Additional Control Based on ANFIS Algorithm to Improve Transient Current of Converter-Side in HVDC Transmission System

The present PhD thesis deals with transformer inrush current in offshore grids including offshore wind farms and High Voltage Direct Current (HVDC) transmission systems. The inrush phenomenon during transformers energization or recovery after the fault clearance is one of important concerns in offshore systems which can threaten the security and reliability of the HVDC grid operation as well as the wind farms function. Hence, the behaviour of wind turbines,Voltage Source Converters (VSC) and transformer under the normal operation and the inrush transient mode is analyzed. For inrush current reduction in the procedure of the offshore wind farms start-up and integration into the onshore AC grid, a technique based on Voltage Ramping Strategy (VRS) is proposed and its performance is compared with the operation of system without consideration of this approach. The new methodology which is simple, cost-effective ensures minimization of transformer inrush current in the offshore systems and the enhancement of power quality and the reliability of grid under the transformer energizing condition. The mentioned method can develop much lower inrush currents according to the slower voltage ramp slopes. Concerning the recovery inrush current, the operation of the offshore grid especially transformers is analyzed under the fault and the system restoration modes.The recovery inrush transient of transformers can cause tripping the HVDC and wind farms converters as well as disturbing the HVDC power transmission. A voltage control design based on VRS is proposed in HVDC converter to recover ali the transformers in offshore grid with lower inrush currents.The control system proposed can assure the correct performance of the converters in HVDC system and in wind farm and also the robust stability of the offshore grid. Esta tesis doctoral estudia las corrientes de energización de transformadores de parques eólicos marinos con aerogeneradores con convertidores en fuente de tensión (VSC) de plena potencia conectados a través de una conexión de Alta Tensión en Corriente Continua (HVDC). Las corrientes de energización pueden disminuir la fiabilidad de la transmisión eléctrica debido a disparos intempestivos de las protecciones durante la puesta en marcha o recuperación de una falta. Para la mitigación de las corrientes de energización durante la puesta en marcha del parque esta tesis propone una nueva estrategia basada en incrementar la tensión aplicada por el convertidor del parque eólico en forma de rampa (VRS). Este método persigue energizar el parque eólico con el menor coste y máxima fiabilidad. La tesis analiza diferentes escenarios y diferentes rampas. Otro momento en que las corrientes de energización pueden dar lugar a un disparo intempestivo de las protecciones es durante la recuperación de una falta en la red de alterna del parque eólico marino. Esta tesis extiende la estrategia VRS, utilizada durante la puesta en marcha del convertidor del parque, para los escenarios de recuperación de una falta.

The evacuation of bulk electrical power over long distances can be done mainly by high voltage alternating current (HVAC) transmission systems. However, where there are large water bodies and long distance between the generation source and demand centres like most islands in European countries, high voltage direct current (HVDC) power transmission is gaining more traction and is proving to be a sustainable solution. Regarding the distance between HVDC and HVAC, the break-even threshold in terms of cost is approximately 600 - 800 km for overhead line systems (OHLs) and 40 - 70 km for systems with cables. The global thirsty to harness the renewable energy sources is diverting the attention towards efficiency and flexibility of power transmission. Traditional HVAC systems are being quickly replaced with HVDC systems in various parts of the world. This report highlights the significance of modern HVDC power transmission networks, topologies, converter varieties, contrasts, and main equipment associated with HVDC systems. Through the use of HVDC converters, renewable energy sources (RES), such as offshore, onshore, coastal wind farms and photovoltaic power plant can be linked to the current existing AC grids. Although the difficulties associated with the upcoming DC grids are also highlighted, the views on multi-terminal HVDC systems and DC grids are also outlined.

This research is to regulate firing angle inverter-side of high voltage direct current (HVDC) system using ANFIS-based controller. Average-value model (AVM) is used to implement the HVDC systems because the AVM is needed in level-system simulation of power systems. The AVM neglects switching-device responses and time scale of the AVM is lower than detailed model. Controller based on ANFIS is implemented to control firing angle of inverter-side HVDC because this controller is simple and easy to be implemented. The HVDC system is constructed by using Matlab/Simulink programming. Initializing the variable and parameter of HVDC is loaded to the Matlab workspace using script programme. Some modules for converter, inverter and their controller are developed. Next, the ANFIS controller that consist of 7 inputs and a single output is applied to inverter module. Membership function (MF) that used for input and output are Gaussian MF and constant MF (Sugeno type 0), respectively. The ANFIS scheme control responses are compared to conventional controller responses to validate the ANFIS scheme controller.

The transfer of bulk electrical power over long distances can be done by high voltage alternating current (HV AC) transmission systems. These HV AC power transmission systems have their perfections and imperfections. To enhance the existing power transmission systems, high voltage direct current (HVDC) power transmission systems are a sustainable solution. The emergence of advanced HVDC systems is so rapid that they are replacing their counterpart HV AC systems in many parts of the world. In this paper, the prominence of modern HVDC power transmission systems is highlighted. The topologies, converter assortments, contrasts, and modeling methods are discussed. The renewable energy sources (RES) including solar photovoltaic (PV) parks and mainland/ onshore/ offshore wind farms can be assimilated into the existing AC grids via advanced HVDC converters. The perceptions of multi-terminal HVDC systems and DC grids are presented, whereas, the challenges concomitant with the future DC grids are also emphasized.

Many kinds of high temperature superconducting (HTS) devices are being developed due to its several advantages. In particular, the advantages of HTS devices are maximized under the DC condition. A line commutated converter (LCC) type high voltage direct current (HVDC) transmission system requires large capacity of DC reactors to protect the converters from faults. However, conventional DC reactor made of copper causes a lot of electrical losses. Thus, it is being attempted to apply the HTS DC reactor to an HVDC transmission system. The authors have developed a 8 mH class HTS DC reactor and a model-sized LCC type HVDC system. The HTS DC reactor was operated to analyze its operational characteristics in connection with the HVDC system. The voltage at both ends of the HTS DC reactor was measured to investigate the stability of the reactor. The voltages and currents at the AC and DC side of the system were measured to confirm the influence of the HTS DC reactor on the system. Two 5 mH copper DC reactors were connected to the HVDC system and investigated to compare the operational characteristics. In this paper, the operational characteristics of the HVDC system with the HTS DC reactor according to firing angle are described. The voltage and current characteristics of the system according to the types of DC reactors and harmonic characteristics are analyzed. Through the results, the applicability of an HTS DC reactor in an HVDC system is confirmed.

High-voltage direct current (HVDC) transmission system is commonly used for transmitting the bulk electric power over long distance and the reason behind this is, its less transmission losses and interconnection between two or more asynchronous power systems. Due to the recent advancement in power electronics devices, the terminal end converter and inverters cost is reduced and capacity is increased which makes the HVDC transmission more effective comparing with HVAC transmission over a long distance. The present scenario of HVDC in India is also given more attention due to more power demand and we have limited resources, it is being fulfilled by only through renewable energy resources, so the scope for HVDC transmission increases day by day. Most of the renewable energy resources generate energy at different frequency levels that fulfil the requirement of demand through islanded mode or connect through grid, it is an obvious option to connect renewable energy resources to grid via HVDC links. So, nowadays increasing HVDC links also increase the probability of fault in the system.

Direct Current (DC) transmission system is an important part of the interregional grid. However, the frequency impact of power grid caused by DC blocking is more serious with the continuous increase of scale of single DC transmission. The frequency coordination control system is applied to the receiving-end power grid in High Voltage Direct Current (HVDC) transmission system. In the cases of single-infeed HVDC blocking, multi-infeed HVDC blocking or fast power drop, the frequency coordination control system can reduce the frequency fluctuation of power grid by instantly promoting the transmission power of the rest normal DC systems and reducing power loss. The frequency coordinated control substation is an executive device of the system at the converter station. It keeps communicating with the coordinated control master station for real-time monitoring the operation of converter station, reports the fault and executes the remote power boost instructions. Taking the frequency coordinated control substation of 500 kV conventional DC systems as a research background, the load of converter station in a regional power grid is analyzed. The working mechanism of frequency coordination control system in the cases of current converter station fault or other station faults is analyzed. The actual effect of a frequency coordination control system on a converter station in East China power grid is investigated by testing the actual power, voltage and current variation of a conventional DC transmission system.

Recent developments in power semiconductor devices have made High Voltage Direct Current (HVDC) transmission systems the most crucial technology for future power systems. Today several HVDC projects have been commissioned around the world. The primary concern for such an advanced HVDC grid is to follow standard grid codes to maintain the safety and stability of the electrical equipment during transient and dynamic circumstances. Although the HVDC system has been recognized as the most prominent technology to transmit electric power, some issues must be addressed, such as selective and quick detection of faults. For selective fault detection, it is essential to look into the several fault detection strategies that fortify the HVDC system with time-frequency signal processing techniques. This research uses a time frequency-based method to identify the different HVDC transmission line faults. The most suitable mother wavelet for various transient states is selected by analyzing fault voltage and current signals based on variance, standard deviation, and range parameters. An experimental setup of a thyristor-based HVDC transmission system has been developed in the MATLAB/Simulink environment, and multiple fault scenarios have been analyzed. The simulation outcomes reveal that the most effective mother wavelet for detecting DC line, symmetrical and unsymmetrical faults occurs at the 6th level of fault signal decomposition. By examining current and voltage signals, our work has determined the best mother wavelet for various faults. Using more than one mother wavelet to detect faults is more reliable than using a single mother wavelet to detect all faults. Additionally, it has been noted that each fault's voltage and current signals have two distinct mother wavelets.

This paper investigates the problem of the insensitivity of synchronous generators out of step protection when they are operating close to a High-Voltage Direct Current (HVDC) transmission system. Commercial out of step protection devices are based on the measurement of the impedance seen at the terminals of the generators and on the fact that this will be reversed when a loss of stability occurs. However, when these generators operate near an HVDC system, the out of step protection is desensitized, because the HVDC controls are able to maintain constant power when a loss of synchronism condition starts. As a consequence, the generators are exposed to serious damage. Our investigations also showed that the academy’s recent proposals to detect the out of step conditions were ineffective to solve the problem. Therefore, investigating the causes of this phenomenon and under what conditions it occurs is fundamental to develop efficient methods to solve it.

High Voltage Direct Current (HVDC) systems interconnect AC grids to increase reliability, connect offshore wind generation, and enable coupling of electricity markets. Considering the growing uncertainty in power infeed and the complexity introduced by additional controls, robust decision support tools are necessary. This paper proposes a chance constrained AC-OPF for AC and HVDC grids, which considers wind uncertainty, fully utilizes HVDC control capabilities, and uses the semidefinite relaxation of the AC-OPF. We consider a joint chance constraint for both AC and HVDC systems, we introduce a piecewise affine approximation to achieve tractability of the chance constraint, and we allow corrective control policies for HVDC converters and generators to be determined. An active loss penalty term in the objective function and a systematic procedure to choose the penalty weights allow us to obtain AC-feasible solutions. We introduce Benders decomposition to maintain scalability. Using realistic forecast data, we demonstrate our approach on a 53-bus and a 214-bus AC-DC system, obtaining tight near-global optimality guarantees. With a Monte Carlo analysis, we show that a chance constrained DC-OPF leads to violations, whereas our proposed approach complies with the joint chance constraint.

In this paper, a new inductive filtering scheme with a four-winding inductive filtering converter transformer (FWIFCT) and its filtering system is proposed to solve the harmonic pollution problems generally existing in the high voltage direct current (HVDC) transmission system. The main circuit topology by series combining the fully tuned filters with the filtering winding of the FWIFCT are described. Based on the inductive filtering method, the voltage transfer mathematical model is established, and then the operation characteristics of the FWIFCT in HVDC system are analyzed in detail. The simulation model in the PSCAD/EMTDC simulator and prototype experiment of the FWIFCT and its filtering system for HVDC transmission system are built. The simulation and experiment results show that the filtering effect of the new inductive filtering scheme is better than that of the traditional filtering scheme, and it can effectively improve the power quality of ac grid-side, which verifies the effectiveness of the proposed theory.

A black-start technology of high voltage direct current (HVDC) power transmission system based on pseudo-deicer startup mode is proposed by analyzing HVDC deicer operation mode, which breaks through the technical bottleneck of HVDC power transmission system could not be firstly involve in the grid black-start due to the half-controlled thyristor device and the weak environment of AC grid. In this paper, the conception of pseudo-deicer startup mode is proposed for the first time, and its black-start implementation process is given. In addition, the startup demand of HVDC system inverter station and the AC grid, and the control and protection requirements of AC and DC systems are researched. Finally, examples for ±500kV Long-Zheng HVDC system engineering of China, the feasibility of the black-start technology of HVDC power transmission system based on pseudo-deicer startup mode is demonstrated by simulation. The proposed technology effectively avoid the impact of HVDC transmission power to the AC grid in the black-start beginning, and the transmission power is continuously adjustable, which could realize the synchronization between the transmission power with the recovery process of AC grid and load. It plays a positive role and has important practical significance to speed up the grid rapid recovery after large-scale blackout accidents.

Conventional fault analysis algorithms may not be suitable for the AC/DC hybrid systems. To solve this problem, a mathematical model for HVDC (High Voltage Direct Current) system, which is suitable for analyzing faults in AC system, is presented in this paper. Firstly, based on the switching functions method and sequence components method, an equivalent model of HVDC converter under the asymmetric AC system is put forward. The unique advantage of the converter model lies in that it has taken harmonic characteristics into account and can improve the calculation accuracy significantly. Secondly, response performances of HVDC control system in cases of AC faults are analyzed and expressions for the control response characteristics are discussed under 3 modes. Finally, a general algorithm is developed to incorporate the proposed HVDC model into the conventional AC short-circuit calculation algorithm. The effectiveness of the proposed HVDC model and algorithm has been verified by simulation studies and by comparing the results with those obtained from the EMTDC simulations.

High voltage direct current (HVDC) transmission systems play an essential role in our modern power grids, not only providing bulk power transmission and asynchronous AC connection, but also enabling renewable energy integration into power grids. Renewable energy integration through HVDC transmission systems leads to the creation of multi-terminal HVDC (MT-HVDC) grids. Recently, the idea of electrifying remote communities by HVDC transmission systems has emerged, and multi-terminal small-scale HVDC grids have been investigated in such applications. Despite numerous applications and significant potential of HVDC transmission systems, various technical and economic challenges in utilizing these systems still persist. In this paper, an extensive literature review is conducted on HVDC transmission systems, particularly focusing on their role in renewable energy integration, and their potential application in remote communities. Future research directions are also recommended.

The fault of electrode line can pose serious threat to the safe and stable operation of the High Voltage Direct Current (HVDC) system, but it is quite difficult to detect the line fault by normal ways because the grounding electrode line is usually at ground potential. This paper presents a novel fault monitoring and location system for HVDC electrode line, which is based on the TDR technique and specially designed according to the characteristics of electrode line. The system injects differential mode voltage pulse to monitor the electrode line, and reduces the cancellation of differential mode signal through series reactance. Theoretically, this system is immune to the interference signal from the HVDC system, and can greatly improve the Signal-to-Noise Ratio (SNR) of the reflected pulse. In addition, the system uses a waveform comparison algorithm to solve the problem of line parameter uniform and slowly changing, which could make much trouble to the normal TDR methods. For this fault monitoring and location system, the output pulse amplitude is designed as up to 500 V, the pulse width range is about 1-5 $\mu$s, and the pulse sampling rate is as high as 10 MHz. Simulation results and field applications show that this system can achieve excellent fault monitoring and location result for the HVDC electrode line, with the fault location accuracy better than 300m.