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The work is based on a collaboration between Hiroshima University and Kure KOSEN College. This paper presents the design concept, hardware, and applications of a single-phase synchronous inverter (SSI), a specially designed grid-forming inverter (GFM) for single-phase micro-grid (SMGs). The SSI is designed for the conventional 100/200 V distribution network and is based on the concept of "Non-Interference Core (NIC) dynamic model". Novel contributions of this paper are: (1) A root mean square (RMS) model of NIC-SSI was developed, combined with the conventional power system model, and verified through the comparison with the hardware-in-the-loop (HIL) simulation and SSI hardware experiments; (2) using the developed RMS simulation tool, the stabilization effect of the SSIs was investigated in condition under which the SSIs are massively installed in a distribution system; (3) off-grid SMG operations using SSIs under various ill-conditioned loads were demonstrated. The results show that the SSI has the considerable ability of grid stability enhancements for frequency, transient, and small-signal stabilities. The proposed SMG using SSIs is promising. [ABSTRACT FROM AUTHOR]

This paper summarizes our recent work on Grid‐Forming Inverter (GFM) application to power systems. We have developed a novel design of GFM, a single‐phase synchronous inverter (SSI) for the conventional 100/200V distribution network based on the concept of "non‐interference core (NIC) dynamic model." This paper first explains the design concept of NIC‐SSI and a simulation model of SSI for power system analysis. Then the stabilization effect is investigated, where the installation of SSI on the single‐phase consumer side is assumed as a new concept. The improvement of frequency and transient stability are evaluated. The SSI model is verified by comparing the developed root mean square (RMS) simulation, hardware‐in‐the‐loop (HIL) simulation, and the experiment using SSI hardware. The simulation results show that the SSI has the considerable ability of grid stabilization. Singlephase micro‐grid (SMG) operations using SSIs are also presented. The effectiveness of SMG operations is shown based on laboratory experiments. [ABSTRACT FROM AUTHOR]

Significant photovoltaic (PV) penetration accelerates voltage problems in distribution networks. We have so far studied what kind of low voltage system configurations are desired in the planning stage. In our previous studies, possible equipment measures were classified into several categories, formulating the 'low‐voltage system configuration determination problem'; in addition, a solution algorithm based on the practical priorities of classified measures was proposed. In this study, we propose a more efficient and comprehensive solution method while considering load distribution. Several numerical simulations were conducted to verify the performance of the proposed method. © 2023 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

This paper proposes a controller for single‐phase synchronous inverters (SSIs) that was designed to stabilize the performance of a grid while providing overcurrent protection during the occurrence of a fault. The proposed controller for an SSI eliminates the multiple feedback control loops consisting of voltage and current controllers under normal conditions to avoid the instability attributed to them. To output the designed voltage precisely calculated using the core circuit, including a swing equation and speed governor, we employ the latched limit strategy for the overcurrent protection, which is combined with a proportional resonance current controller. Numerical simulations and experiments demonstrated that the proposed controller provides the designed performance while ensuring overcurrent protection during normal operation. © 2023 The Authors. IEEJ Transactions on Electrical and Electronic Engineering published by Institute of Electrical Engineer of Japan and Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

Worldwide energy shortages and the green energy revolution have triggered an increase in the penetration of standalone microgrids. However, they have limited generation capacity and are wasteful when excess generated energy is curtailed. This presents an opportunity for the coordinated operation of multiple prosumer microgrids that absorb this waste to enhance their resiliency and reliability. This paper proposes a reliable prosumer model with an inbuilt energy management system (EMS) simulator that considers the mentioned deficiencies for constructing resilient interconnected prosumer microgrids. The EMS simulator operates in a real-time dynamic environment to coordinate the prosumer components and performs flexible switching for (1) prioritizing critical load by shedding the non-critical load and (2) meeting load demand locally or from other interconnected prosumers. The EMS simulator maintains energy balance by setting limits for the battery energy storage system (BESS) to preserve energy during low generation and performs real-time monitoring. The novelty of this model lies in its simplicity and flexibility, which allows interconnected prosumer microgrids to operate in cooperation without the need for communication. The proposed model is evaluated in a post-disaster off-grid scenario using a single-phase average value model that considers reasonable computation time. The simulation results of the proposed system indicate the preservation of stored energy while maintaining critical resources beyond three days of poor weather conditions. [ABSTRACT FROM AUTHOR]

In nowadays deregulated market, total transfer capability (TTC) calculation, which is the basis for evaluating available transfer capability (ATC), has been becoming more significant. During the last decade, transient stability constraints have been included in the optimal power flow approach to maximize TTC. However, no previous work on investigating the effect of generation shedding action to TTC has been reported. Therefore, in this paper, we propose a TTC maximization by means of transient stability constrained optimal power flow considering the generation shedding action. Proper selection of the generator to shed is based on its Lagrange multiplier value of transient stability constraint. Our simulation results show how TTC can be increased to anticipate possible generation shedding. Index Terms--Generation shedding, total transfer capability, transient stability constrained optimal power flow.

Zone 3 impedance relays are sensitive to power-flow distribution due to their overreaching property, and may undesirably operate in stressed system conditions with reactive power shortages or neighboring faults. In this paper, the distance of the apparent impedance to the protection boundary on the R-X plane is defined by the operation margin, which is expressed as the function of bus voltages. The sensitivities of the operation margin to bus voltages, bus powers, and shunt susceptances quantify the influence of various disturbances on the relay performance. The numerical results show: 1) the operation margin is quantified by the sensitivities with satisfying accuracy and less computation effort compared with power-flow schemes; 2) the variation of bus powers and shunt susceptances at different locations may improve or decrease the operation margin; and 3) the variation of active and reactive powers at the same bus may yield opposite effects on the operation margin. Index Terms--Bus power, impedance relay, operation margin, sensitivity analysis, shunt compensation, transmission system.

In recent years, massive introduction of photovoltaic power sources (PV) has compounded the problem of voltage fluctuation in distribution networks. The problem has been recognized from early on, and many research studies have been already reported. However, majority of the previous studies focused on high‐voltage distribution networks. Hence, in this study, the authors investigated appropriate configurations for low‐voltage distribution networks in Japan to address the problem as a planning problem. The on‐site practical measures were examined, and by means of the obtained knowledge, a network configuration problem was formulated for low‐voltage networks. In addition, an effective solution method is proposed, based on the priorities of measures applied to the network equipment. Numerical simulations were conducted for verifying the performance of the proposed method, wherein corroborative results were obtained. © 2021 Institute of Electrical Engineers of Japan. Published by Wiley Periodicals LLC. [ABSTRACT FROM AUTHOR]

The rapid increase in the installation of renewable energy sources, particularly solar photovoltaic (PV) sources associated with unbalanced features of distribution systems (DS), disturbs the classic control strategy of voltage regulation devices and causes voltage violation problems. This paper proposes an effective control strategy for voltage regulators in the DS based on the voltage sensitivity using a multi-agent system (MAS) architecture. The features of the unbalanced distribution system (UDS) with the PV and different types and configurations of voltage regulators are considered in the proposed strategy. The novelty of the proposed method lies in realizing both the control optimality of minimizing voltage violations and the flexibility to accommodate changes in the DS topology using an MAS scheme. An advantageous feature of using the MAS scheme is the robust control performance in normal operation and against system failure. Simulation studies have been conducted using IEEE 34-node and 123-node distribution test feeders considering high PV penetration and different sun profiles. The results show that the proposed voltage control strategy can optimally and effectively manage the voltage regulators in the UDS, which decrease their operation stresses and minimize the overall voltage deviation. [ABSTRACT FROM AUTHOR]

Recent power system operations are faced with the difficulties due to large penetration levels of intermitted renewable energy sources (RESs), for example, photovoltaic power (PV) generations and wind-turbine (WT) generations. Operations of a large number of uncontrollable RES also cause an unforeseen issue for the reliability in power system operation. To tackle the issue, this paper proposes a novel real-time generation schedule (GS) integrated with RES' power curtailment. The proposed real-time GS become to effectively compute the outputs of the associated generating units so as to meet the required electricity load. The GS computation is frequently updated every 5-min in real-time operation. The proposed approach is applied for real-time dynamic economic load dispatch for a real-time GS that completely treats the limited controllable energy resources under the uncertainties. Furthermore, real-time PV forecasting with their covariance matrices of the prediction errors is effectively utilized in the approach. In case of a unforeseen situation when the estimated electricity load cannot match the controllable unit's capability, the approach can be detect a minimum amount of supply-demand mismatch in advance, and can manage it reliably for the considerable time horizon. [ABSTRACT FROM AUTHOR]

This article presents a novel load frequency control (LFC) method using an adaptive internal model of a power system, where model predictive control (MPC) technique is applied to the internal model, which is being updated on‐line. The proposed method will improve the LFC performance by reducing model identification error and by handling the disturbance effectively. Novelty lies in the combination of MPC and the effective use of the internal model to meet the response time of real world LFC control, which is typically equivalent to generation dispatch control cycle. The effectiveness of the proposed control is confirmed by simulations using a three‐area power system model. The results show that the proposed method can accurately identify the target plant and successfully handle disturbances to realize a reliable LFC. © 2019 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

This paper proposes a monitoring and control method for transient stability (TS). First, it is clarified that there may exist various critical patterns of uncertain power flows causing instability due to photovoltaic generations. Then, the use of critical clearing time (CCT) is suggested as a TS index, which can be computed by conventional TS analysis tools. A distribution factor (DF) for controlling CCT is investigated, referred to as CCT‐DF in this paper. Then, three optimization problems using CCT‐DF are formulated based on bilevel optimization to obtain reliable operation planning. The combined use of the optimizations guarantees the power system robust security under uncertainties. [ABSTRACT FROM AUTHOR]

A novel design of a single‐phase synchronous inverter (SSI) having noninterference core dynamic performance is proposed, referred to as noninterference core SSI (NIC‐SSI) in this paper. The proposed inverter consists of "core" and "shell." This novel design is advantageous for a flexible setting of the core controller to enhance power system stability, which is supported by the shell function for keeping robust synchronizing operation of inverters including microgrid operation. The novel points in this paper are summarized as: (a) the proposed controller enables SSIs to implement directly a desired dynamic characteristic such as a synchronous machine; (b) it can connect any single‐phase systems as well as three‐phase systems; (c) an islanded single‐phase microgrid without rotating machines can be stabilized by multiple SSIs. The proposed controller makes it possible to enhance system stability against increasing uncontrollable renewable energy sources (RESs) and static power conversion devices. The developed SSI realizes flexible use in the demand side operations under normal/faulted grid conditions. Hardware‐in‐the‐loop (HIL) testbed including the proposed digital controller and the real‐time simulator validates the effectiveness of the proposed controller in the experimental conditions. [ABSTRACT FROM AUTHOR]

This paper proposes a real‐time generation regulation system named as "on‐demand generation regulation control" system, which is suitable for small independent power grids with unstable renewable energy generators. It has been developed as a part of "robust demand and supply control manager" and plays an important role of flexible frequency control that includes vehicle to grid and generation restriction when necessary. The main feature of the system is the integrated framework to effectively deal with hard‐to‐predict control options in a flexible way by observing the unstable generator outputs and all generators/batteries status including electric vehicles. In addition, a feedback system is also proposed to flexibly adjust the regulation capacity in case unexpected disturbances occur. The performance of the proposed system is verified through numerical simulations based on an actual test site with successful results. [ABSTRACT FROM AUTHOR]

Due to rapid expansions of renewable energy (RE) generations, it becomes more important to assess the feasibility of power system operation under limited controllable resources. Especially, exact evaluation of the system reserve for preserving system security is required under erroneous RE output predictions. This paper proposes a method to evaluate the size of the feasible region of power system operation in control space for the examination of the effective system reserve margin under uncertainties. Predicted RE and demands with their confidence intervals (CIs) are specified to formulate a problem for the evaluation of the size of the worst-case feasible region, where positive size implies feasibility, while negative, infeasibility. The method computes the degree of system security, which is referred to as “Robust Power System Security” in this paper. The problem is formulated as bi-level optimization, which is linearized and transformed into the mixed integer linear programming (MILP) problem. This is a new approach in the treatment of uncertainties. We use DC power flow and linear constrained dynamic economic dispatch problem to demonstrate the effectiveness of the proposed method. The proposed approach is useful in power system planning in analyzing the feasibility of dynamic real-time operation in future circumstance. [ABSTRACT FROM PUBLISHER]

SUMMARY In this paper, we propose three applications to quantitatively evaluate N-1 security with increased uncertainties. By using a robust dynamic security region (RDS), which is based on the concept of robust power system security (RS), the key region for judging the security and a new evaluation index for quantitative analysis, we can evaluate power system security as follows. (1) The calculation of the RDS at the time of yearly peak load demand can be recognized as the benchmark for the N-1 security at present, and this method can be applied to the future power system with increased renewable energy resources (RES) such as PV. (2) We show the yearly declining trend for N-1 security resulting from increased PV deployment year by year. (3) We calculate the necessary amount of additional power supply capacity, including conventional electric power generators or storage batteries, needed for maintaining the N-1 security at the present level. The effectiveness of the proposed method is demonstrated through simulations using a model system with three machines. [ABSTRACT FROM AUTHOR]

The rapid expansion of renewable energy (RE) generation is leading to severe problems related to the reliability of network operation. Power system security is threatened by increased uncertainties of RE outputs and reduced controllable resources. Therefore, it is important to evaluate the robustness of the system controllability in order to preserve system security against uncertainties of RE outputs. We refer to the robustness of the system in this context as 'robust power system security'. This paper proposes a new formulation for evaluating the robust security region in controllable parameter space where the conventional N-1 security criterion suffices in the presence of uncertainties. We specify the confidence interval (CI) of uncertain parameters and obtain the worst case feasible region defined by a set of security constraints, which is a very new approach in the treatment of uncertainties. We use the minimum set of static constraints in the load dispatching problem with CI of RE prediction to evaluate the security region, which is referred to as the robust static security region (RSS) in this paper. The proposed method is useful to assess the degree of difficulty of system planning and operation from the viewpoint of the balance between controllable generator capacity and RE uncertainties. © 2015 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

Impact of increasing penetration of photovoltaic (PV) generation is expected in Japan in the near future; thus a new voltage control scheme is required in order to improve the operation of distribution networks. We propose a new voltage control scheme for distribution networks based on a multi-agent system. The large voltage fluctuations caused by PV generations are effectively regulated by adjusting the conventional types of tap-changing controllers. We achieved an optimal control performance by developing a new optimal control law applicable to decentralized autonomous control. The autonomy is accomplished by using a multi-agent system. The proposed method is applicable to general distribution network and highly efficient for popular radial networks under certain approximations. The effectiveness of the proposed scheme is demonstrated through numerical simulations of radial 6.6-kV test system with successful results. [ABSTRACT FROM AUTHOR]

This paper proposes a new method for obtaining controlling unstable equilibrium point (CUEP) for transient stability analysis in electric power systems. A minimization problem is formulated to attain CUEP by applying the critical trajectory (CTrj) method to the boundary controlling unstable equilibrium point (BCU) method. CTrj method is a unique approach proposed by the authors to obtain critical condition for transient stability problem. The Proposed method simultaneously computes a trajectory on the stability boundary starting from the exit point reaching CUEP. The effectiveness of the method is demonstrated through simulations for various systems. [ABSTRACT FROM PUBLISHER]

Mega penetration of photovoltaic power generation (PV) will be introduced in future electric power systems, dealing with environmental problems such as global warming and energy exhaustion. The authors have pointed out that difficulties will arise in meeting the N-1 security standard if increases in uncertainties cannot be avoided due to PV penetration. Thus, we have proposed a new concept of 'robust power system security' together with several conditions to be satisfied in order to guarantee the N-1 security standard under uncertainties. In this paper, (1) a model of uncertainties due to PV generation is proposed based on robust power system security; a method for proper parameter selection is also provided. Then, (2) the influence on the stability of the PV and load disconnection from the power system is studied through simulation using the model system; (3) the effect of PV fault ride-through (FRT) on stability is investigated along with dynamic voltage support (DVS). Finally, it is shown that (4) complicated and burdensome tasks will be considerably increased in power system operation planning and real-time operation under future circumstances by the mega penetration of PV. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(3): 1-13, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22391 [ABSTRACT FROM AUTHOR]

The power market will be in a highly competitive environment because of power deregulation. The electric power companies have raised business efficiency to reduce the number of employees and centralize business establishments. Furthermore, the installation of photovoltaic generation will increase because of energy policy. However, the electric power companies must maintain power supply reliability. Therefore, to reinforce reliability in distribution systems is a significant problem. In this paper, we propose the optimal placement of distribution section switches to maintain power supply reliability in such a situation, and we present simulation results obtained for the original distribution system using a genetic algorithm. © 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(2): 36-43, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22375 [ABSTRACT FROM AUTHOR]

High penetration of renewable energy in the future power system will pose a big problem to the load dispatch operation. The large disturbance and high forecast error must be considered when scheduling a limited number of controllable generators to follow rapid change in load. This paper proposes a dynamic economic load dispatch (DELD) problem approach based on the concept of a feasible operation region (FOR). FOR is defined as the region that committed generators may operate in to match the load profile without violating the ramp-rate constraints. The DELD problem is solved in two stages. In the first stage, FOR of each generator is computed using recent real-time forecasted load as well as renewable energy generation. In the second stage, a generation schedule is determined by solving the DELD problem interval by interval while considering ramp-rate constraints and FOR constraints. The method can gives feasible solution for feasible load and specify the amount of compensation required for feasible solution for infeasible load. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc. [ABSTRACT FROM AUTHOR]

A large amount of PV penetration may introduce uncertainties in future power system planning and operations. This paper proposes the concept of 'Robust Security (RS)' and the 'RS region' in order to investigate power system security in the presence of a large amount of uncertainties. The RS region is defined as the region of power system operation where the system is secure under uncertainties. It is shown that the region tends to shrink and disappear for a high degree of PV penetration. Emerging problems concerned with security in future power systems are investigated. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 182(1): 30-38, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22289 [ABSTRACT FROM AUTHOR]

This paper presents an optimal allocation method for flexible ac transmission system (FACTS) devices for market-based power systems considering congestion relief and voltage stability. The purpose of the FACTS devices installation is to provide benefit for all entities accomplished by both minimizing annual device investment cost and maximizing annual benefit defined as difference between expected security cost (ESC) with and without FACTS devices installation. Different from previous approaches, the proposed method accurately evaluates the annual cost and benefits obtainable by FACTS devices installation by formulating a large-scale optimization problem that contains power flow analyses for a large number of system states representing annual power system operations. In addition, dynamic state transitions caused by specified contingencies are also simulated in the optimization problem to evaluate the effect of FACTS control actions as well as the other coordinated controls. The expected cost consists of operating cost under normal and contingency states along with their related probabilities to occur. Maximizing social welfare is the objective for normal state while minimizing compensations for generations re-scheduling and load shedding as well as maximizing social welfare are the objectives in case of contingency. Although installation cost of FACTS devices is required, they are useful as cost free means, which can reduce effectively the annual costs for generations re-scheduling and load shedding. [ABSTRACT FROM PUBLISHER]

This paper proposes a fast computation method for Available Transfer Capability (ATC) with respect to thermal and voltage magnitude limits. In the paper, ATC is formulated as an optimization problem. In order to obtain the efficiency for the N–1 outage contingency calculations, linear sensitivity methods are applied for screening and ranking all contingency selections with respect to the thermal and voltage magnitude limits margin to identify the severest case. In addition, homotopy functions are used for the generator QV constraints to reduce the maximum error of the linear estimation. Then, the Primal-Dual Interior Point Method (PDIPM) is used to solve the optimization problem for the severest case only, in which the solutions of ATC can be obtained efficiently. The effectiveness of the proposed method is demonstrated through IEEE 30, 57, 118-bus systems. © 2009 Wiley Periodicals, Inc. Electron Comm Jpn, 92(3): 17–23, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10049 [ABSTRACT FROM AUTHOR]

This paper proposes two types of PSS design methods that take into account robustness for comparably large power systems. The first one is a design method based on ∞ control theory and the second one is a parameter determination method for a standard PSS by using Particle Swarm Optimization (PSO). In order to deal with large-scale systems, a reduced model is developed to get the target system which preserves major oscillation modes only. The major oscillation modes are selected by using the residue concept, and the PSS is designed based on the target system. In order to verify effectiveness, the proposed methods are compared with the other previously proposed method based on a Genetic Algorithm (GA) through many numerical simulations. © 2008 Wiley Periodicals, Inc. Electron Comm Jpn, 91(8): 34–43, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10132 [ABSTRACT FROM AUTHOR]

Zone 3 impedance relays are sensitive to power-flow distribution due to their overreaching property, and may undesirably operate in stressed system conditions with reactive power shortages or neighboring faults. In this paper, the distance of the apparent impedance to the protection boundary on the R-X plane is defined by the operation margin, which is expressed as the function of bus voltages. The sensitivities of the operation margin to bus voltages, bus powers, and shunt susceptances quantify the influence of various disturbances on the relay performance. The numerical results show: 1) the operation margin is quantified by the sensitivities with satisfying accuracy and less computation effort compared with power-flow schemes; 2) the variation of bus powers and shunt susceptances at different locations may improve or decrease the operation margin; and 3) the variation of active and reactive powers at the same bus may yield opposite effects on the operation margin. [ABSTRACT FROM AUTHOR]

Recently, the evaluation of control performances of interconnected power systems has become an important issue with respect to individual load frequency controls. Several performance measures are in practical use such as Area Control Error (ACE), Mutual Aid Criterion (MAC), and Control Performance Standard (CPS). Area Requirement (AR) is an alternative expression for ACE, where AR=-ACE. However, the characteristics of these indices have not been sufficiently studied as far as the authors know. In particular, this is the case for their dynamic behavior against random load disturbances. This paper investigates the performance index, AR, through analysis of two-area systems. Major dynamic elements of interconnected systems affecting the frequency are taken into account to investigate responses of the system against load disturbances. The analysis shows that, when new systems are interconnected, the smaller system tends to require more effort of control against load disturbances to keep the same value of the performance indices. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 154(1): 20–28, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20233 [ABSTRACT FROM AUTHOR]