Michael Suriyah

Simulation Framework for DC Grid Control and ACDC Interaction Studies Based on Modular Multilevel Converters

Summary form only given. The Modular Multilevel Converter (MMC) topology is discussed intensively as a crucial part of future DC grids, especially to integrate offshore wind power. While generalized voltage source converter models are usually utilized for multiterminal investigations, conclusions on transient DC grid behavior and on ACDC interactions during distorted AC grid conditions are hard to draw or limited in their significance. In this paper, a MMC based DC grid framework with a tripartite focus on internal converter, DC and AC grid control is presented. Besides an evaluation of the implemented MMC model that incorporates the available arm sum voltage, a decoupled AC and DC current system control concept suitable to handle unbalanced voltage conditions is derived for HVDC applications. Combined with wind farm arrays, this collocation allows a meaningful examination of hybrid ACDC structures and the implemented DC grid control methodologies. To provide proof of the universal applicability of this framework, simulations are carried out on a five terminal system.

Regionalizing Input Data for Generation and Transmission Expansion Planning Models

To support decision making in the context of restructuring the power system, models are needed which allow for a regional, long-term operation and expansion planning for electricity generation and transmission. Input data for these models are needed in a high spatial and temporal granularity. In this paper, we therefore describe an approach aimed at providing regionalized input data for generation and transmission expansion planning models. We particularly focus on a dynamic assignment of renewable energy sources and electrical load to potential buses of the transmission grid. Following a bottom up approach, we model the existing and potential distributed generation and load at the lowest possible spatial resolution based on various databases and models. Besides large power plants, which are directly connected to the transmission grid, a decentralized grid connection is modeled on the distribution grid level based on Voronoi polygons around the corresponding substations. By simplifying the load flow over the distribution grid to a shortest path problem, we model the feed-in into the transmission grid as a variable, depending on the nearest available transmission grid connection. As a result, the connection to the buses at transmission grid level is kept variable in case of grid expansion measures at substation level.

Volumetric electrolyte flow rate control in vanadium redox flow batteries using a variable flow factor

In flow batteries, efficient operation is strongly related to a sophisticated volumetric flow rate control of the electrolyte. The optimal flow rate is a compromise between prevented losses caused by concentration over-potential and additional pump losses. Beside experimental approaches, model-based studies are often used for flow rate optimization. Therefore, we first present a multi-physical flow battery model which covers ohmic losses, shunt current losses, concentration over-potential and pump losses. The losses introduced by the energy conversion system for grid connection are included as well. A new method of efficiency determination is proposed, which allows for the determination of system efficiency depending on batterys state of charge and power as an alternative to the round-trip efficiency. With the SOC and power dependent efficiency, we develop an optimal flow rate control. While previous works achieved highest efficiencies with variable flow rates but constant flow factors, we propose to use a variable flow factor. It is demonstrated that this allows for a further increase in efficiency and a reduction of the required pump size. Furthermore, the reduced peak volumetric flow rate enables the use of smaller pipe diameters, which saves space and installation costs. Smaller pipe diameters will also decrease shunt current losses, which occur in the outer circuitry if two or more stacks are electrically connected in series.

On efficient computation of time constrained optimal power flow in rectangular form

In this paper, an entire formulation of a multiperiod optimal power flow is presented. The standard timeindependent AC Optimal Power Flow (AC-OPF) includes voltage, line capacity and generator limits for each time step. Additionally, time-dependent constraints like generator rampings, stored energy or energy contracts are crucial for time constrained optimization. A comparison of methods for modeling the latter and its computation efficiency is shown. Rectangular coordinates are used to minimize the computation time of the Hessian. The composition and computation of Jacobian and Hessian is described in detail. There is no conditional software or toolboxes necessary than MATLAB, as it uses a compact open source interior-point-method for solving the nonlinear optimization problem. The functionality is shown with a modified IEEE14 test case.

Characterization of a Countercurrent Injection-Based HVDC Circuit Breaker

The scope of this paper is a precise analysis of countercurrent injection-assisted HVDC circuit breaking. The considered topology consists of an outer diode rectifier for unipolar switching operations and an inner power electronic circuit breaker built using series-connected resonant circuits, which combine the Marx generator principle for voltage scaling and existing thyristor commutation circuitry for enhanced IGCT turn off. Toward the integration of an HVDC system, comparatively unique application demands such as current stresses, energy absorption characteristics, or transient voltage resilience during breaking operations require the evaluation of its fitness, including individual component rating limitations. Therefore, the characterization of the breakers switching behavior at relevant abstraction levels is undertaken within this work, giving a detailed overview of technical features and even some specific dimensioning criteria. The advantageous features of the associated proposal in terms of adaptability are investigated, and the opportunity for an increase in the breakers maximum current interruption capability is thereby outlined. General realization challenges are highlighted and verified by simulations using MATLAB Simulink and PLECS Blockset.

Short circuit detection in HVDC grids

Diverse grid structures in HVDC systems are generally possible. Though HVDC fault detection methods from overcurrent to derivative detection have recently been proposed, a broad discussion about reasonable fault localization is mostly absent. A MATLAB Simulink model with four different transmission line scenarios was developed for application-oriented considerations of short circuit fault detection and resulting tripping orders. Simulations presenting peak derivative value curves over distance and the influence of branching points support discussion.

Active DC fault management of a bipolar full-bridge MMC-HVDC scheme with metallic return

Besides offshore applications, MMC-HVDC is discussed for embedded onshore structures. In contrast to their maritime counterparts, these systems shall be carried out with overhead transmission lines to minimize investment costs. To avoid a full outage in case of a dc fault, bipolar topologies seem advantageous considering reliability aspects. While ground currents during asymmetric operation are unwanted, an additional return path needs to be introduced. This paper presents an active clearing sequence of a bipolar full-bridge MMC-HVDC system with metallic return. Subsequent to a pole to ground fault, detection, short term asymmetric operation and full system restoration are investigated. If blocking of the IGBT modules can be avoided to maintain the converters in a controllable mode, the midpoint voltage shift at the ungrounded terminal has to be explicitly considered in system controls. Transient simulations validate the developed methodology.

HVDC grid protection with integrated fault categorization for selective tripping

HVDC Circuit Breakers require adequate fault detection for selective breaking. In terms of tripping commands, general fault categorization is necessary and, where appropriate, even short circuit localization. Wave propagation phenomena from switching operations, blocking MMCs, lightning strikes and short circuits must be clearly distinguished, which is fundamentally examined here. Analyses have been undertaken by accompanied simulations in MATLAB Simulink.

High Power DC Chargers with Extended Negative-Sequence Current Control for Load Balancing in Low-Voltage Networks

Public three phase low-voltage networks are capable of feeding high power DC chargers. For charger installation in remote network locations, active front-end (AFE) rectifiers using voltage source converters ensure network power quality. A modular parallel current control decoupled by a multiple complex-coefficient filter is proposed to achieve low negative-sequence and harmonic current emission. Furthermore, the AFEs capability of compensating network load unbalance is discussed. As unbalance compensation significantly increases DC-voltage ripple, the proposed current control is extended by a parallel third harmonic current control module to optimize DC-capacitor dimensioning. Simulation and experimental tests validate the proposed current control concept.

Power tracking in a MMC-multi-terminal HVDC system with centralized and decentralized MPC using a black box modeling approach

This paper introduces and compares a decentralized and centralized model predictive control scheme for power tracking in a multi-terminal HVDC system based on MMCs. The MPC scheme requires a prediction, which is implemented by treating the HVDC system as a black box. With this approach it is possible to use an MPC scheme without detailed knowledge about the controlled system, which eases the design of the controller and allows using this method on more complex systems. While a centralized MPC achieves a better performance, a decentralized MPC is more practical, since it does not require communication between the terminals. This paper investigates the performance loss of decentralized MPCs compared to a centralized MPC and introduces a new method to increase the performance of decentralized MPCs by iteratively minimizing undesired control interactions. The proposed method is not limited to HVDC systems.