Flexible Control of Power Flow in Multiterminal DC Grids Using DC–DC Converter

Abstract

Abstract-This paper proposes an efficient control framework that utilizes de-de converters to achieve flexible power flow control in multiterminal de (MTDC) grids. The de-de converter employed in this paper is connected in cascade with the de transmission line, and is therefore named cascaded power flow controller (CPFC). In this paper, a two-layer control strategy is developed for the operation and control of voltage source converter stations and CPFC station in MTDC grids. At the primary control layer, a novel differential voltage droop control is developed, while at the secondary control layer, a modified de power flow algorithm-employing the new CPFC frameworkis implemented. The overall control strategy enables the CPFC to regulate the power flow in the de transmission line. The primary control guarantees the transient stability of the CPFC, and the secondary control system ensures the desired steady-state operation. The proposed voltage droop control framework helps the MTDC grid to remain stable in the event of a communication failure between the primary and secondary control layers. Static analysis and dynamic simulations are performed on the CIGRE B4 de grid test system, in order to confirm the effectiveness of the proposed control framework for power flow regulation in MTDC grids.Index Terms-CIGRE B4 de grid, de power flow control, multiterminal de (MTDC) grid, power system control, voltage droop control.

Flexible HVDC transmission systems small signal modelling: A case study on CIGRE Test MT-HVDC grid

“…An averaged model for the CDC-PFC is shown in Fig.1 [2]. However, for the static studies (e.g., powerflow) the impacts of inductance and capacitor vanish.…”

“…In addition, as the multi-vendor supply of the VSC-HVDCs is expected, future interoperability is a key challenge. Moreover, as the power-flow pattern is passively determined by the MT-HVDC configuration and the resistances seen between the HVDC buses [2] there will be a limited room to adjust the power flow pattern by redacting the set-points of power converters. Therefore, flexible power flow control between the buses is a key factor to enable congestion management, loop flow control, addressing power market requests and avoiding transmission bottlenecks [3].…”

“…The DC-PFC should fulfill requirements regarding its cost, footprint, losses, controllability, and reliability along with fulfillment of its main objective (power flow control) [2], [4,5]. The objective here is to maximize the exploitation of the existing transmission lines inside their safe and reliable boundaries.…”

“…Benefits such as great controllability and fault blocking capability are entitled to CDC-PFCs [2], [9]. On the other hand, Serial Parallel DC-PFC devices should only withstand a partial (typically around %5) rated system voltage.…”

Analysis on impacts of the shunt conductances in multi-terminal HVDC grids optimal power-flow

“…Whereas, control of power converters, direct voltages, and power sharing are assigned to the primary control layer. Indeed, appropriate reference settings are sent to the VSC-HVDC stations in a prescribed time intervals by the power dispatch control center of the MT-HVDC grid [7,8].…”