Khadija Ben Kilani

Synchronverter-Based Emulation and Control of HVDC Transmission

This paper presents a new control strategy for high voltage direct current (HVDC) transmission based on the synchronverter concept: the sending-end rectifier controls emulate a synchronous motor (SM), and the receiving end inverter emulates a synchronous generator (SG). The two converters connected with a DC line provide what is called a synchronverter HVDC (SHVDC). The structure of the SHVDC is firstly analyzed. It is shown that the droop and voltage regulations included in the SHVDC structure are necessary and sufficient to well define the behavior of SHVDC. The standard parameters of the SG cannot be directly used for this structure. A specific tuning method of these parameters is proposed in order to satisfy the usual HVDC control requirements. The new tuning method is compared with the standard vector control in terms of local performances and fault critical clearing time (CCT) in the neighboring zone of the link. The test network is a 4-machine power system with parallel HVDC/AC transmission. The results indicate the contribution of the proposed controller to enhance the stability margin of the neighbor AC zone of the link.

Improvement of transient stability in an AC/DC system with synchronverter based HVDC

The HVDC emulation by the synchronverter concept is investigated in a realistic power system. A specific tuning method for the parameters of the regulators based on the sensitivity of the poles of the neighbor zone of the HVDC with the respect to the latter parameters is used. As consequence, not only the local performances of the HVDC link, but also overall transient stability of the AC zone in which the HVDC is inserted are improved. Extensive tests are provided using Matlab/Simulink implementation of the IEEE 9 bus/3 machines test system.

Improvement of fault critical time by HVDC transmission

This paper investigates the impact of High Voltage Direct Current (HVDC) transmission on the transient stability of a two-machine power system, considering three transmission line configurations: parallel HVAC-HVAC, parallel HVDC-HVDC, and a hybrid HVAC-HVDC operation. The faults are balanced three-phase short-circuits in AC lines, and single phase faults on DC lines, applied in the mid-point of the interconnection. For each configuration, transient stability of the AC systems is assessed in terms of the fault critical clearing time (CCT), and for different DC power levels. The results indicate the contribution of HVDC transmission in increasing the critical clearing time; and therefore enhancing the systems stability margin and operational security.

Primary reserves managment in power systems

This paper deals with the impact of primary reserves management on the performance of frequency control. A parametric study is presented, investigating various primary reserves management strategies, and their impact on the performance of the primary control. Emphasis is on major emergency system faults. Methodologically, many study cases are defined in terms of allocated reserves volume, committed generators characteristics and governor controls. The study is conducted on a simplified model of the Tunisian transmission network. Loss of generation contingencies are simulated for the defined primary reserves settings and the results are compared in terms of frequency deviation critical values, leading to eventual load shedding. The results are discussed and recommendations are made to improve the operational security of the system.

Virtual synchronous generators dynamic performances

In this paper, dynamic performances of virtual synchronous generator are investigated. The virtual synchronous generator is based on the synchronverter (VSG) concept which is an inverter that mimics a synchronous generator (SG). The real and reactive power delivered by VSGs connected in parallel and operated as generators can be automatically shared using the well-known frequency and voltage drooping mechanisms. The VSG method is tested in comparison with the standard SG of the same parameters and capacity on a two bus test network. A comparative assessment is presented in terms of local performances and the fault critical clearing time (CCT). The results indicate good local performances and the contribution of the synchronverter to enhance the stability margin of the neighbouring AC zone.

Enhancement of voltage stability in ultra high voltage electric network by static VAR compensation

This paper investigates the impact of Ultra High Voltage (UHV) transmission on the voltage stability of electric networks, and proposes remedial solutions. The proposed solution is based on using Flexible AC Transmission Technology (FACTS). A Static Var Compensator (SVC) is chosen as a low cost solution to replace a conventional capacitor bank, thus allowing a continuous and flexible nodal voltage adjustment and improve voltage control. The study compares the proposed FACTS to traditional compensators terms of voltage stability dynamic performances of the electric network.

Bifurcation in power systems distribution networks integrating variable speed wind generation

This paper investigates the dynamic stability of a power system distribution network integrating variable speed wind generation based on a bifurcation analysis. A generalized model of the system is developed taking into account multiple wind farms, transformer branches and composite dynamic load model. Bifurcations may be analyzed by a systematic testing of the complete system jacobian matrix. Considering the load voltage, frequency and voltage gradient dependency, three models are studied. Factors of impact are: the generator input mechanical power, the loading factor and the reactive loading. It is shown that generator and load dynamic interaction may trigger nonlinear phenomena, such as voltage oscillations and the appearance of bifurcations.

Static analysis of islanded micro grids: Focus on renewable fraction and cost of energy

The aim of this work is to explore the static conduct of an islanded micorgrid. A technical economic study is performed. Many parameters and aspects such as renewable resources availability, fuel price, microgrid components, and dispatch strategies are examined to see their effect on microgrid operation and cost effectiveness. The study is performed on Desertores Island microgrid example. A comparison is made also with Tunisia using the same system but with Tunisian climate data and fuel price. HOMER (Hybrid Optimization of Multiple Energy Resources) software is very useful to perform such study.

Simplified modelling of wind farms for voltage dip transients

This paper addresses the issue of simplified modelling of large scale wind farms and its impact on the accuracy of dynamic systems responses to grid transient faults. We propose a simplified model consisting of a detailed wind turbine model with scaled-up outputs, versus the conventional aggregate representation of wind farms. The simplification methods are compared for their suitability to grid voltage dip transients of increased duration. The methods are applied to a wind farm split into three coherent clusters. The results show that the aggregate machine model is suitable for static and short transient grid faults, but limited in voltage ride through studies of grid faults of longer clearing time.

Detection of inter area oscillations in the Tunisian interconnected power system using phasor measurement units

The Tunisian power system rely more on tie line power interchange. Unstable inter-area oscillations restrict power transfer between areas and may endanger the system security. In this paper we perform a study on the oscillation phenomena in the interconnected Tunisian power system and their detection by the global positioning system. The detection of the frequency is based on the equivalent relation machine rotor speed- terminal voltage angle. The placement of the phasor measurement units (PMUs) is optimized by the Recursive Security N Algorithm and several power exchanges events are simulated. The results of this paper can help solve some of the existing interconnexion problems and demonstrate how to improve the operation of the overall system behaviour by phasor measurement technology.