Short Circuit Ratio Research Articles

Impact of Grid Strength and Impedance Characteristics on the Maximum Power Transfer Capability of Grid-Connected Inverters

Continuously expanding deployments of distributed power generation systems are transforming conventional centralized power grids into mixed distributed electrical networks. The higher penetration and longer distance from the renewable energy source to the main power grid result in lower grid strength, which stimulates the power limitation problem. Aimed at this problem, case studies of inductive and resistive grid impedance with different grid strengths have been carried out to evaluate the maximum power transfer capability of grid-connected inverters. It is revealed that power grids with a higher short circuit ratio (SCR) or lower resistance-inductance ratio (R/X) provide higher power transfer capability. Moreover, under the resistive grid conditions, a higher voltage at the point of common coupling (PCC) is beneficial to increase the power transfer capability. Based on mathematical analysis, the maximum power curves in the inductive and resistive grids can be found. Moreover, a performance index is proposed in this paper to quantify the performance of the system with different parameter values. Finally, the effectiveness of the analysis is verified by simulation.

Multi-feed System Partitioning Method Based on N-1 Preconceived Fault Set and Local Expansion Theory

In view of the fact that the AC system with multiple LCC-HVDC feeds is prone to simultaneous commutation failure, this paper proposes the scheme of grid division for the multiple feed system to isolate the coupling between multiple HVDC and reduce the risk of occurring simultaneous commutation failure. First, N-1 scan is performed on the multi-feed system to find the set of lines that can cause simultaneous commutation failure of multiple HVDC. The line with high line load rate is removed from the line set. Then, on the basis of local expansion theory, the nodes in the system are aggregated to form the initial partition. In this process, the calculation method of voltage/reactive sensitivity matrix considering PV node and LCC-HVDC is firstly given; moreover, the corresponding indexes were calculated by the node aggregation function and the partition fitness function. Finally, the results of the partition are modified by considering the circuit set and the partition short circuit ratio, and the flow chart of the partition scheme is given. The partitioning process is validated with an IEEE 39-node system as an example. The results show that the partitioning method is reasonable and solves the problem of the failure of commutation effectively.

Developing a Decision Tree Algorithm for Wind Power Plants Siting and Sizing in Distribution Networks

The interconnection of wind power plants (WPPs) with distribution networks has posed many challenges concerned with voltage stability at the point of common coupling (PCC). In a distribution network connected WPP, the short-circuit ratio (SCR) and impedance angle ratio seen at PCC (X/RPCC) are the most important parameters, which affect the PCC voltage (VPCC) stability. Hence, design engineers need to conduct the WPP siting and sizing assessment considering the SCR and X/RPCC seen at each potential PCC site to ensure that the voltage stability requirements defined by grid codes are provided. In various literature works, optimal siting and sizing of distributed generation in distribution networks (DG) has been carried out using analytical, numerical, and heuristics approaches. The majority of these methods require performing computational tasks or simulate the whole distribution network, which is complex and time-consuming. In addition, other works proposed to simplify the WPP siting and sizing have limited accuracy. To address the aforementioned issues, in this paper, a decision tree algorithm-based model was developed for WPP siting and sizing in distribution networks. The proposed model eliminates the need to simulate the whole system and provides a higher accuracy compared to the similar previous works. For this purpose, the model accurately predicts key voltage stability criteria at a given interconnection point, including VPCC profile and maximum permissible wind power generation, using the SCR and X/RPCC values seen at that point. The results confirmed the proposed model provides a noticeable high accuracy in predicting the voltage stability criteria under various validation scenarios considered.

Transient Synchronization Stability Control for LVRT With Power Angle Estimation

Power synchronization control (PSC) is an effective control method for a voltage-source converter (VSC) connected with a weak grid, where the short-circuit ratio is close to 1. The grid voltage sag will induce the synchronization instability of PSC-VSC and then trigger the catastrophic cascading failure in the power system. This letter presents the transient power angle nature of PSC under the grid disturbance by phase portrait, and the critical condition of transient synchronization instability for PSC is derived. The virtual orthogonal power (VOP) is elaborated and the power angle estimation method is presented by VOP. Then, a mode adaptive transient synchronization stability control scheme is proposed for the low-voltage ride through with the power angle estimation scheme. By using the proposed transient synchronization stability control scheme, the PSC-VSC can maintain synchronization with the grid during the fault of grid voltage sag. The control method theoretical analysis is verified by the experimental tests.

Robust Output Feedback Control Design for Inertia Emulation by Wind Turbine Generators

Wind generation has gained widespread use as a renewable energy source. Most wind turbines and other renewables connected to the grid through converters result in a reduction in the natural inertial response to grid frequency changes. The doubly-fed induction generator (DFIG) can be controlled to compensate for this reduction and, in fact, provide faster response than traditional synchronous machines. This paper proposes to design observer based output feedback linear quadratic regulator (LQR) and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$H_{\infty }$</tex-math></inline-formula> control laws to realize the inertia emulation function and deliver fast frequency support. The aim is to track the reference speed by a diesel synchronous generator (DSG) in order to reach the desired inertia. The control signal is computed based on a reduced order model using the balanced truncation technique. A comparison with selective modal analysis (SMA) and balanced truncation model reduction techniques is presented. Comprehensive results show the effective emulation of synthetic inertia by implementing the control laws on a nonlinear three-phase diesel-wind system. The proposed technique is analyzed for different short circuit ratio (SCR) scenarios.

Study on Dynamic Characteristics of UHVDC System under Hierarchical Infeed Mode with STATCOM

This paper focuses on the investigation of the dynamic characteristics of Ultra High Voltage Direct Current (UHVDC) system Under Hierarchical Infeed Mode (UHVDC-HIM) with Static Synchronous Compensator (STATCOM). The STATCOM at the inverter side of UHVDC-HIM is developed in PSCAD/EMTDC, and their power flow equations are derived. Based on these equations, the control schemes for rectifier and inverter side of UHVDC-HIM and STATCOM are properly designed. Several dynamic characteristics indices such as Commutation Failure Immunity Index (CFII), Commutation Failure Probability Index (CFPI), fault recovery time and Temporary Overvoltage (TOV) are evaluated in detail. The impact of different sizes of STATCOM and Short Circuit Ratio (SCR) of AC sources on dynamic characteristics of UHVDC-HIM system during single-phase and three-phase faults are examined. The analysis shows that higher SCR values and greater capacity of STATCOM can make the UHVDC-HIM system less vulnerable to Commutation Failure (CF), considerably enhance the fault recovery time and effectively reduce the TOV.

An Improved AC System Strength Measure for Evaluation of Power Stability and Temporary Overvoltage in Hybrid Multi-Infeed HVDC Systems

We developed a new strength measure, the hybrid multi-infeed effective short-circuit ratio (HMESCR), for evaluating hybrid multi-infeed high-voltage direct current (MIDC) systems. The HMESCR defines the strength of hybrid MIDC systems with line-commutated converter (LCC) high-voltage direct current (HVDC) and voltage-source converter (VSC) HVDC systems. Unlike previous measures, the HMESCR considers the capacity of VSC HVDC systems to provide reactive power and thus, enhance the strength of AC systems connected at different buses with LCC HVDC systems. To demonstrate how the HMESCR reflects power stability (PS) and temporary overvoltage (TOV), we used MATLAB/Simulink software to perform quasi-steady-state analyses and dynamic simulations. The HMESCR can be used to determine the strength of hybrid MIDC systems.

Robust Control of a PMSG-Based Wind Turbine Generator Using Lyapunov Function

This paper proposes a robust finite-time controller (FTC) for a permanent magnet synchronous generator (PMSG)-based wind turbine generator (WTG). An adaptive observer is used for the rotor angle, rotor speed, and turbine torque estimations of the PMSG, thus eliminating the use of anemometers. The robustness of the proposed FTC regarding parameter uncertainty and the external weak power grid is analyzed. The impacts of the power grid short-circuit ratio (SCR) at the point of common coupling (PCC) on the conventional proportional-integral (PI) controller and the proposed FTC are discussed. Case studies illustrate that the proposed observer-based FTC is able to estimate the mechanical variables accurately and provides robust control for WTGs with parameter uncertainty and weak power grids.

Reactive Power Control Strategy for Inhibiting Transient Overvoltage Caused by Commutation Failure

The commutation failure (CF) is the most common fault in line-commutated high voltage direct current (LCC-HVDC) systems that may lead to the transient overvoltage in the sending-side system. In the worst condition, the CF may lead to large-scale wind turbine tripping. To resolve this problem, the mathematical relationship between the reactive power consumed by the rectifier and DC voltage, DC current is derived. Then, a transient overvoltage calculation method is proposed in this paper. Furthermore, the mechanism of transient overvoltage caused by the CF is analyzed; it is revealed that the reason for three times transient overvoltage is the rapid decrease of the reactive power consumed by the rectifier during the CF and the recovery period from the CF. This paper proposes a constant reactive power control (CRPC) to inhibit transient overvoltage of the sending-side AC system. The proposed CRPC can increase the reactive power consumed by the rectifier, reduce the exchange reactive power between AC and DC systems, and suppress the transient overvoltage. A simulation model in PSCAD serves to verify the proposed CRPC on the transient overvoltage suppression in the situation of different fault types, fault duration, fault severity and short circuit ratio (SCR).

Quantifying the Contribution of Dynamic Reactive Power Compensators on System Strength at LCC-HVdc Converter Terminals

This paper presents improved indices to evaluate the contribution of reactive power compensators at HVdc converter terminals. The presence of such compensators impacts converter performance, such as the maximum available power (MAP) limit, susceptance to commutation failure, fault recovery time and temporary over voltage. Traditionally, the Effective Short Circuit Ratio (ESCR) has been used to indicate system strength. However, this index cannot be directly applied when power-electronics based converters are used to provide the reactive power support. The approach of this paper is to use an improved form of the ‘Apparent increase in Short Circuit Ratio’ (AISCR) index to quantify the impact of the compensator on the HVdc system. This index is evaluated by comparing the performance with the reactive power device with a system without the device, but having the same performance. The paper shows that a single AISCR index cannot quantify all behaviors and suggest modifications to AISCR for each of several HVdc phenomena. In this paper two types of compensators are considered – the Synchronous Compensator and the Static Synchronous Compensator (STATCOM).

Novel grid‐forming control of PMSG‐based wind turbine for integrating weak AC grid without sacrificing maximum power point tracking

Traditional permanent magnet synchronous generator (PMSG)-based wind turbine (WT) normally utilises phase lock loop (PLL) for obtaining accurate phase angle of AC grid to make sure of maximum power point tracking (MPPT). However, a weak AC system with a low short circuit ratio (SCR) and high control bandwidth of PLL might have impaired effects on the stability of the system. To overcome these, a novel power synchronous control of PMSG-based WT with grid forming ability of PMSG for integrating very weak AC system is first proposed in this study. Grid-side converter (GSC) of PMSG emulates the inertia response of synchronous generator (SG) by artificially coupling the DC-link voltage with the grid frequency based on the certain droop property, while the converter voltage magnitude is regulated according to DC-link voltage deviation by controlling the modulation index of GSC. This novel scheme can realise the grid synchronisation without PLL by mimicking the natural characteristic of SG. Small-signal analysis of the proposed scheme indicates the DC-link capacitor energy can be exerted for system inertia support during system disturbances, while the system damping can be provided by spontaneously alternating converter voltage magnitude. More importantly, MPPT property of PMSG is not compromised while providing inertia and damping for the system, and the related inertia and damping coefficient of WT with the proposed control can be easily tuned for satisfying the system requirement. Non-linear simulations of one PMSG connected with one large SG considering a sudden change of active power reference and variable wind speeds have been studied to verify the effectiveness of the proposed grid-forming control.

AC system’s strength evaluation of UHVDC system under hierarchical connection mode

The inverter’s end of ultra high voltage DC (UHVDC) system under hierarchical connection mode (HCM) is connected to 500 and 1000 kV AC buses through high end converter and low end converter transformers, respectively. The two AC buses are interconnected via coupling transformer and the tie line. This complex structure of interconnection at inverter’s end can make the understanding unclear about each AC system’s strength of UHVDC-HCM. Therefore, this paper covers the system’s strength evaluation by developing the “Hierarchical mode effective short circuit ratio” (HMESCR) index. The HMESCR incorporates the impact of neighborhood AC networks, impedance of coupling transformer, the tie line and strength of the objective AC network. Based on the proposed index, an equivalent model for UHVDC-HCM system is developed. An equivalent model is verified under the steady state conditions such as maximum available power and the transient conditions such as commutation failure immunity index under AC faults. In addition, three different cases are considered in order to get the in-depth and comparative performance analysis of an equivalent model with UHVDC-HCM system. MATLAB and PSCAD/EMTDC are used as analysis tools for illustrating the steady state and transient state conditions. The analysis indicates that the presented equivalent model has similar behavior as UHVDC-HCM system under both steady state and transient state conditions, which verifies the accuracy of proposed index in assessing the strength of both AC systems of UHVDC-HCM.

Control Structures Implementation to Allow High Penetration of a VSC within an Isolated Power System

: Recently, the increasing occurrence of power instability caused by alternative energy sources has attracted the possibility of implementing Energy Storage System (ESS), capable of supplying the energy necessary to improve the energy quality within the grid. Therefore, the attention of researchers has been drawn on the behaviour that has the connection of the ESS, based on internal controllers such as Virtual Synchronous Machine (VSM) and a Classical Control Cascade (CCC), for different ESS power ranges. In this context, this paper deals with the implementation of an ESS represented by a Voltage Source Converter (VSC) employing control techniques of VSM and CCC simulated in PScad environment in three cases: in the first case, the power system in the steady-state is seen during the description of the controls; in the second case, the power system in the presence of a fault; in the third case, the power system with a load at the PCC. Finally, a fusion CCC-VSM using a grid impedance estimator is implemented with different current reference controls considering Short Circuit Ratio (SCR) values that represent the weak and robust grid

Research on optimizing controller parameters based on transient energy function to improve the stability of weak transmission system

This paper takes the Ximeng-Taizhou UHVDC transmission weak transmission end AC system as the background, and proposes to optimize the low-voltage current limiting control link (VDCOL )to improve the stability of the system. First of all, this paper analyzes the working principle and limitations of traditional VDCOL, and proposes that the short-circuit ratio of the sending end and the voltage of the commutation bus at the receiving end are used as the variables of the VDCOL cut-off voltage to achieve the effect of optimizing VDCOL. Make the cut-off voltage a value that changes with the change of the inverter side commutation bus voltage, which can take into account the recovery speed of the AC and DC systems. The process of constructing the transient energy function considering the dynamic characteristics of the DC system is analyzed in detail, and the real-time digital simulation software RTDS is used to build a single-machine infinite-DC simulation example. Based on time-domain simulation software and transient energy function margin method, the optimization of the low-voltage current-limiting control link in the DC control system is verified to improve the stability of the sending-end AC system.

Optimal Allocation and Sizing of Synchronous Condensers in Weak Grids With Increased Penetration of Wind and Solar Farms

The increasing number of weak-grid-connected renewable energy resources in power systems has created various challenges in recent years. Some examples include un-damped voltage oscillations in the ERCOT and sub-synchronous resonance in the North-China power grid. Several solutions for these challenges have been proposed, such as Static Compensators and Synchronous Condensers (SynCons). SynCons, being synchronous machines without a prime mover, provide several benefits in weak power systems, such as frequency support, system strength, and voltage regulation. Although SynCons are widely-utilized to mitigate the weak grid integration challenges, their installation /operation costs make them a costly solution. Additionally, their lead-time can be more than a year, which means their initial sizing and allocation must be optimal. In this paper, a method for the optimal allocation and sizing of SynCons is proposed. The main objective of this method is maintaining Short Circuit Ratio (SCR) in the system greater than pre-defined values while the investment and operation costs of SynCons, and voltage deviation in the system are minimized. Three meta-heuristic optimization algorithms are used to implement the proposed method, and its performance is evaluated via Electromagnetic Transient (EMT) time-domain simulation in a modified IEEE 39-bus system. The PSCAD/EMTDC software is used for the time-domain simulation. The simulation results confirm that with optimized allocation and sizing, the SCR of the network in all areas is more than pre-defined values, and also, all renewable energy resources can ride through disturbances and comply with given grid codes.

Critical system strength evaluation of the power system with high penetration of renewable energy generations

The power system is experiencing a higher penetration of renewable energy generations (REGs). The short circuit ratio (SCR) and the grid impedance ratio (GIR) are two indices to quantify the system strength of the power system with REGs. In this paper, the critical short circuit ratio (CSCR) is defined as the corresponding SCR when the system voltage is in the critical stable state. Through static voltage stability analysis, the mathematical expression of the CSCR considering the impact of GIR is derived. The maximum value of CSCR is adopted as the critical value to distinguish the weak power system. Based on the static equivalent circuit analysis, it is proved that the CSCR is still effective to evaluate critical system strength considering the interactive impact among REGs. Finally, we find that the GIR can be neglected and the SCR can be used individually to evaluate the system strength when SCR>2 or GIR>5. The correctness and rationality of the CSCR and its critical value are validated on ADPSS.

Power-Synchronization Control for Ultra-Weak AC Networks: Comprehensive Stability and Dynamic Performance Assessment

Power-synchronization control (PSC) is a promising control strategy to improve the stability and performance of voltage-source converters (VSCs) in ultra-weak AC grids. However, evaluation of PSC to date has investigated performance only at single controller operating points, rather than holistically varying multiple controller gains. This paper develops a new methodology, based on small-signal eigenvalue analysis, to comprehensively analyze PSC-VSC stability. The maximum active power transfer of PSC is established across a broad range of controller tunings and the two-way and three-way couplings between the power-synchronization control, AC voltage control and high-pass current filter gains are quantified. A new stable tuning region is introduced, which represents the controller parameter space for stable operation. It is shown that PSC can achieve rated power transfer into an AC grid (short circuit ratio(SCR)=1) at multiple controller operating points, but dynamic performance varies significantly within this region. The robustness of this operating region to SCR changes is also investigated. The stability boundary and dynamic performance are validated using control hardware-in-the-loop experiments with a real-time digital simulator. Practical recommendations arising from this work are a set of controller gains that provide stability and good dynamic performance at high power transfer for ultra-weak grid-connected VSCs employing PSC.

Analysis of the Operating Margin Evaluation of Multi-Infeed LCC-HVDC Systems Based on the Equivalent Impedance

In China, a structure in which multiple line-commutated converter based high-voltage direct current (LCC-HVDC) systems feed into the same AC system has been formed. For the multiple LCC-HVDC infeed system, the electrical strength and real operating condition evaluation of the receiving AC system are of great significance for system operation. The multi-infeed short circuit ratio (MISCR) is generally utilized to assess the strength based on the rated parameters; hence, it cannot analyze the real operating condition under the non-rated status. In this paper, an LCC-HVDC system is equivalent to a grounded impedance without changing the external characteristics. The multi-infeed operating short circuit ratio (MIOSCR) is then deduced from the equivalent impedance and equivalent nodal analysis. Relationships between the MIOSCR and electrical strength indices of AC and AC/DC systems are analyzed, and the MISCR is deduced as a special case of the MIOSCR under the rated status. The multi-infeed operating margin coefficient (MIOMC) is finally proposed, which can be utilized to evaluate the operating margin of the LCC-HVDC system and provide guidance for designing operating strategies. The proposed MIOSCR and MIOMC overcome the previous shortcomings of inaccuracy about operating status assessment by the MISCR. The results of the case study demonstrate the effectiveness of the MIOSCR and the MIOMC for assessing the operating condition and margin of converter buses.

Analysis of Ventilation Efficiency and Effective Ventilation Flow Rate for Wind-driven Single-sided Ventilation Buildings

Natural ventilation can be categorized into single-sided ventilation and cross-ventilation. Based on the air flow mechanism of a single-sided wind-driven natural ventilation, the effective ventilation flow rates in buildings are different from the air flow rates through openings. The mixing coefficient or ventilation efficiency is defined by the ratio of these flow rates, indicating the effective ventilating ability of a single-sided ventilation, similar to the effect of penetration depth of fresh air. This paper provides a detailed analysis of ventilation effectiveness in a building with a single opening based on numerical simulations. The simulation was validated by a study of an existing case. By numerical calculation, the air flow rates through openings and the effective ventilation flow rates in buildings were compared. Effects of various influencing parameters, such as wind speed, opening location, opening area and aspect ratio of windward wall on ventilation efficiency were evaluated. An estimated mixing coefficient has been established by the modelling, which would be beneficial for the predication of the short-circuit ratio of ventilation rate through openings.

On the Tuning of Fractional Order Resonant Controllers for a Voltage Source Converter in a Weak AC Grid Context

This paper proposes a method for tuning the fractional exponent of different types of fractional order resonant controllers for a voltage source converter in a weak AC grid context. The main objective is to ensure the stability of the controlled system in a weak AC grid environment and to achieve an adequate dynamic response under disturbances. Therefore, six commonly used integer order proportional resonant (PR) control structures are selected from the literature and compared with each other according to their frequency behaviour. Afterwards, a rational approximation for the fractional order term is selected based on continuous fraction expansion technique. The inclusion of a fractional exponent in each integer order PR structure generates the fractional order proportional resonant (FPR) control transfer functions. Once the FPR controllers have been obtained, their closed-loop responses are tested via eigenvalue trajectory analysis. For each FPR control structure, a range of the fractional exponent that ensures stability is obtained. The conclusions of eigenvalue trajectory analysis are tested by implementing the FPR control structures in an specific application consisting in a modular multi-level converter (MMC) connected to a weak AC grid with adjustable short-circuit ratio. By means of time-domain simulations, not only the previous eigenvalue analyses are validated, but also new tuning criteria are given for the fractional exponent in combination with other control parameters, such as the damping frequency and the inductance of the complementary feedback branch. Moreover, a sensitivity analysis of the tuning criteria is carried out for other sizes of the AC filter inductance.