Short Circuit Ratio Research Articles

Influencing factor analysis of the short circuit ratio on grid-connected photovoltaic systems

Over the past few years, solar energy conversion technology is sharply developing. An important first step is to make this conversion system more effective and more reliable. The main objective of this paper is to study the influence of the power of the electricity network on the connection of the solar energy source. The photovoltaic source has been examined under the effect of the variation of the parameters of the networks such as the power of short-circuit and the frequency .The results obtained by the simulation have shown that the photovoltaic source has amazing performance if the power system is of high or medium power and with constant parameters.

Harmonic failure in the filter of Static Var Compensator

The Static Var Compensator (SVC) is an important transmission component for the stability of the power system, it has 3 main components: Thyristor- controlled reactor (TCR), Thyristor-switched capacitor (TSC) and filters. The filters are the weakness part in this power system. In this kind of equipment, the worst problem associated with a filter failure is the harmonic damage in the power system, the currently international standards doesńt have enough information about the damage in the power system with high harmonic influence caused by SVC with the filter failure.In this research the main scope is to provide a new assessment procedure for energy and industrial companies for the application of equipment on the utility system, it will illustrate the harmonic assessment levels, at the point of common coupling (PCC).A real failure analysis is made for a better comprehension of the concepts of the PCC, it is very important to know the way of harmonic current flow and other parameters as: Average maximum demand load current, the short circuit ratio (SCR), the total demand distortion (TDD) and total harmonic distortion (THD). I will provide a better understanding and recommendation for the IEEE Standard 519, the dimension should address the time variations and the statistical characteristics of the harmonic levels. The harmonic is important due to levels, control on the utility system. This engineering failure analysis of the filter applications and interface with other energy companies.

Comprehensive small‐signal model and stability analysis of VSC‐based medium‐voltage DC distribution system

DC distribution technology is emerging as a prospective form of future power system owing to its advantages such as high controllability and easy compatibility. In medium- or low-voltage DC distribution systems, the two-level voltage-source converter (VSC) has become one of the preferred topologies for grid-connected converters. A comprehensive small-signal model for VSC-based DC distribution systems, which takes into consideration the AC filter, DC distribution cables, and all the controllers in the VSC is presented in this study. Based on the ideal AC source, an absolutely stable synchronous rotating coordinate system is built to preclude the influence of the short circuit ratio of the AC system. The proposed model is verified using a detailed electromagnetic transient simulation carried out using power systems computer-aided design (PSCAD). To analyse the impact of neglecting the AC filter and phase-locked loop controller, two simplified models are established, and a detailed comparison of the three models is presented. The results indicate that when using the two simplified models, the time-domain responses are significantly different from those in the PSCAD simulation and the obtained feasible domains of the control parameters tend to be either more optimistic or conservative. Thus, to carry out a small-signal stability analysis, the use of the proposed comprehensive model is recommended to ensure the safety of the DC distribution system.

Damping of low-frequency oscillation in power systems using hybrid renewable energy power plants

Global warming, increase in environmental pollution, and high cost of electrical power generation using fossil fuels are considered the most important reasons for the application of renewable energy power plants (REPPs) around the world. In recent years, a new generation of REPPs called hybrid renewable energy power plants (HREPPs) has been implemented in order to have higher efficiency and reliability than conventional REPPs such as wind power plants and photovoltaic power plants. The HREPPs include two or more renewable energy generation units such as wind turbine generation units, and PV generation units. In case of high penetration of these types of power plants, the most common tasks of synchronous generators should be supported by them. One of these tasks is the ability to reduce the low-frequency oscillation (LFO) risk through power oscillation damper such as the power system stabilizers of synchronous generators. In this paper, a novel method is proposed for LFO damping by HREPPs, which is based on the design of an optimal power oscillation damper (OPOD) implemented in the generic HREPP controller model. The structure of the proposed OPOD is a 2nd-order single-input lead-lag controller, and its performance is investigated in a modified two-area test system. The simulation results show the proper performance of the HREPP for LFO damping using the proposed OPOD in the various loading levels and different short circuit ratio values.

Minimum Short Circuit Ratio Requirement for MMC-HVDC Systems Based on Small-Signal Stability Analysis

This paper determines the minimum short circuit ratio (SCR) requirement for a modular multilevel converter based high-voltage direct current (MMC-HVDC) transmission systems. Firstly, a simplified model of MMC is introduced; the MMC is represented by its AC and DC side equivalent circuit. Next, by linearizing the MMC subsystem and the DC network subsystem, the deduction of the small-signal models of MMC subsystem, the small-signal model of the DC network and MMC-HVDC are carried out successively. Thirdly, the procedure for determining the minimum SCR requirement of MMC-HVDC is described. Finally, case studies are performed on a two-terminal MMC-HVDC system under four typical control schemes. The results show that the restraint factors for the rectifier MMC is predominantly the voltage safety limit constraint, and the restraint factors for the inverter MMC are mainly the phase locked loop (PLL) or the outer reactive power controller. It is suggested that the minimum SCR requirement for the sending and the receiving systems should be 2.0 and 1.5 in the planning stage.

Investigating subsynchronous oscillations caused by interactions between PMSG-based wind farms and weak AC systems

Recently, sporadic power oscillations in subsynchronous frequency range were detected in several permanent magnet synchronous generator (PMSG)-based wind farms in Xinjiang, China. Given no existence of fixed series compensation in the system, it can be concluded that this is a new type of subsynchronous interaction (SSI) and different from the previous subsynchronous control interaction (SSCI) occurred in doubly-fed induction generator (DFIG)-based wind farms. Therefore, the characteristics and mechanism behind this phenomenon are so ambiguous that further clarification is needed. To investigate this new type of subsynchronous oscillations (SSO) problem, a real SSI incident is reported, and the equivalent model is established based on multi-machine equivalent aggregation. Refined frequency scan and small-signal analysis are carried out to analyse the mechanism and characteristics of such SSI. Time-domain simulations are conducted to replicate the incident. The results show that the interactions between PMSG-based wind farms and weak grids can trigger negative-resistance and capacitive-reactance effects, which leads to unstable power oscillations. To further investigate the influence of different wind power plants (WPPs), two short-circuit ratio metrics are developed, and time-domain simulations are conducted based on semi-aggregation. The potential hazards of this SSI and its mitigation strategies are also discussed and proposed.

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.

Virtual Inductance for Stable Operation of Grid-Interactive Voltage Source Inverters

In this paper, a feedforward virtual inductance control strategy is developed for the stability of voltage source inverters (VSIs) in weak grids. A weak grid is characterized by a low short-circuit ratio and a low inertia constant, in which VSIs become susceptible to voltage distortions and instability. The proposed feedforward virtual inductance term is integrated into the current control loops of grid-tied VSIs. In this paper, the effectiveness of the virtual inductance control strategy in weak grids is first proved through closed-loop transfer function analysis and then the system's root locus analysis to determine the stability region of the system versus the grid impedance. The developed virtual inductance controller is also verified through laboratory experiments for different weak grid scenarios.

A Novel Data Driven Voltage Control Approach for Grid Connected Wind Power Plants

Due to increasing penetration of wind power plants (WPP), power grids are facing significant power quality challenges at the point of interconnection (POI). To achieve stable and robust power system operation, reactive power plays a vital role. Reactive power needed to compensate voltage fluctuations at the POI of the WPP varies with a short-circuit ratio (SCR). The reactive power capability of a WPP is also limited and largely depends on various operating conditions. In a physical system, it is difficult to find the correlation among critical parameters for voltage control. In this paper, a data-driven voltage control approach is proposed for a grid-connected WPP. Two regression models are developed through surface fitting using MATLAB curve fitting toolbox: One model based on simulation data is to determine the required reactive power for grid voltage compensation; another model based on field measurement data is to determine the reactive power characteristics of the WPP. The reactive power compensation device is capacitors in this study. Two controllers, a central WPP controller, and a capacitor controller are designed, and their effectiveness is validated through several case and sensitivity studies.

Investigation of SSO characteristics in grid‐connected PMSGs system

In recent years, a new type SSO issue, which has occurred in direct-drive permanent magnetic synchronous generators (PMSGs) based wind farm, brings a new challenge to the stability of the power system. In this paper, the SSO issue in grid-connected PMSGs system is studied by the eigenvalue analysis. The linearised state-space model of this system is built firstly. The impacts on the SSO characteristics from short-circuit ratio (SCR), impedance-frequency characteristic, resonant frequency of AC network and control parameters of PMSGs are observed by the root loci analysis. After that, electromagnetic transient simulation is carried out to verify the eigenvalue analysis. Meanwhile, the participation factors of SSO mode are calculated to analyse the subsynchronous control interaction (SSCI) between AC network and PMSG. The result shows that there is a SSO mode which is greatly influenced by AC network and DC capacitor, grid side converter (GSC) control of PMSG. The characteristics of AC network will affect the SSCI between AC network and PMSG.

Evaluation of Strength Measure for Static Voltage Stability Analysis of Hybrid Multi-Infeed DC Systems

This paper presents an evaluation method of strength measure for the static voltage stability analysis of the hybrid multi-infeed dc (HMIdc) system, which comprises both the line-commutated converter-based high voltage dc (LCC-HVdc) and voltage source converter-based HVdc (VSC-HVdc) links. For this purpose, the coupled single-port model proposed originally to assess the static voltage stability of ac power systems is first extended in the HMIdc system. In particular, how the LCC-HVdc and VSC-HVdc links should be modeled in the extended coupled single-port model is further investigated. Then, a generalized effective short-circuit ratio (GESCR) index is proposed as a strength measure for the static voltage stability analysis of the HMIdc system using the extended coupled single-port model. The GESCR can resolve the ambiguity encountered by earlier indices arising from empirical reasoning; thus, it is able to indicate the static voltage stability of the HMIdc system more accurately. Finally, the simulated results based on a classical simplified HMIdc system and a modified IEEE 118-bus system, both including two LCC-HVdc links and a VSC-HVdc link, validate the proposed GESCR.

Control of Droplet Transition in Underwater Welding Using Pulsating Wire Feeding.

Underwater wet welding technology is widely used. Because the stability of droplet transfer in underwater wet welding is poor, the feasibility of improving the droplet transfer mode has been discussed from various technical directions. In this work, the characteristics of pulsating wire feeding were studied in the pulsating wire feeding mode by investigating the effects of changing the pulsating frequency, the wire withdrawal speed, and the wire withdrawal quantity on the droplet transfer process and the welding quality. With the aim of improving weld forming and welding stability, the authors selected the coefficient of variation and the ratio of unstable droplet transfer as the indexes to evaluate the effect of droplet transfer control. The pulsating wire feeding process of underwater wet flux-cored wire was analyzed in depth, and the following conclusions were drawn: using the pulsating wire feeding mode and after comparing and analyzing the pulsed wire feeding process under the same frequency condition, the authors found that the forming and stability were better under the conditions of slower withdrawal speed and smaller withdrawal quantity. The short-circuit transition ratio decreased steadily with the increase of pulsating wire feeding frequency, the rejection transition ratio first rose and then decreased, and the splash ratio first decreased and then rose.

Analysis of Voltage Control Strategies for Wind Farms

PCC voltage control is an important key to deal with upcoming challenges of large-scale wind power integration into electric grid. In this paper, based on the derived transfer functions of the plant and controllers, the dynamic performance of three voltage control strategies (centralized, decentralized, and distributed control) is evaluated in terms of robustness to variation of short-circuit ratio (SCR), disturbance rejection (DR), and impact of communication delay. After showing shortcomings of each strategy, an improved voltage control strategy with lower sensitivity to SCR variation and higher capability on grid voltage DR as well as on reactive current dispatch is proposed. The theoretical analysis is validated by simulation results in MATLAB. Finally, a small wind farm is modeled and simulated in RTDS to demonstrate the possible control interaction and reactive current sharing performance of the studied control schemes.

Study on stability evaluation in multi-send hybrid AC/DC transmission system

Our country has shown the significant features of high voltage direct current (HVDC) including multi-infeed and multi-send. Compared to multi-infeed system, there are a lot of weakness in the multi-send system, such as the grid structure is single and weak, which makes the interaction between AC and DC to a greater extent affect the stability of the multi-send hybrid AC/DC transmission system. In order to evaluating the system strength in multi-send hybrid AC/DC transmission system, for the first time, this paper proposed the general formula and simplified formula for multi-send short-circuit ratio (MSSCR), which accounts for the coupling between multi-dc projects. Considering the control characteristics of rectifier in sending-side and combining the maximum power curve method, the criticality of MSSCR is proposed. Multi-send critical short-circuit ratio (MSCSCR) is deduced by the quasi steady-state equation of rectifier, to determine the multi-send system is strength enough or not. The proposal of MSSCR is of considerable significance for effectively evaluating the stability performance of the multi-send AC/DC hybrid transmission system, completing the short-circuit ratio theoretical system, and consolidating the short-circuit ratio index status.

Active and Reactive Power Injection Strategies for Three-Phase Four-Wire Inverters During Symmetrical/Asymmetrical Voltage Sags

Electric grid codes are expected to change in near future to accommodate an increased number of distributed generation units in the distribution power system without impairing its power quality. It is desired that the generators remain connected during voltage sags and provide ancillary services, such as voltage and reactive power control, ensuring the operational stability of the power system. This paper explores how the existing strategies for active and reactive power injection impact the operation of grid-tied inverters in terms of required power, current flowing, and reduction of active power delivery during the voltage sags. Such inputs are relevant to properly size converters for operation under fault events. In addition, this paper contributes to devise: 1) constant peak current control, 2) constant active current control, and 3) constant average active power control strategies for three-phase four-wire grid-tied inverters considering the natural (abc ) reference frame. The design and implementation of the investigated power injection strategies are discussed, and their effectiveness and technical viability are analyzed through dynamic computational simulations under symmetrical/asymmetrical voltage sags, and variation of the short-circuit ratio.

Comparison study of small-signal stability of MMC-HVDC system in different control modes

The control mode adopted by a modular multilevel converter (MMC) in an HVDC transmission system directly affects both dynamic and steady-state characteristics of the system. This paper, benefiting from a comprehensive dynamic-phasor based state-space model of MMC, presents a comparative analysis on operational stability of the MMC-HVDC station in PV-control mode (i.e., in active-power and AC-voltage-control mode) and PQ-control mode (i.e., in active- and reactive-power-control mode). Firstly, corresponding to the MMC rectifier /inverter in PV- and PQ-control modes, four linearized small-signal models are developed in MATLAB, and validated by electromagnetic transient (EMT) simulations in PSCAD/EMTDC. Then, based on eigen-analysis, the impact of AC-system strength, quantified by the short-circuit-ratio (SCR) and impedance-angle, on the small-signal stability for a MMC rectifier /inverter under PV- and PQ-control are fully compared. Finally, based on the participation factor analysis and parametric sensitivity analysis, this paper digs more deeply into the essential state-variables and critical control-parameters that significantly contribute to the instability of the MMC system under a very weak AC-grid condition. The results show that the stability-margin of the MMC system, either rectifier or inverter, is essentially different when under PV- and PQ-control. This paper provides an effective approach, through adjusting the most sensitive control-parameters, to mitigate the MMC system instability at weak AC-grid conditions, which is a valuable guidance for parametric selection and adjustment in practical engineering.

Small-Signal Modelling and Stability Assessment of Phase-Locked Loops in Weak Grids

This paper proposes a low-complexity small signal model for a 3-leg converter connected to a balanced three-phase, three-wire weak grid and synchronised to this grid using a PLL implemented in a synchronous rotating d-q axis. A thorough analysis of the system stability as a function of the PLL bandwidth and the short circuit ratio (SCR) of the grid is performed based on a linearised model. By using the proposed model, an improved design process is proposed for the commonly used dq-PLL that accounts for the potential stability issues which may occur in weak grids. Using the proposed approach, it is possible to optimise the PLL design to find the fastest PLL that can operate stably considering the SCR of the grid. In addition, the proposed model is very simple, resulting in a straightforward design tool that could also be used for online stability monitoring. The method is validated through simulations and experimental results from a 5kW laboratory system.

Small Signal Stability Analysis of Multi-Infeed Power Electronic Systems Based on Grid Strength Assessment

The increasing penetration of renewable energy resources, such as wind and solar, is changing AC grid dynamics. Potential dynamic stability issues such as small signal instability may occur when these resources are connected to a grid with low strength, i.e., a weak grid. In this paper, the relationships between small signal stability and grid strength measured with short circuit ratio (SCR), along with the relationships between the stability boundary and critical SCR (CSCR) are analytically studied in a single-infeed power electronic system (SIPES). On this basis, these relationships are extended to a multi-infeed power electronic system (MIPES) through eigenvalue decomposition techniques to define the so-called generalized short circuit ratio (gSCR) and critical gSCR (CgSCR) for small signal stability analysis. The defined gSCR and CgSCR are the generalized representation of SCR and CSCR for SIPESs in MIPESs. The gSCR can assess the grid strength of a MIPES in terms of its small signal stability while the CgSCR can characterize the stability boundary of a MIPES. In addition, the condition of CgSCR = CSCR is satisfied for a MIPES with homogeneous power electronic devices. This enables the proposed method based on the gSCR to simplify the complex stability analysis of MIPESs into the stability study of SIPESs. The efficacy of the proposed gSCR and CgSCR is validated with eigenvalue analysis and time-domain simulations.

ANALYSIS OF THE SHOP NETWORKS BASBURS PARAMETERS AT HIGHER HARMONIC CURRENT ACTIONS

Purpose. Research and analysis trolley busbar’s parameters in condition of higher current harmonic actions, with taking into account the structural features of nonlinearity of magnetic and electrical properties of materials, proximity effects, surface and external surface effects.Methodology. The researches were carried out using the electromagnetic field theory methods, the electrical circuit theory, mathematical physics, finite elements, interpolation, approximation and regression analysis.Findings. The voltage drop in the phases of busbar’s trolls from the action of higher current harmonics is researched. It has been evidenced that with permissible values of higher harmonics established by the standard for different short-circuit ratio Rsce, the magnitude of the voltage drop can increase by 3.5 times, and using the steel casing - by 4 times. At the same time, the increment of the magnetic field energy and active losses in the busbar is 20% relative to their values for the main current harmonic, excluding shielding, and up to 23%, including shielding of busbar’s trolleys. The action of higher harmonics at permissible own amplitudes at corresponding values of the short-circuit ratio Rsce also causes an increase in active resistance by 4 times, inductive - by 20 times. Due the features of the placement of the busbar in the workshop, production and technological processes, the shielding of the busbar’s trolleys is required. Therefore, in such cases, to compensate for the asymmetry of the voltage drop in the trolleys and active losses in the busbars, the current load of the busbar’s trolls should be reduced by 5-20% depending on the value of the short circuit coefficient of the shop network.Originality. The mathematical model of electromagnetic processes in the copper busbar’s trolley, which contain design features, nonlinearity of magnetic and electrophysical properties of materials, proximity effects, surface and external surface effects, the effect of current harmonic components on voltage drop and power loss in the power transmission are proposed. It allows with high accuracy and numerical efficiency, to determine the busbar’s trolley parameters for the corresponding values of the amplitudes and frequencies of the current hight harmonics. For the first time, the relationship between the voltage drop and specific active losses from the spectrum of frequencies and amplitudes of current harmonics and the values of the short-circuit coefficient is executed, which allows to determine the degree and character of the effect on the asymmetry of the voltage drop and the parameters of the busbar’s trolley.Practical value. It has been evidenced that with permissible values of higher harmonics established by the standard for different short-circuit ratio Rsce, the magnitude of the voltage drop can increase by 3.5 times, and using the steel casing - by 4 times. At the same time, the increment of the magnetic field energy and active losses in the busbar is 20% relative to their values for the main current harmonic, excluding shielding, and up to 23%, including shielding of busbar’s trolleys. The action of higher harmonics at permissible own amplitudes at corresponding values of the short-circuit ratio Rsce also causes an increase in active resistance by 4 times, inductive - by 20 times. The results of the work can be used in the design or modernization of existing shop power supply systems, as well as to determine the parameters, as well as to estimate the voltage drop in the other busbar’s design.

Assessing Strength of Multi-Infeed LCC-HVDC Systems Using Generalized Short-Circuit Ratio

In this paper, a generalized short-circuit ratio (gSCR) is proposed for assessing the grid strength of multi-infeed LCCHVDC (MIDC) systems by the theoretical analysis of the relationship between static voltage stability and grid strength. The gSCR can resolve the ambiguity when applying the concept of shortcircuit ratio (SCR) for single-infeed LCC-HVDC (SIDC) systems to MIDC systems. Moreover, the gSCR is the generalized presentation of SCR in MIDC systems. Based on the gSCR, our defined critical gSCR (CgSCR) and boundary gSCR (BgSCR) for MIDC systems have the same functions as the critical SCR (CSCR) and boundary SCR (BSCR) used in grid strength analysis for SIDC systems. Also, the CgSCR and BgSCR have the same values as those of CSCR and BSCR (i.e., CgSCR = CSCR ≈ 2 and BgSCR = BSCR ≈ 3) to characterize the stability boundaries of MIDC systems. This allows the analysis experience of using the SCR, CSCR, and BSCR for SIDC systems can be easily transplanted to the application of the gSCR, CgSCR, and BgSCR for MIDC systems. The efficacy of the proposed gSCR along with CgSCR and BgSCR is demonstrated by comparing them with other indices of grid strength assessment for MIDC systems.