Simulation of FACTS Devices as Reactive Power Compensators and Voltage Controllers in the Smart Grid

This paper provides a modeling platform for students to understand and study power system dynamics using Matlab® and PSCAD/EMTDC® when Flexible AC Transmission Systems (FACTS) devices for reactive power compensation and voltage control are implemented. In this study, a proposed power system’s voltage profile has been modeled and analyzed using Matlab® and PSCAD/EMTDC® to determine the optimum parameters for the design of FACTS devices, such as Static VAR Compensator (SVC) and Thyristor Controlled Series Compensator (TCSC). A tutorial was developed as part of the Smart Grid Control Systems course at SUNY University at Buffalo. This tutorial provides the insight for both undergraduate and graduate students as well as practicing engineers. It helps them to understand FACTS devices design and implementation by highlighting their main characteristics and enhancing performance of the power system they are being integrated with. Introduction As an outcome of the deregulation of electric power industry, power transmission operators are now responsible for their own business. Therefore, they must make the best use of their transmission capacity and ensure that transmission losses are minimized 1 . Also, any loss of transmission capacity means loss of income for the power transmission operators. Therefore, all actions must be taken to ensure that reactive circulating power does not obstruct available transmission capacity. In addition, energy congestion in critical transmission corridors must be avoided to eliminate the risk of jeopardizing the grid stability, reliability as well as profitability of business operations. Moreover, to offer the greatest flexibility, transmission operators must ensure the maximum safe operating margins to allow power injection and tapping from its buses without endangering stable operation. Thus, the successes of transmission operation depend on offering the maximum available transmission capacity (ATC) on their transmission lines. It is apparent that in developing the smart grid realm, transmission operators have a greater Page 23068.2 responsibility to make their networks more flexible. Advancements in power-electronics technology now offer new fast, controllable FACTS controllers to assure this required flexibility. The subject matter is an integration of classic power systems, power electronics, control theory and programing among others and is quite complicated for understanding by students and engineers who have no previous exposure to this field. During development and implementation of Control Systems for Smart Grid course it was decided to develop and present a tutorial to assist electrical engineering students and professionals in achieving deeper understanding and obtaining practical skills related to FACTS devices in order to meet the power flow control challenges. Reactive power is an important, but not an easy, concept to understand in electrical power systems. Reactive power compensation is considered as a powerful tool for optimizing the power flow on transmission networks. Inadequate reactive power leads to voltage collapses and has been a major cause of several recent major power outages worldwide 1 . Reactive power compensation can be provided by using FACTS devices, which are power electronics-based devices that control and regulate the power flow within the power system. They are capable to reroute power through the optimum available paths regardless of the dynamics of the power system. Clear understanding of the principles of FACTS devices and how they affect the behavior of the power system becomes easier after grasping the fundamentals of designing primary building blocks of all thyristor based FACTS devices. Since the process of developing and implementing FACTS devices is quite complicated, the use of power systems modeling tools becomes obvious to facilitate the learning process in this field. Two available software packages, Matlab® 2 and PSCAD/EMTC® 3 where chosen to be used in the tutorial and simulation. Matlab® uses a high-level language and an interactive environment for numerical computation, visualization, and programming. Using Matlab®, electrical engineering students can analyze data, develop algorithms, and create models and applications. In the tutorial, Matlab® is used to study the voltage profile dynamics of a proposed power system with varying load profile. Besides that, the PSCAD/EMTC® provides vast possibilities in power system simulation. It contains a comprehensive library of system models ranging from simple passive elements and control functions, to complex electric machines and FACTS devices. In order to understand the dynamics of a power system in the presence of FACTS devices, modeling and simulation approach allows students to appreciate the value of these devices and their role in enhancing the P ge 23068.3 power system performance for different scenarios and case studies. Moreover, when the students grasp the concept of FACTS devices, they will be able to use PSCAD/EMTC models to build their own designs in order to solve complex compensation and voltage control problems. Power System Configuration In order to appreciate the role of FACTS devices, a power system should be specified and its voltage profile analyzed. The representative power system, as shown in Fig. 1, consists of: Power source step up transformer combination with rated voltage of 230 KV at 60 Hz. 300 Km (168.4 miles) high voltage transmission line with impedance Zline = (7.5 + j86.71)Ω Dynamic load with a lagging power factor that changes within well-known limits. Figure 1. Singleline diagram of the specified power system configuration. Figure 2. Power system's voltage profile at different loading and power factors Voltage Profile Study of the Specified Power System The voltage profile is studied at the receiving end of the transmission line, i.e. the bus where the load is connected to the transmission line. The load is dynamic with a varying profile within the limits between 20 to 140 MVA, while the power factor is also changing from unity down to 0.4 lagging. Matlab® is used for simulation of the given power system and for analysis of the voltage profile at the receiving end during load variations. Figure 2 illustrates the voltage profile of such a system. It is obvious that when the load increases the voltage drops. The red line in Fig. 2 shows the minimum standard acceptable voltage limit of 0.95 pu of the rated voltage (or 95% of 230 KV in our case). Therefore the voltage must not drop below this limit. Otherwise, the system will have a voltage quality issue and unacceptable performance. Figure 2 illustrates that P ge 23068.4 the voltage profile of the given power system under different loading conditions has undesirable performance in most cases. Only at 20 MVA load at power factor greater than 0.9 lagging, voltage is acceptable and lies within limits. This means that the loading capability of the system cannot exceed 20 MVA. Therefore, corrective measures should be taken to address this problem. Shunt Compensation One remedy to such a problem is using a shunt compensation at the receiving end. That is, by connecting a shunt capacitor to the load, required reactive power is supplied localy and the power source would not have to provide reactive power to the load. Transmission line is also relieved from transmitting reactive power to the load. Consequently, the system in Fig. 1 is amended as depicted in Fig. 3. The value of the capacitor changes as the reactive power demand varies; the maximum reactive power demand is about 42.15 MVAr as shown in Fig. 4. Therefore, the maximum capacitance value has to be about 15 μF as illustrated in Fig. 5. Figure 3. Single-line diagram for the shunt compensated system