Electromagnetic Transients Including DC Research Articles

Coordinated reactive power control for hybrid cascaded high‐voltage direct‐current links in weak AC grids

AbstractThe hybrid cascaded high‐voltage direct‐current (HC‐HVDC) links are recently introduced for ultra‐high voltage bulky power transmissions. However, as wind power becomes more prevalent, the grid is gradually weakened and will face voltage stability issues. This paper proposes a coordinated reactive power control strategy for HC‐HVDC links to enhance the voltage stability of AC bus. Firstly, the impact of HC‐HVDC on the static voltage stability of the AC bus is studied by the voltage sensitivity coefficient. Then, the limitation of reactive power capability of modular multilevel converter in the HC structure is identified. Based on the analysis results, a coordinated reactive power control strategy based on adaptive voltage droop is proposed. In the strategy, the DC voltage ratio is regulated according to the fluctuation degree of the AC bus voltage to adjust the reactive power of HC‐HVDC, which realized the dynamic voltage support for the AC bus. Finally, electromagnetic simulations in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) are performed, and the results validate the theoretical findings and the effectiveness of the control strategy.

Dynamic State Estimation Based Protection for Flexible DC Grid

Fast and reliable dc line protection is one of the key techniques for flexible dc grid with the development of flexible dc transmission technology. However, the serious overcurrent caused by dc faults has a prominent contradiction with the weak overcurrent capability of power electronics. In this article, the high-fidelity dynamic model of the transmission line is first established based on the frequency-dependent characteristic of line parameters. Then, a new dynamic state estimation based protection (DSEBP) scheme is proposed, which uses the redundancy feature of state estimation to improve the dependability of protection. Finally, the actual measurements and the estimated states are compared with the similarity to identify the fault by the chi-square distribution. The proposed method's effectiveness is verified through simulations power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) and experiments, and the factors affecting the performance of DSEBP are analyzed. Compared with the existing protection methods, the proposed method is more dependable, more sensitive, and available for actual projects.

Fault Location Using the Natural Frequency of Oscillation of Current Discharge in MTdc Networks

This paper discusses a novel fault location approach using single ended measurement. The natural dissipation of the circuit parameters are considered for fault location. A relationship between the damped natural frequency of oscillation of the transmission line current and fault location is established in this paper. The hybrid dc circuit breaker (dcCB) interrupts the fault current and the line current attenuates under the absence of any driving voltage source. The line capacitance discharges into the fault at a specific frequency of oscillation and rate of attenuation. Utilizing this information, the fault location in a multi-terminal direct current (MTdc) network can be predicted. A three terminal radial model of a MTdc is used for performance evaluation of the proposed method using Power System Computer Aided Design (PSCAD)/Electromagnetic Transients including dc (EMTdc).

A Reduced RLC Impedance Model for Dynamic Stability Analysis of PI-Controller-Based DC Voltage Control of Generic Source-Load Two-Terminal DC Systems

For the dynamic stability analysis of PI-controller-based dc voltage control of generic source-load two-terminal dc systems, a simpler <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> parallel impedance model has been proposed. First, by ignoring fast inner current control, the output impedance model of the dc voltage control system can be represented as a parallel <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> circuit, which gives an intuitive insight into the physical sense of parameters in the dc voltage PI controller. Then, a dynamic impedance ratio is introduced to represent the impact of dc voltage droop control and line impedance on the dc voltage control system. With this ratio, a generalized <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> parallel impedance model appropriate for dynamic stability analysis of the considered dc system has been obtained. By simplifying the higher order dynamic impedance ratio model to a first-order high-pass filter in the dc voltage control timescale, a typical second-order model of the total impedance expression can be obtained, by which the dynamic stability of dc voltage control can be investigated in an analytical way. Furthermore, even including low dynamics of the inner current control of the dc voltage control unit and the constant power load (CPL)-side converter, the generic <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> model can still be retained. It should be noted that the proposed <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RLC</i> model can be extended to more complex dc systems. Finally, power system computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC)-based simulation verifications and a small-scale hardware-platform-based experimental validations have been provided.

Dynamic Inertia Response Support by Energy Storage System with Renewable Energy Integration Substation

In recent years, the expansion of renewable energy in electric power systems has been increasing at such a rapid pace that it has started affecting frequency stability. Renewable generators connected to the grid produce variable amounts of power, and in most cases have no inherent inertia response (IR) to the system frequency. Therefore, the high penetration of renewable generators in the system results in low inertia and frequency distortion. If renewable generators account for a high proportion of the supply in a power system, the use of energy storage systems (ESSs) with frequency-support algorithms (in the place of synchronous generators) can stabilize the network. The participation of ESSs in frequency support must be organized precisely, so that they are fully devoted to their own purpose. In this paper, the frequency-support parameters of ESSs are calculated for achieving stable frequency response from a network. An estimation and calibration process is conducted during the active power-order change of the ESSs in the substation, and is verified through electromagnetic-transients-including-DC (EMTDC)-based simulations.

Voltage source control of offshore all‐DC wind farm

The offshore all-DC wind farm with increasing capacity will bring problems such as the weakening of grid frequency stability and the increase of equivalent grid impedance. To overcome this, a coordinated control strategy for the offshore all-DC wind farm is proposed here with two salient features: better performance under weak grid condition and real-time frequency support from the wind farm. The control strategy consists of three parts: the inertia synchronising control of the receiving-end converter, the constant ratio control of the DC transformer and the frequency response of the wind farm. With the proposed strategy, the all-DC wind farm operates like a synchronous generator to the onshore grid, which provides fast frequency support when the onshore grid frequency changes. The effectiveness of the proposed method is validated in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) using a typical IEEE 9 bus system.

A Novel Hybrid Islanding Detection Method for Grid-Connected Microgrids With Multiple Inverter-Based Distributed Generators Based on Adaptive Reactive Power Disturbance and Passive Criteria

In order to obtain the benefits of both passive and active methods, a well-designed hybrid islanding detection method for grid-connected microgrids with multiple inverter-based distributed generators (IBDGs) is presented in this paper. Considering that IBDGs located at different positions can detect the same frequency variation (FV) characteristics, linear reactive power disturbance (RPD) is utilized, which can guarantee the synchronization of RPDs added on different IBDGs in the microgrid without the need for communication. In addition, the disturbance slope can change adaptively to reduce the disturbance in grid-connected mode and enlarge the reactive power mismatch during islanding for effective and rapid detection. Setting principles of slope values for IBDGs operating at the unity power factor and those generating active and reactive power simultaneously are analyzed in detail. Considering different conditions that may occur after islanding, three passive criteria, including voltage variation, voltage unbalance, and rate of change of frequency, are sufficiently utilized to activate disturbance slope adjustment, further shortening the detection time. Moreover, the correlation factor between the RPD and FV is also designed as another frequency-related criterion, and the slope adjustment logic is provided. This method is simple and easy to implement with excellent performance regardless of the IBDG power factor, parallel number, and connection mode. According to the anti-islanding test system recommended by IEEE Std.929-2000 and IEEE Std.1547-2003, the method effectiveness has been validated with several test cases in the power systems computer-aided design (PSCAD)/electromagnetic transients including DC (EMTDC) environment.

Optimum Selection of Circuit Breaker Parameters Based on Analytical Calculation of Overcurrent and Overvoltage in Multiterminal HVDC Grids

This paper proposes a time-domain method to calculate the fault response in multiterminal dc (MTdc) grids and the performance of hybrid dc breakers. The proposed method, based on traveling waves: 1) provides a sound representation of fault performance by considering all created traveling waves; 2) introduces a new approach to estimate the reflection coefficients; and 3) provides an approximation of the worst case fault location. Then, based on the analytical results, three parameters of the hybrid dc circuit breaker, i.e., current limiting reactor, arrester rated voltage, and time delay are optimally selected with respect to maximum overcurrent, maximum overvoltage, fault clearance time, and energy absorption in arresters through multiobjective optimization. Accuracy and performance of the proposed method are evaluated and verified by time-domain simulation studies in the power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) environment using frequency-dependent models. The results confirm reasonable accuracy of the proposed fault performance calculation and represent a further step towards optimized design of hybrid dc circuit breakers.

Wind energy conversion system based on open‐end winding three‐phase PMSG coupled with ac−dc buck‐boost converter

This study proposes a new back-to-back (BTB) current source converter (CSC) configuration and presents its application for variable speed wind energy conversion system. The proposed configuration consists of three cascaded ac−dc non-inverting buck-boost converter. The ac side of each converter is connected to an open-end winding of the three-phase permanent magnet synchronous generator and the dc sides are connected in parallel to a current source inverter (CSI) without the need for a dc-side capacitor. Ac−dc converters operate under discontinuous conduction mode, and they draw sinusoidal input currents at the unity power factor from fixed or variable magnitude and frequency supply. In addition, the proposed BTB CSC addresses one of the main drawbacks of the conventional CSI which is the over-voltage experienced by switches during commutation. Moreover, the switching losses of the proposed CSI are eliminated. The ElectroMagnetic Transients including DC (EMTDC)/Power System Computer Aided Design (PSCAD) software is used to assess the feasibility of the proposed topology during normal operating conditions and during low voltage ride through.

State‐space analysis based pole‐to‐ground line fault isolation strategy for LCC–HVDC systems

The common way to isolate the line fault in line-commutated converter-based high-voltage direct current (LCC-HVDC) transmission grids is regulating the trigger angle of converters on both terminals of the faulted line, which usually depends entirely on the conventional controller and goes with large current peak and lengthy overcurrent duration. In this study, a trigger-angle setting approach with adaptability is, therefore, proposed to provide a theoretical basis for the generation of fault isolation strategies under different scenarios. First, equivalent circuits of solid pole-to-ground line faults at rectifier and inverter terminals are established, and time-domain expressions of the fault and converter currents are obtained. Secondly, based on the analysis of current characteristics, the non-linear programming method is utilised to obtain the optimal trigger-angle sequence after the fault, which can be applied in the following fault isolation. Finally, the proposed approach is verified via power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) simulation. The results show that it can effectively reduce the current peak and the overcurrent duration of the converter, which is of positive significance for maintaining the power system security and stability.

Wavelet‐based EMTR method for fault location of VSC‐HVDC transmission lines

A wavelet-based electromagnetic time reversal method for locating faults in voltage source converter-high-voltage direct current (VSC-HVDC) transmission lines is proposed in this study. This principle, firstly, is to use wavelet decomposition to filter the 1-mode fault currents, which are recorded at ends of the transmission line. Secondly, the filtered currents are mirrored on time axes, which called time reversal. Thirdly, the time-reversed currents are set as current sources at ends of the lossless mirror lines. Through assuming a fault at each point of line, the assumed fault currents are calculated, and the RMS of fault current at actual fault location is maximal according to a strict scientific proof. Theoretically, the Wavelet-based Electromagnetic Time Reversal (WEMTR) method is robust against fault types and resistance. A multilevel converter-HVDC system is established in Power Systems Computer-Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC) to verify the feasibility of the proposed method, and the transmission line is simulated by frequency independent phase model. The results show that, the fault location can be calculated exactly without high sampling rates.

Novel control strategy of grid‐connected photovoltaic power supply for frequency regulation

The maximum power point tracking (MPPT) mode is widely applied in photovoltaic (PV) power generation system. In this article, instead of MPPT, a novel control strategy of grid-connected PV power supply for frequency regulation is proposed. First, the static frequency characteristic curve of PV plant is proposed, and a method of synthesising the PV power output power–voltage characteristic curve and static frequency characteristic curve into frequency regulation curve of PV power supply is introduced. So the PV plant can participate in frequency regulation of the system. Through simulation analysis in power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC), PV plant realises the function of frequency regulation when load changes, so the variation of system frequency can be reduced and the frequency stability of power system can be improved, and the effectiveness of the strategy is verified.

Multivalued Coefficient Prestorage and Block Parallel Method for Real-Time Simulation of Microgrid on FRTDS

The microgrid containing a large amount of high frequency power switches and nonlinear components has put forward high requirements for power system real-time simulation technology. Multivalued coefficient prestorage can reduce the calculation steps in real-time simulation. In order to reduce the storage pressure of the multivalued coefficients, the whole network is divided into multiple subnetworks that can be simulated in parallel, and only the parameters for computing input variables and internal variables are prestored. The multiport hybrid equivalent is performed to reduce the number of simultaneous network equations. The input variables are tied to state variables of the circuit so that the iterative calculation is limited to the local network. The devised methodology is validated through simulation of a low-voltage microgrid on a field programmable gate array (FPGA)-based real-time digital simulation (FRTDS) platform at a 5 μs time step. Comparison with a power systems computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) model shows that the proposed method is effective.

A conventional dynamic voltage restorer with fault current limiting capability

In this paper, a new topology of conventional dynamic voltage restorer (DVR) with fault current limiting capability is proposed. The structure uses a grid-tied transformer which is utilized for both supplying energy to the DVR, and revers voltage supply as a FCL function. In order to limit the fault current, a fast bidirectional switch is employed during fault condition to supply revers voltage to avoid high current levels. However, the ratio of transformer is consequential for the proper operation. Also, for decreasing power losses across isolation transformer and semiconductors, a switch is added to flow the current during normal state. The design methodology of the proposed structure is discussed and the results are carried out based on power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) simulation.

A Novel Protection Method for Single Line-to-Ground Faults in Ungrounded Low-Inertia Microgrids

This paper proposes a novel protection method for single line-to-ground (SLG) faults in ungrounded low-inertia microgrids. The proposed method includes microgrid interface protection and unit protection. The microgrid interface protection is based on the difference between the zero-sequence voltage angle and the zero-sequence current angle at the microgrid interconnection transformer for fast selection of the faulty feeder. The microgrid unit protection is based on a comparison of the three zero-sequence current phase directions at each junction point of load or distributed energy resources. Methods are also included to locate the minimum fault section. The fault section location technology operates according to the coordination of microgrid unit protection. The proposed method responds to SLG faults that may occur in both the grid and the microgrid. Simulations of an ungrounded low-inertia microgrid with a relay model were carried out using Power System Computer Aided Design (PSCAD)/Electromagnetic Transients including DC (EMTDC).

Precise algorithm for frequency estimation under dynamic and step‐change conditions

To achieve fast-response frequency estimation under step-change dynamics as well as accurate frequency estimation under slowly varying dynamics, a dynamic-model-based frequency estimation with step-change detection (DFESD) algorithm is proposed in this study. In DFESD, the derivative of the initial phasor estimation, which is obtained from short-time Fourier transform and the first-order Taylor-expansion phasor model, is used to detect the step changes in signals. Then, the initial estimations are adaptively selected by the data-selection strategy to estimate the frequency, of which the estimation is based on a high-order Taylor-expansion model to express the dynamic characteristics in signal. DFESD is a Taylor-expansion-based algorithm and it avoids the states mixture of both pre- and post- step-change data within one long data window, so it can track the frequency accurately under slowly varying dynamics and give fast-response estimation under step-change dynamics. Performance of the proposed algorithm is evaluated by computer-simulated and electromagnetic transients including DC (EMTDC) generated signals, and the results prove the efficiency of DFESD for accurate or fast-response frequency estimation under various test cases.

Design Considerations of a Fault Current Limiting Dynamic Voltage Restorer (FCL-DVR)

This paper proposes a new fault current limiting dynamic voltage restorer (FCL-DVR) concept. The new topology uses a crowbar bidirectional thyristor switch across the output terminals of a conventional back-to-back DVR. In the event of a load short, the DVR controller will deactivate the faulty phase of the DVR and activate its crowbar thyristor to insert the DVR filter reactor into the grid to limit the fault current. A fault condition is detected by sensing the load current and its rate of change. The FCL-DVR will operate with different protection strategies under different fault conditions. Design of the FCL-DVR involves selecting important parameters, such as DVR power rating, dc link voltage of the DVR, output filter reactors and capacitors, and grid-tied transformers is proposed. The design methodology of the proposed FCL-DVR is fully discussed based on power systems computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) simulation. A scaled-down experimental verification is also carried out. Both modeling and experimental results confirm the effectiveness of the new FCL-DVR concept for performing both voltage compensation and fault current limiting functions.

Novel Coordinated Voltage Control for Hybrid Micro-Grid With Islanding Capability

This paper proposes a new coordinated voltage control (CVC) method with reactive power management scheme (RPMS) for a hybrid micro-grid (MG). The CVC scheme, based on synchronizing the response speeds of different voltage regulating devices, is coordinated with novel RPMS. Two cases, with and without proposed CVC, were simulated in the power system computer aided design (PSCAD)/electromagnetic transients including dc (EMTDC) environment and compared against each other. The case with proposed CVC shows superior performance, when tested for fault triggered islanding, intentional islanding, and MG internal fault. Further, the proposed CVC with RPMS is compared to a voltage regulation method present in literature. The proposed CVC with RPMS provides better voltage regulation, maximizes the fast dynamic reactive power reserve, and improves the transient response and transient stability margin of the hybrid MG.

Single‐stage, single‐phase, ac–dc buck–boost converter for low‐voltage applications

The suitability of a single-stage ac–dc buck–boost converter for low-voltage applications is investigated. In-depth discussion and analysis of the converter's operating principle, basic relationships that govern converter steady-state operation and details of the necessary control structures needed to comply with the grid code are provided. The validity of the proposed system is confirmed using power system computer aided design (PSCAD)/electromagnetic transients including DC (EMTDC) simulations, and is substantiated experimentally. The buck–boost converter under investigation has good dynamic performance in both buck and boost modes, and ensures near unity input power factor over the full operating range, whilst having fewer devices and passive elements than other published versions of the buck–boost converter.