Abstract
With the increasing penetration of renewable energy into the power system, the voltage source converter (VSC) for integrating renewable energy has become the most common device in the electric network. However, the operating stability of the VSC is strongly dependent on its operating control strategy, which is also highly related to the strength of the AC system. Choosing the control strategy of VSC for different strengths of AC systems becomes an essential issue for maintaining the symmetry between high proportion of renewable energy integration and stable operation of AC system. In order to obtain the operation zones of the control strategies of the VSC under different strengths of AC system, in this paper, the two common VSC control strategies, vector current control (VCC) and power synchronization control (PSC), are compared. Firstly, the principle of VCC and PSC are introduced. Then, based on the short circuit ratio (SCR) and the power limit calculation under steady-state conditions of the VSC, the operation zones of the vector current control and power synchronization control are proposed. Finally, a medium voltage modular multilevel converter (MMC) system was built in PSCAD/EMTDC and the proposed operation zones of the VCC and PSC were tested by changing the SCR of the modified IEEE 33 bus system and analyzed via the critical short circuit ratio (CSCR) analysis, the small-signal stability analysis, and transient stability analysis. The results indicate that, as the SCR decreases, the VSC based on VCC is gradually worked into unstable conditions, while the stability of VSC based on PSC gradually increases. The analysis results provide a criterion for the converter operation strategy change that could significantly improve the operating stability of the VSC in the power system and realize the symmetry of the stability of the converter and the change of the strength of the AC system.
Highlights
Due to the onerous environmental responsibility for reducing greenhouse gas emissions, renewable energy is being recognized as the most effective solution by governments and will become the mainstream energy in the world with irresistible trends [1,2]
It is interesting that the voltage source converter (VSC) based on vector current control (VCC) will work into unstable conditions when the AC system has a lower short circuit ratio (SCR) [11] that is generally deemed as an important index of the strength of an AC system with a converter connection [12,13,14]
With the critical short circuit ratio (CSCR) analysis, small-signal stability analysis and transient stability analysis, the proposed operation zones of the VCC and power synchronization control (PSC) are analyzed by changing the SCR of the modified IEEE 33 bus system in a medium voltage modular multilevel converter (MMC) system built in PSCAD/EMTDC
Summary
Due to the onerous environmental responsibility for reducing greenhouse gas emissions, renewable energy is being recognized as the most effective solution by governments and will become the mainstream energy in the world with irresistible trends [1,2]. Due to the system occurring fault, generator tripping or putting into operation will cause the decrease or increase of SCR, in order to clarify the impact of the continuous change process of the SCR on the power system, and provide the theoretical basis for the follow-up study of reliable and flexible control strategies of grid-connected VSC under the complex operating conditions. It is necessary to compare and analyze the system stability of VCC and PSC strategies under continuous SCR change, so as to achieve the best performance of the converter and keep the symmetry of power supply and demand under different strengths of an AC system. 3. With the CSCR analysis, small-signal stability analysis and transient stability analysis, the proposed operation zones of the VCC and PSC are analyzed by changing the SCR of the modified IEEE 33 bus system in a medium voltage MMC system built in PSCAD/EMTDC. Where PdN is the rated active power of the converter, Vt is the converter bus voltage at rated DC power, and Zs is the Thevenin equivalent impedance of the AC system
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