Optimal dynamic voltage support in photovoltaic system integration

Increasing levels of renewable energy sources (RES) in the form of inverter-based resources (IBRs) affect the overall inertia of the power system. Grid-forming (GFM) inverters have the potential to contribute to synthetic inertia and enhance the oscillatory stability of the system. This paper presents a comparative analysis of the performance of GFM inverters and conventional synchronous generators (SGs). The GFM inverters are implemented using a virtual synchronous machine (VSM) control model, whereas conventional SGs use the GENROU model. PowerWorld Simulator and DSATools are employed for steady-state and dynamic (time-domain) simulations. The Single-Machine Infinite-Bus (SMIB) system and the benchmark two-area Kundur system are used to investigate the damping performance of GFM inverters under varying grid strengths. The performance of system damping is evaluated using conventional eigenvalue analysis and the matrix pencil method. The simulation results show that GFM inverters with VSM control provide acceptable damping performance compared to conventional SGs and have the potential to play a key role in future low-inertia power grids.

In recent years, interest in integration of renewable energy (RE) sources (particularly Wind Energy Systems - WES) into the electrical grid for sustainable, innovative solutions and a greener future continues to grow exponentially. To date, most RE sources have been connected to the grid through traditional grid-following inverters (GFL). Although this control method promises grid compatibility and lower complexity, the unpredictable variability of these eco-friendly RE sources leads to undesirable effects when taken into account. With the increasing interaction of renewable energy sources in today's grid, new challenges arise—such as voltage/frequency regulation and the reduction of overall system inertia—that cannot be resolved with conventional grid-following inverters (GFL). Therefore, the concept of grid-forming inverters (GFM) has been adopted. The aim of this study is to examine the comparison between grid-forming inverters and traditional grid-following inverters, particularly focusing on droop control, and to regulate the simulated test results with appropriate control systems. For this purpose, the study first compares GFM—based on different control techniques (droop, virtual synchronous generator, and synchronverter control) in the literature—with the conventional GFL technique. The circuit tested in this study involves a 40 kVA-rated inverter supplying relevant loads and the grid, with GFL and GFM control modes analyzed separately. Finally, the potential effects of inverters on load variation and grid outages will be considered.

The high penetration of renewable energy sources (RES) results in a low‐inertia, weak power grid. To mitigate this and restore system inertia, it has been widely proposed to operate the inverters of RES units to mimic synchronous generators; this technology is known as a virtual synchronous generator (VSG). In weak grids there is, however, strong coupling between active power () and reactive power (), and any VSG technique therefore requires decoupling in order to operate effectively. This article proposes a new decoupling technique based on the transformation of the power circle of a VSG connected to a weak grid: first, the power circle is translated from its designed position to that of a conventional synchronous generator (SG) connected to a strong grid, achieving partial decoupling. Then, to achieve full decoupling, the authors propose further to modulate the radius of the translated power circle; this is achieved using a series resistance‐capacitance–inductance () circuit which is virtually implemented in the VSG controller. The efficacy of the proposed scheme is validated using a modified synchronverter connected to the weak grid in representative loading scenarios. It is demonstrated that the technique achieves a decoupled control for the synchronverter connected in a weak grid. Moreover, the modified synchronverter is capable of supporting frequency and voltage regulation in the grid without inducing large transient grid currents during mild frequency and voltage variations.

For voltage-source converter (VSC) based high-voltage direct-current (HVDC) connected to weak AC grids, the grid current control interacts with phase-locked loop (PLL) under weak grid, via point of common coupling (PCC) voltage. However, the dynamic of PLL might result in system instability under weak ac grid. To mitigate this problem, a phase-optimized PLL (PO-PLL) technique is proposed for VSC-HVDC connected to weak ac grids. Then, the small-signal output impedance models of VSC-HVDC connected to weak ac grids under based-PLL synchronous and based on PO-PLL synchronous mode are derived in the synchronous reference (d-q) frame. The impedance and stability under PLL-based synchronous and based on PO-PLL synchronous mode are analyzed. Finally, numeric simulations on MATLAB/Simulink are conducted to validate the correctness of the proposed PO-PLL synchronous mode.

Active power oscillation in the hybrid grid-connected system with grid-forming (GFM) and grid-following (GFL) inverters can threaten its safe operation. In this paper, considering the coupling between GFM and GFL inverters, the small signal model of the heterogeneous system is established. Based on the model, the influence mechanism of control parameters on the power oscillation under varying short-circuit ratio (SCR) is qualitatively and quantitatively revealed. In particular, the damping of the GFM inverter has a dominant effect compared with other parameters. Therefore, dual mutual damping control (DMDC), containing angular frequency and active power mutual damping, is integrated into the GFM inverter to enhance system damping. Its stability analysis and key parameters design are also presented. Finally, the experimental results validate the effectiveness of the DMDC, demonstrating a reduction in the interaction power oscillation amplitudes by 38.5% and 29.4% under strong and weak grid conditions, respectively.

Power converter operation on high-impedance grids can result in oscillatory behavior as a result of “self-synchronization” of an inverter's phase detection unit (PDU). This phenomenon is particularly of concern in renewable energy applications and it occurs when the converter's injected current changes the voltage angle at the point-of-common coupling (PCC) and synchronizes to itself through its PDU. One of the most prevalent PDUs in industry is based on the synchronous-reference frame (SRF) phase-locked loop (PLL). In this work the large signal stability limit of the SRF-PLL is modified to include the angle of reactive current injection, the phenomena of reduced gain margin as a result of self-synchronization is explained and a method to prevent the effects of self-synchronization is proposed and demonstrated. The proposed method is referred to as the grid-sync (GS) PLL and requires a real-time estimation of the grid impedance. The GS-PLL is shown to improve small-signal stability in order to reduce phase-margin erosion.

The integration of power electronic devices and renewable energy sources in the grid has introduced non-linearities and harmonics in the system. The photovoltaic-based distribution static compensator (PV-DSTATCOM) enhances power quality in the grid by suppressing harmonic components, balancing grid currents under unbalanced load conditions, and stabilizing the voltage at the point of common coupling (PCC) under grid voltage disturbance. For stabilized operation of PV-DSTATCOM in such a scenario, a modified instantaneous reactive power theory (IRPT) based control strategy has been proposed. The designed control strategy is applied to a PV-DSTATCOM, coupled with a weak grid, and aims to achieve multiple control objectives. This control algorithm generates the current reference signal for switching of PV-DSTACOM. Furthermore, proposed algorithm ensures that the power output from the solar PV is first utilized by the connected loads, and balanced energy is transferred to the utility grid. The performance of the proposed controller is assessed in comparison with the conventional IRPT technique and the UVT-based control strategy. The operational efficiency of the control algorithm under dynamic conditions is demonstrated through simulations performed in MATLAB/Simulink. The simulation results have been verified using a real-time simulator, OPAL-RT. The tests are conducted for nonlinear loading, unbalanced loading, grid voltage sag, grid voltage swell, and solar PV insolation changes. The test results are satisfactory, with grid current distortion levels lying within the limits specified by IEEE Standard 519.

Drastic climate changes have shifted power grids to the integrated power system (IPS), i.e., inverter-based resources (IBRs), leading to operational challenges associated with reducing system inertia and intermittent power generation. Improved inverter control schemes are necessary to ensure the stability and robustness of the power system. Different types of inverter schemes are used in IPS: grid-following inverters (GFLIs), grid-forming inverters (GFMIs), and grid-supporting inverters (GSIs). GFLIs may experience frequency and voltage fluctuations as IBR penetration increases. Still, GFMIs can regulate both frequency and voltage at their point of common coupling, mimicking the behavior of synchronous generators. GFMIs is an emerging technology, attracting academia and industry. This work aims to review the role of GFMIs from the perspective of undergraduate electrical engineering research in power systems education. Students have the opportunity to understand the theoretical background while at the same time performing simulations and experiments to enrich their learning experience.

Distributed Generation (DG) sources predominantly renewable energy-based like solar photovoltaic and wind energy sources are considerably penetrated in power system networks. This high penetration of DG sources may lead to the formation of unintentional islands which is hazardous to both the equipment and personnel, if sustained. In this paper, 46 passive parameters are considered to find out which candidate(s) are promising for detecting sustained unintentional islands and avoid false DG trips. These 46 parameters are derived from point of common coupling (PCC) voltage, DG output current, frequency, power factor angle, active and reactive powers. All these parameters are tested on standard IEEE 13 and 34 bus distribution networks integrated with inverter-interfaced DGs at different locations for different penetration levels in MATLAB/SIMULINK environment. The parameters are tested for different islanding events (comprising of various local loads such as conventional parallel RLC resonant load, R, RC, and RL loads) and for different non-islanding events. The parameters are then ranked accordingly by a suitable averaging approach based performance ranking technique. From this analysis, the best candidates are obtained for detecting inverter DG islands at single-point (or single PCC) location and multi-point (or multiple PCCs) locations. Multiple best passive candidates are obtained for different island scenarios from which a set of promising islanding detection indicators are proposed.

In recent years, grid-following inverters (GFLIs) and grid-forming inverters (GFMIs) have established their position as the mainstream synchronization techniques for the grid integration of inverter-based resources into power systems. However, both of these integration techniques have their shortcomings in weak, strong, and/or series-compensated grids. Recently, in the literature, a power synchronized GFLI has been proposed that does not suffer from the issues exhibited in GFLIs and GFMIs. This article provides an analytical output impedance of this power-synchronized GFLI that uses a linear parameter-varying controller (LPV-PSGFLI). This impedance is then used for impedance-based stability analysis via the Nyquist stability criterion. In addition, the derived impedance is validated via system identification, and a comparison between a conventional GFLI and an LPV-PSGFLI is made under various system strengths. It is seen that the LPV-PSGFLI can inject power into the grid, whereas the conventional GFLI fails when both are integrated into a very weak grid. Finally, the accuracy of the impedance-based stability analysis using the derived output impedance is validated in Matlab/PLECS and in experiments in various scenarios.

Increasing penetration of Renewable Energy Sources (RESs) such as wind, solar plants equipped with fast-acting control systems to the weak points of a power system could be concerned about the problem from classical voltage instability to control instability and control interactions. System strength is one of the important concerns in the integration of RES. Short circuit ratio (SCR) is often used as an index of the system strength. However, when RES are installed in an adjacent location, system strength can be lower than evaluation by SCR. Weighted Short Circuit Ratio (WSCR) is an index that reflects RESs interaction by assuming a 100% influence. This paper propose Inverter Interaction Level Short Circuit Ratio (IILSCR) method using inverter-based resource's output tracing. IILSCR can accurately estimate system strength which RESs are connected in adjacent spot by reflecting the interaction between RESs.

This work is associated with the renewable energy based microgrid for a standalone operation. The places where the renewable energy sources such as wind, solar, hydro etc. are in abundance can be used to generate electricity by developing microgrid (MG) such as wind- hydro MG. A synchronous reluctance generator (SyRG) is used as a constant speed hydro turbine and a permanent magnet brushless DC generator (PMBLDC) as a variable speed wind turbine. The wind power is converted into electricity and converted into DC power using three phase diode bridge with a boost converter, a maximum power point tracking (MPPT) based perturb and observe (P&O) technique is applied. This power is fed to a battery bank and the consumer loads. The battery bank is attached at DC bus of VSI (Voltage Source Inverter). The SyRG is directly feeding to the AC loads and VSI is connected at the PCC (Point Of Common Coupling). This VSI is also used for the power quality improvements through harmonic suppression of nonlinear loads, voltage regulation during contingencies like load unbalance, reactive power compensation according to the system requirement etc. at PCC. It is capable of providing power balance with various changes. For the appropriate functioning of VSI, a reweighted zero attractor least mean square (RZALMS) control algorithm is applied to generate PWM switching pulses. A MG model is developed in MATLAB/ Simulink environment to simulate the MG performance in normal and dynamic conditions at linear and nonlinear loads.

The increasing integration of inverter-based resources (IBRs) for renewable energy sources (RESs) generation into power systems has a significant impact on their frequency transient behaviour. This is due to the variability and partially non-programmability of RES power generation and the lack of rotating inertia due to the use of static converters. This research work aims to analyse how frequency transients are affected by the increasing share of generation from RES, e.g. photovoltaics (PV), in the total generation, focusing on the types of inverters used as an interface with the grid, Grid Following (GFL) or Grid Forming (GFM) inverters. A quantitative analysis of the impact on the frequency transient of different GFM percentages on total IBR generation is provided. The study shows that a share of 50% of GFM inverter allows a frequency transient with a similar Nadir and still a higher Rate of Change of Frequency ( RoCoF ) of a system with only synchronous generators (SGs). The frequency-supporting actions of these technologies will be evaluated considering the periods of overgeneration, characterised by the need to cut PV production, creating a reserve of available but untapped energy.

This paper presents a three-phase Active Power Conditioner to improve power quality in microgrids based on renewable energy. A microgrid is a weak electrical grid which can be easily subject to disturbances. The Active Power Conditioner (APC) presented in this paper acts as an interface between renewable energy sources and the AC bus of a microgrid and uses an improved control strategy, which makes possible to inject energy in the microgrid, compensate the current harmonics and correct the power factor. Moreover, the proposed control strategy allows the line current at the point of common coupling (PCC) to be balanced and sinusoidal even when the load is unbalanced. Consequently, the voltage at the PPC becomes balanced. Simulation results show the validity of the innovative control strategy.

A Static Synchronous Compensator (STATCOM) is one of the Flexible AC Transmission System (FACTS) device which plays an vital role in power system network in alleviation of small and large transient disturbances and better voltage regulation. This paper proposed a self tuning Proportional-Integral (PI) controller for STATCOM which provided a voltage support at point of common coupling (PCC). Self tuning of PI controller was achieved by adapting PI controller gains with system parameters by using Bats Echolocation controller which mimics the Bats Echolocation behavior. In the proposed controller, the system voltage (parameter) at point of common coupling compared with reference voltage and then Bats Echolocation Controller updates the PI controller gains according to the variation obtained between them. This paper also proposed a Cascaded Multilevel Inverter (CMLI) instead of conventional Voltage Source Inverter (VSI) for STATCOM to obtain better voltage regulation at point of common coupling. The voltage imbalance between H-bridge cells in CMLI was rectified by prosing Multicarrier Pulse Width Modulation (PWM) techniques, The proposed controller was validated by comparing the simulation results with fixed gain PI controller and Self tuning PI controller using Neural controller and Particle Swarm Optimization.