Abstract
As a variant modular converter configuration, the alternate arm converter (AAC) is well-suited for high-voltage power transmission and large-scale integration of renewables. In contrast to conventional multilevel converters, the director switches in the arms of AAC lead to the introduction of an overlap period, during which circuiting current can flow through the two arms in the same phase. Thus, fixed or variable overlap period control can be implemented in AAC systems so as to dynamically balance stored arm energy. However, the control of overlap period is linked to instability issues that might impede the safe operation of AAC systems, which are yet to be reported. In this paper, the stability of an AAC system is demonstrated based on measured grid and converter impedance, in conjunction with impedance-based stability criterion in the dq frame. The interaction between harmonic sources at AC and DC sides of the AAC system is analyzed to determine resonant frequencies in the AC current when any potential resonance is identified in the dq frame. Novel results with respect to the impact of overlap period on the system stability are obtained by depicting and comparing the Eigenloci in the polar plot, which are validated by real-time simulations.
Highlights
Nowadays, power converters are increasing in medium/high-voltage applications, e.g., HVDC power transmission and renewable energy integration [1]
This paper firstly demonstrates the stability of an alternate arm converter (AAC) converter according to the impedance-based stability criterion in the dq frame
With the stability analysis technique, this paper reveals the impact of overlap period on the Eigenloci of return ratio matrix and the stability of the AAC system
Summary
Power converters are increasing in medium/high-voltage applications, e.g., HVDC power transmission and renewable energy integration [1]. The resonance caused by impedance interaction will affect the control performance of converter systems and amplify specific harmonics in the power system. State-space modeling analyzes the interaction between the converter and the grid based on the details of hardware setup and control designs of converters. This type of model gives deep insights into the relationship between unstable modes and specific state variables [6]. Impedance-based stability analysis can help to facilitate converter design tasks of practical engineers and to set specifications for converter integration required by system operators. As a modular converter configuration used for high-voltage power transmission and large-scale renewable energy integration, the control aspects of alternate arm converter (AAC) have been discussed widely. The analysis is further supported by real-time simulation results using RSCAD and RTDS
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