burden of communication between the transmitter and receiver in proportion to the available energy on the two devices. We achieve this with a novel carrier offload method that dynamically moves carrier generation across end points. While such a design might raise the specter of a high-power, large form-factor radio, we show that this integration can be achieved with no more than a BLE-style active radio augmented with a few additional components. Our design, Braidio is a low-power, tightly integrated, low-cost radio capable of operating as an active and passive transceiver. When these modes operate in an interleaved (braided) manner, the end result is a power-proportional low-power radio that is able to achieve 1:2546 to 3546:1 power consumption ratios between a transmitter and a receiver, all while operating at low power.

Transient stability issues of power converter systems have attracted much attention. Generally, the converter systems behave in high-order nonlinear dynamics due to multiple cascaded control loops, which become obstacles for transient stability analysis. Considering the dynamic coupling between active power and reactive power control loops, the grid-tied virtual synchronous generators (VSG) system is a third-order nonlinear system. Including the equal area criterion (EAC), many conventional methods are only applicable to second-order systems and cannot fully capture damping effects. To fill this gap, a novel iterative EAC method is proposed to handle the transient stability of the third-order VSG system. First, an implicit relationship from the output voltage to the power angle is derived by integral substitution. From energy aspects, the dynamic relations between voltage, power angle, and frequency are approximated by algebraic equations through iterative calculation. The impact of nonlinear damping is also fully captured quantitatively. Both conservatism and quantity of computation are improved compared with previous studies. Furthermore, the critical clearing angles under different disturbance forms and degrees are derived. In addition, some discussions about current limiters’ influence and different controller parameters’ influence are given. Simulation and hardware-in-the-loop experiments based on RT-Lab are conducted to verify the accuracy of the proposed method.

The power electronics’ penetration in modern grids is rapidly increasing due to the growing proportion of renewable energy generation. As the important energy conversion interface, the stable operation of grid-tied converters is vital to the overall safe of the double-high power system. The prior arts have mainly focused on the varying negative damping in the phase-locked loop (PLL). Negative damping imposes energy into the system and is detrimental to system's transient stability and should not be neglected as the constant positive damping in synchronous generators. The deficiencies of the previous studies lie in the large conservatism of the criteria once considering the influence of negative damping. In this article, an elliptic-equal area criterion (EAC) is proposed to assess the damping impacts on the stable domain of the converter systems, which improves conservatism compared to previous transient stability criteria. In addition, a transient stability improved PLL is proposed by eliminating the negative damping term.

With continuous increase of penetration of renewable energies and power electronic equipment, power grid stability incidents are emerging frequently worldwide. In this paper, the transient stability assessment of renewable dominated power systems with multiple PLL-based voltage source converters (VSCs) is studied by calculating critical clearing time (CCT). A tangent hyperplane method is developed, which approximates the boundary of basin of attraction near unstable equilibrium point by linear system theory. The detailed algorithm is presented, accompanying with four typical case studies, including a single machine infinite system, two parallel VSCs, modified IEEE 9- bus, and modified 39- bus systems. Broad EMT simulations are performed to verify the accuracy and computational effectiveness of the algorithm. In all these tests, we find that the hyperplane method can give accurate estimations of CCTs. Hence it is expected to be an efficient method. In addition, we find that generally the CCT in renewable dominated power systems is around tens of milliseconds, which is much shorter than that in traditional power systems around hundreds of milliseconds. Therefore, it might be imperative to adopt faster controls and protections for renewable dominated power systems.

User-side battery energy storage systems (UESSs) are a rapidly developing form of energy storage system; however, very little attention is being paid to their application in the power quality enhancement of premium power parks, and their coordination with existing voltage sag mitigation devices. The potential of UESSs has not been fully exploited. Given the above, this paper proposes a hierarchical power supply strategy for premium power parks (PPPs) based on the coordination of UESSs and dynamic voltage restorers (DVR). Firstly, the topology and control strategy of the UESS for voltage sag mitigation are devised. Further, we construct a prototype device, and experimental tests are conducted to verify its effectiveness in voltage sag mitigation. To avoid the negative effects caused by the UESS control mode switching between the voltage sag mitigation mode and the peak-shaving and valley-filling modes—namely, the long control mode switching time and low state of charge (SOC)—a hierarchical power supply strategy is proposed. The basic idea is to supply three levels of power quality service for loads with different sensitivities in PPPs by coordinating UESS with DVR. Several cases are explored to demonstrate the feasibility of the proposed strategy.