Figure 1. Static Synchronous Series Compensator
Today, modern power system carries very high demand from the load side. Thus, they cause the stability problem. This paper investigates the problem related to the voltage stability in the power system and controlling the power flow through transmission line using a FACT controller known as Static Synchronous Series Compensator (SSSC). In this paper, the FACT controller investigates the effects for controlling the active and reactive power as well as damping power system oscillation that occur because of the heavy loaded system. Under heavy loaded conditions, there may be insufficient reactive power causing the voltage to drop. This voltage may lead to drop in voltage at various buses. Flexible AC Transmission System (FACTS) controller have been mainly used for various power system stability. In this paper, the authors used a Pulse Oscillation Damping controller with SSSC to damp out the system oscillation and maintain the stability of the power system. The SSSC equipped with source of energy in dc link can supply or absorb the active power to or from the line along with the reactive power flow control. The SSSC is represented by a variable voltage injection with associated transformer leakage reactance and voltage source.
In recent years, greater demand have been placed on the transmission network and increase in the demand will rise because of the increasing in the number of non-utility generators and competition among utility themselves. From the energy utilization point of view, the demand of electricity is dramatically increased and the modern power system becomes a complex network of transmission lines interconnecting generating station to the major load point in the overall power system in order to support the high demand of consumers. The complicated network causes the stability problem.
This paper studies the SSSC (FACTS) controller performance in terms of stability improvement. The utility need to operate their power transmission network much more effectively, increasing their utilization degree, reducing the effective reactance of line by series compensation of line is direct approach to increase the transmission capability because of the power electronic switching capabilities in terms of control and speed, more advantages have been done in FACTS device areas [1, 2]. A Static Synchronous Series Compensator is a member of FACTS family that is connected in series with the transmission lines to control the active and reactive power flow. It consists of a solid state voltage converter with GTO thyristor switches or high performance of semi conductor and transformer [2]. The Static Synchronous Series Compensator has the primary to control the power flow in steady state. It can also improve the transient stability of the power system.
The concept of FACTS or flexible AC transmission system was proposed by Hingorani which includes the use of high power electronics, communication links and dynamic control to increase the power transmission capability and stability of power system [1]. SSSC consist of VSC voltage source converter. At the fundamental frequency, it converts dc voltage to three phase ac voltage. The output voltage of SSSC is kept in quadrature with the line current. It is done because there is no real power exchange between the transmission line and SSSC.
Series compensation is an economic method of improving the transmission capability of the lines. Typical series compensation use capacitors to decrease the reactance of transmission line at rated frequency [7] . The connection of capacitor for series compensation generate the reactive power in a self regulated manner [12]. The result is improved for power system through-
The basic scheme of SSSC is shown in Figure 1. The compensator equipped with a source of energy which help in supplying or absorbing active power to or from the transmission line along with the control of reactive power flow.
Figure 1. Static Synchronous Series Compensator
As the device name suggest, it is a series compensator, means, it is connected in series with the transmission line. A three phase series transformer is used to couple the device with the power system.
SSSC contained a voltage source converter. At fundamental frequency, it generates an output voltage. This generated output voltage is kept in phase quadrature with the line current [15]. The SSSC voltage emulates a series capacitor when it legs the current by 90°. When the voltage is lead, the line current by 90° behaves like a series inductor [1]. Here SSSC is considered as a series compensation where by controlling the output voltage of SSSC, the degree of compensation can be varied by adjusting the output voltages [2]. To improve the dynamic performance of a power system, the power flow control is essential.
Figure 2 shows the single line diagram of SSSC with the inductive reactance Xl connecting a sending end voltage source Vs and receiving end voltage source Vr .
Figure 2. SSSC Control Scheme
The real and reactive power is represented by P and Q as,
where Vs and Vr are the magnitude of voltages δs and δr are the phase angle of the voltage source Vs and Vr respectively. Here. Vs = Vr = V
By connecting SSSC, it is capable for the series compensation of the transmission line by a compensating reactance Xq, therefore, expression of power flow becomes,
The compensating reactance Xi is defined to be negative when it is operated on inductive mode, and positive when it is operated on capacitive mode. Figure 3 shows the control system of SSSC.
Figure 3. Control System of SSSC
In modern power system, it is a complex interconnection of transmission lines and the utility generators, which requires careful design of new devices [3]. The creation of new concept that is able to work more efficiently with the existing power system without reduction in system stability and security [13]. The proposed concept is known as Flexible AC Transmission System [1]. The main objective of FACT controller are to increase the transmission capability of power system, minimize the losses, maintaining stability, and reduce the power system cost [10]. The applications of this concept are
The dynamic performance of SSSC is presented by real time voltage and current waveforms. Using MATLAB software, the system shown in Figure 4 has been obtained [4]. In the simulation one SSSC has been utilized to control the power flow in the 500 KV transmission line.
Figure 4. Two Machine 4 Bus Power System
This system has been consisting four buses B1 to B4 connected through the transmission line of length 280, 150, 150 and 50 km respectively [5]. The system has been feeding from two power plants with phase voltage is 13.7 KV. Figure 4 shows the Two machine 4 bus power system.
In this study, SSSC is employed with the Pulse Oscillation Damping Controller to damp out system oscillation. First, we disturb the system at any point with SSSC then transient will occur and system will start oscillating for seconds. POD controller wash out the system oscillation and improve the system stability with the help of Static Synchronous series compensator. SSSC injects a fast changing voltage in series with the transmission line which is independent from the line current. The system is connecting very high load that is responsible for voltage drop at various buses connecting to the line.
According to the results at first, due to the large loads connecting to the system active power got oscillation which keep continuing for seconds. PSS and other stabilizing devices are used to damp out these oscillation [5]. Because of above mentioned reason, reactive power bus 2 got oscillations at first, and then will be damped properly.
After a three phase fault created at first in the system at 1.3 sec to 1.5 for 10 cycle voltage and current waveforms of bus 2 got closer to the sinusoidal waveforms. Voltage amplitude is 1 per unit, but despite the drawn current by loads in system, current amplitude is 6.7 per unit.
First, power system with two machine and four buses has been simulated in MATLAB environment, then power and voltage in all buses has been obtained [5]. The result have been shown in Figures 5,6,7,8 and 9. Measurement is taken across Bus B2.
Figure 5. Injected Voltage by SSSC
Figure 6. Active Power of Bus2
Figure 7. Current of Bus B2
Figure 8. Active Power of Buses B1B2B3B4 without SSSC
Figure 9. Reactive Power of Buses B1B2B3B4 without SSSC
SSSC is controlling the active and reactive power; besides these SSSC with POD controller, could fairly improve the transient oscillation of system. Power system with two machine and four buses after installing of SSSC has been simulated in MATLAB environment [4,7]. By installing of SSSC active power, damping time will be less than the mode without POD controller and it will be damped faster. The results have been shown in Figures 10,11,12,13 and 14.
Figure 10. Injected Voltage by SSSC
Figure 11. Active Power of Bus B2 using SSSC
Figure 12. Current of Bus2 using SSSC
Figure 13. Active Power of Buses B1B2B3B4 using SSSC
Figure 14. Reactive Power of Bus B1B2B3B4 using SSSC
From this study of two machine power system, the authors can conclude that, the SSSC is capable of controlling the flow of power at a desired point in the transmission line. It is also observed that, the SSSC injects a fast changing voltage which is near to sinusoidal and irrespective from the magnitude and phase of the line current. On the basis of result of SSSC with Pulse Oscillation Damping controller which is examined in a multi machine system, and application of SSSC will be extended in future to a complex power system to investigate the various problem related to the stability, power compensation, power flow control and power oscillation.