Abstract
To investigate the oscillation problems in power systems with modular multilevel converters (MMCs), a disturbance-state model of the MMC under P/Q control mode is established based on the harmonic state-space (HSS) method. Firstly, the basic structure of the MMC with P/Q control is presented, including the circuit structure and the controller structure. Next, the dynamic characteristics of each module in the MMC are described with the time-domain state-space model or the s-domain transfer function. The complete controller structure is considered, including the P/Q outer-loop controller, the inner-loop control, the phase-locked loop (PLL), the circulating current suppression control (CCSC) and the control delay. On the basis, the disturbance-state HSS model of the MMC and its corresponding steady-state condition are established by the HSS method. Lastly, the case studies are performed to verify the accuracy of the proposed model, based on an MMC based test system in PSCAD/EMTDC. Moreover, one oscillation phenomenon in test system is illustrated by analyzing the AC-side impedance characteristics based on the proposed model. The results indicate that the MMC system under P/Q control mode has certain oscillation risk due to its negative resistance effect and the proposed model could be applied to its oscillation analysis.
Highlights
The modular multi-level converter (MMC) has become one of the most promising topologies of high voltage direct current (HVDC) systems, due to its more flexible control scheme and lower manufacturing difficulty [1,2]
The structure schematic diagram of the MMC under P/Q control mode is shown as Figure 1. It consists of three parts, namely the converter itself, the converter transformer and the control system. Both the converter itself and the converter transformer belong to the circuit structure, which are used in power transmission; the control system belongs to the controller structure, which is used in signal control
Under P/Q control mode, the phase-locked loop (PLL) is adopted to keep the MMC synchronized with AC system, which is very important for the stable operation
Summary
The modular multi-level converter (MMC) has become one of the most promising topologies of high voltage direct current (HVDC) systems, due to its more flexible control scheme and lower manufacturing difficulty [1,2]. In [12,13], the oscillation problem in the asynchronous systems connected by MMC-HVDC is investigated In these literatures, the frequency-domain impedance model is widely adopted to represent the MMC. The mid-point voltage of the DC side is usually ignored in the circuit structure of the MMC [23] It has an important influence on the disturbance components with zero-sequence characteristics [24]. Based on the disturbance-state model, the AC-side impedance characteristics could be analyzed to investigate the oscillation phenomenon in MMC-based power system. This remainder of this paper is organized as follows: In Section 2, the common structure of the MMC under P/Q control mode is introduced in details, including its circuit structure and controller structure. Both the converter itself and the converter transformer belong to the circuit structure, which are used in power transmission; the control system belongs to the controller structure, which is used in signal control
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