Short Circuit Ratio Research Articles

The Dual-Mode Combined Control Strategy for Centralized Photovoltaic Grid-Connected Inverters Based on Double-Split Transformers

Centralized photovoltaic (PV) grid-connected inverters (GCIs) based on double-split transformers have been widely used in large-scale desert PV plants. However, due to the large fluctuation of short circuit ratio (SCR) under high-penetration PV power plants, the stability of GCIs controlled in current source mode (CSM) is seriously affected. Reducing the bandwidth of the phase-locked loop (PLL) will improve the stability of CSM-controlled GCI, but the dynamic response performance will be degraded. Even if the PLL bandwidth is designed to be very low, the CSM-controlled GCI still cannot operate stably in very weak grids. The voltage source mode (VSM) controlled GCI can operate stably in very weak grids, but is not suitable for strong grids. Therefore, this article proposes a dual-mode combined control strategy: When SCR gradually decreases, the GCIs operate in full CSM (FCSM), hybrid mode (HM), and full VSM (FVSM) in turn, which effectively improves the stability of GCIs when SCR fluctuates greatly. The stability region of GCIs under the constraints of multiperformance-index is obtained, and the switching boundaries between the above three modes are also quantitatively analyzed in this article. Finally, the correctness of the proposed control strategy is verified by experiments.

Research on steady state control strategies of wind farm integration by VSC-LCC hybrid HVDC transmission

Wind farm is integrated by a hybrid high voltage direct current (HVDC) system with line commutated converter (LCC) and modular multilevel converter (MMC). Power sources at the sending end include wind farm and AC system, which can complement each other. Basic control strategies of each converter of the system are proposed. Considering the capacity limitation of MMC, basic control strategy under fluctuating wind power is made and the system is not able to transmit rated power under the strategy when the wind power is low. In order to enlarge the range of wind power when the system can transmit the rated power, an improved control strategy that lower the voltage of MMC is devised. Under the improved strategy, performance of the system is improved. Operation characteristics are compared under different short circuit ratio (SCR) of AC system at the sending end, and additional control is introduced when SCR is low.

A Fundamental Study on the Transient Stability of Power Systems with High Shares of Solar PV Plants

The last decade has seen an immense growth in renewable energy sources such as solar photovoltaic (PV) plants due to environmental concerns. Due to this rapid growth, solar PV plants are starting to have a larger influence on power system stability and thus their dynamic behavior cannot be ignored in stability studies. The lack of well-established models and parameter sets is the primary reason solar PV plants are not modeled with dynamic characteristics. This paper presents a method to define a standard parameter set for representing large-scale and aggregated solar PV plants in stability studies from the perspective of the transmission system operator (TSO). The method takes into account primarily the conditions provided in the grid connection requirements; for illustrative purposes, the connection requirements of the Netherlands are used. Additionally, a relationship defined as short-circuit current (SCC) PV ratio is proposed to estimate the effect of solar PV plants on transient stability. To illustrate the workings of the proposed ratio, the transmission network of the TenneT TSO B.V. in the Netherlands is used. The analysis demonstrated that high values of SCC PV ratio are an indicator that solar PV plants affect the transient stability while low values of SCC PV ratio showed that solar PV plants have minimal effect on the transient stability. Additionally, methods to improve the transient stability are provided which include limiting the operation regions of critical generators, increasing short-circuit ratio by adding a synchronous condenser or static compensator (STATCOM) and decreasing the reactance between the critical synchronous generator and faulted bus.

Dynamic Performance Comparison between STATCOM and SVC

This paper presents an investigation to the dynamic performance of a PI-type controlled STATCOM for various faulty conditions as well as for different system short circuit ratios SCR. Furthermore, the performance of the STATCOM is thereby simulated using the PSCAD/EMTDC software. The obtained results reveal superior dynamic performance and reduced system short circuit ratio SCR.

Improved Control Strategy of MMC–HVDC to Improve Frequency Support of AC System

With the continuous development of power electronics technology, variable-speed offshore wind turbines that penetrated the grid system caused the problem of inertia reduction. This study investigates the frequency stability of synchronous, offshore wind-farm integration through a modular-multilevel-converter high-voltage direct-current (MMC–HVDC) transmission system. When full-scale converter wind turbines (type 4) penetrate the AC grid, the AC system debilitates, and it becomes difficult to maintain the AC system frequency stability. In this paper, we present an improved inertial-response-control method to solve this problem. The mathematical model of the synchronous generator is based on the swing equation and is theoretically derived by establishing a MMC–HVDC. Based on the above model, the inertia constant is analyzed using a model that integrates the MMC–HVDC and offshore synchronous generator. With the new improved control method, a more sensitive and accurate inertia index can be obtained using the formula related to the effective short-circuit ratio of the AC system. Moreover, it is advantageous to provide a more accurate inertial control evaluation for AC systems under various conditions. Furthermore, the impact of the MMC–HVDC on system safety is assessed based on the capacitor time constant. This simulation was implemented using the PSCAD/EMTDC platform.

Line Inductance Stability Operation Domain Assessment for Weak Grids With Multiple Constant Power Loads

Weak grids are gaining considerable attention since power generation resources are remote from constant power loads (CPLs), which results in low-frequency/harmonic oscillation. Meanwhile, due to the play, and plug demand of modern power system, the line inductance of weak grids often changes, which is also regarded as the variation regarding short circuit ratio (SCR). Based on this, the conventional impedance-based stability operation point assessment approaches should be expanded into stability domain assessment approach considering the line inductance variation. Therefore, the stability-oriented line inductance stability domain assessment approach for weak grids with CPLs is proposed in this paper. Firstly, the source impedance matrix of weak grid, and load admittance matrix of CPLs are separately built. Secondly, an improved stability forbidden domain criterion is proposed through related mapping transformation process, which has lower conservatism than two previous improved stability criteria. Thirdly, the improved stability forbidden domain criterion is switched into the condition that the intermediate matrices are Hurwitz. Meanwhile, the line inductance stability domain is directly obtained through these intermediate matrices, and guardian map theory. Finally, the simulation, and experiment results illustrate that the proposed approach has less conservatism, and high efficiency.

Transient stability analysis and stability margin evaluation of phase‐locked loop synchronised converter‐based generators

As the penetration level of converter-based generators (CBGs) continues to grow in electric power systems, their impact on power system dynamics and control becomes increasingly significant. Maintaining a stable operation of the CBGs is becoming more and more critical. Since their controls and dynamic behaviour differ from synchronous generators, in this study, the transient stability mechanism of CBGs is analysed and the key influencing factors are identified and investigated. Moreover, a stability margin index, which can be obtained analytically, is proposed to identify potential transient stability issues of CBGs and can be used as a screening technique before conducting time-consuming electromagnetic simulation studies. This index can be used for both planning and operations studies and it outperforms the state-of-the-art index of short circuit ratio that is presently used to identify potential CBG instabilities due to weak grid scenarios. Dynamic simulations on a real power system model have been conducted to validate the accuracy and applicability of the proposed stability margin index.

Opportunities for Embedded High-Voltage Direct Current: Evaluating the Benefits for the Legacy ac Grid

Power system engineers and planners often consider high-voltage dc (HVdc) systems for bulk power transmission over long distances. There are two technologies within HVdc: line-commutated converters (LCC-HVdc) and voltage-source converters (VSC-HVdc). Each type has its own suitable range of applications. LCC-HVdc is a mature technology that has been available since the 1970s and uses thyristor valves. LCC-HVdc is commonly applied for high-capacity power transmission between two strong power systems (high short circuit ratio) over long distances with limited dynamic requirements (e.g., features such as fast power reversal, dynamic reactive power support, or black start are not available or are available only with limited performance). Currently installed systems reach ratings of 12 GW at a transmission voltage of ?1,100 kV and may cover distances beyond 3,000 km. The introduction of VSC-HVdc in the late 1990s initiated the use of self-commutated valves employing insulated-gate bipolar transistors with a significant improvement in dynamic performance. Presently, the ratings of VSC-HVdc a relower than those of LCC-HVdc. VSC-HVdc systems are now installed with a rating of 3,000 MW at a transmission voltage of ±525 kV, and higher ratings are under development. For special applications such as the connection of offshore wind power plants or connection to/between weak grids, VSC-HVdc often offers the only technoeconomically viable solution.

Evaluating Strength of Hybrid Multi-Infeed HVDC Systems for Planning Studies Using Hybrid Multi-Infeed Interactive Effective Short-Circuit Ratio

The strength is an important concept in planning studies for voltage stability analysis of hybrid multi-infeed HVDC (HMIDC) systems, which include both VSC- and LCC- HVDC links. However, the accuracy of earlier strength indices is very limited because they are developed by the empirical reasoning rather than rigorous theoretical derivation. In this paper, a virtual single-infeed model of HMIDC systems is firstly established utilizing the reduced-order equivalent nodal assessment of the inverter bus current-voltage equations. While incorporating the inter-inverter interactions, the virtual model is a strict dual-bus equivalent of the original complicated multi-infeed model. This enables easier and simpler analysis to be implemented. Secondly, the virtual model is employed to propose the hybrid multi-infeed interactive effective short-circuit ratio (HMIESCR) strength index for HMIDC systems by the rigorous theoretical derivation. Compared to earlier indices, the HMIESCR can depict the voltage stability much more accurately. This is because the critical HMIESCR (CHMIESCR) indicative of the stability limit is explicitly and solely evaluated to 1.5 according to the PV sensitivity based stability criterion, while the critical values of earlier indices are ambiguous. Finally, case studies by extensive numerical simulations on the classical hybrid four-infeed HVDC system validate the HMIESCR along with good robustness.

Analysis of stable operation region of the hybrid AC/DC distribution systems

Hybrid alternating current (AC)/direct current (DC) distribution is a promising technology emerging for future power systems owing to its advantages, such as high controllability and compatibility. The stability of grid-connected converters is essential for the operation of an AC/DC distribution system. This study analyses the stable operation requirements of converters under different control modes based on the short-circuit ratio (SCR). Then, a method to calculate the critical SCR (CSCR) of a specified converter is proposed. Based on the CSCR of each converter, the stability criterion to judge the stability of multi-terminal DC (MTDC) distribution networks is proposed. To verify the accuracy and effectiveness of the proposed method and criterion, detailed electromagnetic simulations using PSCAD are conducted. The simulations indicate that the analysis results obtained using the proposed method and criterion are accurate, and the presented stable operation requirements of the converters can help to enhance the controllability of hybrid AC/DC distribution systems.

Transmission Capability Assessment of Distributed Energy Resources Using Measurement-Based Site-Dependent Single-Port Model

In the recent advance, the site-dependent single-port model has been utilized to define a site-dependent short circuit ratio for the transmission capability assessment of distributed energy resources (DERs). However, miscalculations have been founded in the extensive simulations using the site-dependent short circuit ratio. Such miscalculation is due to the mismatch between the DER's injection real power sensitivity of the existing site-dependent single-port model and the one measured in the power grid. In this paper, the measurement-based site-dependent single-port model is proposed. The proposed model is constructed through modifying the equivalent parameters of the existing site-dependent single-port model by using a measurement-based alleviation multiplier. Accordingly, the mismatch in the injection real power sensitivity can be indeed eliminated in the proposed measurement-based site-dependent single-port model. The associated measurement-based site-dependent short circuit ratio will follow and be further defined for the transmission capability assessment of DERs. The simulations on IEEE 14-bus and IEEE 57-bus test systems are presented to validate the accuracy of the proposed measurement-based site-dependent single-port model.

Classification and dynamic assessment of droop-based grid-forming control schemes: Application in HVDC systems

This paper introduces a novel classification for droop-based control strategies towards emerging interest of utilizing grid-forming control schemes in high voltage direct current (HVDC) systems. Classical droop control for grid-forming operation does not conventionally depend on a phase-locked loop (PLL). It is demonstrated that by including a PLL in droop control, it is possible to define two other droop-based control variants with different capabilities and functionalities. To highlight the differences in performance of each control scheme, the active power response of the power converter is evaluated in case of variation in grid short circuit ratio (SCR) as well as the PLL response time by considering an ideal DC bus. Accuracy in active power regulation, frequency support capability and virtual inertia emulation are the functionalities that have been addressed for the proposed three different control schemes. In a next step, the introduced droop-based grid-forming schemes are assessed by considering the dynamics of both AC and DC side of the power converter at one terminal of an HVDC link. Finally, based on the simulation results, a summary of features for all introduced grid-forming control schemes is presented11This work is supported by the project “HVDC Inertia Provision” (HVDC Pro), financed by the ENERGIX program of the Research Council of Norway (RCN) with project number 268053/E2, and the industry partners; Statnett, Statoil, RTE and ELIA..

Coherence Analysis of System Characteristics and Control Parameters for Hybrid HVDC Transmission Systems Based on Small-Signal Modeling

Hybrid high-voltage direct current (HVdc) system, which comprises a line-commutated converter (LCC) at rectifier side and a voltage source converter (VSC) at inverter side, is drawing wide attention due to its high reliability and economic benefit. For such a new transmission technology, the system characteristics are not comprehensively studied, and inappropriate control parameters may be coherent to system instability. This article puts forward a novel coherence analysis procedure that analyzes system stability based on a verified hybrid HVdc small-signal model. For the first time, the procedure is proposed to capture the coherence relationships between the system characteristics and the control parameters. The analysis results reveal that the circuit and control parameters can highly affect the oscillatory modes and even cause instability. Moreover, the variation of grid short-circuit ratio (SCR) at LCC rectifier side trades off the response speed and damping of the oscillatory modes—under low SCR, large proportional and small integral coefficients of the LCC dc current control are suggested to enhance response performance. Also, the increase of VSC dc capacitance reduces the mode response speed—for a large capacitance, large proportional and small integral coefficients of the VSC dc voltage control shall be set to guarantee system stability.

A General Steady-State Voltage Stability Analysis for Hybrid Multi-Infeed HVDC Systems

As large-scale offshore wind farms are integrated into the power grid, a hybrid multi-infeed HVDC (HMIDC) system including both LCC-HVDC and VSC-HVDC links is facing a critical steady-state voltage stability problem which mainly stems from a weak AC system under high-level DC power injections. To overcome this challenge, this paper proposes a general steady-state voltage stability analysis to accurately measure the critical voltage stability point of HMIDC systems. In this paper, a typical HMIDC system is firstly modeled as a general dual-infeed system composed of an LCC-HVDC, a VSC-HVDC, and a virtual equivalent voltage source behind a virtual equivalent impedance at the AC side. Then a hybrid power sensitive factor (HPSF) is established to indicate the critical voltage stability point with the consideration of dynamic power-voltage characteristics of HVDC links, as well as operational changes in AC system (e.g. dynamics of excitation voltage control in generator and network topology change). Finally, a general voltage stability index, called hybrid generalized short circuit ratio (HGSCR), is further derived to effectively explain the critical voltage stability point of HMIDC systems. Moreover, multiple comparative studies with existing voltage stability indices are conducted in this paper to verify the effectiveness of the proposed general steady-state voltage stability analysis by using both PSCAD/EMTDC™ power flow and time-domain simulations.

Modeling and Stability Analysis of Weak-Grid Tied Multi-DFIGs in DC-Link Voltage Control Timescale

The DC-link voltage control (DVC) timescale (i.e., the frequency dynamics covering converter outer controls) instabilities in wind generation have gained increased attention recently. This paper presents DVC timescale modeling and stability analysis for multi doubly-fed induction generators (DFIGs) connected to weak AC grids. A reduced-order, small-signal model of a grid-tied multi-DFIG system, designed for DVC dynamics analysis, is firstly proposed. The model allows for the dynamic interactions among the DC-link voltage control, active power control (APC), terminal voltage control (TVC) and phase-locked loop (PLL). Eigenvalue and participation factor analyses are conducted to explore the potential instabilities and correlated critical factors for such a multi-machine system. The sensitivity studies find that instability can occur at high levels of power generations or low short-circuit ratio (SCR) conditions. In addition, the dominant mode is identified to be highly related to the PLL, and its modal damping is decreased when the bandwidths of PLLs in different generators are close. Detailed model-based time domain simulations verified the analysis above.

Enhanced performance controller for high power wind converters connected to weak grids

This study proposes a control scheme for high power grid-connected wind power converters, which is oriented to enhance their performance when connected to weak grids with low short circuit ratio. The proposed controller consists of an outer current reference generation loop and an inner current loop, working in stationary reference frame. In the outer loop, the current reference is calculated to comply simultaneously with the grid code requirements, the control of the DC link, and the operational safety margins of the converter during faulty conditions. On the other hand, the proposed inner current loop consists of a proportional resonant controller, a capacitor voltage feedforward and a phase shifter. Moreover, simulation results considering different weak grid conditions, as well as experimental results of a full-scale 4 MW converter test-bench are presented to validate the good performance of the proposed method.

Improved practical method for low‐inertia VSC‐HVDC stability analysis in weak system

The physical mechanism of the instability of the low-inertia voltage source converter (VSC) caused by the phase-locked loop (PLL) in a weak system is clearly revealed in a simplified way. Based on the complex torque method, the joint influence of the PLL and the system strength on the VSC is studied through the defined key stable component. To keep the VSC stable, the value of the key stable component must be positive. The component is decided by AC voltage, short-circuit-ratio (SCR), reactive power, and PLL parameters. Improper PLL parameters and weak AC system could make the key stable component negative. To keep the VSC stable in a weak system, the PLL parameters can be adjusted according to the curve of the key stable component, which means suitably decreasing k p and increasing k i. Finally, simulations are implemented to validate the theory, which matches well with the findings.

A Cascaded Converter Interfacing Long-Distance HVdc and Back-to-Back HVdc Systems

This paper proposes a cascaded converter dedicated to a long-distance high-voltage direct current (HVdc) infeed and asynchronous back-to-back interconnection of receiving grids. The cascaded converter consisted of modular multilevel converters (MMCs) in series and parallel connection, meeting the high dc voltage and power demand of the HVdc system. It realizes the hierarchical feeding and asynchronous interconnection of receiving grids, optimizing the multi-infeed short-circuit ratio (MISCR) and improving the flexibility of the receiving grids. The topology and operating characteristics of the cascaded converter are introduced in detail. The MISCR and the maximum power infeed of the cascaded converter-based HVdc systems are analyzed. Various feasible operating modes with online switching strategies of the cascaded converter are studied to improve the operational flexibility of the system. The simulation results verify the effectiveness of the control strategy of the HVdc system embedding the cascaded converter. The dc faults clearing strategy and operating modes switching strategies are also validated.

Online Diagnosis of Slight Interturn Short-Circuit Fault for a Low-Speed Permanent Magnet Synchronous Motor

Online diagnosis of slight interturn short-circuit fault (ISCF) has always been difficult in low-speed permanent magnet synchronous motors (PMSMs). It is found that the residual zero-sequence voltage exists in healthy PMSMs due to the inherent unbalance of a low-speed motor. This article proposes a new ISCF diagnosis method based on zero-sequence voltage vector decomposition to solve this problem. The voltage vector whose phase angle is closest to the ISCF phase angle is extracted from the zero-sequence voltage vector, which improves the sensitivity of fault phase identification. Furthermore, the method of extracting phase voltage from PWM duty-cycle control voltage is proposed, which can reduce the error caused by measuring zero-sequence voltage with three-phase balanced resistance. Experiments demonstrate that the online detection limit of ISCF is reduced from 5% to 1% (short-circuit ratio) for a low-speed PMSM.

Generic characterization of electrical test benches for AC- and HVDC-connected wind power plants

Abstract. The electrical test and assessment of wind turbines go hand in hand with standards and network connection requirements. In this paper, the generic structure of advanced electrical test benches, including grid emulator or controllable grid interface, wind torque emulator, and device under test, is proposed to harmonize state-of-the-art test sites. On the other hand, modern wind turbines are under development towards new features, concerning grid-forming, black-start, and frequency support capabilities as well as harmonic stability and control interaction considerations, to secure the robustness and stability of renewable-energy-based power systems. Therefore, it is necessary to develop new and revised test standards and methodologies to address the new features of wind turbines. This paper proposes a generic test structure within two main groups, including open-loop and closed-loop tests. The open-loop tests include the IEC 61400-21-1 standard tests as well as the additional proposed test options for the new capabilities of wind turbines, which replicate grid connection compliance tests using open-loop references for the grid emulator. In addition, the closed-loop tests evaluate the device under test as part of a virtual wind power plant and perform real-time simulations considering the grid dynamics. The closed-loop tests concern grid connection topologies consisting of AC and HVDC, as well as different electrical characteristics, including impedance, short-circuit ratio, inertia, and background harmonics. The proposed tests can be implemented using available advanced test benches by adjusting their control systems. The characteristics of a real power system can be emulated by a grid emulator coupled with real-time digital simulator systems through a high-bandwidth power-hardware-in-the-loop interface.