Nonlinear control of voltage source converters in AC–DC power system P.K. Dash n , N. Nayak Department of Electrical Engineering, Siksha O Anusandhan University, Bhubaneswar, India article info Article history: Received 9 January 2014 Received in revised form 4 March 2014 Accepted 27 March 2014 This paper was recommended for publication by Jeff Pieper. Keywords: VSC-HVDC link Lyapunov direct method Sliding mode control Parallel AC line Super twisting algorithm Higher order sliding modes abstract This paper presents the design of a robust nonlinear controller for a parallel AC–DC power system using a Lyapunov function-based sliding mode control (LYPSMC) strategy. The inputs for the proposed control scheme are the DC voltage and reactive power errors at the converter station and the active and reactive power errors at the inverter station of the voltage-source converter-based high voltage direct current transmission (VSC-HVDC) link. The stability and robust tracking of the system parameters are ensured by applying the Lyapunov direct method. Also the gains of the sliding mode control (SMC) are made adaptive using the stability conditions of the Lyapunov function. The proposed control strategy offers invariant stability to a class of systems having modeling uncertainties due to parameter changes and exogenous inputs. Comprehensive computer simulations are carried out to verify the proposed control scheme under several system disturbances like changes in short-circuit ratio, converter parametric changes, and faults on the converter and inverter buses for single generating system connected to the power grid in a single machine infinite-bus AC–DC network and also for a 3-machine two-area power system. Furthermore, a second order super twisting sliding mode control scheme has been presented in this paper that provides a higher degree of nonlinearity than the LYPSMC and damps faster the converter and inverter voltage and power oscillations. & 2014 ISA. Published by Elsevier Ltd. All rights reserved. 1. Introduction Recent developments in HVDC transmission technologies like the pulse width modulated (PWM) IGBT converters have led to VSC-HVDC transmission systems. The world's first VSC-based HVDC system using pulse width modulation (PWM) IGBT con- verters was installed in March 1997 and from thereon, VSC-HVDC systems have attracted increasing attention, resulting in a number of installations in operation worldwide. Their applications are widespread from transporting power from offshore wind farms, connecting asynchronous power systems, distributed generation and power quality improvements, feeding remote passive loads, etc. The principal characteristic of VSC-HVDC transmission is that it can provide numerous advantages that include enhancement of voltage stability of AC power network, reduction of fault currents, independent control of active and reactive power on the AC network. Furthermore, this ensures a rapid reversal of active power flow in the network, enabling short-term transactions in the electric power markets. These features of the VSC-HVDC systems make them attractive for connecting weak AC systems, island networks, and renewable sources to the main distribution grid. The normal operating modes of the VSC-HVDC systems include (i) constant DC voltage control mode, (ii) constant active and reactive power control mode, and (iii) constant AC voltage control mode [1–3]. The control inputs for the VSC-HVDC trans- mission link are the modulation signal magnitude and phase angle at both the rectifier and inverter stations comprising voltage source converters and PWM control. The VSC-HVDC has different control requirements depending on specific applications. The most common approach in designing controllers for the VSC-HVDC system has been to use PI control. A properly tuned PI controller can damp out the system oscillation during a variety of disturbances and is easier to implement. However, when multiple oscillatory modes are present, PI control is ineffective and it requires several lead-lag blocks and coordinated tuning strategy [4,5]. Also when the system operating condition moves from the operating point around which the PI controller tuning was under- taken, the performance of PI controllers deteriorates considerably [4]. Other approaches have been presented in recent years that include the classical control, optimal control, and digital control strategies for small signal stability studies, LMI-based robust control [6], H-infinity control [7], adaptive control [8], decoupling control in converter stations using feedback linearization [9,10], active and reactive power control [11], input–output linearization with sliding mode control [12], frequency control [13], etc. The variable structure control strategy provides a robust controller once the system state reaches the sliding surface, is insensitive to parametric variations and uncertainties of the VSC-HVDC system Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/isatrans ISA Transactions http://dx.doi.org/10.1016/j.isatra.2014.03.011 0019-0578/& 2014 ISA. Published by Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: pkdash.india@gmail.com (P.K. Dash). Please cite this article as: Dash PK, Nayak N. Nonlinear control of voltage source converters in AC–DC power system. ISA Transactions (2014), http://dx.doi.org/10.1016/j.isatra.2014.03.011i ISA Transactions ∎ (∎∎∎∎) ∎∎∎–∎∎∎