Grid Code Requirements Research Articles

A Cooperative Approach of Frequency Regulation Through Virtual Inertia Control and Enhancement of Low Voltage Ride-through in DFIG-based Wind Farm

Doubly-fed induction generator (DFIG)-based wind farms (WFs) are interfaced with power electronic converters. Such interfaces are attributed to the low inertia generated in the WFs under high penetration and that becomes prevalent in a fault scenario. Therefore, transient stability enhancement along with frequency stability in DFIG-based WFs is a major concern in the present scenario. In this paper, a cooperative approach consisting of virtual inertia control (VIC) and a modified grid-side converter (GSC) approach for low voltage ride-through (LVRT) is proposed to achieve fault ride-through (FRT) capabilities as per the grid code requirements (GCRs) while providing frequency support to the grid through a synthetic inertia. The proposed approach provides LVRT and reactive power compensation in the system. The participation of the VIC in a rotor-side converter (RSC) provides frequency support to the DFIG-based WFs. The combined approach supports active power compensation and provides sufficient kinetic energy support to the system in a contingency scenario. Simulation studies are carried out in MATLAB/Simulink environment for symmetrical and unsymmetrical faults. The superiority of the proposed scheme is demonstrated through analysis of the performance of the scheme and that of a series resonance bridge-type fault current limiter (SR-BFCL).

Dual Control Strategy for Grid-tied Battery Energy Storage Systems to Comply with Emerging Grid Codes and Fault Ride Through Requirements

Battery energy storage systems (BESSs) need to comply with grid code and fault ride through (FRT) requirements during disturbances whether they are in charging or discharging mode. Previous literature has shown that constant charging current control of BESSs in charging mode can prevent BESSs from complying with emerging grid codes such as the German grid code under stringent unbalanced fault conditions. To address this challenge, this paper proposes a new FRT-activated dual control strategy that consists of switching from constant battery current control to constant DC-link voltage control through a positive droop structure. The results show that the strategy ensures proper DC-link voltage and current management as well as adequate control of the positive- and negative-sequence active and reactive currents according to the grid code priority. It is also shown that the proposed FRT control strategy is tolerant to initial operating conditions of BESS plant, grid code requirements, and fault severity.

Stability Assessment and Enhanced Control of DFIG-Based WTs During Weak AC Grid

According to the latest grid codes, wind turbines (WTs) are required to inject certain amount of reactive and active current into the faulty grid during voltage sags. However, studies have shown that the doubly fed induction generator (DFIG)-based WTs may lose stability under weak grid condition. So the impact of grid code requirement on DFIG system stability is worthy of investigation. To address such issue, a comprehensive small-signal admittance model of DFIG system is established in this paper. Then the influence of reactive/active current settings during grid fault is analyzed in detail. Furthermore, the stable operation region is derived, considering three key factors, i.e., the current demand of grid codes, capacity of WTs and stability constraint. Besides, in order to extend the stable operation region of the DFIG system, an enhanced control strategy based on voltage disturbance compensation is put forward. Finally, simulations and experiments are performed to validate the correctness of the theoretical analysis and effectiveness of the proposed control.

Coordinated Control of Grid-Connected PMSG Based Wind Energy System With STATCOM and Supercapacitor Energy Storage

In this paper, two coordinated control schemes have been designed for a grid-connected permanent magnet synchronous generator-based wind energy conversion system with static synchronous compensator (STATCOM) and supercapacitor energy storage (SCES) under various operating conditions. The coordinated control scheme-I (CCS-I) integrated a SCES into the DC-link through a bidirectional DC/DC converter to stabilize the DC-link voltage under varying wind speed and grid disturbances. The coordinated control scheme-II (CCS-II) implemented a STATCOM at the point of common coupling (PCC) to enhance the grid voltage stability during the grid voltage sag or swell. In CCS-II the SCES balances the active power while reactive power support is extracted from the grid side converter (GSC). The STATCOM is only activated when the reactive power requirement by the PCC exceeds the support capacity of the GSC. The proposed coordinated control schemes have been experimentally implemented with a dSpace MicroLabBox. Results show that CCS-I and II ensured the uninterrupted operation of the wind energy system and satisfied grid code requirements under various operating conditions and grid disturbances.

Artificial Intelligence Based Control Strategy of a Three- Phase Neutral-Point Clamped Back-to-Back Power Converter with Ensured Power Quality for WECS

This paper provides an in-depth investigation into the state-of-the-art power converter control approaches utilizing artificial intelligence that ensure the power quality for wind energy conversion systems (WECS). The most promising and feasible wind energy conversion configuration has been elected to be evaluated in an attempt to reduce the computing cost and time, as well as meet the grid code requirements. For this purpose, in this work, a back-to-back neutral-point clamped power converter is controlled with high precision using a machine learning algorithm. The machine is trained offline by data acquired from the well-known voltage-oriented control (VOC) technique. The majority of the computational load is moved from online to offline mode. Thus, there is no need for accurate development of the PI controller parameters and bandwidth, and hence, the cost and time of calculation will be considerably reduced. As a consequence, the recommended machine learning-based technique can take over the conventional VOC responsibilities. To accomplish this, the training dataset is applied to learn the behavior of the system using a locally weighted lasso regression approach. The cost function is then minimized using stochastic gradient descent, batch gradient descent, Broyden–Fletcher–Goldfarb–Shanno (BFGS), and limited-memory BFGS optimizers, successively. The comparative analysis reveals that the BFGS family of optimizers outperforms the counterparts in terms of computation time, accuracy, and THD performance of WECS.

Security Enhancement of Network Constraint Grid-Edge Energy Management System

Network constrained grid edge energy management system (EMS) provides economic solution for active and reactive power dispatch of distributed energy resources (DERs) at the grid edge level. Grid edge EMS ensures secure interconnection of a circuit segment to the distribution system by maintaining grid code requirements (e.g. IEEE 1547-2018). Grid edge EMS is dependent on communication to receive load measurement, which brings a risk of unobservable false data injection attacks (FDIAs). To mitigate the risk, this paper proposes a framework to enhance resilient operation of grid edge EMS by detecting the unobservable FDIAs on loads and replacing them with forecasted values. In this work, a two-step detection algorithm is proposed. In first step, conventional residual based algorithm is deployed. Autoencoder (AE) based data driven mechanism is included in second step to detect the presence of unobservable FDIAs. After ensuring the presence of FDIA, its specific location is detected by checking the maximum residue values till the predefined threshold value is reached. Detected false data injected loads are then replaced with forecasted load values following long- short term memory (LSTM) based forecast to ensure resilient performance of grid edge EMS in the presence of attacks. This proposed security enhancement framework for grid edge EMS is evaluated in IEEE 13 bus system with three integrated DERs. Numerical simulation shows the validation of the proposed framework by reducing voltage violation in real operation of grid edge EMS.

Evaluation of New Grid Codes for Converter-Based DERs From the Perspective of AC Microgrid Protection

The ever-increasing penetration levels of converter-based distributed energy resources (DERs) in medium and low voltage (MV/LV) distribution networks necessitates new and revised standards or connection requirements of generating units. These standardized connection requirements to be met by DERs are called grid codes. This paper reviews new European and IEEE grid codes of MV/LV converter-based DERs and presents an evaluation of selected grid codes applicable for AC microgrid protection. The selected grid codes are evaluated for both grid-connected and islanded modes of AC microgrid operation. The standardized settings of different protection schemes including symmetrical components-based protection schemes are evaluated for quick fault detection and isolation during the most challenging unbalanced faults in grid-connected and islanded modes. The extent of dynamic reactive power (Q) injection by DERs according to European grid codes is evaluated and the effect of Q-injection on fault detection and protection coordination is observed. The generic grid-forming DER model with coupled dq-control has been enabled to provide enough reactive current during the unbalanced faults with the help of an additional Q-source. It is concluded that the grid code requirement of dynamic Q-injection can be met by converter-based DERs with an additional Q-source of minimum capacity equal to twice the apparent power of individual DER in the grid-connected mode. The same extent of Q-injection is also useful for fault detection and reliable directional element design in the islanded mode. A new five-cycle overvoltage ride-through (OVRT) curve is also suggested for smooth transition to the islanded mode.

FRT and Transient Stability Augmentation of Grid-Connected PV Station Using Virtual Synchronous Generator

Due to the extensive integration of renewable energy sources (RESs), i.e., photovoltaic (PV) system, the future power system is developing into an inverter-based system from a dominated alternator-based power system. This massive penetration of inverter-based PV system reduced the system inertia and damping characteristics of the power grid, impacting the fault ride-through (FRT) capability and causes frequency instability. Modern grid codes require that PV systems should work in the same way as conventional power plants and assist the system during transient state. However, most of the conventional inverter control mechanisms failed to fulfill the requirements of grid codes, especially when the penetration ratio of the PV system is close to the conventional unit. Therefore, this paper proposes a virtual synchronous generator (VSG) control mechanism of PV system inverter to augment FRT competency and frequency stability. The proposed VSG control system mimics the behavior of conventional power plants. To observe and evaluate the proposed controller behavior, simulation analyses were executed in the PSCAD/EMTDC software for both proposed and conventional controllers. The simulation results clearly indicate that the proposed VSG control system has sufficient damping characteristics to ensure FRT capability and frequency stability.

A phase-locked loop with an improved dynamic response under abnormal grid conditions

The cascaded delayed signal cancelation (CDSC) method has attracted much attention recently as an efficient filtering technique to improve synchronous reference frame phase-locked loop (PLL) performance in grid synchronization under adverse grid conditions. The CDSC-based PLL suffers from slow transient response. Therefore, this study proposes a solution for grid synchronization under abnormal conditions utilizing an arbitrary time-delay operator to mitigate the DC offset and frequency adaptive CDSC operators for harmonic filtering. An accurate small-signal model was derived, opening the door to design the loop filter. The coefficient diagram method was adopted to systematically design the gains of the proportional–integral controller and a lead–lag compensator, yielding a faster dynamic response than the symmetrical optimum method-based design. The proposed PLL effectiveness was verified numerically under abnormal conditions, showing better dynamic and steady-state performance than other related PLLs. The proposed PLL requires less than two grid cycles to synchronize without ripples, meeting the grid code's requirement.

Enhancement in fault ride through capabilities with inertia control for DFIG-wind energy conversion system

Doubly fed induction generator (DFIG)–based wind energy conversion system (WECS) has been found very effective in harnessing wind power. Conferring to grid code requirements, DFIG needs to stay connected to the grid under fault conditions. This implicates the requirement of enhancing the transient stability through augmenting the low-voltage ride through (LVRT) capability of DFIG. There are many software and hardware solutions for the improvement of LVRT. Fault current limiter is one of the frequently used hardware solutions for the argumentation of fault ride through requirements. Instead of LVRT, DFIG-based WECS also faces low inertia problems at fault condition. In this article, a combined approach of virtual inertia control with an external retrofit adaptive hysteresis control–based bridge-type fault current limiter (AHC-BFCL) is proposed to improve the LVRT capability as well as the inertia of DFIG-WECS. The results are compared with that of a conventional logic control-BFCL and proportional–integral-BFCL.

Facilitating the operation and integration of DER and microgrids using the IEC 61850-7-420 standard data models, especially for meeting IEEE 1547 and EN 50549 grid code requirements

Utilities, aggregators/VPP's, DER, microgrid operators and DER manufacturers are facing increasing challenges to interoperate in a seamless manner, especially in implementing grid code requirements and/or in setting-up microgrids. This paper describes how to face such a challenge based on the coming IEC 61850-7-420 object model standard, especially for meeting IEEE 1547 and EN 50549 grid code requirements. A specific focus explains how to benefit from the IEC 61850 based data models in non-IEC 61850 based systems and highlights the potential benefits.

A Real-Time Energy Management System Design for a Developed PV-Based Distributed Generator Considering the Grid Code Requirements in Turkey

Each country must determine the Grid Code conditions and apply these criteria to integrate distributed generation (DG) systems into the existing electricity grid and to ensure a stable power system. Thus, experimental studies are required to provide an effective, national, and specific Grid Code. In this study, the Turkish Grid Code’s electrical criteria were examined, and the application of these criteria was carried out on a developed PV-based DG. A real-time energy management system (RTEMS) was proposed in the study. Electrical parameters on the developed DG were monitored in real-time by considering IEEE 1547, IEEE 929–2000, and Turkey’s electrical criteria. A practical grid code study was firstly investigated in detail about the Turkish Grid Code by a developed real-time monitoring-control and protection system. The proposed RTEMS method in the study is implemented as an inverter-resident system; thus, it provides advantages over many energy management systems embedded in the inverter. The degradation in power quality and non-detection zone (NDZ) problems encountered in active and passive island mode detection methods developed embedded in the inverter are eliminated in the proposed method. With the RTEMS method, where under and over-voltage, under and over voltage frequency, and unintentional island mode events can be detected in real-time, both the existing grid-code requirements are met, and the existing power quality and NDZ problem is eliminated with the recommended inverter-independent RTEMS method.

Dynamic Voltage Support for Low-Voltage Ride-Through Operation in Single-Phase Grid-Connected Photovoltaic Systems

This article presents a dynamic voltage support (DVS) scheme for achieving low-voltage ride-through (LVRT) with a grid-connected photovoltaic (PV) inverter during the voltage sag fault. The DVS scheme is achieved by formulating an additional reactive active current control mode which is developed from a conventional reactive current control approach. This provides stable operation of the system and achieves higher effectiveness due to the lower <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">X</i> / <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</i> ratio at the point of common coupling in low-voltage networks. Further, the performance of the proposed controller is assessed by simulating a 4 kW system for voltage sag faults. Further, the proposed controller tested with the real-time simulations and experimental setup for voltage sag conditions. The results presented demonstrate that the active and reactive power is regulated in concordance with grid code requirements. The controller achieves LVRT within the time limits of grid standard during symmetrical faults, which makes it appropriate for fast transient events. This operates the PV system under its nominal capacity, avoiding unwanted grid disconnection events.

Short-circuit modeling of three-phase 4-wire unbalanced networks in presence of single-phase photovoltaic systems

Due to the growing diffusion of single-phase Photovoltaic Systems (PVSs), the analysis of active unbalanced three-phase 4-wire distribution networks in short-circuit operating conditions is nowadays of great interest. However, this analysis is not trivial due to various factors (e.g., environmental conditions, inverter control system, limits of the inverter current, location and type of the fault, PVS self-protection systems) influencing the contribution of PVSs to the short-circuit conditions. Among these factors, the greatest challenge is to model the complex behavior of the inverter control system that must comply with actual and future grid requirements, or with national and international standards, in both normal and abnormal operating conditions.An analytical model based on the phase-coordinates approach is proposed in this paper to evaluate the short-circuit contributions of single-phase PVSs connected to unbalanced three-phase 4-wire LV distribution networks. The model of the PVSs takes into account the main factors that influence the PVS contributions, with particular attention to the model of the control system which injects reactive power to support grid voltage in normal operating conditions and to perform low-voltage ride-through (LVRT) dynamic grid support during the fault.In this paper, control systems for next-generation single-phase PVSs able to satisfy grid code requirements are explored; they include constant average active power control, constant active current control and constant peak current control. Extensive numerical experiments are presented with reference to the Cigrè European LV Distribution Network Benchmark, considering different fault types, effects of various control strategies and the evaluation of the impact of the PVSs in LVRT conditions w.r.t. the passive network, i.e., the network under fault with no installed PVSs.

Model validation of a high-speed flywheel energy storage system using power hardware-in-the-loop testing

Low-inertia power systems with a high share of renewables can suffer from fast frequency deviations during disturbances. Fast-reacting energy storage systems such as a Flywheel Energy Storage System (FESS) can help limit the frequency deviations by injecting or absorbing high amounts of active power, with almost no degradation concerns. But for an accurate evaluation of the benefits of using a FESS in power systems, an accurate and validated model is necessary, which not only reflects its advantages, but also shows the practical limitations of this technology, such as its losses, its auxiliary power requirements, and its limitations during the whole operational range. In this paper, an accurate model for a high-speed FESS is presented, and then experimentally validated by means of Power Hardware-in-the-Loop (PHIL) testing of a full-scale commercial high-speed FESS in several frequency deviation scenarios. The PHIL testing setup allows testing the FESS during extreme and realistic frequency deviation, including the major frequency disturbance of August 9, 2019, in the power system of the UK. In each scenario, the same inputs that are sent to the real FESS are also given to the FESS model, running in real time, for model validation, and an excellent match between the results of the real FESS and its model is observed. Moreover, the PHIL testing results demonstrate the quick response of the FESS, following a frequency deviation, and its compliance with the latest grid code requirements in Germany for frequency support.

Modified Selective Harmonics Mitigation PWM for a Switched Diode Multilevel Inverter

A modified selective harmonic mitigation (SHM) technique for multilevel inverters considering the RMS output voltage magnitude is presented. The harmonic contents in the output voltage of these inverters must satisfy the specified grid code requirement standards. In conventional SHM techniques, the firing angles of the multilevel inverters have been derived by taking into account grid code harmonic reduction standards. When the multi-level inverters are driven with these firing pulses generated, it results in reduction of the magnitude of the inverter output voltage. In order to overcome this issue of output voltage reduction, the modified SHM optimization problem includes another constraint on the RMS output voltage limits, which results in different set of firing angles. This facilitates the use of firing angles, which takes into account the grid code standards of harmonic mitigation without compromising the value of the RMS output voltage of the inverters. The proposal has been simulated and validated in MATLAB Simulink and the experimental results are obtained for a single-phase seven level inverter with Silicon made semiconductor switches. By using the proposed method, output voltage THD (upto 40th harmonics were considered) of 5.9% was obtained, which is well below the harmonic standards specified by EN 50160.

Promoted supercapacitor control scheme based on robust fractional-order super-twisting sliding mode control for dynamic voltage restorer to enhance FRT and PQ capabilities of DFIG-based wind turbine

Aside from the importance of power quality issue, fault ride-through capability is also of greater importance which must be required for doubly-fed induction generator to meet the grid code requirements. Both of these issues can be relatively enhanced using a series-connected power electronic-based compensator styled dynamic voltage restorer. This compensator can present the best compensation performance in case that both the power supply and multilevel inverter parts to be upgraded. In this regard, this paper aims to suggest two novelties:1) New asymmetrical mutual E-type clamped X-type multilevel inverter to provide high-step staircase sinusoidal voltage containing low number of switches2) New robust fractional-order super-twisting sliding mode control for supercapacitor-based power supply to provide steady and smooth DC voltageAs for the creative structure of the proposed inverter, special geometric progression of DC voltage source and low switch count, high-step staircase sinusoidal voltage can be provided. It has been compared with the traditional and recently proposed inverters to validate its step-creation capability.As for the chattering free scheme of the proposed controller, increase of the system freedom degree and robustness against the uncertainties, nonlinearities and perturbations, steady and smooth DC voltage can be provided. It has been compared with PID, SMC and FLC controllers to validate its tracking capability.Both these suggestions have effectively enhanced the AC voltage synthesizer of DVR to accurately and promptly enhance both the FRT and PQ capabilities of DFIG-based wind turbine. The compensation capability of the proposed DVR has been evaluated under different voltage disturbances, and also compared with the conventional DVR and improved series resistive limiter along with superconducting based DC-chopper circuit. Finally, all the disturbance conditions have been perfectly compensated with trivial THD by proposed DVR, and also the DFIG-based wind turbine has been well stabilized.

Internal mode control based coordinated controller for brushless doubly fed induction generator in wind turbines during fault conditions

Brushless double fed induction generator (BDFIG) based machines have gained popularity in wind turbine applications because of their easily accessible design. Low voltage ride through (LVRT) is critical for the reliable integration of renewable energy with the power grid. The refore, LVRT capability of brushless DFIGs makes them an attractive choice for maintaining voltage stability in grid. The existing works on BDFIG control suffer from two major drawbacks. Firstly, the methodology does not consider LVRT as a design metric, and secondly, these techniques do not have any means for coordinating between a machine side inverter (MSI) and grid side inverter (GSI). This results in sub-optimal controller design and eventually result in the violation of grid code requirements. To solve these issues, this paper proposes the use of brushless DFIGs in wind turbines using a control technique based on analytical modeling. Moreover, employing internal model control (IMC), the proposed technique can effectively coordinate the control between the MSI and GSI resulting in reduced oscillations, overshoots and improved stability under fault conditions. Furthermore, the simulation results for wind turbine generators show that the proposed scheme significantly improves the stability and compliance of grid codes ascompared to the existing hardware techniques.

European and Indian Grid Codes for Utility Scale Hybrid Power Plants

The aim of this paper is to review and compare present European and Indian grid code requirements imposed to hybrid power plants (HPPs) combining wind, solar and storage technologies. Since there are no grid codes specifically for HPPs, the paper will review grid codes for the power plant based on individual renewable technology in the HPP. European grid codes specifies ranges for parameters inside which each national transmission system operators (TSO) has to specify the set of national parameters (Danish specifications in this paper). The comparisons are performed with respect to fault-ride-through capability, frequency and voltage operation ranges, active power control/frequency support as well as reactive power control/voltage support.

Decentralized Optimal Control for Photovoltaic Systems Using Prediction in the Distribution Systems

The high penetration of photovoltaic (PV) systems and fast communications networks increase the potential for PV inverters to support the stability and performance of microgrids. PV inverters in the distribution network can work cooperatively and follow centralized and decentralized control commands to optimize energy production while meeting grid code requirements. However, there are older autonomous inverters that have already been installed and will operate in the same network as smart controllable ones. This paper proposes a decentralized optimal control (DOC) that performs multi-objective optimization for a group of PV inverters in a network of existing residential loads and autonomous inverters. The interaction of independent DOC groups in the same network is considered. The limit of PV inverter power factor is included in the control. The DOC is done by the power flow calculation and an autoregression prediction model for estimating maximum power point and loads. Overvoltage caused by prediction errors resulting in non-optimal commands from the DOC is avoided by switching to autonomous droop control (ADC). The DOC and ADC operate at different time scales to take account of communication delays between PV inverters and decentralized controller. The simulation of different scenarios of network control has proved the effectiveness of the control strategies.