Hybrid High Voltage Direct Current Research Articles

A Novel Topology of Hybrid HVDC Circuit Breaker for VSC-HVDC Application

The use of high voltage direct current (HVDC) circuit breakers (CBs) with the capabilities of bidirectional fault interruption, reclosing, and rebreaking can improve the reliable and safe operation of HVDC grids. Although several topologies of CBs have been proposed to perform these capabilities, the limitation of these topologies is either high on-state losses or long time interruption in the case bidirectional fault current interruption. Long time interruption results in the large magnitude of the fault current in the voltage source converter based HVDC (VSC-HVDC) system due to the high rate of rise of fault current. This paper proposes a new topology of hybrid CB (HCB) with lower conduction loss and lower interruption time to solve the problems. The proposed topology is based on the inverse current injection method, which uses the capacitor to enforce the fault current to zero. In the case of the bidirectional fault current interruption, the capacitor does not change its polarity after identifying the direction of fault current, which can reduce the interruption time accordingly. A switching control algorithm for the proposed topology is presented in detail. Different operation modes of proposed HCB, such as normal current mode, breaking fault current mode, discharging, and reversing capacitor voltage modes after clearing the fault, are considered in the proposed algorithm. The proposed topology with the switching control algorithm is tested in a simulation-based system. Different simulation scenarios such as temporary and permanent faults are carried out to verify the performance of the proposed topology. The simulation is performed in the Matlab/Simulink environment.

Hybrid, Multi-Megawatt HVDC Transformer Topology Comparison for Future Offshore Wind Farms

With the wind industry moving further offshore, High Voltage Direct Current (HVDC) transmission is becoming increasingly popular. HVDC transformer substations are not optimized for the offshore industry though, increasing costs and reducing redundancy. A suggested medium frequency, modular hybrid HVDC transformer located within each wind turbine nacelle could mitigate these problems, but the overall design must be considered carefully to minimize losses. This paper’s contribution is a detailed analysis of the hybrid transformer, using practical design considerations including component library minimization. The configurations investigated include combinations of single phase H-Bridge and Modular Multilevel Converter topologies operating under minimum switching frequency control strategies. These were modelled in the MATLAB/Simulink environment. The impact of the minimum switching control strategy and converter topology on power transfer stability and overall efficiency is then investigated. It was found that the H-Bridge converter generated the lowest overall losses, but there was a trade off with power flow sensitivity due in part to the additional harmonics generated.

Assembly HVDC Breaker for HVDC Grids With Modular Multilevel Converters

The modular multilevel converter (MMC) with half-bridge submodules (SMs) is the most promising technology for high-voltage direct current (HVDC) grids, but it lacks dc fault clearance capability. There are two main methods to handle the dc-side short-circuit fault. One is to employ the SMs that have dc fault clearance capability, but the power losses are high and the converter has to be blocked during the clearance. The other is to employ the hybrid HVDC breakers. The breaker is capable of interrupting fault current within 5 ms, but this technology is not cost effective, especially in meshed HVDC grids. In this paper, an assembly HVDC breaker and the corresponding control strategy are proposed to overcome these drawbacks. The assembly HVDC breaker consists of an active short-circuit breaker (ASCB), a main mechanical disconnector, a main breaker, and an accessory discharging switch (ADS). When a dc-side short-circuit fault occurs, the ASCB and the ADS close immediately to shunt the fault current. The main breaker opens after a short delay to isolate the faulted line from the system and then the mechanical disconnector opens. With the disconnector in open position, the ASCB opens and breaks the current. The proposed breaker can handle the dc-side fault with competitively low cost, and the operating speed is fast enough. A model of a four-terminal monopolar HVDC grid is developed in Power Systems Computer Aided Design / Electromagnetic Transients including DC, and the simulation result proves the validity and the feasibility of the proposed solution.

Hybrid high‐voltage direct current topology with line commutated converter and modular multilevel converter in series connection suitable for bulk power overhead line transmission

In this study, a hybrid high-voltage direct current (HVDC) topology with line commutated converter (LCC) and modular multilevel converter (MMC) in series-connection is proposed, which is suitable for bulk power overhead line transmission. This topology is of operational flexibility in terms of active and reactive power controls, and is able to withstand ac and dc faults by the cooperative control of LCC and MMC. First, the operation principle and mathematical model are presented. Then, the control strategies for ac faults at rectifier and inverter side are discussed, which can prevent current cut-off under ac fault at rectifier side as well as maintain part of active power if commutation failure of LCC occurs under ac fault at inverter side. In addition, the feasibility on dealing with dc fault is theoretically demonstrated by analysing the characteristic of MMC under blocking state. A detailed control strategy for dc fault is further proposed combined with a test system. Finally, the effectiveness of the control strategy for ac and dc faults is verified and further compared with LCC-based HVDC topology through time-domain simulation.

Commutation failure and voltage sensitivity analysis in a hybrid multi-infeed HVDC system containing modular multilevel converter

Summary A typical hybrid dual-infeed high-voltage direct current (HVDC) system includes two types of links—line-commutated converter (LCC) HVDC and voltage-source converter (VSC) HVDC in proximity. When faults occur in the LCC-HVDC side, the VSC-HVDC link sufficiently can compensate power to the LCC-HVDC system; thus, voltage fluctuation is suppressed. In recent years, the modular multilevel converter (MMC) has been a promising type of VSC. This paper presents a novel hybrid dual-infeed HVDC system consisting of an LCC-HVDC link and an MMC-HVDC link, and the two links are interconnected through a tie line between buses in the inverter side. The positive contribution of the MMC-HVDC link to the LCC-HVDC link in the area of commutation failure immunity index, effective short circuit ratio, voltage sensitivity factor, temporary overvoltage, and DC line fault recovery are investigated. Investigations show that the stability of system has been enhanced significantly with MMC, and the contributors are physical structure change and the control strategy of the MMC-HVDC system. The modelling and simulation of the steady state and transient performance of hybrid HVDC system are conducted in PSCAD/EMTDC. Finally, the simulation results verify the enhancement in sensitivity and stability of the dual-infeed HVDC system. Copyright © 2016 John Wiley & Sons, Ltd.

Hybrid HVDC circuit breaker with self‐powered gate drives

The ever increasing electric power demand and the advent of renewable energy sources have revived the interest in high-voltage direct current (HVDC) multi-terminal networks. However, the absence of a suitable circuit breaker or fault tolerant VSC station topologies with the required characteristics (such as operating speed) have, until recently, been an obstacle in the development of large scale multi-terminal networks for HVDC. This paper presents a hybrid HVDC circuit breaker concept which is capable of meeting the requirements of HVDC networks. Simulation results are presented which are validated by experimental results taken from a 2.5kV, 700A rated laboratory prototype.

Optimal power flow tool for mixed high‐voltage alternating current and high‐voltage direct current systems for grid integration of large wind power plants

This study presents a tool for solving optimal power flows (OPFs) in hybrid high-voltage direct current (HVDC) and high-voltage alternating current (HVAC) systems for grid integration of large wind power plants, located either offshore or onshore. The OPF departs from the assumption that the power being produced from the wind power plants is known, as well as the demand from the AC grid. To model the interaction between the DC and AC grids, the active power conservation is expressed between the AC side and DC side of each converter, taking into consideration converter losses (modelled as a second-order polynomial). The tool developed determines the voltages and the active and reactive power in each bus and branch that ensure the selected objective function. All the electrical variables are limited. Moreover, the currents flowing in each DC and AC branch are also limited. The maximum AC voltage that can be applied by the converters is also limited. To develop the tool, both HVDC and HVAC grids need to be represented appropriately through its impedances and admittances. The tool has been implemented through MATLAB® optimisation toolbox and through a more specific optimisation software GAMS®, leading to the same results for the study case presented.

Technical Assessment of Load Commutation Switch in Hybrid HVDC Breaker

The development of a large-scale high-voltage direct current (HVDC) power grid requires a reliable, fast, and low-loss circuit breaker. The load commutation switch (LCS) is an essential part of ABB's 1200-MW hybrid HVDC breaker concept, which builds up a low-loss conducting path for the load current. The technical requirements for the LCS are expressed in this paper by studying the operation principle of the hybrid HVDC breaker. The voltage stress over the LCS is determined based on a dc grid with 320 and 2 kA rated voltage and current. A system model of the hybrid HVDC breaker is developed in PSCAD/EMTDC to study the design criteria for snubber circuit and arrester blocks. It is observed that conventional snubber circuits are not suitable for a bidirectional LCS as the current of snubber capacitors prevent the fast interruption action. A modified snubber circuit is proposed in this paper along with two more alternatives for the LCS to overcome this problem. Moreover, the power loss model for a semiconductor device is discussed in this paper based on the 4.5-kV StakPak IGBT. The model is used to calculate the conduction power losses for different LCS topologies. Ultimately, a matrix of 3 $\times$ 3 IGBT modules is selected to provide a reliable LCS design which can handle several internal fault cases with no interruption of operation. A full-scale prototype has been constructed and tested in ABB HVDC Center, Ludvika, Sweden. The experimental test results are also included in the paper in order to verify the calculation and simulation study.