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In practical installation of FACTS in the power system, there are three common requirements as follows: What Kinds of FACTS devices should be installed?. Where in the system should be placed?. How much capacity should it have?. This paper focus on the first two of these requirements. Choosing the type of FACTS devices and deciding the installation location and control of FACTS. We presented a simple algorithm based in heuristic and practical rules to seek the optimal location of two types of FACTS, shunt compensation ‘SVC’ and series compensation ‘TCSC’. The system loadability and loss minimization are applied as a measure of power system performance. Results show the impact of optimal operating points of FACTS (SVC and TCSC) devices under various conditions of power system.
The proposed methodology to install and operates facts devices properly is verified on the 9-bus system where FACTS devices operated under normal and abnormal condition. Key words —Power flow, Static Voltage Stability, FACTS, SVC, TCSC, Optimal Location, Power Quality, Heuristic Methods. I. INTRODUCTION The capacity of transmission lines is becoming the main bottleneck of electricity transmission in the deregulated power industry. The competition of electricity may aggravate loadability of some transmission lines. To meet the load demands in a power system and to satisfy the stability and reliability criteria, the existing transmission lines must be utilized more efficiently. The purpose of the transmission network is to pool power plants and load centers in order to supply the load at a required reliability and maximum efficiency at a lower cost. A technically attractive solution to above problems is to use some efficient controls with the help of FACTS (Flexible AC Transmission Systems) devices. The conception of FACTS as a total network control philosophy was first introduced by N. G.
Hingorani  from the Electric power research institute (EPRI) in the USA in 1988, although the power electronic controlled devices had been used in the transmission network for many years before that. The application of FACTS in electric power System is intended for the Control of power flow, improvement of stability, voltage profile
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1-4244-0088-0/06/$20. 00 ©2006 IEEE 1068 management, power factor correction, and loss minimisation. Thyristor Controlled Series Capacitors (TCSC) and Static Var Compensators (SVC) are the most popular devices of the FACTS. The main functionality of the SVC is to regulate the voltage at a chosen bus by controlling the reactive power injection at the location. Maintaining the rated voltage levels is important for proper operation and utilization of loads. Under voltage causes deregulation in the performance of loads such as induction motors, light bulbs, etc.. , whereas over voltage causes magnetic saturation and resultant harmonic generation, as well as equipment failures due to insulation breakdown. These devices are characterised by rapid response, wide operational range and high reliability. Series capacitor compensation is another approach to improve stability limits and increase transfer capabilities.
The transmitted power through a line is inversely proportional to the transfer impedance. For example, considering other parameters constants, 50% series compensation approximately doubles the steady-state transmitted power, whereas 75% series compensation would increase the transfered power to about four times the original value. In this paper we propose a simple approach based in heuristic and practical rules for the optimal location of two different FACTS devices, SVC and TCSC, with specific characteristics. The system loadability and loss minimization are applied as an objective function and a measure of power system performance. They are modeled for steady-state analysis  and located in order to maximize the security margin of the system in terms of branch loading and voltage levels. The proposed approach is verified on the 9 bus system with FACTS devices under normal and abnormal condition. II. MODELLING OF FACTS CONTROLLERS The FACTS devices can be categorized into three types, namely series controllers, shunt controllers and combined series-shunt controllers.
Fig. 1 shows a schematic diagram outlying the basic model of FACTS devices . In principle, series controllers inject voltage in series with the line and the shunt controllers inject current into the system at the point of connection. The combined series-shunt controllers inject current into the system with the shunt part of the controllers and voltage in series in the line with the series part of the controllers. Bmax Vpq X/2 X/2 ~ Iq Iq ~ Vs ~ Vm Vm+Vpq ~ + Vr Vref Fig. 4 SVC type . 1 regulator Fig. 1 Basic model of FACTS Bsvc Kr Tr p + 1 V Bmin B. Thyristor Controlled Series Capacitors (TCSC) A. Static VAR Compensator The steady-state model proposed in  is used here to incorporate the SVC on power flow problems. This model is based on representing the controller as a variable impedance, assuming an SVC configuration with a fixed capacitor (FC) and Thyristor-controlled reactor (TCR) as depicted in Fig. 2.
Applying simultaneously a gate pulse to all thyristor of a thyristor valve brings the valve into conduction. The valve will block approximately at the zero crossing of the ac current, in the absence of firing signals. Thus the controlling element is the Thyristor valve. The thyristors are fired symmetrically, in an angle control range of 90 to 180 with respect to the capacitor (inductor) voltage. The steady-state control law for the SVC is the typical current-voltage characteristic, illustrated in Fig. 3. The TCSC may have one of the two possible characteristics: Capacitive or inductive respectively to decrease or increase the reactance of the line. -jxc rij +jxij j Ui Uj Fig . 5 Series compensator line 1) TCSC Regulators TCSC regulators are depicted in Fig. 6. The system undergoes the algebraic equations: Pkm = ? VkVm Be sin(? k ? ? m ) . Vr (7) (8) Pmk = ? Pkm . 2 Qmk = ? Vm Be ? VkVm Be cos(? k ? ? m ) . Be, +Q . (10) Where the indexes k and m denote the two buses at which the TCSC is connected. -Q Filte TSC TCR The output signal can be interpreted in two different ways, as a total reactance Xe or, as the firing angle of the power electronics switching control system.
The device is then represented by the following set of equation: Fig. 2 SVC steady-state circuit representation 1) SVC Regulators One of two SVC regulators is implemented in the program. The first one assumes a time constant regulator as depicted in Fig. 4. In this model a total reactance Bsvc is assumed and the following differential equation holds: Bsvc = (Kr (Vref ? V )? Bsvc ) ?V k V m B e sin( ? k ? ? m ) ? P = 0 Tr (12) 2 ? V m B e + V k V m B e cos( ? k ? ? m ) ? Q m = 0 (5) (11) ? V k2 B e + V k V m B e cos( ? k ? ? m ) ? Q k = 0 (13) 1 ( 2? ? 2? + sin( 2? )) ? ? Be = 0 Xc ?X L The model is completed by the algebraic equation expressing the reactive power injected at the SVC node: (6) Thus the reactance Bsvc is locked if one of its limits is reached. The second model not implemented in the algorithm takes in account the firing angle. Thus the model can be developed with respect to a sinusoidal voltage. V (14) 2 P2 + Qk ? IVk = 0 k Q = ? BsvcV 2 Where Be = 1 Xe .
The control mode is defined by either one of the following equations: -constant reactance control: -constant power control: X set ? X e = 0 Pset ? Pe = 0 -constant current control: XL (15) I set ? I = 0 (16) (17) (18) XSL max x max ,? max Vref _ min Pref XC + K sTw p Tw p ? 1 1 T1 p ? 1 P IC (9) Qkm = ? Vk2 Be ? VkVm Be cos(? k ? ? m ) max IL max Fig. 6 TCSC type 1 & type 2 regulators. Fig. 3 Typical steady state V-I characteristics of SVC 1069 T2 p ? 1 T3 p ? 1 x min ,? min xe , ? III. B. TCSC Location and Control STRATEGY OF OPTIMAL LOCATION OF FACTS DEVICES A. SVC Location A 9-bus system is used to study a methodology for the placement and design of SVC and then a TCSC. The behaviour of the system is investigated with and without FACTS devices under different loading conditions.
Voltage stability studies are performed by starting from an initial stable operating point and increasing the loads by a factor until the singular point of the power flow linearization is reached. The loads in this case defined as P L = ? Po Q L = ? Qo (19) Where Po and Qo are the active and the reactive base loads, whereas PL and QL are the active and reactive loads at a bus L for the current operating point. The voltage profiles for chosen buses are presented in Fig. 7 assuming a uniform load increase at all buses for P and Q. At the bifurcation point, which corresponds to the maximum parameter value = 2. 905.
Based on this result the optimal location of the SVC was chosen to be at bus 9, with = 3. 0436 and power loss P=0. 04733. Fig. 8 shows the effects of placing SVC at bus 9, to improve voltage stability. Generate initial solution Evaluate objective function Stop criterion reached ? Create new solution No yes Return best solution The most important advantage of heuristic methods lies in the fact that they are not limited by restrictive assumptions about the search space like continuity, existence of derivative of cost function . In a very general manner, the principle of a heuristic method may be represented with the Fig.
The specificity of each method lies mainly in the way of moving from the current solution to the new solution. 0. 9 Voltage Magnitude Define solution Fig. 9 General principal of a heuristic method 1 0. 8 0. 7 1) Heuristic Methods Heuristic methods may be used to solve combinatorial optimization. These methods are called “intelligent”, because the move from one solution to another is done using rules close to the human reasoning. The heuristic algorithms search for a solution inside a subspace of the total search space. Thus they are able to give a good solution of a certain problem in a reasonable computation time, but they do not assure to reach the global algorithm. VBus 5 VBus 7 VBus 8 VBus 9 0. 6 0. 5 0. 4 0. 5 1 1. 5 2 Loading Parameter ? (p. u. ) 2) Description of the approach proposed The goal of this proposed approach which based in practical rules is to find the best location and the type of FACTS devices in accordance with a defined criterion. In this approach the choice of the initial configuration based on the continuation power flow. The algorithm is simple and easy to implement. One of the drawbacks is the possibility to converge prematurely to a suboptimal solution. 2. 5 Fig. 7 Voltage profiles without FACTS.
Voltage Magnitude 0. 8 This approach includes three phases: the first phase determine the initial condition which represent the weak bus based in voltage collapse, the second phase which represent the core of the method, a load flow is applied continuously to identify all groups formed with bus connected to the weak bus, and the third phase is a selection of the efficient location based in load factor, voltage magnitude and total power losses. 0. 7 0. 6 0. 5 0. 4 0. 3 VBus 5 VBus 7 VBus 8 VBus 9 0. 5 1 1. 5 2 Loading Parameter ? (p. u. ) 2. 5 3 Power losses are used as a final rule to choose between solutions that have approximately the same value. This approach has an advantage that guaranteed the exploration of the global space search. Fig. 8 Voltage profiles with SVC at BUS 9 1070 For TCSC location we must have the first candidate bus from SVC location, this bus considered as the initial condition for the TCSC search location , then a sequence of practical rules are proposed to find the efficient location of TCSC, this rules based on loadability margin, voltage magnitude and power losses are applied for each search level.
The idea in its simplest form may be described as follows. -Let W be the solution space and S ( , P) a measure of power system performance or a selection criteria. -Select an initial solution X0 based in SVC location Pmin). max, -Create the first candidate group. Table I shows all solution, obtained during the phase generation, for this network test two group selected, a load flow is applied continuously and a selection of the efficient location of the TCSC is obtained. We can conclude that line 8-9 with load factor 3. 2322 and minimum voltage 0. 67231 is chosen as an efficient candidate line to place and control the TCSC. -Check the performance of this group based in ‘S’ -To pass from level to another:
We introduce a voltage test: j k j If Vmin < Vmin and ? max < ? k max then: the search pass to the group formed by bus k, eliminate the element of this group from group candidate selection. j k j If Vmin < Vmin and ? max > ? k then: the search pass to max the group formed by bus j, save this group as a candidate selection in Candidate-G. -Choose the best Candidate-G that maximize minimize power losses in normal condition. and – X * =Solution: the line which assure the best performance ( max and Pmin). 1 0. 9 Voltage Magnitude ( with the TCSC located at line (8-9), we found that the bifurcation in this case occurs at = 3. 2322 and power losses P=0. 03025. Load margin is lightly greater than the SVC case, but the power losses are reduced than the SVC case. In order to compare the effects of placing a TCSC at various locations, another location was considered at lines (6-7), (4-9), (1-4). It can be noticed from Fig. (14, 15 and 16) that when the loading parameter changes, the TCSC Power regulation change to assure an optimal power quality indexes, for example, when TCSC installed at Line (8-9), in normal condition, the optimal Power regulation Preg=1 pla.
Branch I-J 8 9 2. 5 VOLTAGE MAGNITUDE 3 6 1 1. 5 2 Loading Parameter ? (p. u. ) Fig. 12 Voltage profiles with TCSC at line 8-9 2 7 0. 5 Group J 3 7 Group KII 3 8 9 Fig. 11 Global space search 2 IV. RESULTS DISCUSSIONS The voltage profiles for a TCSC placed at line (8-9) are shown in Fig. 12. Using fixed power (Preg=0. 4) control 1071 loadability margin and losses power are applied for each search level. The tests performed on the 9bus test system are found to be quite encouraging and promising. Both devices exhibits the fact that insertion of these devices in power system can eventually increase the power limit, line power and loading capability of the network as well as enhancing the system stability. The optimal location allows to increase the system loadability.
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