Modern Grid Codes Research Articles

Optimizing active distribution microgrids with multi-terminal soft open point and hybrid hydrogen storage systems for enhanced frequency stability

The recent increased interest in active distribution microgrids has complicated the process of reaching stable generation with the presence of distributed generators (DGs). These challenges can be handled by employing multi-terminal soft open points (SOPs), which can further boost system performance compared to the conventional two-terminal SOP, especially with present system disturbances. At the same time, hydrogen energy storage has drawn increased attraction to strengthen power grid stability and flexibility. This paper uses a hybrid-based energy storage device that employs an electrolyzer and fuel cell means with a hydrogen tank to absorb or generate power through multi-terminal SOP based on desired grid requirements. This work also offers a novel implementation of a hybrid jellyfish search and particle swarm optimization (HJPSO) applied to model predictive controllers (MPCs) to provide a solution regarding frequency control issues for multiple microgrids comprised of hybrid micro-turbines and renewable generators with the presence of storage devices. Furthermore, different forms of nonlinearity and actual data measurements for the implemented renewable generators are incorporated to attain further realistic analysis. The transient behavior of the studied microgrid is substantially augmented via the suggested control scheme. Referring to the modern grid codes and standards, the frequency operation limits in a specific range. This requirement has been preserved throughout the entire simulations, including uncertainty analysis, assessing the capabilities of multi-terminal SOP in providing frequency disturbance ride-through for the operating active hybrid distribution microgrid under various operating conditions.

Transmission line protection using local information in the presence of inverter‐interfaced renewable energy sources

AbstractAlthough inverter‐interfaced renewable energy sources (IIRESs) have great economic and environmental benefits, their fault current characteristics differ from those of synchronous generators. IIRESs also comply with modern grid code (GC) requirements for injecting reactive current to compensate for voltage drop during fault conditions. Moreover, the limits of renewable energy‐side fault current contribution and the phase angle difference between the local and infeed fault current from conventional sources lead to incorrect reactance estimation by the traditional distance relay at the IIRES substation and negatively affect relay dependability. As a result, the IIRES‐side distance protection is prone to incorrect operation. Furthermore, an improved relaying scheme is proposed to alleviate the adverse effects of IIRES on the reactance calculation. This research aims to improve the performance of the ground distance protection scheme in the case of a single‐phase to ground fault in zone one. The proposed method, with more than 99% accuracy, is solely dependent on the local information and, therefore, no communication link is required to implement the algorithm. Moreover, the algorithm is compatible with the existing pilot protection methods used in the industry. The simulation studies are modeled in the Digsilent Power Factory environment.

A Blind Spot in the LVRT Current Requirements of Modern Grid Codes for Inverter-Based Resources

Modern grid codes (GCs) require that inverter-based resources (IBRs) inject both positive- and negative-sequence currents during asymmetrical low-voltage ride through (LVRT) conditions. This GC provision prioritizes the reactive currents and also demands maximizing the active positive-sequence current if the IBR has unused current generation capacity when the required reactive current is generated. A variety of inverter control schemes are available to generate positive- and negative-sequence active/reactive currents, and satisfying these GCs seems to be straightforward. However, this paper reveals that the reference current generation methods of existing inverter control schemes fail to fulfil some important requirements of recent GCs. For example, they do not fully utilize the inverter capacity to generate the maximum active and/or reactive current. It is shown that these so-far hidden GC violations can result in a large untapped generation capacity during asymmetrical faults. This paper also develops an algorithm that satisfies recent GCs by deriving the positive- and negative-sequence currents that maximize the IBR’s reactive and active currents while the reactive current is prioritized. The simulation of a grid with high IBR penetration verifies that this new algorithm can unlock the full potential of recent GCs by significantly increasing the power generated during LVRT.

A new fault type classification method in the presence of inverter-based resources

Fault type classification is a critical stage in modern distance relays for activating the effective phase and ground distance elements. It is also used for some supplementary functions such as single-pole tripping and reclosing, teleprotection, and fault location. Traditional fault type classifiers are based on the symmetrical or superimposed components of the current signal. However, owing to the unconventional behavior of inverter-based resources (IBRs) compared to the synchronous generators, traditional methods are not reliable in the presence of IBRs. For example, modern grid codes impose some restrictions in terms of injected current during asymmetrical fault which put the accuracy of conventional fault classification methods in danger. Fault resistance could also affect the accuracy of fault type classification method. In this paper, first, the fault current signature of IBRs is analytically derived. Then, impacts of modern grid code and fault resistance on the conventional fault classification are properly evaluated by executing some simulation studies. Finally, a new fault classifier is proposed which is based on symmetrical components of the local voltage and current during asymmetrical faults. The proposed classifier presents a satisfactory degree of security in the faulted phase identification in the presence of IBRs under different host grid codes. Performance of the proposed method is properly analysed with different simulation scenarios such as different fault locations, fault resistances, pre-fault condition, test systems, and host grid codes in PSCAD/EMTDC software.

Dynamic response and low voltage ride-through enhancement of brushless double-fed induction generator using Salp swarm optimization algorithm.

A brushless double-fed induction generator (BDFIG) has shown tremendous success in wind turbines due to its robust brushless design, smooth operation, and variable speed characteristics. However, the research regarding controlling of machine during low voltage ride through (LVRT) need greater attention as it may cause total disconnection of machine. In addition, the BDFIG based wind turbines must be capable of providing controlled amount of reactive power to the grid as per modern grid code requirements. Also, a suitable dynamic response of machine during both normal and fault conditions needs to be ensured. This paper, as such, attempts to provide reactive power to the grid by analytically calculating the decaying flux and developing a rotor side converter control scheme accordingly. Furthermore, the dynamic response and LVRT capability of the BDFIG is enhanced by using one of the very intelligent optimization algorithms called the Salp Swarm Algorithm (SSA). To prove the efficacy of the proposed control scheme, its performance is compared with that of the particle swan optimization (PSO) based controller in terms of limiting the fault current, regulating active and reactive power, and maintaining the stable operation of the power system under identical operating conditions. The simulation results show that the proposed control scheme significantly improves the dynamic response and LVRT capability of the developed BDFIG based wind energy conversion system; thus proves its essence and efficacy.

Adaptive super-twisting terminal sliding mode control and LVRT capability for switched reluctance generator based wind energy conversion system

Wind Energy Conversion systems (WECS) based on the Switched Reluctance Generator (SRG) devoid of rotor windings and permanent magnets are becoming more and more an attractive generator in wind energy market. Currently, the applicability of this machine in the WECS is now possible thanks to the mitigation of its drawbacks, mainly its high torque fluctuations and its low power factor. In this paper, an Adaptive Super Twisting Terminal Sliding Mode (AST-TSM) control is proposed for SRG-based WECS interfaced to the grid via a full-scale back-to-back converter. The main purpose of the proposed scheme is controlling the generator speed in order to achieve a Maximum Power Point Tracking (MPPT) with better performances. The second issue addressed consists of exploring the Low Voltage Ride Through (LVRT) capability of SRG-based WECS for compliance with recent grid code requirements. In modern grid codes, wind farms are only allowed to be connected to the grid if they have the capacity to remain in service during a short grid disturbance. As a matter of fact, under grid faults, the main tasks of the LVRT method consist in feeding the required reactive power into the grid in order to facilitate the recovery of the grid voltage. In this work, the investigated LVRT scheme, classified as a software method, is responsible for swapping the back-to back converter duties without external devices under grid voltage sags. Thereby, the generated power is stored as kinetic form, preventing strong overvoltage on the DC-bus. The proposed AST-TSM control scheme and the adopted LVRT method are substantiated by simulation results. All simulation case studies show that the AST-TSM control achieves good performance and extreme robustness as compared with the Proportional Integral (PI) and Integral Backstepping (IBC) controllers. Furthermore, the proposed controller is able to improve the suppression of chattering effect when its parameters are adjusted carefully. Also, simulation under grid faults conditions has proven that the LVRT method is effective for SRG-based WECS.

An intelligent robust cascaded control scheme for renewable energy based microgrid

AbstractHandling the uncertainties in the utility grid and uncertainties in renewable energy sources is a main challenge to accomplish the modern grid code requirements. This article proposes a new control strategy based on interval type‐2 fuzzy sets for controlling the grid‐connected dispersed generation (DG) system tied to the distorted electric grid. The proposed control technique can easily model and handle the uncertainties in the system parameters and renewable energy sources. The existence of three‐dimensional membership functions of type‐2 fuzzy sets offers an additional degree of freedom to counter the uncertainties in the system. The proposed strategy is effective in improving the dynamic response during system uncertainties such as distortions in grid voltage, variations in grid frequency, variations in renewable energy sources and due to presence of non‐linear and unbalanced loads. Also, it improves the quality of the current being injected to the grid during uncertainties. The proposed control strategy is applied to control the dc side capacitor voltage as well as to control the current loop of the grid connected inverter. The proposed system is simulated in MATLAB Simulink environment and the performance is compared with traditional controllers to show the effectiveness of the proposed control strategy during system abnormalities.

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.

Design and Performance Analysis of a PV Control Scheme to Improve LVRT of Hybrid Power System.

Solar power stations worldwide have been rising every year as well as the usage of sustainable power has increased. The addition into the traditional electricity grid networks of renewable sources such as the photovoltaic (PV) system is an essential challenge, given its erratic power generation. The major issue is to detach PV from the faulty grid, which leads to fluctuation in the system's interconnected PV system and system-wide energy interruptions. The low voltage ride through (LVRT) enables the PV scheme should stay linked to the grid even under fault situations in short periods and must assist the grid return to regular situations according to the modern grid codes. Hence, this paper develops and analyzes five aspects of control strategy to increase PV station's LVRT capability. The in-depth simulation of the independent units of the grid-connected large-scale PV farm is accomplished using PSCAD/EMTDC to assess the feasibility of the suggested control tactic for the modified IEEE nine-bus system.

Dual-Mode Control for Brushless Doubly Fed Induction Generation System Based on Control-Winding-Current Orientation

Brushless doubly-fed induction generator (BDFIG) has huge potential due to its high reliability and low maintenance costs. In order to meet the requirements of the modern grid code, the BDFIG system should have dual-mode (i.e., operating in both stand-alone and grid-connected modes) capability. Rather than using two separated systems for two modes, this article proposes a unified control architecture for dual-mode operation. Such a unified architecture is based on the stand-alone control-winding (CW) current orientation, and the grid-connected functions are realized by adding three plug-in functional units: 1) the phase-lock function is realized by adding an increment adjustment controller into the CW-angle control; 2) the inrush current during mode switching is suppressed by adding a high-pass feedforward controller into the inner-loop controller; 3) the active power and reactive power are regulated by adding an amplitude-angle controller of CW current. With the proposed unified architecture, the high-performance dual-mode operation is achieved without breaking the original CW-current orientation system, facilitating the overall system design a lot. The detailed analysis and designs are given in this article, and the experiments based on a BDFIG prototype are given to verify the feasibility.

Power control strategies for grid connected converters applied to full-scale wind energy conversion systems during LVRT operation

Some grid codes establish that during voltage sags, the generation units employing static converters must favor the electrical mains support. Unbalanced voltage sags generate grid power oscillations with twice the fundamental frequency. This way, the converter control can be modified to mitigate the unbalanced sag effects to the grid converter. The present work analyzes different power control methods presented in literature, in order to study how they affect the active and reactive power oscillation and the generated current waveform. A case study with a simulation of a 2(MW) WECS is performed in order to analyze the strategies behavior in terms of power fluctuation and converter dc bus impacts. It is evaluated the system behavior over different operation points of reactive current injection curve, defined in the modern grid codes.

Evaluation of the Use of Class B LoRaWAN for the Coordination of Distributed Interface Protection Systems in Smart Grids

The adoption of the distributed generation paradigm is introducing several changes in the design and operation of modern distribution networks. Modern grid codes are becoming more and more complex, and the adoption of smart protection systems is becoming mandatory. However, the adoption of newer and smarter units is only half of the story. Proper communication networks must be provided as well, and the overall costs may become critical. In this work, the adoption of the Long-Range Wide Area Network (LoRaWAN) technology is suggested as a viable approach to implement the coordination of Interface Protection Systems. A proper communication architecture based on the LoRaWAN Class B technology was proposed and evaluated in order to assess its feasibility for the considered application. A scalability analysis was carried out, by computing the number of devices that can be handled by a single LoRaWAN Gateway (GW) and the maximum expected time of response between a triggering event and the arrival of the related coordination command. The results of the study showed that up to 312 devices can be managed by a single GW, by assuring a maximum response time of 22.95 s. A faster maximum response time of 6.2 s is also possible by reducing the number of managed devices to 12.

Harmonic Analysis and Control of Grid-Connected Solar PV Inverter under Normal and LVRT Operating Modes

Environmental factors and active involvement in grid-connected solar PV inverter ancillary operations may impact the quality of the current injected into the grid. The future grid-connected solar PV system with ancillary facilities (e.g., low voltage ride-through (LVRT)) will be more active and intelligent, which will degrade grid current reliability. The grid current distortions are specifically caused by the dc-link voltage variations and the modulation of pulse width (PWM) control applied to the PV inverter. This article analyzes the current harmonic distortion under the two-stage grid-connected PV system's regular (MPPT) and fault (LVRT) condition. Furthermore, a dc-link voltage variation control system for the two-stage photovoltaic (PV) inverter is presented during low voltage ride-through (LVRT) operation mode..The dc-link voltage differences are regulated under the fault condition to preserve the high modulation ratio in order to considerably mitigate the distortion rate of the grid current. Besides, the proposed system of control is designed to protect the PV inverter from the overcurrent failure under the faults to meet the modern LVRT grid codes. The conducted simulation tests have confirmed that the proposed control scheme leads to reduce a grid currents harmonics level by controlling the dc-link voltage variations.

Grid-Connected Wind Power Plants: A Survey on the Integration Requirements in Modern Grid Codes

In recent years, the integration of wind power generation facilities, and especially offshore wind power generation facilities, into power grids has increased rapidly. Therefore, the grid codes concerning wind power integration have become a major factor in ensuring power system reliability. This work compares grid codes about wind power integration around the world. The grid codes of Denmark, Ireland, the U.K., Germany, Spain, China, the U.S., Canada, and other countries are considered. The most important of these grid codes concern reactive power, frequency regulation, fault ride through, and power quality. Several grid codes also address communication, ramp rate, and offshore wind power plants. This work provides information on the future of grid code requirements for offshore wind power integration, which helps the system operators ensure the safe operation of a power system with a high penetration of wind power generation.

Current Reference Generation Based on Next-Generation Grid Code Requirements of Grid-Tied Converters During Asymmetrical Faults

Increased penetration of converter-based power generation has enforced system operators to require ancillary services from distributed generation in order to support the grid and improve the power system stability and reliability. Recent and next-generation grid codes require asymmetrical current provision during unbalanced faults for optimal voltage support. To address this, based on the highly used flexible positive- and negative-sequence control method for current reference generation, this paper presents a general current reference strategy for asymmetrical fault control, where a direct and explicit method is proposed to calculate power references and controller gains while simultaneously complying with converter current limitation and fulfilling the next-generation grid code requirements. The proposed method is tested for three distinct asymmetrical grid faults considering the requirements for dynamic voltage support of the recently revised German grid code as well as the next-generation grid codes. It is shown that the proposed method can improve the fault ride-through performance during asymmetrical faults compared with conventional solutions and comply with modern grid code requirements in a general and flexible manner.

A novel control solution for improved trajectory tracking and LVRT performance in DFIG-based wind turbines

This paper presents a new control strategy for the rotor side converter of Doubly-Fed Induction Generator based Wind Turbine systems, under severe voltage dips. The main goal is to fulfill the Low Voltage Ride Through performance, required by modern grid codes. In this respect, the key point is to limit oscillations (particularly on rotor currents) triggered by line faults, so that the system keeps operating with graceful behavior. To this aim, a suitable feedforward-feedback control solution is proposed for the DFIG rotor side. The feedforward part exploits oscillation-free reference trajectories, analytically derived for the system internal dynamics. State feedback, designed accounting for control voltage limits, endows the system with robustness and further tame oscillations during faults. Moreover, improved torque and stator reactive power tracking during faults is achieved, proposing an exact mapping between such quantities and rotor-side currents, which are conventionally used as controlled outputs. Numerical simulations are provided to validate the capability of the proposed approach to effectively cope with harsh faults.

A State-Space Dynamic Model for Photovoltaic Systems With Full Ancillary Services Support

Large-scale photovoltaic (PV) integration to the network necessitates accurate modeling of PV system dynamics under solar irradiance changes and disturbances in the power system. Most of the available PV dynamic models in the literature are scope-specific, neglecting some control functions and employing simplifications. In this paper, a complete dynamic model for two-stage PV systems is presented, given in entirely state-space form and explicit equations that takes into account all power circuit dynamics and modern control functions. This is a holistic approach that considers a full range of ancillary services required by modern grid codes, supports both balanced and unbalanced grid operation, and accounts for the discontinuous conduction mode of the dc/dc converter of the system. The proposed dynamic model is evaluated and compared to other approaches based on the literature, against scenarios of irradiance variation, voltage sags, and frequency distortion. Simulation results in MATLAB/Simulink indicate high accuracy at low computational cost and complexity.

Optimal reactive power dispatch of permanent magnet synchronous generator-based wind farm considering levelised production cost minimisation

As wind power penetration increases, large wind farms (WFs) need to provide reactive power according to modern grid codes. Permanent magnet synchronous generator-based wind turbines (WTs) can generate reactive power, by assigning the appropriate reactive power to each WT to meet the reactive power requirements of the grid. This is a more economical method than setting up additional reactive power compensation equipment. This study proposes an optimal reactive power dispatch strategy for minimising a levelised production cost, and is implemented in two ways: minimising the power loss of a WF, and maximising the lifetime of WTs. The reactive power references of each WT are chosen as the optimisation variables, and a particle swarm optimisation algorithm is adopted to solve the optimisation problem. The proposed and traditional reactive power dispatch strategies are demonstrated and compared on a WF with 25 WTs to validate the effectiveness of the proposed approach.

Stability Analysis and Enhancement of Type-4 Wind Turbines Connected to Very Weak Grids Under Severe Voltage Sags

Modern grid codes require wind turbines (WTs), e.g., type-4 WTs, to have low voltage ride-through (LVRT) capability, and recently, great attention has been attracted on the stability issues of WTs during the period of the LVRT. In this paper, we investigate the influences of the LVRT control on the stability of type-4 WTs under severe voltage sags in very weak grids. First, by proposing a novel quasi-static modeling method, we explore the existence of the WT's equilibrium during the LVRT. It is shown that the WT can lose the equilibrium under certain circumstances, leading to instability of the whole WT system. Furthermore, we use the small-signal analysis to illustrate that even there exists an equilibrium, the WT can become linearly unstable due to the dynamics of the current references generated by the LVRT control. We propose an enhanced LVRT control strategy to deal with these instability problems. Simulations verify our analysis and effectiveness of the proposed control strategy.

Robust disturbance rejection control of grid-connected fuel cell converters with grid support ability under unbalanced faults

This work proposes a high-performance, robust and low-complexity control strategy for grid-connected fuel cell converters that operate under several performance limiting conditions such as grid faults, parametric and modeling uncertainties, unknown disturbances, step changes and noise. The strategy enables the fuel cell-based distributed generators to ride through balanced and unbalanced grid faults and fulfills the modern grid code requirements that mandate them to actively support the grid in case of contingencies. The controller is built by combining disturbance rejection control and repetitive control, both having simple structures and attractive performance features. Moreover, during grid faults, the strategy manages to keep the active power delivered to the grid constant and the phase currents sinusoidal, without a phase locked loop or the positive and negative sequence components of the unbalanced grid voltages or currents. As a result, the controller’s computational and structural complexity are reduced without compromising its performance. Several test cases are run using the $$\hbox {SimPowerSystems}^{\mathrm{TM}}$$ toolbox of MATLAB/Simulink computing environment to demonstrate the response of the proposed strategy and ascertain its performance under grid faults, parametric uncertainties, noise and unknown disturbances.