In this study, an extinction angle predictive (EAP) control strategy is proposed to mitigate commutation failure (CF) in high voltage direct current (HVDC) systems. The proposed strategy works ...effectively during both AC system faults and commutation voltage distortions. At first, an index is put forward to evaluate the prospective commutation area drop and quantify the upcoming commutation stress considering the commutation mechanism. Then, the extinction angle is predicted based on the detected commutation area drop incident. Further, the predicted extinction angle is introduced into the HVDC inverter control to design the EAP control strategy, ensuring that the CF can be prevented due to a sufficient commutation margin. The advantage of the proposed strategy is that it evaluates the upcoming commutation stress based on the transient commutation voltage waveform. Thus, a better sensitivity for commutation voltage distortion can be ensured. The proposed strategy is simulated in the PSCAD/EMTDC platform and the results are compared with that of the conventional schemes, verifying the performance of the proposed strategy in mitigating the CF during AC faults and voltage distortion.
Commutation failure (CF) occurs frequently under inverter AC system faults. Around 70% of power system faults are Single-Line-to-Ground (SLG) faults that affect not only magnitudes but also phase ...angles of non-faulty phases. This has not been thoroughly analyzed in existing research. Therefore, this paper proposes a new commutation voltage calculation method where the commutation voltages under asymmetrical faults in Multi-Infeed LCC-HVDC systems are derived. The proposed method considers the impact of transformer connections, fault location, fault impedance, and voltage magnitude change and voltage phase shift of faulty and non-faulty phases that have not been considered in previous publications. Secondly, based on this proposed commutation voltage calculation method, CF prediction criteria based on the Voltage-Time Area (VTA) theory are proposed. Thirdly, a calculation method of the extra commutation voltage needed for each inverter to avoid CF is proposed to measure how close the commutations are to being successful under SLG faults. The effectiveness of the proposed methods is demonstrated by simulation results using the CIGRE LCC-HVDC Benchmark system and a six-machine-two-area (6M2A) test system simulated by the Real Time Digital Simulator (RTDS).
This paper investigates the in-depth mechanism of commutation failure for a line-commuted converter-based high-voltage direct current (LCC-HVDC) system. The commutation failure prevention control ...(CFPREV) and the initial fault voltage angle (IFVA) are considered from the view of the voltage-time area (VTA) in the analysis. It is revealed that the IFVA is among the dominant factors for commutation failures when the voltage drop of the inverter bus is relatively small, and CFPREV further intensifies the impact of the IFVA on commutation failures, while the fluctuation of the direct current plays a dominant role in commutation failures under a greater voltage reduction at the inverter bus. A quantitative division of the severity of AC faults is proposed to determine dominant factors for commutation failures. The relationship between the chance of commutation failures to occur and the IFVA is built, and the method used for computing probability of commutation failures is proposed. The influence of the dynamic of CFPREV output on our research is studied. Simulations based on a typical monopole LCC-HVDC system using PSCAD/EMTDC software are conducted to verify the correctness of the theoretic analysis and the effectiveness of the proposed computing methods.
This letter presents a novel pseudo extinction angle (PEA) for the HVDC inverter, as well as its calculation method. The proposed PEA extends the measured extinction angle (EA) into a real-time ...calculated value. As a result, the non-measurability of the EA between two adjacent EA updating times, which is likely to cause commutation failures (CFs), can be avoided effectively. Simulation results verify the validity of the proposed PEA calculation method and prove the capability of the PEA to mitigate the CFs.
Abstract
Commutation failure (CF) threatens system safety with traditional HVDCs. To improve the recovery performance from CF, a novel recovery strategy is proposed, where a dynamic current ramp ...limiter is introduced to reduce the probability of current overshot during the recovery. The dynamic ramp limiter is designed based on the idea of judging the system recovery state by the dc current level, where a high level corresponds to a high risk of CF. The ramp limit is calculated by the frequency constraint of ac systems. Case studies based on a real-world system are used to verify the effectiveness of the proposed method, and the results show that the proposed method can improve the recovery performance and avoid the continuous CFs.
Procedure analysis of UHVDC commutation failure Wang, Chenqing; Zhang, Chi; Kong, Xiangping ...
Journal of engineering (Stevenage, England),
March 2019, Letnik:
2019, Številka:
16
Journal Article
Recenzirano
Odprti dostop
There may be a commutation failure during a fault or start-up of the UHVDC inverter side. The law of commutation failure process in UHVDC converter is studied here. First, the cause of inverter ...commutation failures is briefly introduced. Then, the general law for the bridge arm conduction of the inverter in the process of the commutation failure is revealed. Finally, the system debugging recorded data of some commutation failures of Taizhou UHVDC inverter in China is used to verify the law. Research shows that the commutation failure will go through three stages which has obvious features: (i) the commutation failure stage, (ii) the bypass circuit formation stage, and (iii) the commutation failure recovery stage. In the last stage, the AC currents of two conducted phases increase instantly in opposite directions, which are easy to recognise. In the second stage, only the DC current increases, and the AC currents equal 0 approximately. Because of the three stages, the commutation failure valve number is easily recognised. The study on the law of commutation failure process lay the foundation for commutation failure process analysis and identification for relay protection.
Any permutation w of the symmetric group can be generated by a product of adjacent transpositions, and a reduced word for w is a sequence of generators of minimal length whose product is w. The main ...result in this paper gives a formula to compute the diameter of a commutation class of the graph G(w), whose vertices are reduced words for w and whose edges are braid relations. To do so, we use a set-valued metric on the set of all reduced words of a given permutation which turn out to induce the usual distance in any commutation class. If a permutation is fully commutative, i.e. it has only one commutation class, then the formula gives the diameter of G(w). The diameter for a Grassmanian permutation is also given in terms of its Lehmer code.
Commutation torque ripple of brushless DC motor (BLDCM) is caused by the different current slew rates between switching-in phase and switching-out phase. If the slew rates were equal, commutation ...torque ripple could be eliminated. A simple full speed range method was presented in this paper to eliminate commutation torque ripple by dividing each pulsewidth modulation (PWM) period into three segments during commutation time. By selecting proper proportions of these three segments, the average current slew rate of switching-in phase can be kept equal with the average rate of switching-out phase in each PWM period. Because the commutation time is controllable in the presented method, commutation process which is bad for the operation of BLDCM can be finished in its highest speed. The theoretical derivation is given, the experimental results show that the commutation torque ripple can be eliminated in the minimum commutation time, and output power of BLDCM under rated load can be improved.
This letter proposes a comprehensive zero-voltage switching analysis (ZVSA) to illustrate the commutation mechanism of full-bridge (FB) mosfets in resonant converters. Inherent contradiction between ...two conventional ZVSA techniques is discussed with specific consideration of a full-bridge LLC (FBLLC) resonant converter. An analytical approach is proposed to determine the accurate zero-voltage switching (ZVS) criterion for both zero- and finite-phase overlap operations of an FB network. Impact of the proposed ZVSA on power losses of FBLLC is explained analytically. Experimental results are presented not only to validate the proposed ZVSA, but also to validate its effect on FBLLC. Low load efficiency is improved by 6% with the proposed approach.
The commutation failure immunity index (CFII) recommended by CIGRE is an useful indicator for quantifying the immunity of LCC-HVDC inverters to the CF. It should be calculated by the electromagnetic ...transient (EMT) simulations under the worst fault condition that doesn't cause the CF as per its definition. Moreover, the symmetrical three-phase line-to-ground fault has been empirically claimed by CIGRE as the worst one in the previous calculation method. However in this letter, it is firstly found that the unsymmetrical double-phase fault rather than the three-phase one is actually the worst as clearly observed from large numbers of the EMT simulations. This is achieved by the comprehensive analysis of the simulated CF characteristics when taking into account the diversified fault conditions such as fault type, property, severity, initiation time. Secondly, the above interesting finding motivates a corrected CFII calculation method to be proposed considering the double-phase fault. The proposed method is further shown to be more superior than the previous method by the case studies under various ac grid strength.
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