Series Compensated Transmission Lines Research Articles

A new least squares error based method for accurate phasor estimation of fault currents in series-compensated transmission lines

Phasor estimation of the fundamental component of the fault current is of great importance for the fault location unit in distance relays. In series-compensated transmission lines, the transient component of the fault current may appear in the complicated wave shape of an exponential-sinusoidal function. Thus, phasor estimation is a major challenge owing to the fact that more unknown parameters should be found, especially during off-nominal frequency conditions. Therefore, in this paper, a new method is put forward, on the strength of the least squares error (LSE), in order to accurately estimate the fundamental component phasor of the fault current in a series-compensated transmission line for off-nominal frequency conditions. The performance of the established method is also investigated through extensive simulation studies, taking care of different fundamental components, harmonic contents, and transient components. Moreover, the performance of the proposed method is evaluated for off-nominal frequencies. The results obtained in this study demonstrate that the mean absolute error of the estimated amplitude and phase for the fundamental component is less than 1% and 1°, respectively. Furthermore, the accuracy of the proposed method is not affected by the noise up to a signal-to-noise ratio of 30 dB. Thus, the established method is capable to effectively be employed for the phasor estimation in fault location applications.

The Analysis of Sub-Synchronous Resonance in a Wind Farm for a Doubly-Fed Induction Generator Using Modern Analytical Method

The occurrence of Sub-Synchronous Resonance (SSR) phenomena can be attributed to the interaction that takes place between wind turbine generators and series-compensated transmission lines. The Doubly-Fed Induction Generator (DFIG) is widely recognized as a prevalent generator form employed in wind energy conversion systems. The present paper commences with an extensive exposition on modal analysis techniques employed in a series of compensated wind farms featuring Doubly Fed Induction Generators (DFIGs). The system model encompasses various components, including the aerodynamics of a wind turbine, an induction generator characterized by a sixth-order model, a second-order two-mass shaft system, a series compensated transmission line described by a fourth-order model, controllers for the Rotor-Side Converter (RSC) and the Grid-Side Converter (GSC) represented by an eighth-order model, and a first-order DC-link model. The technique of eigenvalue-based SSR analysis is extensively utilized in various academic and research domains. The eigenvalue technique depends on the initial conditions of state variables to yield an accurate outcome. The non-iterative approach, previously employed for the computation of initial values of the state variables, has exhibited issues with convergence, lack of accuracy, and excessive computational time. The comparative study evaluates the time-domain simulation outcomes under different wind speeds and compensation levels, along side the eigenvalue analysis conducted using both the suggested and non-iterative methods. This comparative analysis is conducted to illustrate the proposed approach efficacy and precision. The results indicate that the eigenvalue analysis conducted using the proposed technique exhibits more accuracy, as it aligns with the findings of the simulations across all of the investigated instances. The process of validation is executed with the MATLAB program. Within the context of the investigation, it has been found that increasing compensation levels while simultaneously decreasing wind speed leads to system instability. Therefore, modifying the compensation level by the current wind speed is advisable.

Unstable Subsynchronous Interaction Induced by Blocking Filters in a Wind-Thermal Combined Power System

Blocking filters (BFs) are applied to reduce the risk of shaft related subsynchronous resonance (SSR) in thermal power plants connected to series compensated transmission lines. However, when doubly-fed induction generator (DFIG)-based wind farms are connected in parallel with the existing thermal power plants, the same BFs may cause unstable subsynchronous interaction (SSI) with the DFIGs. This paper investigates the potential risk of this new type of SSI caused by BFs, which are originally designed to reduce the SSR risk. First, a representative system model of thermal power units with BFs and DFIGs integrated into grid is established. Then, impedance-based analyses and time-domain simulations are carried out to analyze the mechanism and characteristics of the SSI. The results indicate that the interaction between BFs and DFIGs may induce unstable oscillation in the system. In particular cases where series compensation in the transmission line is bypassed, the risk of SSI instability still exists. With series compensation operating normally, the interaction among BFs, DFIGs, and series compensated lines becomes even more complex. The paper also investigates the effects of system parameters such as the number of online DFIGs, wind speed, and series compensation level on the SSI characteristics.

A modified mutual impedance based backup protection for series-compensated transmission lines

A modified mutual impedance based backup protection for series-compensated transmission lines

An enhanced sensitive power differential protection for series compensated transmission lines in renewable generation intensive power systems

Series compensation is widely used to improve the electrical characteristics of long transmission lines and increase the power transfer capability. However, the insertion of a series capacitor introduces various phenomena that affect the performance of traditional distance relays. Moreover, the fault characteristics of renewable generation systems coupled with generation outputs that vary with weather patterns produce additional complications to the protection of transmission lines. This paper proposes the application of an enhanced sensitive power differential protection (SPDP) algorithm to the protection of series compensated transmission lines. The algorithm incorporates a criterion that improves the security of the protection by inhibiting the SPDP algorithm from operating for faults external to the protected line. Performance of the enhanced SPDP algorithm has been evaluated on a test system built in Digsilent Powerfactory software. Various in-zone and out-of-zone fault cases were simulated at different compensation levels and fault resistances. The results showed that the enhanced SPDP protection technique can detect in-zone faults along practically the entire transmission line (99.99 %) for compensation levels up to 90 %. The method operated reliably for faults with resistances up to 330 Ω.

Improving protection of compensated transmission line using IoT enabled adaptive auto reclosing scheme

Abstract Protection of critical components in the power sector especially the transmission line system is very important to minimize loss of revenue, resources, and productivity. Improvement of power transfer capacity and voltage profile in smart the power grid is achieved by Series Compensation (SC). However, it poses many serious issues in the protection of compensated transmission lines. The majority of faults occurring in transmission lines are transient in nature for which service continuity can be regained by an auto-reclosing scheme. An efficient auto-reclosing scheme that can provide adaptive protection with reliable operation is required to prevent transient and permanent faults in the series compensated transmission line (SCTL). This work highlights the emulation of an Internet of Things (IoT) aided adaptive auto-reclosing scheme in association with an ESP32 controller that provides adaptive dead time control. The reclosing instance has been identified by means of a synchro-check relay. A real-time IoT-aided auto-reclosing scheme has been realized using the ESP32 controller that can generate a trip/reclosing signal instantaneously using the concepts of edge computing. The projected system has been also corroborated by simulating the modeled smart power grid network with wide variations in fault context in PSCAD. The validation of the set of rules is done in MATLAB software for the proposed auto-reclosing scheme. The extracted test outcomes of the simulation and its laboratory hardware implementation indicate an appreciable reduction in dead time which reflects the efficacy of the developed protection scheme.

Parallel Impulse Current Shunt Performance of Multiple MOVs for 500 kV Series Compensation

As an important component of the transmission line series compensation device, MOVs have the strong nonlinear characteristics to limit the overvoltage caused by line fault and protect the capacitor banks in 500kV lines. We studied the parallel impulse current shunt performance of multiple MOVs and the influence of the mixed-use of new MOVs with served MOVs for different numbers of MOVs on impulse shunt performance. We conducted 500 kV MOV parallel impulse shunt tests under three conditions of 9-served+2-new, 11-served and 9-served. We found that when MOVs with different V-I characteristics are connected in parallel, the impulse current action characteristics of a single MOV will not be affected. When the number of MOVs in parallel is changed from 11 to 9, the dispersion of impulse current does not change significantly. When two served MOVs are replaced with two new MOVs (with a 1% increase in DC U <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2mA</sub> ), the average dispersion of impulse current increases by 2.81%, and the average maximum deviation increases by 5.93%. If the margin is sufficient when MOVs in the whole group are damaged, they can be replaced with matched new MOVs.

Traveling wave-based fault locators: Performance analysis in series-compensated transmission lines

In this paper, the performances of one-ended and two-ended traveling wave (TW)-based fault location techniques in series compensated transmission lines (SCTLs) are thoroughly investigated, highlighting the impact of the series capacitors (SCs) and their associated overvoltage protection on such routines. To do so, a diversity of fault simulations were carried out on a realistic Alternative Transients Program (ATP) power system model of a 500 kV/60 Hz double-circuit SCTL located in the north region of Brazil. From the carried out studies, it is demonstrated that the two-ended algorithm is not impacted when the SCs are installed at the line ends, but the single-ended routines based on correlation functions are more affected by the SCs banks, even though some promising results may be obtained depending on their adjustments.

A Generalized Harmonic Compensation Control Strategy for Mitigating Subsynchronous Oscillation in Synchronverter Based Wind Farm Connected to Series Compensated Transmission Line

With the increasing application of synchronverter technology in renewable energy power generation, power system stability issue becomes more complicated. Due to the impedance interaction between the synchronverter based wind farm and the series compensated transmission line, the synchronverter based wind farm is prone to cause sub-synchronous oscillation (SSO). To suppress this SSO, a generalized harmonic compensation control strategy of active power filter (APF) with supercapacitor is proposed, which counteracts SSO by injecting the generalized harmonic current including reactive and oscillation current. Since the generalized harmonic current can be calculated by subtracting the sinusoidal active current from the sampling current, a first-order inertial controller of the voltage loop for APF is proposed to accurately obtain the active current. In addition, the supercapacitor of APF is used to reduce the dc-side voltage fluctuation caused by SSO. As this paper does not need to directly extract the SSO component, the proposed method does not need to know the system oscillation frequency in advance, which can be used as a defensive control method of SSO. Finally, experimental results verify the effectiveness of the proposed strategy.

An intelligent protection scheme for series-compensated transmission lines connecting large-scale wind farms

Series-compensated transmission lines (SCTLs) are increasingly preferred for transmitting bulk amounts of electricity generated from the present-day large-scale wind farm to the utility grid due to several technical and economic benefits. However, when a fault occurs in such a wind farm-integrated SCTL, the impedance across the metal oxide varistor (MOV)-protected series capacitor varies non-linearly. Also, the fault current contributed from the wind farm side is quite different compared to the grid side. Consequently, the widely used fixed impedance-based distance relaying schemes showed limitations when used for protecting such crucial TLs. In this paper, the impacts of series compensation and wind farm integration on distance relay are investigated, and this paper proposes an intelligent relaying scheme using only the current measurements. In the proposed scheme, the fault detection task is performed using the signs of the half-cycle magnitude differences of the line end positive-sequence currents, and the fault classification task is performed using only the local current measurements processed through the Fourier–Bessel series expansion (FBSE) bagging ensemble (BE) classifier. The non-stationary components present in the current signal at the initiation of a fault are captured by calculating FBSE coefficients, and the singular value decomposition is applied for dimensionality reduction of the feature set. Finally, the extracted features are used by the BE classifier for fault classification. The method is evaluated in MATLAB/Simulink® on numerous fault and non-fault data simulated in two-bus systems and also validated through the OPAL-RT (OP4510) manufactured real-time digital simulation platform. The obtained results (response time for fault detection and classification &lt;10 ms), including the comparative assessment results (fault detection accuracy =100% and fault classification accuracy =99.37%), justify the effectiveness of the proposed relaying scheme in protecting the wind farm-integrated SCTLs.

Analysis and Damping of Sub-Synchronous Oscillations in a 250 MW DFIM Hydro Unit Connected to Series Compensated 765 kV Transmission Lines

Increasing energy consumption has required a greater need for connections between the grid and nonconventional energy sources. Recently, doubly fed induction machine (DFIM) fed hydro power units are increasingly connected to the power system due to the requirement of large energy storage option and cost-effective converter solution in the variable speed units. On the other hand, stability related issues are raised in the power system due to the sub synchronous control interaction (SSCI) that occurs between radially connected DFIM, and series compensated transmission line. This article focuses on analysis of resonance stability in a doubly fed induction machine (DFIM) based hydropower system. The controller of the rotor side converter is tuned using a dampness optimization approach. Additionally, optimized SSCI damping controller in the grid side converter is designed to attenuate the sub-synchronous oscillation (SSO) of the DFIM. Time domain simulation analysis is conducted for this research to validate the SSCI mitigation in the DFIM-based hydro units. In MATLAB/Simulink environment, the efficacy of the proposed system with 250 MW DFIM and a series compensated 765 kV extra high voltage transmission line is tested. To scale down the model and check SSO in the 2.2 kW DFIM system, a practical demonstration is carried out in the Lab.

Management Relay for Assisting Distance Relays in Series Compensated Transmission Lines

Management Relay for Assisting Distance Relays in Series Compensated Transmission Lines

Evaluation of Inherent Damping Introduced by Thyristor Controlled Series Compensators

Series compensation of transmission lines increase the risk of Sub-Synchronous Resonance (SSR) problems. Use of Thyristor Controlled Series Compensators (TCSC) in place of Fixed Series Capacitors (FSC) has benefited in terms of damping SSR. SSR Damping with TCSC can be achieved with supplementary controls or through its inherent damping capability. Use of discrete time models for small signal stability analysis of TCSC is well established. This paper uses a simplified Dynamic Phasor (DP) model of the TCSC for an in-depth evaluation of the inherent SSR damping capability through small signal stability analysis. Source of inherent damping in open loop operation and the representation of it in the DP model is elaborated and the small signal model is validated against Electro Magnetic Transient (EMT) simulations. Effect of inductive-resistive nature of the TCSC on Induction Generator Effect (IGE) and Torsional Interactions (TI) is shown through small signal stability assessment. It is shown that the network resonant frequency exists only in a narrow range of firing angles of the TCSC and thus IGE and TI are avoided in most part of its operating range. IEEE first benchmark system for SSR studies is used to demonstrate the inherent damping capability of a TCSC.

ANFIS-Based Fault Distance Locator With Active Power Differential-Based Faulty Line Identification Algorithm for Shunt and Series Compensated Transmission Line Using WAMS

ANFIS-Based Fault Distance Locator With Active Power Differential-Based Faulty Line Identification Algorithm for Shunt and Series Compensated Transmission Line Using WAMS

How to avoid the impact of TCSC on protection schemes

Because of its undoubted advantages, the Thyristor Controlled Series Capacitor (TCSC) is one of the most widely used power-electronic controllers in power systems. However, the TCSC has also been a cause of concern in protective relaying because of its impact on the impedance seen by conventional distance relays, which may lead to an incorrect assessment of fault distance and, consequently, an erroneous relay tripping. To overcome these difficulties, this paper proposes using schemes based on traveling waves for protecting TCSC series compensated transmission lines. Since the method avoids using impedance-based techniques, it is not prone to under-reach or over-reach problems, among others. The approach is based on traveling wave analysis using the discrete wavelet transform. The different case studies show the effectiveness and accuracy of the proposed algorithm in a TCSC environment.

DFIG-Based Wind Plant Coupled Series Compensated Transmission Line: Modeling and SSR Stability Analysis

DFIG-Based Wind Plant Coupled Series Compensated Transmission Line: Modeling and SSR Stability Analysis

Analytical methods for series compensation of a transmission line

The paper proposes an analytical approach to evaluate the effects of series compensation of an overhead transmission line (OHL) concerning the increase of (1) loadability and (2) practical loading of the line. The first part of the paper focuses only on the line characteristics, and proposes an analytical approach to determine the loadability curves of the line as a function of the compensating capacitive reactance. In the second part of the paper, the perspective shifts to examine the power loading of the selected OHL in a meshed electrical network. The target is to evaluate the compensation degree required to achieve a desired power flow increase in the line. To this purpose a fast analytical method, based on the compensation theorem, is presented. Preserving the linearity of the network model, the proposed method allows computation of the needed compensation degree by means of a closed form equation. Numerical applications performed on two simple test networks allow to compare the method with an ad hoc implemented classical optimal power flow. The results validate the method and highlight its effectiveness as a simple and useful tool to support power system planners and designers.

Fast Detection of SSR for Wind Parks Connected to Series-Compensated Transmission Systems

The interactions between wind parks and the series-compensated transmission lines can bring about sub- or super-synchronous resonance (SSR) incidents which jeopardize the safe operation of the entire network. In practice, such incidents may emerge under various conditions as different types of wind turbines with various parameters can be deployed at several locations in the network. Hence, it is crucial to efficiently identify the conditions that lead to adverse interactions and their subsequent SSR incidents. This paper proposes a simulation-based method, namely disturb and scan (DaS), for fast and automated detection of SSR. The proposed technique uses small-scale disturbances in time-domain electromagnetic transient (EMT) simulations to perform spectral analysis along with positive-sequence impedance scans. Numerical results are presented and validated for benchmark systems that utilize type-III and type-IV wind turbines. The system operators can adopt the developed methodology to quickly assess the risks of SSR, evaluate conditions of instability, and improve their network protection and control schemes.

Control stability of inverters with series-compensated transmission lines: analysis and improvement

Control stability of inverters with series-compensated transmission lines: analysis and improvement

A Hybrid Time-Domain Protection Scheme for Series Compensated Transmission Lines

In this paper, a hybrid time-domain protection scheme for series compensated lines using incremental quantity combined with traveling wave principle is proposed. Each principle is assigned a zone of the protected section of the line to achieve fast protection. The incremental quantity-based method is assigned for close-in faults up to 35% and the traveling wave from 33.33% to 66.67% of the line from the relay end. The incremental quantity-based method compares a dynamic restraining quantity obtained from voltage and current to the operating quantity using current for the protection decision. The traveling wave-based method derives a decision by comparing the calculated line length, obtained using the first three arrival times measured at the relay location, to the actual line length. The proposed hybrid approach exploits the advantages of both principles to achieve better reliability and speed of protection for series compensated transmission lines. The proposed scheme is verified for a 400-kV, 300 km series compensated transmission line and results are compared with traditional impedance and incremental quantity-based methods.