Operational concepts of HVDC in the context of security assessment in the German transmission grid

1 Introduction

Increased loading of the German transmission grid caused by the regional imbalance of generation and load as well as increasing cross-border flows result in the necessity for grid expansion to maintain grid security.This expansion measures include the realization of HVDC connections parallel to the existing AC grid in Germany.Therefore,a need for reasonable operational concepts of HVDC connections will play an important role in the context of grid security aspects and the planning of remedial measures.In this article,different operational concepts for HVDC connections are evaluated under consideration of grid security aspects.Furthermore,the simulation framework as well as possible evaluation criteria for the evaluation of the different concepts are introduced.

2 Analysis

2.1 Operational Concepts

Different operational concepts vary in their complexity,beginning with the least complex one,the maximum usage of the HVDC connections.In this concept,a maximum power flow on the HVDC connections in southbound direction should be reached.Another concept,which does not necessarily require an optimization,is the voltage angle control.Depending on the voltage angles of the synchronous AC grid,the power flow of the HVDC lines adjusts.

More complex concepts like the minimization of losses or the minimization of (n-1)-warnings as well as (n-1)-indications require an optimization.In the next section,the simulation approach,which also includes the objective function to model the more complex concepts,is introduced.

2.2 Simulation Framework

Optimal operating points of HVDC converters are determined under consideration of operational limits.This is done with the help of an optimization framework,formulated as a so-called security constrained optimal power flow (SCOPF) [1].The input of the optimization framework (see Fig.1) is given by a model of the European transmission grid,a pre-calculated power plant dispatch,redispatch costs of generation units and the HVDC operational limits [2].In this framework,as described before,the different operational concepts of HVDC lines can be analyzed using respective objective functions.

At the beginning of the methodology,a power flow in normal operation as well as in contingency situations((n-1)-state) is simulated followed by the determination of sensitivities from generation units on the respective grid elements.The problem is solved using successive linear optimization.

Fig.1 Overview of Security Constraint Optimal Power Flow

As a result of the optimization,(n-1)-loading of each grid element before and after the usage of remedial measures,redispatch volumes as well as the usage of grid related remedial measures are given.These results can be used to evaluate different operational concepts.

2.3 Evaluation Criteria

Due to many stakeholder interests,a straightforward evaluation is complicated.Possible criteria can be distinguished in the categories technical,acceptance,political,transparency and performance aspects.As the focus of this article addresses grid security,the technical aspects will be evaluated.Possible technical criteria are power losses,congested energy and full load hours.Based on these evaluation criteria a simulation is performed for an entire year for each of the operational concepts is conducted.

3 Exemplary Results

The exemplary results in this article are based on two simulations each representing an entire year of operation for a future scenario in 2025.The simulations include the two operational concepts minimization of losses and minimization of (n-1)-warnings.In Fig.2 the results of one exemplary HVDC line in Germany are given by a qualitative load duration curve for each operational concept.

Fig.2 Qualitative HVDC Load Duration Curve

The results show,that the usage of the HVDC line is potentially higher under minimization of (n-1)-warnings compared to the minimization of losses.Under the minimization of losses,there are no situations without usage of the respective HVDC line.Furthermore,a combination of operational concepts could be a reasonable approach to benefit from advantages of both concepts.

In Fig.3 an excerpt of different evaluation criteria for the two operational concepts is given.Focusing on grid security aspects,minimization of (n-1)-warnings results in lower congested energy,which means less overloaded lines before usage of other remedial measures.Nevertheless,in hours without usage of the respective HVDC line to minimize (n-1)-warnings,it can be used to minimize losses for an efficient operation of HVDC lines.

Fig.3 Evaluation Method

4 Conclusion

As operational concepts of HVDC lines become more important in Germany in future,this article focuses on how to determine and evaluate different operational concepts.

In general,the determination of operational concepts highly depends on the definition of reasonable evaluation criteria.Therefore,evaluation criteria have been developed and evaluated using the above methodology.

The evaluation shows that in the context of the overall system,the concepts of reducing losses in combination with minimizing (n-1)-warnings or -indications1 as well as the sole minimization of (n-1)-warnings and -indications are the most reasonable ones.Under grid security aspects best results can be achieved by reducing operational limits and therefore,by the minimization of (n-1)-warnings.

Next to the determination of the operational concepts,further research issues result.One of them would be the question,how these operational concepts can be integrated into day-to-day and real-time transmission grid operation.