Abstract
To mitigate commutation failures (CFs) and minimize the instability of HVDC systems due to AC faults or control mode ambiguity, a constant extinction voltage-time area based control strategy with virtual resistance is proposed. In this strategy, using sine-cosine components detector and considering zero-crossing phase shift of commutation voltage, the extinction angle setting value can be adjusted dynamically. Meanwhile, to reflect the characteristics of DC current during the fault and recovery process, a virtual resistance is introduced into the control system and DC voltage considering the voltage drop of the virtual resistance is taken as the input of voltage-dependent current order limiter (VDCOL). Through theoretical analysis, the proposed strategy not only reduces the firing angle dynamically, but also reduces DC current by lowering the current order on the rectifier side immediately when the AC voltage disturbance is detected, thereby further reducing the occurrence of CFs. Therefore, the control strategy can effectively suppress successive and intermittent CFs. The effectiveness of the proposed control strategy is verified by simulation of the single HVDC and multi-infeed HVDC model based on the CIGRE HVDC benchmark system, in which AC-DC current criterion of identifying CF and suppression ratio index are adopted. The simulation results show that the proposed control strategy can effectively mitigate CFs under single-phase and three-phase faults to a certain extent. Comparing with the existing control strategies based on controller modification, i.e. commutation failure prevention, DC current predictive control, smooth logic switching control and DC current limitation control strategy based on virtual resistance, the proposed control strategy is superior in mitigation effects. The average suppression rates of these strategies are 1.66%, 3.21%, 6.11%, 3.35%, and 8.33% under single-phase fault, respectively; with the rates of 1.4%, 2.72%, 4.97%, 0%, and 6.8% under three-phase fault, respectively.
Highlights
Commutation failures (CFs) caused by AC faults are the most common and unfavorable dynamic events, which have been recorded in several practical systems [1]
voltage-dependent current order limiter (VDCOL) has been widely used in HVDC systems to reduce the risk of CFs and improve fault recovery performance according to DC voltage [39]
In order to solve this problem, inspired by the compensation resistance in the CIGRE HVDC Benchmark [40], this paper considers the limiting effect of resistance on DC current, the concept of virtual resistance is introduced to reflect the characteristics of the DC current during the fault and recovery process
Summary
Commutation failures (CFs) caused by AC faults are the most common and unfavorable dynamic events, which have been recorded in several practical systems [1]. The authors in [21], [29]–[36] directly used the inverter AC voltage to calculate the required firing angle; While the authors in [24] used the harmonic characteristics of commutation voltage to calculate These improved strategies can mitigate successive CFs to some extent, but it should be noted that by decreasing the inverter firing angle, the inverter reactive power consumption will increase, which will lead to further AC voltage drops. Based on the above concerns, in order to dynamically adjust the setting value of extinction angle and take advantage of the DC current characteristics during the fault and recovery process, a constant extinction voltage-time area (CEVTA) control strategy with virtual resistance is proposed.
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