Smart Photovoltaic Inverters Research Articles

Characterization of Voltage Stabilization Functions of Residential PV Inverters in a Power Hardware-in-the-Loop Environment

The exponential growth of Photovoltaic (PV) technology is creating concerns for electric grid operators. As PV penetration increases, overvoltage in the distribution network can occur due to a mismatch between PV generation and load demand. However, PV smart inverters can be part of the solution to stabilize grid voltage. By providing reactive power and other grid supporting functions, PV inverters in a distribution network can mitigate this problem and enable a higher integration of renewable energy. To accomplish this, characterization and testing of advanced functions must be performed at a small scale before deploying these strategies in a field. In this work, we described in detail the components and communication interfaces of a Hardware-in-the-Loop testbed that includes two 3.8 kW PV inverters from different manufacturers. We conducted efficiency tests on the inverters and characterized the grid supporting functions for grid voltage stabilization, specifically constant power factor, volt-var, and volt-watt. We identified some abnormalities in the operation of the volt-var-watt control in one of the inverters and presented a method to overcome the limitation in remote control of another inverter using Modbus communication. Identifying, understanding, and overcoming shortcomings on the operation of PV smart inverters that provide grid supporting functions is key for the quick adoption of this technology and can help regulatory agencies to determine what is the appropriate control mode that will facilitate higher PV capacity. Additionally, we discuss the economic and technical implications of operating the inverter in active or reactive power grid control.

Adaptive Volt–Var Control in Smart PV Inverter for Mitigating Voltage Unbalance at PCC Using Multiagent Deep Reinforcement Learning

Modern distribution networks face an increasing number of challenges in maintaining balanced grid voltages because of the rapid increase in single-phase distributed generators. Because of the proliferation of inverter-based resources, such as photovoltaic (PV) resources, in distribution networks, a novel method is proposed for mitigating voltage unbalance at the point of common coupling by tuning the volt–var curve of each PV inverter through a day-ahead deep reinforcement learning training platform with forecast data in a digital twin grid. The proposed strategy uses proximal policy optimization, which can effectively search for a global optimal solution. Deep reinforcement learning has a major advantage in that the calculation time required to derive an optimal action in the smart inverter can be significantly reduced. In the proposed framework, multiple agents with multiple inverters require information on the load consumption and active power output of each PV inverter. The results demonstrate the effectiveness of the proposed control strategy on the modified IEEE 13 standard bus systems with time-varying load and PV profiles. A comparison of the effect on voltage unbalance mitigation shows that the proposed inverter can address voltage unbalance issues more efficiently than a fixed droop inverter.

Mitigation of overvoltage due to high penetration of solar photovoltaics using smart inverters volt/var control

The modern photovoltaic (PV) inverters are embedded with smart control capabilities such as Volt/Var and Volt/Watt functions to mitigate overvoltage issues. The Volt/Var control has gained a significant attention in regulating grid voltage through reactive power compensation. However, the reactive power capability of a PV inverter is limited during peak irradiance and could be improved by curtailing the active power generation and by oversizing the PV inverter. This paper analyzes the performance of Volt/Var function of smart PV inverters in mitigating overvoltage issues due to high PV integration and thus increasing the hosting capacity of low voltage distribution networks (LVDNs). The study is conducted on a real Malaysian LVDN considering two different Volt/Var set points under different PV penetration levels. Results demonstrate that the oversized smart PV inverter could enhance the Volt/Var functionality by increasing its reactive power capability than a typical smart PV inverter. Further it reveals that adaptation of sensitive Volt/Var set points with shorter deadbands increase the PV hosting capacity of LVDNs.

Grid-friendly power control for smart photovoltaic systems

The still increasing penetration of power electronics into the modern power systems challenges the entire system stability, which requires more advanced control strategies to address the issues. One of the challenges is the variability of renewable energies, including photovoltaic (PV) systems, which are generally operating with uncertainty and intermittency (non-dispatchable). In this regard, flexible power control solutions are of interest for PV systems, as an essential function of smart PV inverters, to minimize the adverse impact in grid-integration and operation. On the other hand, PV systems can be adapted to provide ancillary services, e.g., voltage and frequency support through the power control. This paper thus presents an overview of the recent advances in flexible active power control (FAPC) that enables the grid-friendly integration of smart PV systems. The demands for the FAPC are introduced from the grid’s perspective. Then, various FAPC schemes are reviewed, among which the control strategy by modifying the maximum power point tracking (MPPT) is the most feasible and efficient without any hardware modifications. This is known as the flexible power point tracking (FPPT), which is further illustrated by case studies. Additionally, a power reserve control (PRC) facilitating comprehensive voltage and frequency support to the grid is discussed in detail. Future research perspectives are also presented.

Autonomous reactive power support for smart photovoltaic inverter based on real-time grid-impedance measurements of a weak grid

A large share of renewable energy production is connected to a weak grid with significant grid impedance. The transmission impedance causes unintended flow of reactive power to grid, coupling the grid reactive power to the active power fed from the inverter. The reactive power causes transmission losses, strains the grid with reactive power requirements, and can even compromise system stability. Requirements on reactive power support were recently imposed on new photovoltaic inverters, which are often implemented with proportional power factor control or droop control for local voltage regulation. The present work proposes a method for real-time compensation of the unintended reactive power, which decouples the reactive power from the active power of a photovoltaic inverter. Based on real-time measurement of the grid impedance, the unintended reactive power is estimated and autonomously compensated in the inverter. The method removes the fluctuating reactive power component, while still permitting unrestricted manual control of the reactive power. Unlike conventional methods, the proposed method requires no prior knowledge on grid impedance values or delicate tuning. The method outperforms conventional power factor control even when the conventional method is tuned optimally with known grid inductance. The method is validated with simulations and experiments on three-phase photovoltaic inverter connected to a weak grid.

EVALUATION OF DISTRIBUTED ENERGY RESOURCE INTERCONNECTION CODES AND GRID ANCILLARY SERVICES OF PHOTOVOLTAIC INVERTERS: A CASE STUDY ON DUBAI SOLAR PROGRAMME

This paper evaluates the technical aspects of grid-connected photovoltaic (PV) systems and distributed energy resources (DERs) interconnection grid codes. The advanced functions of smart PV inverters and smart grid solutions are discussed as well as the gaps of the existing grid codes that hinder DER ancillary services. An online survey targeted the key stakeholders and industry experts have been conducted to investigate advanced inverters potential of providing DER ancillary services to distribution grids. The survey results are discussed in details and recommendations for the reactive power support of DER inverters and DER interconnection codes enhancements are presented. Keywords: Distributed Energy Sources, Distributed Energy Resource Interconnection Grid Codes, Distributed Energy Resource Ancillary Services, Distributed Energy Resource Integration, Advanced Inverters Capabilities, Volt/Var Control JEL Classifications: P48, Q42, Q48 DOI: https://doi.org/10.32479/ijeep.8758

Developing Power Hardware-in-the-Loop Based Testing Environment for Volt-Var and Frequency-Watt Functions of 500 kW Photovoltaic Smart Inverter

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.

Active power control to mitigate voltage and frequency deviations for the smart grid using smart PV inverters

Large-scale deployments of variable renewable energy sources, such as photovoltaic (PV) systems, are increasing around the world. PV systems are a potential source of clean energy but they experience power intermittency and lack controllability. However, smart PV inverters provide grid-friendly functionalities that control the power output of PV systems. Power intermittency of PV systems causes major problems such as voltage fluctuations and frequency deviations in an electric power grid. Together with varying loads and other renewable distributed generations, the grid frequency and voltage become difficult to manage. A smart PV inverter allows for active power curtailment, volt-watt, and frequency-watt control systems to mitigate the voltage and frequency deviations for a smart power grid. Therefore, smart PV inverters can reduce the installation of voltage and frequency control devices in an electric power grid, which in turn can reduce installation and maintenance costs. Real-time analyses of active power curtailment, volt-watt control, frequency-watt control using smart PV inverters and their effects on voltage and frequency are presented in this paper. Power curtailment operations were executed in low and high PV penetration areas. From the experimental analyses, distributed voltage deviations can be mitigated using active power curtailment and volt-watt control systems, and the over-frequency of an electric power grid can be reduced using frequency-watt control method. Controller performances of smart PV inverters are also evaluated in this paper. The experimental analyses were executed in the Maui Advanced Solar Initiative Project.

Reconfigurable and flexible voltage control strategy using smart PV inverters with integrated energy storage for advanced distribution systems

A novel circuit topology is proposed for utility-owned photovoltaic (PV) inverters with integrated battery energy storage system (BESS) and compared to two state-of-the-art configurations. The proposed topology offers flexibility and can be applied to a range of distribution networks for tight voltage regulation. During BESS maintenance, the solar-storage system reconfigures itself for a self-run mode of operation, and actively compensates high penetration induced voltage fluctuation without activating overcurrent protection of the inverter, which is an added advantage of this strategy. This advantage is achieved by slightly increasing the inverter size to reserve a portion of inverter's current-carrying capability. A dynamic model of the new configuration is also developed to analyse its performance in providing fast response for high ramp up/down solar irradiance variation. As the proposed control strategy is implemented at the device level, the local voltage regulation is quite guaranteed to be in the permissible range. Results from the analysis performed on a modified IEEE 33 bus medium voltage distribution network with multiple inverters show evidence that the proposed strategy has the potential to mitigate voltage fluctuation in several extreme cases.

Combined Impact of Network Reconfiguration and Volt-VAR Control Devices on Energy Savings in the Presence of Distributed Generation

Reduction of energy consumption and energy losses is a major concern of the distribution network operator in the present scenario. Traditionally, on-load tap changers, a shunt capacitor bank, and a voltage regulator have been employed as volt-var control (VVC) devices for savings in energy consumption and losses. In this paper, an efficient and optimally coordinated operation of traditional VVC devices, a distribution network reconfiguration (DNR), and a photovoltaic smart inverter (PVSI) for energy savings has been proposed. In order to achieve the optimal solution, a modified binary gray wolf optimization (MBGWO) algorithm has been proposed. Besides, the proposed method in association with PVSI droop control has been employed to control the voltage violations during cloudy day condition. Furthermore, the proposed method has been employed for service restoration considering voltage regulation and peak demand reduction under faulty condition. For validation, the performance of the proposed algorithm has been tested on balanced as well as unbalanced distribution systems. The test results demonstrate the significance of the DNR associated with VVC devices and PVSI for energy savings under different load models. Outcome of the proposed algorithm has been compared with other existing metaheuristic algorithms and test results demonstrate the benefit of the proposed method.

Smart Inverter Volt-Watt Control Design in High PV-Penetrated Distribution Systems

Advanced control techniques such as volt-watt and volt-VAR control have been developed for high integration of distributed renewable energy, such as photovoltaic (PV) resources, on an electric distribution system. However, designing these control parameters, which yields the best results in the system, is complicated and depends on feeder conditions. This paper proposes a method to properly design the volt-watt control parameters in smart PV inverters to increase the benefit of their control action. The intention of this control design is to mitigate the voltage violations in a high PV-penetrated distribution feeder, while evenly distributing the weight of energy curtailment among all PV systems. Test results are provided from simulation-only scenarios and a hardware-in-the-loop test platform.

Efficacy of a Smart Photovoltaic inverter as a virtual detuner for mitigating Network Harmonic Resonance in Distribution Systems

As the grid is becoming smarter, more challenges are encountered with the integration of Photovoltaic plants in distribution systems. The real power generation varies based on the insolation from the sun. As a result, the fluctuations in voltages are seen as transient adversaries in the system causing the Photovoltaic inverters to trip. Smart Photovoltaic inverters nowadays are equipped with specialized controllers for exchanging reactive power with the grid based on the available capacity of the inverter, after the real power generation. With the ability of operation in VAR mode, Smart Photovoltaic inverters could regulate the voltage based on volt/VAR control, facilitate the power factor correction of the local loads and provide VAR support to ride through the voltage deviations during faults. Although the research that are undergoing are pertaining to all the aforesaid applications of Smart Photovoltaic inverters, employing a smart Photovoltaic inverter for resonance mitigation is a novel approach. U.S. based standard IEEE 519 for power quality describes the network resonance as a major contributor that has an impact on the harmonic levels. This paper proposes a new application in utilizing a Smart inverter to act as a virtual detuner in mitigating network resonance, based on the VAR mode control strategy with anti-windup and droop characteristics scheme that are incorporated with the design of Smart inverters. This novel application would further serve as a pioneering approach for researchers and planning engineers working in distribution systems as the grid gets more and more smarter.

Peer-to-peer-based integrated grid voltage support function for smart photovoltaic inverters

The increasing penetration of renewable energy sources is impacting the operation of the distribution grid. Whilst they are currently placing a burden on the distribution grid, it is generally agreed that these sources could also be used for active grid control, thereby contributing to a stable and secure grid. Smart photovoltaic inverters can contribute to active grid control, by expanding their features with added functions. In this paper, we develop a novel grid voltage support algorithm for smart photovoltaic inverters, based on distributed optimization and peer-to-peer communication. The Jacobi-Proximal Alternating Direction Method of Multipliers is applied in this paper to locally optimize reactive power compensation and active power curtailment of each inverter participating in the voltage control. We propose the use of a push-sum gossip protocol to enable peer-to-peer data interchange between inverters. We also use strategies to improve the robustness of the proposed algorithm: an acceleration strategy, a feedback strategy and an anti-windup strategy. A case study is presented, demonstrating the fast convergence of the algorithm and its ability to solve the voltage problems.

Distributed dynamic grid support using smart PV inverters during unbalanced grid faults

A dynamic voltage support strategy using smart photovoltaic (PV) inverters during unbalanced grid faults events is proposed. It uses Karush-Kuhn-Tucker condition for finding optimal solutions to calculate the inverter's active and reactive current references. The proposed methodology also takes the X/R ratio into consideration which allows the inverter to differentiate weak or strong grid conditions and adjust its reference currents. Existing multiple-complex coefficient-filter based phase locked loop is used to extract the positive and negative sequence components. The proposed strategy deploys existing dual vector current control to ensure optimal current injection and low voltage ride through. A distributed ride-through coordination approach among multiple inverters is also proposed based on different optimisation goals - either fundamental positive or fundamental negative sequence voltage support. The strategy is simulated, and inverter's transient performance is experimentally verified on a modified IEEE-13 bus test feeder using controller hardware-in-the-loop approach. Results show substantial evidence that the proposed method can be successfully applied to support the grid during an unbalanced fault event.

Coordination of Transformer On-Load Tap Changer and PV Smart Inverters for Voltage Control of Distribution Feeders

The increasing penetration of photovoltaic (PV) generation in distribution systems has entailed dramatic deterioration of distribution system voltage quality due to power injection by PV systems. This paper presents a coordinating control strategy of on-load tap changer (OLTC) of the main transformer in a substation and PV smart inverters to maintain good service voltage quality for the distribution network with high penetration of PV systems. The voltage level at the end-point of each distribution feeder and the power generation by each PV system are collected via a renewable management system. The hourly tap position of OLTC is then derived based on the daily profiles of feeder voltage variation and the injected power generated by all PV systems along the feeder. The PV smart inverters execute an autonomous control of reactive power compensation so that the voltage at the point of common coupling (PCC) can be fixed at the specified voltage level. A main transformer, which serves six feeders with high PV penetration in Taiwan Power Company (Taipower), was selected for this case study. With the proposed coordinating control of OLTC and PV smart inverters, the impact of service voltage quality due to the high penetration of PV systems in the distribution system can be effectively mitigated.

Transformer Management System for Energy Control of Customer Demand Response and PV Systems

This paper proposes an innovative transformer management system (TMS) for energy control of customer loads and photovoltaic (PV) systems to solve the problems of transformer overloading and overvoltage due to power injection by PV renewable energy. The operation status of distribution transformers is monitored by transformer terminal unit (TTU) and reported to the control master station via the hybrid communication system. The demand response of customer load control is activated to reduce the loading level of transformers for mitigation of overloading problem. For the residential customers with installation of roof top PV system, the TTU is applied to perform the power factor control of smart inverter to prevent the overvoltage problem. The hybrid communication system consisting of power line carrier, LoRa radio frequency, and optical fiber is used to provide two-way communication for data collection and controls of customer loads and PV system. According to the field testing of the proposed TMS in Taiwan Power Company, the adaptive energy control of customer loads by TTU is executed when the overloading problem of distribution transformer occurs. The TTU also performs the power factor control of PV smart inverters to solve the overvoltage problem due to too much of PV power being injected during the solar peak period. The overvoltage problem due to PV power injection has also been solved by TTU for control of PV smart inverters to perform the proper reactive power compensation.

Challenges in analysis of antihypertensive exposures and birth defect rates in women with chronic hypertension: an evaluation of the German Embryotox Database

Distributed photovoltaic (PV) generator systems (especially rooftop PV) have been increasing significantly in electrical power systems. However, due to the variability of their output power, it is challenging to integrate large number of these PV generator systems into the existing electrical grid. These distributed PV generator systems can cause large voltage fluctuations due to the oscillation in their power output and reverse power flow in highly penetrated areas. While real power curtailment is an option to manage voltage issues, it reduces the production of this clean energy and is governed by policies and contracts. Traditionally, the use of reactive power to control voltage on a power grid is done through maintaining the voltage within a tolerable range by using transformer tap-changers (TPC), capacitor banks (CB), voltage regulators (VR), static synchronous compensator (STATCOM), and static Var compensators (SVC). Installation and maintenance costs of these devices can be quite high and some have relatively slow response times on the order of many seconds. A smart PV inverter can help regulate voltage by absorbing and injecting reactive power (Var) to/from the grid by using the Volt-Var control function. This paper presents an experimental analysis of the inverter Volt-Var control method for voltage regulation. The capacitive (i.e., Var injection) and inductive (i.e., Var absorption) effects of using a smart inverter and its ability to influence the voltage at the distribution level is investigated in this paper. When the smart PV inverter injects reactive power, it increases the distribution voltage. Conversely, voltage is reduced when the smart inverter absorbs reactive power. As a result, electric power utilities can control the distribution voltage without installing additional devices on the power network. The analyses from a field deployment under the Maui Advanced Solar Initiative Project in Hawaii is discussed here.

Economic Evaluation of Smart PV Inverters with a Three-Operation-Phase Watt-Var Control Scheme for Enhancing PV Penetration in Distribution Systems in Taiwan

The paper presents an economic evaluation, including a cost-benefit analysis and a sensitivity analysis, of smart photovoltaic (PV) inverters with a novel Watt-Var control scheme for enhancing PV penetration in distribution systems in Taiwan. The novel Watt-Var control scheme with three operation phases is utilized to avoid the voltage violation problem during peak solar irradiation period and increase the PV real power injection, and thus can get higher PV penetration in distribution systems. To evaluate the benefit and cost of the PV investment project, the annual revenue of PV power sales, the initial capital investment cost for a PV project with or without a smart inverter, and the operating and maintenance (O&M) cost are taken into account. The paper demonstrates the analyses of net present value (NPV) and benefit-cost ratio (BCR) for the PV project. In addition, the paper also presents a sensitivity analysis to deal with the project uncertainty with respect to some affecting parameters. The analyzing results show that, under the feed-in tariffs (FITs) policy, with proper selection of PV and smart inverter capacities, the investment can be profitable, and the smart PV inverter can greatly enhance the PV penetration in distribution systems in Taiwan. These results can provide some useful information for making policy to encourage investment in solar PV industry.

Distributed voltage regulation using Volt-Var controls of a smart PV inverter in a smart grid: An experimental study

Distributed photovoltaic (PV) generator systems (especially rooftop PV) have been increasing significantly in electrical power systems. However, due to the variability of their output power, it is challenging to integrate large number of these PV generator systems into the existing electrical grid. These distributed PV generator systems can cause large voltage fluctuations due to the oscillation in their power output and reverse power flow in highly penetrated areas. While real power curtailment is an option to manage voltage issues, it reduces the production of this clean energy and is governed by policies and contracts. Traditionally, the use of reactive power to control voltage on a power grid is done through maintaining the voltage within a tolerable range by using transformer tap-changers (TPC), capacitor banks (CB), voltage regulators (VR), static synchronous compensator (STATCOM), and static Var compensators (SVC). Installation and maintenance costs of these devices can be quite high and some have relatively slow response times on the order of many seconds. A smart PV inverter can help regulate voltage by absorbing and injecting reactive power (Var) to/from the grid by using the Volt-Var control function. This paper presents an experimental analysis of the inverter Volt-Var control method for voltage regulation. The capacitive (i.e., Var injection) and inductive (i.e., Var absorption) effects of using a smart inverter and its ability to influence the voltage at the distribution level is investigated in this paper. When the smart PV inverter injects reactive power, it increases the distribution voltage. Conversely, voltage is reduced when the smart inverter absorbs reactive power. As a result, electric power utilities can control the distribution voltage without installing additional devices on the power network. The analyses from a field deployment under the Maui Advanced Solar Initiative Project in Hawaii is discussed here.

Distributed Control of Voltage Regulating Devices in the Presence of High PV Penetration to Mitigate Ramp-Rate Issues

A distributed voltage control scheme is proposed in an active distribution network in the presence of a large scale photovoltaic (PV) farm/plant to mitigate high ramp-rate issues. Multiple voltage regulating devices, such as on-load tap changers, step voltage regulators (SVRs), and switched capacitors banks are controlled in various ways based on either a centralized or a distributed coordination management scheme. This paper presents the relationship of the active and reactive power outputs of a PV plant in the presence of the conventional voltage regulating devices, and proposes a distributed control strategy to coordinate the SVRs with the PV smart inverter’s capability to improve power quality. The distributed voltage control schemes are tested on an unbalanced medium voltage feeder located in a California utility service territory with the PV integrated at the far end of the feeder.