Abstract Nowadays, because of the deregulation of the power industry the continuous increase of the load increases the necessity of calculation of available transfer capability (ATC) of a system to analyze the system security. With this calculation, the scheduling of generator can be decided to decrease the system severity. Further, constructing new transmission lines, new substations are very cost effective to meet the increasing load and to increase the transfer capability. Hence, an alternative way to increase the transfer capability is use of flexible ac transmission system (FACTS) controllers. In this paper, SSSC, STACOM and UPFC are considered to show the effect of these controllers in enhancing system ATC. For this, a novel current based modeling and optimal location strategy of these controllers are presented. The proposed methodology is tested on standard IEEE-30 bus and IEEE-57 bus test systems with supporting numerical and graphical results. 2015 Faculty of Engineering, Ain Shams University. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( -nc-nd/4.0/).
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One of the major advantage of competitive electricity market is the availability of power is open to all consumers to access power from the transmission system. This open access of power system network may create the overload on the power system network more frequently. In power system network, since ATC is an available transfer capability, and unless the calculations of ATC are being used optimally by the power transmission companies, huge amount of power losses will occur in the power system network. The result of this will be a challenging task for power system operation people to manage the system in secured conditions.
To determine the ATC for bilateral and simultaneous transactions between seller and buyers OPF models are formulated for most of the FACTS devices viz. STATCOM, SSSC, and UPFC. In bilateral transaction a buyer bus demands real power from seller bus. This transaction can be maximized by maintaining equality and inequality constraints [16]. In general Power flows can be calculated between any seller and buyer buses. Since ATC is a function of power flow sensitivity, this will provide a better location for FACTS device in finding best possible ATC values. To enhance the ATC values in a power system network, the sensitivity factors known as Power Transfer Distribution Factors (PTDFs), will provide optimal locations for any FACTS device [17]. For effective increase of transmission system capacity, several studies have found that, by using FACTS devices, the current through a line can be controlled at a reasonable level, which will enable the increase of existing transmission lines [18,19].
From the careful review of the literature it is identified that, the evaluation of ATC using sensitivity approach is one of the effective methods. In this paper, ATC is evaluated by formulating power transfer distribution factors (PTDFs). Further, the system ATC is enhanced using the FACTS controllers. To increase the effectiveness of the problem, the FACTS controllers are placed in an optimal location. A methodology based on the total power loss minimization is presented to identify an optimal location of FACTS. From the literature, it is also identified that, voltage source converter type FACTS controllers are powerful and more effective when compared to the variable impedance type FACTS controllers. Hence, in this paper, the static synchronous series compensator (SSSC), static compensator (STATCOM) and unified power flow controller (UPFC) devices are considered. To identify the effect of these controllers on system performance and on OPF problem, a novel current based model of these controllers is also developed. Using this model, these controllers can be easily incorporated in a given system with decreased computation burden. The proposed methodology is tested on standard IEEE-30 bus and IEEE-57 bus test systems with supporting numerical and graphical results.
The generalized form of optimal power flow (OPF) problem can be formulated by considering total power losses as an objective, by adjusting the system control variables while satisfying a set of operational constraints. Therefore, the OPF problem can be formulated as follows:
where 'g' and 'h' are the equality and inequality constraints respectively and 'x' is a state vector of dependent variables such as slack bus active power generation (Pg,siack), load bus voltage magnitudes (VL) and generator reactive power outputs (Qg) and apparent power flow in lines (Si) and V is a control vector of independent variables such as generator active power output (PG), generator voltages (VG), transformer tap ratios (T) and reactive power output of VAr sources (Qsh).
SSSC is an important series FACTS device which controls the active or/and reactive power flow in a line where it is connected by compensating the voltage drop in the respective transmission line. In general, it consists of voltage source converter connected through a coupling transformer into a transmission line. This voltage source converter is supplied through an auxiliary DC source. The converter controls the compensating voltage by varying the firing angles of the solid state devices. Based on the voltage compensation in terms of voltage magnitude and voltage angle, the respective active and reactive power flow in a transmission line is controlled. The schematic representation of the SSSC connected in a transmission line is shown in Fig. 1.
STATCOM is important shunt FACTS devices which control the voltage magnitude at bus where it is connected by injecting/absorbing the reactive power at that bus. In general, it consists of voltage source converter connected through a coupling transformer at the connected bus. This voltage source converter is supplied through an auxiliary DC source. The converter controls the compensating voltage by varying the firing angles of the solid state devices. Based on the control settings of the converter, the system parameters such as voltage magnitude and voltage angles are controlled which results in controlling of active and reactive power flows in the transmission lines which are attached to this bus. The simple schematic
UPFC is an important series-shunt FACTS device which controls the active or/and reactive power flow in a line where it is connected by compensating the voltage drop in the respective transmission line and controls the voltage magnitude at the connected bus by absorbing/injecting the reactive power. In general, it consists of voltage source converters connected through a coupling transformer into a transmission line and the common bus. These voltage source converters are connected together and operating in a coordinated manner. The converter controls the compensating voltage by varying the firing angles of the solid state devices. Based on the voltage compensation in terms of voltage magnitude and voltage angle, the respective active and reactive power flow in a transmission line and voltage magnitude at the common bus are controlled. The schematic representation of the UPFC connected in a transmission line is shown in Fig. 10.
The consolidated OPF results when ATC is maximized and the respective TPL values are tabulated in Table 3. In this table, ATC values are evaluated for the transactions for each of the generators minimum and maximum transaction obtained from the earlier analysis. From this table, it is identified that, maximization of ATC using OPF increases the ATC value when compared to load flow; further, the ATC value is enhanced using FACTS controllers. Out of which, maximum ATC value is obtained with UPFC when compared to the remaining FACTS controllers. It is also identified that, TPL value increases with OPF when compared to load flow due to increase of transfer capability. It is identified that, with UPFC, the ATC value further increases when compared to without device. This is due to the redistribution of power flows with UPFC. It is also observed that, maximum ATC variation is observed in the transactions 8-12 and 13-7; this is due to the nearness of UPFC to these transactions.
[3]Gyugyi L, Rietman TR, Edris A, Schauder CD, Torgerson DR, [25 Williams SL. The unified power flow controller: a new approach to power transmission control. IEEE Trans Power Deliv 1995;10 [26 (2):1085-97.
Galiana GD, Almeida K, Toussaint M, Griffin J, et al. Assessment and control of the impact of FACTS devices on power system performance. IEEE Trans Power Syst 1996;11(4):1931-6. Verma KS, Singh SN, Gupta HO. Facts devices location for enhancement of TTC. In: Power engineering society winter meeting, vol. 2. IEEE; 2001. p. 522-7.
M. Venkateswara Rao received his B.Tech in Electrical Engineering from SV University, Tirupati & M.Tech in High Voltage Engineering from JNTU Hyderabad. He is currently pursuing Ph.D. at JNT University, Kakinada. His research interest area is FACTS controllers, power system security, and Power quality.
Abstract:In power system operations, unforeseen energy imbalances commonly occur, resulting in unexpected constraints on the system. This leads to a disturbance in normal operation. In systems with integration of large intermittent wind power resources, additional complications are imposed on the system, especially under heavy winds that require immediate measures to minimize possible impact of abrupt wind power fallout. Effective power system fortifications have to be put in place to address the challenges. Wind varies more on the sub-hourly time scales; therefore, sub-hourly dispatch is bound to address more of these issues than commonly used hourly methods. Hybrid power system operation with wind necessitates the use of fast start-up generation and storage to improve quality of power. In this work, the impact of intermittent wind power curtailment on power system operation is addressed to prevent system instability. A modified wind turbine power curve is used to restrict the onset of the normal cut-off point, thereby allowing sufficient time for effective power switchover with pumped hydro generation. This improves the voltage stability of the power system during curtailment. Singular value decomposition matrix of the power system network is employed to evaluate the performance of the proposed method.Keywords: dispatch; generation; integration; intermittence; power; pumped hydro; sub-hourly; wind 2ff7e9595c
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