# Analysis of Six Active Power Control Strategies of Interconnected Grids with VSC-HVDC
## 1 Introduction
However, the stability issues mentioned above can be resolved by the robust compensator design mentioned by many authors [12; 13; 14; 15]. The robust PI (Proportional and Integral) parameters, feedforward controllers, and adjusted PLL parameters enable the stable operation of VSC-HVDC, and the damping condition which occurs at a certain frequency range can be mitigated. Therefore, in this work, the VSC-HVDC system is deployed without considering the small signal stability issues, and the main contribution of this paper is to analyze the impact of six active power control strategies on the generator angle stability of two interconnected grids. Excluding the contingency event, the fixed power control for two grids is commonly used to lessen the operation burdens of TSOs. However, during a contingency event, the power should be adjusted to provide grid services such as frequency support or transient stability support. According to the previous works related to VSC control for grid service, Reference [16] demonstrated the AC transmission emulation control strategy, which acts like an AC line when a contingency event occurs. It is able to mitigate the possible overloading of adjacent AC transmission, and maintain power balance between metropolitan regions. However, the transferred power is not exactly estimated since the output power is varied depending upon contingency event types; thus, it is not suitable for interconnected grids since there is a clear exchange clause in their agreement. In Reference [17], the flexible operation of the generator tripping scheme was achieved without a large decelerating energy as the generators trip, and it was confirmed that a simple converter control strategy that transfers the maximum power reserve instantly to the fault area surely contributes to the stability of the AC network. However, this paper only addressed one kind of step control. In References [18; 19], the DC voltage droop, local frequency control, and weighted-average frequency control are compared in detail; however, this analysis was performed in an embedded MTDC (multi-terminal DC) environment. In References [20; 21], the kinds of frequency-power modulation control strategies for the converter to enhance the system transient stability are introduced. As can be observed, there has been no detailed analysis of interconnected grids depending upon the several converter power control schemes in a point-to-point environment. Therefore, in this paper, two step control and four ramp-rate control scenarios are specifically defined, and then simulated to provide useful feasibility studies results for grid operators. A dynamic control model of the VSC-HVDC is developed written by Fortran language in the PSS\({}^{\text{\textregistered}}\)E program (Power Transmission System Planning Software), and the ideal averaged equivalent VSC model is used. The MMC (modular multilevel converter) is not used since the AC system stability is the major observation target. To perform this analysis, the GVIF index, meaning the Generators-VSC Interaction Factor, is newly defined in Section 2. In Section 3, the introduction of the VSC-HVDC model serves to illustrate the configuration of the active power controller. In addition, six active power control strategies are defined in Section 4. Lastly, a simulation of the transient stability regarding the control schemes is performed.