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D. Menniti, A. Pinnarelli
Electronic, Computer and System Science Department

U. De Martinis, A. Andreotti
Electric Engineering Department


Abstract –
Flexible AC Transmission Systems (FACTS) use power electronic components to enhance controllability and capability of electrical power system. FACTS devices are able of opportunely modify voltage, phase angle and/or impedance and then the power flows at particular points in power systems [1-2]. One of the more intriguing and potentially most versatile class of FACTS device is the Unified Power Flow Controller (UPFC).
Two are the main purposes of this paper. The first one is to illustrate a UPFC model with two separate control systems for the shunt and the series inverters realising an appropriate co-ordination between them. The second purpose is to implement this UPFC model using the PSpice (Simulation Program with Integrated Circuit Emphasis) as simulation program.

INTRODUCTION
The deregulation and competitive environment in the contemporary power networks will imply a new scenario in terms of load and power flows condition and so causing problems of line transmission capacity. But, nowadays some problems exist to change the present structure of transmission system. So, the need for new power flow controllers capable of increasing transmission capacity and controlling power flows through predefined transmission corridors will certainly increase.
For this reason, as well known in recent years a new class of controllers, Flexible AC Transmission System (FACTS) controllers has rapidly met with favour. Indeed, the two main objectives of FACTS technology are to control power flow and increase the transmission capacity over an existing transmission corridor. Nowadays, the improvements in the field of power electronics have had a major impact on the development of this technology. So, a new generation of FACTS controllers has emerged. The devices of this generation are based on the use of high power electronic components such as GTO (Gate Turn-Off thyristors) and IGBT (Insulated Gate Bipolar Transistor) which makes them respond quickly to the control inputs. So, these FACTS devices are able to act almost instantaneously to changes in power system.

Representative of the last generation of FACTS devices is the Unified Power Flow Controller (UPFC). The UPFC is a device which can control simultaneously all three parameters of line power flow (line impedance, voltage and phase angle). Such "new" FACTS device combines together the features of two "old" FACTS devices: the Static Synchronous Compensator (STATCOM) and the Static Synchronous Series Compensator (SSSC). In practice, these two devices are two Voltage Source Inverters (VSI’s) connected respectively in shunt with the transmission line through a shunt transformer and in series with the transmission line through a series transformer, connected to each other by a common dc page link including a storage capacitor. The shunt inverter is used for voltage regulation at the point of connection injecting an opportune reactive power flow into the line and to balance the real power flow exchanged between the series inverter and the transmission line. The series inverter can be used to control the real and reactive line power flow inserting an opportune voltage with controllable magnitude and phase in series with the transmission line. Thereby, the UPFC can fulfil functions of reactive shunt compensation, active and reactive series compensation and phase shifting. Besides, the UPFC allows a secondary but important function such as stability control to suppress power system oscillations improving the transient stability of power system.

As the need for flexible and fast power flow controllers, such as the UPFC, is expected to grow in the future due to the changes in the electricity markets, there is a corresponding need for reliable and realistic models of these controllers to investigate the impact of them on the performance of the power system. Different UPFC models have been investigated by several authors. The main purposes of the paper has been firstly to illustrate a UPFC model with two separate control systems for the shunt and the series inverters realising an appropriate co-ordination between them, and then to describe the implementation of that UPFC model using PSpice (Simulation Program with Integrated Circuit Emphasis) as simulation program.
P-Spice has been used as programming environment for its peculiarities in transient analysis, and its ease in the implementation. This permits more simple but at the same time realistic simulations to perform a preliminary analysis of the impact of a UPFC on the performance of a power system.

In particular, in the paper the UPFC characteristics are delineated, the proposed UPFC model is described, the control systems of the shunt and series inverters are discussed and, finally the P-spice implementation of UPFC model and some simulation results to test the proposed UPFC P-Spice model are illustrated.

for more:
http://docs.googleviewer?a=v&q=cache:8SaFwFf_riIJ:ipstTechPapers/2001/IPST01Paper205.pdf+Modelling+of+Unified+Power+Flow+Controller+into+Power+Systems+using+P-Spice+pdf&hl=en&gl=in&pid=bl&srcid=ADGEESjT5bpgBT9ez-Wo5TAcFzymxfGoJn3qqvxmhxN5QtquMTYItKOaURlpDJFvYrAoca34LWQWul8nzjzQxq9M0I4l_BmWW0aHaXO3D8v6sneoLNm7qSQxeuEVa-f3EzfodSAECCWc&sig=AHIEtbRf9WdHyXrnh9LiaFr3konDiXCgrA

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