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This paper proposes a novel wide-area control strategy to enhance the transient stability of a large inter-connected power grid. The proposed control strategy achieves maximal damping of the inter-area oscillations by calculating the optimal number of cluster centers based on a novel online coherency grouping. To achieve this objective, the critical electro-mechanical mode is selected using localness index method during transient conditions. In addition, the online identification of the full model is performed to estimate the states to be controlled during the grid disturbances. Thereafter, a discrete linear quadratic regulator (DLQR) is directly applied to the estimated states, and the corresponding signals are channelized through the loop of most controllable and observable machines of each coherent group. The efficacy of the proposed method is tested on IEEE 39 bus and IEEE 68 bus standard benchmark model. The results demonstrate faster damping with minimal computation burden compared to existing solutions such as a classical slow coherency and a multi-input multi-output (MIMO) based wide area damping controllers. [ABSTRACT FROM AUTHOR]

This study presents a H∞ mixed‐sensitivity robust control design to improve the overall damping performance of low‐frequency oscillations for power systems with doubly fed induction generator‐based (DFIG) wind power generation. The supplementary damping controller is designed in the linear matrix inequality framework which allows the placement of the closed‐loop poles inside a pre‐defined region for improved stability. The damping signal is implemented at the reactive power modulation of the rotor‐side converter of the DFIG wind turbine (DFIG‐WT). The damping contribution of the power oscillator damper is compared with that provided by the power system stabiliser (PSS) of the synchronous generator (SG). State‐space system analysis and time‐domain simulations show the capability of DFIG‐WT to enhance the overall damping of the oscillatory modes even if PSSs are not installed at SG s. Also, the designed controller shows a robust performance to wind speed variations. The performance evaluation is based on the revised two‐area four machines system and the revised New England 39‐bus system. [ABSTRACT FROM AUTHOR]

This letter introduces a unit delay compensation scheme for Type-2 phase locked loop (Type-2 PLL) to eliminate the steady state phase angle error during the frequency ramp. The proposed PLL (ZPLL) uses an adaptive delayed signal cancellation (DSC) filter, along with a compensation scheme, to ensure effective operation under unbalanced grid. The proposed compensation scheme acts as an open-loop compensator, hence, ZPLL functions as Type-3 PLL with Type-2 dynamics features while preserving its second order characteristics. The proposed scheme is mathematically and experimentally validated under different scenarios of grid disturbances. The obtained results demonstrate the capability of the proposed compensation scheme to obtain a zero steady state phase angle error during the frequency ramp while employing symmetrical and asymmetrical voltage dip. [ABSTRACT FROM AUTHOR]

Representing a major technology toward the proliferation of sustainable energy resources, droop controlled microgrids have attracted numerous modeling approaches with a view to improving their operation and applicability. Acknowledging the low inertia of distributed generators and the network characteristics at the distribution level, this paper proposes a reduced order model for inter-inverter oscillations in autonomous droop controlled microgrids. At first, a simplified equivalent network on the basis of Kron reduction is suggested as an effective topology for the study of inter-inverter oscillating currents. The focus shifts toward low-frequency modes following the detailed small signal model and its simplification according to the singular perturbation theory with multiple time scales. Assuming accurate and instantaneous tracking of the voltage by the inner controllers on their local reference frame, the quasi steady state study results in a simple closed-loop model. This model is successfully benchmarked against the detailed model and other germane models over the full range of operational damping ratios. [ABSTRACT FROM AUTHOR]

This paper introduces a new technique to characterize the ac grids stability connected to multiterminal high-voltage direct current networks. The proposed method identifies the maximum active powers that converters can exchange with an ac network without losing the transient stability using trajectory sensitivity analysis. Therefore, the generators' rotor angles variations due to various converters active powers exchange are measured and normalized. Consequently, the norm of each trajectory vector is used as an indicator of the system proximity to instability. To enhance the accuracy of the method for online stability assessment applications, the steady-state initial conditions of the sensitivity network variables are precisely obtained using the Newton shooting method. For this purpose, the detailed differential and algebraic models representing the dynamics of the ac and dc networks have been formulated and analytically solved simultaneously with the sensitivity network. Comprehensive simulation studies on the IEEE 68-bus benchmark system are carried out using PSCAD/EMTDC for validation purposes. Results show that the modified approach provides more accurate results compared to the normal trajectory sensitivity analysis with less time compared to multiple time-domain simulations. [ABSTRACT FROM PUBLISHER]

This paper introduces an efficient algorithm to adaptively determine the droop coefficients of generating units in hybrid ac–dc networks. These generating units include conventional synchronous generators and multiterminal high-voltage direct-current (MTDC) converters. The proposed algorithm relies mainly on trajectory sensitivity to reschedule power generation according to the new calculated droop coefficients to ensure and/or system stability margin for set of credible contingencies with different load conditions. The Newton shooting method is used to find the new steady-state values for the systems when load is changed. MTDC converters are modeled in such a way to emulate the inertia behavior of the synchronous generators. This virtual emulation is achieved by providing two additional control layers that link and control the converter powers through inertia constants similar to synchronous generators. Also, an expression for generated powers controlled by the droop coefficients is developed to be utilized in the algorithm. Finally, comprehensive simulation studies on the IEEE 68-bus benchmark system are carried out using PSCAD/EMTDC interfaced with MATLAB to validate the proposed algorithm. [ABSTRACT FROM AUTHOR]

High concentrations of induction motor loads can impose stress on transmission and distribution systems, leading to voltage instability in some situations. Properly sized and coordinated reactive power sources will provide for improved operation. We present a strategy for finding an optimal mix (type, size, and location) of dynamic shunt reactive compensation devices. The planning strategy is subject to satisfying steady-state, dynamic and transient performance criteria such as fault-induced delayed voltage recovery limits, as well as criteria related to single ( N–1) contingency and load disturbance events. Shunt reactive power compensation devices considered include mechanically switched capacitor banks, static reactive power compensators and static synchronous compensators. The proposed strategy employs a large number of multitimescale time-domain simulations suitable for use with high performance computing clusters and a genetic algorithm to solve the mixed-integer nonlinear programming formulation using parallel computation capabilities. The method is applied to a New England IEEE 39-bus system with assumed high penetration of induction motors. A comprehensive study shows that performance enhancement and significant cost reduction can be achieved using an optimum combination of various shunt compensator technologies. [ABSTRACT FROM PUBLISHER]

The low cost of diesel generators have led to their increased presence in hybrid microgrids. The presence of low-inertia devices in such microgrids makes the stability of hybrid microgrids a pressing concern. This paper analyzes the impact of equal and unequal active power sharing conditions on the dynamic stability of a hybrid microgrid. The sensitivity of low-frequency eigenvalues to changes in active power droop gains is studied and a “domain of stability” region is proposed to assess the stability of hybrid microgrids under different power sharing conditions. The different operating regions within the domain of stability are defined. Based on the sensitivity analysis and the domain of stability, the optimal droop gain for the diesel generator to improve the stability of the microgrid is determined. Time-domain simulations are carried out in the MATLAB/Simulink software environment and are used to validate the small-signal stability analysis results. [ABSTRACT FROM AUTHOR]

This paper proposes a novel configuration and transient management scheme for Voltage Source Converter-High Voltage Direct Current (VSC-HVDC) connecting Large Scale Wind Farm (LSWF). The proposed configuration aims to fully utilize the HVDC converters with shunt and series reconfiguration during steady state and fault conditions respectively. Thus, it targets the employment of the available converters controls capabilities to enhance the Fault Ride-Through (FRT) performance of the VSC-HVDC and achieve smooth power evacuation to the electric grid. The control strategy is built in such a way to guarantee three factors: 1) enhancing the voltage profile of the electrical grid during symmetrical and asymmetrical grid faults, 2) providing smooth power evacuation from the LSWF, and 3) mitigating the DC link oscillations caused by unbalanced faults. Hence, the proposed scheme demonstrates superior performance of the VSC-HVDC system during steady state and transient conditions. Comprehensive simulation studies are carried out to demonstrate the gorgeous performance of the proposed VSC-HVDC configuration using PSCAD/EMTDC. In addition to that, a lab-scaled experimental set up is constructed to validate the functioning of the proposed VSC-HVDC configuration. [ABSTRACT FROM PUBLISHER]

This paper proposes the notion of “critical cluster” in inverter based droop controlled microgrids, thus representing the neighborhood of the distributed generation units that determines the small signal stability margin of the entire system. The clustering starts from the lowest impedance electrical connection between distributed generators, termed as “critical line”. At first, the systematic study of few similar microgrids by means of the eigenvalue analysis of their small signal models reveals the correlation between the location of their dominant low frequency modes and the individual connections between neighboring inverters. Second, the sensitivity analysis of active power droop gain with respect to network parameters confirms the previous findings and shows the dominating effect of the critical line's impedance. Simulations and small signal analysis studies assess the impact of the choice of various interconnection points when two individual droop controlled islanded microgrids are connected to form a single microgrid. At the same time, simplified models on the basis of critical clusters allow the effective approximation of the stability margin corresponding to the original systems. The results reveal that different interconnection points lead to strong variations in the performance of the coupled microgrid both in terms of transient response and small signal stability. [ABSTRACT FROM PUBLISHER]

This paper introduces a novel framework of coordinated voltage and frequency control strategy for islanded microgrid (MG) operation. The proposed control schemes rely on local measurements as communication-free control approach. Therefore, the distributed controllers of the MG components have been deployed based on their slow, medium, and fast dynamic responses to maintain the voltage and frequency in adherence to IEEE Standards 1547 and 929. The various voltage and frequency control responses associated with reactive power management scheme are efficiently utilized based on well-defined states of operation and transient management scheme. In each state, the roles of each device for voltage and frequency regulations are defined with its regulation capability, and response time based on its local measurements. Consequently, the fast reactive power compensation and rapid frequency regulation are ensured based on the inverter-based devices at challenging operating conditions. As a result, the proposed control strategy improves the voltage and frequency regulation, transient response, and MG stability. A comprehensive simulation study has verified the superior performance of the communication-free approach during steady state and in response to severe disturbances. [ABSTRACT FROM PUBLISHER]

This paper investigates a special class of dynamic power system problem, namely subsynchronous resonance (SSR) resulted from a series-compensated network connecting doubly-fed induction generator (DFIG) based wind farms. A novel two-degree-of-freedom (2DOF) control strategy combined with a damping control loop is designed and analyzed for enhancing the system stability and alleviates the SSR that may arise due to the induction generator effect (IGE). The proposed control strategy is tested at different operating conditions of series compensation levels and low wind speeds to ensure the system stability. The doubly-fed induction generator based wind farms without the proposed control strategy leads to overall system instability during high series compensation and low wind speeds. Hence, the mitigation of the SSR and damping enhancement are critical to the entire power system stability. A reliable way of analyzing the system and designing effective control strategies against SSR based on the eigenvalue analysis and impedance based stability criterion is deployed. Moreover, analytical explanations have been elaborated to verify the procedure of the controller design. Fault ride-though capability has also been investigated with the proposed control strategy that is flexible to be integrated with the FRT schemes so as to assist the wind farm in mitigating the SSR during the fault recovery stage. Finally, time domain simulations are carried out to demonstrate the effectiveness of the proposed control strategy for mitigating the SSR and damping power system oscillations. [ABSTRACT FROM PUBLISHER]

This paper introduces a novel coordinated voltage-control (CVC) scheme for distributed generations (DGs) that relies on adaptively changing the roles (master or slave) of the devices [inverter-based DG, diesel generator, and online tap changer (OLTC)] within the smart grid, depending on system conditions. In addition, the proposed scheme imposes different control response and bandwidth on the devices to coordinate the reactive power among distributed generations (DGs) and OLTC steps. The main objective of the proposed method is twofold: 1) to maximize the reactive power reserve of DGs and, hence, facilitate reaction during contingency situations and 2) to provide voltage regulation during normal operating conditions. The simulated distribution system includes inverter-based DGs (photovoltaic and wind turbine), diesel generator, and OLTC and the potential of the CVC scheme is evaluated and analyzed in view of improving voltage profile, maximizing the reactive power reserve, enhancing fault ride through and improving the transient stability margin. The control algorithm is examined under steady state, load excursion, and three-phase-to-ground fault conditions. The results demonstrate the ability of the proposed CVC scheme to satisfy the targeted objectives with significant improvement in the maximum critical clearing time. The proposed scheme is independent of real-time measurements and is widely adaptive to the dynamics of power systems, thus making it quite suitable for utility implementation. [ABSTRACT FROM AUTHOR]

Hybrid micro‐grids are one of the most viable solutions to the present day problem of integration of renewable energy into existing power systems. With various options available for implementation of hybrid micro‐grids, the stability of such systems has to be ensured before they can be implemented. Further, conventional micro‐grid operational studies do not take into consideration the constraints set by the grid codes. This study analyses three types of interconnections, possible for implementation of hybrid micro‐grids, taking into consideration the transient stability and grid code constraints. The hybrid micro‐grid considered comprises a synchronous diesel generator and an inverter‐based distributed generation. The results and analysis are conducted by time‐domain simulations using MATLAB/Simulink software. The results provide the actual operational limits of possible micro‐grid interconnections under the set of given constraints. [ABSTRACT FROM AUTHOR]

This paper presents new series grid interface topologies for enhancing the low voltage ridethrough capability of self-excited induction generator-based wind turbines. Two proposed schemes: static synchronous series compensator (SSSC) and controllable series braking resistor (CSBR) have been designed and simulated. The test system simulated represents a wind turbine connected to an electric grid with alternatively employing SSSC and CSBR. The potential of the two schemes is evaluated and analyzed using positive and negative-sequence reference frames in response to balanced and unbalanced fault scenarios. The simulation results using PSCAD/EMTDC demonstrate the performance of the proposed schemes for improving the fault ridethrough capability and transient stability margin in response to severe symmetrical and asymmetrical grid faults. The schemes have shown successful and fast mitigation of voltage disturbance consequences on the mechanical system. The schemes helped the wind turbine to remain connected with considerable isolation from grid faults. [ABSTRACT FROM PUBLISHER]

The paper proposes a novel fault ride-through (FRT) configuration with a transient management scheme to provide dynamic grid supports for self-excited induction generator (SEIG)-based wind turbines. The new configuration is capable of supporting both shunt and series compensation modes for enhancing FRT capability and transient stability. The proposed control scheme, utilizing positive and negative sequence controllers, provides a transient management solution for both balanced and unbalanced fault conditions. An active damping control loop for the shunt compensation configuration is developed to mitigate system oscillation and improve system performance during and following faults. A comprehensive simulation has verified the FRT capability of the new configuration and transient management scheme, and showed that the proposed topology increases system stability under various short circuit ratios, even in case of a weak grid. The experimental results are also carried out to show the effectiveness of the proposed shunt-series reconfiguration topology. [ABSTRACT FROM PUBLISHER]

Integration of dynamic grid support is required for distributed power systems that are interconnected with medium voltage grids. This study proposes a comprehensive control solution to enhance fault ride through (FRT) capability for utility‐scale photovoltaic (PV) power plants. Based on positive and negative sequence control schemes and PV characteristics, the approach alleviates dc‐bus double‐line‐frequency ripples, reduces voltage stress on inverter power switches and DC‐link capacitors, and minimises undesirable low‐order voltage and current harmonics that are presented on the ac side. The study proposes a new feature to achieve superior FRT performance by using the overload capability of grid‐tied inverters. A weak electric grid is used for the test case including a wind turbine induction generator, diesel engine driven synchronous generators and various loads. A comprehensive simulation verified the capability of the proposed control schemes for mitigating the voltage dip, enhancing the voltage response and further improving the stability of interconnected distributed generation in reaction to severe unbalanced voltage conditions because of asymmetrical grid faults. [ABSTRACT FROM AUTHOR]