Converter-based High-voltage Direct Current System Research Articles

An improved prediction method of subsequent commutation failure of an LCC-HVDC considering sequential control response

Subsequent commutation failure (SCF) can be easily generated during the first commutation failure (CF) recovery process in a line-commutated converter-based high voltage direct-current system. SCF poses a significant threat to the safe and stable operation of power systems, and accurate prediction of CF is thus important. However, SCF is affected by the operating characteristics of the main circuit and the coupling effects of sequential control response in the inverter station. These are difficult to predict accurately. In this paper, a new SCF prediction method considering the control response is proposed based on the physical principle of SCF. The time sequence and switching conditions of the controllers at different stages of the first CF recovery process are described, and the corresponding equations of commutation voltage affected by different controllers are derived. The calculation method of the SCF threshold voltage is proposed, and the prediction method is established. Simulations show that the proposed method can predict SCF accurately and provide useful tools to suppress SCF.

A novel partitioning method for the power grid restoration considering the support of multiple LCC-HVDC systems

The restoration of the modern alternating current-direct current (AC–DC) hybrid power grid after a blackout is difficult and complex. By using the partitioning restoration strategy and the support of high voltage direct current (HVDC) system, the restoration speed of the blackout power grid can be greatly accelerated. Aiming at this problem, this paper proposes a partitioning method for the power grid restoration with multiple line commutated converter-based HVDC (LCC-HVDC) systems. Firstly, by introducing the system strength indices to quantify the interaction between AC and LCC-HVDC systems in the restoration process, a partitioning model considering multiple LCC-HVDCs is proposed. Wherein, the transmission power is optimized to make full use of LCC-HVDCs’ support capability. Then, by combining the system strength characteristic and community structure characteristic of the power grid, a partitioning method based on an improved Girvan Newman (GN) algorithm is proposed. Finally, the results are studied based on a modified IEEE 39-bus system and a realistic partial power grid of southwest China. Compared with two typical partitioning methods for the AC power grid restoration, the results show that the proposed method can effectively use the DC support power and speed up the restoration progress.

Optimal DC Voltage and Current Control of an LCC HVDC System to Improve Real-Time Frequency Regulation in Rectifier- and Inverter-Side Grids

High-voltage direct-current (HVDC) systems for constant or intermittent power delivery have recently been developed further to support grid frequency regulation (GFR). This paper proposes a new control strategy for a line-commutated converter-based (LCC) HVDC system, wherein the dc-link voltage and current are optimally regulated to improve real-time GFR in both rectifier- and inverter-side grids. A dynamic model of an LCC HVDC system is developed using the dc voltage and current as input variables, and is integrated with feedback loops for inertia emulation and droop control. A linear quadratic Gaussian (LQG) controller is also designed for optimal secondary frequency control, while mitigating conflict between the droop controllers of HVDC converters. An eigenvalue analysis is then conducted, focusing on the effects of model parameters and controller gains on the proposed strategy. Simulation case studies are performed using the models of a real HVDC system and a CIGRE benchmark system. The case study results confirm that the proposed strategy enables the HVDC system to improve GFR, in coordination with generators in both-side grids, by exploiting the fast dynamics of HVDC converters. The proposed strategy is also effective under various conditions for the LQG parameters, inertia emulation, and droop control.

A Modular Resonant DC–DC Converter With High Step-Down Ratio for Tapping Power From HVDC Systems

In this article, a new circuit topology of DC-DC converter for high step-down ratio is proposed. The proposed DC-DC converter can be used to tap power from high-voltage direct current (HVDC) system to feed the local loads that are located in the vicinity of HVDC transmission corridor. The proposed DC-DC converter consists of a large number of identical submodules each having one controllable switch, two diodes, and one capacitor. The operation of DC-DC converter is based on repeated charging and discharging of capacitors through a resonant circuit. Zero-current switching is achieved for all the switches used in the converter. In case of failure of one submodule, the failed submodule could be bypassed without affecting the normal operation of the HVDC tap. Equations governing the behavior of the proposed converter and the design of the components have been included in this article. The operational performance of the line commutated converter based HVDC system along with the HVDC tapping scheme with the proposed converter is investigated using a simulation model developed in PSCAD/EMTDC. The proposed circuit topology has also been verified with a down scaled experimental prototype.

Enhanced Fault Current-Limiting Circuit Design for a DC Fault in a Modular Multilevel Converter-Based High-Voltage Direct Current System

The main weakness of the half-bridge modular multilevel converter-based high-voltage direct current (MMC-HVDC) system lies in its immature solution to extremely high current under direct current (DC) line fault. The development of the direct current circuit breaker (DCCB) remains constrained in terms of interruption capacity and operation speed. Therefore, it is essential to limit fault current in the MMC-HVDC system. An enhanced fault current-limiting circuit (EFCLC) is proposed on the basis of fault current study to restrict fault current under DC pole-to-pole fault. Specifically, the EFCLC consists of fault current-limiting inductance L F C L and energy dissipation resistance R F C L in parallel with surge arrestor. L F C L reduces the fault current rising speed, together with arm inductance and smoothing reactor. However, in contrast to arm inductance and smoothing reactor, L F C L will be bypassed via parallel-connected thyristors after blocking converter to prevent the effect on fault interruption speed. R F C L shares the stress on energy absorption device (metal oxide arrester) to facilitate fault interruption. The DCCB requirement in interruption capacity and breaking speed can be satisfied effortlessly through the EFCLC. The working principle and parameter determination of the EFCLC are presented in detail, and its effectiveness is verified by simulation in RT-LAB and MATLAB software platforms.

Study on fault ride‐through control of islanded wind farm connected to VSC‐HVDC grid based on the VSC converter AC‐side bus forced short circuit

Wind farms integrated to an AC power system through a voltage source converter-based high-voltage direct current system (VSC-HVDC) are viewed as islanded wind farms. The wind farm side converter of VSC-HVDC maintains a constant frequency of the wind farm collection network and also controls its voltage. However, the transient responding time of VSC-HVDC is very short, and its fault responding feature is at the millisecond (ms) level. As a result, under grid fault conditions, the VSC station could be overloaded and even operation could be quitted because of blocking if islanded wind farms fail to limit their power generation in time. This study proposes a fault ride through control method for VSC-HVDC connected with islanded wind farms based on the forced short circuit of the AC-side bus of the VSC station. When there is a fault in the VSC-HVDC and the protection equipment is not able to cut off the excessive power at the ms level, the short-circuit resistance installed on the AC side of the VSC station would be switched to make a forced short circuit. That would activate the low-voltage ride through mode of islanded wind farms, and therefore reduce the power flowing into the VSC quickly. The HVDC fault and short circuit on the AC side would be cleared at the same time. The VSC-HVDC system accomplishes fault ride through.

Power quality control of voltage source converter-based HVDC system using particle swarm optimisation

Power quality control of voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system is proposed in this paper. The particle swarm optimisation (PSO) technique is used for selection of optimum controller gain which results quick response to various transient conditions such as load perturbation, voltage sag, nonlinear load, and tapped load fault at points-of-common coupling (PCC). The conventional PSO approach is modified to get various power quality indices such as harmonic distortion, power factor, and DC-link voltage ripple within the limits for an 18-pulse VSC-based HVDC system. The effectiveness of proposed PSO approach is compared with conventional PI tuning, i.e., Zeigler-Nichols approach. The complete design and optimisation process are presented using illustrations, equations and flow charts in a simplified manner so as to provide a good support for readers.

Power quality control of voltage source converter-based HVDC system using particle swarm optimisation

Power quality control of voltage source converter (VSC)-based high voltage direct current (HVDC) transmission system is proposed in this paper. The particle swarm optimisation (PSO) technique is used for selection of optimum controller gain which results quick response to various transient conditions such as load perturbation, voltage sag, nonlinear load, and tapped load fault at points-of-common coupling (PCC). The conventional PSO approach is modified to get various power quality indices such as harmonic distortion, power factor, and DC-link voltage ripple within the limits for an 18-pulse VSC-based HVDC system. The effectiveness of proposed PSO approach is compared with conventional PI tuning, i.e., Zeigler-Nichols approach. The complete design and optimisation process are presented using illustrations, equations and flow charts in a simplified manner so as to provide a good support for readers.

Coordinated Voltage Control in Offshore HVDC Connected Cluster of Wind Power Plants

This paper presents a coordinated voltage control scheme (CVCS) for a cluster of offshore wind power plants connected to a voltage-source converter-based high-voltage direct current system. The primary control point of the proposed voltage control scheme is the introduced Pilot bus, which is having the highest short-circuit capacity in the offshore AC grid. The developed CVCS comprehends an optimization algorithm, aiming for minimum active power losses in the offshore grid, to generate voltage reference to the Pilot bus. During the steady-state operation, the Pilot bus voltage is controlled by dispatching reactive power references to each wind turbine (WT) in the wind power plant cluster based on their available reactive power margin and network sensitivity-based participation factors, which are derived from the dV/dQ sensitivity of a WT bus w.r.t. the Pilot bus. This method leads to the minimization of the risk of undesired effects, particularly overvoltage at the terminals of the WT located far away from the AC collector substation, by dispatching lower reactive power references compared with the ones nearer to the substation. In addition, this paper proposes a control strategy for improved voltage ride through capability of WTs for faults in the offshore grid, thus leading to improved dynamic voltage profile in the offshore AC grid.

Hybrid Design of Modular Multilevel Converters for HVDC Systems Based on Various Submodule Circuits

The modular multilevel converter (MMC) has become the most promising converter technology for high-voltage direct current (HVDC) transmission systems. However, similar to any other voltage-sourced converter-based HVDC system, MMC-HVDC systems with the half-bridge submodules (SMs) lack the capability of handling dc-side short-circuit faults, which are of severe concern for overhead transmission lines. In this paper, two new SM circuit configurations as well as a hybrid design methodology to embed the dc-fault-handling capability in the MMC-HVDC systems are proposed. By combining the features of various SM configurations, the dc-fault current path through the freewheeling diodes is eliminated and the dc-fault current is enforced to zero. Several MMC configurations based on the proposed hybrid design method and various SM circuits, that is, the half-bridge, the full-bridge, the clamp-double, and the five-level cross-connected SMs, as well as the newly proposed unipolar-voltage full-bridge and three-level cross-connected SMs, are investigated and compared in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements. The studies are carried out based on time-domain simulation in the PSCAD/EMTDC software environment for various SM configurations and dc-fault conditions. The reported study results demonstrate the proposed hybrid-designed MMC-HVDC system based on the combination of the half-bridge and the proposed SM circuits is the optimal design among all evaluated systems in terms of the dc-fault-handing capability, semiconductor power losses, and component requirements.

Modeling and Control of a Modular Multilevel Converter-Based HVDC System Under Unbalanced Grid Conditions

The modular multilevel converter (MMC) is an emerging and attractive topology for the high-voltage direct-current (HVDC) transmission system. This paper presents a generalized mathematical model for MMC in HVDC applications under balanced and unbalanced grid conditions. The dynamics of the positive-, negative-, and zero-sequence components are derived from the model. Then, a dual current control scheme with positive- and negative-sequence current controllers is applied to MMC. The power controller to eliminate negative-sequence current components and the other one to eliminate double-line-frequency voltage ripple are compared. Moreover, a zero-sequence current controller is proposed in addition to the positive- and negative-sequence current controllers. Time-domain simulations on a 61-level MMC-HVDC test system are performed in the PSCAD/EMTDC software environment. The results demonstrate that the MMC-HVDC system with or without converter transformer is able to operate under unbalanced conditions by the use of the proposed control scheme.

Modeling and Control of IGBT Converter-Based High-Voltage Direct Current System

This paper presents modeling and control for the emerging IGBT converter-based high-voltage direct-current system (IGBT-HVDC). This paper adds to the representation of the IGBT-HVDC system in the dq-synchronous reference frame and its decoupled control scheme. Additionally, since the IGBT-HVDC is able to actively support the grid due to its capacity to control independently active and reactive power production, a reactive power control scheme is presented in order to regulate/contribute to the voltage at a remote location by taking into account its operational state and limits. The ability of the control scheme is assessed and discussed by means of simulations using ahybrid power system, which consists of a permanent magnetic synchronous-generator (PMSG) based wind turbine, an IGBT-HVDC, and a local load.