Flexible HVDC Research Articles

Impact of Wind Generators Number and Location on the Resonance Risk of Wind Farm Integration through Flexible HVDC System

Resonances or instability phenomena may occur in wind farms integration through a flexible HVDC transmission system. The relationship between the resonance risk and wind turbines number under different lengths of the line from wind farms to the flexible HVDC converter station is studied in this paper. First, the effect of the line length on the impedance characteristics of interconnected system is analyzed, and the aggregated RLC circuit approach is applied for the quantitative assessment of potential resonance risk. Then, the variation trend of system impedance characteristics caused by different wind turbines number is revealed, and resonance risk affected by the number of wind turbines integrated from the different location is evaluated by calculating system resonance damping around the resonant frequency. Finally, the simulation results on the RT-LAB platform validate the theoretical analysis.

Fault diagnosis and fault tolerance control of multilevel sub-module based on HONBM self-inclusion

The mathematical relation between the sub-module capacitance and the DC system operation characteristics has been analyzed from 4 aspects including the steady energy exchange process of modular multilevel flexible HVDC system, the dynamic response characteristics of active power control, the transient energy exchange process and the bridge arm protection requirement when the DC bipolar short circuit fault. A fault diagnosis and error prevention strategy for the multilevel sub-module system based on the higher order naive Bayes model (HONBM) is proposed. Firstly, analyze the model structure of the multilevel transmission sub-module, and give the Bayes model structure for the fault analysis of the electrical components in the multilevel transmission sub-module model. Then, based on the theoretical analysis results, the selection principle and the calculation method of the modularized multilevel sub-module capacitance are given. The sub-module capacitance is usually calculated according to the requirements of the steady-state sub-module voltage fluctuation and the dynamic response characteristics of the active power control, and then the calibration is made based on the transient sub-module voltage fluctuation and the bridge arm protection requirements.

Modelling and resonance analysis of wind power integration via flexible HVDC considering wind speed and wind farm location

Field experience has shown that harmonic resonance may occur in wind farms integration through a flexible HVDC transmission system. The impact on resonance characteristics from wind speed and length of the connection line from wind farm to flexible HVDC converter station is studied in this paper. First, the system model of wind farm integration through flexible HVDC system is established, including the whole structure of electrical model, control structures of converters and the impedance measurement model. Then, the effect of the line length and wind speed on the impedance characteristics of interconnected system is analyzed, and the aggregated RLC circuit approach is used to evaluate the risk of potential resonance quantitatively. Finally, the simulation results on the RT-LAB platform validate the theoretical analysis.

Study on VSC-HVDC grid topology of offshore wind farms

In this paper, the mathematical model of VSC-HVDC is established in dq synchronous rotating coordinate system, and the double closed loop vector control strategy for wind farm networking is discussed. The control strategy and model of the feed end converter station and the receiving end converter station are studied in the light of different working conditions. This paper investigates the development of the wind power industry in Cangzhou and the wind power absorption capacity in South of Hebei. It analyzes the necessity to use VSC-HVDC networking access methods to improve the wind farm and raise the wind power absorption capacity in South of Hebei. What’s more, the possibility of supplying several wind farm power to south of Hebei power grid through the flexible HVDC transmission system is analyzed, and a simple multi-terminal flexible direct current transmission model is established according to the characteristics of the power grid. This paper provides theoretical support for a larger scale wind farm in Cangzhou.

Enhancing Frequency Stability of Asynchronous Grids Interconnected with HVDC Links

Frequency stability management of asynchronously interconnected grids is becoming a great challenge. This paper investigates how asynchronous sending- and receiving-end grids can be operated in a synergistic and interactive manner to guarantee frequency stability following the worst case HVDC bipole block while still ensuring operational economy. A novel enhanced frequency stability constrained multiperiod optimal power flow model (EFOPF) with flexible fast-acting HVDC corrective control is proposed. EFOPF dynamically schedules the distribution of primary reserves and HVDC corrective actions to simultaneously handle postcontingency over- and under-frequency disturbances occurring in the sending- and receiving-end grids, respectively. An alternating direction method of multipliers based distributed algorithm was developed to solve this problem. A case study of a modified two-area RTS-96 system demonstrates the effectiveness of the proposed EFOPF.

AC Filterless Flexible LCC HVDC With Reduced Voltage Rating of Controllable Capacitors

This paper presents a further significant development to the developed flexible LCC HVDC system with controllable capacitors [1] , which can provide ac voltage/reactive power control [2] . The development involves the installations of fixed parallel capacitors at the valve side of converter transformer, which brings the following significant benefits: First, ac filter banks at the ac side of converter transformer are not needed as a better harmonic filtering performance can be achieved; second, significant reduction of the HVDC station land requirement (compared with traditional LCC HVDC), as the ac filters together with the switchgear can occupy over 50% of the HVDC station footprint; third, up to 50% reduction of the required voltage rating and more than 60% reduction of the capacitance of controllable capacitors for commutation failure elimination can be achieved while similar power system dynamic performance (ac voltage/reactive power control) compared with that in [1] can be demonstrated. Detailed analyses are presented to illustrate the effective commutation process and superb harmonic filtering performance with the fixed parallel capacitors. Selections of the component values are presented. Simulation results in real-time digital simulator are presented to verify the effectiveness of commutation failure elimination, power system dynamic performance, harmonic filtering performance, and show voltage/current stress of the fixed parallel capacitors.

Supporting Capacity-Building Project Characteristics Test of Zhoushan Multi-Terminal HVDC Flexible Demonstration Project

In this paper, a flexible Zhoushan multiterminal HVDC test demonstration projects supporting capacity-building project for the study, the DC cables through the flexible testing ground field measurements of its surrounding environment, the DC cables of a flexible environment affect the characteristics of the test site. The results show: the project on the surrounding environment has little effect of electromagnetic radiation, which affect the field strength generated by the project on the environment is negligible, the project generated frequency electric field, power frequency magnetic fields, DC magnetic field, radio interference strength, noise are smaller than the corresponding standard limits.

Harmonic suppressing control strategy for MMC–HVDC

Modular multi-level converter based high-voltage direct current transmission (MMC–HVDC), which combines the high-power capacity of conventional HVDC and the excellent dynamic performance of flexible HVDC, is the hotspot of research and application. Recent research finds MMC may produce harmonic current on its DC side and as the same as a line-commutated-converter, there is also harmonic coupling phenomenon between an MMC's AC side and DC side. In order to suppress the harmonics in MMC–HVDC and prevent the occurrence of AC–DC harmonic instability, this study analyses the current path of harmonics in an MMC. On this basis, a harmonic suppressing control strategy (HSCS) for MMC–HVDC is proposed, which includes an AC harmonic suppressing strategy and a DC harmonic suppressing strategy. PSCAD/EMTDC show that the proposed control strategy can effectively suppress the harmonics in MMC–HVDC and block the transmission of harmonics through the converter between AC and DC side, which is beneficial to the stable and reliable operation of MMC–HVDC.

Research on fault diagnosis for MMC-HVDC Systems

With the development of flexible HVDC technology, the fault diagnosis of MMC-HVDC becomes a new research direction. Based on the fault diagnosis theory, this paper proposes a robust fault diagnosis method to study the fault diagnosis problem of MMC-HVDC systems. By optimizing the gain matrix in the fault observer, fault detection with good sensitivity and robustness to disturbance is achieved. In the MMC-HVDC system, because of the inherently uncertain system and the presence of various random disturbances, the study of robust fault diagnosis method is particularly important. Simulation studies during various AC faults have been carried out based on a 61-level MMC-HVDC mathematical model. The results validate the feasibility and effectiveness of the proposed fault diagnosis method. So this fault diagnosis method can be further applied to the actual project, to quickly achieve system fault diagnosis and accurately complete fault identification.

Study on a New Type of HVDC Flexible Digital Pulse Width Modulated Carrier Frequency Selection

For multi_level multi_module in the flexible HVDC converter (MMC) module capacitor control algorithm is commonly based on nearest voltage level modulation (NLM), this algorithm requires the decimal part of the input sine wave for analog-to-digital conversion by digital signal (digital pulse modulation, referred to as DPWM),in the choice of hysteretic DPWM control signal is better, if the input triangle carrier wave frequency selection is not good, in each digital signal level will cause the output frequency of DPWM is not stable, some DPWM frequency is too high, and some DPWM has no output. In order to solve this problem, to meet the requirements of control precision, the carrier frequency change DPWM triangular wave, the output ideal control signal, the output control signal is analyzed, put forward how to carry out DPWM frequency selection. Through the simulation certified the feasibility of this method, the control method of downloading to the field programmable gate array (FPGA) chip, the results of the actual control operation show that this method can optimize the control of flexible HVDC.

Hysteretic Control Technology in New Flexible HVDC

The nearest level modulation (NLM) is commonly used in the multi-level multi-module converter (MMC), which need to convert the fractional part of the input sine wave to the digital signal (DPWM). However, the narrow pulse width of digital signal , the frequent change of the digital signal and the frequent instability of output are caused by the sensitive fractional portion of the input, which push Multilevel converter into the position where it has to operate in high frequency and be unstable. In order to solve the problem mentioned above, this paper provides a way called hysteresis comparison to meet the control requirements of precision, adopting the comparison between the fractional part and triangular carrier wave. The approach this study provides is called hysteresis comparison which not only can ensure the stability of the output of the digital signal but also can be able to obtain better digital pulse width by adjusting hysteresis threshold trigger. At the same time, it could ensure the stability and pulse width of the output by means of reducing the frequency of the digital output. The feasibility of this approach has been certified by the simulation of field-programmable gate array (FPGA). Apparently, the actual results show that this method can optimize the control of HVDC Flexible.

Research of High-Voltage IGBTs for Series Connection with Active Voltage Clamping

Because of the high voltage, high current and easy to be driven, IGBT become the optimal selection of the power convert system. As the improvement of power electronics, the voltage becomes higher and higher, such as high inverter, HVDC and Flexible HVDC system. The voltage should be dozens of kilo-volt. Single device cannot be used in these occasions. Thus, IGBTs for series connection is necessary.In the series connection of IGBTs, as the result of the different parameters and the delay of the drive signals, the main problem is unequal sharing of voltage respectively. The devices which withstand over voltage might be broken down and result in the fault of the whole circuit. Therefore， it is necessary to protect the over voltage of the IGBT.The method of active voltage clamping for the High-Voltage IGBTs for series connection is presented. The transient voltage suppressor(TVS) lied between the collector and gate of IGBT is adopted as active clamping circuit, and the clamping voltage value is decided by rated IGBT voltage and the working voltage. When turning of on the IGBT, TVS is broken down by over voltage, and the voltage between collector and emitter is clamped．

Study on Flexible HVDC Transmission System Based on PSO-ADRC

This paper expounds the control principle of active disturbance rejection controller (ADRC). The traditional flexible HVDC transmission system has a lot of disturbance factors and uncertainty,so Particle Swarm Optimization ( PSO ) algorithm is proposed to optimize the theory of ADRC which is used to design out-loop control system of flexible HVDC.The simulation results prove that control performance of system has been effectively improved .

The Research of the Photovoltaic DC Grid-Connected Converter Applied in Flexible DC Grid

With the rapid development of flexible HVDC technology in China, the demand for the DC grid of new energy generation projects growth. This paper studies the topology of the DC grid photovoltaic converter, a topological scheme is proposed and detailed analysis of the topology of the work mode is presented. For the device extension problem of equipment used in high-voltage direct current power level, a connecting scheme based on modular IPOS program and a control strategy of uniform duty cycle are given. In this paper, the equivalent controlled source method for small-signal dynamic modeling is established, and the stability of the control scheme is analyzed using Bode diagram. Finally, a 10kW prototype platform topology programs is built and control strategies experimentally is verified. This paper has important reference value for the HVDC grid-connected research of new energy power generation and the research of small power scale AC-DC hybrid microgrid network.

A Novel Modulation Method for Modular Multilevel Converters

Modular Multilevel has been widely applied in Flexible HVDC, electric locomotives and PET(Power Electronics Transformers). The modulation method is important for the converter performance. This paper presents an improved modulation method for Modular Multilevel Converter.

Development of Back-to-Back Flexible HVDC Prototype Based on MMC

With the development of flexible HVDC technology in the world, back-to-back HVDC system based on MMC topology have more attention. A high power back-to-back flexible HVDC prototype has been developed to assess performance and reliability of construction of converter valve segment. Valve normal working condition (rated conditions rated voltage, current, temperature), valve transient working condition (temporary overvoltage, overcurrent), valve fault protection and anti-interference ability are accessed. Performance and reliability of control and protection equipment are assessed. The design of control and protection strategy is optimized. The correctness of main circuit design is tested.

Voltage Stability Analysis of Wind Power Integrated System Based on Flexible HVDC

With the scale of integration of wind power increases gradually, the voltage stability and the fluctuation problem of the system caused by integration have become more and more prominent. Traditional AC power transmission mode cannot alleviate the adverse effects of integration of wind power, Flexible DC power transmission has many advantages due to the use of voltage source converters self-commutated characteristics, Flexible DC power provides a new technical method and also provides a solution for the integration. This paper introduces the basic structure and the basic control principle of Flexible DC power transmission technology. Then, through comparing the transient stability level and voltage supporting ability of AC power transmission with that of Flexible DC power transmission, an isolated system is used as an example to elaborate the beneficial contribution that the new integration of wind power make to the voltage stability, which indicates that Flexible DC power transmission technology has broad application prospect in the transmission area of wind power, and gives very good reference value for the Flexible DC power used in integration in the future.

DC faults ride-through capability analysis of Full-Bridge MMC-MTDC System

Full-Bridge Modular Multilevel Converter (FBMMC) has strong ability to ride through serious DC faults, thus it is very suitable for multi-terminal flexible HVDC applications. However, no references have reported the locating and isolating of DC faults and corresponding DC faults ride-through capability evaluation index. This paper introduces the topology mechanism of FBMMC and its loss reduction operation mode, theoretically certifies that the universal decoupled control strategy of Voltage Source Converter (VSC) and the similar modulation strategies of Half-Bridge MMC (HBMMC) can be applied to FBMMC for constructing complete closed-loop control system. On the basis of the existing DC faults locating and isolating schemes of 2-level VSC based Multi-Terminal HVDC (VSC-MTDC) system and the particularity of FBMMC, this paper proposes the DC faults wire selection “handshaking” method of the FBMMC-MTDC system, and proposes the DC Fault Ride-Through Capability Index (DFRTI) for evaluating the DC faults suppressing capability of the VSC-MTDC systems, including FBMMC-MTDC. Simulations of FBMMC-MTDC in PSCAD/EMTDC validate the correctness and effectiveness of the proposed control strategy and evaluation index.

Processing of Symmetrical Components in Time-Domain

A novel system for decomposing a set of three-phase signals into its constituting symmetrical components is proposed. The system also estimates the frequency, the magnitudes and phase-angles of the sequence components. The system is useful for online estimation of instantaneous symmetrical components and their attributes in the presence of frequency variations. The proposed system can be considered as a fundamentally improved version of the conventional three-phase Phase-Locked Loop (PLL) system which is widely used in power system applications. From this viewpoint, the proposed system obviates the inherent shortcoming of the conventional PLL system which is its erroneous response in the presence of negative-sequence component. Mathematical derivations and computer simulations are presented to describe the principles of operation and the performance of the proposed system. Examples of applications of the proposed system are in Flexible ac Transmission Systems (FACTS), HVDC systems, and Custom Power Controllers