Two-terminal High Voltage Direct Current Research Articles

Fault Analysis and Traveling Wave Protection Based on Phase Characteristics for Hybrid Multiterminal HVDC Systems

By fully combining the advantages of line commutated converter (LCC) high-voltage direct current (HVDC) and modular multilevel converter (MMC)-HVDC systems, hybrid multiterminal HVDC (hybrid-MTDC) transmission technology has become a new technical means to overcome the shortcomings of conventional two-terminal HVDC systems. However, the conditions of high fault resistances and more sophisticated network topologies have brought considerable challenges to the existing traveling wave (TW) protection principles. In this article, a parallel hybrid-MTDC system is taken as the research object, and its mathematical analysis model is established. Based on the mathematical model, the TW characteristics that the phase of the high-frequency voltage TW during the occurrence of an internal and external fault approaches approximately 90° and 0°, respectively, are clarified. Therefore, considering the influences of reflection and refraction of TWs and line propagation, the phase-characteristics-based criterion is constructed. Using the proposed criterion, a protection scheme for hybrid-MTDC systems is proposed, which realizes the identification of line faults with high fault resistance. The proposed method is verified in detail with the electromagnetic transient simulation model using PSCAD/EMTDC, and the test results show that the proposed protection scheme is feasible and has high sensitivity, reliability, and adaptability to different operation modes.

Optimal reactive power flow of power systems with two-terminal HVDC and multi distributed generations using backtracking search algorithm

This paper deals with optimal reactive power flow (ORPF) of power systems with two-terminal high voltage direct current (HVDC) and multi distributed generations (DGs) using backtracking search algorithm (BSA). For the first time with this study, the power system is modeled closest to the real world and the power loss of both test systems are minimized by BSA, while DGs and HVDC links are existing simultaneously in the power systems. Although simulations on modified IEEE 30-bus and modified IEEE 57-bus test systems have given successful results by using BSA, genetic algorithm (GA) was also used to compare the outcomes of different algorithms. This study was performed for five and three different cases of the modified 30-bus and modified 57-bus test systems, respectively. The obtained results show that although the power loss does not vary much according to the connection points of HVDC link, the optimal replacement of the DGs to the power system reduces the power loss considerably. In addition to this, when the obtained results are compared with those in the literature, it is seen that the proposed algorithm is better than the other algorithms in terms of performance and effectiveness.

Controlled transition bridge converter: Operating principle, control and application in HVDC transmission systems

This paper employs an amplitude modulation with sinusoidal plus third harmonic injection instead of trapezoidal modulation to operate a controlled transition bridge (CTB) converter as ac/dc and dc/ac converter terminals. With such an operation, the CTB converter may require small ac filters; thus attractive for high-voltage direct current (HVDC) transmission systems. To facilitate ac voltage control over a wide range and black-start capability, the injected 3rd harmonic allows the cell capacitor voltages of the CTB converter to be regulated independent of the modulation index and power factor. The insertion of 3rd harmonic into modulating signals achieves two objectives: extends the regions around voltage zeros so that the total voltage unbalanced can be distributed between the cell capacitors, thereby exploiting the bipolar capability of the full-bridge cells in each limb; and to ensure that each limb can be clamped to the positive and negative dc rails every half fundamental period independent of the modulation index to allow recharge of the cell capacitors from the active dc link. The suitability of the CTB converter for HVDC type applications is demonstrated using a two-terminal HVDC link that employs a 21-cell CTB converter, considering normal operation and ac faults.

Modelling, analysis and virtual parallel resistor damping control of VSC‐based DC grid using master–slave control mode

This study presents a generic linearised modelling method and a virtual parallel resistor damping control for a voltage source converter-based high-voltage direct current (HVDC) system including a direct current (DC) grid. The generic modelling method is composed of DC network's equivalent node-admittance matrix and the Thevenin equivalent matrix of all converters. A simplified two-terminal HVDC system is used to identify the key factors influencing the stability of the DC grid easily. Those key factors are proved by two methods, namely, root locus and participation factors analysis. The parameters of the virtual parallel resistor damping control are designed using the simplified HVDC system. The performance of the proposed damping control is studied employing the linearised model of the DC grid. Time simulations such as instability suppressing, power-controlled station N − 1 fault, and DC system faults are carried out to validate the effectiveness of the damping control.

Application of Grey Wolf Optimizer Algorithm for Optimal Power Flow of Two-Terminal HVDC Transmission System

This paper applies a relatively new optimization method, the Grey Wolf Optimizer (GWO) algorithm for Optimal Power Flow (OPF) of two-terminal High Voltage Direct Current (HVDC) electrical power system. The OPF problem of pure AC power systems considers the minimization of total costs under equality and inequality constraints. Hence, the OPF problem of integrated AC-DC power systems is extended to incorporate HVDC links, while taking into consideration the power transfer control characteristics using a GWO algorithm. This algorithm is inspired by the hunting behavior and social leadership of grey wolves in nature. The proposed algorithm is applied to two different case-studies: the modified 5-bus and WSCC 9-bus test systems. The validity of the proposed algorithm is demonstrated by comparing the obtained results with those reported in literature using other optimization techniques. Analysis of the obtained results show that the proposed GWO algorithm is able to achieve shorter CPU time, as well as minimized total cost when compared with already existing optimization techniques. This conclusion proves the efficiency of the GWO algorithm.

A Hybrid Reliability Evaluation Method for Meshed VSC-HVDC Grids

High-voltage direct current (HVDC) grids are emerging, and their reliability has been an increasing concern for the utilities. HVDC grids are different from typical two-terminal HVDC transmission systems due to the loops in their topology, which makes it difficult to evaluate the reliability by conventional analytical methods. This paper proposes an innovative hybrid method to evaluate the reliability of meshed HVDC grids. First, steady-state models and reliability models are established for the components in HVDC grids, especially for converters and power flow controllers. In the models, virtual buses are introduced to represent the external AC connections to the HVDC grid. Then a hybrid reliability evaluation method is proposed based on an analytical approach and Monte Carlo simulation. One innovation of the paper is the application of an analytical analysis method to accelerate state evaluation in Monte Carlo simulation by skipping unnecessary optimization. The proposed models and methods are verified on two HVDC grids. Test results show that HVDC grids under most failure states (approximately 70%) tend to shed no load except on buses connected to faulted converters, and the application of the analytical method could promote evaluation efficiency significantly.

An improved teaching-learning-based optimization with differential evolution algorithm for optimal power flow considering HVDC system

In this paper, a modified teaching–learning based optimization (TLBO) algorithm is introduced in order to solve the optimal power flow (OPF) considering the high voltage direct current (HVDC) link in power systems. In TLBO, there is an improper diversity among search learners; therefore, its convergence speed is lower in comparison with some other evolutionary algorithms. Hence, in order to improve the quality of the solutions and speed up the velocity convergence, the teacher phase is modified. Moreover, to balance the global and local search capability of TLBO, one of the most common mutation operations of the differential evolution algorithm is incorporated into learner phases. With these modifications, the trapping to the local minima of traditional TLBO is vastly improved and the trade-off between the global searching ability and the convergence rate is retained. In order to demonstrate the efficiency of the proposed optimization method, it is applied to the OPF problem of two different two-terminal HVDC systems, including the modified 5-bus system and the modified WSCC 9-bus system. The behavior of some optimization methods such as the Genetic Algorithm, Backtracking Search Algorithm, and Artificial Bee Colony algorithm as well as the CPU running time for the objective function is presented. Comparison results indicate that the proposed optimization method is reliable with higher quality solutions among other applied evolutionary algorithms.

Artificial bee colony algorithm based optimal reactive power flow of two-terminal HVDC systems

This paper presents a solution technique for the optimal reactive power flow (ORPF) of alternating currentdirect current (AC-DC) power systems using the artificial bee colony (ABC) algorithm. A high voltage direct current (HVDC) transmission link is one of the most important network components in AC-DC power systems. In recent years there has been an increasing number of studies related to power systems with HVDC transmission links. One of them is the reactive power flow (RPF) optimization. The aim of ORPF in alternating current (AC) or AC-DC power systems is to minimize transmission line power loss under equality and inequality constraints. The relationships related to the HVDC link are incorporated into AC power system equations considering its power transfer characteristics. This paper contributes to the solution of the problem by using the ABC algorithm for the first time. The proposed method is examined on three test systems and its effectiveness in these kinds of applications is shown by the comparative results.

Optimal power flow of two-terminal HVDC systems using backtracking search algorithm

Abstract This paper presents a solution technique for optimal power flow (OPF) of high-voltage direct current (HVDC) power systems using a backtracking search algorithm (BSA). BSA is a population-based evolutionary algorithm (EA), and it is not sensitive to initial conditions, contrary to most other meta-heuristic algorithms. The proposed algorithm is applied to three different test systems as follows: the modified 5-bus test system, the modified WSCC 9-bus test system, and the modified New England 39-bus test system. As a result of the simulations, minimum, maximum, and average production costs and CPU times are obtained for different cases of each of the three test systems. These results are also compared to those of the Artificial Bee Colony (ABC) algorithm, the Genetic Algorithm (GA), and the unified method provided in literature. In regard to the comparative results, it can be said that the proposed method has a shorter CPU time and is more efficient than the others. Thus, the applicability and efficiency of the proposed method in this field are demonstrated.

Optimizing reactive power flow of HVDC systems using genetic algorithm

Due to the usage of high voltage direct current (HVDC) transmission links extensively in recent years, it requires more studies in this issue. Two-terminal HVDC transmission link is one of most important elements in electrical power systems. Generally, the representation of HVDC link is simplified for optimal reactive power flow (ORPF) studies in power systems. ORPF problem of purely AC power systems is defined as minimization of power loss under equality and inequality constraints. Hence, ORPF problem of integrated AC–DC power systems is extended to incorporate HVDC links taking into consideration power transfer control characteristics. In this paper, this problem is solved by genetic algorithm (GA) that is an evolutionary-based heuristic algorithm for the first time. The proposed method is tested on the modified IEEE 14-bus test system, the modified IEEE 30-bus test system, and the modified New England 39-bus test system. The validity, the efficiency, and effectiveness of the proposed method are shown by comparing the obtained results with that reported in literature. Thus, the impact of DC transmission links on the whole power systems is shown.

Optimal power flow solution of two-terminal HVDC systems using genetic algorithm

The usage extensively of high voltage direct current (HVDC) transmission links in recent years makes it necessary further work in this area. Therefore, two-terminal HVDC transmission link is one of most important elements in electrical power systems. HVDC link representation is mostly ignored and simplified for optimal power flow (OPF) studies in power systems. OPF problem of purely alternating current (AC) power systems is defined as minimization of the fuel cost to subjected equality and inequality constraints. Hence, OPF software of purely AC power systems is extended by taking into consideration power transfer control characteristics of HVDC links. In this paper, OPF problem of integrated AC–DC power systems is first solved by genetic algorithm that is a heuristic algorithm based on evolutionary. The proposed method is tested on two test systems which are the modified 5-node test system and the modified WSCC 9-bus test system. In order to show effectiveness and efficiency of the proposed method, the obtained results are compared to that reported in the literature.

Study of HVDC controls through efficient dynamic digital simulation of converters

The authors describe a converter model for 6/12 pulse operation and present its application to the study of the performance of converter controls. The simulation is simplified by considering a time-varying equivalent circuit on the DC side, which is derived on the basis of graph theory. Elimination of the need to store connection matrices and a fast and efficient way of generating the converter equations are further innovations introduced here. For the simulation of the HVDC (high-voltage direct-current) system the approach used is to model each component separately and in a modular fashion. These models are interconnected using appropriate interface variables. The control of converters based on digital techniques is considered along with voltage-dependent current order limit (VDCOL). Results for a two-terminal HVDC system under normal operation and abnormal conditions are presented to illustrate the capability of the simulation method.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>