Weak Grid Research Articles

Transient Synchronization Stability of an Islanded AC Microgrid Considering Interactions Between Grid-Forming and Grid-Following Converters

Microgrid has been rapidly developed for the integration of distributed renewable energy, owing to its superiority of flexible operation and high reliability. Without frequency support from the main grid, the microgrid under islanded operating mode is more likely to lose stability in the presence of large-signal disturbances. However, previous studies mainly focus on weak grid conditions and ignore the interaction effects between different-type converters. Therefore, this paper investigates the transient synchronization stability of an islanded AC microgrid considering dynamic interactions between grid-forming and grid-following converters. Firstly, a simplified nonlinear model of the islanded microgrid is established, which reveals the nonlinear interaction and damping impact on system stability. Based on the passivity principle and LaSalle’s theorem, an explicit stability criterion is formulated to reveal the instability mechanism. With the proposed stability criterion, the impacts of different system parameters are analyzed, which points out the direction of parameters optimization to enhance stability. To validate the effectiveness of the proposed method, simulations and experiments are carried out.

Improving the power sharing transients in droop-controlled inverters with the introduction of an angle difference limiter

This paper derives a mathematical model suitable for analysis of connection behavior of the inverter-based grids and, using that model, it presents guidelines to control grid-forming inverters’ rate of change in frequency as function of output power (virtual inertia), or in other words “measured power filters”. Furthermore, in order to improve the transient response, an angle difference limiter control is proposed. An angle difference limiter is a control algorithm which is constantly checking the angle difference between the inverter and the grid in the connection point limiting power peaks during the transients. The performance of the angle difference limiter is analyzed in detail and validated with an experimental microgrid consisting of two three-phase inverters and a variable load. The paper analyzes the connection of inverters in weak and strong grids validating the performance of the proposed angle difference limiter.

Smart power management strategy controlling domestic solar solutions in sub-Saharan countries

AbstractLimitations such as maximum power consumption during peak hours, scheduled load shedding and unplanned brownouts are problems of weak and stressed electricity grids. To compensate for such shortage of energy, the usage of renewable energies is a solution delivering increased reliability for consumers. Although sub-Saharan African countries suffer from unreliable grids, they benefit from large amounts of solar radiation throughout the year. Therefore, domestic solar systems including photovoltaic panels, battery storage and solar water heating are attractive solutions to supply affordable and reliable energy and hot water for consumers. However, to deliver the best management of the electrical loads in the face of unplanned brownouts, a smart power management strategy is required. Consequently, the proposed strategy relies on the predicted values of power generation and power consumption to autonomously control the system in both on-grid and off-grid modes. This method is evaluated using a case study relying on the measured electrical load and the hot water consumption data of a low-income house in Botswana. Results show that in addition to delivering sustainable support for the utility grid by decreasing the power consumption in peak hours, the proposed method reduces annual consumer electricity bill by 64% and increases the reliability of electricity supply from 95.5 to 99.5%. Thereby providing affordable and reliable solution to unreliable power supply due to stressed grids.

Oscillation Suppression Strategy of Three-Phase Four-Wire Grid-Connected Inverter in Weak Power Grid

As the penetration of renewable energy increases year by year, the risk of high-frequency oscillation instability increases when a three-phase, four-wire split capacitor inverter (TFSCI) is connected to the grid with complementary capacitors in weak grids. Compared to the three-phase, three-wire inverter, the TFSCI has an additional zero-sequence current loop. To improve the accuracy of the modeling and stability analysis, the effect of the zero-sequence loop needs to be considered in the impedance-based stability analysis. Therefore, a correlation model considering multi-perturbation variables is first established, based on which the inverter positive, negative, and zero sequence admittance models are derived, solving the difficult problem of impedance modeling under small perturbations. Secondly, an admittance remodeling strategy based on a negative third-order differential element and a second-order generalized integrator (SOGI) damping controller is proposed, which can improve the stability of positive, negative, and zero-sequence systems simultaneously. Finally, the effectiveness of the oscillation suppression strategy is verified by simulation and experiment.

Adaptive fault ride through control of VSM Grid-forming converters

Recently, many high voltage direct current (HVdc) transmission lines are being constructed with the intention of becoming “backbones” of future power grids. Grid-forming (GFM) controlled Voltage Source Converters (VSCs) are anticipated to provide significant stability advantages compared with Grid-following (GFL) VSCs in HVdc converters connected to weak ac grids or to grids with high penetration of renewable inverter-based resources (IBRs). This paper proposes an adaptive fault ride through controller for Virtual Synchronous Machine (VSM) GFM control, which has significantly improved fault ride through capability over the more conventional GFM control with cascaded or switchable current limiting. The performance of the proposed adaptive control is investigated using Electromagnetic Transient (EMT) simulation. The results show that the proposed adaptive control has superior post disturbance recovery from ac faults as well as phase angle shifts, and overcomes the instability reported in GFM converters connected to strong ac networks.

Two-coordinate decomposed SISO models-based stability analysis of three-phase grid-connected inverter systems with asymmetric control

In this paper, the Single-Input Single-Output (SISO) theory-based stability analysis method for weak grids with a three-phase grid-connected inverter (GCI) incorporating asymmetrical control is proposed in dq-frame and sequence domain, respectively. Initially, the paper presents two-coordinate decomposed models for Multi-input Multi-output (MIMO) systems. These models reveal that the stability of the MIMO system can be evaluated by analyzing two open-loop gains in the dq-frame or a single gain in the sequence domain using Nyquist/Bode plots. Next, the proposed analytical method is compared with existing state-of-the-art techniques. Theoretical analysis demonstrates that the proposed method enables the separate design of asymmetrical control in the d-axis and q-axis using the dq-frame approach. Furthermore, it enables the identification of the system's instability axes, namely the d-axis and q-axis. On the other hand, the proposed method in the sequence domain offers a lower computational complexity while accurately identifying the system's frequency-coupled resonances. Finally, the effectiveness of the proposed method is validated through simulations and experiments, consolidating its practical applicability.

Analysing the performance of incremental quantity based directional time-domain protection near HVAC cables and VSC HVDC converters

New grid elements such as Voltage Source Converters (VSC) and cables are being increasingly used in today’s power grid. These new grid elements behave differently compared to synchronous machines and overhead lines during transients and faults, impacting the present system’s legacy protection. An increased share of underground cables affects the system’s resonance frequencies whereas VSCs introduce actively controlled current phase shifts during short-circuit faults. Existing research mainly focuses on VSC impacts on phasor-based legacy protection. This paper studies the impact of these new grid elements on incremental quantity-based directional protection, which operates in the time domain, using EMT-type models and hardware experiments. First, cables introduce low frequency oscillations for remote, single phase faults and additionally under weak grid conditions. At low frequencies, these oscillations are not filtered out by the protection, which may result in a decrease of dependability margin (23% of the tested cases) and potential security issues. For the majority of studied cases, the relays worked correctly. Second, fast reactive current injection by a VSC can cause malfunction of incremental quantity protection. Malfunction occurred in 1 case and a decrease of dependability in 10% of the tested cases. This is especially problematic for grids with relatively low synchronous infeed.

Grid-Forming Controller Based on Virtual Admittance for Power Converters Working in Weak Grids

The high penetration of distributed energy resources, based on power electronics, is giving rise to stability issues in the electrical network, as initially those plants were not required to provide either voltage or frequency support. Due to this, TSOs and DSOs have set new requirements for them to provide grid support functionalities. This paper presents a grid forming control strategy for power converters which is based on a virtual admittance loop. By means of this strategy it is possible to use a conventional current control based inverter, just adding this outer loop. By doing so, the converter is not only able to perform current control, but also frequency and voltage support functionalities. In this work, the proposed control is described and analyzed, checking as well its stability boundaries. After this analysis, the results obtained in a hardware-in-the-loop platform, where faulty scenarios, islanding or black start conditions, will be shown. Finally, the results obtained in an experimental true scale workbench will show its effectiveness for providing such services.

Fault Coordination Control for Converter-Interfaced Sources Compatible With Distance Protection During Asymmetrical Faults

In this article, distinctive fault characteristics of converter-interfaced renewable energy sources (CIRESs) will result in the misoperation of traditional distance relays. To handle this issue, a new fault coordination control method is proposed to ensure the correct operation of distance relays. Combined with sequence boundary conditions, the relationship between the apparent reactance and the CIRES positive-sequence current angle is revealed. Based on this, a reasonable current angle is generated by adjusting current references of the controller to make the apparent reactance equal to the actual line replica reactance, so distance relays can detect the fault distance correctly. The proposed control method shows good performance for high resistance faults and operation in weak grids. Meanwhile, the proposed method is economical because it does not require revising the controller structure, and only local measured data are used. Furthermore, fault ride through requirements such as current limitation and reactive current injection are still being realized. Simulation analysis is performed in PSCAD and real-time digital simulator to validate the recommended scheme.

Small Signal Stability of Phase Locked Loop Based Current-Controlled Inverter in 100% Inverter-Based System

Present-day inverter control mainly employs phase locked loop (PLL) based current-controlled strategies. With growing penetration of inverter-based resources in power systems, stability issues associated with inverter control have been reported in weak grid scenarios. A natural and fundamental question to answer is whether this instability is inherent to PLL based current-controlled strategies or it can be resolved with proper frequency and voltage control loop design. This paper addresses this question by analyzing the small signal stability of the current-controlled inverter in island and weak grid operation scenarios. Torque analysis, which is well known for synchronous machine stability analysis, is adapted to uncover the stability enhancement effect of properly designed and parameterized outer-loop controls. Different types of loads are considered and investigated in the study. Electromagnetic transient (EMT) simulations are conducted on a test network to validate the analytical results. The results from this study indicate that with appropriate outer-loop voltage and frequency control design, the PLL based current-controlled inverter can be small signal stable with high inverter penetration, even in a 100% inverter-based system.

Re-examination of small-signal instability in weak grid-connected voltage source converters

Weak grid sub-synchronous oscillation (SSO) is an important research topic. Although some analysis and mitigation schemes have been conducted, misperceptions still exist. This paper first aims to update the understanding of weak grid instability of voltage source converters (VSCs) and then points out that instability risks originate in the super-synchronous frequency range rather than in the sub-synchronous frequency range. Moreover, the resonance frequency can be even larger than the double fundamental frequency under certain parameter conditions. This paper also refines the impacts of VSC control parameters on weak grid instability mechanism and resonance frequency. Based on the results, the classification of this phenomenon should be revised.

High-Bandwidth Grid-Connected Inverter to Enhance System Robustness Against Weak Grid

The MIMO matrix of the multi-inverter paralleled system based on different parameters is established, and three criteria to ensure the stability of the total grid current and interactive current are put forward. Furthermore, the robustness of the system with different switching frequencies for modules is analyzed, and the introduction of a high-bandwidth inverter into the system is found to be effective in suppressing low-frequency resonance. Then, the interaction stability between the high- -frequency and low-frequency module is analyzed, and the switching frequency of the high-bandwidth inverter is selected. Finally, a prototype of the high-bandwidth inverter based on GaN is built in the lab. The effectiveness of the high-bandwidth grid-connected inverter to improve the robustness of the system in weak grid is verified by the experiment.

Hidden Modes of Instability for Inverters in Weak Grids

This article introduces a hidden instability mode of grid-following inverters at low power levels. It is known that grid-following inverters become unstable due to weak grid conditions and improper control parameters. However, this article elaborates on a hidden instability mode, which can occur at a power level below the inverter's nominal value. The nonlinearity of PQ control loops explains how the inverter becomes unstable for a narrow range of active power setpoints below the inverter's nominal value. This phenomenon is explained using a full-order dynamic model of PQ-controlled inverters. This work also presents a stability criterion, identifying a margin for this hidden instability mode. This phenomenon and the stability criterion are verified by a set of experiments using a 3kVA inverter feeding a programmable 208V, 12kW power grid emulator.

3P4W Grid Interactive PV-BES System With Seamless Transition Capability

For seamless transition of 3P4W (three phase four-wire) grid interactive solar photovoltaic (PV) system, a suboptimal finite impulse response (FIR) filter with approximation based on infinite impulse response filter (IIR) is employed here. The need for power quality improvement is essential and in order to guarantee an improved PQ, the grid currents are analyzed in covenant with the IEEE-519 standard. The power transfer proficiency with suboptimal filter utilization reduced intermittency associated with renewable sources along with improved power quality. Moreover, with the rise in the usage of distributed energy resources involving renewable energy sources, the need for power quality improvement is paramount. The need for improving PQ, stems from the fact that an uninterrupted supply is essential to the critical loads along with an improved power quality. Therefore, during operating modes such as grid linkage/interruption, the utilization of an efficient control is mandatory. Moreover, the behavior is required to be in agreement with the IEEE-519 and IEEE-1547 standards and in off-grid mode, a voltage controller is utilized for the power transfer. Thus, with adequate performance during the weak grid conditions, corroboration of satisfactory system behavior is obtained.

Identification model for weak areas of transient energy balance in EESs based on dynamic grid partitioning

In response to the high uncertainty of large-scale new energy output in the electrical energy system (EES) and the weak controllability of energy output at multiple time scales, this paper proposes a weak grid identification model for transient energy balance in EESs based on grid partitioning, which has an increasingly complex impact on the weak areas of transient energy balance in the sending-end network. First, the accumulation of port energy during transient faults and the propagation mechanism of port energy in the sending-end system were studied, and an EES transient energy propagation mechanism model was established. Then, considering the energy balance support requirements of nodes, an EES grid partitioning model was established. Afterward, based on the characteristics of transient energy propagation and a grid partitioning model, an identification model for weak areas of transient energy balance in EESs was constructed. Finally, based on actual operating data, numerical simulations were conducted, and the results showed that the proposed weak grid identification model for transient energy balance can meet the requirements for transient stability analysis and transient energy balance characteristic analysis during actual operation of power grids.

Stability analysis of hybrid synchronization controller based grid forming control

Synchronization units are vital for the stable operation of grid-tied converters in generation and transmission. There are two main synchronization units, i.e., phase-locked loop (PLL) and power synchronization controller (PSC), with the corresponding drawbacks in weak and strong grid respectively. Recently, the hybrid synchronization controller (HSC) is proposed in order to make up the deficiencies of PLL and PSC in weak and strong grid respectively. This paper deduces the detailed impedance characteristics of PSC and HSC and compares their cons and pros to show the stability mechanism of HSC. Moreover, two types of HSCs are also presented and compared in this article with slight difference in structure. It is found that the impedance of Type II based HSC has smaller amplitude and better phase margin than PSC around fundamental frequency, while Type I based HSC has larger amplitude and less phase margin than PSC. Type II based HSC can improve the stability and Type I based HSC can worsen the stability compared with PSC. Realtime experiments are carried out to verify this.

Robust H∞ controller based on linear matrix inequalities for grid-connected inverters in weak grids

Robust H∞ controller based on linear matrix inequalities for grid-connected inverters in weak grids

Detailed Wideband Impedance Modeling and Resonance Analysis of Grid-Connected Modular Multilevel Converter

Modular multilevel converter (MMC) tends to cause resonance instability in interconnected systems. Current studies on the stability of MMC mainly focus on high-voltage and large-capacity power systems, while the impedance modeling process of MMC ignores the harmonics of the submodule (SM) capacitor voltages and the influence of voltage feedforward control. The applicability of the MMC impedance model with fewer submodules is doubtful. According to the circuit structure and control mode, the AC-side sequence impedance model of the MMC is established, which takes into account the capacitor voltage balance control of submodules and voltage feedforward control. Based on the RT-Lab control hardware-in-loop (CHIL) test platform, the MMC impedance frequency scanning was carried out to verify the accuracy of the impedance modeling method. The influence of control parameters in different frequency bands on the impedance characteristics of MMC was studied. The AC terminal voltage feedforward causes phase lag in the mid-/high-frequency range and increases the risk of resonance. In the low-frequency range, the dynamics and control of capacitor voltages reduce the impedance magnitude of MMC. Using the resonance phenomenon of an MMC connected to a weak grid as an example, the high-frequency resonance mechanism caused by control parameters is analyzed from the perspective of the negative damping effect. The simulation results show that the detailed wideband impedance model can improve the accuracy of the stability analysis results.

Optimization method of power supply mode and connected to external power grid of CZ railway traction station

Aiming at the problem of long slopes and weak power grids along the railway in high altitude mountainous areas, an optimization method of power supply mode and access to external power grid of CZ railway traction station is proposed. First, combined with the terrain and altitude along the railway and the characteristics of the regional power grid, a sample library of the power supply mode and access to external power grid of CZ railway traction station is designed. Secondly, combined with the train operation diagram and line ramp parameters, the full-day periodic load of each power supply arm is simulated. Then, according to the sample library, a “source-net-load” integrated simulation model is constructed in Simulink. Then, the three-phase voltage unbalance evaluation model of the power system is constructed in combination with relevant standards. Finally, the optimal wiring scheme is selected by using the evaluation model and the simulated voltage unbalance index.

Voltage and frequency instability in large PV systems connected to weak power grid

The voltage and frequency control of photovoltaic (PV) systems are influenced by coupled nonlinear factors. It has been discovered that frequency control stability is threatened by voltage regulation methods in PV systems. However, the frequency instability caused by voltage regulation methods has not been fully investigated. This paper investigates the voltage and frequency stability problems in PV systems connected with weak power grids. The voltage problems caused by grid impedance, comprising inverter AC voltage and DC voltage, are first analyzed. Then, methods for improving voltage stability, such as reactive power compensation, and the benefits and drawbacks of various compensation methods are investigated. Finally, the effect of reactive power compensation on frequency control stability is investigated and resolved. Simulations and experiments are used to validate the theory’s correctness.