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This study comprehensively analyzes the discontinuous conduction mode (DCM) of a two-phase interleaved buck converter (IBC) with an inversely coupled inductor (ICL). With ICL, nine circuit configurations are possible when the IBC operates in the DCM. Depending on the duty ratio and load condition, DCM can also be classified into seven types. Each type of DCM has a corresponding combination of circuit configurations. Consequently, the application of the combinations for distinguishing DCM types is proposed to analyze the DCM operation of two-phase IBCs with ICL. Different DCM types have different DCM characteristics, such as state space averaged models and time interval calculations, which are needed to be analyzed for small-signal modeling. Therefore, the state space averaged models and time intervals equations are derived for each DCM type, and boundary conditions between adjacent DCM types are analyzed. Experimental tests on the hardware prototype of the IBC are conducted to verify the practical effectiveness of the proposed analysis.

The separation of transmission and distribution systems raises a variety of questions concerning the integration of many distributed generators (DGs) into future grids. They are difficult to solve by using current energy management methods. Especially, the cooperative control between transmission system operator (TSO) and distribution system operator (DSO) has been increasingly emphasized to manage the line congestion. This paper proposes a new cooperative control of TSO-DSO based on the generation-load power sensitivity analysis. To minimize the required computational effort and data communication, the information of TSO and DSO is processed separately in the generation-load power sensitivity matrix of power system. The proposed cooperative control is implemented by three-step process, which is aggregation, specification, and local distribution. Firstly, the DSOs aggregate the flexibility area of DGs in their networks, and they inform the TSO of the feasible operation regions (FORs). Then, the TSO sends the power references at the boundary buses to many DSOs. Thereafter, the DSOs use these references as loads to calculate the detailed power references for their generators. As the result, the net power of DSOs satisfies the power references requested by the TSO. The case studies are carried out to verify the effectiveness of proposed control. The results show that when the load is increased by 20%, the overall average of line loading is decreased by 4.34% with the proposed control. Also, when the generator of 634 MW is disconnected, the frequency nadir is increased by 0.1 Hz compared with the $P$ - $f$ droop control.

This paper proposes a new coordinated control for multiple wind power plants (WPPs) based on two stepwise inertial control (SIC) methods to effectively release the kinetic energy of the permanent magnet synchronous generators (PMSGs) and improve their inertial response. When a large disturbance occurs, the conventional SIC method increases the output power from the PMSG instantly to arrest the frequency nadir (FN). However, in low wind speed conditions, it may not be able to provide a sufficient inertial response. Therefore, the proposed coordinated control applies the new SIC method to some WPPs by decreasing the output power from their PMSGs for a short period before increasing it. This results in initially accelerating the rotor speed of the PMSGs and reserving their releasable kinetic energy. This means that they are ready to extract more powers without causing the over-deceleration (OD) problem. Also, the proper selection of two SIC methods for multiple WPPs is able to prevent the rate of change of frequency (RoCoF) from increasing. The effectiveness of the proposed coordinated control is verified with several case studies on the IEEE benchmark 39-bus test system. The results show that it effectively improves the frequency stability of the power system without raising the RoCoF in various conditions. Moreover, this enhancement becomes more apparent when the penetration level of wind power is high.

The high penetration of converter-based distributed generations (DGs) to power system can give rise to the lack of rotational inertia while replacing the conventional synchronous generators (SGs), which provide the primary frequency reserve (PFR) in power systems. As the result, the decrease in PFR aggravates the frequency stability. To overcome this problem, the droop coefficients of governors in the remaining conventional SGs must be re-determined newly and properly. This paper proposes a new solution based on the grey wolf optimization (GWO) method to optimally select the droop coefficients of SG governors in the low-inertia large-scale power system due to the high penetration of renewables. The proposed solution is very effective for reducing the computational effort significantly, and it is able to recover not only the steady-state but also the transient frequency stability. To verify the effectiveness of proposed optimization solution based on the GWO method, several case studies are carried out on the practical Korea electric power system with the penetration of wind power plants of 4 GW.

In this paper, a new constant power generation (CPG) method for photovoltaic (PV) systems is proposed to mitigate the power variations due to changes in external conditions. A power grid with the high penetration of renewables might face both voltage and frequency instability when the power variations from PV systems are large. The proposed CPG method estimates the constant power point (CPP) in real-time without requiring the time step used in the conventional methods. Thus, it can rapidly track the CPP, and it is able to generate the constant power effectively even for sudden changes in external conditions. The practical effectiveness of proposed CPG method is verified by experimental and hardware-in-the-loop tests. The results clearly show that the proposed CPG method provides a much faster response with a less tracking error than the conventional methods in different operating conditions. Thus, it can be an effective solution to further increase the penetration level of PV systems to power grid.

This paper proposes an improved fast charging strategy for electric vehicles (EVs) by considering available battery capacity. According to previous research and battery experiment reports, the energy capacity of batteries is not fixed, and it can decrease temporarily depending on the magnitude of charging or discharging power. This paper addresses the decreased capacity in proportion to the magnitude of the charging power which leads to a reduction in the driving range. For effective and practical use for EV users, the change in battery capacity is expressed through an equation, and a new state of charged indicator is proposed. To improve the conventional charging method that uniformly supplies power regardless of the battery capacity, this paper proposes an optimal charging strategy injecting constant power. The proposed charging strategy provides an optimal charging power reference to minimize costs considering charged energy, charging time, and usable energy loss based on billing system of EV charging. To verify the effectiveness of the proposed charging method, the optimal charging power reference for each battery is calculated based on capacity and characteristics, and the total cost is compared. The results show that the performance of the proposed charging strategy is effective in minimizing both the reduced battery capacity and economic burden on EV users.

Recently, renewable energy has received considerable attention worldwide due to environmental problems such as climate change. However, there are still many problems to be solved to increase the penetration level of renewable energy sources in a power system. Above all, a power system with a high renewable penetration level requires an entirely novel operational strategy and control approach to achieve reliable operation by considering the intermittency of variable renewable energy sources. This paper proposes power-sensitivities-based indirect voltage control of renewable energy generators with power constraints. The proposed method realizes seamless control of the voltage by compensating with the reactive power if the real power of the distributed generator (DG) is insufficient due to its intermittency. To achieve the proposed method, the real and reactive power references of DGs are initially operated based on the power sensitivities between generation and loads if all the DGs operate within the power constraints. However, if at least one of the DGs reaches its constraint limit, the references can be modified using other sensitivities between real and reactive power generation enabling indirect voltage control of DGs. Therefore, a high level of renewable penetration in power systems can be accomplished with the proposed method. The proposed method is verified with several case studies that are based on a microgrid with practical data from a real island power system. Verification is carried out using the power system computer aided design and electromagnetic transient including DC (PSCAD/EMTDC™) simulation software.

To ensure the frequency stability of a power grid, the settling frequency following a frequency event should be stabilized within the maximum steady-state frequency deviation (SSFD) band. If the frequency is stabilized at a value beyond the maximum SSFD band for high penetrations of variable renewable energy, the frequency cannot be restored to the nominal frequency because automatic generation cannot be activated. This paper proposes a primary frequency control support scheme of a doubly-fed induction generator (DFIG) to improve the settling frequency following an event. In the proposed scheme, if the frequency following an event is stabilized at a value beyond the maximum SSFD band, to reduce the output power of synchronous generators, the output power of a DFIG is instantly increased by a constant and maintained until the frequency increases to within the maximum SSFD band. At the same time, the mechanical input power of the synchronous generators is maintained so that the difference between the mechanical input power and electrical output power of the synchronous generators increases the frequency to a value within the maximum SSFD band. The simulation results demonstrate that the proposed scheme can improve the settling frequency while avoiding additional system defense plans when the settling frequency is stabilized at a value beyond the maximum SSFD band.

The implementation of distributed generation (DG) becomes more common while causing a challenge to maintain the voltage stability in a power system. The optimal placement of multiple DGs in a planning stage can effectively solve this problem. In many cases, the best placement has been determined by the small-signal study like the modal analysis. However, it is acceptable only if the linearization of large-scale nonlinear power system is accurate. To overcome this, this paper proposes the new optimal placement algorithm for multiple DGs based on the model-free Lyapunov exponent estimation to maintain the voltage of system stable. In other words, the individual Lyapunov exponents of all buses are firstly estimated to determine the candidates for optimal placement of DGs. Then, the maximum Lyapunov exponent for each candidate is calculated to decide which bus is the best place to improve the voltage stability of system. Several time-domain simulation studies are carried out to verify the effectiveness of proposed algorithm. In particular, its performance is compared with that of the conventional method. The results show that the optimal placement of multiple DGs determined by the proposed algorithm improves the voltage stability of system much more effectively.

This study proposes the practical power management of integrated system with photovoltaic (PV) and energy storage system (ESS) to solve the line congestion and voltage problems in a distribution system while considering the electricity price and state of charge (SOC) of ESS. In addition, the proper power management of PV/ESS integrated system (PEIS) enables to increase the hosting capacity of renewable energies in a distribution system and to maximize the profit of independent power producer (IPP). In this study, the real power management of PEIS of 100 kW is implemented in practice and tested with the actual measurements for 3 days. The results show that the proposed method deals with the line congestion problem effectively and efficiently while increasing the profit of IPP by 11%. In particular, the reactive power control based on the international electrotechnical commission (IEC) standard, IEC 61850-90-7 is applied to the PEIS, and it can successfully mitigate the variations of voltage in a distribution system.

As renewable energy penetrates the power system, system operators are required to curtail output power from generation units to balance the power supply and demand. However, large curtailment from wind power plants (WPPs) may instantly cause excessive output power decrement, causing system frequency to drop significantly before reaching its nominal value. In order to solve this problem, this paper proposes a cooperative control framework to determine the operation of WPPs in two control methods, which are the stepwise inertial control (SIC) method and the curtailed control method. The proposed framework first determines the WPPs to operate in the curtailed control method to provide the required power curtailment. Next, it determines the WPPs to operate in the SIC method considering their releasable kinetic energy to provide an effective inertial response and compensate for the sudden excessive output power decrement caused by other WPPs operated in the curtailed control method. Therefore, each WPP is operated in one of two control methods to provide required power curtailment while reducing the sudden excessive output power decrement. To verify the effectiveness of the proposed cooperative control framework, several case studies are carried out on the practical South Korea electric power system.

Abstract Background Cluster headaches (CH) are recurrent severe headaches, which impose a major burden on the life of patients. We investigated the impact of CH on employment status and job burden. Methods The study was a sub-study of the Korean Cluster Headache Registry. Patients with CH were enrolled from September 2016 to February 2018 from 15 headache clinics in Korea. We also enrolled a headache control group with age-sex matched patients with migraine or tension-type headache. Moreover, a control group including individuals without headache complaints was recruited. All participants responded to a questionnaire that included questions on employment status, type of occupation, working time, sick leave, reductions in productivity, and satisfaction with current occupation. The questionnaire was administered to participants who were currently employed or had previous occupational experience. Results We recruited 143 patients with CH, 38 patients with other types of headache (migraine or tension-type headache), and 52 headache-free controls. The proportion of employees was lower in the CH group compared with the headache and headache-free control groups (CH: 67.6% vs. headache controls: 84.2% vs. headache-free controls: 96.2%; p = 0.001). The CH group more frequently experienced difficulties at work and required sick leave than the other groups (CH: 84.8% vs. headache controls: 63.9% vs. headache-free controls: 36.5%; p

The high penetration of wind power decreases the system inertia and primary frequency reserve while replacing the conventional synchronous generators (SGs). Therefore, if the system operator does not take appropriate action on the remaining generation units (GUs) operation, high penetration of wind power will aggravate the frequency stability. To solve this problem, wind power plants (WPPs) may provide the inertial response and primary frequency response (PFR) to support the frequency stability. However, due to the variability of renewable energy, WPPs may not provide adequate frequency response whenever it is required. This paper proposes an algorithm to determine the operation of GUs to provide appropriate PFR for a power system with high penetration of wind power. Through the proposed algorithm, it calculates the required PFR to restore the decreased frequency stability caused by the high penetration of wind power. Then, while considering the available PFR from WPPs, it redetermines the droop coefficient of SGs governor to provide the sufficient PFR to recover the frequency stability. Finally, the effectiveness of the proposed algorithm is verified on the practical Korean electric power system.

This paper proposes an algorithm for fault detection, faulted phase and winding identification of a three-winding power transformer based on the induced voltages in the electrical power system. The ratio of the induced voltages of the primary-secondary, primary-tertiary and secondary-tertiary windings is the same as the corresponding turns ratio during normal operating conditions, magnetic inrush, and over-excitation. It differs from the turns ratio during an internal fault. For a single phase and a three-phase power transformer with wye-connected windings, the induced voltages of each pair of windings are estimated. For a three-phase power transformer with delta-connected windings, the induced voltage differences are estimated to use the line currents, because the delta winding currents are practically unavailable. Six detectors are suggested for fault detection. An additional three detectors and a rule for faulted phase and winding identification are presented as well. The proposed algorithm can not only detect an internal fault, but also identify the faulted phase and winding of a three-winding power transformer. The various test results with Electromagnetic Transients Program (EMTP)-generated data show that the proposed algorithm successfully discriminates internal faults from normal operating conditions including magnetic inrush and over-excitation. This paper concludes by implementing the algorithm into a prototype relay based on a digital signal processor.

In South Korea, the existing diesel generators are being replaced with renewable energy sources (RESs) in several stand-alone microgrids. However, their reliability and stability are not still guaranteed by fluctuations of the system caused by RESs, arising from unpredictable changes in environmental factors. Moreover, since the inertia that is originally provided by the diesel synchronous generators is reduced, the overall system becomes highly sensitive to frequency variations. As a result, the installation of RESs must be well planned while considering the upper limit of their installed capacity. In this paper, a new method for evaluating the maximum penetration capacity of photovoltaic (PV) generators to ensure the stable operation of a microgrid is proposed. For the analytical approach, two frequency stability indexes, which are the minimum instantaneous frequency and the quasi-steady-state (QSS) frequency after a disturbance, are used. The capacity of PV to be installed is limited by considering the characteristics of other generators. In addition, the efficiency of the energy storage system (ESS) is also analyzed to determine the optimal capacity of both PV and ESS. The effectiveness of the proposed method is firstly proven through the mathematical analysis. Then, case studies on a practical stand-alone microgrid in South Korea are carried out using a time-domain simulation based on the DIgSILENT PowerFactory® software.

The high penetration of distributed energy resources (DERs) in the distribution system brings new challenges related to voltage variations and reverse power flow. The sudden changes in electric power generation from the DERs due to climate conditions or their internal malfunction might cause high fluctuations in system voltage. To enhance the voltage stability when a disturbance occurs, this paper proposes a new reactive power management method to control each DER based on voltage sensitivity analysis. The voltage variation has the different features in both steady-state and transient conditions. In particular, its transient behavior depends on the type of DERs. Therefore, the particular optimal control for improving the dynamic response of voltage is difficult to apply for all types of DERs. In contrast, the voltage variation in steady-state can be controlled with the reactive power management by the sensitivity analysis between reactive power generation and system voltage. Even though this paper focuses on the reduction of voltage variation in steady-state, the relationship between the voltage variations in steady-state and transient conditions is also analyzed. The effectiveness of the proposed method is verified with several case studies on the practical distribution system in South Korea by using the time-domain simulation based on the PSCAD/EMTDC® software. The results show that the proposed reactive power management method can improve the dynamic voltage responses in both steady-state and transient conditions when the distribution system has the high penetration of renewables.