R. Teixeira Pinto

Impact of HVDC Transmission System Topology on Multiterminal DC Network Faults

This paper compares how a dc fault affects a multi-terminal dc (MTdc) network depending on the HVDC transmission system topology. To this end, a six-step methodology is proposed for the selection of the necessary dc fault protection measures. The network consists of four voltage-source converters converters radially connected. The converters natural fault response to a dc fault for the different topologies is studied using dynamic simulation models. For clearing of the dc faults, four different dc breaker technologies are compared based on their fault interruption time, together with a current direction fault detection method. If necessary, the converters are reinforced with limiting reactors to decrease the peak value and rate of rise of the fault currents providing sufficient time for the breakers to isolate the fault without interrupting the MTdc network operation. The study shows that the symmetric monopolar topology is least affected by dc contingencies. Considering bipolar topologies, the bipolar with metallic return exhibits better fault response compared to the one with ground return. Topologies with ground or metallic return require full semiconductor or hybrid breakers with reactors to successfully isolate a dc fault.

Optimization of social welfare and transmission losses in offshore MTDC networks through multi-objective genetic algorithm

A multi-objective approach, for an envisioned future DC independent system operator (ISO), on how to optimally operate an offshore multi-terminal DC network is presented in this paper. A pool market is used, in which the ISO receives the bids, of both producers and consumers connected to the offshore network, and determines the electricity spot price. A trade-off between maximization of the social welfare and the minimization of transmission losses is analyzed. The offshore multi-terminal DC (MTDC) network here implemented is based on recent studies [1]. The multi-objective optimization algorithm (MOOA) determines an optimal power flow (OPF), which guarantees the network constrains - e.g. DC voltages boundaries, maximum DC cable current and power produced at the offshore wind farms - as defined by the ISO are all respected. Even on DC Networks the system losses, capitalized over a year, can be in the range of tenths of millions of euros [2]. Consequently, a fair power losses allocation among loads and generators has an important impact on their benefits. Therefore, a losses allocation technique is implemented in the algorithm. System security is also taken into consideration. In order to enhance the DC system stability with regard to predictable changes - demand and generation evolution - and unpredictable events - e.g. an outage at of one of the DC voltage controlling stations - the results of the OPF are also tested to make sure that the offshore network always remains at least N-1 secure [3].

Providing dc fault ride-through capability to H-bridge MMC-based HVDC networks

This paper proposes a framework to achieve dc fault ride-through capability in multi-terminal dc networks (MTdc), when H-bridge multilevel modular converters (MMC) are used. The studied network consists of four voltage-source converters (VSC) for high voltage direct current (HVdc) transmission. Two of these VSC converters connect two offshore wind farms (OWF) to the main HVdc link between two asynchronous onshore grids, in a radial configuration. In case of a dc fault, H-bridge MMCs are able to block the fast developing currents and drive them to zero, allowing for fast mechanical disconnectors to isolate the faulty cable segment and reconfigure the grid layout. In this paper, the effect of the dc fault location to the grid behavior is analysed both at the fault isolation phase, as well as at the grid restoration phase. Moreover, the worst-case dc fault scenario for the studied network is identified. Finally, the total fault recovery time of the MTdc network is estimated. The study showed that H-bridge MMCs are unable to isolate the faulty part of the network without de-energizing the MTdc grid. However, the proposed framework allows for fast grid restoration within 3.6 s without the need for expensive dc breakers.

Optimization of limiting reactors design for DC fault protection of multi-terminal HVDC networks

In multi-terminal dc networks (MTdc) reactors are required to limit the rate of rise and the peak values of the fast developing currents in case of a dc fault. In this way, dc breakers have more time to isolate a fault and the system can restore its post-fault operation. This paper proposes a methodology to optimize the design of limiting reactors used for the protection of voltage-source converter (VSC) based MTdc networks. The limiting reactors were optimized using the covariance matrix adaptation evolution strategy (CMA-ES) optimization algorithm with two design objectives: first, the minimization of the reactor inductance value at the output of each VSC station to achieve N-1 security and second, the minimization of the reactors cost and mass and the peak dc fault current. Following the methodology steps, the effect of the dc fault location and the pre-fault power level of the converters on the dc fault network response are investigated in a four-terminal radially-connected grid. It resulted that the VSC stations controlling the dc voltage of the MTdc network are the first to respond to a dc fault and thus, limiting reactors higher than 97 mH are required in their dc output, in combination with dc breakers faster than 5 ms, to successfully protect the grid of the present case study.

Description and Comparison of DC Voltage Control Strategies for Offshore MTDC Networks: Steady-State and Fault Analysis

Abstract Estimates are that circa 40 GW of offshore wind power capacity is going to be installed throughout Europe by the end of this decade. In this scenario, a pan-European offshore grid network is needed in order to efficiently integrate large amounts of offshore wind into the different European countries’ transmission networks. In this paper, the dynamic model of a multi-terminal HVDC (MTDC) transmission system composed of voltage-source converters is presented. Afterwards, the dynamic models are used to compare four different methods for controlling the DC voltage inside MTDC networks, viz.: droop control, ratio control, priority control and voltage margin method. Lastly, a case study is performed in a four-node MTDC network and the different control strategies are compared during steady-state and an onshore three-phase fnult.

Sustainable DC-microgrid control system for electric-vehicle charging stations

As renewable energies and electric-vehicles increase in popularity, their interaction with existing grids becomes inevitable. This paper proposes a power management system to distribute acquainted power in a dc-microgrid supplying energy to a fleet of electric vehicles (EVs) used as taxis. A dynamic dc-microgrid model is used to validate the algorithm, including the dc-dc converters that interact with the grid and a PI controller to regulate the dc voltage. The proposed algorithm and voltage control are validated with several test cases.

Grid code compliance of VSC-HVDC in offshore multi-terminal DC networks

This paper studies grid code compliance of offshore wind farms (OWFs) when connected to a multi-terminal dc network (MTDC) using VSC-HVDC technology. As the wind farms become decoupled from the ac grids via the dc network, the onshore VSC-HVDC terminals are responsible for complying with the grid codes. However, the onshore VSCs are an active part of the MTDC network control. Voltage-source converters can independently control active and reactive power, but only within their rated capability. If reactive power requirements are too strict, it detriments the converter active power capability, influencing the MTDC network operation. The work is divided as follows: at first, the most common grid codes requirements are introduced. Secondly, the capability chart of a VSC-HVDC terminal is analyzed. Afterwards, the operation of VSC-HVDC terminals inside MTDC networks is explained. Lastly, dynamic simulations verify whether VSC-HVDC terminals fulfill the grid codes requirements when inside MTDC networks.

Optimal power flow of VSC-based multi-terminal DC networks using genetic algorithm optimization

Europe is rapidly expanding its offshore wind energy capacity. Hence, the construction of a multi-terminal dc (MTDC) infrastructure to accommodate the generated electrical energy brings several advantages, but also comes with many challenges. Operation and control of a MTDC network is one of these challenges. This paper explains the operation and control of MTDC networks. Moreover, a study is carried on how to optimally operate and control an offshore VSC-based MTDC network. It focus on the development plans for an offshore transnational grid in the North Sea. A genetic algorithm (GA) will be employed to obtain an optimal power flow inside the offshore network. The MTDC grid is composed of 19 nodes, interconnecting 9 OWFs to 5 European countries. The optimal power flow results obtained from the genetic algorithm are tested in a simulation model for three case studies.

Fast DC fault recovery technique for H-bridge MMC-based HVDC networks

This paper proposes a post-fault control technique for H-bridge multilevel modular converters (MMC), which in combination with a restoration framework, can achieve fast operation recovery in multi-terminal direct current (MTdc) networks under different dc fault types. The studied network consists of four voltage-source converters (VSC) for high voltage dc (HVdc) transmission. A meshed MTdc grid topology is used for the connection of two asynchronous grids with two offshore wind farms (OWFs). The effect of different dc fault types on the grid restoration time is evaluated. Following the proposed restoration steps, the grid is able to restore its operation within 7.4 s in case of a pole-to-ground fault. This time is reduced to only 34 ms, if a pole-to-pole fault occurs. To overcome the main time constraint of the restoration framework, a control technique is proposed that allows momentarily the connection of the two dc pole cables through the MMC-VSC switch valves. In this way, the dc grid is discharged faster and thus, the operation can be faster restored. The study showed that, with the proposed strategy, an H-bridge MMC-based network can restore its operation after a pole-to-ground dc fault within 158 ms without the need for expensive dc breakers.

Multi-terminal network options for the interconnection of offshore wind farms: A case study between Britain and The Netherlands

The increasing global energy needs have led to the realization of interconnectors to strengthen regional electricity systems. As interconnectors are often placed offshore, combining those with wind farm grids can increase profitability compared to separate infrastructure and improve the grid integration, offering higher security of supply and controllability. However, a systematic approach for selecting the best transmission options for offshore multi-terminal networks is still missing in literature. This paper investigates options for interconnecting two offshore wind farms (OWFs) to two onshore asynchronous grids in a multi-terminal network. The options are presented for a case study comprising the Dutch and the British grid along with two OWFs in the coastal zone of each country. A review of the available technologies regarding high-voltage ac (HVac) and high-voltage dc (HVdc) transmission systems is given and, on this basis, six technical scenarios are identified and evaluated. For each scenario, the technical specifications are elaborated and a rough cost calculation is made. Based on the design possibilities for a four-terminal network, the technical bottlenecks and the most important research challenges for its realization are presented.