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Technical Constraints in Using Controlled Series Compensation for the Optimization of Electrical Power Networks Operation MOTI BIRJANDI A.A. , RYZHOV Yu.P . Electrical power systems department Moscow power engineering institute (Technical university) RUSSIA Abstract : -The main technical constraints are considered which limit the range of controlled series compensation of electric transmission line. The compensation limits are determined by the conditions of line conductor heating, capacitor voltages and line efficiency. Keywords: - FACTS, Controlled series compensation, Degree of compensation, Technical constraints, Power systems. 1 Introduction Transmission systems are becoming increasingly loaded because of growing demand and restrictions on new line construction.However most high voltage transmission systems operate below their capacity. Electric power systems (EPS) development is caused by the increase in demand and power generation, and it is accompanied by growing complexity of existing network. Increasing power flows in high voltage EPS and distribution networks result in a number of new problems connected with the further development of power systems. These problems are as follows: • The necessity of efficient distribution of powerflows between high voltage lines that have higher transmission capacity, and lines with lower voltage rating, which operate in parallel with the high voltage lines. • Increase in power transmission capacity of the lines in transmission and distribution networks, since it can be more economical, than construction of new lines, however it is necessary to note that such increase is required only when there is maximum load period. • Existing rural and urban areas and unfavorable land topography constitute problems facing new transmission lines construction. Load demand continues to increase despite these difficulties. This growth has been practically stabilized in developed nations, but political and environmental constraints make the construction of new lines difficult and forces electrical utilities to a better use of the existing network. The problems mentioned above can be solved by FACTS (Flexible AC Transmission Systems). It may be noted, that FACTS changes transmission line load flows and control EPS reactive power balance. Hence FACTS can stabilize nodal voltages [1]. However the use of FACTS in EPS, gives rise to technical difficulties, which in turn limit the control range of the devices. Technical constraints arising from the use of CSC, TCSC, STATCOM, UPFC are considered below. 2 Configuration of controlled series components At present there exists a number of ways to compensate line reactance. The first of them changes the degree of compensation by shunting separate sections of series connected capacitor banks by thyristor valves (CSC), see Fig. 1, a, [2]. The second choice is a device consisting of capacitor banks shunted by controlled reactors which are switched in series to the line (TCSC), see Fig. 1,b. Reactor control can be affected by a thyristor unit, magnetic filed saturation, etc[3]. According to the third method of controllable series compensation, the source of variable voltage is introduced in the line which corresponds to the capacitor voltage in phase and value and it is varied in accordance with the system operational requirements. This voltage can be introduced into the line with the help of a transformer connected in series to the line. Any controllable source of reactive power can be connected to the secondary winding of the transformer, see Fig.1 ,c (UPFC)[4]. Fig. 1: Controllable series components, a) CSC, b) TCSC, c) UPFC The majority of works on flexible lines are devoted, basically to the analysis of the devices' characteristics. At the same time, the networks and systems aspect of using these devices have not been given enough attention. In known literature, as a rule, the active resistance, capacitive susceptances, distributed parameters of the lines, were not taken into consideration. An attempt to take into account parameters of actual lines, including long lines, is outlined below. 3 Constraints Consider the constraints, which arise when controlling the degree of compensation. When degree of compensation increases then line transmission capacity increases too and consequently phase currents and temperature of conductors increase. The maximum permissible value of current can depend on cooling conditions of conductors and of course on the ambient temperature. Therefore permissible degree of compensation should be selected with due account for such constraints. Another known constraint is that increase in the degree of compensation gives rise to the increase in voltage at the terminals of the condenser, which under certain conditions can reach the maximum permissible value. The same phenomena will be observed when using transformer device (Fig.1,c). Two factors determine the increase in voltage. First of them, is the operation conditions of a capacitor bank with a variable capacity connected in series with the inductance of a line. Second one is due to the fact that with increasing degree of compensation, compensated portion of a line inductance increases, however line capacitance remains uncompensated, which also results in voltage increase at the condenser. Shunt reactors are used for voltage reduction at the terminals of the condensers. In series controlled compensators, the reactors, obviously, must be controllable. But the application of controllable reactors will increase the price of the device. Therefore, in all the cases compensation limits without reactors are considered. One more constraint on maximum compensation of reactive power is minimum permissible efficiency of controlled transmission line. As it was already noted, with the increase in the degree of compensation, the line loses increase and its efficiency is reduced. It is necessary to consider it even in the cases when the current in the line has not reached its thermal limits. This constraint becomes important when the line with increased transmission capacity is working for a long period. Thus, the technical constraints to the degree of compensation in EPS, which are considered below, are as follows: • thermal constraint, • permissible voltage at the condenser terminals constraint, • line efficiency constraint. 4 Description of simulation study Single line 220 and 500 kV without intermediate substation were studied, for the length range characteristic to the corresponding voltage classes: for 220 kV lines 100-500 km; for 500 kV tines 100- 1000km. The calculations were carried out with and without account of line resistance for four types of conductors. Corona power losses in all cases were not taken into account. Usually, in study of the effects of a controllable serial capacitor, controlled transmission line is considered as connecting two buses with constant voltages. But, this assumption is true only in the cases when these two buses infinite ones and also, the internal reactance of the systems connected to these two buses are equal to zero. In real conditions, the network cannot be presented by infinite bus and the buses, which the transmission line connect are only network nodes. Since the reactance of the network can affect the operation of the transmission line, it is essential that they should be introduced by certain equivalent values in the related calculations. The quantity of this reactance can be obtained on the basis of the three-phase short circuit capacity in the related bus. For example, this capacity for a bus with 500 kV voltage is about 20-28*103 MVA and for a bus with voltage 220 kV about 10-12*103 MVA. Therefore, the equivalent inductance of the system at these buses will be about 9-12 Ohm and 4-6 Ohm, respectively. For smaller systems, this inductance will be more. In the conditions that the transmission line is connected to these buses with an autotransformer, it is obvious that this equivalent reactance could be obtained by high voltage winding of the autotransformer. For example, when two parallel autotransformers are used for connecting to constant voltage buses two sides of the transmission line, the reactance at high voltage side will be about 20-30 Ohm for 500/220 kV autotransformers and about 12-50 Ohm for 220/110 kV autotransformers. Consequently, in calculating operating condition the transmission line, this equivalent reactance will be considered in the 1050 Ohm range. Since the lines, as a rule, are separated from system buses by a certain reactance, it was taken into account in calculations. Its value varied between 0-50 Ohms. The lower limit corresponds to constant voltage at terminal substation; the upper corresponds to the case when the line is connected to the buses via autotransformer. The circuit diagram is shown in Figure 2. Fig.2: Circuit diagram Of CSC in transmission line, a) general circuit, b) single line diagram The simulation studies are based on a simple power system model in which two local networks A and B are connected by a long transmission consisting of a line with controllable bank capacitor in the middle. Thus the left-hand and right-hand boundaries correspond to a line connected to system buses through an autotransformer with constant voltage, UA and UB, (see Fig.2), equivalent reactance of system being equal to 50 Ohm (Xeq=50 Ohm). Degree of compensation, KC %, varied in the range 0 - 60%, (1) X c *100 K % c X Line where: XC - reactance of capacitor banks Xline - line reactance Degree of compensation here is understood as the ratio of the reactance of capacitor banks to the transmission line reactance expressed in per cent. Higher degree of compensation was not considered, as it tends to resonance and sharp increased in voltage across capacitor banks. Besides certain constraints may be imposed by 8500 by a line at bus A may be calculated by the following expression: * cos 11 U a *U b K z *Z12 6500 5500 (2) Pmax [MW] Pmax 12 U a2 K z *Z11 Pmax- 500 кВ - Ас 3*400\51- L=100,500,1000 [км] 100-Xeq=0 7500 .. Where: Ua and Ub - constant voltages at buses A and B Z11,Z12 - desiring point and mutual impedances of the lines 4500 500-Xeq=0 3500 +25 c -5 c 2500 500-Xeq=50 1000-Xeq=0 100-Xeq=50 1500 1000-Xeq=50 500 11 - Phase angle of Z11 -500 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 Degree of compencation 5 Results of calculations a) Uc-500 кv - Ас 3*400\51 - L=100,500,1000 [км] 700 100-Xeq=0 650 1000-Xeq=50 600 Uc1 [kv] Kz =1.2 - power stability margin Under normal operating conditions the typical case UA = UB was considered. Compensation modifies values of Z11 and Z12 acts and correspondingly changes maximum active power. Efficiency of 90% was taken as minimum level. Lower values are not acceptable for continuous operation. However in extreme situations in an electric power system the lower values of efficiency are probably admissible. 525 kv 1000-Xeq=0 550 500 100-Xeq=50 450 500-Xeq=50 Some results of calculations for 220 kV line are presented in a Table 1, Fig.3 and for 500 kV line in a Table 2, Fig.4. 500-Xeq=0 400 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 Degree of compencation b) Efficiency [%]- 500 кВ , Ас 3*400\51, L=100,500,1000 [км] 100 Table 1: Maximum Degree of 220kv line compensation a) Xeq=0 b)Xeq=50 500-Xeq=50 95 Max. degree of Compencation [%] , Conductor-1*Ac 400/51 , Xeq=0 , 220 [kv] 90 90 % 1000-Xeq=0 85 Efficiency [%] length Length L. Length L. L. [km] [km] [km] 100 300 500 Constraint 100-Xeq=50 1000-Xeq=50 500-Xeq=0 80 100-Xeq=0 75 Thermal Cond. Voltage Comp. 0 0 5 44 0 43 0 42 0 70 0 Efficiency 0,1 0,2 0,3 0,4 0,5 0,6 0,7 Degree of compencation c) a) Max. degree of Compencation [%] , Conductor-1*Ac 400/51 , Xeq=50 , 220 [kv] length L. Length L. Length L. [km] [km] [km] 100 300 500 Constraint Thermal Cond. Voltage Comp. Efficiency b) 0 30 52 60 60 60 0 60 0 Fig. 3: Variation max. power transmission, voltage at the condenser terminals and efficiency in the 220 kV line with compensation degree for length 100,300,500km. As can be seen from these tables and curves, for 220 kV lines in the studied length range (100500 km) for zero equivalent reactance at the ends of the line the greatest possible transmitted capacity lies beyond the limit determined by the heating of line at any degree of compensation. The results are shown in Table 1. Only for 300 km lines and with equivalent reactance of 50 Ohm, it 8500 Pmax- 500 кВ - Ас 3*400\51- L=100,500,1000 [км] 100-Xeq=0 7500 6500 Pmax [MW] 5500 4500 500-Xeq=0 3500 +25 c 500-Xeq=50 1000-Xeq=0 100-Xeq=50 1500 1000-Xeq=50 500 -500 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 Degree of compencation a) Uc-500 кv - Ас 3*400\51 - L=100,500,1000 [км] 700 100-Xeq=0 650 1000-Xeq=50 600 525 kv 1000-Xeq=0 550 Table 2 : Maximum Degree of 500kv line compencation a) Xeq=0 b)Xeq=50 -5 c 2500 Uc1 [kv] is possible to work below the level of the degree of compensation up to 30 %. Voltage limit at the condenser terminals for 220kV lines occurs only for lines with constant voltages at the ends for the degree of compensation equaling 42-44 %. Thus the allowable degree of compensation practically it does not depend on the length of the line. The transmission lines that are far from constant voltage buses of the system can operate with any degree of compensation. Minimum allowable efficiency for lines without the account of equivalent reactance corresponds to the degree of compensation of about 5% and in this case it does not depend on the length of the line. It reduces when the degree of compensation is increased. When equivalent reactance is taken into account the lines with 100 km length can operate with acceptable efficiency for any degree of compensation. Thus the increase in the degree of compensation practically does not effect line efficiency. 500 100-Xeq=50 450 500-Xeq=50 500-Xeq=0 400 0 Max. degree of Compencation,Conductor-3*Ac 400/51 , Xeq=0 , 500 [kv] 0,5 0,6 0,7 b) Efficiency [%]- 500 кВ , Ас 3*400\51, L=100,500,1000 [км] 100 100-Xeq=50 1000-Xeq=50 0 22 500-Xeq=50 48 90 90 % 48 10 44 13 25 16 1000-Xeq=0 85 500-Xeq=0 80 a) 100-Xeq=0 75 length L. Length L. Length L. [km] [km] [km] 100 500 1000 Constraint Efficiency 0,4 95 Max. degree of Compencation,Conductor-3*Ac 400/51, Xeq=50 [ohm] , 500 [kv] Thermal Cond. Voltage Comp. 0,3 Efficiency [%] Efficiency 0,2 Degree of compencation length L. Length L. Length L. [km] [km] [km] 100 500 1000 Constraint Thermal Cond. Voltage Comp. 0,1 0 58 60 60 60 60 60 0 48 b) Fig. 4: Variation Max. Power transmission, voltage at the condenser terminals and efficiency in the 500 kV line with compensation degree for length 100,500,1000 km. For lines of 500 kV, with no equivalent reactance, the limit of compensation due to heating depends on the line length. For a line of 500 km it is 22%, and for a line of 1000 km - 48%. Transmission line with equivalent reactance may operate for any degree of compensation in all 70 0 0,1 0,2 0,3 0,4 0,5 0,6 Degree of compencation c) lengths range. Voltage at the condenser terminals for the line of 1000 km without reactors exceeds permissible limit for any degree of compensation whether equivalent reactance is introduced or not. For lines of 100 to 500 km without equivalent 0,7 References reactance the maximum permissible degree compensation is 48-44%. But the line with equivalent reactance can operate without exceeding voltage limit for any degree of compensation. As for efficiency of the line, it is possible to see that (Figure 4), in the absence of equivalent reactance, increase in compensation leads to the reduction in efficiency to a value below accepted minimum. At the same time, the presence of equivalent reactance, allows transmission line of more than 100 km to operate at allowable value of efficiency in all the range of compensation. The exception are lines of length close to 1000 km, for which maximum degree of compensation is about 50%, which can be considered sufficient. Conclusion 1. The analysis of the effect of controlled series compensation on the operating condition of EPS shows that it is necessary to take into account technical constraints on the degree of compensation. 2. It is necessary to consider the equivalent reactance at the ends of the lines with controlled series compensation. 3. It is necessary to determine the maximum degree of compensation for every given line with the consideration of the type of conductors, line length and its location in the network. [1] [2] [3] [4] [5] [6] Ram Adara, "Facts system studies'', Journal IEEE Power Engineering Review, pp. 17 - 22, December 2002. J. Deuse, M. Stubbe, B. Meyer, P. Panciatici, "Modeling of facts for power system analysis", CIGRE, Sym. Tokyo, Session 320-04,1995 E. Lerch, D. Povh, R. Witzamann, B. Helebcar, R. Mihalic "Simulation and performance analysis of unified power flow controller", CIGRE, Session 14-205, 1994. E. V.Larson, K.Clark, S.A. Miske,Jr , J.Urbanek, "Characteristics and rating considerations of thyristor controlled series compensation", IEEE Trans. Power Delivery.Vol.9,No. 2,pp. 992-1000, April 1994. 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