Aniruddha M. Gole

Efficient Modeling of Modular Multilevel HVDC Converters (MMC) on Electromagnetic Transient Simulation Programs

The number of semiconductor switches in a modular multilevel converter (MMC) for HVDC transmission is typically two orders of magnitudes larger than that in a two or three level voltage-sourced converter (VSC). The large number of devices creates a computational challenge for electromagnetic transient simulation programs, as it can significantly increase the simulation time. The paper presents a method based on partitioning the systems admittance matrix and deriving an efficient time-varying Thévenins equivalent for the converter part. The proposed method does not make use of approximate interfaced models, and mathematically, is exactly equivalent to modelling the entire network (converter and external system) as one large network. It is shown to drastically reduce the computational time without sacrificing any accuracy. The paper also presents control algorithms and other modelling aspects. The efficacy of the proposed method is demonstrated by simulating a point-to-point VSC-MMC-based HVDC transmission system.

A platform for validation of FACTS models

The paper presents a platform system for the incorporation of flexible ac transmission systems (FACTS) devices. The platform permits detailed electromagnetic transients simulation as it is of manageable size. It manifests some of the common problems for which FACTS devices are used such as congestion management, stability improvement, and voltage support. The platform can be valuable for the validation of reduced order models such as small signal or transient stability models. The paper presents details on the development and validation of a small signal based model with the inclusion of a Unified Power Flow Controller. The validated model is then used successfully for the design of a feedback controller for improved damping.

Digital Simulation of DC Links and AC Machines

This paper discusses the digital simulation of HVDC transmission and ac machines by an electromagnetic transients computer program. Ease in interfacing user written control system models with the network solution is presented along with discussion of means to simplify dc valve group representation in simulation. A working digital simulator has been developed enabling flexible in-house studies to be undertaken. Test results of its performance are presented.

Electromagnetic transients simulation models for accurate representation of switching losses and thermal performance in power electronic systems

This work presents an electrothermal model of an insulated-gate bipolar transistor (IGBT) switch suitable for the simulation of switching and conduction losses in a large class of voltage-sourced converter (VSC)-based flexible ac transmission systems (FACTS) devices. The model is obtained by mathematical derivation of loss equations from the known submicrosecond device switching characteristics, and through the selection of appropriate differential equation parameters for representing the thermal performance. The model is useful in determining the devices heat generation, its junction temperature, as well as the cooling performance of the connected heat sinks. The model provides accurate results without recourse to an unreasonably small time step.

Dynamic System Equivalents: A Survey of Available Techniques

This paper presents a brief review of techniques available for reducing large systems to smaller equivalents. The paper is divided into High Frequency Equivalents, Low Frequency Equivalents, and Wide-band Equivalents.

Commutation Failure Analysis in Multi-Infeed HVDC Systems

The commutation failure vulnerability of multi-infeed HVDC converters to ac side faults is analyzed. Faults in multi-infeed systems can cause commutation failure events in the nearby local converter or concurrently in both converters. The immunity of the system to both these events is investigated and suitably parameterized. “Commutation Failure Immunity Index” is calculated for multi-infeed systems with different ratings. Anomalous failure in valve current commutation due to voltage distortion has been investigated.

Optimization-enabled electromagnetic transient simulation

Electromagnetic transient simulation programs (EMTP-type programs) are powerful tools for the study of a wide range of power system transient problems. This paper introduces a novel tool in which an emtp-type program becomes the objective function evaluator for a nonlinear optimization algorithm. In this approach, the nonlinear optimization program is given control to perform several consecutive runs with a view to minimize (or maximize) the desired objective function, which is computed from the results of each simulation run. Since the optimization algorithm strategically selects the parameters for the emtp run, the overall design process is orders of magnitude faster than that possible from sequential or random (Monte-Carlo-type) multiple-runs of the EMTP-type program. The paper discusses the mechanics of the interface as well as design of objective functions. The power of the proposed method is demonstrated through two examples for the design of power-electronic converters.

A Novel Configuration for a Cascade Inverter-Based Dynamic Voltage Restorer With Reduced Energy Storage Requirements

This paper introduces a new configuration for a cascade (H-bridge) converter-based dynamic voltage regulator in which the basic cascade converter is supplemented with a shunt thyristor-switched inductor. The proposed topology is shown to possess the ability of mitigating a severe and long duration voltage sag with a significantly smaller energy demand from the cascade converter. A suitable control system is designed, and the operation of the new device is analyzed using electromagnetic transients simulation as well as mathematical analysis. Simulation and experimental results are presented to demonstrate the feasibility and the practicality of the proposed novel dynamic voltage restorer topology.

Optimized Partial Eigenstructure Assignment-Based Design of a Combined PSS and Active Damping Controller for a DFIG

The paper applies the method of eigenstructure assignment for the design of a controller for a wind generation scenario in Northern Scotland based on doubly-fed induction generators (DFIGs). The designed controller serves the combined purpose of a conventional power system stabilizer (PSS) and an active damping controller and provides a contribution to both network and shaft damping. This novel approach is superior because all available degrees of freedom are fully exploited by selecting not only the new eigenvalue locations but also certain elements of the left eigenvectors. These elements are obtained by solving a multiobjective nonlinear optimization problem (MONLOP). Examples are presented to demonstrate that optimizing the eigenvectors yields a better performing controller in comparison with one designed using mere eigenvalue relocation.

A Wide-Band Multi-Port System Equivalent for Real-Time Digital Power System Simulators

This paper describes a method of developing wide-band multi-port system equivalents for use with real-time digital power system simulators. The proposed equivalent combines a frequency dependent network equivalent (FDNE) for the high frequency electromagnetic transients and a transient stability analysis (TSA) type simulation block for the electromechanical transients. The frequency dependent characteristic for FDNE is obtained by curve-fitting frequency domain admittance characteristics using the vector fitting method. The paper also introduces an approach for approximating the frequency dependent characteristic of large power networks from readily available typical power-flow data. The paper shows how the TSA algorithm can be adapted to a real-time platform. The validity of this method is confirmed with examples, including the study of a multi in-feed HVDC system based network.