It is well established that the UPFC is the most versatile PFC that was ever developed. A detailed comparative analysis of the ST and UPFC is given in Chapter 6, Section 6.3 (Comparison Among the VRT, PAR, UPFC, and ST). The life‐cycle costs (installation, operation, and maintenance) of the ST are less than the competing FACTS Controller, such as UPFC for the most utility applications due to the following reasons:
For a one per unit (pu) power through the ST, the installed transformer rating may be as much as two pu, whereas the “all electronic” UPFC requires more than a four pu‐rated transformer and more than eight pu of installed power electronics, which translates into a higher installation cost for the UPFC.
The ST rides through the fault current, but the UPFC requires a protection scheme with an additional electronic bypass‐switch, which translates into a higher installation cost for the UPFC.
The power loss in the ST is less than 1% of its rated power whereas the power loss in the two coupling transformers and two intermediate transformers of a UPFC are 1–2% of the power flowing through the UPFC, which translates into a higher operating cost for the UPFC.
There is no switching loss in the ST, whereas the switching and conduction losses in the two inverters of the UPFC can be 2–6% of the power flowing through the UPFC, depending on their configuration, which translates into a higher operating cost for the UPFC.
The ST requires the use of LTCs whose contacts are immersed in the transformer oil. The maintenance expertise for ST is readily available in the industry. However, the power electronics inverter‐based UPFC consists of semiconductor switches with appropriate snubber circuits that create power loss. To remove the heat generated from this loss, deionized water cooling and heat exchangers are needed. The failed switches need to be replaced, requiring specialized expertise. Therefore, the operational/maintenance cost of the UPFC is much higher than that of ST.
The ST uses traditional but redesigned transformer and LTCs technology that has been proven to be efficient, simple, reliable, and robust in utility applications for decades. The UPFC uses thousands of electronic components that are constantly becoming obsolete. Therefore, the cost due to component obsolescence in a UPFC is far greater than that in an ST.
The footprint of an ST is a fraction of that of a UPFC. Therefore, the ST is relocatable as the system needs change. The power electronics inverter‐based UPFC is not practical to be relocated.
The ST uses off‐the‐shelf transformer/LTC technology from any manufacturer and therefore, it is interoperable. The ST can be manufactured and serviced anywhere in the world. In contrast, there is no manufacturing standard established for the VSC‐based FACTS Controllers. Since each manufacturer establishes its own unique design, the VSC‐based FACTS Controllers are not interoperable. Its maintenance depends on the expertise of a specific manufacturer.
Impedance Regulators, such as the UPFC and ST, are capable of injecting a compensating voltage in series with the line in the entire range of 360°. However, in many instances, the capability of connecting a compensating voltage in series with a line within its entire range of 360° is not needed. The active power flow can be increased to the maximum possible level within the first 120° of the 360°‐range of the relative phase angle. The active power flow can be decreased to the minimum possible level within the next 120° of 360°‐range of the relative phase angle. The cost of the ST can be further reduced with its simpler design per the functional requirements to operate in a “limited angle” configuration, instead of the full 360°‐range of operational configuration. There is no such cost advantage in the design of a UPFC. Hence, the ST is adequate and economically attractive to meet most of the utility’s present need for independent control of the active and reactive power flows in the transmission lines.
In another unique operation, the ST with an autotransformer is the most cost‐effective option that allows interfacing of two transmission systems with different voltage levels and implementing independent power flow control as shown in Figure 6‐86.
The ST, in its basic design, uses three primary windings and nine secondary windings with either nine single‐phase LTCs or three three‐phase LTCs that are in direct contact with the transmission line. Therefore, the LTCs, in the basic design, are required to carry a high line current as well as even a higher fault current. The readily available LTCs may be challenging for use in Extra High Voltage (EHV) and Ultra High Voltage (UHV) applications. In these cases, the applications with greater than 230‐kV voltage level require a two‐core design where the taps are not exposed to high voltages as shown in Figure 6‐72. A comparison of the sizes and footprints of the world’s first UPFC and a comparably rated prototypic ST is shown in Figure 1-25.
The compensating voltage in an autotransformer is in‐phase (0°) or out‐of‐phase (180°) with the line voltage and, therefore, regulates the magnitude of the transmission line voltage. The compensating voltage in the PAR is in quadrature (90° or –90°) with the line voltage and, therefore, regulates the phase angle of the transmission line voltage. The ST creates a series‐compensating voltage that is variable in magnitude and phase angle and can control the transmission line voltage in both magnitude and phase angle simultaneously in order to achieve independent control of active and reactive power flows in the line. This compensating voltage may be thought of as two separate orthogonal compensating voltages of an autotransformer and a PAR (asym). Therefore, in the ST, the functions of the autotransformer and the PAR (asym) are combined in a single unit that results in a reduced amount of hardware from what is required separately in an autotransformer and in a PAR as shown in Figure 1-32.
Both the ST and UPFC are suitable for independent control of active and reactive power flows in a single transmission line in which they are installed. However, several transmission lines in close proximity may be connected to a common voltage bus. Therefore, any change in the power flow in one line will affect the power flows in the other lines as well. Thus, the excess power from one specific line cannot be transferred directly to another specific line. In a multiline transmission network, it would be advantageous to be able to transfer power from an overloaded to an underloaded line with minimum undesirable impact on the power flows in the other uncompensated lines.
Figure 1-32 Autotransformer/PAR (asym).
Figure 1-33 Multiline power flow concepts.
The common DC‐link concept can be extended for power exchange between transmission lines with series–series‐connected VSCs. The BTB‐SSSC, also called interline power flow controller (IPFC), shown in Figure 1-33, consists of at least two VSCs; each VSC is connected in series with a transmission line. All the VSCs are connected at their common DC link. The BTB‐SSSC transfers active power from one or more transmission lines, referred to as “leader” lines, to the others, referred to as “follower” lines,