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Owing to the considerable shortage of traditional energy sources, utilization of renewable energy is gaining attention. Thus, the high voltage inverters for solar and wind power generation systems are in huge demand. Modular inverters are more preferred over singular capacity inverters due to their numerous benefits such as relieving of thermal management, fault tolerance, reduced component stress and modularity. This paper proposes a parallel-configured improved Z-source inverter (ZSI) based on transformer. The proposed circuit has increased voltage gain with the combined benefits of paralleling inverters. Also, it offers lower harmonic distortion and lower filter requirements by employing the interleaved pulse-width-modulation (PWM) strategy. Two parallel-connected transformer-based improved ZSI is firstly analyzed in detail, and the similar concept is then applied to N parallel-connected transformer-based improved ZSI. A hardware prototype is implemented in laboratory, and it is tested experimentally for high power applications.

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 article, we present a new type of Z -source inverter (ZSI) that replaces two inductors in the original quasi-ZSI with two transformers. It can produce a very high output voltage with the high value of the modulation index. Moreover, it enables both to provide a better power quality of output and to reduce the switching loss. In particular, the proposed quasi-clamped ZSI with two transformers give a unique solution to obtain high gain by reducing the turns ratio, rather than increasing it. Also, the leakage inductance of the transformer might cause huge voltage spikes across the inverter bridge. To deal with this problem, an effective clamping technique is proposed. As a result, this increases the overall efficiency of the system by utilizing the low-power switching devices. The characteristics of the proposed quasi-clamped ZSI with two transformers are comprehensively discussed with the detailed theoretical analysis. Thereafter, its performances are evaluated by both simulation and experimental tests.

This article proposes the new noninverting buck–boost ac–ac converter, which can minimize the reverse recovery loss by disabling the body diodes of switches and using the external fast recovery diodes for its current freewheeling. In particular, the proposed ac–ac converter takes advantages of metal-oxide semiconductor field effect transistors and external fast recovery diodes to achieve the high efficiency. Also, it has the high reliability without the shoot-through and dead-time considerations in the circuit. Moreover, it can be simply implemented without employing the conventional commutation control. This results in decreasing the control complexity. As its proper application, a new type of dynamic voltage restorer (DVR) based on the proposed noninverting buck–boost converter is described. It enables to solve the voltage sag and swell problems by utilizing both positive and negative gains of converter. Then, the hardware prototype of proposed ac–ac converter and DVR are implemented to verify their practical effectiveness.

This article proposes a family of single-phase transformer based Z-source ac–ac converters (ZSACs). The proposed converters inherit all the benefits associated with the classical single-phase ZSAC, which are providing the buck–boost operation, maintaining or reversing the phase angle, and improving reliability. The proposed converters also have the improved input profile, continuous input current, and shared ground between input and output. Moreover, they use the turns ratio of transformers to produce the required voltage gain, which can be tuned by either increasing or decreasing the turns ratio of transformer depending on the configuration of Z-source structure. A safe-commutation strategy is also presented to achieve the soft commutation, which eliminates the voltage and current spikes without any additional snubber circuits. The features of proposed converters are comprehensively discussed with the detailed theoretical analysis. Thereafter, its performances are evaluated by both simulation and experimental tests.

This article proposes the partially signed binary voltage control (PSBVC) method for a hybrid modular multilevel converter (MMC) (HMMC), which has a mixture of half-bridge submodules (SMs) (HB-SMs) and full-bridge SMs (FB-SMs). With the proposed PSBVC method, the HMMC can generate a large number of voltage levels with much fewer SMs than the MMC and enhanced MMC (EMMC) by having different voltages for each FB-SM. That is, all HB-SMs have the same voltage and their sum in an arm is the same as the dc-link voltage. On the contrary, the voltages of FB-SM are sequentially decreased by half from HB-SM voltage. As the voltage level becomes more sophisticated, the circulating current decreases effectively because the voltages of HB-SMs are tightly regulated. As the result, the HMMC with the proposed PSBVC method is superior in not only harmonic performance but also circulating current. To verify the effectiveness of the HMMC with the proposed PSBVC method, its controller is firstly designed by the electromagnetic transients program (EMTP). Then, it is experimentally verified with the hardware prototype of HMMC. Finally, the main advantages of HMMC with the proposed PSBVC method are emphasized by comparing it with MMC and EMMC in detail.

This paper proposes a frequency-smoothing scheme of a doubly-fed induction generator (DFIG) that can significantly eliminate the frequency fluctuations caused by continuously fluctuating wind speeds. To achieve this, a proportional-integral (PI) control loop depending on the frequency deviation instead of a proportional control loop used in conventional schemes is added to the maximum power point tracking loop. The P control loop in the proposed frequency-smoothing scheme eliminates the alternating component in the frequency deviation, whereas the I control loop eliminates the slowly-varying component. The gains for the P control loop and I control loop are suggested for effectively using the rotating masses of a DFIG while avoiding over-deceleration of the rotor speed. The simulation results evidently demonstrate that the proposed scheme nearly removes the frequency deviation under various wind conditions, by using the proposed PI control loop, especially in high penetration levels of wind.

This article proposes a new control method for the synchronous rectifier (SR) of phase-shifted full-bridge (PSFB) converter to improve its efficiency in light-load condition. In general, the PSFB converter operates in the discontinuous conduction mode (DCM) in light-load condition to reduce the effective duty-ratio and core loss. However, the conventional control methods in the DCM cause undesirable conduction loss in SR because the output inductor current flows through the body-diode instead of the channel of SR. To figure it out, the proposed control method modulates the turn- on time of SR to eliminate the body-diode conduction. Moreover, it can be easily implemented without any auxiliary circuits. Therefore, it can improve the efficiency in light-load condition while keeping the advantages of DCM. The operating principle of PSFB converter in the DCM is first described. Thereafter, the proposed control method for SR is theoretically analyzed. Finally, its practical effectiveness is verified by experimental tests on the hardware implementation.

The output voltage of renewable energy sources is influenced by environmental conditions. To compensate for the variations of voltage, a buck–boost power conditioning system can be used. This article presents a full-bridge single-inductor-based buck–boost inverter. Its output voltage can be greater or lower than the input voltage depending on the controllable duty ratio. This inverter features bidirectional and reactive power flow operations with no high-frequency common-mode voltage. The unique feature of the proposed inverter is using a single inductor in the power train, which enhances the power density. Moreover, only two switches can operate at high frequency. This enables the inverter to be made with the high efficiency and simple circuit configuration. Based on the proposed full-bridge inverter, a novel cascaded buck–boost inverter is also presented. It retains all the benefits of the proposed full-bridge inverter. The theoretical analysis of the proposed full-bridge inverter is first explained in detail. It is then extended to a cascaded inverter. Thereafter, the practical effectiveness of the proposed inverters is verified with the experimental tests by implementing the hardware prototypes of 110 Vrms, 60 Hz, and 500 W for full-bridge single-inductor-based buck–boost inverter, and 220 Vrms, 60 Hz, and 1000 W for the cascaded inverter.

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.

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.

This article proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can maintain the frequency within a narrow range under normal operations. The proposed scheme uses an additional control loop based on the frequency deviation that operates in conjunction with maximum power point tracking (MPPT). To improve the energy-absorbing capability in the overfrequency section (OFS) and energy-releasing capability in the underfrequency section (UFS), the scheme suggests separate control gains, which are determined based on the ratio of the output of the additional loop to that of the MPPT loop. The ratio in the OFS is set to decrease with respect to the rotor speed whereas the ratio in the UFS is set to increase with respect to the rotor speed. The simulation results demonstrate that the proposed scheme can significantly lessen the WTG output power fluctuations by adjusting the control gains of the UFS and the OFS even though the WTG output power is severely fluctuating, thereby mitigating the frequency fluctuations. The scheme effectively uses the heavy rotating masses of a WTG as an energy storage system for enhancing the power grid flexibility in terms of frequency, particularly in high penetration levels of variable renewable energy.

Conventional cascaded dual-buck dc–ac inverters and dual-boost ac–dc converters can achieve high efficiency by using power MOSFETs without causing reverse recovery issues. In addition, these topologies can provide high reliability by eliminating shoot-through issues. As a result, dead-time in the switching signals can be eliminated and higher switching frequencies can be used. However, these topologies have a major and serious drawback of having more inductors in them and a high magnetic volume. This paper presents a family of improved dual-buck dc–ac inverters and dual-boost ac–dc converters with fewer inductors that can effectively mitigate the high magnetic volume and large number of inductors problems without adding any complexity to the control or hardware. The proposed dual-buck inverters (DBI) and dual-boost converters have all the aforementioned benefits of conventional DBIs and dual-boost converters, respectively. In addition, they reduce the magnetic volume, cost, and complexity of the circuit layout through the use of fewer inductors. To validate the feasibility of the proposed topologies, experimental results of the proposed 2-kW cascaded dual-buck dc–ac inverter with 310 $V_{\mathrm {dc}}$ input voltage and $420~V_{\mathrm {rms}}$ output voltage are provided.

The widespread adoption of feature phones and smartphones has previously led researchers to test the assumption that users always have their phones ready at hand. We believe the current ubiquity of other smart devices has changed this. Using a mixed methods approach inspired by past work in this area, we reevaluate the physical proximity relationship between individuals and their smartphone, as well as the role of other smart devices in users’ lives. We find that, on average, participants have their phone within arm's reach 51% of the time, similar to the findings of the prior work. However, if smartphones are not within arm's reach, they are now much further away because of the use of wearables and other devices. We also show how social, personal, and external factors result in users relying on other devices for functions (e.g., information seeking, checking the weather, setting timers and alarms, and playing music) once performed by their smartphones.

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 paper proposes the new clamping angle control (CAC) pulse width modulation (PWM) method to restore its maximum linear modulation range of voltage source inverters (VSIs) reduced by the duty ratio limitation in hardware implementation, which makes the VSIs to operate nonlinearly in the high modulation range. The deadtime and the bootstrap gate driver circuit mainly cause this duty ratio limitation in practice. In particular, the bootstrap gate driver circuit is used in many industrial applications by avoiding the additional power supply to apply the gate-source voltage of power switches, and therefore making the VSIs to be small size with the low cost. First, the proposed CACPWM method is theoretically analyzed. Then, it is applied in the high modulation range to overcome the limitation of the duty ratio due to the use of the bootstrap gate driver circuit. Thereafter, its practical effectiveness is verified by both simulation and experimental tests. Also, the resulting harmonic distortions are compared with those by the conventional PWM methods.

The kinetic energy stored in the rotating mass of wind turbine generators can provide the effective power quality solution to the inertia-free stand-alone (IFSA) microgrid by improving its stability. In conventional power systems, the power released from the inertia of synchronous generators regulates the system frequency. However, the frequency does not vary much even when a large disturbance or a load change occurs in the IFSA microgrid with only converter-based generators. This paper proposes a new inertia control method for a type-4 permanent magnet synchronous generator wind turbine system in the IFSA microgrid by utilizing the measurement of the dc-link voltage of the converter and its derivatives, not by the system frequency. Several case studies are carried out to evaluate the performance of the proposed inertia control in the IFSA microgrid with electromagnetic transients program-based simulation. Also, its effectiveness is verified with the experimental results from the hardware-in-the-loop test implemented by the controller interface with the digital signal processor, which is connected to the real-time digital simulator.

The inertia-free stand-alone (IFSA) microgrid is the new small power grid without synchronous generators equipped in conventional power systems. In other words, this system has the only converter-based generators (CBGs) based on energy storage devices and renewable energies. Therefore, it requires a new CBG control to provide strong voltage reference for the CBGs connected to the point of common coupling (PCC). This is because the IFSA microgrid is subject to a weak system such that it cannot maintain its rated frequency and/or bus voltage with the conventional real and reactive power controllers, which usually get their references from the PCC. This paper proposes the innovative conceptual approach for distributed coordination in the IFSA microgrid by applying the synchronized global positioning system time to the CBGs. Moreover, it is verified that the proposed physical structure of the IFSA microgrid makes it possible for all CBGs to share the real and reactive powers effectively. As the result, the proposed CBG control can provide the robustness, which is strongly required in the IFSA microgrid, based on timely outage management and restoration process. Several case studies are carried out with the microgrid of 36 MW to verify the effectiveness of the proposed CBG controller. The simulation and hardware-in-the loop tests are carried out using the power system computer-aided design/electromagnetic transient design and control (PSCAD/EMTDC) and the real-time digital power system simulator (RTDS), respectively.

Recently, microgrids have faced a great challenge for the stable operation of converter-dominant system with very high renewable penetration. The converter-based-generator reduces inertia constant. In this case, very small power imbalance can cause large frequency variation. Therefore, the accurate load sharing between generators is important for the stable operation. This paper proposes the virtual multi-slack (VMS) droop control based on the power sensitivity analysis. In other words, one physical slack generator directly controls the magnitude and phase angle of its bus voltage. At the same time, the other generators indirectly control the magnitudes and phase angles of their bus voltages by using the VMS droop control. This results in the proper load sharing between the generators in the stand-alone microgrid. That is, all nonslack generators operate as virtual slacks so that they solve the power imbalances together with one physical slack generator. The proposed VMS control is realized with the multi-slack power flow analysis, which enables adaptive droop based power allocation between the generators. Then, its effectiveness is verified with several case studies on the practical stand-alone microgrid in South Korea by using the electromagnetic transient program.

This paper describes the development of a new buck cascaded buck–boost power factor correction (PFC) converter of 2 kW with a soft-switching technique. For its wide range of input voltage, it operates in both buck and boost modes. The parameters are properly selected to endure voltage and current stress in all operating ranges. In addition, the electromagnetic interference (EMI) filter is used to reduce the EMI noise and guarantee continuous input current in buck operation. Moreover, the zero-voltage-transient pulse-width-modulation (ZVT-PWM) method is applied to improve the overall efficiency of the converter. The performance of the proposed PFC converter with the widest range of input voltage is evaluated by the hardware experimental test including harmonics analysis based on the International Electrotechnical Commission standard in all operating ranges. Also, the variations of power factor are theoretically analyzed in both buck and boost modes to determine the widest input range of the proposed PFC converter of 2 kW with an EMI filter. These are strongly required to commercialize it in practice. Finally, the efficiency of proposed PFC converter is compared with that of a conventional buck cascaded buck–boost PFC converter under various conditions.

This paper proposes a flexible reactive current-to-voltage ( $I_{Q}-{V}$ ) scheme of a doubly fed induction generator (DFIG) for the rapid voltage regulation of a wind power plant (WPP). In the proposed scheme, the WPP controller dispatches different voltage set points to the DFIGs depending on their rotor voltage margins. The DFIGs inject different reactive powers with the flexible $I_{Q}- V$ schemes implemented in the rotor-side and grid-side converters. The $I_{Q}- V$ characteristic, which consists of the gain and width of a linear band and $I_{Q}$ capability, varies with time depending on the $I_{Q}$ capability of the converters and a voltage dip at the point of interconnection (POI). To increase the $I_{Q}$ capability during a fault, the active current is reduced in proportion to a voltage dip. If the IQ capability and/or the POI voltage dip are large, the $I_{Q}- V$ gain is set to be high, thereby providing rapid voltage regulation. To avoid an overvoltage after the fault clearance, a rapid $I_{Q}$ reduction scheme is implemented in the WPP and DFIG controllers. The performance of the proposed flexible scheme was verified under scenarios with various disturbances. The proposed scheme can help increase wind power penetration without jeopardizing voltage stability.

This paper proposes a power-smoothing scheme for a variable-speed wind turbine generator (WTG) that can smooth out the WTG's fluctuating power caused by varying wind speeds, and thereby keep the system frequency within a narrow range. The proposed scheme employs an additional loop based on the system frequency deviation that operates in conjunction with the maximum power point tracking (MPPT) control loop. Unlike the conventional, fixed-gain scheme, its control gain is modified with the rotor speed. In the proposed scheme, the control gain is determined by considering the ratio of the output of the additional loop to that of the MPPT loop. To improve the contribution of the scheme toward maintaining the frequency while ensuring the stable operation of WTGs, in the low rotor speed region, the ratio is set to be proportional to the rotor speed; in the high rotor speed region, the ratio remains constant. The performance of the proposed scheme is investigated under varying wind conditions for the IEEE 14-bus system. The simulation results demonstrate that the scheme successfully operates regardless of the output power fluctuation of a WTG by adjusting the gain with the rotor speed, and thereby improves the frequency-regulating capability of a WTG.

Increase in power demand has required providing infrastructure, such as huge power plants and transmission lines. However, the recent trends in power system planning and operation are being toward maximum utilization of existing electricity infrastructure with tight operating margins because of new constraints placed by economic, political, and environmental issues. This might cause serious stability problems when a severe fault or a sudden outage of critical lines occurs. Therefore, the special protection scheme (SPS) has been used by generator tripping and successive load shedding to prevent a large-scale blackout by the cascading effect. If the number of tripping generators in the SPS operation is reduced, the enormous cost of power outage can be saved. This paper analyzes the enhancement of SPS via the transient stability improvement by the static synchronous compensator (STATCOM) applied to a generation side of Korea electric power system. First, its characteristic related to the SPS is described. Then, the proper capacity of STATCOM is determined with the equal area criterion to reduce the number of tripping generators. Also, the new control method for STATCOM is proposed for its full utilization. Finally, its effectiveness is verified by the case study with the practical data of Korea electric power system.

This paper proposes an adaptive hierarchical voltage control scheme of a doubly-fed induction generator (DFIG)-based wind power plant (WPP) that can secure more reserve of reactive power ( ${Q}$ ) in the WPP against a grid fault. To achieve this, each DFIG controller employs an adaptive reactive power to voltage ( $\textit{Q}-\textit{V}$ ) characteristic. The proposed adaptive $\textit{Q}-\textit{V}$ characteristic is temporally modified depending on the available ${Q}$ capability of a DFIG; it is dependent on the distance from a DFIG to the point of common coupling (PCC). The proposed characteristic secures more ${Q}$ reserve in the WPP than the fixed one. Furthermore, it allows DFIGs to promptly inject up to the ${Q}$ limit, thereby improving the PCC voltage support. To avert an overvoltage after the fault clearance, washout filters are implemented in the WPP and DFIG controllers; they can prevent a surplus ${Q}$ injection after the fault clearance by eliminating the accumulated values in the proportional-integral controllers of both controllers during the fault. Test results demonstrate that the scheme can improve the voltage support capability during the fault and suppress transient overvoltage after the fault clearance under scenarios of various system and fault conditions; therefore, it helps ensure grid resilience by supporting the voltage stability.

This paper proposes a torque limit-based inertial control scheme of a doubly-fed induction generator (DFIG) that supports the frequency control of a power system. If a frequency deviation occurs, the proposed scheme aims to release a large amount of kinetic energy (KE) stored in the rotating masses of a DFIG to raise the frequency nadir (FN). Upon detecting the event, the scheme instantly increases its output to the torque limit and then reduces the output with the rotor speed so that it converges to the stable operating range. To restore the rotor speed while causing a small second frequency dip (SFD), after the rotor speed converges the power reference is reduced by a small amount and maintained until it meets the reference for maximum power point tracking control. The test results demonstrate that the scheme can improve the FN and maximum rate of change of frequency while causing a small SFD in any wind conditions and in a power system that has a high penetration of wind power, and thus the scheme helps maintain the required level of system reliability. The scheme releases the KE from 2.9 times to 3.7 times the Hydro-Quebec requirement depending on the power reference.

This paper proposes a computationally effective method for optimal operation of multiple distributed generations (DGs) in dc distribution systems to improve their efficiency. There have been many research works to select the optimal sizes of multiple DGs. However, these optimal sizes are valid only in the particular operating point. For optimal operations of power systems, the DG sizes must be determined all the time according to the various load conditions. The proposed method can maintain the distribution system operating in its optimal condition with only single optimization process, without regard to load variations. Therefore, it can moderate the serious problems, derived from inaccurate load forecasting. To verify the performance of the proposed method, the dc system is first implemented with the real distribution system of Do-gok area in Seoul, Korea. Then, the real hourly-load data, acquired from SCADA system in Korea, are used to simulate the practical load variations.

If a large disturbance occurs in a power grid, two auxiliary loops for the inertial control of a wind turbine generator have been used: droop loop and rate of change of frequency (ROCOF) loop. Because their gains are fixed, difficulties arise in determining them suitable for all grid and wind conditions. This paper proposes a dynamic droop-based inertial control scheme of a doubly-fed induction generator (DFIG). The scheme aims to improve the frequency nadir (FN) and ensure stable operation of a DFIG. To achieve the first goal, the scheme uses a droop loop, but it dynamically changes its gain based on the ROCOF to release a large amount of kinetic energy during the initial stage of a disturbance. To do this, a shaping function that relates the droop to the ROCOF is used. To achieve the second goal, different shaping functions, which depend on rotor speeds, are used to give a large contribution in high wind conditions and prevent over-deceleration in low wind conditions during inertial control. The performance of the proposed scheme was investigated under various wind conditions using an EMTP-RV simulator. The results indicate that the scheme improves the FN and ensures stable operation of a DFIG.

This paper proposes a computationally efficient technique for estimating the composite load model parameters based on analytical similarity of parameter sensitivity. When the model parameters are updated in the optimization procedure to best fit the actual load dynamics, i.e., measurements, parameters of similar sensitivity representation in the given mathematical model structure are updated in the same manner at every iterative step. This research allows for practically reducing the number of load model parameters to be identified in the estimation process and the overall computational cost while preserving the desired complexity and accuracy of the original model. This approach consequently facilitates the parameter estimation in the optimization process and helps manage increased number of parameters often criticized for adopting the dynamic composite load model via measurement-based approach. Case studies for the real power system demonstrate the computational efficiency and intact accuracy of the proposed method with reference to the existing methods of estimating all the parameters of the given composite load model independently.

This paper proposes the new energy management method based on the photovoltaic (PV) hybrid power conditioning system of 4 kW with an energy storage device (ESD). The use of the ESD such as a lithium-ion battery improves the energy efficiency of the overall system depending on time and weather conditions. In addition, the proposed system provides the new function for energy management in real time without weather forecasting while it considers the power generations from the PV system, residential load variations, and electricity price. In other words, the proposed energy management system (EMS) can reduce the electricity price by charging the energy of nighttime electric power. In addition, it can suppress the grid voltage variations caused by the large amount of PV generations during daytime with respect to relatively less load demand. The performances of the proposed EMS system based on the 4-kW PV HPCS with a lithium-ion battery are evaluated by time-domain simulation. Thereafter, the experimental prototype is implemented in hardware to verify the usefulness of the proposed system.

This paper presents an effective optimization scheme for the measurement-based load modeling based on the sensitivity analysis of composite load model parameters. Each parameter of load model has different effects on its dynamic response. Moreover, some parameters are insensitive to the change of others. To estimate the dynamic interactions between parameters, their sensitivity is analyzed by using the eigenvalues of Hessian matrix used in the optimization algorithm. Also, the linear dependence between two load model parameters is then identified by examining the condition number of Jacobian matrix. With this parameter analysis, the performance of optimization process for measurement-based composite load modeling is improved by reducing the number of necessary parameters to consider. The performance of proposed method is verified with the practical data measured at a feeder in a real substation.

The islanding detection is one of the most important issues for grid-connection of the distributed generation (DG) systems. Several recent researches have developed two types of islanding detection methods (IDMs) with high-performance. One is the power and voltage (P-V) characteristic of load based IDM, and the other is the inverter's switching frequency based IDM. Although the above IDMs have no non-detection zone (NDZ) theoretically, their usefulness mainly depends on the control performance of the inverter in practice. In other words, the inverter must achieve the decoupled real and reactive control and successful operation of phase-locked loop (PLL). To deal with these problems, this paper describes the design of inverter controller to improve both system stability and islanding detection performance with the NDZ. The proposed controller uses the magnitude and phase of measured voltage directly instead of using the direct and quadrature axes voltages and currents. Additionally, it does not need an instant phase of point of common coupling (PCC) voltage, which is required in the conventional controller, and therefore makes it possible for the PLL independent control. Several case studies are carried out to verify that the proposed controller has better performance than the conventional controller.

The selection of placement and size for multiple distributed generators based on renewable energies becomes an important issue in operating modern power systems safely and reliably. This paper extends the previous study in our earlier paper by treating the problems for their optimal locations and sizes in more realistic way based on the power loss sensitivities and Kalman filter algorithm, respectively. In particular, the proposed method is implemented by the simulation on a part of practical distribution network of 60 MVA scale, which is located at the Do-gok area in Seoul, Korea. The new factor, called as the optimal locator index, is developed to determine the optimal locations in a systematic and effective manner by the power loss sensitivities. Also, the Kalman filter algorithm is applied to determine the optimal sizes by choosing the step size of 10 KW, which is a much smaller increment than the 10-MW step used in our earlier paper, without requiring high computational efforts. This selected step size of 10 KW is realistic and suitable to apply in practical test systems.

This paper describes a power management method in a feeder with distributed generations (DGs) and energy storage devices. To ensure stability, reliability, and safety of the system, the proposed power management method deals with the DGs in two groups, which are utility and independent power producer (IPP). When the feeder is islanded from grid, the utility-owned DG receives fault signal from the protection relay. Thereafter, it is allowed to operate itself when the feeder is islanded because it is directly controlled by the utility. In contrast, the IPP-owned DG should be stopped to avoid several problems, which can be caused by the islanding operation. Then, the proposed power management method operates the above DGs to maximize the efficiency with no serious impact to grid. To do so, the inverter controller of DG is designed to implement it in a part of practical distribution system, which is located at the Do-gok area in Seoul, Korea. Several case studies are carried out to verify the effectiveness of proposed method.

This paper presents the Euclidean norm based new power quality index (PQI), which is directly related to the distortion power generated from nonlinear loads, to apply for a practical distribution power network by improving the performance of the previous PQI proposed by the authors. The proposed PQI is formed as a combination of two factors, which are the electrical load composition rate (LCR) and the Euclidean norm of total harmonic distortions (THDs) in measured voltage and current waveforms. The reduced multivariate polynomial (RMP) model with the one-shot training property is applied to estimate the LCR. Based on the proposed PQI, the harmonic pollution ranking, which indicates how much negative effect each nonlinear load has on the point of common coupling (PCC) with respect to distortion power, is determined. Its effectiveness and validity are verified by the experimental results from its prototype's implementation in a laboratory with a single-phase 3 kW photovoltaic (PV) grid-connected inverter, which contributes to a small distortion in voltage at the PCC, and practical nonlinear loads. Then, the harmonic current injection model based time-domain simulations are carried out to prove the effectiveness of the proposed PQI under the other conditions with different nonlinear loads.

The number of photovoltaic (PV) generators has been significantly increased worldwide as a main renewable energy resource. The output power of PV generators mainly depends on the solar irradiance, module temperature, and field conditions. Therefore, the maintenance of the PV system is necessary to improve the reliability in its use. In this paper, a novel method to diagnose output power lowering in a PV array is proposed to keep the good performance of its output power based on the relationships between PV module temperature, current, and voltage under maximum power point conditions. In particular, the proposed method does not require the accurate information in specification on PV module and sensors for measuring irradiation levels, which are required in conventional monitoring systems. This results in the reduction of cost. Then, it is effectively applied to a practical PV system by using the Kalman-filter algorithm. The performances of the proposed diagnosis method are evaluated and verified with the laboratory experiment and field tests as well as the PSCAD/EMTDC-based simulation test.

This paper presents a novel method by the multilevel analysis (MLA) to estimate the composition rate of electric loads with respect to measured total current waveform by improving the performance of a previous estimation scheme proposed by the authors. The proposed MLA makes it possible to analyze the electric load composition rate (LCR), which indicates the portions of several typical loads connected to a single point of common coupling (PCC), with the partition of several levels. Then, the Kalman-filter (KF) algorithm is applied to solve the estimation problem of LCR based on formulation with current waveforms in each level. The effectiveness of MLA based on the KF in practice is verified by the experimental implementation based on the prototype's setup in laboratory. It consists of a 3-kW photovoltaic grid-connected inverter, which contributes to a small distortion in voltage at PCC, and practical nonlinear loads connected to PCC. Also, the harmonic-current-injection-model-based time-domain simulations are carried out to prove the potential of the proposed method under various circumstances with different nonlinear loads.

This paper presents design and control methods for fault-tolerant operations with redundant converter modules, one of the most prominent features in modular multilevel converter (MMC) topology. In fully implementing MMC functionalities, a nearest level control is applied as a low-switching modulation method. A dual sorting algorithm is newly proposed for effectively reducing the switching commutations of each power module as well as for voltage balancing control. Built upon these primary MMC topological and control features, its redundant operation is comprehensively investigated for fail-safe energy transfer. In particular, a novel spare process is proposed to handle an emergency situation when the number of faulty power modules exceeds the module redundancy. Since topological redundancy may cause the switching commutations of power modules in an arm to be unevenly distributed, a practical and effective mitigation measure is incorporated to keep the energy balance while avoiding the undesired switching stresses. Rigorous simulation studies for MMC and its application for high-voltage direct current are performed to demonstrate the validity and effectiveness of the proposed spare process under normal and emergency conditions.

To enhance performance of flash memory-based solid state disk (SSD), large logically chained blocks can be assembled by binding adjacent flash blocks across several flash memory chips. However, flash memory does not allow in-place overwriting and thus the operations that merge writes on these blocks suffer a visible decrease in performance. Furthermore, when small random writes are spread over the disk address space, performance tends to be degraded significantly. We thus present a technique to manage random writes efficiently to achieve stable SSD performance. In this paper, we propose a pattern adaptive SSD structure, which classifies access patterns as either random or sequential. The structure primarily consists of a write cache and a flash translation layer that separates groups of writes by access pattern (S-FTL). Separately managing the two types of write patterns enables greater parallelism and reduces the cost of large block management, thus enhancing the performance of the proposed SSD. Simulation experiments show that the proposed pattern adaptive structure can provide 39 percent decrease in extra flash block erase overhead on the average, and write performance can be improved by around 60 percent, compared with a basic FTL applied to existing parallel SSD structures.

When the wind-turbine generator system (WTGS) is connected to a radial distribution system, it can improve reliability of system by achieving the appropriate voltage regulation via its suitable control action. In this case, protective relay coordination and voltage stability issues become important factors in determining the optimal size of the WTGS, and moreover in evaluating its maximum capacity. In this paper, the proposed algorithm for selecting the optimal size of WTGS is applied based on a power flow analysis by treating the WTGS as a regulated voltage source, which is added to the related utility substation. Also, it considers the coordination time interval (CTI) of existing relays to maintain their present settings, and therefore provides an efficient solution in power system restructuring process by connecting the WTGS with its optimal size. The case study is carried out to verify that the proposed method can effectively determine the optimal size of WTGS under tightly constrained conditions resulting from voltage regulation and required protective relay coordination problems.

This paper describes a study to determine the optimal resistance of a superconducting fault current limiter (SFCL) connected to a wind-turbine generation system (WTGS) in series considering its protective coordination. The connection of the WTGS to the electric power grid may have serious effects on the stability and reliability of the overall system during a contingency due to the increase in fault currents. Moreover, it causes malfunction in the protective devices such as overcurrent relays (OCRs). To deal with this problem, the SFCL is applied to reduce the level of fault current increased by the connection of the WTGS. Then, to determine the optimal parameter of the SFCL, protective coordination between the existing multiple OCRs is considered based on the computation of fault current by the equivalent voltage source method. The effectiveness of the optimal resistance for the SFCL is evaluated by the time-domain simulation with the power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) software. The results show that protective coordination is well maintained without changing the previous settings of existing OCRs by applying the SFCL with its optimal parameter. Therefore, the system can keep its suitable reliability even when the level of fault current is increased by connecting the WTGS to the power system.