With ever increasing demand of electric power, the existing transmission networks are found to be weak resulting in poor quality of unreliable supply. Also, it is realized that for expanding or enhancing the power transfer capability of the present transmission networks, huge sum of finances are required and sometimes even difficulties are encountered in finding right-of way for the new transmission lines.
Lot of research has been done for developing new technologies over the past few years to gain increased efficiency and reliability from the existing transmission network. As a result, new transmission system, known as Flexible AC Transmission System (abbreviated as FACTS) has been developed.
The new technologies make use of high speed thyristors for switching in or out transmission line components such as capacitors, reactors or phase shifting transformer for some desirable performance of the system. The main objective of this system is to replace the existing slow acting mechanical controls needed to react to the changing system conditions by rather fast acting electronic controls.
These recently developed systems are likely to be employed for ac interconnecting lines and long ac lines. A long flexible ac transmission system (FACTS) is illustrated in Fig. 13.4. It has intermediate substations at an interval of 250-350 km. 1, 1 are controllable series capacitors and 1′, 1’ are controllable shunt compensation (SVS).
In a typical FACTS, controllable series capacitor installation is obtained by means of bypass thyristor switch (Fig. 13.5). The amount of series compensation is controlled by controlling the bypass current IB through the thyristor controlled switch (TCS) and current IC through series capacitor Cse.
Power flow is given by expression:
where XL is series reactance of the transmission line and XCF is the controllable series reactance of series capacitor of FACTS.
ADVERTISEMENTS:
So, by controlling XCF by means of bypass thyristor switch, the power flow can be controlled. The control over power flow P and power angle δ is provided by the phase control of bypass thyristor switch. The basic limitation of ac transmission line and series compensation had been the lack of controllability, which has been overcome by controllable series capacitors of FACTS.
In addition to controllable series capacitor; controllable shunt compensation is obtained by means of SVS in FACTS. Thus, voltage at both ends of transmission line is controlled dynamically. This provides voltage stability and improves dynamic stability of the transmission link.
In a typical FACTS, controllable series capacitors and controllable SVS combinations are installed at an interval of about 250 km along a long ac transmission line.
ADVERTISEMENTS:
Single line schematic diagram of FACTS is shown in Fig. 13.4. Figure 13.5 shows a schematic diagram of the details of controllable series capacitors 1. All the equipment of controllable series capacitors are installed on insulating platform. The control unit senses the phase-to-ground voltage V at the location of controllable series capacitors and adjusts phase angle of thyristor control switch (TCS) and so bypass current IB is controlled.
By varying IB, IC is controlled because IL = IC + IB. This results in variable series compensation which can be controlled from load control centre through compensation, the power flow P is controlled. By controlling static VAR source (SVS), the voltage at sending end, receiving end and at intermediate substations are controlled.
Series and shunt compensations are controllable and control is fast and over a wide range. Hence FACTS improves the dynamic stability of the ac transmission system.
By simultaneous control of 1, 1, … and 1′, 1′ … the power flow P, sending end voltage VS, receiving end voltage VR and power angle δ between VS and VR are controlled. The oscillations in P, V, δ are damped by controlling 1 and 1′.