Commutation Failure In HVDC System Research Articles

Analysis of Asymmetric Fault Commutation Failure in HVDC System Considering Instantaneous Variation of DC Current

HVDC is an important part of reducing energy transmission losses and maintaining energy sustainability. Commutation failure is the most common fault in HVDC systems, but existing commutation failure analysis approaches for HVDC systems do not consider the effects of instantly increasing direct current on the turn-off angle after an asymmetric fault in the AC system. To address this problem, we developed a commutation failure analysis approach that considers instantaneous variation of the DC current and AC voltage after asymmetrical faults. Firstly, the effects of the AC voltage and the DC current on the turn-off angle and the coupling relationship between the two are analyzed. Secondly, an equivalent mathematical model of the DC line, which covers the reactance, is built in Laplace space. Combined with the phase angle offset generated by the voltage after an asymmetric fault, a single relation expression containing only the AC voltage and turn-off angle is obtained by decoupling the DC current and AC voltage. The critical instantaneous AC voltage leading to system commutation failure is then derived based on the critical turn-off angle. Lastly, based on the CIGRE HVDC model built in the PSCAD electromagnetic transient simulation software (PSCAD v46), the accuracy of the proposed commutation failure analysis method compared with the other two methods is verified via simulation experiments under different grounding impedance values, and the applicability of the proposed method is further verified using simulation experiments with different smoothing reactor parameters.

Commutation failure prediction method based on characteristic of accumulated energy in inverter

Concerning the difficulty in predicting commutation failure in HVDC transmission system, a commutation failure prediction method based on the energy accumulation characteristic of 12-pulse inverter is proposed. First, the operation characteristic of valve-side current in the commutation process is analyzed. And then, according to the characteristic of cross commutation between Y bridge and D bridge of 12-pulse inverter, the model of accumulated energy difference between commutation inductances is built. On this basis, considering the instantaneous values of real-time firing angle and valve-side current in the DC system, the commutation failure prediction criterion is constructed, which combined with the variation rate of valve-side current can predict commutation failure. Simulation tests in RTDS verify that, the proposed method can effectively predict commutation failure in HVDC system without phasor calculation. Besides, it is strongly immune to disturbance and fault resistance and unaffected by the type of AC system fault.

Research on Double-Deck Traceability Identification Method of Commutation Failure in HVDC System

In AC/DC hybrid power system, AC system failures and commutation valve trigger pulse disorder will lead to commutation failure, which may lead to DC voltage fluctuations, power transmission interruption and other serious consequences. In order to accurately and effectively identify the specific causes of commutation failures, a double deck traceability identification method is proposed in this paper. The surface identification based on wavelet entropy and affinity propagation (AP) algorithm can distinguish internal and external faults. The deep identification uses convolution neural network which can further lock the specific cause of commutation failures. In this paper, 1) the various factors leading to commutation failures are analyzed; 2) the fault feature space consists of the wavelet analysis components of DC voltage signal, and the AP algorithm is used to identify the surface source; 3) the DC current, AC voltage and current signals are added into the sample matrix of fault time-space, and the convolution neural network is used to identify the deep traceability. Finally, the accuracy of the method is verified by using the typical HVDC model.

A criterion for accurate identification of commutation failure in HVDC systems

Commutation failures (CFs) caused by AC faults are the most frequent and adverse events in HVDC systems. Accurately confirming CFs has great significance in guaranteeing the safety and stability of AC/DC power systems. The requirements of CF criteria are as follows: not only can whether CF occurs be judged but also the beginning and ending instants can be accurately determined. For the problem that a CF cannot be accurately determined in an electromagnetic transient (EMT) simulation of an HVDC system, in this paper, based on further investigation of commutation processes and CF mechanisms, an AC-DC current criterion (ADCC) is proposed to accurately identify single, double and successive CFs in HVDC systems. The ADCC is derived from Kirchhoff's current/voltage law and is based on the DC current and the valve-side AC current. Detailed explanations and justifications are given that are strictly proven. Moreover, for the convenience of studying CF problem, a method is presented to quickly determine the valves that suffer CF. The validity and accuracy of the ADCC are also demonstrated through EMT simulations under different scenarios, and the bridge arm short circuit phenomenon verifies the accuracy of the criteria. Compared with the ADCC, the applicability of existing CF criteria is reexamined.

Assessment of commutation failure in HVDC systems considering spatial-temporal discreteness of AC system faults

This paper presents a novel commutation failure (CF) assessment method considering the influences of voltage magnitude drop, phase shift, and spatial-temporal discreteness of AC system faults. The commutating voltage-time area is employed to analyze the spatial-temporal discreteness of AC system faults causing CF in high-voltage direct current systems, and the influences of fault position and fault time on CF are revealed. Based on this, a novel CF criterion is proposed, further considering the influence of voltage phase shift and the spatial-temporal discreteness. Then this research develops a new CF assessment method, which does not rely on electromagnetic transient simulations. A real case from the China Southern Power Grid is used to verify the practicability of the proposed method by comparing with simulation results obtained using PSCAD/EMTDC.

Application of SFCLs to Inhibit Commutation Failure in HVdc Systems: Position Comparison and Resistance Recommendation

Commutation failure is one of the most common failures that may lead to serious consequences in high-voltage direct current (HVdc) systems. Superconducting fault current limiters (SFCLs) have been proved effective to inhibit commutation failures. In this paper, different installation positions of SFCLs are compared through three proposed evaluation indexes. The position between ac and dc systems is proved to have the best performance, and the position at the end of dc line performs moderately but costs less. Then, methods for calculating the convergence resistance of the SFCL are recommended. The theoretical method can draw the lower limit of the resistance quickly and reduce simulation time drastically when SFCLs are installed between ac and dc systems. In addition, an improved structure is presented to ensure fast recovery of an SFCL, so that the system can be restored quickly when the fault disappears. Simulations are carried out on PSCAD/EMTDC against the CIGRE HVdc benchmark system to verify the validity of the proposed installation scheme.