Electrical Engineering 79 (1996) 3o3-3io 9 Springer-Verlag z996 An advanced unified power flow controller M. Aredes, K. Heumann Contents This paper describes the implementation of a new control approach for the Unified Poet Flow Controller (UPFC), which provides active filtering capabilities to this equipment, besides the functions of power flow control and voltage con- trol, as originally proposed by L. Gyugyi [ 1,2]. The power circuit is made up from two PWM converters which share a common dc link without the need of dc power supplies. An integrated controller attends to both series and shunt converters of the UPFC. It is based on the theory of instantaneous active and reactive power defined in the ~-fi-0 reference frame [3,4]. A complete model of the new UPFC has been implemented in a digital simulator. Simulation results have confirmed that the new approach has higher performance than those UPFCs which use traditional definitions of powers in the controller. Ein universeller Lastflu~regler Ubersicht In dieser Arbeit wird ein neues Regelungsverfahren ftir den Lastflut~regler (Unified Power Flow Controller-UPFC) beschrieben. Dieses neue Regelungsverfahren bietet nicht nur die M6glichkeit der Spannungs- und Lastfluflregelung, wie ursprtinglich bei L. Gyugyi [1,2] vorgeschlagen, sondern besitzt auch aktive Filterungseigenschaften. Das Leistungstellglied besteht aus zwei PWM-Umrichtern mit gemeinsamem Gleich- spannungszwischenkreis, ohne zus/itzlicher Gleichspannungs- quelle. Der gemeinsame Regler tibernimmt in diesem UPFC die Ansteuerung des Serien- und des Shuntumrichters. Das Regel- verfahren basiert auf der Theorie der Momentanwerte fiir Blind- und Wirkleistung im c~-fl-0 Koordinatensystem [3,4]. Ein funktionsttichtiges Modell des UPFC wurde fiir einen Digitalsimulator implementiert. Die Simulationsergebnisse haben best~itigt, dat~ der UPFC rnit dem vorgeschlagenen Regelungsverfahren eine h6here Leistungsf~ihigkeit aufweist, als vergleichbare Einrichtungen, deren Regelungsverfahren die traditionelle Leistungsdefinition zugrunde liegt. 1 Introduction The concept of flexible ac transmission system (FACTS) can be understood in a general way as comprising all equipments which can influence the load flow of a power system. Three principal parameters voltage, impedance, and phase angle, determine the power flow in ac systems. At present, there is Received: 14 February 1996 M. Aredes, K. Heumann Technische Universit~it Berlin, Institut fiir Allgemeine Elektrotechnik, Einsteinufer 19, D-10587 Berlin, Germany Correspondence to: K. Heumann a common sense to restrict the use of the name "FACTS device" only for those equipments that can control in real-time one or more of such parameters. For instance, SVC controls the volt- age at a selected terminal of the transmission line. Advanced series compensator control the equivalent impedance, whereas static phase shifter control the phase angle. The main goal is to increase the usable power transmission capacity to its thermal limits [1]. An interesting approach of FACTS device is the unified power flow controller (UPFC) [1,2,5,6]. It can control all three principal parameters (voltage, impedance and phase angle) which determine the power flow of a transmission line. A unified power flow controller is a high power electronic device. It was proposed by Gyugyi [1]. Its power circuit consists of two three-phase static converters connected back-to-back through a dc link. The ac output of a converter is connected in series with the power system, through insertion transformers, whereas the other one is connected in parallel (shunt). The same power circuit configuration of the UPFC [2] was used in [7] to realize a combined approach of series and shunt active power filters, also known as Unified Power Quality Conditioner (UPQC). The switching frequency of the PWM converters of a UPQC must be higher than that of a UPFC. The UPFC generates compensating voltage and current at the fundamental frequency, whereas the UPQC compensates harmonic currents of a non-linear load and harmonic voltages of the power supply as well. This work proposes a new control to the UPFC that joins active filtering capabilities, power flow control and voltage control into a single device. One advantage of this new control is the possibility of enhancing the response of the UPFC by using active filtering control techniques that are based on the instantaneous active and reactive power defined in the c~-fl-0 reference frame [3,4]. Due to the additional compensation characteristics of the new equipment, it has been named advanced Unified Power Flow Controller (advanced UPFC). Simulation results of a complete model of advanced UPFC are presented to validate the proposed control strategy. 2 General description of the advanced UPFC It has been imagined a scenario where there are nonqinear loads connected to an already distorted power supply. This subsystem is connected to another one, through a transmission line, in which a UPFC should be installed to control the voltage at the end-terminal, as well as to control the active and reactive power flow through this transmission line. Further, harmonic voltages and currents of one subsystem should not affect the other one. Figure 1 presents this problem. The harmonic generating 303