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Recently, transformerless (TL) inverters are being extensively used in small-scale photovoltaic (PV) systems due to their compact-size, lighter-weight, lower-cost and higher-efficiency compared to their counterpart with transformer. However, the growing penetration of these small-scale based PV systems and low-inertia induction motor (IM) loads in low-voltage distribution networks (DNs) make the grid more vulnerable to short-term voltage stability (STVS). Hence, this paper thoroughly examines the STVS of DN with high-penetration of TL-PV units, and provides countermeasures by TL-PV systems to mitigate any short-term voltage instability. The detailed dynamic model of the TL inverter is developed first. Next, three control strategies: (1) constant peak current (CPC), (2) constant active current (CAC), and (3) constant active power (CAP) with low voltage ride through (LVRT) and dynamic voltage support (DVS) capabilities are proposed to improve STVS. The impacts of different level of PV penetrations and LVRT capability of TL-PV inverters on the STVS are explored. Moreover, countermeasures, such as LVRT with CAC and CAP controls and DVS by the TL-PV systems are designed and implemented. Several case studies are carried out on an IEEE 4 bus system first, and later extended to IEEE 13 node test feeder. The results show that DVS can further improve STVS in residential DNs. In addition, CAP and CAC control strategies can speed-up the postdisturbance voltage recovery compared with the conventional CPC control. [ABSTRACT FROM AUTHOR]

This paper presents a new robust nonlinear excitation controller design for synchronous generators in multimachine power systems to enhance the transient stability. The mismatches between the original power system model and formulated mathematical model are considered as uncertainties, which are modeled through the satisfaction of matching conditions. The exogenous noises appearing from measurements are incorporated with the power system model including the two-axis model of synchronous generators. The partial feedback linearization technique is used to design the controller which transforms the original nonlinear multimachine power system model into several reduced-order linear and autonomous subsystems. The desired control law is obtained for each subsystem and implemented in a decentralized manner provided that the dynamics of the autonomous subsystems have no effects on the overall stability of the system. The analysis related to the dynamics of noisy autonomous subsystems is also included and the proposed controller has the excellent capability to decouple these noises. Finally, the performance of the proposed control scheme is evaluated on an IEEE 39-bus benchmark power system following different types of large disturbances. The performance of the proposed controller is compared to that of a partial feedback linearizing controller, which is designed without robustness properties, to verify the effectiveness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

This paper makes two contributions. Firstly, it is shown that there may exist oscillations in distribution networks with physically close photovoltaic (PV) units due to their control interactions. The main reason for oscillations is changes in the dynamics of PV units with varying solar irradiance and the decentralized tuning of control parameters for the nominal operating condition. Eigenvalue analysis, nonlinear interaction index, and time-domain simulations are used to assess the degree and impacts of dynamic interactions. Secondly, a decentralized robust control design is proposed to ensure a non-interacting and well-damped response under varying operating conditions for physically close PV units. The control design process uses an estimate of changes in the model due to variations in solar irradiations, changing electric loads, and the dynamics of the physically close PV units. A minimally conservative method is used to capture the estimate as a norm-bounded uncertainty. Time-domain simulations on a test distribution system verify the predictions of the interaction analysis and demonstrate the performance of the proposed robust controller under different system contingencies. [ABSTRACT FROM AUTHOR]

In this paper a new nonlinear excitation controller design to enhance transient stability of multimachine power systems is presented. Partial feedback linearization is first used to transform the nonlinear power system model into a partially linear system comprising a reduced-order linear part and a nonlinear dynamic autonomous part. Then a linear state feedback stabilizing controller is designed for the reduced-order linear part using optimal control theory to enhance the stability of the whole system. In this way, the performance of the stabilizing controller would be independent of the operating points of the power system and therefore is superior to those designed for completely linearized systems. It is shown that the controller design method ensures the stability of the nonlinear dynamic autonomous part. The design method is applicable to multimachine power systems but tested on a 3-machine 11-bus two-area test system. The performance of the proposed control scheme to large disturbances is evaluated, through computer simulation, and compared with a conventional power system stabilizer and an exact feedback linearizing controller. [ABSTRACT FROM AUTHOR]

This paper presents a new nonlinear current control scheme for a single-phase grid-connected photovoltaic (PV) system. The controller is designed using partial feedback linearization which linearizes the system partially and enables the controller design scheme for reduced-order PV systems. The reference current is calculated from the maximum power point tracking system. The proposed current control approach introduces the internal dynamics and the stability of the internal dynamics is a key requirement for the implementation of the controller. In this paper, an energy-based Lyapunov function is used to analyze the stability of internal dynamics of a PV system. The performance of the controller is evaluated based on the tracking of grid current to the reference current by considering the changes in atmospheric conditions. To ensure the suitability of the proposed controller in a real system, a large system similar to a practical system is simulated under different operating conditions such as changes in atmospheric conditions and faults on various parts of the system and compared with conventional controllers. The experimental validation of the proposed control scheme is also presented in this paper. [ABSTRACT FROM PUBLISHER]

The complexity of power systems has increased in recent years due to the operation of existing transmission lines closer to their limits, using flexible AC transmission system (FACTS) devices, and also due to the increased penetration of new types of generators that have more intermittent characteristics and lower inertial response, such as wind generators. This changing nature of a power system has considerable effect on its dynamic behaviors resulting in power swings, dynamic interactions between different power system devices, and less synchronized coupling. This paper presents some analyses of this changing nature of power systems and their dynamic behaviors to identify critical issues that limit the large-scale integration of wind generators and FACTS devices. In addition, this paper addresses some general concerns toward high compensations in different grid topologies. The studies in this paper are conducted on the New England and New York power system model under both small and large disturbances. From the analyses, it can be concluded that high compensation can reduce the security limits under certain operating conditions, and the modes related to operating slip and shaft stiffness are critical as they may limit the large-scale integration of wind generation. [ABSTRACT FROM PUBLISHER]