Degree Of Series Compensation Research Articles

Research on the Torsional Vibration Response Characteristics of Parallel Operation Steam Turbine Generator Unit Shafting

With the continuous increase of unit capacity and the widespread application of series compensation devices in power transmission systems, the risk of coupling oscillation between units and power grids and between units has increased significantly, which seriously endangers the safe operation of units and power grids. In this paper, the torsional vibration response of each unit after grid failure is calculated by establishing a lumped mass model of the shafting torsional vibration of the parallel unit. The effects of fault type, series compensation degree and fault location on the torsional vibration of parallel unit shafting are studied. The results show that the three-phase short-circuit fault in the grid has a greater impact on the unit; the influence of series compensation degree on the torsional vibration of the shaft system decreases first and then increases; when the transmission line fails, it will produce a certain torsional vibration excitation effect on the shafting of all units; when a short-circuit fault occurs at the outlet of a generator, the torsional vibration excitation of the unit is the largest, but it will also produce a certain degree of torsional vibration excitation to other units in the same network.

Sub-Synchronous Oscillation Suppression Strategy Based on Impedance Modeling by Attaching Virtual Resistance Controllers for Doubly-Fed Induction Generator

A sub-synchronous oscillation (SSO) suppression strategy of attaching virtual resistance controllers to the rotor-side converter (RSC) of the doubly-fed induction generator (DFIG) is proposed in this study to suppress sub-synchronous oscillation (SSO) caused by series compensation and grid connection of DFIG. A DFIG-based frequency domain impedance model considering RSC control under small signal perturbations is developed in a three-phase stationary coordinate system. Subsequently, the factors and mechanisms of SSO in the system with different phase sequences are analyzed in combination with the equivalent RLC resonant circuit of a DFIG-based series-compensated grid-connected system (SCGCS). SSO occurs when RSC and rotor winding generate a large equivalent negative resistance at the SSO frequency, resulting in a negative total system resistance. Additionally, the influences of series compensation degree (SCD) of line and inner loop parameters (ILPs) of RSC related to the total impedance of the system on the SSO characteristics are analyzed to optimize the parameters and improve the system stability. Based on the causes of SSO, virtual resistance controllers are attached to RSC to provide positive resistance to the system and to offset the equivalent negative resistance of RSC and rotor winding at the SSO frequency, thereby avoiding SSO of the system. Finally, time-domain simulations using power system computer aided design/electromagnetic transients including dc (PSCAD/EMTDC) show that the SSO of the system is effectively suppressed.

Adaptive Damping Control of Subsynchronous Oscillation in DFIG-Based Wind Farms Connected to Series-Compensated Network

The frequency of subsynchronous oscillation (SSO) in doubly-fed induction generator (DFIG)-based wind farms connected to series-compensated networks is determined by system-wide operating conditions, including the wind speed, number of in-service wind turbines, and degree of series-compensation. This paper proposes an adaptive mitigation scheme to damp the SSO for a full range of subsynchronous frequency variations due to different operating conditions. The proposed scheme comprises 1) a subsynchronous frequency estimator (SSFE) and 2) an adaptive subsynchronous damping controller (ASDC). The SSFE detects the SSO mode and tracks its subsynchronous frequency accurately. The ASDC utilizes voltage signals to extract the SSO at the estimated subsynchronous frequency, generates appropriate current signals, and injects them into the grid through a shunt-voltage sourced converter (SVSC). The ASDC behaves like a variable impedance, which essentially reshapes the system's impedance response by adding a positive resistance at the estimated subsynchronous frequency. The parameters of the phase-shifters are computed using Brent's root-finding method such that the desired phase-shift at the estimated subsynchronous frequency is always maintained. The control's adaptiveness and damping performance under different operating scenarios is verified through extensive EMT simulations on a real-world wind power system facing SSO using PSCAD.

Research on Mechanism and Damping Control Strategy of DFIG-Based Wind Farm Grid-Connected System SSR Based on the Complex Torque Method

The subsynchronous resonance (SSR) of a double-fed induction generator (DFIG) and its suppression method are studied in this paper. The SSR may be aroused by the interaction between the double-fed induction generator and the series-compensated transmission lines. This paper proposes an expression of the electrical damping for assessing the SSR stability based on the complex torque method. The expression is derived by linearizing the DFIG model at the operating point. When the mechanical damping is neglected, the expression can be used to calculate whether the electrical damping is positive or negative to judge the SSR stability. The expression can quantitatively analyze the impact of the wind speed, the compensation degree, and the parameters of the rotor speed controller and the rotor-side converter controller on the SSR stability. Furthermore, a subsynchronous damping control (SDC) strategy is designed to suppress the SSR. The parameters of the SDC are optimized by particle swarm optimization (PSO) based on the electrical damping. Finally, the above research is verified by the PSCAD/EMTDC time-domain simulations. The results show that the stability of SSR decreases with the decrease of wind speed, the increase of series compensation degree, the increase of proportional coefficient, and the decrease of integral coefficient in rotor speed controller and rotor-side converter. The designed subsynchronous oscillation controller can suppress the SSR of a DFIG.

Study on the characteristics of secondary arc current of UHV high compensation degree TCSC line under the fine-tuning mode

Abstract In the transmission line, the series compensation device is often used to improve the transmission capacity. However, when the fixed series capacitor (FSC) is used in high compensation series compensation device, the stability margin cannot meet the requirements. Therefore, thyristor controlled series compensator (TCSC) is often installed in transmission lines to improve the transmission capacity of the line and the stability of the system. For cost considerations, the hybrid compensation mode of FSC and TCSC is often adopted. However, when a single-phase grounding fault occurs in a transmission line with increased series compensation degree, the unreasonable distribution of FSC and TCSC will lead to the excessive amplitude of secondary arc current, which is not conducive to rapid arc extinguishing. To solve this problem, this paper is based on 1000 kV Changzhi-Nanyang-Jingmen UHV series compensation transmission system, using PSCAD simulation program to established UHV series compensation simulation model, The variation law of secondary arc current and recovery voltage during operation in fine tuning mode after adding TCSC to UHV transmission line is analyzed, and the effect of increasing series compensation degree on secondary arc current and recovery voltage characteristics is studied. And analyze the secondary arc current and recovery voltage when using different FSC and TCSC series compensation degree schemes, and get the most reasonable series compensation configuration scheme. The results show that TCSC compensation is more beneficial to arc extinguishing under the same series compensation. Compared with several series compensation schemes, it is found that with the increase of the proportion of TCSC, the amplitude of secondary arc current and recovery voltage vary greatly. Considering various factors, the scheme that is more conducive to accelerating arc extinguishing is chosen.

Study and Solution of Current Differential Protection Disability for High Series Compensation Transmission Line

With the increasing day by day integration of large-size generators and tighter among the power systems, in internal fault, the fault current of Series Compensated (SC) lines could also be upturned. Therefore, the differential protection of SC transmission lines may stop working to operate because of less sensitivity. In this research article the causes for the current overturning in SC lines and its effects differential protection. The working characteristics of differential protection are analyzed by focusing on multiple-factors, for example, different series compensation degrees, types of fault, system operation mode, the difference of power angle, points of the fault, and transition resistance. An enhanced standard based on the current amplitudes and phases on both sides of the series compensated lines is proposed. Furthermore, to make stronger the operation characteristics of differential protection, the triple-fold line is chosen to get better sensitivity for internal faults in the article. The results are efficient to improve the protection sensitivity of SC lines during the internal fault. The PSCAD/EMTDC simulation and the power system dynamic physics simulation demonstrate that protection sensitivity is increased in the improved scheme. Keywords : Differential Protection, Series compensation, EHV/ UHV, Current reverse, sensitivity analysis. DOI: 10.7176/CTI/10-06 Publication date: August 31 st 2020

Influence of Series Compensation Degrees on Secondary Arc Current Characteristics under the Different Series Compensation Mode

AbstractThe high compensation degree series compensation has a broad development prospects in long distance and large-capacity UHV transmission lines. But it is generating secondary arc current contains low frequency components with high amplitude when the single-phase grounding fault occurs in the series compensated transmission line, which is not conducive to rapid arc extinguishing. To solve this problem, this paper based on the mechanism of generating secondary arc by series compensation transmission lines, and using ATP – EMTP electromagnetic transient program to establish the simulation model of UHV series compensation device. And the suppression effects of small resistance short connection fault series compensation and linkage bypass series compensation measures on secondary arc current and recovery voltage under different series compensation layout mode are studied respectively. The results show that the attenuation speed of the secondary arc current is related to the series compensation arrangement modes and the time when the series compensation device is short connected or bypassed. Series compensation device short connected or bypassed before circuit breaker tripping which can accelerate the arc extinction speed, and the suppression effect is better when the series compensated double platform segmentation layout is distributed on both sides of the line. The results can be used as a reference for the engineering design of UHV with large capacity and long distance and high compensation series compensation transmission lines.

Stability Assessment of DFIG Subsynchronous Oscillation Based on Energy Dissipation Intensity Analysis

Concerning the subsynchronous oscillation caused by the integration of doubly-fed induction generator (DFIG) to the power grid via the series compensation circuit, a method to assess the stability of subsynchronous oscillation based on the dissipation intensity is proposed in this article. First, the transient energy that DFIG generates in one oscillation period is defined as the dynamic energy of DFIG, and the model of DFIG dynamic energy containing internal control and external networks is constructed. And then, according to the Lyapunov stability theory, the negative gradient of dynamic energy is defined as the dissipation intensity, which reflects the dissipation effect of DFIG on the energy generated during oscillation. On this basis, the effect of the series compensation degree, phase-locked loop, and converter control parameters on subsynchronous oscillation is assessed, and real-time assessment of the stability level of DFIG-integrated power system is realized. Finally, a simulation model is built in RT-LAB to verify the proposed method. Simulation results demonstrate that the stability level of the system can be assessed accurately according to the value of dissipation intensity. When the value of dissipation intensity is positive, system oscillation converges. When the dissipation intensity is zero, the system oscillates with constant-amplitude oscillation. When the value of dissipation intensity is negative, system oscillation diverges.

The Analysis and Solution of Current Differential Protection Maloperation for Transmission Line with High Series Compensation Degree

With the increasing integration of huge-size generators and tighter interconnection among power grids, in internal fault, the fault current of Series Compensated (SC) lines may be reversed. At this time, the differential protection of SC lines may fail to operate due to insufficient sensitivity. In this research analyzes the reasons for the current reverse in SC lines and its effect on differential protection. The operation characteristics of differential protection are studied by focusing on multiple factors, including different series compensation degree, fault types, system operation mode, power angle difference, fault points, and transition resistance. An improved criterion based on the current amplitudes and phases on both sides of the series compensated lines is proposed. Further, to strengthen the operation characteristics of differential protection, the triple-fold line is chosen to improve the sensitivity for internal faults in this paper. The criterion is effective to improve the protection sensitivity of SC lines during internal fault. The PSCAD/EMTDC simulation and the power system dynamic physics simulation demonstrates that protection sensitivity is obviously increased in the improved scheme.

A hybrid method for fault location estimation in a fixed series compensated lines

This paper proposes a fault distance estimation scheme for fixed series capacitor compensated parallel transmission lines using discrete wavelet transform and decision tree regression. The purpose of the data mining based scheme is to avoid the complicated equation based methods that have been suggested by researchers to overcome the drawbacks of conventional fault location scheme. Although decision tree has inherent advantage over other methods like artificial neural network and support vector machines to work with large data sets, it has not been used in fault location estimation in series compensated (SC) transmission line so far. Decision tree is chosen to locate the faults because of its ability to work with large data set and high accuracy in associating the fault pattern to the fault distance using regression analysis. The discrete wavelet transform processed signals makes the decision process of decision tree regression easy by providing appropriate features. The proposed method is evaluated with variation of fault location, fault type, pre-fault load angle, location of series capacitor, degree of series compensation, fault inception angle, line parameters, inter-circuit faults and fault resistance. The test result of decision tree regression based location estimation scheme ensures that, it can estimate the fault distance accurately.

An artificial neural network-based solution to locate the multilocation faults in double circuit series capacitor compensated transmission lines

During thunder storms, multilocation faults may occur at different locations in different phases of the 3 phase transmission lines at same or different time. This paper proposes an artificial neural network-based solution to locate the multilocation faults in double-circuit series capacitor compensated transmission lines (SCCTLs), unlike previous works that only locate the fault at single location. Although various fault location schemes have been proposed for normal shunt faults occurring at 1 location in SCCTL, nevertheless, finding the locations of multilocation faults in double circuit SCCTL has not been addressed so far. The proposed artificial neural network-based method determines the location of multilocation faults by using current and voltage signals of 1 end of line only, thus avoiding the need of communication link. The signals are preprocessed by using discrete wavelet transform. A comparative study of various neural networks such as feed-forward back-propagation network with Levenberg-Marquardt algorithm, Elman recurrent neural network with gradient descent algorithm, radial basis function neural network has been carried out. The proposed method is not affected by variation in different parameters viz. fault type, fault location, fault inception angle, fault resistance, degree of series compensation, and location of series capacitor. The key advantage of the method is that it correctly estimates the locations of multilocation faults as well as single-location fault, thus making it more reliable and accurate as compared to conventional fault location schemes.

Research on the sub-synchronous oscillation in wind power connected to series compensated power system and its influencing factors

When wind farms, which is based on double fed induction generator (DFIG), are connected to the series compensation power system, the phenomenon of sub-synchronous oscillation (SSO) may occur. In order to study the sub-synchronous oscillation in wind power connected to series compensated power system and its influencing factors, we used RT-LAB to establish a simulation model concerning wind power connected to series-compensation power system. This model take a wind power connected to series compensation power system in north China as prototype. All influence factors of wind power SSO are simulated and analyzed by using time domain analysis method. The simulation results show that the effects of wind speed, series compensation degree and proportional control coefficient of rotor side converter (RSC) are most obvious.

Small-signal Modeling and Modal Analysis of a PMSG-based Wind Power System

A unified small-signal model of a wind power system is developed in this paper to study the small-signal stability of permanent magnet synchronous generator (PMSG)-based wind turbines connected to the power grid. The model is composed of both mechanical system and electrical system. A two-mass shaft model for the mechanical system is provided to analyze the dynamic and steady state behaviors. Meanwhile, PMSG, converter system and transmission line of the electrical system are modeled separately to build a unified small-signal model of the PMSG-based wind power system. Then, based on the unified small-signal model, eigenvalue analysis for the whole system is presented, which helps discover the relation between different modes and state variables. Cases involving various series compensation degrees, length of transmission lines and the stiffness of shafts are carried out to study how the modes are influenced by relevant variables specifically. The relatively independent modules of the unified small-signal model allow for easy expansion and modification for various research purposes. The simulation results show that the mechanical and electrical systems of PMSG-based wind turbines are independent to some degree with the implementation of a full-scale converter system.

Optimal Location of Facts Device for Improved Power Transfer Capability and System Stability

The work entailed in this paper is to develop a model for the optimal location of shunt Flexible Alternating Current Transmission System (FACTS) along a transmission line so as to enhance controllability and increase power transfer capability of the transmission network. Mathematical models for maximum power transfer and transmission angles for transmission line were developed. The investigation was done for both lossless and actual transmission lines. MATLAB software was used for the simulation of the models. Aloaji – Itu transmission 132 KV transmission line in South-eastern Nigeria was used as a case study. Performance analysis was conducted on the various maximum power and transmission angles data for different degree of series compensation and FACTS locations along the transmission lines to determine the optimal location of the FACTS device for both lossless and actual transmission lines. The results obtained showed that the optimal location of the shunt FACTS device is not fixed, but changes with the change in degree of series compensation. Both the power transfer capability and stability of the system can be improved much more if the shunt FACTS device is placed at the new optimal location instead of the mid-point of the line.

Investigating SSO risks of large‐scale photovoltaic generation with the series compensated transmission lines

With the increasing of photovoltaic (PV) installed capacity, large-scaled development of PV and centralized grid connection has become an important form of solar energy resources utilization. The series compensated transmission lines could enhance the long-distance transmission capacity of large-scale PV plants. Some PV plants in China are planned to adopt this technology. However, series capacitor compensation transmission lines are one of important sources of sub-synchronous oscillation (SSO). This SSO risk is a potential threat for the operation of PV plants and the stability of the utility grid. It is urgent to investigate the SSO risk of large-scale PV generation with the series compensated transmission lines. This study first developed a dynamic model of large-scale PV generation connected to the utility grid with series compensated lines on PSCAD platform. The PV dynamic model is built with detailed circuits of the power converter. Using the proposed model, the interaction of PV inverter control parameters, reactive power compensation device and the series compensation degree is investigated in detail. Then, the dominant parameters that affect the SSO are identified. From the results above, the SSO damping expression is deduced to reveal the SSO performance of large-scale PV generation with series compensated transmission lines.

Impact of series compensation on operation performance of large‐scale PV plants

Application of series compensation transmission lines to enhance the long-distance transmission capability of photovoltaic (PV) plants is the most economical technical measure. To investigate the impact of series compensation on operating performance of large-scale PV plants, the simulation research is performed in this study. The detailed model of large-scale PV plants connected to the grid through series compensated lines is developed using power systems computer aided design (PSCAD)/electromagnetic transients inccluding DC (EMTDC) software. Using the detailed model, the performances of active and reactive power outputs, voltage and current profiles of PV plants are investigated for different levels of series compensation and strengths of AC system during short-circuit faults. The simulation results show that series compensation and strong AC system can effectively improve the transient operation performance of the PV power plant. With the increase of the series compensation degree and the strength of AC system, the fault ride-through performance of PV plants is improved.

Bifurcation Analysis of a Series Compensated System with STATCOM-ES

Abstract The energy storage devices incorporated into FACTS controllers increase the operational flexibility and reduce capital cost. This paper investigates the effect of Static Synchronous Compensator with energy storage (STATCOM-ES) on dynamic bifurcations of Subsynchronous resonance (SSR). The test system considered is IEEE first benchmark model (FBM). The line is provided with fixed series compensation and active shunt compensation at the electrical center of the line. The degree of series compensation is taken as the bifurcation parameter. The results of bifurcation analysis are validated by performing transient simulation of the system.

Study of Sub-Synchronous Interaction in Doubly-Fed Wind Power Systems

When doubly fed wind power generation system transmits power to power grid through the series compensation, once the external disturbances is close to the natural sonant frequency, there is a trouble of synchronous oscillation between the rotor side and the series capacitor. The paper analyzes the condition of resonance and its mechanism. According the actual argument provide by north china wind power plant, we use PSCAD to build a 1.25MW doubly fed wind power generation model to analyze the synchronous oscillation. We adopt time-domain simulation method to analyze that as the change of series compensation degree the system subsynchronous resonance frequency changes as well as the influence to doubly fed fan current and power. The simulation result is basically the same as the fault wave record monitoring data result of actual site when wind power plant take off the network .Consequently to verify the validity of the simulation platform. To verify the improvement of the converter control strategy to build simulation platform.

English

Static VAR Compensator (SVC) and Static Synchronous Compensator (STATCOM) are important devices in FACTS family, and is widely recognized as an effective and economical means to solve the power system stability problem. SVC and STATCOM are used as shunt in transmission system. In the present work a series compensated Distributed transmission line (345KV, 450km, 50Hz) with a shunt FACTS devices SVC and STATCOM is considered to the optimal location of the shunt FACTS devices to get the highest possible benefits of maximum power transfer and system stability. Effect of change in degree of series compensation on the optimal location of the shunt FACTS device to get the highest possible benefit is studied. It is found that the optimal location of the shunt FACTS device varies with the change in the level of series compensation to get the maximum benefit in terms of power transfer capability and stability of the system. All the simulations for the above work have been carried out using MATLAB /SIMULINK software.

Study of Sub-Synchronous Control Interaction (SSCI) Based on the Test Signal Method

Large-scale wind turbine generators with power electronic converters that operate near series compensated transmission lines are susceptible to un-damped sub-synchronous oscillations. This sub-synchronous oscillation is called Sub-synchronous Control Interaction (SSCI). Based on PSCAD / EMTDC simulation platform the establishment of Double Fed Induction Generator (DFIG) equivalent inverter controlled sources and network model, and verify the correctness of the equivalent model. Analysis the impact of series compensation and rotor-side converter PI parameters characteristic of the SSCI based on test signal method and time-domain simulation method. The results show that increasing the degree of series compensation , inner gain constant increases and inner integral time decreases will help increase SSCI, outer ring of PI parameters has little effect on the SSCI .