Willem Leterme

Nonunit Protection of HVDC Grids With Inductive DC Cable Termination

This paper deals with nonunit protection of HVDC grids by proposing a set of parameters that characterizes the open protection zones together with an efficient method to determine the thresholds on these parameters. Selective HVDC grid protection schemes must detect and discriminate faults within the first milliseconds of the fault transient and consequently differ considerably from existing ac protection schemes. Due to the accompanying speed requirement, primary protection is expected to be based on open protection zones as communication delay impedes fast operation. In this paper, the principles of the nonunit protection scheme are developed based on reflection of a traveling wave at an inductive termination. Next, the method to obtain the protection scheme thresholds is elaborated. The method accurately calculates the thresholds for HVDC grids with an arbitrary topology. A sensitivity analysis of these thresholds toward grid and fault parameters demonstrates the applicability of the proposed protection scheme in cable-based HVDC grids with inductive cable termination. The results obtained with the reduced grid model are validated by comparison against simulations using a detailed model implemented in PSCAD.

Overview of Grounding and Configuration Options for Meshed HVDC Grids

This paper provides an overview and comparison of the possible grounding and configuration options for meshed HVDC grids. HVDC grids are expected to play a key role in the development of future power systems. Nevertheless, the type of grounding and the base configuration for the grid have not yet been determined. Various studies related to multiterminal HVDC or meshed HVDC grids often assume one specific configuration and grounding scheme and take it for granted. However, since a large number of options exist, an overview is needed to balance the pros and cons. In this paper, the influence of the different grounding options on fault behavior is investigated for point-to-point connections. Furthermore, the impact of the grounding type on the system fault behavior is investigated with electromagnetic transient simulations. Next, the suitability of a configuration to serve as a base configuration in a meshed dc grid is investigated and compared in terms of extensibility and flexibility. In this evaluation, the grounding type, the number, and location of grounding points in a grid are considered as well. Finally, an overview of the most important conclusions is given in a summarizing table.

Demand side management of electric vehicles with uncertainty on arrival and departure times

Uncertainty on arrival and departure times makes the scheduling of plug-in hybrid electric vehicles an intrinsically stochastic optimization problem. To take the stochastic nature of this problem into consideration, a scalable stochastic optimization strategy has been formulated. Generally, stochastic programming methods are computationally demanding and become impractical for large-scale problems. This work reduced the dimensionality of the scheduling problem with techniques from approximate dynamic programming. To illustrate the advantage of the stochastic algorithm a deterministic method has been formulated. Compared to the deterministic method, the proposed stochastic method can help an aggregator to reduce its expensive peak charging or avoid penalties for not fully charging the batteries of its clients.

A Flexible Stochastic Optimization Method for Wind Power Balancing With PHEVs

This paper proposes a flexible optimization method, based on state of the art algorithms, for the smart control of plug-in hybrid electric vehicles (PHEVs) to balance wind power production. The problem is approached from the perspective of a balance responsible party (BRP) with a large share of wind power in its portfolio. The BRP uses controllable PHEVs to minimize the imbalance of its portfolio resulting from wind power forecast errors. A Markov Decision Process (MDP) formulation in combination with dynamic programming is used to solve the multistage stochastic problem. The main difficulty for applying MDPs to this problem is to efficiently include time interdependence of the wind power forecast error. In the presented approach, the probability distribution and time interdependence of the forecast error are represented by a scenario tree. Because of the MDP formulation, the algorithm is adaptable to deal with different transition models and constraints. This feature enables to use the algorithm in a dynamic environment such as the future smart grid. To demonstrate this, a generic charging model for PHEVs is used in the BRP wind balancing case. The flexibility of the algorithm is shown by investigating the solution for different degrees of complexity in the charging model.

Configurations and earthing of HVDC grids

HVDC grids are considered to be essential building blocks for the future upgrade of the existing AC power system and as a means to transport the expected massive amounts of renewable energy from remote sources to the load centers. HVDC systems exist for over 50 years, yet meshed DC grids do not exist so far. For point-to-point HVDC connections, there is a certain freedom in choosing the configuration and earthing scheme. For a grid, different converter arrangements and earthing schemes can be considered. The choices made will influence how the grid will look like, the components in the grid and their rating, the operating principles, the protection philosophy, the degree to which the grid is extensible and the overall reliability and inherent redundancy. Clearly, it will influence investment and operating costs as well. This paper provides an qualitative overview of potential grid configurations for DC grids (symmetrical monopole, asymmetrical monopole, bipolar schemes, with and without metallic return and combined systems). The possible earthing options for a meshed HVDC grid are part of this discussion. The extensibility and reliability of the HVDC grid are specifically dealt with.

A Local Backup Protection Algorithm for HVDC Grids

DC faults in HVDC grids lead to quickly increasing currents which should be interrupted sufficiently fast to prevent damage to power-electronics components. Although several primary relaying algorithms for HVDC grids have been proposed, fast backup relaying algorithms are needed to ensure system reliability when primary protection fails. This paper proposes a local backup relaying algorithm for HVDC grids, which leads to a short delay between primary and backup protective actions. The proposed algorithm, consisting of breaker and relay failure subsystems, uses classifiers which detect primary protection malfunctions based on the voltage and current waveforms associated with dc breaker operation. The algorithm initiates the detection of uncleared faults during primary protection operation, which results in accelerated actions by the backup protection after primary protection failure. The proposed algorithm is applied to a four-terminal HVDC grid. Study results show that the proposed algorithm accurately detects uncleared faults, identifies the source of primary protection malfunction, and expedites backup protective actions by operating during the fault current interruption interval of the primary protection.

Stochastic portfolio management of an electric vehicles aggregator under price uncertainty

This paper considers the portfolio management problem of a flexibility aggregator under uncertainty on real-time prices. Solving this stochastic optimal control problem in a reasonable time, considering overall scalability, comfort settings and grid constraints, is a challenging task. This paper tackles these problems by making use of a Three-Step Approach (TSA). Two control approaches are considered in the second step of the TSA: Model Predictive Control (MPC) and Approximate Dynamic Programming (ADP). The performance of both controllers for different temporal autocorrelated price profiles is illustrated for an aggregator with a fleet of 1000 electric vehicles. The simulations show that the TSA extended with a stochastic controller can reduce the cost of the aggregator compared to a certainty equivalent approach. The paper concludes by discussing the strength and weaknesses of MPC and ADP in a smart grid setting.

Equivalent circuit for half-bridge MMC dc fault current contribution

The modular multilevel converter (MMC) is currently the preferred converter topology for HVDC point-to-point links and the likely choice for future meshed HVDC grids. For breaker dimensioning or protection system design, thorough knowledge of the dc fault currents supplied by these converters is required. In this paper, the dc fault current supplied by the half-bridge MMC is analyzed and an equivalent circuit model is proposed. The proposed equivalent circuit has a low complexity and accurately models the MMCs contribution to dc faults. Furthermore, the paper proposes guidelines for the values of the equivalent circuits parameters. Study results show that a high level of accuracy is achieved for a wide variation in converter and fault parameters.

Generic circuit partitioning method for efficient simulation of modular multilevel converter topologies

A generic circuit partitioning method is proposed which reorders the equations and variables of power electronic circuits with idealized switches and diodes in a specific bordered block-triangular form, where each block on the diagonal is dependent only on a small number of switches. The number of floating-point operations for sparse matrix triangularization, needed after a change of switch configuration, is reduced significantly by using the possibility to individually store the triangular LU factors of the diagonal blocks in memory for each possible switch configuration. The method is applicable to any power electronic circuit, but is especially beneficial for electromagnetic transient simulation involving modular multilevel converter (MMC) HVDC topologies where the number of switching components is usually very large.