Dragan Jovcic

Phase locked loop system for FACTS

This research addresses the special requirements of phase locked loops (PLLs) for a typical application with FACTS elements. A new PLL system that uses adaptation algorithms is developed with the aim of improving speed of responses, robustness to AC voltage depressions, and harmonic rejection. The adaptive PLL consists of the three control units that individually control frequency, phase angle, and voltage magnitude. The voltage controller output is used to compensate for reduced gain caused by the AC voltage magnitude depressions. The output phase angle and its derivative, the frequency signal, are controlled in two independent control systems in order to enable elimination of frequency and phase error without compromising transient responses. The simulation results are compared with a PLL available with the PSB MATLAB block-set and noticeable improvements are demonstrated. In particular, settling time and overshooting are significantly lower with conditions of reduced AC voltage magnitude.

Analysis and Design of an Offshore Wind Farm Using a MV DC Grid

This paper outlines the design of an offshore wind farm using a dc offshore grid based on resonant dc-dc converters. Multiphase resonant dc-dc converters are studied to step up the dc voltage from individual wind generators to a Medium Voltage (MV) dc bus, and from the MV bus to an HVDC line that will connect the wind farm to shore. Compared to an equivalent ac grid-based wind farm, a dc grid-based wind farm has slightly higher losses, but the weight of the magnetic components and cables is substantially lower. The analysis of operating permanent-magnet synchronous generators at variable and constant dc voltages shows that a fixed dc voltage has marginally higher efficiency than a variable dc voltage. However, using a variable dc voltage gives lower harmonics at the generator facing the voltage-source converter and the dc-dc step-up converter. An aggregated model of multiple parallel connected wind generators is developed and shown to accurately approximate a detailed PSCAD model during varying wind conditions and transients.

Bidirectional, High-Power DC Transformer

This paper studies a bidirectional dc-dc converter concept which is capable of achieving very high stepping ratios with MW level power transfers. The converter can find potential application in connecting high-power dc sources, interfacing to high-voltage dc transmission or to flexible ac transmission system elements. The converter is based on two resonant circuits which share a common ac capacitor. The topology is simple and utilizes thyristors with potentially all soft switchings. Complete analytical modelling is presented which enables systematic design procedure for the converter. The detailed digital simulation on PSCAD platform confirms satisfactory operation on a 5-MW test system, which connects 4-kV dc source to an 80-kV high voltage dc grid. The converter shows good responses to rapid changes in power magnitude/direction and it is concluded that robustness to terminal voltage disturbances is excellent. Since the highest-power phase-control thyristors are employed, the converter can potentially be used at much higher power levels. The passive components are of reasonable size.

Feasibility of DC transmission networks

This paper examines the current status of technology and discusses technical options for developing DC transmission grids. The fast advances in VSC HVDC, the recent offshore VSC projects, the experience with multiterminal HVDC and recent development of fast DC circuit breakers bring large meshed DC grids closer to reality. The most important and most difficult remaining technical challenge is the system level protection of DC grids. The article further discusses some of the ongoing research directions like the use of travelling wave detection for fast protection or deployment of DC/DC converters for isolation of DC faults. One of the main work packages in EU funded Twenties project studies the major prerequisites for operation of DC grids. This project has delivered some major studies of DC grids and two hardware demonstration systems are under development: a mock-up DC grid at University of Lille and fast DC Circuit Breaker at ALSTOM.

Interconnecting offshore wind farms using multiterminal VSC-based HVDC

This paper presents a large off shore wind farm interconnected to the grid using a multiterminal HVDC link. The 200 W wind farm consists of 100 individual 2 MW turbines connected using 25 VSC (voltage source converter) converters to a common DC bus. The transmission system converters enable variable speed operation and therefore additional converters are not needed with individual generators, implying savings in converter costs. The paper presents PSCAD simulation of the proposed concept for various changes in wind speeds. The results confirm the ability to operate at optimum coefficient of performance and no synchronization problems occur even for severe wind speed changes. Further tests with faults on AC grid demonstrate satisfactory recoveries. The proposed concept may enable integration of large offshore wind farms at considerable distances, and using optimal number of converters

Analytical modeling of TCSC dynamics

This paper presents an analytical, linear, state-space model of a thyristor-controlled series capacitor (TCSC). First, a simplified fundamental frequency model of TCSC is proposed and the model results are verified. Using frequency response of the nonlinear TCSC segment, a simplified nonlinear state-space model is derived, where the frequency of the dominant TCSC complex poles shows linear dependence on the firing angle. The nonlinear element is linearised and linked with the ac network model and the TCSC controller model that also includes a phase-locked-loop (PLL) model. The model is implemented in MATLAB and verified against PSCAD/EMTDC in the time and frequency domains for a range of operating conditions. The model is sufficiently accurate for most control design applications and practical stability issues in the subsynchronous range.

Stability of a Variable-Speed Permanent Magnet Wind Generator With Weak AC Grids

The operation of high-power wind generators with weakened ac grids has historically been difficult because of stability and power quality issues. This paper presents an analytical stability study of a variable-speed directly-driven permanently-excited 2-MW wind generator connected to ac grids of widely varying strength and very weak grids. The generator includes two back-to-back full-scale vector controlled 3-level neutral-point-clamped (NPC) voltage-source-converters (VSC). A 47th order small-signal analytical wind generator model is developed within MatLab, and a summary of the model structure and controls is given. Model verification is demonstrated for fast and slow system variables employing detailed simulation software PSCAD/EMTDC. An eigenvalue stability study for weak ac networks is presented, and qualitative conclusions about inherent system dynamics and stability characteristics are given. These insights are employed to study the design of an ac voltage controller for weak ac networks. Two alternative controller designs are studied for their potential to enhance system robustness to changes in ac grid strength. Testing on the detailed simulator PSCAD/EMTDC is employed throughout to confirm conclusions from analytical studies.

VSC transmission model for analytical studies

This paper presents a model of voltage source converters (VSC) based transmission that can be used for wide range of stability studies and controller design with MATLAB software. The model is intended for analytical studies in the frequency domain above 5 Hz. The system model has modular structure and includes three separate units in state-space form: AC rectifier model, AC inverter and DC system model. The converters, controllers and phased locked loop (PLL) units are included in the DC model and the model is linked with other units using input-output matrices in the state-space models. The converter non-linear model is linearized assuming small deviations around the steady-state. The model is tested against the PSCAD/EMTDC non-linear model and the results show satisfactory accuracy. The tests also confirm that the analytical model well represents multivariable system properties. An example of the model used is given showing the eigenvalue study of the influence of PLL gains on the system stability. It is concluded that increased PLL gains deteriorate system stability.

Developing DC Transmission Networks Using DC Transformers

This paper studies principles of developing dc transmission grids based on high power dc/dc converters. There has been much research on dc/dc converters and it is likely that some megawatt size units will achieve commercialisation stage soon. In this study, we assume that electronic dc transformers can achieve three functions: 1) voltage stepping, 2) voltage (or power) regulation, and 3) fault isolation. The location, sizing, and control of dc transformers is first analyzed using a simple 4-terminal, 1.8 GW dc grid. It is postulated that this grid would be a better alternative to a point-to-point HVDC. Detailed simulations on PSCAD/EMTDC demonstrate the capability to independently regulate power flow in each dc branch. The simulations of worst case faults on dc lines and ac grids show that dc transformers can isolate the faulted segments enabling the remaining part of the grid to operate normally. The generic principles of developing more complex dc grids with meshed power flows, are also presented. It is concluded that there are no significant technical barriers in developing dc transmission grids but the cost and losses of dc transformers remain as the primary challenges.

Analytical modelling of HVDC-HVAC systems

A new HVDC-HVAC analytical model is presented in this paper. The model comprises three subsystems: AC system, PLL and DC system. The model is structured in such a manner to enable small signal analysis of HVDC-HVAC interactions and possible problems arising from these interactions, CIGRE HVDC Benchmark model is used as a test system. Model verification, performed using PSCAD/EMTDC simulations, showed good response matching for all DC and AC system variables. As an example of application of this model the influence PLL dynamics on the system stability is studied offering the important rules for tuning of PLL gains.