The modular multilevel converter (MMC) has been a subject of increasing importance for medium/high-power energy conversion systems. Over the past few years, significant research has been done to address the technical challenges associated with the operation and control of the MMC. In this paper, a general overview of the basics of operation of the MMC along with its control challenges are discussed, and a review of state-of-the-art control strategies and trends is presented. Finally, the applications of the MMC and their challenges are highlighted.

Operation, Control, and Applications of the Modular Multilevel Converter: A Review

The paper reviews different approaches, technologies, and strategies to manage large-scale schemes of variable renewable electricity such as solar and wind power. We consider both supply and demand side measures. In addition to presenting energy system flexibility measures, their importance to renewable electricity is discussed. The flexibility measures available range from traditional ones such as grid extension or pumped hydro storage to more advanced strategies such as demand side management and demand side linked approaches, e.g. the use of electric vehicles for storing excess electricity, but also providing grid support services. Advanced batteries may offer new solutions in the future, though the high costs associated with batteries may restrict their use to smaller scale applications. Different “P2Y”-type of strategies, where P stands for surplus renewable power and Y for the energy form or energy service to which this excess in converted to, e.g. thermal energy, hydrogen, gas or mobility are receiving much attention as potential flexibility solutions, making use of the energy system as a whole. To “functionalize” or to assess the value of the various energy system flexibility measures, these need often be put into an electricity/energy market or utility service context. Summarizing, the outlook for managing large amounts of RE power in terms of options available seems to be promising.

https://research.aalto.fi/files/6557604/lund_et_al_review.pdf

Review of energy system flexibility measures to enable high levels of variable renewable electricity

Levelized costs of electricity (LCOE) are a common metric for comparing power generating technologies. However, there is qualified criticism particularly towards evaluating variable renewables like wind and solar power based on LCOE because it ignores integration costs that occur at the system level. In this paper we propose a new measure System LCOE as the sum of generation and integration costs per unit of VRE. For this purpose we develop a conclusive definition of integration costs. Furthermore we decompose integration costs into different cost components and draw conclusions for integration options like transmission grids and energy storage. System LCOE are quantified from a power system model and a literature review. We find that at moderate wind shares (~20%) integration costs can be in the same range as generation costs of wind power and conventional plants. Integration costs further increase with growing wind shares. We conclude that integration costs can become an economic barrier to deploying VRE at high shares. This implies that an economic evaluation of VRE must not neglect integration costs. A pure LCOE comparison would significantly underestimate the costs of VRE at high shares. System LCOE give a framework of how to consistently account for integration costs and thus guide policy makers and system planers in designing a cost-efficient power system.

System LCOE: What are the Costs of Variable Renewables?

http://www.mcc-berlin.net/fileadmin/data/pdf/Publikationen/Ueckerdt_Hirth_Luderer_Edenhofer_System_LCOE_2013.pdf

System LCOE: What are the costs of variable renewables?

Different HVDC grid types and the respective protection options are discussed. An earthed bipole HVDC grid was modeled in PSCAD, and using simulation results, the necessity of di/dt limiting inductors to contain the rise of fault currents within the capacity of current hybrid dc breakers is demonstrated. The impact of different inductor sizes on current rise was studied. A fault detection and localization scheme using the rate of change of voltage measured at the line side of the di/dt limiting reactors is proposed. The protection system was modeled and tested under different fault types and locations. The results show that the proposed method of HVDC grid protection is feasible using the current hybrid dc breaker technology. A systematic procedure for setting the necessary protection threshold values is also demonstrated.

Fault Detection and Interruption in an Earthed HVDC Grid Using ROCOV and Hybrid DC Breakers

This paper investigates the influence of dc network topology with respect to two important aspects: the steady-state losses and the transient fault currents. For this, optimal power flow in a combined ac-dc multiterminal HVDC cable system based on voltage-source converters and the development of prospective fault currents during a pole-to-ground fault is simulated. The results prove that an evaluation of grid topologies must include both of the aforementioned aspects and that no network topology can optimize both at the same time.

/pdf/multiterminal-hvdc-networks-what-is-the-preferred-topology-1bwgru7okh.pdf

Multiterminal HVDC Networks—What is the Preferred Topology?

This paper presents important results of the European HVDC Study Group founded on an initiative by the German Commission for Electrical, Electronic and Information Technologies in September 2010. The main task of the Study Group has been defining “Technical Guidelines for first HVDC Grids”: 1) to gain a common understanding of basic operating and design principles of HVDC grid systems, and 2) to prepare the ground for more detailed standardization work. The HVDC systems are moving from the stage of point-to-point (PTP) connectors to the stage of the transmission systems interconnecting more than two stations and utilizing multiterminal (MT), multivendor (MTMV) HVDC voltage-source converter (VSC) systems. The development will be modular, for example, in steps, starting with interconnecting PTP connectors and establishing three-pod, radial systems. Then, such simple systems may gradually be interconnected into radial and meshed HVDC-VSC systems comprising more HVDC lines and dc converter stations. This paper describes understanding, specification, and standardization of design and operation principles of the HVDC grids seen as the first needed steps toward such MTMV systems.

/pdf/technical-guidelines-and-prestandardization-work-for-first-rw1s5ymya4.pdf

Technical Guidelines and Prestandardization Work for First HVDC Grids

The scope of this paper is the investigation of the development of fault currents in a circuit breaker (CB) during a pole-to-ground fault in a generic multiterminal HVDC cable system based on voltage-source converters. It aims to contribute to the current discussion of which requirements on breaking time and peak current HVDC CBs need to fulfill in such networks. Therefore, the fault current is broken down into the individual contributions from the different network components, and the influence of the key parameters on the development of the fault current in the CB of the faulted cable is illustrated.

/pdf/contribution-of-fault-current-sources-in-multiterminal-hvdc-2z34arze68.pdf

Contribution of Fault Current Sources in Multiterminal HVDC Cable Networks

In a multiterminal HVDC network, dc circuit breakers (CBs) are required at each end of a line to selectively isolate a fault. Several CB concepts have been implemented in PSCAD, which differ significantly in their structure and performance. This paper describes the interaction of the different HVDC CB topologies with a meshed four-terminal network and assesses their performance in terms of maximum currents and voltages. The tradeoff between network parameters and CB requirements is analyzed.

/pdf/fault-current-interruption-in-multiterminal-hvdc-networks-4ox2wdplf9.pdf

Fault Current Interruption in Multiterminal HVDC Networks

The unavailability of adequate HVDC circuit breakers is often named as one of the key inhibitors for building HVDC multi-terminal networks. Instead of only discussing what needs to be done with respect to improving CB technology, we discuss what other options in principle exist. None of the options is optimum in itself, thus the discussion must be mainly driven by reviewing the pros and cons from a fault current clearing point of view and also from a network planning and operation point of view. The manuscript aims at triggering and stimulating the discussion on fault clearance options other than only CBs.

Matthias Bucher

Papers

Multiterminal HVDC Networks—What is the Preferred Topology?

Technical Guidelines and Prestandardization Work for First HVDC Grids

Contribution of Fault Current Sources in Multiterminal HVDC Cable Networks

Fault Current Interruption in Multiterminal HVDC Networks

Options for ground fault clearance in HVDC offshore networks