A Controllable Thyristor-Based Commutation Failure Inhibitor for LCC-HVDC Transmission Systems

Commutation failure is one of the most frequent failures in line-commutated converter based high-voltage direct-current (LCC-HVdc) systems which may lead to the outage of HVdc links, and thus affect the performance of the power system. Most commutation failures are caused by voltage reduction due to ac system faults. In this paper, a commutation failure prevention module (CFPM) is developed, which inserts the optimal value of resistance proportional to the reduced extinction angle in series with the fault current path using a simple switching method. When a fault occurs at the inverter ac side, a self turn- off switch starts switching with a pre-specified frequency and controlled modulation index, which limits the fault current to the desired value and inhibits commutation failure. The salient feature of the proposed CFPM is that it is fully controllable. In addition, despite the available controller modification methods which are not effective for low-impedance faults, the proposed CFPM can prevent commutation failure regardless of the fault severity. The practical performance and feasibility of the developed CFPM is validated through laboratory testing, using the real-time Opal-RT hardware prototyping platform. The experimental results demonstrate that the proposed strategy can effectively eliminate the commutation failure or repetitive commutation failures under different fault types.

The HVDC converter valve is an important equipment to realize power transmission under high-voltage direct-current condition. The reliability of high-voltage direct current (HVDC) converter valve directly affects the operation of HVDC converter valve. At first, the typical situations inside the converter valve over-voltage are analyzed and summarized in this article, and then analyzes the broadband model of each component in the converter valve, and establishes the broadband model of the valve tower based on this. Then this paper analyzes the over-voltage distribution of the converter valve under several fault conditions through model simulation, such as HEMP, operating over-voltage and lighting over-voltage. Through the summary of several over-voltage distributions, the over-voltage mechanism of HVDC converter valve is more deeply understood and the theoretical guidance for improving the reliability of the HVDC converter valve during high-voltage power transmission is provided.

The commutation failure of high-voltage direct current (HVDC) transmission system will have a huge power impact on the AC system. It is necessary to accurately identify the commutation failure of the HVDC for the security and stability control device (stability control device). At present, the stability control device which has already been applied in engineering for HVDC identifies the commutation failure of HVDC depends on the command of DC control and protection system, and there is a certain risk. The valve side current of converter transformer and the DC current of valve group that can be directly collected by the engineering are studied. Starting from the self-demand of the stability control device, new HVDC commutation failure and continuous commutation failure criteria are proposed based on the original commutation failure protection principle. Also, RTDS test verification was carried out.

Coordinated power control of electrochemical energy storage for mitigating subsequent commutation failures of HVDC

The electromagnetic interference (EMI) from the valve hall as an important consideration in the design of high voltage direct current (HVDC) converter stations is analyzed by electromagnetic field numerical method and measurements. By using moment method, the EMI filed strength level as well as radio interference level (RIL) of the valve hall during the normal operation are computed after an antenna model is built for the valve tower. According to the character of EMI obtained, the practical shielding measures for valve hall are discussed to satisfy the relative standards. The test results for the ±500 kV converter station show that both the numerical method and shielding technique used in this paper are practical.

This paper proposes a novel control method for controlling voltage source converter (VSC) based high voltage direct current (HVDC) converters present in a network consisting of a Doubly Fed induction generator (DFIG) - wind farm (WF) connected to infinite bus via an HVDC line and a parallel tie line. A new idea has been presented where inertia emulation capability of capacitors present in HVDC network has been utilized with a nonlinear control such as sliding mode control method. In this method the DC voltage reference for the control is allowed to vary with the frequency deviations in the system for emulating the inertia characteristics of the DC link capacitance and thus controlling the power oscillations. The results of the proposed method are then compared with that of conventional PI control. The simulations results obtained in MATLAB proved the effectiveness in stabilizing the oscillations in case of various transient events such as fault and power reference variations of the DC link.

Commutation failure is one of the commonest events in high voltage direct current (HVDC) systems. In the process of commutation failure, the DC voltage, current and power change drastically and impact the sending-side grid. At present, the related research lacks the quantitative analysis of transient overvoltage, which is caused by commutation failure in sending-side DC systems. Aiming at this problem, the equivalent model of the sending-side system is established. Considering characteristics of reactive power losses caused by the rectifier during commutation failure, analytical expression of transient voltage at the commutated bus is derived, then a quantitative calculation method of transient overvoltage and influencing factors are obtained. The results show that the weaker the AC system, the higher the transient voltage, and the transient over-voltage rises nonlinearly with the increase of the steady-state transmission capacity. Finally, the accuracy and effectiveness of the method are verified with the HVDC digital-physical hybrid simulation model based on Hypersim.

In the hybrid dual-infeed High Voltage Direct Current (HVDC) system, simulation methods are mainly used to study the inhibitory effect of Modular Multilevel Converter based HVDC (MMC-HVDC) on the commutation failure of Line Commutated Converter based HVDC (LCC-HVDC), but there is no mechanism analysis, which leads to a lack of universality. To solve the problem, this paper derives the expressions of LCC inverter bus voltage and short-circuit current after fault, quantitatively analyzes the support effect of MMC-HVDC on LCC inverter bus voltage, which provides theoretical support for MMC-HVDC to suppress the commutation failure of LCC-HVDC by providing reactive current. On this basis, a commutation failure suppression strategy for hybrid dual-infeed HVDC system is proposed. The reactive power compensation is obtained according to the required reduction of firing angle order and added to the MMC’s reactive-power outer control. This method can improve the reactive power support from MMC to ac system and suppress commutation failure. Finally, the mechanism analysis is proved correct and the control strategy is proved effective in PSCAD.

In a high voltage direct current (HVDC) transmission converter station, the purpose of the Reactive Power Control (RPC) is to facilitate the switching of alternating current (ac) filters to meet the reactive power demand with the ongoing dc power flow. In addition to it, the RPC will also ensure that the required ac filters are connected to prevent harmonics from entering into the AC system. In a comparatively smaller grid like North Eastern Region (NER) of India, with RPC in automatic mode, the switching in of ac filter sub banks of HVDC system due to sudden rise of transmitted dc power through the HVDC link during oscillations in the connected ac grid causes significant rise in ac voltage was a challenge and this challenge has been solved practically. By using the manual scheme of RPC without compromising the quality of power and by connecting bus reactors the problem of sudden rise of ac voltage during oscillations have been resolved.

The high voltage direct current (HVDC) transmission can significantly influence the transient stability of the power system to which it is connected. This work carries an investigation in which the active and reactive powers exchange by the HVDC converter with the system are modulated to improve the transient stability. For this study, a point to point (PTP) HVDC system connected to WSCC 3 machine 9 bus system is considered. The topology of the HVDC converters are considered to be hybrid modular multilevel converters (MMCs). The simulations are performed on PSCAD/EMTDC platform.

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.

More and more high voltage direct current (HVDC) converters are being located in a load area in the Yangtze River Delta Region in China. It’s large transmission capacity and intensive placement are hardly seen in other countries. Accurate and reliable study tools and methods are extremely needed for power system engineers and researchers to deal with such problems which were never met before and otherwhere. A novel approach on electromagnetic modeling of alternating current / direct current (AC/DC) system is proposed and simulation is carried out to replay a real AC 3-phase to ground fault on a multi-infeed high voltage direct current (MIHVDC) system. AC system is specially simplified. Dynamic models and real parameters are adopted in main AC/DC equipments concerned and retained. Modeling and simulation are based on the power system computer aided design/electro-magnetic transient in direct current (PSCAD/EMTDC) system software package. Comparisons between simulation results and the records from the fault recorders are studied including both AC and DC quantities.

Accurate detection and effective control strategy of commutation failure (CF) of high voltage direct current (HVDC) are of great significance for keeping the safe and stable operations of the hybrid power grid. At first, a novel detection method for consecutive CF is proposed. Concretely, the 2nd and higher orders’ derivative values of direct current are summarized as the core to judge CF by analyzing the physical characteristics of the direct current waveform of the converter station in CF. Then, the Daubechies wavelet coefficient that can represent the 2nd and higher order derivative values of direct current is derived. Once the wavelet coefficients of the sampling points are detected to exceed the threshold, the occurrence of CF is confirmed. Furthermore, by instantly increasing advanced firing angle β in the inverter side, an additional emergency control strategy to prevent subsequent CF is proposed. Eventually, with simulations of the benchmark model, the effectiveness and superiorities of the proposed detection method and additional control strategy in accuracy and rapidity are verified.

Several failures of high voltage direct current (HVDC) converter valve occurred in recent years. After carefully investigations, it was found that the thyristor voltage monitoring (TVM) board was responsible for many of them. In this paper, the working principle of TVM board is first introduced and a circuit simulation model is then built to carry out the simulation. After that, artificial contamination tests of TVM board are performed to verify the simulation. The results show that when the TVM board is seriously contaminated, the soluble salt components in the surface contamination layer can dissolve under high relative humidity conditions. This will reduce the equivalent balancing resistance which can result in check-back signals offset or even disappear. To crack this problem, regular cleaning is proposed to solve the TVM failure. This method has been successfully applied in Baoan converter station even the relative humidity is as high as 90%.

Soft computing likes ANFIS now penetrating the control system, especially for power semi-conductor and power system controls. Control of trigger angle is one of an important method to deliver power through high-voltage direct current (HVDC) transmission system effectively. Higher transient of current is one problem of HVDC on start-period, for up-ramp at 20 pu/s or higher. To cove this problem, the ANFIS control scheme is proposed to suppress the transient current in this research. The ANFIS control is built by using data train that these data are obtained from the simulation of HVDC system equipped by PI control under various loads and up-ramp rates. Training of ANFIS controller is done by input–output in 5000 data points. After training stage, the ANFIS controller is embedded into HVDC converter through fuzzy logic controller tool. Simulation results show that the proposed control is more effective to reduce the transient current in all scenarios compared to conventional PI control. Where the peak overshoot of transient current is achieved at the value of 1.11 pu for the proposed control. While the peak overshoot of transient current for conventional PI control is obtained at the value of 1.148 pu.