Session E2.1: HVDC Transmission System

The commonly used double-tuned filter and triple-tuned filter in HVDC transmission system can usually be equivalent to the parallel single-tuned filters. The parameters of double-tuned filter and triple-tuned filter can be obtained from the parameters of parallel single-tuned filters by equivalent transformation. In addition, there is no model of double-tuned filter or triple-tuned filter in some common simulation software, which brought some difficulties to the harmonic analysis of HVDC. Therefore, it is significant to transform the double-tuned filter and the triple-tuned filter to the parallel single-tuned filters equivalently. Based on the circuit equivalence of double-tuned filter, triple-tuned filter and single-tuned filter, a parameters equivalent transformation method for AC filters in HVDC system is proposed. The forward and backward equivalent transformation formulas between the parameters of double-tuned filter, triple-tuned filter and parallel single-tuned filters are given respectively. Finally,  examples are given to verify the feasibility and validity of the parameters equivalent transformation method for AC filters in HVDC system.

Wen-tao Lyu was born in March 1989. He received the B.S. degree in electrical engineering from Zhejiang University, Hangzhou, China, in 2014. His research focus is on FACTS, and power quality of power grid.

In the multi-infeed-direct-current receiving-end transmission power systems, because of the interactions among alternating-current (AC) and direct-current (DC) subsystems and large-scale infeed DC power, there exists many problems needing to be solved, such as insufficient voltage-support ability, standard-exceeding short-circuit current, probabilities of simultaneous commutation failures for multi-infeed DC channels caused by AC faults. With its fast and flexible power regulation, effective bus-voltage support and no induced additional short-circuit currents, flexible DC transmission technology has been widely used in long-distance large-capacity power transmission, system integration for long-distance large-scale offshore wind farm, asynchronous system construction and so on. As a typical multi-infeed-direct-current receiving-end transmission power system, the safety and stability of Jiangsu power grid has been threatened by the integrations of sharply increased large-scale DC infeed power and long-distance offshore wind farms. In this paper, affections from application of the flexible DC transmission technology to the safety and stability of Jiangsu power grid are analyzed from two aspects as short-circuit current and ability for voltage support. With application of the flexible DC transmission technology, it can not only to optimize the power flow distribution of Jiangsu power grid, but also effectively improve its safety and stability.

Dr. Hui Cai received his bachelor, master and PhD degrees from Southeast University, China; RWTH Aachen University, Germany and Queen's University Belfast, UK. Now he is working in State Grid Jiangsu Economic Research Institute, Nanjing, P.R. China. His research interests are including transmission system planning, renewable energy integration and flexible transmssion devices application.

This paper develops an impedance-based small-signal model for the two-terminal VSC-based HVDC system. At first, the recent developments of impedance models of the converters involving mirrored frequency coupling effects are presented. Then, the small-signal perturbation-response characteristics of HVDC system are investigated, considering the frequency coupling effects of both the rectifier and inverter, as well as the dynamics of the DC-link. Based on that, a new sequence impedance model that consists of eight transfer functions is proposed for the whole HVDC system, which can not only represent the perturbation-response characteristics at each terminal, but also the transfer characteristics between the two terminals. Finally, the application of the new model for the modeling of complex multi-converter system is discussed.

Haijiao Wang received the B.S. and Ph.D. degrees from Zhejiang University, Hangzhou, China, in 2009 and 2014, respectively, both in electrical engineering. He is currently with the China Electric Power Research Institute. His research interests include the modeling and stability analysis of power system with renewable generations.

LCC-MCC hydrid HVDC system combines the advantages of both LCC-HVDC and MMC-HVDC, and will be widly used in the future.

The sub-module will withstand a very high voltage if a fault occurs in MMC converter station. In order to reduce this voltage, the LCC converter must operation in inversion state through gate-shift (GS) control stategy as quickly as possible. Normally, the fault signal was send to LCC station through communication and GS control strategy was triggered due to this signal. However the time lag between fault time and triggering time is 60ms, LCC converter station connot receive a fault signal as soon as possible.  In this way, ths SM voltage will be as high as 4270V which beyond the maximum withstanding voltage of IGBT. It will endanger the safety of devices.To solve this problem, a new topology of sub-module which combined with distributed energy dissipation resistance was proposed. 

In this paper, the basic principle of LCC-MMC system and parameters is introduced firstly. Secondly, the MMC converter station fault characteristics are analysed. Thirdly, the topology of sub-module combined with dissipation resistance was proposed, and the coordination control strategy is also studied. Finally, the model of LCC-MMC system is carried out in PSCAD simulation software.The simulation results shows that the maximum SM voltage could be reduced below 3578V.

Dingwen Hu, graduated from Xi'an Jiao Tong University and achieved his master degree at 2015. He majoed in electrical engineering. Now, he is mainly engaged in the research of MMC,  Flexible DC Distribution Network, and Hybrid DC Transmission System.

 Traditional grid-connected converter usually relies on phase-locked loop (PLL) to obtain the accurate phase of the grid in the normal operation. However, this PLL-based grid following control might induce a negative damping and result in the instability of the overall meshed AC/DC system when the grid short circuit ratio (SCR) is low or the control bandwidth of PLL is high. To overcome these drawbacks of traditional PLL based control, this paper proposes a novel virtual synchronous control for VSC based HVDC system without the need of PLL for inter-area interconnection of the power grid. In the proposed control, the droop property between the DC-link voltage and the frequency of the receiving-end system is artificially coupled. As a result, the emulated inertia support can be provided from the DC-link capacitor for the system under the severe system frequency deviations. Moreover, the sending-end AC system can sense the frequency disturbance of the receiving-end system without the remote communication by utilizing the local DC-link voltage and current variations of the VSC based rectifier. Accordingly, the sending-end system can provide the primary frequency control for the other side disturbed system by fully emulating the primary frequency regulation of traditional synchronous generator (SG). Compared with the traditional PLL based control , the proposed control  stands out itself by utilizing the grid-forming and grid-support potential of VSC for the disturbed AC system without relying the PLL and remote communication, and the stability of the receiving-end system is largely improved. 

Xiaotian Yuan received the B.S. degree in Xi’an Jiaotong University, Xi’an, China, in 2017, where he is currently working towards P.hD. His fields of interests include power system stability and control.

In recent years, HVDC has developed rapidly due to its long-distance, large-capacity and low-loss characteristics. Nowadays, China has built a large number of HVDC projects. However, due to the big use of non-linear components such as converters, most of the HVDC transmission systems have complicated harmonic problems. These harmonics may lead to the system resonance, threatening the operation safety and stability of the systems. For HVDC transmission systems with higher voltage levels, the greater harm caused by the resonance, so the analysis of system resonant characteristics is required.

This paper takes the ±1100kV Changji-Guquan UHVDC transmission project as an example to research on the resonant characteristics of HVDC transmission system. Analysis of the impedance frequency characteristics of the system under normal operation by using the test signal method and FFT transform, find the frequencies that may cause the system to resonate. And at the same time, the influence of lines, AC filters, smoothing reactors and other equipment on the impedance frequency characteristics in HVDC transmission systems is also studied. Based on this, measures to suppress harmonics are given to reduce the possibility of system resonance.

Feihong Yu, Master's degree, Department of Electrical Engineering, Xi'an Jiaotong University