Damping of power swings using a FACTS device of the TCPS type: modelling and laboratory experiments

International Journal of Electrical Engineering Education, Jul 2007 by Robak, S, Rasolomampionona, D D, Januszewski, M

Abstract Considerations concerning the damping of power swings using a FACTS device like TCPS are presented in this paper. The case of a generator-infinite bus test system is taken into consideration. Phenomena have been analysed both by performing computer simulations of the mathematical model and experimental analysis in laboratory conditions. A short description is given of the laboratory set up equipped with a TCPS device used for the experimental analysis.

Keywords FACTS device; power swings; power system stability

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Damping of oscillations is widely considered an important issue in electric power system operation. During the past decades many utilities have reported serious problems in maintaining synchronous operation of generators in their power system. In extreme conditions undamped oscillations have been the cause of serious events, resulting in a total block-out of a quite important part of a power system (several load groups).1

In a general case, the oscillations occurring in power systems can be classified into four main categories: (i) local plant mode oscillations, (ii) interarea mode oscillations, (iii) torsional mode oscillations, (iv) control mode oscillations.2,3

The first two categories induce power swings in electrical generators and transmission lines. These oscillations must be effectively damped to maintain the system's stability, otherwise they can lead either to a loss of power system stability or to an unnecessary action of power system protection. One of the first conventional methods used to enhance power system stability was the addition of a supplementary feedback control loop called a power system stabiliser (PSS) to the automatic voltage regulator (AVR) system of the generator. The basic function of a power system stabiliser is to extend stability limits by modulating generator excitation to provide damping to the oscillation of a synchronous machine.4 Increasing the transmission line loading over long distances could lead to such a situation that the use of conventional power system stabilisers might in some cases not provide suf- ficient damping.5 In these cases, other effective solutions need to be studied.

Currently the intensive development of power systems has provided an opportunity to introduce new devices like FACTS, used to increase power transmission flexibility6,7 in power transmission networks. FACTS devices have the advantage of flexibility, as they can be located at the most suitable places to achieve the best control results.6-10

The possibility of controlling the transmittable power implies the potential application of these devices for damping of power system electromechanical oscillations. Recent developments have shown that they can also be used for stability enhancement by increasing the damping of power oscillations.11,12

The FACTS primary function consists in steady state power transmission and/or node voltage control. This primary function is performed simultaneously with a damping function, which is aimed at enhancing the stability of the power system. It has been recognised that such devices as FACTS can be utilised to control inter-area modes via supplementary controls.7,13,14

Among different types of FACTS devices special attention should be paid to the Thyristor Controlled Phase Shifter (TCPS). A typical application of a TCPS device consists in its switching in one or more parallel transmission lines in order to increase their power flow.15 It is also possible to increase the power flow in networks of different voltage levels or situated in different system areas.12

This paper is focused on the analysis of phenomena regarding damping of local plant mode oscillations by means of TCPS. Despite the huge number of papers dealing with the different aspects of FACTS device operation,5-8, 15-17 the originality of this paper is that it tackles both the mathematical model design and the physical model implementation. A few results of the power system stability analysis performed on the power system laboratory model, and then compared with theoretical background assumptions are also included in this paper.

The system study

Local plant mode oscillation problems are the most commonly encountered in power systems. These oscillations are associated with the swing of synchronous generator units at a generating station with respect to the rest of the power system. For the analysis of this type of oscillations the well-known generator-infinite bus model is used. The generator-infinite bus model is a useful starting point for designers to evaluate the design and dynamic performance of the individual controllers. This system quite faithfully reproduces the features and phenomena occurring in a full system only when the disturbance and transient states occurring in the analysed power plant do not influence the operation of the rest of the power system. This situation occurs only when the power of the analysed power plant is much smaller than the total generated power of the rest of the system. In contemporary power systems, the amount of power generated in different power plants is quite high but comparable to one another. Disturbances often influence the majority of generators. Hence in practice, such a situation in which the model of the whole power system can be reduced to a generator-infinite bus model is very rare. However this type of model plays an important role from the point of view of electrical engineering teaching methodology. A wide range of power system stability problems and appropriate features of selected phenomena occurring in power systems can be presented easily using such a simplified model.