EMT simulation of an MTDC system integrating Modular Multilevel DC/DC converter with DC voltage control

Abstract

The increasing demand to utilize renewable energy necessitates the transmission of power over long distances. HVDC technology has emerged as the optimal solution for this purpose due to fewer losses and good economic factors. Multi-terminal DC (MTDC) systems are being more focused nowadays, as they offer more advantages over Point to Point (P2P) HVDC scheme because the MTDC network adds more reliability and flexibility to the system. DC/DC converters are emerging as an important device for future MTDC transmission systems. They are required to interconnect HVDC links with different system characteristics such as different DC voltage levels, grounding schemes, and technologies. In addition to this, DC/DC converters are capable of providing additional features in the system like grid protection, DC voltage control, and power flow control. The majority of studies in the literature on DC/DC converters are predominantly focused on the context of either exploring different DC/DC converter topologies or their control and operation in constant power mode for interconnecting HVDC links, suitable for future MTDC grids. This paper presents an MTDC test case integrating a DC/DC converter where the converter is working with a DC voltage controller and participating in the DC voltage management system. The influence of voltage-controlled DC/DC converter is studied by introducing power disturbances in the MTDC system. The system is modeled and simulated in EMTP software. The droop control technique known for the VSC converter for DC voltage control is extended to obtain a dual droop controller which is used with a DC/DC converter for controlling both DC grid voltages simultaneously. However, this control approach involves designing two droop coefficients for their respective DC grids, complicating the examination of their interaction. Another possibility is to use a new technique called “virtual resistance DC voltage control” which requires tuning only one parameter. The objective is to control DC grid voltages and establish a link between the interconnected networks. The control approach is validated through electromagnetic transient (EMT) simulations. Through the virtual resistance DC voltage control, the interconnected DC grids can share the power disturbance in the system and maintain the DC voltages under their specified limits. This makes the MTDC system more reliable and reduces the stress on the DC voltage management system. Modular multilevel converter (MMC) based topologies are used for DC/DC converters, namely F2F-MMC (front-to-front MMC) which can provide galvanic isolation between the two links and MMC-DC (M2DC) which does not provide galvanic isolation. A comparison analysis has also been made to compare their behavior with a virtual resistance controller. All converters are modeled using reduced order modeling methodology and the DC cables are modeled with wideband models. The observations in this paper indicate that by employing the virtual resistance DC voltage controller, a connection has been established between interconnected networks, enabling HVDC links to actively participate in and share the power disturbances within the MTDC system. Apart from this, the virtual resistance control behavior remains consistent regardless of the topology of the DC/DC converter, thus demonstrating its robustness as a DC voltage controller.