Electric Power Systems Research 112 (2014) 65–73 Contents lists available at ScienceDirect Electric Power Systems Research j o ur na l ho mepage: www.elsevier.com/locate/epsr Sliding mode active and reactive power decoupled control for Distributed Power Flow Controllers Ivo M. Martins a,∗ , Fernando A. Silva b , Sónia F. Pinto b , Isménio E. Martins a a INESC-id, DEE, ISE, University of Algarve, 8005-139 Faro, Portugal b INESC-id, DEEC, IST, TU Lisbon, 1049-001 Lisbon, Portugal a r t i c l e i n f o Article history: Received 20 May 2013 Received in revised form 4 February 2014 Accepted 19 March 2014 Available online 7 April 2014 Keywords: FACTS DFACTS UPFC DPFC a b s t r a c t Unified Power Flow Controllers (UPFC) are one of the most useful Flexible AC Transmission Systems (FACTS). They can be used for power flow control in AC transmission grids, allowing simultaneous con- trol of the bus voltage and line active and reactive power. However, due to high costs and reliability concerns, UPFCs have experimented limited use in such applications. Recently, the concepts of Distributed FACTS (DFACTS) and Distributed Power Flow Controller (DPFC) have been introduced as a low cost, high reliability alternative for power flow control. However, DPFCs present cross-coupled (interdependent) and limited regulation of active and reactive power. Therefore, this paper contributions include: (1) a third-harmonic output voltage controller for a full-bridge converter, able to extract active power from third-harmonic currents, to maintain the converter DC voltage constant; (2) DPFC sliding-mode con- trollers to simultaneously inject active and reactive power at the fundamental frequency, to achieve cross-decoupled (independent) control of active and reactive power flow; (3) applying the sliding mode controlled DPFC to a part of the Portuguese distributed generation and transmission network under study. To provide the required active power to each DPFC device, a PI controlled full-bridge converter acting as a virtual resistance is proposed to extract active power from zero-sequence harmonic frequency currents injected into the line. DPFC models including semiconductor switching, together with line transmission models, were simulated in Matlab/Simulink environment and in PSCAD for comparison purposes. Simu- lations results show the effectiveness of the full-bridge converter sliding mode controllers in decoupling P and Q control while simultaneously extracting active power from the injected zero-sequence injected currents. © 2014 Elsevier B.V. All rights reserved. 1. Introduction Nowadays it is essential to improve the performance of power lines and the optimization of power flow on existing lines. Flex- ible AC Transmission Systems (FACTS) offer the possibility of power flow control both in static and dynamic conditions, improv- ing transmissions systems [1]. FACTS devices can be inserted in existing transmission lines to achieve control functions, includ- ing enhancement of power transfer capacity, decrease line losses and generation costs, and increase the stability and security of the power system [2,3]. The most powerful and versatile FACTS device is the Unified Power Flow Controller (UPFC), proposed by L. Gyugyi in 1992 [4,5]. It uses solid-state power semiconductors and can be used for power ∗ Corresponding author. Tel.: +351 962058737. E-mail address: immartin@ualg.pt (I.M. Martins). flow control, improvement of transient stability and damping oscil- lations or active filtering [6,7]. With a UPFC it is possible to control a local bus voltage and active and reactive power flows of a trans- mission line. The UPFC series converter may have also other control modes such as direct voltage injection, phase angle shifting and impedance control modes [5]. A UPFC consist of two AC–DC converters, a Static Synchronous Compensator (STATCOM) and a Static Synchronous Series Compen- sator (SSSC), connected back to back via a common DC link allowing active power exchange between them. Both converters AC sides are connected to the transmission line through coupling transformers. The line series converter performs as a synchronous AC-voltage source, providing the main function of the UPFC, by injecting a controllable voltage vector, while the shunt converter acts as a syn- chronous source, controlling the voltage of the DC capacitor (U dc ). Fig. 1 illustrates the simplified diagram of UPFC. Although FACTS devices offer several benefits, they have not seen widespread commercial acceptance due to a number of http://dx.doi.org/10.1016/j.epsr.2014.03.026 0378-7796/© 2014 Elsevier B.V. All rights reserved.