Detailed Electromagnetic Transient Simulation Research Articles

Short-Circuit Modeling of DFIG-Based WTG in Sequence Domain Considering Various Fault- Ride-Through Requirements and Solutions

The doubly fed induction generator (DFIG) based wind turbine generator (WTG) can activate different fault ride through (FRT) control schemes and circuits to comply with various grid code or utility interconnection requirements. These FRT solutions play a key role in the resulting fault current of the DFIG-based WTG as well as the voltages and currents distributed in faulted networks. This paper accounts for their impact by proposing a new generic sequence domain model of the DFIG-based WTG for static short circuit calculations. In this paper, the FRT configurations are classified into three types. The proposed model first calculates the positive- and negative-sequence current phasors under the specified FRT control. Then based on the control, the desired currents and voltages are calculated by explicit equations to determine whether the crowbar could operate during FRT operation. By comparing with detailed electromagnetic transient (EMT) simulations using an EPRI benchmark system, the proposed model, incorporated into an iterative steady state solver, is shown to be accurate to calculate voltage and current phasors in an efficient manner. The proposed model provides a rapid tool for short circuit computations and protective relaying studies considering various grid code requirements and FRT implementations.

Design of a robust and practicable SSI damping controller using H[formula omitted] technique for series compensated DFIG-based wind farms

This paper designs a robust and practicable subsynchronous interaction (SSI) damping controller using H∞ technique for the safe operation of series capacitor compensated wind farms (WFs) with doubly-fed induction generator (DFIG) wind turbines (WTs). Mixed sensitivity control design together with pole placement are formulated into a set of linear matrix inequalities (LMIs) to obtain the controller parameters. The LMI technique allows to include both desirable frequency and time domain specifications. The proposed damping controller is integrated into the WT controller (WTC) and receives the DFIG converter currents as inputs. The implementation of the proposed controller does not require any communication links between the WTs and WF secondary control layer. The controller output signals are applied to the inner control loops of DFIG converters and are dynamically limited for the desired fault-ride-through (FRT) performance. The effectiveness of the damping controller is verified through detailed electromagnetic transient (EMT) simulations. In these simulations, the complete medium-voltage (MV) collector grid is modeled with all details, and it is assumed that the wind speed at the location of each turbine follows a Gaussian distribution. The collected results confirm the accuracy of the modeling of the entire WF as an aggregated WT with the average wind speed.

Provably-Stable Overload Ride-Through Control for Grid-Forming Inverters Using System-Wide Lyapunov Function Analysis

A key challenge associated with a grid-forming (GFM) inverter based resource (IBR) is its behavior during severe grid disturbances: since a GFM inverter regulates voltage in the fast timescale instead of current or power, it may experience a transient overload of current, power and/or energy during a severe grid disturbance. While many promising control strategies for overload ride-through have been proposed over the past two decades, transient stability of the system during and after the transition to an overload ride-through control mode remains difficult to guarantee. In this article, a novel overload ride-through control strategy is proposed for a system of grid-forming inverters that takes both self-protection and system-wide transient stability into account. A proposed system-level supervisory control uses slow communication to pre-emptively assign a set of local ride-through control parameters to individual GFM IBR, including a current-limiting virtual reactance, that guarantees that synchronism is still preserved for any set of anticipated grid disturbances. At the core of the supervisory control lies a Lyapunov-function-based routine capable of establishing a strong, albeit conservative, transient stability guarantee for the system. The proposed overload ride-through control strategy is validated via numerical integration of a reduced-order model, as well as through detailed electromagnetic transient (EMT) simulation.

A Data-Driven Method for Prediction of Post-Fault Voltage Stability in Hybrid AC/DC Microgrids

Faults are extreme events that canadversely affect the voltages in islanded microgrids. This paper provides a new data-driven methodology for timely prediction of the post-fault voltage stability in hybrid AC/DC microgrids. The proposed method performs a binary classification with delay constraints by processing sequences of the short-time mean squared deviations using a deep learning system. The deep learning system consists of a bidirectional long short-term memory network whose output is a probabilistic voltage instability indicator. When the value of the indicator is non-zero, persistent voltage disturbances are most likely to occur even after the fault clearance. The proposed method enables the microgrid to carry out remedial or preventive actions, such as event-triggered protection and control of distributed energy resources (DERs), which are advantageous to the resilient operation of the microgrids. Extensive and detailed electromagnetic transient (EMT) simulations of a low-voltage hybrid AC/DC microgrid benchmark are analyzed, and the results confirm the effectiveness of the proposed method for online prediction and fast voltage regulation.

Assessment of energy storage and dynamic response of modular multilevel converters for frequency support control actions

Modular Multilevel Converters (MMCs) inherently possess an energy storage that can be used to help mitigate power imbalances due to perturbations in the system. Control systems use this reserve to release or absorb additional energy, from which additional power contributions are manifested based on the time derivative. However, this requires not only the controller response speeds to be sufficiently fast, but also the MMC to be capable of reaching the set point demanded from its controllers within sufficient time. Furthermore, large energy storage elements in the MMC have implications on its response speed. Detailed electromagnetic transient (EMT) simulations may be used to determine the natural MMC response speed capability, but this comes at high computational costs, especially in larger MMCs. This paper derives a closed-form equation for the response speed of an MMC. It is obtainable without the construction of waveforms or iterative calculations, which makes it computationally efficient. Parametric studies are conducted and investigate the effect of different MMC parameters on its settling time. Results obtained from the model have been compared against those from EMT simulations. The model may be used as an initial design tool, particularly when the natural response speed of the MMC is a concern.

Structure Preserving Aggregation Method for Doubly-Fed Induction Generators in Wind Power Conversion

An aggregation method is proposed that transforms the multiple DFIGs into an equivalent DFIG model that retains the major collective dynamic characteristics of a group of DFIGs. It is intended for Electromagnetic Transients Simulation (EMT). The aggregated machine can take into account different speeds and parameters of each of the individual DFIGs and the connecting impedance of individual DFIGs. Starting with a State Variable (SV) model of an individual DFIG, aggregation is carried out recursively, by combining two DFIGs at a time and then reducing the order of the aggregate to match the state variable equations of a single DFIG so that the steady state performances are identical. Validation is carried out by comparing the detailed electromagnetic transient (EMT) simulation of the unreduced system with the reduced aggregate system. It is shown that the proposed aggregation method accurately matches the steady state response and also accurately reproduces the dominant transient response of the unreduced system. As the aggregate DFIG has the same number of state variables as a single DFIG, the overall wind farms order is reduced significantly to increase the modelling and simulation efficiency.

A Cyber Attack Mitigation Scheme for Series Compensated DFIG-Based Wind Parks

Subsynchronous Interaction (SSI) phenomenon is known to be one of the most frequent and severe stability issues of a Wind Park (WP), and can potentially lead to a significant loss of power generation. The broad impacts of this phenomenon on a power grid have made WPs interesting targets for cyber attacks. To initiate the SSI, an adversary can target either the power grid (external attacks) or the cyber system of WPs (internal attacks). This paper proposes a mitigation scheme for attacks that initiate the SSI phenomenon in series compensated doubly-fed induction generator (DFIG)-based WPs. External attacks are addressed by employing a robust static-output-feedback Subsynchronous Damping Controller (SSDC), which is designed based on the insensitive strip region and Linear Matrix Inequality (LMI) techniques. Internal attacks, however, are detected by comparing the estimated and measured converters’ currents. Once the compromised measurements are detected, the designed SSDC is restructured to mitigate the attacks. The effectiveness of the proposed method is demonstrated using detailed Electromagnetic Transient (EMT) simulations for both internal and external cyber attacks. Additionally, the performance of the proposed method is corroborated using a real-time co-simulation framework.

Hierarchical Modeling Scheme for High-Speed Electromagnetic Transient Simulations of Power Electronic Transformers

The extensive application of the power electronic transformers (PETs) in power systems poses a challenge as the accurate and high-speed electromagnetic transient (EMT) simulation of PET has been a critical issue. The computational time of the detailed EMT simulation of PET on EMT programs is unacceptable due to the large electrical node count, microsecond-range simulation steps, and high switching frequency of the devices and transformers. This article proposes a hierarchical modeling scheme for PET. Unlike the existing modeling methods, the proposed technique recursively decreases the dimension order of the admittance matrix to obtain the generalized Norton equivalent of each phase leg. The final admittance matrix overlaid onto the external system admittance matrix has a dimension order of magnitude remarkably smaller than that of the unreduced structure. By comparison with a detailed EMT model of a medium-voltage dc system, the performance of the proposed scheme has been assessed in PSCAD/EMTDC under various working conditions. With negligible loss of accuracy, approximately one to two orders of magnitude speedup over a straightforward EMT program is achieved.

Improved Coordinated Control Approach for Evolved CCC-HVDC System to Enhance Mitigation Effect of Commutation Failure

The evolved capacitor commutated converter (ECCC), embedded with anti-parallel thyristors based dual-di-rectional full-bridge modules (APT-DFBMs), can effectively reduce commutation failure (CF) risks of line-commutated con-verter-based high voltage direct current (HVDC) and improve the dynamic responses of capacitor-commutated converter-based HVDC. This paper proposes an improved coordinated control strategy for ECCC with the following improvements: 1 under normal operation state, series-connected capacitors can accelerate the commutation process, thereby reducing the overlap angle and increasing the successful commutation margin; 2 under AC fault conditions, the ability of ECCC to mitigate the CF issue no longer relies on the fast fault detection, since the capacitors inside the APT-DFBMs can consistently contribute to the commutation process and further reduce the CF probability; 3 the inserted capacitors can output certain amount of reactive power, increase the power factor, and reduce the required reactive power compensation capacity. Firstly, the proposed coordinated control approach is presented in detail, and the extra commutation voltage to mitigate the CFs provided by the proposed control approach and an existing approach is compared. Secondly, the mechanism of the improved control approach to accelerate commutation process and improve the power factor is analyzed theoretically. Finally, the detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC is conducted to validate the effectiveness of the proposed coordinated control. The results show that the proposed approach can present a further substantial improvement for ECCC, especially enhancing the CF mitigation effect.

Protection Strategies for the Connection of Diode Rectifier-Based Wind Power Plants to HVdc Interconnectors

The connection of diode rectifier (DR) based wind power plants to existing or planned High Voltage dc (HVdc) interconnectors can lead to important savings on cost and system robustness. Since the DR station usually operates in a bipolar configuration, its connection to symmetric monopoles is particularly challenging. However, there are no published detailed studies on the protection of DR connection wind power plants to symmetric monopole interconnectors or even to bipolar interconnectors. This paper includes the comparative study of five different protection strategies for such systems, including both solid and resistive DR station grounding and strategies with and without the use of dc-circuit breakers. An analytical study allows for the calculation of fault current during fault on-set for both half-bridge and hybrid Modular Multi-level Converter (MMC) stations. Using detailed Electromagnetic Transient (EMT) simulation studies, the different protection strategies are evaluated in terms of current, voltage and isolation requirements of each element, as well as the need for dc-circuit breakers, fast communication or larger surge arresters. Moreover, a distance fault detection algorithm is included for the wind turbine converters to distinguish between local ac-grid and dc-cable faults. From the simulation results it is possible to conclude that DR high impedance grounding, together with wind turbine distance protection can be used for the protection of DR based off-shore wind power plants connected to symmetric monopole interconnectors without requiring dc-circuit breakers.

Analysis of low frequency interactions of DFIG wind turbine systems in series compensated grids

Recent field events and subsequent studies have indicated that doubly-fed induction generator (DFIG) based wind farms might be susceptible to adverse interactions with series compensated systems. This paper focuses on the analysis of the interaction phenomenon. A new benchmark network is proposed to address the investigation of low frequency oscillations between wind generation and series compensated systems based on realistic system parameters. Nonlinear components and detailed representation of wind turbines are taken into account. An analytical model for a generic DFIG based wind farm is developed considering all inner and outer loops as well as its mechanical parts. Participation factor analysis is applied to the model to identify states most contributing to the critical modes and to address suitable mitigation. Sensitivity analysis is used to support redesign of control parameters. Investigations indicate that resonance can be avoided with proper control tuning. Finally, the capabilities of a screening technique for SSR risk assessment suitable for early project stages are also investigated and its results compared to the modal analysis. All conclusions are verified with detailed electromagnetic transient (EMT) simulations.

Analytically Validated SSCI Assessment Technique for Wind Parks in Series Compensated Grids

This work highlights limitations in existing screening approaches to assess low-frequency interactions during interconnection studies of grid-connected inverters (GCIs), such as wind power plants. It then proposes a new screening methodology capable of addressing these limitations for realistic representations of power system components and GCIs. It demonstrates that the asymmetric input admittance characteristics of GCIs, whose consideration is required for proper stability assessment, can be accurately extracted with the proposed adequate scanning method implemented in an electromagnetic transient (EMT) software package. This is verified through rigorous validation against analytical formulations developed for field validated type-III and type-IV wind turbine models. It shows that the technique is capable of correctly assessing stability in all presented cases. All results are validated against detailed EMT simulations.

Lagrangian Model of an Isolated DC-DC Converter With a 3-Phase Medium Frequency Transformer Accounting Magnetic Cross Saturation

This article presents a nonlinear equivalent circuit model of an isolated dc-dc converter with a 3-phase medium frequency transformer. The model takes into account the magnetic cross saturation of the 3-phase core-type magnetic circuit. The model is suitable in detailed electromagnetic transient simulations of power systems involving isolated dc-dc converters. The model is developed using the Lagrange energy method. It involves a matrix of dynamic inductances containing a nonlinear term resulting from core magnetization and a linear term resulting from leakage flux. The model parameters are determined based on a series of magnetostatic finite element method simulations. This approach is convenient when applied to high power transformers offering a limited characterization effort, or if the transformer prototype does not exist. The experimental validation performed on a novel 3-phase MFT prototype in a 100 kW 1.2 kV 20 kHz dual active bridge converter has proved the validity of the model and model parameters. The no-load steady-state and inrush tests and the full-load test show a very good fit between the simulated and experimentally measured waveforms. The comparison with a classical simplified model neglecting magnetic cross saturation shows a significant difference in the no-load inrush test.

Single-Input–Single-Output Feedback Control Model and Stability Margin Analysis for Hybrid Dual-Infeed HVDC System

To quantitatively investigate the system stability margin and provide systematic design guidance for the hybrid dual-infeed high-voltage direct current (HVdc) system, this article develops a dynamic multi-input–multi-output (MIMO) transfer function model for the hybrid dual-infeed HVdc system, consisting of a line-commutated converter-based HVdc (LCC-HVdc) link and a modular multilevel converter-based HVdc (MMC-HVdc) link located in close mutual proximity at the receiving side. Then, based on the individual channel analysis and design (ICAD) theory, the equivalent single-input–single-output (SISO) feedback control model is developed without structural information loss, which has the advantage of quantitative evaluation for the system stability margin. Then, the impacts of the ac system strength and control system on the system stability margin are quantitatively evaluated by gain margin (GM) and phase margin (PM) indexes based on the derived SISO model. Moreover, the coupling effect between the ac system strength and the control parameters is also studied. The obtained results from the theoretical analysis and the detailed electromagnetic transient (EMT) simulation in PSCAD/EMTDC validate the accuracy of the derived SISO feedback control model and the correctness of the theoretical findings.

Capacitor Energy Storage Requirements in Mixed-Submodule Hybrid Cascaded MMCs

This article investigates energy storage requirements in the submodules of a class of converters known as hybrid cascaded modular multilvel converters (HC-MMCs). The complexities arising from the usage of different submodule types and their non-conventional arrangement necessitate detailed analysis of submodule capacitor voltage ripples, which are needed for development of guidelines for sizing submodule capacitors. This paper characterizes electric charge variations in the submodule capacitors to derive expressions for capacitor voltage ripples, and to determine the energy storage requirements of the converter. The paper uses detailed electromagnetic transient (EMT) simulations and experimental results on a laboratory-scale MMC setup to verify and validate its findings.

The Voltage Clamping Based DC Circuit Breaker With Decoupled Fault Isolation and Energy Dissipation Processes

This paper proposes a DC Circuit Breaker (DCCB) topology for the DC fault clearance of the Modular Multilevel Converter based High Voltage Direct Current (MMC-HVDC) grid. By utilizing the voltage clamping and current limiting functions of the pre-charged capacitors in the DCCB, the DC-side short-circuit fault is isolated. First, the topological mechanism and operational principle of the unidirectional topology are analyzed, and the bidirectional topology is provided for backup protection. Then, the residual energy dissipation branch of the DC transmission line is introduced and the reclosing strategy after the deionization process is provided. Further, a step-insert method of the clamping branch is designed to relieve the sudden change of the electrical stresses during the voltage clamping based fault clearance process. Compared to the conventional DCCB schemes, the main advantage of the proposed topology is the decoupling of fast fault isolation and slow energy dissipation processes, which will accelerate the power recovery speed of the healthy parts of the HVDC grid. The proposed approaches are verified first by fully detailed Electro-Magnetic Transient (EMT) simulations on PSCAD/EMTDC then by a down-scaled hardware experiment.

Analysis of the Harmonic Transmission Characteristics of HVDC Transmission Based on a Unified Port Theory Model

The application of power electronics can introduce flexibility into power systems, but it can also cause harmonic problems. As a typical application of power electronics, high-voltage direct current (HVDC) transmission exhibits very prominent harmonic problems. In recent years, many HVDC protection maloperation events caused by harmonic problems have occurred in the field. The phenomena of harmonic transfer and amplification in HVDC transmission have an undeniable impact on the overall operation of a power system. Previous studies of these phenomena have relied mainly on detailed electromagnetic transient simulations. However, these simulations normally suffer from a large computational burden and low efficiency due to the complexity of the models and the small simulation time steps used. Therefore, an efficient analysis model of harmonics for various HVDC operating modes is presented in this paper. This model is based on port theory; every part of the HVDC transmission system is represented by an equivalent two-port component to construct a unified equivalent circuit. The proposed model is simple to calculate, and only the connections of the two-port components need to be changed to model different operation modes. The proposed two-port model is applied for the harmonic analysis of an HVDC transmission system, and the validity and accuracy of the model are confirmed by detailed electromagnetic simulations. Moreover, the model can be easily applied to HVDC systems for harmonic transfer and amplification, and the effects of the HVDC transmission power and smoothing reactor on harmonic transfer and amplification are analyzed.

Short-Circuit Model for Type-IV Wind Turbine Generators With Decoupled Sequence Control

The power system planning and protection studies are becoming more challenging due to the rapid increase in penetration levels of converter-interfaced renewables. Type-IV wind turbine generators (WTGs) and photovoltaic panels are interfaced to the grid through a full-scale converter, and their short-circuit current contributions are mainly designated by the converter control and associated current limits. This paper proposes a new phasor domain modeling approach for the wind parks (WPs) with Type-IV WTGs, using the concept of control-based equivalent circuits. The proposed model precisely represents the detailed electromagnetic transient (EMT)-type model in steady state, and is able to account for the fault-ride-through function of the WTG control as well as its specific decoupled sequence control scheme in addition to the traditional coupled control scheme. Although the collector grid and WTGs inside the WP are represented with their aggregated models, the overall reactive power control structure of the WP is preserved by taking the central WP controller into account. The accuracy of the proposed model is validated through detailed EMT simulations.

Comprehensive small‐signal model and stability analysis of VSC‐based medium‐voltage DC distribution system

DC distribution technology is emerging as a prospective form of future power system owing to its advantages such as high controllability and easy compatibility. In medium- or low-voltage DC distribution systems, the two-level voltage-source converter (VSC) has become one of the preferred topologies for grid-connected converters. A comprehensive small-signal model for VSC-based DC distribution systems, which takes into consideration the AC filter, DC distribution cables, and all the controllers in the VSC is presented in this study. Based on the ideal AC source, an absolutely stable synchronous rotating coordinate system is built to preclude the influence of the short circuit ratio of the AC system. The proposed model is verified using a detailed electromagnetic transient simulation carried out using power systems computer-aided design (PSCAD). To analyse the impact of neglecting the AC filter and phase-locked loop controller, two simplified models are established, and a detailed comparison of the three models is presented. The results indicate that when using the two simplified models, the time-domain responses are significantly different from those in the PSCAD simulation and the obtained feasible domains of the control parameters tend to be either more optimistic or conservative. Thus, to carry out a small-signal stability analysis, the use of the proposed comprehensive model is recommended to ensure the safety of the DC distribution system.

SSCI Damping Controller Design for Series-Compensated DFIG-Based Wind Parks Considering Implementation Challenges

The use of supplementary controllers for mitigating subsynchronous control interaction (SSCI) in doubly-fed induction generator based wind parks is quite promising due to their low investment costs. These SSCI damping controllers are typically designed and tested using an aggregated wind turbine (WT) model that represents the entire wind park (WP). However, no research has been reported on their implementations in a realistic WP. This paper, first presents various implementation schemes for a linear-quadratic regulator based SSCI damping controller, and discusses the corresponding practical challenges. Then, an implementation scheme that obviates the need for high rate data transfer between the WTs and the WP secondary control layer is proposed. In the proposed implementation, the SSCI damping controller receives only the WT outage information updates from the WP controller, hence it is not vulnerable to the variable communication network latency. The SSCI damping controller parameters are also modified when there is a change in WT outage information for the ultimate performance. The effectiveness of the proposed implementation scheme is confirmed with detailed electromagnetic transient simulations, considering different wind speeds at each WT and WT outages due to sudden decrease in wind speeds.