Virtual flux direct power-backstepping control of 5-level T-type multiterminal VSC-HVDC system

Abstract

Summary In this study, a nonlinear control of a 5‐level T‐type converter topology‐based multiterminal voltage source converter high‐voltage direct current system is proposed. The idea of the proposed control is to combine the backstepping control and direct power control with virtual flux concept into one controller able to improve the performance of the transmission system. In the other side, the use of a 5‐level space vector modulation with a balancing strategy based on effective use of the redundant switching states of the converters voltage vectors guarantees the objective of maintaining balanced voltages in DC‐capacitors. Finally, simulations of the 5‐level T‐type multiterminal voltage source converter high‐voltage direct current system validate the effectiveness of the proposed control law. The obtained results are compared with those performed by a conventional Proportional Integrator (PI) controller. These outcomes allow to exhibit excellent transient response during a range of operating conditions.

Virtual Flux Predictive Direct Power Control of Five-level T-type Multi-terminal VSC-HVDC System

“…The adopted control method used to balance the DC capacitor voltages is called the minimum energy property [9]. The balancing algorithm selects a combination of three adjacent redundant switching vectors that minimize the total energy stored in the DC-link capacitors; this energy reaches its minimum when all capacitor voltages are balanced [30].…”

“…Recently, the advanced power electronic devices with turn-off capability like Insulated Gate Bipolar Transistor (IGBT) have led to the development of VSC based HVDC transmission. The principal advantages of VSC-HVDC system is an independent control of active and reactive powers and faster power flow control with benefits in grid stability, bidirectional active power transmission without changing DC voltage polarity, operation with a weak or passive AC system, and no line commutation problems [4,8,9]. Specifically, multi-terminal HVDC (MTDC) is ideal for offshore platforms and it can also be used for connecting national AC grids with different frequencies [10].…”

“…A multi-terminal VSC-HVDC system consists of more than two converters connected through DC line cables and sharing the same DC bus. It has advantages over point-to-point HVDC transmission system one in many aspects such as control flexibility, security, reliability, and economy [9,11].…”

“…In DPC, there are no internal current loops or modulator block. Indeed, the converter switching states are selected via a switching table [9]. The main drawbacks of this control scheme are the variable switching frequency and fast sampling required for a digital implementation of the hysteresis controllers [27].…”

“…These drawbacks can be eliminated by replacing the hysteresis comparators and the switching table by a PI controllers and Space Vector Modulation (SVM) approach. This method is considered an effective way to reduce power ripple and it also has the advantage of a fixed switching frequency [9].…”