Impact of Grid Support Functionality on PV Inverter Response to Faults

In electric power systems, integration of more Distributed Generators (DGs) in the network increase the short circuit level due to the short circuit current contribution of the DGs during faults. As compared to the synchronous and induction machine based generators, the inverter based generators, such as Photovoltaic (PV) solar systems, contributes lower fault current to the network due to the characteristics of PV panels and inverter operation. Although, each PV solar farm may contribute short currents as above, the total amount of fault current contribution may become unacceptably large for a feeder which has several PV systems connected. It is apprehended that short circuit current contributions from multiple solar systems in the distribution feeders may add up to levels that could be damaging to the circuit breakers. As a first step, adequate modeling of PV solar plants for predicting their short circuit contributions during network faults is essential. So far, the above fast faults detection techniques have been used for protection of network and DGs; and for unsymmetrical fault detection in fault current limiters (FCL). However these techniques have not been used to prevent any short circuit current contribution in excess of the rated or utility-acceptable current output of PV solar inverters. In this paper, a new fast detection module for short-circuit current detection has been discussed based on the rate of rise of current together with the current magnitude in a PV solar system based DG. There can be two control operations, as per the applicable grid code in that region, one of which is the disconnection of the PV inverter before the current exceeds the rated output current of the inverter. The second one discusses about transforming the PV inverter into a dynamic reactive power compensator STATCOM and provide grid support functions. A new concept of utilizing PV solar farms as STATCOM (PV-STATCOM) both during nighttime and daytime for different grid support functions was introduced in this paper.

Grid integration of photovoltaic (PV) inverters has been increasing in the past decade. As a result of the uncertainties introduced with high penetrations of PV, better monitoring and control of the PV inverters becomes crucial for improving overall system stability. This paper focuses on the communications capability of the inverter controller and on enabling interoperability. Multiple standards are available to enable interoperability in PV inverters. In this paper, an interoperable controller, enabled by Distributed Network Protocol 3 (DNP3) communications protocols, is developed for a grid-connected, three-phase PV inverter. The DNP3 server for the PV inverter is programmed on the real-time layer of the field-programmable gate array (FPGA)-based inverter controller. Set points for advanced inverter control functions, such as volt/VAr curves, ride-through curves, are sent from a DNP3 client, a simulated distribution management system application, to the PV inverter through DNP3. This communications capability of the inverter controller is validated using a controller-hardware-in-the-loop experimental setup. The code developed to achieve the interoperability is available in the public domain through open-source software licensing. This interoperability will enable smoother grid integration of smart PV inverters with advanced grid-support functions as well as allow better monitoring and control of PV inverters for grid stability.

With the proliferation of Distributed Energy Resources (DER), Grid Support Functions (GSFs) have been widely deployed. With the object to maintain the frequency and voltage stability during the grid-connected operation, GSFs, however, has raised great concerns on the effectiveness degradation of islanding prevention. Focusing on it, this paper evaluates the performance of the widely used islanding detection method, Slip-Mode Frequency Shift (SMS) operating together with GSFs. First, a small-signal model for this control combination is firstly derived, facilitating the investigation of interaction mechanism between GSFs and SMS. Accordingly, the optimal range of SMS coefficient is identified to guarantee both a successful islanding detection when an island forms and a stable operation during the grid-connected condition. It is observed that several key parameters, especially the slope and the open-loop response time of GSFs, as well as the SMS coefficient, jointly dominate the effectiveness of the islanding detection and should be deliberately designed. Finally, extensive simulations and controller hardware-in-the-loop (HIL) tests are performed to validate the above analysis.

Due to the increased penetration in Distributed Energy Resources (DERs), especially in Photovoltaic (PV) systems, voltage and frequency regulation has become a topic of interest. Utilities have been requesting DER voltage and frequency support for almost two decades. Their request was addressed by standards such as the IEEE Std 1547-2018. With the continuous improvements in inverters’ ability to control their output voltage, power, and frequency, a group of advanced techniques to support the grid is now required by the interconnection standard. These techniques are known as Grid Support Functions (GSF), and they allow the inverter to provide voltage and frequency support to the grid as well as the ability to ride-through abnormal events. Understanding how a GSF behaves is challenging, especially when multiple GSFs are combined to help the utility to control the system voltage and frequency. This paper evaluates the effects of GSF’s on the IEEE Std 1547.1-2020 Unintentional Islanding Test 5B by comparing simulation results from a developed PV inverter model and experimental results from a Power Hardware-in-the-Loop platform.

This paper presents the impact of inverter grid-support functions (GSFs) on photovoltaic (PV) customer energy production on a real distribution feeder in Oahu, HI. These autonomous GSFs based on local voltage measurements are good alternatives to increase PV hosting capacity. However, these functions can result in PV energy curtailment to the customer, and this study addresses the concerns about the impact of inverter GSFs through detailed quasi-static time series (QSTS) simulations. It proposes four metrics: maximum and average GSF curtailment, average increased generation and average net generation change, to assess the impact of a given control on PV systems located on customer sites. It was found that curtailment of PV production is negligible for customers where peak voltage is within ANSI C84.1 range. A reliable relationship between curtailment and peak customer voltage is demonstrated, suggesting that peak voltage could be used as an indicator of customer curtailment.

Distributed energy resources (DER) such as photovoltaic (PV) systems, when deployed in a large scale, are capable of influencing significantly the operation of power systems. Looking to the future, stakeholders are working on standards to make it possible to manage the potentially complex interactions between DER and the power system. In 2009, the Electric Power Research Institute (EPRI), Sandia National Laboratories (SNL) with the U.S. Department of Energy (DOE), and the Solar Electric Power Association (SEPA) initiated a large industry collaborative to identify and standardize definitions for a set of DER grid support functions. While the initial effort concentrated on grid-tied PV inverters and energy storage systems, the concepts have applicability to all DER. A partial product of this on-going effort is a reference definitions document (IEC TR 61850-90-7, Object models for power converters in distributed energy resources (DER) systems) that has become a basis for expansion of related International Electrotechnical Commission (IEC) standards, and is supported by US National Institute of Standards and Technology (NIST) Smart Grid Interoperability Panel (SGIP). Some industry-led organizations advancing communications protocols have also embraced this work. As standards continue to evolve, it is necessary to develop test protocols to independently verify that the inverters are properly executing the advanced functions. Interoperability is assured by establishing common definitions for the functions and a method to test compliance with operational requirements. This document describes test protocols developed by SNL to evaluate the electrical performance and operational capabilities of PV inverters and energy storage, as described in IEC TR 61850-90-7. While many of these functions are not currently required by existing grid codes or may not be widely available commercially, the industry is rapidly moving in that direction. Interoperability issues are already apparent as some of these inverter capabilities are being incorporated in large demonstration and commercial projects. The test protocols are intended to be used to verify acceptable performance of inverters within the standard framework described in IEC TR 61850-90-7. These test protocols, as they are refined and validated over time, can become precursors for future certification test procedures for DER advanced grid support functions.

Grid codes around the world are requiring grid-support functions (GSFs) and standardized interoperability interfaces for distributed energy resources (DERs) to address the rapid increase of renewable energy. However, these new GSFs need to be assessed to ensure the desired power and communication capabilities exist in the field. The IEEE 1547.1 standard outlines the conformance test procedures for DER devices and is currently undergoing a major revision to align it with IEEE 1547–2018. Once it is published (anticipated in mid-2020), GSFs in commercial PV inverters in USA and Canada will be certified to the IEEE 1547.1 conformance test procedures. Several international research laboratories are collaborating to develop a versatile open-source DER testing platform that performs automated testing of DER devices. This community of laboratories is developing open-source IEEE Std. 1547.1 test scripts to lower barriers to DER vendor internal equipment evaluations, ease product compliance testing at certification laboratories, and provide research institutions a tool to study DER behaviors. In this work, test scripts were used for test verification of GSFs, including limit active power, constant reactive power, active power-reactive power (watt-var), and prioritization of GSF response for several DER devices. Sample test results for these DER GSFs and test protocol recommendations are presented in this paper.

The high penetration of utility interconnected photovoltaic systems is is leading to a need for inverters to include grid support functions, to minimize the negative impact these variable distributed energy resources may have on system voltage and frequency. Unfortunately, grid support functions may interfere with island detection algorithms; specifically, it may be difficult for an island detection scheme to detect voltage and frequency deviations if that converter and other converters on the same bus actively modulate their real and reactive power outputs in response to voltage and frequency deviations while also tolerating greater deviation. This report provides analysis and simulation evidence to investigate the effect of advanced inverter functions on the performance of island detection schemes. A mitigation scheme is also presented and shown to be effective in simulation.

This research focused on the development of a methodology for calculating the optimal power factor (OPF) in microgrids with the photovoltaic generation, in order to use solar inverters as reactive compensators, which will change their power factor according to the needs of the load. The developed methodology proposes a convex optimization model with multiple constraints to solve the OPF problem. Wirtinger's linearization in the power balance equation was implemented. The stochastic behavior of solar radiation was considered using the average sampling approach (ASA) to generate solar scenarios, which are used to calculate the magnitude of the generation of photovoltaic systems for specific hours of the day. Finally, the algorithm was run on CIGRE's 19-node test grid. The proposed methodology showed that as the radiation level increases during the day, more radiation scenarios can be tested, which increases the accuracy of the power factor value for each PV system. Although the general idea in power systems is to have a unity power factor, the algorithm resulted in power factors with values less than one in some inverters. This represents an injection of reactive power from the inverters to meet the reactive needs of the loads connected close to said PV generators, which is reflected in a variation in the magnitude of the power factor.

In 2011, EPRI began a four-year effort under the Department of Energy (DOE) SunShot Initiative Solar Energy Grid Integration Systems - Advanced Concepts (SEGIS-AC) to demonstrate smart grid ready inverters with utility communication. The objective of the project was to successfully implement and demonstrate effective utilization of inverters with grid support functionality to capture the full value of distributed photovoltaic (PV). The project leveraged ongoing investments and expanded PV inverter capabilities, to enable grid operators to better utilize these grid assets. Developing and implementing key elements of PV inverter grid support capabilities will increase the distribution system’s capacity for higher penetration levels of PV, while reducing the cost. The project team included EPRI, Yaskawa-Solectria Solar, Spirae, BPL Global, DTE Energy, National Grid, Pepco, EDD, NPPT and NREL. The project was divided into three phases: development, deployment, and demonstration. Within each phase, the key areas included: head-end communications for Distributed Energy Resources (DER) at the utility operations center; methods for coordinating DER with existing distribution equipment; back-end PV plant master controller; and inverters with smart-grid functionality. Four demonstration sites were chosen in three regions of the United States with different types of utility operating systems and implementations of utility-scale PV inverters. This report summarizes the project and findings from field demonstration at three utility sites.

This paper discusses the design, development, and testing of a flexible combined heat and power (F-CHP) system. It includes two parts: the power conditioning system (PCS) converter, which connects the system to the grid and is the key component to provide grid support services, and the F-CHP central controller, which accommodates the control of local sources, local loads, and the PCS converter. The functions and structures of the central controller are discussed. The central controller’s system-level performances are verified with hardware-in-the-loop (HIL) tests, and the mode transition and grid transient performances are validated with power electronic converter-based hardware testbed (HTB) tests. The PCS converter is designed with 10 kV SiC MOSFETs. The grid requirements are considered in the 13.8 kV/1 MW PCS converter design, and the impact of different requirements on the converter design is identified. Also, converter scalability is studied with the paralleling operation. Two 13.8 kV/100 kW converter prototypes are developed to verify the converter design and scalability. The second prototype is an improved design of the first one; the volume is reduced by 49% and the efficiency at the rated output power is increased from 96.4% to 98.4%. The test results of the converter performances, grid support functions, and parallel operation are also discussed.

The objective of this CRADA between Sandia National Laboratories (SNL) and SunSpec Alliance (SunSpec) was to create a software test platform called the System Validation Platform (SVP) that performs end-to-end certification testing of photovoltaic (PV) inverters, energy storage systems, and other distributed energy resources (DERs). The SVP automates interactions of test equipment (PV and grid simulators), data acquisition systems, and interoperable DERs to certify the devices for advanced grid-support functions and interoperability. To assist electricity grid, operators maintain voltage and frequency, new interconnection requirements in the U.S. and Europe require DERs to have grid-support functions with adjustable settings. Automating the certification process for DER devices impacts the bottom line of NRTLs because they do not need to rely on test engineers running days (or weeks) of experiments to validate the performance of the equipment. These savings can then be passed down to the DER vendors so the certification process becomes less expensive and lowers the barrier to market for PV inverters, charge controllers, and other grid-interconnected equipment. This technology can also be used as a research platform for SNL, other national labs, and universities that would like to run hardware tests with power electronics equipment.

This paper addresses grid interconnection tests of a 3 kW transformer-less photovoltaic (PV) inverter to verify the effectiveness of the PV inverter and promote its wide use. The 3 kW transformer-less PV inverter, which was equipped with intelligent protection and control system, was manufactured. A hardware test bed was constructed for performance tests of the PV inverter. Control performance and grid protection tests were carried out using the test bed. Test results verify the performance of the power control and grid protection functions of the developed transformer-less PV inverter.

The aim of this paper is to study the behavior of a three-phase inverter with an RL filter powered by a renewable energy source and connected to a grid under fault conditions. The novelty of the work lies in proposing a mathematical model which can be solved analytically. So, a comprehensive analytical solution is given, which can be used to describe the behavior of a three-phase inverter under both symmetrical and unsymmetrical faults. Firstly, a complete mathematical model of the system is given. Secondly, a control is applied for both symmetrical (three-phase faults) and unsymmetrical (one-phase and two-phase faults) conditions, making use of the positive- and negative-sequence components for the latter. Finally, a mathematical model for the whole system Is developed. The analytical approach is valid for any fault conditions, either symmetrical or unsymmetrical, which helps in the understanding of the behavior of three-phase inverters connected to a faulty grid and simplifies its study. The analytical model is validated through simulations carried out in Matlab-simulink™.

This paper presents a novel fast frequency response and power oscillation damping control by large-scale PV plants controlled as STATCOM, termed PV-STATCOM, to simultaneously enhance frequency regulation and small signal stability of power systems. Frequency deviations typically occur together with power oscillations in large power systems. The proposed controller comprises: first, power oscillation damping controller based on reactive power modulation and second, fast frequency response controller based on real power modulation, both of which are applied to the plant level controller of PV-STATCOMs. The proposed composite control is shown to successfully reduce frequency deviations, damp power oscillations, and provide voltage regulation both during over-frequency and under-frequency events. The proposed smart inverter control makes effective utilization of the PV inverter capacity and available solar power. For large power flows, the proposed control is shown to be superior than the conventional droop control recommended by North American Electric Reliability Corporation for generating plants. MATLAB/Simulink-based simulations are conducted on two-area power system using generic PV plant dynamic models developed by Western Electricity Coordinating Council, for a wide range of system operating conditions. Such grid support functionality is expected to bring new revenue making opportunities for PV solar farms.