Flatness-based feed-forward control of an HVDC power transmission network

Abstract-An efficient and well-established technology for power transmission across long distances is high voltage direct current transmission (HVDC). However, HVDC is up to now almost completely limited to peer-to-peer connections or networks with peers situated closely to each other. This contribution introduces the flatness-based design of a feedforward control of tree-like, i.e. cycle-free, HVDC transmission networks comprising two or more converter stations. The resulting control concept allows a flexible determination of the power distribution within the network. Furthermore, effects like power losses and delays due to wave propagation, which are related especially to long transmission lines, can be easily taken into account. Numerical simulations for an example network are included to prove the value of the results

The transition towards a sustainable energy system in Europe: What role can North Africa's solar resources play?

Securing energy supply and speeding up the transition towards a reliable, sustainable, low-carbon energy system are among the major current and future challenges facing Europe. Importing dispatchable solar electricity from North Africa is considered as a potential and attractive option. Nevertheless, as things currently stand, the European Commission focuses mainly on the exploitation of the existing wind power potential in the North Sea, largely ignoring the solar power potential in the Sahara region of North Africa. After discussing the major challenges and issues facing Europe to achieve the assigned ambitious objectives, the paper emphasises the importance of North Africa's solar resources in helping Europe to successfully address the challenge of decarbonising its electricity system, in particular with regards to the security of supply and sustainability. Within these two major challenges, the paper explores the issues of access, barriers and opportunities. The paper highlights why the EU’s energy and climate goals will not be achievable without adequate grid expansion and grid-scale energy storage facilities, as well as other innovative measures to manage demand and ensure a secure energy supply. In this respect, the paper shows how the import of dispatchable electricity from North Africa via specific HVDC links could play a key role in helping the EU achieve its energy targets in a cost effective way without recourse to significant investments in transmission infrastructure and storage facilities. The paper then attempts to identify and analyze the main barriers that continue to inhibit the export of solar electricity from North Africa to Europe. Finally, to make the project more attractive and achievable in the near future, the paper proposes a systematic approach for setting up energy import scenarios. A promising import scenario is presented where energy import via Italy is shown to be a more viable and effective solution than via Spain.Peer reviewe

Modular multilevel converter losses model for HVdc applications

Multi-terminal high voltage dc (HVdc) grids can eventually became a feasible solution to transport energy to remote and/ or distant areas and its exploitation depend, among other things, on the performance of the converter terminals. Therefore, to optimize the power transmission strategy along such a grid, it is necessary to recognize the efficiency of all the converters in all points of operation, namely with the different load conditions. In this vision, the aim of this work is to provide the methodology to model the modular multilevel converter (MMC) efficiency by means of a mathematical expression that can describe, over a broad range of active and reactive power flow combinations, the power losses generated by the semiconductors. According to the presented methodology, a polynomial-based model with a reduced number of coefficients is deducted, in such a way that can be directly used for optimal power flow (OPF) studies. The accuracy of the proposed model is characterized by an absolute relative error, at the worst scenario, approximately equal to 3%.Postprint (author's final draft

Power system security enhancement by HVDC links using a closed-loop emergency control

In recent years, guaranteeing that large-scale interconnected systems operate safely, stably and economically has become a major and emergency issue. A number of high profile blackouts caused by cascading outages have focused attention on this issue. Embedded HVDC (High Voltage Direct Current) links within a larger AC power system are known to act as a “firewall” against cascading disturbances and therefore, can effectively contribute in preventing blackouts. A good example is the 2003 blackout in USA and Canada, where the Québec grid was not affected due to its HVDC interconnection. In the literature, many works have studied the impact of HVDC on the power system stability, but very few examples exist in the area of its impact on the system security. This paper presents a control strategy for HVDC systems to increase their contribution to system security. A real-time closed-loop control scheme is used to modulate the DC power of HVDC links to alleviate AC system overloads and improve system security. Simulations carried out on a simplified model of the Hydro-Québec network show that the proposed method works well and can greatly improve system security during emergency situations.Peer reviewedFinal Accepted Versio

Fully Decou pled Controller Models for Voltage Source Converter based High Voltage DC Transmission

VSC-HVDC has two distinct advantages over its earlier generation thyristor based High Voltage DC transmission. Synchronous voltage source is not required to commutate against, for its operation and it does not suffer from commutation failures under adverse conditions in interfacing ac system. These two properties make it amenable to wider application areas. To make it adapt to operational conditions imposed on it in various applications, its controller parameters need to be assessed and tuned through extensive simulation studies. To facilitate this, two alternative controllers viz. a fully decoupled controller model and also an instantaneous theory based fully decoupled hybrid controller model are developed in the thesis. The decoupling is achieved by exploiting similarity transformation in both the controllers. In the first controller model, the Park's currents and voltages are directly obtained from the measured network variables and the reference park's currents for the inner current loop are obtained from the instantaneous measured power. In the second one, both the feedback as well as reference Park's currents for the inner current loop are obtained from the Clarke's variables. An AC system interfacing electronics based power transmission or distribution network experiences non-sinusoidal voltage and current waveforms. Instantaneous power theory being suitable for steady as well as transient states, is used for handling measured inputs. The performance of the models is assessed through SIMULINK Power system Blockset aided simulations on a VSC-HVDC link interfacing an ac system, having normal fault level, low fault level and also witnessing a single line to ground fault on its rectifier transformer primary side

Short circuit analysis of an offshore AC network having multiple grid forming VSC-HVDC links

This article presents the short circuit analysis of an offshore AC network which consists of wind power plants interconnected using HVAC cables. The power generated in the offshore AC network is transmitted to several onshore grids using VSC-HVDC system. The offshore AC network is formed by the VSC-HVDC systems using frequency and voltage droop control. A coordinated control scheme is proposed for wind turbines and offshore VSCs during short circuit conditions in the offshore grid to ensure fault ride through (FRT) without compromising the system stability. The theoretical analysis used for developing this control scheme allows to calculate the system limits taking into consideration the active and reactive power capability. In order to verify the proposed control scheme, three phase symmetric faults have been applied on a wind turbine busbar, HVAC busbar, and at the AC cable that interconnects the VSC-HVDC system. Additionally, a frequency coordination control scheme without communication between wind power generation and VSC-HVDC system has been proposed. The methodology and control system have been validated by performing a nonlinear simulation.Postprint (author's final draft

OFFSHORE WIND ENERGY STUDY AND ITS IMPACT ON SOUTH CAROLINA TRANSMISSION SYSTEM

As one of the renewable resources, wind energy is developing dramatically in last ten years. Offshore wind energy, with more stable speed and less environmental impact than onshore wind, will be the direction of large scale wind industry. Large scale wind farm penetration affects power system operation, planning and control. Studies concerning type III turbine based wind farm integration problems such as wind intermittency, harmonics, low voltage ride through capability have made great progress. However, there are few investigations concerning switching transient impacts of large scale type III turbine based offshore wind farm in transmission systems. This topic will gain more attention as type III wind generator based offshore wind farm capacity is increasing, and most of these large scale offshore wind farms are injected into transmission system. As expected to take one third of the whole wind energy by 2030, the large offshore wind energy need to be thoroughly studied before its integration particularly the switching transient impacts of offshore wind farms. In this dissertation, steady state impact of large scale offshore wind farms on South Carolina transmission system is studied using PSSE software for the first time. At the same time, the offshore wind farm configuration is designed; SC transmission system thermal and voltage limitation are studied with different amount of wind energy injection. The best recommendation is given for the location of wind power injection buses. Switching transient also impacts is also studied in using actual South Carolina transmission system. The equivalent wind farm model for switching transient is developed in PSCAD software and different level of wind farm penetration evaluates the transient performance of the system. A new mathematical method is developed to determine switching transient impact of offshore wind farm into system with less calculation time. This method is based on the frequency domain impedance model. Both machine part and control part are included in this model which makes this representation unique. The new method is compared with a well-established PSCAD method for steady state and transient responses. With this method, the DFIG impact on system transients can be studied without using time-domain simulations, which gives a better understanding of the transient behaviors and parameters involved in them. Additionally, for large scale offshore wind energy, a critical problem is how to transmit large offshore wind energy from the ocean efficiently and ecumenically. The evaluation of different offshore wind farm transmission system such as HVAC and HVDC is investigated in the last chapter