Enabling multi-terminal HVDC grid for asynchronous AC system support

In the future grid scenario, the multiterminal high-voltage direct current (MTDC) system is a promising alternative for interconnecting asynchronous AC grids and harvesting power from offshore wind farms. With the immense evolution of semiconductor technology, a modular multilevel converter (MMC) based MTDC system offers a cost-effective transmission of bulk power over long distances with higher efficiency. This paves the way for MMC-MTDC transmission to participate in energy trading markets with the capability of providing ancillary services like frequency regulation. However, the operation of a large AC-MTDC grid with a complex topology faces several key challenges. Lack of reliable DC grid protection schemes, AC grid frequency stability issues, and DC grid instability during contingencies are a few of the significant concerns. This thesis presents three main contributions to address the above-mentioned research challenges for facilitating the MTDC grid with asynchronous AC system support.

The first contribution of this research is to propose an adaptive droop control (ADC) strategy to improve the frequency regulation of asynchronous AC areas connected by an MTDC grid. The droop coefficients are adjusted to sharing the active power adaptively among multiple asynchronous AC grids based on the characteristics of frequency deviation and rate of change of frequency. This results in a cogent allocation of imbalance power in multiple asynchronous grids from the frequency variation perspective.

The second contribution of this research is to investigate the impact of the DC grid protection strategies on the dynamic performance of an AC-MTDC system and propose a transient stability index to quantify the DC grid stability. Based on the scale of a circuit breaker's operating time, the performance of three different protection strategies is examined and the impact on grid dynamics is analyzed extensively. Moreover, to assess the frailty of the MTDC grid, a transient energy stability index (TESI) is proposed considering the voltage variation in the pre-state and post-state fault clearing interval. The capability of enduring the DC grid fault eventually enhances the reliability and improves the dynamic performance of the grid.

The final contribution of this research is to develop a fast, simple, and thresholdfree non-unit protection scheme for DC fault identification in an MTDC grid. The continuous operation of large-capacity remote power transmission over an augmented DC network is imperative for implementing a reliable MTDC grid. A fast and selective DC fault identification method with improved reliability eventually enhances the operational continuity and flexibility of the DC grid. Based on the reactance value and reactor voltage gradient (RVG) of the current limiting DC reactor, a simple DC fault identification method is proposed without the threshold value setting. Therefore, the detection and discrimination of a DC fault are achieved by four sampling instants of the reactor value with improved accuracy.

The performance of the proposed techniques is evaluated in a modified multimachine power system using DIgSILENT Power Factory. For the DC test system, both topologies of the MTDC grid configuration, (i.e., radial and mesh) are adopted in the simulation model. Simulation results under significant frequency disturbances caused by credible contingencies are presented to demonstrate the effectiveness of the proposed frequency regulation (ADC scheme) approach. It is found that the proposed ADC scheme provides an excellent and robust frequency response under different operative conditions.

Regarding the operational impact of the DC protection strategies on AC-MTDC systems, the simulation results explicitly reveal that the dynamic performance of the MTDC grid significantly deteriorates with the slow functioning of the protection schemes, followed by a DC grid fault. Besides, prolonged recovery time causes a substantial loss of power infeed and affects the grid stability. To validate the effectiveness of the proposed TESI, relevant case studies are performed on the MTDC grid using an analytical approach. The non-linear simulations along with a detailed comparison study exhibit the efficacy of the proposed TESI.

In terms of the proposed relaying scheme for the MTDC grid, simulation results demonstrate that the effective identification of a DC fault strictly satisfies the speed requirement with a distinct feature of bus fault and line fault discrimination. Moreover, the relevant sensitivity analyses reveal that the proposed scheme remains invariant for a wide range of fault locations and resistances, and performs efficiently, even with a lower sampling frequency. In addition, the transient events apart from DC faults fail to trigger the protection, indicating the high robustness of the proposed scheme.