AC Grid Research Articles

Impedance Characteristic Analysis and Phase-locked Angle Feedforward-based Stability Improvement for LCC-HVDC in Sending AC Grid

Impedance Characteristic Analysis and Phase-locked Angle Feedforward-based Stability Improvement for LCC-HVDC in Sending AC Grid

Enhancing Wind Power Plant Integration Using a T-NPC Converter with DR-HVDC Transmission Systems

This paper introduces an innovative control technique tailored for the seamless integration of wind power plants (WPP) into onshore grids. Our approach utilizes Diode-Rectifier-based High Voltage Direct Current (HVDC) system and incorporates a T-type Neutral Point Clamped (T-NPC) converter on the HVDC back, providing solutions for current challenges in wind power integration. While the use of Diode Rectifier (DR) as the Offshore converter for HVDC transmission has proven advantageous in terms of cost and loss reduction, it also introduces new challenges, such as maintaining AC grid voltage and frequency on the Offshore side and achieving the synchronization of the wind energy conversion system (WECS). This paper proposes a comprehensive control strategy to adresse these challenges by developing frequency and AC voltage control loops, ensuring the synchronization of WECS with the DR AC side. A notable distinctive feature of our proposed technique is to ensure the regulation of the DC voltage of the HVDC cable, accomplished through the onshore inverter. This crucial aspect ensures stable and efficient power transmission from wind power plants to the onshore grid.

Dynamic Short Circuit Ratio for Stability Assessment of Grid Connected VSC-HVDC Systems Considering the Impact of Time-Varying Phase Shift

As more renewable resources are integrated into the receiving-end AC grid through VSC-HVDC, the grid connected VSC-HVDC system is facing a critical dynamic stability issue involving multiple-frequency oscillatory behaviors, which mainly stems from the low-system-strength and the time-varying phase shift. This paper proposes a general stability analysis method to measure the dynamic stability of grid connected VSC-HVDC system from the point of view of input to state stability (ISS). A nonlinear time-varying discrete-time state space model (NTDS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> M) of the grid connected VSC-HVDC system is built considering the impact of the time-varying phase shift. Then, the NTDS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> M is decomposed into several interconnected subsystems. The general asymptotic gain matrix (GAGM), whose elements are derived from the proposed dissipative-form sum-type inequalities (DSIs), is constructed to quantify the interactions between multiple subsystems. Finally, a general dynamic stability analysis index, called dynamic short circuit ratio (DSCR), is proposed to effectively explain the critical dynamic instability point of the grid connected VSC-HVDC system, based on the spectral radius property of the GAGM. Moreover, the real-time simulations and multiple comparative studies are implemented in this paper to verify the effectiveness of the proposed NTDS <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> M and stability analysis method by using RT-LAB platform.

Accurate Distinction of Weak and Strong AC Grids for Emerging Hierarchical-Infeed LCC-UHVDC Systems

Accurate Distinction of Weak and Strong AC Grids for Emerging Hierarchical-Infeed LCC-UHVDC Systems

Optimal Energy Utilization of MMC-HVDC System Integrating Offshore Wind Farms for Onshore Weak Grid Inertia Support

Modular multilevel converter-based high-voltage direct current (MMC-HVDC) systems have the potential to support grid frequency by utilizing the electrostatic energy stored in sub-module capacitors. However, existing energy control schemes cannot effectively use the MMC capacitor energy to provide sufficient inertia support. Additionally, those control schemes are based on grid following control, suffering from stability issues when the AC grid short circuit ratio (SCR) decreases. Thus, a grid-forming control scheme capable of active energy management for MMC-HVDC systems integrating offshore wind farms (OWFs) is developed, achieving flexible energy regulation and stable operation under weak grid conditions. To improve inertia support performance, an optimal two-stage energy utilization method is further proposed to overcome the drawbacks of existing grid-forming methods. Finally, comparative simulations are performed in a 2-area 4-machine weak grid to verify the effectiveness and advantages of the proposed method.

Minimisation of AC Grid Side Input Power Factor Angle for Three-phase AC to Three-phase AC Matrix Converter under Varying Load

Abstract Minimisation of AC grid side input power factor angle for a ‘matrix converter (MC)’ improves the efficiency of the grid. Input volt-ampere requirement is minimum if the current drawn by the ‘MC’ is sinusoidal and input displacement power factor (IDPF) is unity. A MC is inherently capable of maintaining a unity displacement power factor (UDPF) angle at its input terminals. However, the input currents drawn from the grid are not sinusoidal. The high-frequency ripples are suppressed by input current filters (ICFs).These filters additionally introduce a leading phase angle for the current which varies with the loading. This phase lead can be compensated by adjusting the angle between the input current space vector and the input voltage space vector of the MC. The computation of this adjustment angle depends on the estimation of power losses in the switching devices. A simple method is proposed in this paper to estimate the switching losses without measuring device voltages and currents using the perturbation technique. The perturbation logic depends on input current, instantaneous active and reactive power computed at regular intervals of time. The proposed method effectively minimises the IDPF angle very close to zero. The experimental results are included for validation,

Simplified Single-Stage Buck-Boost Rectifier With Proportional-Integral and Cascaded Proportional-Resonant Control Method

This paper presents a simplified single-stage buck-boost rectifier as well as a proportional-integral (PI) and cascaded proportional-resonant (PR) control methods. The design of the proposed rectifier is based on a recently introduced buck-boost inverter with the elimination of low-frequency switches in the H-bridge. It is verified that the low-frequency switches are redundant, and the rectifier can operate when these switches are replaced with diodes. Elimination of four switches as well as their drivers not only simplifies the modulation scheme, but also reduces the overall cost of the proposed rectifier. Moreover, the proposed PI and cascaded PR control method ensures almost zero steady-state error in the dc-link voltage as well as in the grid current. The correct operation of proposed rectifier together with the proposed control method during buck and boost modes are confirmed with experimental results obtained from a 500W prototype. The steady-state and transient results are obtained to verify that the proposed rectifier has a wide ac grid voltage and dc-link voltage operation ranges.

AC Fault Ride-Through Method of HVDC Interconnector for Islanded AC Grid with Offshore Wind Farm and Loads

AC Fault Ride-Through Method of HVDC Interconnector for Islanded AC Grid with Offshore Wind Farm and Loads

Impact of short-duration voltage variations on VSC-HVDC performance

The growing load demand globally necessitates increasing the penetration of renewable energy sources into electrical grids as well as interconnecting grids from different countries and even continents through HVDC transmission systems. Since these applications rely on power electronics devices, several power quality issues arise, namely voltage sags and swells. This paper analyzes the response of a VSC-HVDC transmission system that interconnects two asynchronous AC grids to short-duration voltage variations like sag and swell by adjusting the voltage of the controllable AC source. The system is simulated with the help of MATLAB/Simulink. The study records the effect of the manipulated AC voltage on the active/reactive powers and AC/DC voltages at both converter stations to evaluate the system stability due to these prevalent power quality challenges. The obtained results reveal that the system hardly withstands voltage variation for a short period.

Current Source Strategy for Energy Injection from a CapMix Cell

Circulation of salty and fresh water through the electrodes of a deionization cell produces a voltage between the electrodes caused by the Capacitive Donnan Potential (CDP). The voltage so generated is very low (100 mV), but this work demonstrates that it is possible to develop a power converter suitable to inject this energy into the grid or into energy storage systems; this is a relevant aspect of this paper, for most works in the literature simply dissipate this energy over a resistor. To increase the input voltage, a stack of electrodes is connected in series. A bridgeless rectifier that uses a dual buck–boost converter to operate with both the positive and negative cycles is used to extract the energy from the cell. The topology chosen, which is operated as a current source, can work at extremely low voltage levels and provide power factor correction. After this stage, an H-bridge inverter can be included to inject the energy into the AC grid. The whole system implements a hysteresis control system using the current through the inductor of the power converter as control variable. This paper investigates the influence of such current on the efficiency of the total system.

A Novel SEPIC-Ćuk-Based High Gain Solar PV Microinverter for Grid Integration

A SEPIC-Ćuk based transformer-less micro-inverter capable of interfacing a 35 V, 250 W solar PV module to a single phase 220-230 V ac grid is proposed in this paper. The circuit employs only one high frequency switch and four line frequency switches thereby reducing the switching losses. This also enhances the overall reliability of the system. The circuit is made to operate in discontinuous conduction mode (DCM) under all possible operating conditions to achieve high gain and at the same time ensures negligible turn on loss for the high frequency switch. The direct connection existing between PV neutral and the utility ground makes the magnitude of the leakage current to be zero. In order to reduce the size of the capacitor across the PV module, an active power decoupling circuit is employed. Hence, this capacitor can be realized by a thin film capacitor instead of an electrolytic capacitor thereby improving the reliability of the system. The detailed analyses of the proposed micro-inverter is carried out. The effectiveness of the proposed scheme is verified by performing detailed simulation studies. A 250 W laboratory prototype of the inverter is fabricated, and detailed experimental studies are carried out to confirm the viability of the proposed scheme.

Small-Signal Synchronization Stability of Grid-Forming Converters With Regulated DC-Link Dynamics

The dc-link voltage dynamics can be directly used to synchronize grid-forming (GFM) converters with ac grids. However, such control scheme is prone to unstable low-frequency oscillations (LFO), owing to the constant-power dynamics at the dc link. This paper analyzes first the small-signal stability of dc-link voltage-synchronized GFM converters, and proposes a control method for enhancing the system stability under different operating scenarios. In the approach, a <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> -axis voltage-feedforward control is introduced in parallel with the dc-link voltage control for the grid-synchronization purpose. The LFO problem with the conventional dc-link voltage-synchronization control is mitigated. Compared to conventional cascaded/paralleled dc-link voltage- and active power-based synchronization, the method mitigates the synchronous oscillation problem. Experimental tests are performed for the GFM converter operating in both rectifier and inverter modes and with different grid strengths. The results corroborate theoretical analyses and validate the effectiveness of the approach.

Enhanced Hybrid Converter Topology for PV-Grid-EV Integration

Due to the continuous price reductions in PV mod- ules and significant development in electric vehicles (EV), the integration of solar photovoltaic (PV) into the EV charging system has been inevitable. This paper proposes the control of a hybrid converter in a EV charging station. This system consists of a PV fed Enhanced Hybrid Converter (EHC) integrated with a single-phase grid. The control of the EHC to perceive maximum power from PV, dc voltage regulation for EV charging and AC power control for feeding the AC grid is presented. A testbed under different circumstances i.e., AC and DC power distribution from the solar PV under different insolation conditions, feeding PV power to grid under the absence of EV, feeding PV power to EV charging port in the absence of grid and feeding EV power to grid under the absence of PV power is developed under MATLAB/Simulink environment for validation. Also, a switching logic is proposed for the operation of the EHC converter under no-load condition. The hardware prototype is developed with TI F28379D processor in the laboratory has validated the proposed EHC on various conditions.

A Multi-Terminal Control Method for AC Grids Based on a Hybrid High-Voltage Direct Current with Cascaded MMC Converters

The hybrid high-voltage direct current (HVDC) transmission system with cascaded MMC converters has become a promising alternative for possessing the technical merits of both line-commuted converter (LCC) and voltage source converter (VSC), resulting in favorable characteristics and potential control of good prospect. This paper pays heightened attention to the feasible power and DC voltage control modes of a hybrid HVDC system; characteristics of master–slave control show higher flexibility than the LCC-VSC HVDC system, which demonstrates that the exceptional potential can serve to stability support the AC power grids. To optimize the control effect, besides damping level to attenuate power oscillations, the robustness suitable for various faults is also considered to obtain a multi-objective control problem. A detailed solution is proceeding using the TLS-ESPRIT identification algorithm and H2/H∞ hybrid robust control theory. This motivates multi-terminal controllers in the LCC rectifier and MMC inverters, which immensely improve the stability of both sending and receiving girds at the same time. According to the parameters of the actual hybrid HVDC project, the simulation model is established in PSCAD v4.6.2 software, and proposed control methods have been verified to satisfy damping objectives and perform well in multiple operating scenarios.

Grounding and Isolation Requirements in DC Microgrids: Overview and Critical Analysis

DC microgrids, along with existing AC grids, are a future trend in energy distribution systems. At the same time, many related issues are still undefined and unsolved. In particular, uncertainty prevails in isolation requirements between AC grids and novel microgrids as well as in the grounding approaches. This paper presents a critical technical analysis and an overview of possible grounding approaches in DC systems and the feasibility of avoiding isolation between AC and DC grids.

Model of quasi-Z-source inverter-based PV power systems for stability studies of multi-terminal AC grid-connected PV power systems

Photovoltaic (PV) power systems are becoming one of the most popular green power systems. Power converters are used in these systems. Among them, quasi-Z-source inverters (qZSIs) are currently on the rise. However, instabilities can appear in multi-terminal AC grid-connected power electronics, such as PV power systems, due to the interaction between power electronic controls and the grid. Modeling of qZSI-based PV power systems is lacking in the literature and stability assessment of grid-connected qZSI-based PV power systems is not yet well understood. Accordingly, the present paper contributes with an s-domain dq-real admittance transfer matrix model of qZSI-based PV power systems derived from their small-signal state-space averaged model. The paper also describes a systematic procedure to study multi-terminal AC grid-connected qZSI-based PV power systems by the nodal admittance method, where the proposed qZSI-based PV power system model is incorporated into the nodal admittance matrix to study stability concerns. Thus, AC grid dynamics in the presence of qZSI-based PV power systems can be investigated using impedance-based stability criteria. The proposed model and the procedure are applied in a stability study of two applications where solutions for improving stability issues are presented. In these applications, the two main causes of instabilities in AC-grid connected PV-qZ systems are analyzed in detail. The study is validated by PSCAD/EMTDC simulation. .

Direct current power system stabilizers for HVDC grids: Current status

AbstractA power system stabilizer (PSS) is a control system integrated into the control structure of specific generation units within AC grids. It monitors current, voltage, and machine shaft speed. Analysing these variables, the PSS generates appropriate control signals to the voltage regulator unit, aiming to damp system oscillations. With the advancement of high‐voltage direct current (HVDC) overlaid high‐voltage alternative current (HVAC) grids, it is anticipated that direct current power system stabilizers (DC‐PSS) will be developed to perform a similar role as their AC counterparts. DC‐PSS will be responsible for monitoring and controlling DC voltage levels, ensuring stable operations. This paper focuses on DC‐PSS in HVDC grids, designed to ensure stable operation and mitigate voltage fluctuations. Unlike conventional AC power systems, HVDC includes only DC voltage and power. The input signal for DC‐PSS is the variations in DC voltage, and the output signal is proportional to the power changes at the specific bus where the DC‐PSS is installed, aiming to minimize DC voltage oscillations. These characteristics pose significant challenges in DC‐PSS. The paper addresses the challenges and highlights issues such as inertia and low‐frequency oscillations associated with DC‐PSS. Various control methods are presented and a comparison is made among these methods.

The Cost and Benefit of Enhancing Cybersecurity for Hybrid AC/DC Grids

As critical interfaces of AC grids and DC grids inside a hybrid AC/DC grid, the voltage-sourced-converter (VSC) has been demonstrated to be vulnerable to false data injection (FDI) cyber-attacks. As a result, the cyber-attack-induced AC grid frequency deviations and DC grid voltage deviations threaten the secure operation. To enhance cybersecurity in a not only feasible but also cost-effective manner, this paper proposes a cost-benefit-based cyber-defense strategy for a hybrid AC/DC grid. First, this paper establishes a spatial-temporal dual cyber-attack evaluation model, in which the cyber-attack-induced frequency and voltage deviations are modelled in both a spatially and temporally dual manner. Then, the proposed cost-benefit-based cyber-defense strategy is modelled as a VSC commitment problem to achieve the trade-off between maximizing the cyber-defense benefits and minimizing the cyber-defense costs. The VSC commitment problem is then mathematically convexified into a mixed-integer second-order cone programming (MISOCP) problem, which could be efficiently solved in an event-triggered manner against unfolding cyber-attack events. Simulation results on a test hybrid AC/DC grid verified the feasibility and the cost-effectiveness of the proposed cyber-defense strategy.

Impact of LVRT Control on Transient Synchronizing Stability of PLL-Based Wind Turbine Converter Connected to High Impedance AC Grid

With large scale of wind turbines connected to high impedance AC grid, transient synchronizing stability of PLL (phase locked loop) based WTC (wind turbine converter) during LVRT (low voltage ride through) is increasing concerned. However, the existing research is usually carried out at the assumption of constant current injection. The influence of dynamic current injection adjusted by LVRT control is neglected. Yet, in high impedance AC grid, the dynamic current injection significantly affects the terminal voltage tracked by PLL. Then LVRT control has much impact on the transient synchronizing stability, which is concerned especially in this paper. First, an novel equivalent motion model is developed, in which PLL is modelled in the form of rotor motion and LVRT control adjusts its equivalent driving force similar as governor in synchronous generator. Then based on the developed model, accelerating-decelerating area method is employed to reveal the transient stability mechanism from energy's transformation and dissipation. Transient stability region method is employed to present the visualized stability region. Furtherly, the influence of K-factor, grid impedance and grid voltage sag are studied from the influence of them on accelerating-decelerating area and stability region. Finally, simulated results are presented to verify the analytical results.

LoRaWAN-based IoT protocol for three levels central protection scheme in MT-HVDC networks with ANFIS-PSO restoration algorithm

A new three-level adaptive central protection scheme has been introduced for multi-terminal high voltage direct current (MT-HVDC) transmission grids. The primary protection is based on a differential relay, while two backup protection levels are proposed: a local dv/dt under-voltage relay and a centralized overcurrent relay at the AC grid side. The coordination of these three protection levels is managed based on a convenient coordination time. The communication between the central protection unit and the Fault Isolation Devices (FIDs) is established using the LoRaWAN-based Internet of Things (IoT) protocol. In case of fault clearance triggered by the third protection level, the faulty line is detected and isolated using the adaptive neuro-fuzzy inference system (ANFIS) algorithm. The restoration process of the healthy lines focuses on minimizing load shedding and line power loss. The protection scheme proposed in this article has been implemented and tested under various fault conditions using the MATLAB/Simulink® environment. The results demonstrate the effectiveness and efficiency of the proposed protection scheme in detecting and isolating faulty lines in MT-HVDC transmission grids, along with a rapid restoration technique.