As the penetration rate of renewable enery resources (RES) in the power system increases, uncertainty and variability in system operation increase. The application of energy storage systems (ESS) in the power system has been increased to compensate for the characteristics of renewable energy resources. Since ESS is a controllable and highly responsive power resource, primary frequency response and inertia response are possible in case of system contingency, so it can be utilized for frequency regulation (FR) purposes. In frequency regulation, reduction of the Rate of Change of Frequency (RoCoF) and increase the frequency nadir by improving the response characteristics are important factors to secure frequency stability. Therefore, it is important to control ESS with proper parameters according to changing system situation. In this paper, we propose a method to calculate and apply a frequency droop, which is basically required according to the power system condition based on swing equation and effective inertia assessment. In addition, a method to estimate RoCoF droop according to the correlation with frequency by estimating the systematic inertia in the current situation is proposed. The case study for verification of the proposed method was performed through dynamic simulation using actual Korean power system data. The results show that the proposed method is more effective than the governor-free of the conventional thermal generator and conventional droop control-based FR-ESS.

The variability and uncontrollability of wind power increases the difficulty for a power system operator to implement a wind power system with a high penetration rate. These are more serious factors to consider in small and isolated power systems since the system has small operating reserves and inertia to secure frequency and voltage. Typically, this difficulty can be reduced by interconnection with another robust power system using a controllable transmission system such as a high-voltage direct current (HVDC) system. However, the reliability and stability constraints of a power system has to be performed according to the HVDC system implementation. In this paper, the method for calculation of maximum wind power penetration in an island supplied by a HVDC power system is presented, and the operational strategy of a HVDC system is proposed to secure the power system reliability and stability. The case study is performed for the Jeju Island power system in the Korean smart grid demonstration area.

This paper presents a method for evaluating the harmonic contributions of multiple harmonic sources to voltage distortion at the point of common coupling (PCC). The proposed method quantitatively represents harmonic contributions based on the equivalent voltage models of harmonic sources. The equivalent voltage models can be estimated using the recursive least square (RLS) algorithm and measured voltage and current at the PCC. The assessment of harmonic contribution heavily depends on the accuracy of equivalent model estimation. Many existing studies have assumed that the equivalent model parameters of harmonic sources are not changed. However, because harmonic source parameters and system operating conditions are not constant in real power systems, this assumption can lead large errors in harmonic contribution assessment. In this paper, we propose a new assessment method based on variable forgetting factor RLS and parameter change detection to solve the problem. The proposed method is also effective for evaluating comprehensive harmonic contributions as well as individual harmonic contributions. The case studies show that the proposed method is superior to the previous one even under parameter changing conditions.

The importance of the auxiliary control of power electronic devices such as HVDC and FACTs is growing owing to the increase in the generation capacity of renewable energy sources. This paper presents a temporary frequency support strategy using the electrostatic energy in voltage-sourced converter-type high-voltage direct current systems. An analytical derivation for selecting the optimal droop constant to improve the frequency nadir was established. The optimal droop constant was derived via system frequency response analysis by using the support termination time. Furthermore, the optimality of the support was computed using numerical equations. Modified outer control loops were implemented in the master controller to utilize the proposed method in voltage-sourced converter control. The effect of the fast frequency support control strategy was verified via a time-domain electromagnetic transient program simulation based on case studies. To examine the practical applicability of the proposed strategy, the parameter robustness was investigated by performing deviated system parameter simulations. The results validated the proposed strategy.

Stable system operation is being actively attempted by introducing grid-forming inverters (GFMs) which mimic synchronous generators (SGs). The introduction of GFMs, intended to replace traditional grid-following inverters (GFLs) that provide system inertia and contribute significantly to fault current, paradoxically exhibits unstable output characteristics under high short circuit ratio (SCR) conditions. In this paper, a mathematical plant model of the basic GFM structure is implemented along with the model integrating grid-connected GFM and the SCR of the system for maintaining the grid dynamics. For this end, a single input single output (SISO) model of an GFM with active power control (APC) is proposed for analyzing the stability in small grid events, and the system frequency response model considering the dynamic performance of SG and GFM APC was also configured for simulating large grid events. The performance of the proposed APC for step response and frequency response characteristics is compared with an existing APC method, such as droop and virtual synchronous machine (VSM). Simulation results using MATLAB and PSS/e verify the performance of the proposed method and stable operation can be expected by actively composing the controller during the SCR that continuously changes in both small and large grid events.

This paper provides a comparison of a designed method of a fault ride through (FRT) circuit, i.e., switch-type fault current limiter (STFCL) and bridge-type fault current limiter (BFCL), to optimize the electrical parameters of grid-connected solar systems (PVSs) under asymmetric single line-to-ground fault and symmetric three-phase fault. The main differences between switch- and bridge-type fault current limiters is the electric component devices such as the bridge rectifier, snubber capacitor, energy absorption bypass and current-limiting inductors. In addition, the designed FRT performance with the inverter control are analyzed in-depth, e.g., a well-adjusted proportional integral (PI) and proposed steepest descent (SD) controller are compared in the fault condition. To compare the proposed method with the conventional method, the AC power and voltage on a common coupling point (PCC) and DC link voltage of the PV system are analyzed with a MATLAB/Simulink model of a 100 kW three-phase grid-connected photovoltaic system. The simulation results of the proposed FRT circuit and SD controller verify the stability improvement and vibration-free and fast and robust responses of electrical parameters on both PV grid sides during asymmetric disturbances.