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Coordinated Control of Power Systems with HVDC Links


A. Fuchs


Today's power systems are large networks of electrical energy sources and load components, connected over long distances mainly through alternating current (AC) transmission and distribution grids. This basic structure currently undergoes significant changes to ensure the stability, availability and sustainability of the future electrical energy infrastructure. As a result, power system operation and development faces several challenges, including volatile injections from distributed renewable energy sources, variable energy demands and tighter security margins.

The development of high voltage direct current (HVDC) links provides an alternative solution for the efficient and flexible transmission of electrical energy, that can support the future power system along multiple dimensions. First, during normal operation, HVDC links provide an increased controllability of the AC power system's operating point. Power system operators can use HVDC links to optimize the AC power flow in the network in order to avoid congestions and to achieve an economic gain. Secondly, during dynamic situations, HVDC links can be used by a fast automatic grid controller to support the power system's transient stability.

This thesis studies the control of HVDC injections in power systems during dynamic scenarios. Coordinated HVDC control has a large potential for the dynamic performance of power systems, for instance by increasing the damping of power oscillations, but is currently not exploited in a systematic way. The aim is to develop a framework for power system control through HVDC transmission links. Starting with results for classical AC networks, the thesis presents power system models, operation approaches and network planning methods in the context of dynamically controlled HVDC links.

The modeling of power systems with HVDC links has to incorporate several physical and operational constraints imposed by the HVDC links and the surrounding AC network. A characterization of the resulting constraints on the HVDC injections is particular important if the HVDC links are to be used for dynamic power system control. Classical capability charts of HVDC links assume a strong AC network connected to a single HVDC link with diminishing impedances in the AC transmission system. This results in simple active and reactive power bounds on the HVDC injections. In this thesis, it is shown that this concept can be generalized to constraint sets that are characterized without these simplifying assumptions. The constraint set of admissible HVDC injections is found to be convex, but may deviate strongly from the classical capability charts.

Most operation schemes for HVDC links use constant power references for the HVDC converters. The references are typically updated in hourly or 15-minute-intervals according to global load flow adjustments of the grid operator. One existing control approach for frequency oscillation damping uses local converter measurements and a linear controller to manipulate the active HVDC power transmission. This thesis presents a general strategy for a coordinated power system control using HVDC links and global power system measurements. First, all available models, constraints and objectives are summarized in a power system control problem. The corresponding optimization is then repeatedly carried out over a time horizon to determine the HVDC injections that best support the dynamic power system performance. This approach, known as Model Predictive Control (MPC), can react flexibly to disturbances and changes of the power system operating conditions. It is shown that HVDC links with a global MPC based grid controller effectively damp power system oscillations, keep the system in synchronism and accompany global power system set point changes.

The effectiveness of HVDC links for power system control depends on the location of the HVDC links in the surrounding AC network. The primary criterion for the selection of new HVDC locations concerns the mitigation of load flow congestions and the potential economic gain. In this thesis, it is shown how these aspects can be combined with a novel criterion evaluating the suitability of a given HVDC location for dynamic power system control. A performance measure is introduced to quantify the power system controllability under constraints and general disturbances. To allow the efficient evaluation, the performance measure is reformulated as a semidefinite program. The resulting placement algorithm ranks HVDC locations according to the performance improvement they bring to a given power system.

In conclusion, even power systems with a relatively small share of HVDC transmission capacity can have a large benefit from the coordinated control of the HVDC injections. The simulation studies show a significant improvement of the dynamic power system performance after disturbances, both in small benchmark systems and a large model of the European power system.


Type of Publication:

(03)Ph.D. Thesis

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% Autogenerated BibTeX entry
@PhDThesis { Xxx:2014:IFA_5007,
    author={A. Fuchs},
    title={{Coordinated Control of Power Systems with HVDC Links}},
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