Power Systems Computer Aided Design/electromagnetic Transients Including DC Research Articles

Modelling and control of a back‐to‐back MMC–HVDC system using ADPSS

Back-to-back modular multilevel converter–high-voltage direct current (MMC–HVDC) can realise the asynchronous interconnection of two regional power grids. It can effectively solve the power grid stability problems caused by AC–DC power transfer and avoid the risk of large-scale blackouts. Existing MMC–HVDC modelling methods are usually based on Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC), immensely simplifying the AC system. The simplified model is incapable to reflect the interaction characteristics accurately. To solve this problem, a hybrid simulation model of an actual back-to-back MMC–HVDC system is built using Advanced Digital Power System Simulator (ADPSS). ADPSS is a software developed by China Electric Power Research Institute that can realise multi-time scale hybrid simulation to simulate the mutual coupling characteristics accurately. The model includes the electromagnetic transient simulation model of the DC system, and the control system is designed. Steady-state and transient simulations are carried out to verify its function. In summary, the study provides a better reference for the analysis and designing of the practical system.

Coordination Control Method Suitable for Practical Engineering Applications for Distributed Power Flow Controller (DPFC)

To control multiple series units of distributed power flow controller (DPFC), a hierarchical control method is proposed. This coordination control system consists of a coordination controller and multiple series unit controllers. According to the demand of power flow ordered by a dispatch center, the corresponding series-compensated voltage is calculated by a high-level controller and transferred to each series unit controller. Comparing the targeted compensated voltage with actual injected voltage, the modulation signal of the converter will be modified to change the power flow accurately. The DPFC system model is built in Power Systems Computer Aided Design/ Electromagnetic Transients including DC (PSCAD/EMTDC). The simulation result indicates that the proposed hierarchical control method is effective and can be considered as an option for practical engineering applications in the future.

Applicability analysis of travelling wave protection for the hybrid HVDC system

The hybrid high-voltage direct current (HVDC) system, in which the rectifier is made up of the line commutated converter (LCC) and the inverter is composed of the modular multilevel converter (MMC), has quite different fault characteristics from the symmetrical LCC–HVDC and MMC–HVDC systems. Therefore, it is necessary to analyse the applicability of the existing line protection principles to the hybrid HVDC system. First, the fault characteristics of the travelling wave on both ends of the hybrid HVDC line are obtained by analysing the boundary characteristics. The applicable conditions of travelling wave protection to the hybrid HVDC line are given based on the criteria of travelling wave protection and the fault characteristics. Second, an electromagnetic transient simulation model of the hybrid HVDC system is established in Power System Computer-Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) in order to verify the theoretical analysis results. A large amount of simulation results show that the travelling wave protection has good applicability in the hybrid HVDC system, but it has poor sensitivity for high-resistance grounding faults; an appropriate sampling rate should be chosen according to the parameters of the boundary element for a better performance of the protection.

EEG-based brain connectivity analysis distinguishes children with low math achievements from their schoolmates

A mature mathematical technique to model power systems, aimed at computation of harmonics dynamics in a step-by-step fashion, is the extended harmonic domain (EHD). The EHD is capable of accounting for an arbitrary number of linearly-spaced sequentially-numbered harmonics; however, the dimensions of an EHD-based model are prone to explode if a large number of harmonics are considered. This fact makes computationally impossible the harmonics dynamics study when high-frequencies, inherent of electronic devices, are involved.This paper presents a flexible extended harmonic domain (FEHD) approach that permits to include an arbitrary number of (non-sequential) harmonics in any state variable of the system. The reasoning to use distinct harmonic content for each state variable is that in a general network (especially those involving alternative energy sources), state-variables exhibit distinct harmonic content. Such different contents mainly depend on the network topology, e.g., switched devices involved, filter arrangements utilized, etc.As shown in the paper, the proposed FEHD approach is able to provide reduced-order EHD models including high-frequency ripple information; thus, leading to computational savings while keeping accuracy compared to traditional EHD-based models and averaged-value models. A case study, involving a three-phase photovoltaic system in closed-loop operation, is presented to validate the proposed methodology. The results provided by the proposed methodology are verified with those given by the power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) simulation tool.

Smoothly Transitive Fixed Frequency Hysteresis Current Control Based on Optimal Voltage Space Vector

This paper proposes a smoothly transitive fixed frequency hysteresis current control (ST-FHCC) scheme applied to an active power filter (APF). First of all, a switching fixed frequency hysteresis current control (S-FHCC) is introduced, which is based on phase-to-phase decoupling and switching the control strategies under mode 0 or mode 1, and its weakness is described in detail. To enhance it, an improved approach of regulating the hysteresis bandwidth is presented to fix the switching frequency with switch phases being regulated, based on the optimal voltage space vector (OVSV). Furthermore, a flexible division of the voltage-space-vectors diagram is developed to divide the original voltage-space-vectors diagram into six sub-regions, upon which the control strategies under mode 0 and mode 1 can be switched alternately in order to obtain a smooth transition. As a consequence, ST-FHCC can thoroughly avoid the inherent weakness of S-FHCC of switching that is not smooth as a result of the low control accuracy of current errors. Case studies are carried out through power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) while simulation results verify the effectiveness and superiority of ST-FHCC compared to S-FHCC.

A new fault detection and fault location method for multi-terminal high voltage direct current of offshore wind farm

This paper proposes a novel protection scheme for multi-terminal High Voltage Direct Current (MTDC) systems incorporating offshore wind farm based on high-frequency components detected from the fault current signal. This method can accurately detect the fault on each line and classify the fault types. Using the post-fault current time series, both single-ended measurements (detection and classification) and double-end measurements (location), the frequency spectrum is generated to measure the gaps between the contiguous peak frequencies giving a robust and comprehensive scheme. Unlike the previous travelling wave based methods, which must identify the travelling wavefront and require a high sampling rate, the new gap-based approach is able to give accurate fault detection and fault location using any appropriate range of post-fault signals. Furthermore, the proposed method is fault resistance independent and thus even a very high fault impedance has no effect on the fault location detection. By immediately tripping the faults, the fault-caused disturbance to the offshore wind farm is minimized. A three-terminal voltage sourced converter HVDC (VSC-HVDC) system connection of offshore wind farm is modelled in PSCAD/EMTDC (Power Systems Computer Aided Design/Electro-Magnetic Transients including DC) software, which is used for obtaining the fault current data for the transmission line terminal. The algorithm is verified by studying a range of cases, by varying the fault resistance fault locations and also including external faults. The results show that the proposed method gives an accurate and reliable fault detection, classification and location on the test MTDC system.

AC-Chopper-Based Inrush Current Suppressor in a Wind Power Generation System with Squirrel-Cage Induction Machines

This paper proposes the inrush current suppressor using an AC chopper in a large-capacity wind power generation system (WPGS) with two squirrel-cage induction machines (SCIMs), which are switched over depending on the wind speed. The input side of the AC chopper is connected to the source in parallel. The output side of the AC chopper is connected in series with the SCIM through matching transformers. In the proposed inrush current suppressor, the output voltage of the AC chopper is the same as the receiving-end voltage before connecting the SCIM. By gradually decreasing the output voltage of the AC chopper, the applied voltage of the SCIM is gradually increased without the inrush current. The basic principle of the proposed inrush current suppressor is discussed in detail. A computer simulation is implemented to confirm the validity and practicability of the proposed inrush current suppressor using a power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC). Simulation results demonstrate that the proposed inrush current suppressor can suppress the inrush current.

Simplified Analytic Approach of Pole-to-Pole Faults in MMC-HVDC for AC System Backup Protection Setting Calculation

AC (alternating current) system backup protection setting calculation is an important basis for ensuring the safe operation of power grids. With the increasing integration of modular multilevel converter based high voltage direct current (MMC-HVDC) into power grids, it has been a big challenge for the AC system backup protection setting calculation, as the MMC-HVDC lacks the fault self-clearance capability under pole-to-pole faults. This paper focused on the pole-to-pole faults analysis for the AC system backup protection setting calculation. The principles of pole-to-pole faults analysis were discussed first according to the standard of the AC system protection setting calculation. Then, the influence of fault resistance on the fault process was investigated. A simplified analytic approach of pole-to-pole faults in MMC-HVDC for the AC system backup protection setting calculation was proposed. In the proposed approach, the derived expressions of fundamental frequency current are applicable under arbitrary fault resistance. The accuracy of the proposed approach was demonstrated by PSCAD/EMTDC (Power Systems Computer-Aided Design/Electromagnetic Transients including DC) simulations.

An Online Faulty Sub‐Feeder Identification Method for the NUGS Single‐Phase Earth Fault Based on the Faulty Phase Current Feature Recognition

Single‐phase earthing is a type of fault that occurs frequently in a distribution network. To improve the reliability of the power supply, neutral un‐effectively grounded system (NUGS) is adopted in many countries. However, it will lead to the difficulty of identification of the faulted components. Existing faulty sub‐feeder identification methods place severe limit on field feasibility in various aspects when dealing with this problem. In this paper, an online single‐phase earth faulty sub‐feeder identification method based on the faulty phase current feature recognition is introduced and assessed according to the universal distribution characteristics of the high‐frequency transient components. In comparison with existing location methods, the proposed method is independent of zero‐sequence current measurement and has the natural faulty feeder selection capability. Besides, it can be implemented economically by virtue of the existing distribution automation system in the NUGS. The effectiveness of the proposed method under various fault conditions is verified by Power Systems Computer Aided Design/Electromagnetic Transients including DC (PSCAD/EMTDC) simulation and comparison studies. © 2017 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

A New Structure of Fault Current Limiter Based on the System Impedance With Fast Eliminating Method and Simple Control Procedure

In this paper, a new type of fault current limiter (FCL) called impedance-based fault current limiter (IBFCL) is proposed. The structure has the capability to limit the fault current to an extent predefined value by means of the system impedance. The control circuit uses voltage and current data to calculate the impedance value. In order to limit the fault current, several eliminating resistors, which are in parallel with auxiliary switches, are bypassed by a main semiconductor switch. In addition, the proposed FCL has a fast response time with good current-limiting characteristic. The analysis of normal and fault conditions of the IBFCL circuit using power system computer-aided design/electromagnetic transients, including dc (PSCAD/EMTDC) software have been presented. Moreover, an experimental prototype is provided to validate simulation results and the feasibility of structure to be implemented in power systems.

Flexible extended harmonic domain approach for transient state analysis of switched systems

A mature mathematical technique to model power systems, aimed at computation of harmonics dynamics in a step-by-step fashion, is the extended harmonic domain (EHD). The EHD is capable of accounting for an arbitrary number of linearly-spaced sequentially-numbered harmonics; however, the dimensions of an EHD-based model are prone to explode if a large number of harmonics are considered. This fact makes computationally impossible the harmonics dynamics study when high-frequencies, inherent of electronic devices, are involved.This paper presents a flexible extended harmonic domain (FEHD) approach that permits to include an arbitrary number of (non-sequential) harmonics in any state variable of the system. The reasoning to use distinct harmonic content for each state variable is that in a general network (especially those involving alternative energy sources), state-variables exhibit distinct harmonic content. Such different contents mainly depend on the network topology, e.g., switched devices involved, filter arrangements utilized, etc.As shown in the paper, the proposed FEHD approach is able to provide reduced-order EHD models including high-frequency ripple information; thus, leading to computational savings while keeping accuracy compared to traditional EHD-based models and averaged-value models. A case study, involving a three-phase photovoltaic system in closed-loop operation, is presented to validate the proposed methodology. The results provided by the proposed methodology are verified with those given by the power system computer-aided design/electromagnetic transients including DC (PSCAD/EMTDC) simulation tool.

Modeling of HVDC System to Improve Estimation of Transient DC Current and Voltages for AC Line-to-Ground Fault—An Actual Case Study in Korea

A new modeling method for high voltage direct current (HVDC) systems and associated controllers is presented for the power system simulator for engineering (PSS/E) simulation environment. The aim is to improve the estimation of the transient DC voltage and current in the event of an AC line-to-ground fault. The proposed method consists primary of three interconnected modules for (a) equation conversion; (b) control-mode selection; and (c) DC-line modeling. Simulation case studies were carried out using PSS/E and a power systems computer aided design/electromagnetic transients including DC (PSCAD/EMTDC) model of the Jeju– Haenam HVDC system in Korea. The simulation results are compared with actual operational data and the PSCAD/EMTDC simulation results for an HVDC system during single-phase and three-phase line-to-ground faults, respectively. These comparisons show that the proposed PSS/E modeling method results in the improved estimation of the dynamic variation in the DC voltage and current in the event of an AC network fault, with significant gains in computational efficiency, making it suitable for real-time analysis of HVDC systems.

Subsynchronous Torsional Interaction of Wind Farms with FSIG Wind Turbines Connected to LCC-HVDC Lines

High-voltage direct current (HVDC) lines with line-commutated converter (LCC) are being increasingly employed to transmit bulk wind power over long distance. However, this may cause the sub-synchronous torsional interaction (SSTI) between the wind farms and the LCC-HVDC system. The SSTI characteristics of wind farms with fixed-speed induction generator (FSIG) wind turbines connected to LCC-HVDC are investigated in this paper. To simplify the calculations, a modular modeling method is proposed for building the small-signal mathematical model of the investigated system. Small-signal analysis, participation factor analysis, and impact of dominant factors analysis are then applied to investigate the SSTI characteristics under different operating conditions. Three oscillation modes associated with the SSTI are identified in the entire system through small-signal and participation factor analysis, comprising two torsional modes and an electromechanical mode. Impact of dominant factors analysis shows that the system becomes less stable as the wind farm capacity grows and the distance between FSIG wind farm and the rectifier station increases. Moreover, the above analysis suggests that wind farms with FSIG connected to LCC-HVDC lines may not cause unstable SSTI. The electromagnetic transient simulations based on PSCAD/EMTDC (Power Systems Computer-Aided Design/Electromagnetic Transients including DC) verify these results.

A Current Frequency Component-Based Fault-Location Method for Voltage-Source Converter-Based High-Voltage Direct Current (VSC-HVDC) Cables Using the S Transform

This paper proposes a fault-location method for voltage-source converter (VSC)-based high-voltage direct current (VSC-HVDC) systems. This method relies on the current frequency components generated by faults in the cable, and requires the arrival time of the frequency components at two terminals. The S transform is a time–frequency analysis tool that is superior to the wavelet transform in some respects. Therefore, the S transform was employed to determine the arrival time in this paper. To obtain a reliable criterion, a novel phase-mode transform method for bipolar cables was developed, and the propagation characteristics of the current frequency components through out the cable were analyzed. A two-terminal VSC-HVDC system was modeled in power system computer aided design/electromagnetic transients including DC (PSCAD/EMTDC). Various faults under different conditions were simulated on the basis of this model, and the simulation results verified a high accuracy, robustness against fault-resistance, and noise immunity of the proposed method.

A Novel High-Frequency Voltage Standing-Wave Ratio-Based Grounding Electrode Line Fault Supervision in Ultra-High Voltage DC Transmission Systems

In order to improve the fault monitoring performance of grounding electrode lines in ultra-high voltage DC (UHVDC) transmission systems, a novel fault monitoring approach based on the high-frequency voltage standing-wave ratio (VSWR) is proposed in this paper. The VSWR is defined considering a lossless transmission line, and the characteristics of the VSWR under different conditions are analyzed. It is shown that the VSWR equals 1 when the terminal resistance completely matches the characteristic impedance of the line, and when a short circuit fault occurs on the grounding electrode line, the VSWR will be greater than 1. The VSWR will approach positive infinity under metallic earth fault conditions, whereas the VSWR in non-metallic earth faults will be smaller. Based on these analytical results, a fault supervision criterion is formulated. The effectiveness of the proposed VSWR-based fault supervision technique is verified with a typical UHVDC project established in Power Systems Computer Aided Design/Electromagnetic Transients including DC(PSCAD/EMTDC). Simulation results indicate that the proposed strategy can reliably identify the grounding electrode line fault and has strong anti-fault resistance capability.

Islanding detection using wavelet transform and rate of change of frequency relay method in presence of different distributed generation technologies

In this paper, islanding detection in the presence of different technologies of distributed generations (DGs) is discussed. Three types of DGs, namely a synchronous generator (SG), a wind energy conversion system (WECS), and a photovoltaic (PV) system, with distribution network parameters as well as linear and nonlinear loads are simulated using the Power System Computer Aided Design/Electromagnetic Transients Including DC (PSCAD/EMTDC) software. To detect the island state, a comprehensive islanding detection algorithm based on wavelet transform (WT) and rate of change of frequency (ROCOF) relay method is considered. To determine the islanding threshold value, the voltage and frequency values of sensitive buses are sampled at regular intervals, and the signals are analyzed with WT using MATLAB. The proposed algorithm could detect the exact island state definitively with the consideration of various disturbance signals such as capacitor bank switching, induction motor switching, and the sudden disconnection of power sources and loads from different buses of the distribution network. The results of the simulation are validated with the logic of ROCOF relay islanding detection method, which determines the accuracy threshold value when the power generation and load consumption difference is negligible. In addition, the proposed algorithm based on the rested DG's output power in the islanding state can supply prioritized loads based on thier importance. © 2016 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

Extended Harmonic Domain Model of a Wind Turbine Generator for Harmonic Transient Analysis

This paper presents an extended harmonic domain (EHD) model of a wind turbine generator (WTG) system based on a doubly fed induction generator. The proposed EHD WTG model includes both electrical and mechanical subsystems and permits monitoring step-by-step the harmonics time variation; this allows, among other applications, to detect contingency scenarios, such as resonance problems caused by a specific harmonic. Moreover, control efficiency can be improved by means of the power-quality indices which are instantaneously available by the EHD WTG model. The proposed model is capable of accounting for an arbitrary number of harmonics; however, the more harmonics that are included, the larger the system becomes. This affects the central-processing unit time by the transient simulation. An efficient heuristic remedy is proposed in this paper which consists of accounting for “high-impact” harmonics only, based on the converter's frequency spectrum. The EHD WTG model is validated with the power system computer-aided design/electromagnetic transients including dc (PSCAD/EMTDC) software tool via a case study.

Reduced-Capacity Inrush Current Suppressor Using a Matrix Converter in a Wind Power Generation System with Squirrel-Cage Induction Machines

This paper describes the reduced capacity of the inrush current suppressor using a matrix converter (MC) in a large-capacity wind power generation system (WPGS) with two squirrel-cage induction machines (SCIMs). These SCIMs are switched over depending on the wind speed. The input side of the MC is connected to the source in parallel. The output side of the MC is connected in series with the SCIM through matching transformers. The modulation method of the MC used is direct duty ratio pulse width modulation. The reference output voltage of the MC is decided by multiplying the SCIM current with the variable control gain. Therefore, the MC performs as resistors for the inrush current. Digital computer simulation is implemented to confirm the validity and practicability of the proposed inrush current suppressor using PSCAD/EMTDC (power system computer-aided design/electromagnetic transients including DC). Furthermore, the equivalent resistance of the MC is decided by the relationship between the equivalent resistance and the capacity of the MC. Simulation results demonstrate that the proposed inrush current suppressor can suppress the inrush current perfectly.

Design of a custom power park for wind turbine system and analysis of the system performance under power quality disturbances

This study presents an operation of custom power park (CPP) with wind turbine system (WTS). The classical power system stability, changes and challenges in the power system with installation of wind energy sources are given. The CPP is composed by static transfer switch (STS) for WTS. An electromagnetic transient model of a system is composed by WTS and STS in power system computer aided design/electromagnetic transient including DC (PSCAD/EMTDC) programme. The entire model of the proposed system is developed and simulating results are verified in PSCAD/EMTDC programme at different fault types. The system responses of the different fault types are given and obtained results are evaluated in detail for power quality problems. Moreover, of the different type faults are implemented to the WT feeder and the response of the proposed system is observed for all cases. Then, the simulating results are evaluated. The currents and voltages of all systems are investigated during normal, transfer and post-transfer periods and also these simulating results are evaluated.

Analysis of a back flashover across insulator strings on a 115 kV transmission line tower by PSCAD

Lightning striking on a transmission tower induces high ground potential rise and high voltage at tower arms in which potential is normally at ground level, and subsequently causes overvoltage across an insulator string. If this overvoltage is higher than the withstanding voltage of the insulator string according to the v-t (voltage-time) curve, back flashover phenomena will occur and this event may cause outage. The main objective of this paper is to study the factors influencing the back flashover phenomena. The computer program PSCAD/EMTDC (Power System Computer Aided Design/Electromagnetic Transients including DC) is used to simulate lightning striking on a transmission tower 115kV. Lightning current, transmission towers, ground resistance, insulator strings and back flashover phenomena are modeled. Main simulations are lightning striking on different towers, different soil resistivity, different lightning current magnitudes and wave shapes, different locations, and different phase angles of source voltage. Simulation results show that the higher tower encounters higher induced voltage. A back flashover occurs at the top tower arm easier than at the middle and lower arms. The higher soil resistivity induces higher voltage. The larger lightning current magnitude impacts on higher induced voltage. The longer rise time of lightning current generates lower induced voltage. Lightning strikes directly on tower generate higher voltage than that of striking on overhead ground wires.