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This paper presents a methodology for optimizing the short-term operation of electric vehicle (EV) charging and discharging while considering the potential curtailment of active power due to volt-var control (VVC) prioritizing reactive power output. The proposed approach involves exchanging information between the EV aggregator and the distribution system operator (DSO). This approach allows the EV aggregator to optimize EV charge/discharge schedules while considering voltage-related constraints in the distribution system (DS). Initially, the aggregator shares the optimized schedule with the DSO to estimate the anticipated active power reduction through power flow analysis. Subsequently, the aggregator revises the constraint on active power output to avoid its expected curtailment and performs a second optimization for EV charging and discharging operation. Numerical simulations conducted on a realistic DS model in Japan validate the effectiveness of the proposed method in enhancing profitability in the day-ahead market while ensuring the quality of DS voltage. The results demonstrate an increase in profit by shifting the time of EV charging and discharging based on shared information from the DSO.

Grid-forming inverters (GFMs) have emerged as crucial components in modern power systems, facilitating the integration of renewable energy sources and enhancing grid stability. The significance of GFMs lies in their ability to autonomously establish grid voltage and frequency, enabling grids to form and improve system flexibility. Discussing control methods for grid-forming inverters is paramount due to their crucial role in shaping grid dynamics and ensuring reliable power delivery. This paper explores the fundamental and advanced control methods employed by GFMs, explaining their operational principles and performance characteristics. Basic control methods typically involve droop control, voltage and frequency regulation, and power-balancing techniques to maintain grid stability under varying operating conditions. Advanced control strategies encompass predictive control, model predictive control (MPC), and adaptive control, which influence advanced algorithms and real-time data for enhanced system responsiveness and efficiency. A detailed analysis and performance comparison of different control methods for GFM is presented, highlighting their strengths, limitations, and suitability for diverse grid environments. Through comprehensive studies, this research interprets the ability of various control strategies to mitigate grid disturbances, optimize power flow, and enhance overall system stability.

Due to the increasing burden on the traditional control systems, inverter-based resources (IBR) is required to take part in grid stability efforts. This is the key to increasing the share of IBRs in the power grid and ensuring that issues related to reduced inertia are handled properly. There have been some efforts towards developing frequency control for smart inverters such as frequency–watt​ mode. However, a fully functioning synthetic inertia mode combined with a smart inverter has not been developed yet. In this paper, a novel function is developed for implementing synthetic inertia in smart inverters as a combination of several control modes. Then, the operating curves of these control modes are varied to investigate their impact on the synthetic inertia’s performance and stability. These operations are tested both with lab equipment and controller hardware in the loop (CHIL) testing to study the accuracy of the latter. Once accuracy is established, further CHIL tests have been performed to investigate the interaction between the developed synthetic inertia function and other smart inverter functions such as Volt-Var Control. Results show that there is no negative impact on either of the functions and they can be safely activated simultaneously.

The increasing share of renewable energy-based resources in the power grid has a detrimental impact on the power system stability. As these resources are inverter-based, they decrease the overall inertia of the grid. To mitigate this problem, recent research focuses on developing novel techniques to provide synthetic inertia with inverters. In this fashion, inverter-based resources (IBR) can also contribute to power system inertia, hence, its stability. Some of the approaches for providing synthetic inertia are discussed in the standards. However, despite its potential, frequency-derivate-based (df/dt) solutions are not studied to adapt grid code in Japan. This paper addresses this need by developing a full df/dt control approach for IBRs. Several intricacies of this design is presented along with operation curves and system designs. Considering the special needs of the Japanese Electrical Grid, and the speed in which inertia support should be provided, a novel rate of change of frequency (RoCoF) measurement approach is also developed and presented. Lab tests are performed with hardware-in-the-loop tests and the results validate the operation and performance of both novel RoCoF measurement and df/dt control model.

Microgrids consisting of renewable energy based distributed generators have become popular as a way of energizing off-grid systems. Due to their low-inertia, these distributed generators require a robust frequency control system to maintain it. This problem is amplified by the fact that there is no centralized grid that anchor the system voltage and frequency. For this reason, such off-grid microgrid employs storage systems and diesel generators to provide some flexibility. Flywheel energy storage systems (FESSs) have very quick reaction time and can provide frequency support in case of deviations. To this end, this paper develops and presents a microgrid frequency control system with FESS. The system performance tests are performed with real-equipment where FESS is connected to digital real time simulator. Several frequency events are triggered and the frequency profile is observed to compare microgrid’s performance with and without the developed frequency control. Results show that flywheel energy storage system provides a much smoother frequency profile with less deviations.

Implementation of renewable energy sources (RESs) in power systems can reduce the dependence on fossil-fuel-based thermal power generation systems. At the same time, however, the system inertia decreases as synchronous generators decrease; this is crucial for maintaining the stability of the power system. Virtual inertia control (VIC) can regulate the output of an inverter-based resource (IBR) by increasing the inertia. For a wind turbine generator (WTG), output control factors such as pitch angle control and maximum power point tracking (MPPT) significantly affect the performance of the VIC. This paper theoretically clarifies that the pitch angle control contributes to improvements in the performance of the VIC and helps avoid a loss of operation of the WTG in frequency drop events by analyzing the movement of the operational point of the WTG based on the physical characteristic of the WTG and control characteristics of the pitch angle control and MPPT; an electromagnetic transient (EMT) simulation, performed to verify the analysis, is also presented.

With the growing shift toward renewable energy, attention focused on the disadvantages of current control inverter-based distributed generators, such as lack of grid-forming ability and inertia. To provide inertia support and grid-forming ability, virtual synchronous generators (VSGs) have been proposed. Various control methods related to VSGs have been reported, and various research challenges have been evaluated. However, only a few experimental demonstrations have been addressed. Recommended practices for VSGs were launched as IEEE Standard Project 2988 on March 25, 2021, with VSGs considered to be a new technology that is nearly ready to be put into practical use. Therefore, experimental demonstrations on a real-scale grid are important for establishing a technical standard for VSGs. In this study, a grid was built by simulating an actual microgrid on an approximately 1:1 power capacity scale. In the simulated grid, 125 kVA was provided by a diesel SG, five 20 kVA inverters with a battery energy system (BES) were used, and the power generation penetration of the inverter BES units achieved 44.4%. Six different penetration scenarios for operation of a grid forming inverter with BES (GFM-BES) and a grid following inverter with BES (GFL-BES) were evaluated comparatively. System inertia, and transient and steady state power sharing characteristics affected by different degrees of GFM-BES penetration under a sudden load change were demonstrated. Moreover, characteristics related to grid forming, synchronization, frequency regulation, and power regulation were demonstrated through field tests. The simulations and experiments also evaluated the inertia constant of a diesel SG, which is expected to be helpful for future simulations on this topic. The findings of this paper provide important instructions for engineers to model, design, and test distributed generators.

Renewable energy-based generators are constantly being deployed to future grids. It is expected that their share in overall generation will increase in the future. These novel devices have unknown characteristics and cause novel issues in power system operation. Traditional distribution networks have been operated as passive networks. These devices, such as smart inverters, change this paradigm completely. Due to these considerations, grid operators insist on enforcing strict grid-integration requirements. These rules are developed to ensure the impact of the connected devices is minimized and their behavior can be accounted for, at least to some extent. Testing different devices for different grid codes is a daunting task. Since such tests are undertaken in lab environment with manual control and data collection, they are prone to errors, time-consuming and inefficient. A solution is required to standardize and automate such tests. This will provide consistent testing ability and minimize testing times and errors due to human-intervention. This article presents the design and implementation of an integrated testing platform. Steps of lab equipment integration and associated challenges are presented along with their solutions. Several smart inverter behavior tests are executed, and results are presented. The test durations are compared with traditional test durations and the benefits are reported. It is discovered that use of such platform can increase the system testing efficiency by 85 % while minimizing human-errors, inconsistencies and man-hours required to run the tests.

The ratio between the photovoltaic (PV) array capacity and that of the inverter (INV), PV-INV ratio, is an important parameter that effects the sizing and profitability of a PV project. It is important to find the balance between cutting down costs by under-sizing the inverter and maximizing profits by generating more energy. This becomes much more important where smart inverters with voltage control abilities are utilized. Smart Inverters have the ability to actively take part in voltage control and, thus, increase the injected power. This would require a larger capacity than usual, as some reactive power flow needs to be accommodated to limit the voltage rise. Excess capacity can be utilized to implement smart inverter functionalities and inject more energy under conditions where conventional inverters would cap their generation. Furthermore, PV-INV ratio studies in the literature focus on large-scale, grid-connected PV systems. This paper focuses on investigating PV-INV ratio for residential PV systems with smart inverters. These are connected to low-voltage distribution systems where voltage rise issue is more apparent. A new simulation tool that can model smart inverter functionalities is utilized to investigate the impact of PV-INV ratio on overall power generation. Different smart inverter functions are implemented for comparison. Based on simulation results, the overall costs and power generation are documented for different PV-INV ratios. Finally, a cost-benefit analysis is performed to find out which PV-INV ratio yields the maximum profits with minimum costs.

The power hardware-in-the-loop (PHIL) simulation has become a popular testing approach due to the flexibility it provides and the high-fidelity of its results. It is expected to be utilized as an advanced laboratory testing scheme to validate the grid support functions of distributed energy resources (DERs) because it can evaluate the interaction between the power system and DERs. Despite the strong demand to utilize the PHIL simulation for such testing, the literature that elaborates on the practical design of PHIL simulation based testing (hereafter called “PHIL testing”) environment including laboratory device setup, power system models, and test procedures is very limited. The simulation models, interfacing with the tested equipment, and data collection approaches are all different parameters that need to be fine-tuned for the successful execution of PHIL testing. It is vital for such successful test experiences to be shared to build universal knowledge around PHIL testing. In order to fill this knowledge gap, this paper presents such practical and essential techniques for the PHIL testing to share the knowledge for promotion of the PHIL simulation utilization. The development of PHIL testing environment to validate the smart inverter functions, i.e., volt-var function and frequency-watt function, is focused on in terms of laboratory setup, power system modeling, interfacing, and test procedure. The volt-var and frequency-watt functions of a 500 kW smart inverter of photovoltaic are validated on the basis of the presented techniques. Detailed test configurations, test procedures, and simulation models are presented along with obtained test results.

Penetration of renewable energy based distributed generators into power networks is limited. This is due to several reasons such as intermittency of the generation, low-inertia during disturbances and high variation of local voltage and frequency. Traditionally, distribution networks are designed for a finite voltage drop with distance from feeder connection. When PV panels are deployed at households, their impact on the local voltage profile becomes very substantial. Especially, when there is high solar radiation, generated power increases and the local voltage exceeds the permittable limits. Smart inverters are introduced to tackle this problem. Unlike other inverters, they can follow the local network measurements and provide auxiliary voltage and frequency support. With these features, smart inverters have the ability to push the renewable energy penetration level further. To investigate this phenomenon, custom smart inverter models are developed. Power flow calculation steps are modified to include smart inverter's active inputs. Then, feeder hosting capacity studies were run with conventional and smart inverters to investigate the improvement. Developed models, modified power flow algorithm and results of simulations are reported. Findings show renewable energy share can be increased in networks with smart inverter deployments only, without making substantial changes to rest of the network.

The increase in inverter-based resources associated with the increased installation of PV sources is a concern because it reduces the inertia of the power system during peak PV generation periods. As a countermeasure to reduce grid inertia, the addition of pseudo-inertia using virtual synchronous machines can be selected, and PV generation can cooperatively contribute to the stable operation of the power system by using the suppressed output as reserve power. However, few studies have analyzed VSMs that do not use batteries and use PV as a resource (PV-VSM) in simulations, including grid interconnection and solar radiation fluctuations, and it is necessary to clarify the issues and discuss countermeasures. In this study, electromagnetic transient response analysis was applied to a VSM connected to a two-generator system, simulations were performed, and the following findings were reported and countermeasure methods for the problem were proposed. When the PV capacity is insufficient for the output required by the VSM inverter, the PV-VSM control system may become unstable. This is caused by a drop in the capacitor voltage of the DC/DC converter due to insufficient PV output. The limiter control system is designed to address this problem by combining the headroom estimation system with the current limiting algorithm. The proposed limiter control system is validated on solar radiation ramp fluctuations as a test case and found that the system was effective in supressing PV-VSM instability. In our simulation case, the PV-VSM with our limiter control can continue to operate stably even if the PV available power is 0.03 [p.u.] short of the inverter’s reference power by the solar power ramp fluctuation, as long as the inverter installation rate is less than 50%.

Virtual inertia control is a methodology to make inverter-based resources (IBR) behave like a synchronous machine. However, an IBR cannot fully emulate the response of synchronous machine because of its low-current capacity. When the inertial response of an IBR is affected by the current limitation, the synchronization of the synchronous machine simulated virtually inside the IBR controller with the other synchronous generators in the grid is affected, which may cause step-out of the simulated generator. We propose a methodology which can keep the synchronization by modifying internal induced voltage of the simulated generator to follow the system voltage change. The proposal is validated by the simulation using a nine-bus transmission system model including two synchronous generators and a large-scale IBR. The result of the generator trip simulation shows that the proposed method suppresses the phase angle variation while the current is limited, and avoids the instability regarding the synchronism. Furthermore, the impact of the current limitation on frequency stability is also evaluated through the simulation study and it is found that as the amount of output suppression increases, the frequency nadir falls, but the rate-of-change of frequency is hardly affected.

The high penetration of the distributed energy resources (DERs) encourages themselves to implement grid-supporting functions, such as volt-var control. The quasi-static time-series (QSTS) simulation is an essential technique to evaluate the impact of active DERs on the grid. Meanwhile, the increase of complexity on the circuit model causes a heavy computational burden of QSTS simulation. Although circuit reduction methods have been proposed, there have been few methods that can appropriately handle the distribution system (DS) with multiple voltage control devices, such as DERs implementing volt-var control. To address the remaining issues, this paper proposes an offline bus aggregation method for DS with volt-var control. The method determines the volt-var curve for the aggregated bus on the basis of historical data to reduce error in the aggregated model, and its offline process solves the computational convergence issue concerned in the online one. The effectiveness of the proposed method is validated in the simulation using a Japanese low-voltage DS model. The simulation results show that the proposed method can reduce the voltage error and computational time. Furthermore, the versatility of the proposed method is verified to show the performance does not heavily depend on how to select historical data for model-building.

Inertia reduction due to inverter-based resource (IBR) penetration deteriorates power system stability, which can be addressed using virtual inertia (VI) control. There are two types of implementation methods for VI control: grid-following (GFL) and grid-forming (GFM). There is an apparent difference among them for the voltage regulation capability, because the GFM controls IBR to act as a voltage source and GFL controls it to act as a current source. The difference affects the performance of the VI control function, because stable voltage conditions help the inertial response to contribute to system stability. However, GFL can provide the voltage control function with reactive power controllability, and it can be activated simultaneously with the VI control function. This study analyzes the performance of GFL-type VI control with a voltage control function for frequency stability improvement. The results show that the voltage control function decreases the voltage variation caused by the fault, improving the responsivity of the VI function. In addition, it is found that the voltage control is effective in suppressing the power swing among synchronous generators. The clarification of the contribution of the voltage control function to the performance of the VI control is novelty of this paper.

Required functions of a microgrid become divers because there are many possible configurations that depend on the location. In order to effectively implement the microgrid system, which consists of a microgrid controller and components with distributed energy resources (DERs), thorough tests should be run to validate controller operation for possible operating conditions. Power-hardware-in-the-loop (PHIL) simulation is a validation method that allows different configurations and yields reliable results. However, PHIL configuration for testing the microgrid controller that can evaluate the communication between a microgrid controller and components as well as the power interaction among microgrid components has not been discussed. Additionally, the difference of the power rating of microgrid components between the deployment site and the test lab needs to be adjusted. In this paper, we configured the PHIL environment, which integrates various equipment in the laboratory with a digital real-time simulation (DRTS), to address these two issues of microgrid controller testing. The test in the configured PHIL environment validated two main functions of the microgrid controller, which supports the diesel generator set operations by controlling the DER, regarding single function and simultaneously activated multiple functions.

Under the auspices of the New Sunshine Program and continuous R&D programs by the New Energy Development and Industrial Technology Organization (NEDO), the authors have been implementing a measurement and evaluation program for photovoltaic (PV) systems since the fiscal year 1997. In this program, a total of 100 residential PV systems, equipped with special data acquisition systems, have been set up over seven years. The purpose of this study was to clarify the operating performance of the grid‐connected PV systems on the rooftops of residential houses in Japan and to develop a simulation methodology in order to estimate the electricity generation and costs in the actual housing environment. The validity of the simulation methodology was assessed by using the actually monitored data from some hundreds residential PV systems. Simulation results were also used to optimize the PV system design as well as to diagnose their operating conditions. The mean value of the final annual yield was around 1000 h; 975 h in 2000, 982 h in 2001 and 975 h in 2002, and the mean value of the performance ratio was over 70%; 73·3% in 2000, 71·8% in 2001 and 72·5% in 2001. Copyright © 2004 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Two types of crystalline silicon (c-Si) photovoltaic (PV) modules have been tested in the cold-dry climate of the Gobi Desert of Mongolia, from 2002 to 2012, to verify the preliminary estimation of PV power generation. This study mainly focuses on the analysis of the long-term performance of the PV modules using indices such as energy yield, reference yield, performance ratio (PR), and characterization indices of the main electrical parameters as well as degradation rate. Average annual energy yields were 1880 h for Module 1 and 1789 h for Module 2. Thus, PRs were 0.85 and 0.80, while the average annual reference yield was 2223 h. On the other hand, degradation rates of Modules 1 and 2 were −1.28 and −0.86%/year, respectively. The electrical degradation observed in both modules was due to the loss of short-circuit current as open-circuit voltage reduction was not considered in this field test. [ABSTRACT FROM AUTHOR]