D.V. Nicolae

DC current source to voltage source CT-based converter

As generally known there is no energy transfer between a dc current and a current transformer. After a brief overview of other methods for tapping energy from the high voltage direct current transmission line, this paper presents the concept of a new dc to ac current source converter utilizing a current transformer. The advantage of this converter is that it is connected in series to the dc line and that at any moment in time the current flow is kept continuous. This paper presents the concept of the converter and simulation results.

Modeling and control of a cascaded NPC/H-Bridge inverter with LCL filter in PV- Grid application

The output voltage and current of renewable energy sources such as Photo- voltaic are not controlled. Consequently PV- Grid interface system is employed condition output voltage and current into the grid. This call for efficient and well designed control strategies to ensure reliability, stability and quality power output. This paper proposes nine- level cascaded NPC/H-bridge inverter as the PV- Grid interface. Cascaded NPC/H-Bridge inverter is modeled and a control law for realizing nine- level output with reduced harmonic content is developed. Modeling is easily achieved by considering the whole inverter as having four cascaded three level legs, two positive and two negative. Then a mathematical model is formulated for LCL filters assuming that the resistances associated with the inductances have negligible effect on the system steady state variables. Finally a control technique is developed for the whole system and its robustness tested by carrying out several simulations tests in MATLAB under different operating conditions.

Tapping power from high voltage transmission lines for the remote areas: a review of the state of the art

This paper takes an overview of various electric energy tapping methods from the high voltage transmission lines. Some of these have been studied and implemented previously, some of them are in advance stage of implementation and other new methods have just been started and they are at the initial stage of modeling and simulation. This paper briefly covers the tapping methods for ac transmission lines including two new methods that are at the final stage of research. DC transmission presents a more challenging situation, but this paper manages to present a few new methods for energy tapping

Power sensitivity and algebraic technique for evaluation of penetration level of photovoltaic on DC link of VSC HVDC transmission

With the increased advent of VSC HVDC with long DC transmission link in power systems, situations have arisen and will be even more frequent in the future, where several distributed generation will be connected on the DC-link for more power transfer capability. As penetration level increases, there is need to predict the limit before violation of voltage and power instability on the DC transmission link and ensure that it does not interfere with the main VSC HVDC system control. In this paper, power sensitivity and algebraic technique is proposed to predict the maximum DG penetration that can be accepted at a particular location on the dc link of VSC HVDC transmission system before violation of voltage and power stability.

A cascaded NPC/H-Bridge inverter with simplified control strategy and minimum component count

Multilevel topologies have emerged as a viable technique for medium and high voltage inverters but in the low voltage range they have failed to attract much interest due to their increased cost associated with the additional components and complicated control. This paper proposes a new and simplified control strategy for a nine- cascaded Neutral-Point —Clamped (NPC)/H-Bridge PWM inverter. The new control strategy is achieved by decomposing the nine level output into four separate and independent three- level NPC PWM output. By combining the three- level NPC PWM back to back using DC sources and properly phase shifting the modulating wave and carrier a simplified control strategy is achieved with reduced number of components. The control strategy is applied on cascaded NPC/H-bridge inverter that combines features from NPC inverter and cascaded H-Bridge inverter. A theoretical harmonic analysis of the proposed control is carried out using double Fourier principle. Verification of the analytical results is done using MATLAB simulation.

Boost converter with improved transfer ratio

A variation on the classic boost topology is presented. The improved transfer ratio is obtained by storing energy in parallel inductors and releasing the energy in series. State-space analysis, signal flow graph and simulation validation are presented for a double inductor topology. This analysis is then compared with the experimental results on the same double boost topology.

Study of direct energy tapping from electric field

A method of extracting small amounts of power directly from the ambient electromagnetic field around a 50 kV AC single phase line is described using a new approach to coupling capacitance. The amount of the power that may be extracted is limited. The proposed solution is vastly more cost effective than the alternatives of a transformer or a directly connected capacitive tap. The modelling and design limitations of the method are described and demonstrated on a scaled model.

The performance of distance protection relay on series compensated line under fault conditions

This paper presents the behaviour of the distance protection relays on a series compensated transmission line under fault conditions. Series compensation presents distance protection elements with unorthodox line impedance and as a result the relays may operate incorrectly. Sub-harmonic oscillations and non-linearity of the line impedance are some of the challenges introduced by series compensation of transmission lines. The study is carried out in a Digsilent power factory environment and results pertaining to the response of the distance relay on a 400 kV series compensated line under fault conditions are presented.

Development and implementation of a virtual laboratory for fundamentals on power systems

In almost any engineering department, a laboratory is an integral part of the educational process. The laboratory is a key component in the field of engineering in general, and electrical power engineering in particular. The virtual laboratory (VL) is a simulated experiment run by students on a computer station program; it can transform the conventional laboratory experiments into an interactive virtual environment, allowing simple and complex phenomena to be investigated by students just by changing the parameters and conditions of the experiments. This paper describes the development and the implementation of a VL, named VLABPOWER, for fundamentals on power systems. Mainly, electrical power engineering students will using to increase their understanding and to bridge the gap that exists between the theoretical part in the class and the practical at the traditional (real) laboratory and to better prepare before going to the traditional laboratory. The purpose of this work is to develop and implement a VL in order to enable students to have a better understanding of the experiments done in the traditional laboratory and to familiarize themselves with the different components used for those experiments. It presents a virtual interactive experiment environment that simulates the physical conditions on the transmission line. Moreover, it discusses and explains different types of short circuit faults such as symmetrical and unsymmetrical faults, modeling of different type of lines, and load condition analysis.

Stability improvement of a HVDC transmission link between weak AC systems by multi-terminal scheme

But, the problem of voltage stability for weak AC systems interconnected by a DC link is critical especially during islanding conditions. The approach to improve more on the stability of such system would be to device a means of injecting locally controlled dc power on the dc-link transmission corridor forming a radial multi-terminal HVDC. However, continuous injection of DC power on the dc line of the VSC HVDC link though will increase the power transfer capability of the system but should have a limit otherwise it will lead to instability of the system. In this paper, a detailed VSC HVDC model and a simple analytical technique using the principle of uniform loading to determine penetration limit is presented. The techniques is applied to our case study and validated with a simulation result. Critical contingencies such as sudden island conditions, three-phase to ground fault are simulated with and without DC power penetration. Results show the stability support on the AC side networks by DC power injection on the dc-link.