Weak AC Network Research Articles

Optimizing the performance of a new current source type grid-forming modular multilevel converter using genetic algorithms

Modular Multilevel Converters (MMC) have found extensive application in connecting Renewable Energy Sources to the power grid. Recently, MMCs operated in the Grid Forming (GFM) mode have garnered significant interest due to their superior performance in weak AC networks. The Virtual Synchronous Generator (VSG) is a GFM technology designed to emulate the behavior of a real synchronous generator (SG). There is flexibility in selecting the VSG’s parameters as they are not constrained by the physical attributes of an actual SG. This paper proposes employing the Genetic Algorithm (GA) to select these parameters, underscoring its simplicity and effectiveness in meeting diverse constraints and objectives. Small Signal (SS) models are constructed at potential operational points, with GA leveraging the SS model’s eigenvalues to select parameter values that enhance dynamic performance, ensuring resilience across a wide range of operating conditions. The effectiveness of these designed parameters is validated through Electromagnetic Transients (EMT) simulations.

Impedance network-based oscillatory stability analyzer (INOSA) – A frequency domain tool for investigating IBR-grid interactions

Unstable subsynchronous oscillations (SSOs) may be excited due to adverse interactions between inverter-based resources (IBRs) and series-compensated or weak ac networks. The frequency-sweep impedance modeling and analysis method is widely used for scanning grid-tied IBRs against the risk of SSO. The stability of such a system is heavily influenced by system-wide parameters such as IBR dispatching conditions (input resource, and different dispatch scenarios), and network status (short-circuit and/or series-compensation levels). The variation in the operating conditions must be taken into account when conducting frequency-scanned impedance-based modeling and analysis. This paper proposes a tool, named impedance network-based oscillatory stability analyzer (INOSA), which integrates decades of research on impedance-based analysis to investigate the risk of SSO under varying operating conditions. The INOSA tool is developed as a MATLAB application package. It takes the available system parameters and frequency-scanned impedance data as input and processes it to carry out impedance network-based stability analysis under all operating conditions. The developed tool outputs key stability information such as damping and frequency of each oscillation mode, and a multi-dimensional security boundary/region plot that can be used to assess the risk of SSO and take appropriate measures. A case study on an actual power system is presented to demonstrate the features and capability of the analysis tool.

Robust Transmission Expansion Planning of Ultrahigh-Voltage AC–DC Hybrid Grids

With the ever-increasing installation of large-capacity ultrahigh-voltage direct current (UHVdc) links, the “strong dc link and weak ac network” issue has become prominent for many regional grids in China. The transmission expansion planning (TEP) problem of such grids has been facing a significant challenge to cope with the coordination between the UHVdc links and the ultrahigh/high voltage alternating current (UHVac/HVac) network. To handle this issue, this article proposes a multistage robust TEP (RTEP) model of ultrahigh-voltage ac–dc hybrid grids. This model explicitly takes into account multiple types of uncertainties, i.e., credible contingencies occurring in the ac network, large power imbalances due to the UHVdc bipole blocking, as well as the long-/short-term uncertainty. Wasserstein metric is used to reformulate the uncertainty set and the worst-case constraints. A column-and-constraint generation based solution approach is developed in order to ensure tractability of the RTEP model. The effectiveness of the proposed RTEP is verified by numerical testing on a practical large-scale power grid.

Interaction paths analysis and stability-enhancing methods for LCC-HVDC system with weak receiving-end grid

The stability problem becomes more prominent when line-commuted converter based high voltage direct current (LCC-HVDC) system is connected to weak receiving AC network. Although the impacts of controller parameters on the stability of LCC-HVDC system have been well studied, the interaction mechanism between LCC and weak AC network has not been totally understood and revealed. As a result, the proposal of corresponding solutions is restricted. To remedy such insufficiency, firstly, the sketchy interaction paths between LCC and AC network are extracted and analyzed in this paper based on the motion equation model. The results reveal that the phase interaction path provides negative damping and is the main contributor to the instability. Moreover, since the extinction angle (EA) is the critical node in phase path, the impacts of two coefficients of EA on the system stability are analyzed, namely phase tracking error (PTE)-EA coefficient kδγ and firing angle order (FAO)-EA coefficient kαoγ. The theoretical analysis shows that increasing kδγ or decreasing kαoγ will strengthen the system damping. Next, the comprehensive physical explanations for the above discoveries are presented. Then, two methods that have the same effects on modifying these coefficients are proposed to enhanced the system stability. Finally, the effectiveness of these two methods is verified by the simulations in PSCAD/EMTDC.

Modal Analysis on Control Impact of Fully Rated Converters-Based PMSG-WECS Connected to Turbine-Generator

The power converter-based wind energy conversion systems (WECS) connected to weak AC networks may cause subsynchronous interactions that can potentially lead to instability. The interaction between the WECS converters and a nearby turbine-generator (TG) may lead to severe torsional vibrations. This article investigates the impact of a fully rated converters-based permanent magnet synchronous generator WECS (PMSG-WECS) on the subsynchronous torsional interactions (SSTI) of a nearby multimass TG connected to series compensated transmission line. A simple but effective systematic approach of modal analysis using open-loop systems is proposed. The proposed approach is used to evaluate the impact of PMSG-WECS on SSTI. Further, the impact of PMSG-WECS is analyzed through the damping torque, eigenvalue analysis and transient simulation under various control modes of PMSG-WECS converters. The results demonstrate that the inclusion of PMSG-WECS increases the subsynchronous resonance frequency of the network, thereby reduces the damping of the torsional modes in the high-frequency range of subsynchronous resonance frequency. The negative damping effect of PMSG-WECS significantly reduces the damping of subsynchronous network mode, thereby can destabilize the system. Also, the negative damping impact of PMSG-WECS significantly reduces the damping of high-frequency torsional modes. As a result, the torsional interactions aggravate at lower compensation levels.

Analysis on subsynchronous oscillation stability for large scale offshore wind integration

As the offshore wind installations are moving further from shore, the transmission distance also increases. Synthesize the cost of construction and maintenance, as well as the technical maturity, grid-connection via AC network will still be a preferred power delivery method for offshore wind power projects in the near future. However, according to the “8•9” blackout in the UK, there is sub-synchronous oscillation (SSO) risk in the AC network connected offshore wind projects, which was attributed to the mismatch between the power electronic source and the power grid. In this article, both eigenvalue analysis and time-domain simulation method are used to evaluate the operating SSO stability of grid integration of large scale offshore wind power. It suggests that transmission system via long distance cables naturally forms a weak AC network, and the SSO characteristics of AC grid connected offshore wind farm is closely related to the wind turbine parameters, offshore distance and the active power output level. At last, the principle of parameter configuration in offshore wind turbines is proposed based on different offshore distances.

Limitations of voltage source converter in weak ac networks from voltage stability point of view

A weak ac grid could be envisioned as a voltage source behind a large synchronous impedance; thus the phase and magnitude of its terminal voltage vary significantly with the magnitudes of active and reactive powers it sinks or sources. Such characteristics present formidable challenges to control and stability of voltage source converters in weak ac grids. Therefore, this paper presents a theoretical analysis which establishes the maximum active power transfer that the voltage source converter (VSC) can exchange with a weak ac grid. The presented analysis reveals that the maximum active power that the VSC can exchange with a weak ac grid is determined by voltage stability limit. The validity of the presented theoretical analysis is confirmed by simplified simulation, and further corroborated by detailed simulations from a VSC model, developed in MATLAB-SIMULINK that incorporates all necessary control systems.

Investigation on SSCI between PMSGs‐based wind farm and AC network

The sub-synchronous control interaction (SSCI) has frequently occurred in the direct-drive permanent magnetic synchronous generators (PMSGs) based wind farm integrated to the weak AC network. The system strength, i.e. short-circuit ratio (SCR) has been believed as the critical factor so far. However, the impedance-frequency characteristics of the practical network exhibit a strong resonance that might have impacts on the SSCI as well. In this study, considering the network resonance, the eigenvalue- and impedance-based analyses are conducted to study the SSCI phenomenon in the grid-connected PMSG system. The impacts on SSCI from system strength, network resonance and network-resonant frequency are investigated firstly. Then, the parameter limits of PMSG control for the stable region are calculated to discuss the effects of network resonance on the parameter design. The results show that the network resonance plays a critical role in the SSO mode. When the network resonance is considered in the PMSG system, the SSO would arise more easily, and the stable region of the control parameter will be reduced. Meanwhile, the frequency of the SSO mode matches well with the network resonance frequency in dq frame. The analytical results are all verified by simulation.

Sub-synchronous control interaction between direct-drive PMSG-based wind farms and compensated grids

Previous researches suggested that the direct-drive permanent magnetic synchronous generator (PMSG)-based wind farms are immune from sub-synchronous control interaction (SSCI). However, since the occurrence of emerging sub-synchronous oscillation (SSO) caused by PMSG-based wind farm connected to a weak AC network, it is of great significant to re-investigate this SSCI problem. To address it, a small-signal model of target system is established first, then modal analysis is conducted to delve into the dynamic interaction between PMSG and compensated network, and the time domain simulation is performed to verify modal analysis results. The results of the paper reveal that PMSG connected to compensated grid could excite an emerging SSCI, which is caused by the dynamic interaction of converters and series compensated line, and the oscillation frequency is strongly related to the electrical resonance frequency. The major contributions of the paper include: (1) re-discussion of the SSCI immunity of PMSG-based system and re-identification of the turbine types which are susceptible to SSCI, (2) analysis on frequency characters of the emerging SSCI, (3) investigation of decoupling effect between machine and grid.

Subsynchronous oscillation of PV plants integrated to weak AC networks

The grid-connected photovoltaic (PV) power is booming, and large-scale PV power is mostly integrated to grid through long transmission lines; however, PV systems may face the threat of subsynchronous oscillation (SSO) when AC system strength is weak. This study establishes the sequence impedance model of grid-connected PV system; mechanism and characteristic of SSO in PV plants integrated to weak AC networks are delved into through impedance-based analysis method. The results show that, under certain conditions, the impedance of PV system is capacitive in the subsynchronous frequency domain, when the networks are weak and the PV capacity is high, the resonance may occur between the capacitive PV system and inductive AC grid. In addition, a smaller proportional gain of current loop may increase the risk of SSO; however, a larger proportional gain and integral gain of phase-locked loop can ease the SSO. Finally, time domain simulation based on PSCAD/EMTDC is conducted to validate the results of the impedance-based analysis. The research here can provide guidance to the project of scaled PV plants integrated to weak AC networks in some sense.

Current Error Based Compensations for VSC Current Control in Weak Grids for Wind Farm Applications

A novel current control strategy is proposed for voltage source converters connected to weak grids using conventional current vector control with additional current error based voltage angle and magnitude compensations. For connecting to very weak ac network, the combination of vector control and grid synchronization with a conventional phase-locked loop is proved to be unstable, whereas the proposed current error based compensations can significantly improve system stability. In this way, the proposed control can still benefit from the presence of closed-loop current control without the need for control switching during large ac voltage variations. A comprehensive frequency domain model is established to analyze stability performance. Comprehensive time domain simulations are further carried out to validate its effectiveness and robustness by demonstrating its current control performance during a three-phase fault, multiple-converter situation, and various grid strength conditions.

A Study on an Operation Strategy of Dual-Infeed HVDC System

This paper deals with the operation strategy of reactive power in a multi-infeed HVDC (MIHVDC) system, in which several converters are connected to the same or nearby separate AC buses. The potential problems concerning a MIHVDC system when feeding a weak AC network are as follows: the need for coordination of the recovery control, the possibility of voltage instability or low quality of the area connected to the MIHVDC system, and the risk of mutual commutation failures. These problems in MIHVDC systems are similar to those in single-infeed HVDC (SIHVDC) systems, but the differences with the phenomenon of the SIHVDC system are the interactions between converters. The main reason for the potential problems of HVDC systems (MIHVDC or SIHVDC) is voltage variation; therefore, to mitigate the voltage variations, the performances of the HVDC system should be enhanced. Consequently, to mitigate the potential problems of MIHVDC systems, several solutions are suggested in this study, including installing STATCOM and installing a line arrester on the tower. The study results will be applied to a multi-infeed HVDC system in Korea.

Generalized Frequency-Domain Controller Tuning Procedure for VSC Systems

This paper proposes a robust frequency-domain method to tune the $d$ – $q$ decoupled control system used in Modular Multilevel Converter-type Voltage Source Converter (MMC-VSC) systems. A linearized state-space model of the MMC-VSC system is developed and used to calculate the stability-related frequency-domain attributes. The controller design problem is formulated as an optimization problem. In this paper, the simulated annealing optimization technique is applied to find the proportional-integral (PI) controller parameters that give desired damping for the oscillatory modes and desired values for decaying exponential modes. The efficacy of this method is tested on the electromagnetic transient model of a two-terminal MMC-VSC system on the real-time digital simulators, and the results are provided in this paper. Finally, tuned controller parameters for different ac system strengths are discussed and it is shown that this mathematical model is suitable to tune the PI-controller parameters for MMC-VSC systems connected to strong as well as weak ac networks.

ANALYSIS OF MULTI-TERMINAL HVDC TRANSMISSION SYSTEM FEEDING VERY WEAK AC NETWORKS

This paper presents a line commutated converter (LCC) based multi-terminal HVDC transmission (MTDC) system feeding very weak AC networks with hybrid reactive power compensators (RPC’s) at the inverter AC side. The hybrid compensator is accomplished by the equal mixing of any two of the following compensators: synchronous compensator (SC); static var compensator (SVC); static synchronous compensator (STATCOM). The four-terminal HVDC transmission system model is implemented in the Matlab with the firefly algorithm based optimal proportional integral (PI) controller for rectifiers and inverters control. The transient performances of hybrid RPC’s (SC+SVC, SVC+STATCOM and SC+STATCOM) are studied under various fault conditions and the results are compared with the performance of the SC, SVC and STATCOM to focus the high quality of the hybrid compensators. The simulation results authorize that the equivalent mixture of SC and STATCOM has a steady and fastest response. The results also reveal the supremacy of the firefly algorithm based optimal PI controller over the conventional PI controller. The harmonic present in the inverter side AC quantities is also calculated under steady state operation to assure the quality of power supply.

The Impact Study of a Statcom on Commutation Failures in an HVDC Inverter Feeding a Weak AC System

The Impact Study of a Statcom on Commutation Failures in an HVDC Inverter Feeding a Weak AC SystemThe Static Synchronous Compensator (STATCOM) devices are pure power electronics devices that use voltage source, IGBT, IGCT or GTO based converters to generate reactive current. This paper illustrates the effect of STATCOM connected whit an HVDC inverter feeding a weak AC network, on the recovery from commutation failures following AC side disturbances. MATLAB/SIMULINK simulation results have demonstrated the robust performance of the proposed system based on the first CIGRÉ HVDC Benchmark model against commutation failures.

Power Oscillation Damping using VSC-based Multi-terminal HVDC Grids

This paper presents a Power Oscillation Damping (POD) controller for use with Voltage Source Converter based Multi-terminal Direct Current (VSC-MTDC) grids. The controller has been tested on a large meshed AC network into which a five bus, six line VSC-MTDC network has been incorporated. The controller is designed using a Modal Linear Quadratic Gaussian (MLQG) approach allowing targeted control action on critical low-frequency oscillatory modes. By taking multiple delayed wide area signals and modulating active power injection by multiple VSC-MTDC converter stations, significant improvement in system stability is seen. The effect of blocking the POD controller's active power modulation signals to specific VSC-MTDC converter stations is assessed. Such a blocking feature may be desirable at points of connection with weak AC networks which might be temporarily unable to tolerate modulation in active power injection and instead require a fast return to steady state constant power injection. Even with modulation signals blocked, the post disturbance system performance is significantly enhanced, demonstrating the redundancy in using VSC-MTDC for oscillation damping. The controller performance has been assessed through both small and large disturbance analysis.

Stability of a Variable-Speed Permanent Magnet Wind Generator With Weak AC Grids

The operation of high-power wind generators with weakened ac grids has historically been difficult because of stability and power quality issues. This paper presents an analytical stability study of a variable-speed directly-driven permanently-excited 2-MW wind generator connected to ac grids of widely varying strength and very weak grids. The generator includes two back-to-back full-scale vector controlled 3-level neutral-point-clamped (NPC) voltage-source-converters (VSC). A 47th order small-signal analytical wind generator model is developed within MatLab, and a summary of the model structure and controls is given. Model verification is demonstrated for fast and slow system variables employing detailed simulation software PSCAD/EMTDC. An eigenvalue stability study for weak ac networks is presented, and qualitative conclusions about inherent system dynamics and stability characteristics are given. These insights are employed to study the design of an ac voltage controller for weak ac networks. Two alternative controller designs are studied for their potential to enhance system robustness to changes in ac grid strength. Testing on the detailed simulator PSCAD/EMTDC is employed throughout to confirm conclusions from analytical studies.

Optimal Integration of an Offshore Wind Farm to a Weak AC Grid

This paper studies the behavior of a high-voltage direct current link based on voltage-source converters, which feeds a weak ac network with power produced from an offshore wind farm (WF) of induction generators. Its control system, which is based on adaptive fuzzy controllers, manages to offer very satisfactory performance, without the need for a detailed mathematical model, but just prior knowledge of the behavior of the electrical system. Using the simulation program PSCAD/EMTDC, the study was performed under both steady-state and transient conditions. The results show that the link supplies the variable power of the WF to a weak grid, keeping the ac voltage fluctuations in the point of common coupling at an acceptable level. Moreover, due to the ability of the control system to adjust the stator frequency of the induction generators in relation to the wind velocity, maximum wind power acquisition is achieved.

Frequency domain based control design for an HVdc converter connected to a weak AC network

A small-signal frequency domain model of an HVDC system connected to a weak AC network is used for an operating point-based rectifier direct current control design. The frequency response of the open-loop system is systematically shaped, clearly showing the controller design tradeoffs involved in achieving the objectives of damped system oscillations and disturbance rejection. The performance of the controller is verified by time-domain simulation of the CIGRE Benchmark HVDC test system. This validates the use of small-signal frequency response models for HVDC system control analysis and design. The further developments necessary for a practical controller for large signal excursions are discussed.

Digital Simulation of a Multiterminal HVDC Transmission System

A 4-terminal HVdc system with imbalanced inverters and weak ac networks has been simulated digitally by EMTP on a mainframe computer in parallel with a microcomputer containing a detailed control model and user interface. Recovery from different faults has been examined and the performance evaluated.