Grid Support Functions Research Articles

Field Verification of Autonomous Anti-Islanding Schemes and Grid Support Functions of an Inverter-Based Microturbine Distributed Generator

Canada-wide climate change plans have been put in place in the form of legislation and emission reduction programs. As part of these measures, reduced amounts of natural gas can be vented, incinerated, or flared. These programs have a direct impact on the Canadian oil and gas industry. Other measures mandate the decommissioning of high-emission thermal power plants, such as coal-fired generation. At the same time, governments are encouraging the wide-scale adoption of distributed energy resources (DERs). Because of these changes, natural gas fired DERs have been proliferating in oilfields to consume the byproduct natural gas and most major oil and gas companies are diversifying their portfolio to include generation assets. It is a more sustainable and environmentally friendly scenario as compared to the previous status quo. From an electric utility standpoint, this represents a change in paradigm, because distribution systems are traditionally planned to be radial and only experience power flowing from the substation to the load centers. This change requires utilities to adapt to new concerns and requirements, which span from operation to health and safety hazards. Among these, anti-islanding is likely the most important concern to allow the safe and reliable operation of a distribution system. The standard method to avoid unintended islanded operation is the direct transfer trip. Under exceptional circumstances, the utility may waive this requirement if an alternative acceptable method is in use. This article addresses existing anti-islanding schemes and covers in detail an alternative anti-islanding method native to many inverters. This article contains the philosophy of such method and measurement results captured during commissioning of a DER containing the scheme. Furthermore, this article highlights the evolution of inverter-based generator technologies and how the revision of applicable standards (IEEE 1547) can improve the reliable operation of such DERs.

Enhancement of Solar Farm Connectivity With Smart PV Inverter PV-STATCOM

This paper presents an innovative smart PV inverter control as STATCOM (PV-STATCOM) for obviating the need for a physically connected STATCOM in a distribution network for controlling steady-state voltage and temporary over voltages (TOVs) resulting from unsymmetrical faults. Two 10-MW PV solar systems are already connected in the distribution feeder of a utility in Ontario, Canada. A STATCOM is installed to prevent the steady-state voltage and TOV issues arising from the connection of a third 10-MW PV solar farm at same bus. It is demonstrated from PSCAD electromagnetic transient studies that if the proposed PV-STATCOM control is implemented on the incoming third 10-MW PV solar farm, all the above voltage issues are mitigated satisfactorily as required by the utility grid code. This proposed smart inverter PV-STATCOM control therefore eliminates the need for the physical STATCOM, saving an enormous cost for utilities dealing with voltage rise and TOV issues with grid-connected PV systems. Such a control can effectively increase the distributed generator hosting capacity of distribution feeders at more than an order of magnitude lower cost under similar network conditions. Moreover, this novel grid support functionality can open new revenue making opportunities for PV solar farms.

Load Frequency Control of HVDC Link Interconnected Power System Using Genetic Algorithm

Advances in power electronics have improved grid support functions such as tie-line power control and frequency control, making renewable generation and High Voltage DC (HVDC) links more common in power system applications. Load Frequency Control (LFC) systems handle the complex interactions between the distributed generator and the control area with the HVDC link. In this work, LFC of a two-zone system including parallel AC/DC transmission links has been analysed. The parameters of this system are optimised using advanced genetic algorithm resulting in improved performance of system in terms of reduction in peak overshoots and settling time of frequency measurement, tie line power flow and area control error signals in an interconnected power system. The advantage of having parallel HVDC links is also demonstrated when performance is compared to system interconnected with only AC tie line.

A Multi-Mode Flexible Power Point Tracking Algorithm for Photovoltaic Power Plants

Flexible power point tracking (FPPT) is the control of active power generated by grid-connected photovoltaic power plants (GCPVPPs) to provide grid-support functionality. An FPPT algorithm for the reduction of the extracted power from photovoltaic (PV) strings during voltage sags was previously proposed by the authors. An advantage of this algorithm, compared to conventional FPPT algorithms, was its fast dynamics facilitated by use of a simple PI controller that dynamically modifies the PV voltage reference. The previously proposed scheme could only be employed for the short duration in which the power system experiences a voltage sag. A novel modification to this algorithm with multi-mode operation is introduced in this letter, which provides FPPT capability for continuous operation of GCPVPPs. Unlike the previous algorithm, which was able to only move the operation point to the right-hand side of MPP, the proposed algorithm in this letter is able to move the operation point to both right- and left-hand sides of the MPP that provides the flexibility to operate in the optimum operation region for both single- and two-stage GCPVPPs. Experimental results are provided to demonstrate the performance of the proposed algorithm under dynamic irradiance conditions.

Flexible Control of Interlinking Converters for DC Microgrids Coupled to Smart AC Power Systems

This paper presents a multifunctional control system for interlinking converters interfacing microgrids and ac mains. Applicable standards pose power quality and grid-support function requirements for converters that interface energy resources of dc nature with an ac utility grid. At the same time, the forthcoming smart grid scenario calls for additional functionalities, enabling the effective integration of hybrid (i.e., dc or ac) microgrids, that are applicable to both grid-connected and islanded modes of operation. This requires novel interlinking converter controllers with extended flexibility. The control system described in this paper fulfills both connected and islanded operation needs and provides seamless transitions between the two modes. Such valuable functionalities are attained owing to an underlying, large-bandwidth, triple-loop control organization. The target converter is meant to be connected to a local dc subgrid at the dc side and to a local ac subgrid at the ac side. The controller manages the connection of the ac-side subgrid with the utility grid, ensuring a fast response control of the ac grid current. Therefore, it allows interfacing of renewable sources and loads, aggregated in dc and ac nanogrids or microgrids, with smart power systems, providing enhanced power quality, stability, and flexibility of operation.

Guest Editorial Joint Special Section on Power Conversion & Control in Photovoltaic Power Plants

The papers in this special section focus on power conversion and control in photovoltaic power plants. The power grid is undergoing significant changes with the rapid adoption of renewable energy. Solar power generation, and photovoltaics (PV) in particular, has been one of the fastest growing sectors in the power industry and has now reached price parity with conventional generation in many jurisdictions. Despite recent trends in reduced costs, several technical challenges remain. This includes but is not limited to the following research needs: i) novel converter designs for medium-voltage energy conversion and enhanced efficiency, ii) grid-support functions and fault-detection schemes that ensure reliable system operation, iii) comprehensive modeling strategies for analyzing system stability, iv) modeling techniques which capture complex system interactions, and v) strategies for system planning, siting, and operations.

Power system effects and mitigation recommendations for DER cyberattacks

Extensive deployment of interoperable distributed energy resources (DER) is increasing the power system cyber security attack surface. National and jurisdictional interconnection standards require DER to include a range of autonomous and commanded grid-support functions, which can drastically influence power quality, voltage, and bulk system frequency. Here, the authors investigate the impact to the cyber-physical power system in scenarios where communications and operations of DER are controlled by an adversary. The findings show that each grid-support function exposes the power system to distinct types and magnitudes of risk. The physical impact from cyber actions was analysed in cases of DER providing distribution system voltage regulation and transmission system support. Finally, recommendations are presented for minimising the risk using engineered parameter limits and segmenting the control network to minimise common-mode vulnerabilities.

Simultaneous Fast Frequency Control and Power Oscillation Damping by Utilizing PV Solar System as PV-STATCOM

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.

Analysis of Power Quality Variations in Electrical Distribution System with Renewable Energy Sources

The increasing u se of Power Electronics along with Distributed Generation Systems (DGs) has been playing a predominant role in the efficient operation of Electric Power Systems. Thus, studies regarding identification and classification of Power Quality (PQ) events have been a subject of recent interest for researchers from the view point of initiating suitable control actions in DGs for achieving improved performance. This requires carrying out a detailed analysis of the various characteristics of real time electrical signals through a set of signal processing techniques . PQ distortions due to environmental factors, such as change in wind speed and solar irradiations are considered in this work. Signal features are extracted for various voltage sag/swell signals using S-Transform, while signal-classification is done by Least Square Support Vector Machine (LS-SVM) technique. A 17-bus test system is modeled using the open source software, Open Distribution System Simulator (OpenDSS). Smart converter control is realized with inputs received from signal classifier, so as to initiate proper grid-support functions. Controlled actions realized are Volt-VAr and Volt-Watt functions. The performance of the control functions is tested for varying levels of solar and wind penetrations. The efficacy of the results is imminent based on control actions being in line with the set reference.

A Grid-Supporting Photovoltaic System Implemented by a VSG with Energy Storage

Conventional photovoltaic (PV) systems interfaced by grid-connected inverters fail to support the grid and participate in frequency regulation. Furthermore, reduced system inertia as a result of the integration of conventional PV systems may lead to an increased frequency deviation of the grid for contingencies. In this paper, a grid-supporting PV system, which can provide inertia and participate in frequency regulation through virtual synchronous generator (VSG) technology and an energy storage unit, is proposed. The function of supporting the grid is implemented in a practical PV system through using the presented control scheme and topology. Compared with the conventional PV system, the grid-supporting PV system, behaving as an inertial voltage source like synchronous generators, has the capability of participating in frequency regulation and providing inertia. Moreover, the proposed PV system can mitigate autonomously the power imbalance between generation and consumption, filter the PV power, and operate without the phase-locked loop after initial synchronization. Performance analysis is conducted and the stability constraint is theoretically formulated. The novel PV system is validated on a modified CIGRE benchmark under different cases, being compared with the conventional PV system. The verifications demonstrate the grid support functions of the proposed PV system.

Dynamic Analysis and Improved LVRT Performance of Multiple DG Units Equipped With Grid-Support Functions Under Unbalanced Faults and Weak Grid Conditions

Due to the increased integration of multiple distributed generation (DG) units into the distribution network, riding through short-term faults and supporting the host grid have been requested by the new grid codes in many countries. However, the literature lacks the detailed dynamic analysis and control coordination of multiple grid-connected converter-based DG units, equipped with advanced controllers in the synchronous reference frame. To fill this gap, this paper initially presents a detailed small-signal modeling framework for typical medium-voltage multi-bus power distribution systems with multiple DG units equipped with grid-support functions to operate under the unbalanced conditions. This model encompasses the positive and negative sequences of the current and voltage and is developed for three stages of the fault (i.e., before, during, and after the fault) to cover a wide range of system operating points. In addition, to precisely study the interactions among DG units, four different control modes in DG units are considered to study the system dynamics under low-voltage and unbalanced conditions and at different grid strengths. Using the proposed detailed state-space models and based on the small-signal stability analyses, different control parameters are redesigned using the eigenvalue analysis on the complete multi-DG system. As a second contribution, sensitivity analyses are performed to study the effects of different system parameters, such as line characteristics, loading levels, and unbalanced fault characteristics, on the stability of the multi-DG system under unbalanced faults. Comparative simulation and experimental results are also reported to show the accuracy and effectiveness of the theoretical analyses.

Fuzzy Control for Smart PV-Battery System Management to Stabilize Grid Voltage of 22 kV Distribution System in Thailand

This paper presents a fuzzy logic based algorithm developed to bring smart functionality to an ordinary PV-battery system in order to maintain the grid voltage stability of the 22 kV distribution system in Thailand. This research focuses on minimizing grid voltage fluctuations by converting a typical PV system into a smart PV-battery system (SPVs-BSS). A SPVs-BSS will be able to control the electrical power from a PV system to maintain the grid voltage in case of unexpected events or emergencies. Grid support functions such as a variable reactive power control and active power control will be discussed, leading to strategies for charging and discharging the battery system in response to the status of grid voltage. Fuzzy Logic was used to develop this control algorithm, which is named the Voltage Stability Fuzzy Logic Algorithm (VSFL Algorithm). The methodology of this research consists of three parts. First, testing the grid inverter operated on grid support functions. Second, the VSFL algorithm was developed to manage both the grid inverter and the battery system. Third, a SPVs-BSS equipped with the VSFL algorithm was simulated by using DIgsilent PowerFactory software. Results showed that the SPVs-BSS equipped with the VSFL Algorithm successfully maintained grid voltage in target range.

Interconnection Standard Grid-Support Function Evaluations Using an Automated Hardware-in-the-Loop Testbed

Grid operators are increasingly turning to advanced grid-support functions in distributed energy resources (DER) to assist with distribution circuit voltage regulation, bulk system frequency control, and power system protection. The U.S. DER certification standard, Underwriters Laboratories (UL) 1741, was revised in September 2016 to add test procedures for multiple grid-support functions. Sandia National Laboratories, SunSpec Alliance, and growing community of collaborators have undertaken a multiyear effort to create an open-source system validation platform (SVP) that automates DER interconnection and interoperability test procedures by communicating with grid simulators, photovoltaic (PV) simulators, data acquisition systems, and interoperable equipment under test. However, the power hardware required for generating the test conditions may be untenable for many organizations. Herein, we discuss development of the SVP testing capabilities for UL 1741 tests utilizing a controller hardware-in-the-loop testbed that precludes the need for power hardware using a 34.5 kW Austrian Institute of Technology smart grid controller. Analysis of normal ramp rate, soft start ramp rate, specified power factor, volt-VAr, and frequency-watt advanced grid functions, and the effectiveness of the UL 1741 test protocols are included.

Power System Support Functions Provided by Smart Inverters—A Review

Renewable energy is seen as a viable alternative to traditional energy sources, and distributed generation (DG) based on renewable energy sources has experienced rapid growth worldwide. High penetration of renewable energy based DG systems makes the grid more vulnerable, and stricter standards have been issued for grid interconnection of DG systems. DG systems are expected to be controllable with high flexibility and reliability. Provision of grid support functions and ancillary services, such as reactive power control, fault ride-through and harmonic compensation, is the key to attaining higher utilization of DG. Such functionalities are implemented in new generation smart inverters, which can contribute to the reduced cost of energy and need for additional system resources. The state-of-the-art power system support functions are summarized in this paper for the purpose of enhancing operation in low-voltage networks. Experimental results are given to better understand the implementation of the functions.

A PV Residential Microinverter With Grid-Support Function: Design, Implementation, and Field Testing

Microinverter-based photovoltaic (PV) systems now represent about 8% of the U.S. residential market, and offer many advantages including safety, performance, and simplified installation. The next-generation of PV microinverter will include more ancillary functions to support grid stability and reliability in more distributed generation smart-grid systems. A commercial ready PV microinverter not only focuses on efficiency and cost, but also on reliability, manufacturability, compliance of various grid-code, and electromagnetic interference regulations. This paper presents a detailed design and development process of a microinverter system from concept all the way to final commercial-ready prototype. Various design tradeoffs such as topology, control, filter solutions and power supplies, and mechanical packaging are provided. The required prototype testing and final system field tests are also presented. The presented design and test process intends to accelerate the future microinverter system design and development toward a commercial ready product.

Review on Distributed Energy Storage Systems for Utility Applications

Energy storage systems (ESSs) can improve the grid's power quality, flexibility and reliability by providing grid support functions. This paper presents a review of distributed ESSs for utility applications. First, a review of the energy storage market and technology is presented, where different energy storage systems are detailed and assessed. Then, ESS grid support functions are presented and seven types of functions are described. Finally, the power electronic converters for distributed ESSs are reviewed and the corresponding features and performances are evaluated. The main objective of this paper is to provide an updated reference regarding ESSs for utility applications to researchers and practitioners in the field of power electronics.

Design and Evaluation of SunSpec-Compliant Smart Grid Controller with an Automated Hardware-in-the-Loop Testbed

With increasing penetrations of inverter-based, renewable energy resources on electrical grids around the world, new distributed energy resource (DER) interconnection and interoperability requirements have been introduced to address emerging power system operator needs. The inverter-based power conversion systems are capable of communicating with grid operators, providing voltage and frequency support, and supporting the grid during faults. However, DER vendors are under pressure to quickly and reliably update the interoperability and electrical capabilities of their equipment for different jurisdictions with the rapidly changing landscape of disparate codes and standards. The necessary power hardware required for testing power systems under the wide variety of operational conditions may be untenable for many organizations. Therefore, we introduce an approach for the concurrent development of controls and application software through a controller hardware-in-the-loop (CHIL) testbed integrated with an automated testing platform that allows for the cost-effective, flexible evaluation of advanced grid support functions without the need for large and expensive power hardware. We show this CHIL capability through the demonstration and automation of interconnection tests with a 34.5 kW Austrian Institute of Technology (AIT) smart grid converter (SGC) connected to a Typhoon HIL system. We have demonstrated the CHIL system with regards to connect/disconnect, active power curtailment, fixed power factor, reactive power control, volt-var, and frequency-watt advanced grid functionality tests. For all tests, the automated CHIL testing protocols for advanced functions were sufficient to demonstrate and evaluate the grid support behavior of the equipment under test.

Solid-State-Transformer-Interfaced Permanent Magnet Wind Turbine Distributed Generation System With Power Management Functions

The higher penetration of wind energy poses increasing demand for grid support and power management functions of a wind energy conversion system (WECS). This paper investigates a medium-voltage solid-state transformer (SST)-interfaced permanent magnet synchronous generator system with integrated active power management and reactive power compensation functions. Specifically, a WECS consisting of wind turbines, SSTs, and dc loads is presented. In addition, a distributed power management algorithm is proposed for a dc network with local wind turbine controls incorporated to achieve a self-contained power-balanced condition without the need for energy storage or communication devices. Scenarios considered include the grid-connected mode, the islanding mode, and the mode transitions. Simulation results are provided to verify the effectiveness of the proposed strategy. Additionally, the concept is experimentally verified using a scaled-down laboratory prototype.

Impact of phase-locked loop on system dynamics of a CCGT incorporated diverse source system employed with AC/DC interconnection

Nowadays, research on renewable generation penetration in a power system has facilitated the use of power electronics with improved grid-support functionalities. This article addresses the automatic generation control of a three-area interconnected system with combined cycle gas turbine and two thermal generating units in area1 and a wind and two thermal units in other areas. All areas are employed with delayed communication channels, and appropriate generation rate constraints and governor deadband are provided in thermal systems. A recent metaheuristic algorithm, namely stochastic fractal search (SFS), is applied for optimization of secondary controller gains and other system parameters. The performance of various two-degrees-of-freedom proportional-integral-derivative (2DOF PID) controller structures are compared with single-degree-of-freedom proportional-integral-derivative controllers, and results explore the superiority of 2DOF PID controllers. An attempt to explore the impact of AC/DC links considering delayed phase-locked loop (PLL) dynamics during coordination with diverse source interconnected AC systems is presented and discussed. Results clearly outline overshoots and settling time of frequency, tie-power, and temperature deviation increases owing to the inclusion of PLL. A detailed numerical analysis is presented to understand the system dynamic responses due to parametric changes in PLL function. However, with optimum parametric selection, the deviations and settling time decrease. Last, sensitivity analysis implies that SFS optimized controller parameters need not be reset for ±25% changes in loading condition; inertia constant; and ±10%, ±20%, and –25% changes in tie-power synchronizing co-efficient. But system responses deteriorate during +25% changes in the latter owing to very strong ties between the areas, which are not recommended.

A novel control strategy for Reflex-based electric vehicle charging station with grid support functionality

This paper proposes a novel control strategy for a charging station which provides reflex-based fast charging service for electric vehicles through a number of connection ports. Reflex method has distinct advantages compared to other bulk charging methods. It applies a three-stage charge sequence, a positive, negative, and zero battery current for each cycle. Due to uninterrupted drastic changes in the battery currents during the cycles of this method, the charging station encounters an important power quality issue in which phasor of the AC-side terminal current changes significantly and level of the delivered power to the batteries has considerable variations. An ultra-capacitor is connected to it according to a particular topology and then, is controlled according to the novel control strategy in order to resolve the power quality problem.