Читать книгу Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers - Kalyan K. Sen - Страница 26

The independent control of active and reactive power flows requires the simultaneous control of the magnitude and phase angle of the transmission line voltage, which can be achieved by a shunt‐compensating voltage, using a Shunt–Shunt Compensator as shown in Figure 1-26. This concept dates back to the time when rectifiers and inverters were introduced to convert AC power from one voltage and frequency level to another with active power (P_link) transfer through a DC link. The most frequently‐used topology is an AC–DC rectifier followed by a DC–AC inverter for variable speed motor drives and, if combined with a local energy storage, an Uninterruptible Power Supply (UPS). To improve the power quality at the rectifier’s AC terminal and to accomplish a bidirectional power flow, two DC–AC inverters are connected back‐to‐back via their DC links as shown in Figure 1-26. This configuration in electric utility applications is known as Back‐To‐Back STATic synchronous COMpensator (BTB‐STATCOM).

Table 1-1 Economic Analysis of ST versus UPFC over a 45‐year time period.

Compensators	UPFC	ST
Base Equipment size	100 MVA	100 MVA
Life	15 years	45 years
Equipment First Cost	$50 M	$10 M
Project First Cost	$100 M	$20 M
Annual Operation & Maintenance (O&M) Costs	$X	$0.1 X
Discount Rate	10%	10%
Equivalent Present project Cost over 45‐year life	$129.66 M	$20 M
Equivalent Present Project Cost over 45‐year life ratio	6.483	1
Equivalent Annual Cost over 45‐year life	$13.147 M	$2.0278 M
Equivalent Annual Cost over 45‐year life ratio	6.483	1
Annual Savings of ST over UPFC		$11.1192M
Present Value of Savings of ST over UPFC		$109.66 M

Figure 1-26 Point‐to‐point transfer of power with local reactive power compensation using a Shunt–Shunt Compensator‐based BTB‐STATCOM.

Assuming that the sending‐end voltage (V_s = V_s ∠ δ_s), receiving‐end voltage (V_r = V_r ∠ δ_r), and the line reactance (X) remain unchanged, the compensated active and reactive power flows (P_r and Q_r) at the receiving end are

(2‐74)

and

(2‐75)

where the magnitude of the voltage (V_s′) at the modified sending end is V_s′ and the corresponding phase angle is δ_s′,

(2‐61)

and the modified power angle is given by

(2‐62)

Combining Equations (2-74) and (2-75) and using Equation (2-61), it can be found that for given active and reactive power flows (P_r and Q_r) at the receiving end, a PFC with a Shunt–Shunt configuration applies a voltage at the modified sending end, such that

(2‐81)

and

(2‐82a)

Therefore, a simultaneous regulation of the magnitude ( V_s′ ) and the modified power angle ( δ′ ) of the line voltage is needed for an independent control of active and reactive power flows in the line.

The transfer of power flow from one line to another can be achieved with the use of a BTB‐STATCOM that consists of at least two VSCs, each of which is connected in shunt (parallel) with the transmission line through a coupling transformer. All the VSCs are connected at their common DC link. The shunt‐compensating voltage is of variable magnitude and phase angle, and it is also at any phase angle with the prevailing line current. Accordingly, it exchanges active and reactive powers with the line. The exchanged active power by one VSC flows bidirectionally through the common DC link to another VSC. Each shunt‐connected VSC can also provide an independent shunt reactive power compensation at its AC terminal and, thereby, regulates the voltage at the POC. Each shunt‐connected VSC is rated for the full line voltage and carries the full line current and, therefore, is rated for its full transmitted power. As an example, in order to increase the power flow in a line from its natural flow of 800 MVA to 1000 MVA, the rating of each of the two units of the Shunt–Shunt Compensator would be 1000 MVA. In some special cases for point‐to‐point transfer of power between two isolated networks with different voltages, phase angles, or frequencies, the use of the Shunt–Shunt Compensator may be the preferred solution.

The Shunt–Shunt Compensator may also be implemented, using a common electromechanical or magnetic link in which case, the compensating voltage is generated from either an electrical machine or a transformer with LTCs. In the case of a magnetic link, both active and reactive powers flow through the link. The point‐to‐point transfer of power from one line to another with different voltages, phase angles, or frequencies can be accomplished with the use of shunt–shunt‐connected electrical machines. The Sen Transformer can also generate a shunt‐compensating voltage for the interconnection of two nearby grids with different voltages and phase angles, but the frequencies of the two grids must be the same.

The Shunt–Shunt Compensator is capable of controlling the modified power angle (the phase angle between the modified sending‐end voltage and the receiving‐end voltage of the transmission line) in the range of 0^°≤ δ′ ≤ 360^°. The range of maximum transfer of power along a lossless transmission line (with a quality factor Q = X/R = ∞) between the modified sending and receiving ends takes place at δ′ = 90° as shown in Figure 1-27. At the same time, the actual δ′ is significantly lower and depends on the line length, system characteristics, and load flows. A transmission line with the natural (uncompensated) power angle (δ) in the range of 15 to 20^° may have a possible range of compensation of an additional 5 to 10^° as discussed in Chapter 2, Section 2.2.2. As an example, using a series‐compensating voltage of 0.2 pu, a compensation range (ψ_max) of ±11.54° with an uncompensated power angle (δ) of 30° is shown in the shaded area in Figure 1-27. Therefore, the Shunt–Shunt Compensator is severely restricted to operate within the first quadrant when used as a PFC. Then the question arises: “Is it possible to design a new PFC that can control the same amount of transmitted power that is controlled by the Shunt–Shunt Compensator and operate within the allowable range of power angles, while the power rating of this new PFC is a fraction of the conventional Shunt–Shunt Compensator?” For the same example of increasing the power flow in the line from its natural flow of 800 MVA to 1000 MVA, the rating of each of the two units of the Shunt–Series Compensator might be only 200 MVA, resulting in a saving of 80% of the power rating, compared to a Shunt–Shunt Compensator.

Figure 1-27 Ranges of Q_r versus P_r at the receiving end of the transmission line for the range of modified power angle from 0º to 90º when V_s = V_r = 1 pu, X = 0.5 pu, and R = 0 (X/R = ∞) using Shunt–Shunt and Shunt–Series configurations, respectively.

In an alternate approach to the Shunt–Shunt compensation, the same active and reactive power flows (P_r and Q_r) can be obtained using a PFC with a Shunt–Series configuration that applies a series‐compensating voltage (), such that

(2‐94b)

or when δ_s = 0°,

where the phase‐shift angle,

(2‐51b)

or

(2‐51c)

The magnitude (V_s′s) and the relative phase angle (β) of the series‐compensating voltage (V_s′s) are

(2‐100)

and

(2‐101)

For a Shunt–Shunt Compensator, the relationship between the active and reactive power flows (P_r and Q_r) at the receiving end is shown by a quarter circle, the radius (a′) of which is varied within the upper and lower limits by the modified sending‐end voltage (V_s′) as shown in Figure 1-27. The relationship is given by the following equation:

(2‐79)

where

(2‐61)

Equation (2‐79) represents the locus of a circle, centered at with a radius of a′.

For a Shunt–Series Compensator, the relationship between active and reactive power flows (P_r and Q_r) at the receiving end is shown by the small circle in Figure 1-27 and is given by the following equation:

(2‐136)

where P_rn and Q_rn are the natural active and reactive power flows at the receiving end of the line and

(2‐132)

Equation (2‐136) represents the locus of a circle, centered at [P_rn, Q_rn], with a radius of a_r that is given in Equation (2‐132).

An ideal PFC controls the values of the power flow control parameters (line voltage magnitude, its phase angle, and line reactance) to regulate the magnitude and the phase angle of the line voltage simultaneously by adding a series‐compensating voltage to the original voltage with the use of a Shunt–Series Compensator as shown in Figure 1-28. The compensating voltage is variable in magnitude and phase angle with respect to the line voltage. The series‐connected VSC is rated for a fraction of the line voltage, but carries the full line current. The shunt‐connected VSC is rated for the full line voltage, but carries only a fraction of the line current. Therefore, each VSC carries only a fraction of the full transmitted power. In an actual installation, both shunt‐ and series‐connected VSCs are designed to be of the same voltage and current ratings, which reduces the inventory of spare parts. Therefore, their different interface voltages with the line are accomplished with selection of proper turns‐ratios of the respective coupling transformers. For example, in the world’ first UPFC at the AEP Inez substation, the two VSCs were designed identically to be rated at ±160 MVA and 37 kV phase‐to‐phase voltage. However, the shunt VSC was connected to a 138 kV‐line through a coupling transformer and the series VSC was designed to inject 13.33 kV (~16% of the phase voltage) in series with the line through a coupling transformer.

Figure 1-28 Independent active and reactive power flow controller with local reactive power compensation using a Shunt–Series Compensator‐based UPFC.

The Shunt–Series Compensator connects a compensating voltage in series with the line at any relative phase angle in the range of 0^° ≤ β ≤ 360^° with respect to the line voltage at the POC. Figure 1-27 shows that a series‐compensating voltage of 0.2 pu modifies the power angle by 11.54^°, which may be near the allowable limit. The most important and unique feature of the Shunt–Series configuration is that for a given amount of transmission line power, the series‐connected VSC has a large leverage between its own rating and the controlled transmission line power. The series‐compensating voltage needs to be rated for only a fractional amount of transmitted power, whereas the shunt‐connected VSC in the Shunt–Shunt configuration has no such leverage and it needs to be rated for the full amount of transmitted power. Because of this uniqueness, the Shunt–Series configuration is a preferred topology for a PFC. However, in some special cases for point‐to‐point transfer of power between two isolated networks with POC voltages (V_s and V_s′) as shown in Figure 1-26 or interconnection of two transmission lines with different voltages or phase angles (or frequencies), Shunt–Shunt configuration may be the preferred solution. One such system, called the North American Electric Reliability Corporation (NERC) Interconnections, consists of Eastern Interconnection, Western Interconnection, and Electric Reliability Council of Texas (ERCOT) Interconnection, which are three separate systems of 60 Hz frequency that are asynchronous to each other. Another such system exists in Japan, connecting a 50 Hz frequency system in the North and the East with a 60 Hz frequency system in the South and the West, that is asynchronous to each other.

The UPFC consists of two VSCs with a common DC link capacitor. The two VSCs are connected to the same transmission line through two coupling transformers: one is connected in shunt and the other is connected in series with the line. The series‐compensating voltage is of variable magnitude and phase angle and it is also at any phase angle with the prevailing line current. Therefore, it exchanges active and reactive powers with the line. The exchanged active power (P_link) flows bidirectionally through the common DC link to and from the same line under compensation. Both shunt‐ and series‐connected VSCs can also provide independent reactive power compensation at their respective AC terminals. The compensating voltage, being at any phase angle with the prevailing line current, emulates a four‐quadrant, series‐compensating impedance (Z_se = R_se − jX_se) that consists of a resistance (R_se = +R or −R) and a reactance (X_se = X_C or − X_L) in series with the line. Therefore, the series‐compensating voltage (V_s′s) acts as an IR. A positive resistance (+R) absorbs active power from the line; a negative resistance (−R) delivers active power to the line. The quadrature component of the compensating voltage with respect to the line current emulates a capacitive reactance if the compensating voltage lags the prevailing line current or an inductive reactance if the compensating voltage leads the prevailing line current. A capacitive reactance (X_C) decreases the effective line reactance between its two ends and, in the process, increases the power flow in the line; an inductive reactance (X_L) increases the effective line reactance between its two ends and decreases the power flow in the line; since the power flow in the line is inversely proportional to the effective line impedance, which is assumed to be inductive.

As a special case, when the DC link capacitors of the two VSCs are not connected together, each of the shunt‐connected VSC (STATCOM) and the series‐connected VSC (SSSC) provides only a reactive power compensation that is independent of each other. Since there is no exchange of active power between the STATCOM and the SSSC, they act as RRs (X_sh or X_se = X_C or − X_L).

In 1998, Westinghouse installed a ±160 MVA, 138 kV‐rated FACTS Controller, namely UPFC, at the AEP’s Inez substation in Kentucky, USA. This UPFC demonstrated for the first time that an impedance could be emulated by a series‐compensating voltage whose phase is at any angle with respect to the prevailing line current. As a result, the magnitude and phase angle of the line voltage at the modified sending end could be regulated to the desired values by the UPFC, which is an IR; active and reactive power flows in a transmission line could be regulated independently while maintaining a fixed line voltage at the POC. While maintaining a unity power factor load, the active power flow in the line at the modified sending end is varied at different levels, such as 145 MW, 65 MW, 240 MW, and 145 MW, respectively, as shown in Figure 1-29. The active power flow in a line is reduced by making the effective line impedance higher than its natural value and increased by making the effective line impedance lower than its natural value while maintaining the reactive power flow to zero. Note that the utility application allowed the response time to be adjusted in seconds, even though the power electronics‐based VSC is capable of providing faster responses in ms.

As a special case, the IR can be reconfigured to operate as a RR by connecting the SSSC only. The reactance emulation technique changes the active and reactive power flows simultaneously, meaning both powers either increase or decrease as shown in Figure 1-30; therefore, the line cannot be optimized for the highest amount of active power flow that generates the most revenue at the lowest amount of reactive power flow by using a RR alone.

It was demonstrated in the TVA‐STATCON project (shown in Figure 1-13) that the line voltage can be regulated with a response time in ms; however, the fast response in ms cannot be utilized in power flow control in the AEP UPFC project in order to assure continued operation under various contingencies (i.e. all the possible variations in the number of lines connected together as a network at different times of a day, week, month or a year). Nevertheless, the cost of a FACTS controller is about the same, whether it is used in slow‐response or fast‐response applications. This was the motivation to develop the Sen Transformer that meets the functional requirements to provide independent control of active and reactive power flows with responses in seconds and at a fractional amount of the cost of a VSC‐based FACTS controller.

Figure 1-29 Independent power flow control by impedance regulation (field performance) (Sen and Keri 2003).

Figure 1-30 Simultaneous power flow control by reactance regulation (field performance) (Sen and Keri 2003).

In 1998, a patent was granted to General Electric Company, which proposed to implement the independent control of active and reactive power flows such that the compensating voltage was generated using electrical machines (U.S. patent number 5,841,267, titled “Power Flow Control with Rotary Transformers”).

The Sens (Kalyan and Mey Ling) proposed the idea of independent control of active and reactive power flows, using an IR, called the Sen Transformer, in a radically low‐cost way by using redesigned transformer/LTC technology. The reason is that the transformer/LTC technology has been proven to be efficient, simple, and reliable in utility applications for decades. This implementation of an IR is completely different from the original Westinghouse and the GE concepts. The Sens were awarded five U.S. patents (four patents in 2002, all titled “Versatile Power Flow Transformers for Compensating Power Flow in a Transmission Line” and numbered 6,335,613, 6,384,581, 6,396,248, and 6,420,856, and one patent in 2005, titled “Multiline Power Flow Transformer for Compensating Power Flow Among Transmission Lines,” and numbered 6,841,976). The Sen Transformer is fundamentally different from the conventional transformer, in a sense that it modifies both the magnitude and the phase angle of the line voltage while the conventional transformer only modifies the magnitude of the line voltage. Using a Sen Transformer, the active and reactive power flows in the line can be regulated independently as desired.

The Sen Transformer, shown in Figure 1-31, uses a Shunt Unit (Exciter Unit) and a Series Unit (Compensating‐Voltage Unit) and creates a series‐compensating voltage (V_s′s) that is variable in magnitude and phase angle to modify the sending‐end voltage (V_s) of the line to the modified sending‐end voltage (V_s′) and, in turn, controls both the magnitude and phase angle simultaneously; as a result, the active and reactive power flows in the line are controlled independently as desired. The compensating voltage of the Series Unit can be made to look like the effect of a positive resistance or a negative resistance and a capacitive reactance or an inductive reactance in each phase. The series‐compensating voltage (V_s′s) is at any phase angle with the prevailing line current (I) through the line reactance (X). Therefore, it exchanges active and reactive powers with the line, which is equivalent to emulating a four‐quadrant, series‐compensating impedance (Z_se = R_se − jX_se) that consists of a resistance (R_se = +R or − R) and a reactance (X_se = X_C or − X_L) in series with the line. Therefore, the series‐compensating voltage (V _s′s ) acts as an IR. The ratio of the compensating voltage (V _s′s ) and the prevailing line current (I) is a measure of a virtual four‐quadrant emulated impedance. Note that these exchanged powers pass through the magnetic link as (P_link and Q_link).

Figure 1-31 Sen Transformer (ST).

The LTCs are preferably mechanical with vacuum or oil‐immersed taps. These taps can respond in seconds, which is usually fast enough for utility power flow control needs. If a faster response is needed, the taps can be based on power electronics thyristors, which once turned on in a positive half‐cycle of the voltage across it, commutate naturally in the negative half‐cycle of the voltage. These taps can respond in a few power cycles, which is a 50‐fold decrease in response time. Note that the thyristor technology also faces component obsolescence, albeit with a life cycle of 25–30 years, which is a decade or more longer than the life cycle of the VSC‐based FACTS Controllers. The response time can be further reduced to < 0.010 s if a power electronics inverter‐based FACTS controller is used. However, this type of fast response is almost never needed in utility applications. Besides, as the response speed of the solution increases from slow (3–5 s) to medium speed (< 1 s) to fast (< 0.010 s), there is a corresponding increase in the solution’s life‐cycle costs (installation, operation, and maintenance), complexity, and impracticability of relocation and decrease in the reliability significantly.

The VSC‐based technology has the capability of providing fast (sub‐cycle) dynamic response for a given transmission line impedance, although in a PFC the dynamic response of at least a few cycles of power supply frequency is necessary to operate safely under various contingencies. Most utility applications in the AC system allow regulation of the power flow in the line(s) in a “slow” manner as permitted by the speed of operation of the mechanical LTCs. If faster response is needed, the mechanical LTCs can be replaced with faster TC LTCs. The ST, shown in Figure 1-31, provides simultaneous voltage regulation at the POC and almost the same independent control of active and reactive power flows as the UPFC, albeit at a reduced dynamic rate, which is acceptable in most utility applications.

The STs with both types of LTCs (mechanical and TC) cover a wide range of requirements for power flow control in electric transmission lines. If the LTCs are too coarse for an IR, the number of taps on the winding may be increased. In comparison to a UPFC, which uses power electronics‐based VSCs, the ST uses reliable and proven transformer and LTC‐based technology that results in an order of magnitude less in operational/maintenance cost and equipment cost.

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, and provides an independent series reactive power compensation in each line. A BTB‐SSSC selectively controls the active and reactive power flows in each line in a multiline transmission system and provides a power flow management for the transmission system by decreasing the power flow in an overloaded line and increasing the power flow in an underloaded line. The Multiline Sen Transformer, shown in Figure 1-33, provides the same functionality.

In summary, mechanically or electronically switched compensators are used as PFCs, but each of these compensators can control only one of the three power flow control parameters: line voltage magnitude, its phase angle, and line reactance. Although the active power flow in the line is regulated, the undesirable reactive power flow is also affected simultaneously, but the optimization of power flow that generates the most revenue can be achieved through independent control of active and reactive power flows in the transmission line. Therefore, the power industry’s present need requires the use of PFCs that can independently control both active and reactive power flows in a transmission line, decrease the power flow in an overloaded line, and increase it in an underloaded line, while at the same time keeping the system voltage within the allowable upper and lower limits. The summary of choices for transmission line power flow control equipment is shown in Figure 1-34 in chronological order of their introduction in the industry.

Figure 1-34 Choices for transmission line control equipment.

Table 1-2 Various features of all Shunt–Shunt and Shunt–Series configurations.

Features	VSC	EM	Transformer/LTCs
	Shunt–Shunt configuration
Independent P‐Q flow control	Yes	Yes	Yes
Different frequency system	Yes	Yes	No
Different phase angle system	Yes	Yes	Yes
Intermediate DC transmission	Yes	No	No
	Shunt–Series configuration
Independent P‐Q flow control	Yes	Yes	Yes
One frequency system	Yes	Yes	Yes
Cost	High	Medium	Low

The various features of all Shunt–Shunt and Shunt–Series configurations are summarized in Table 1-2. The VSC‐based solutions provide more features than the electrical machine (EM)‐based solutions, which provide more features than the transformer/LTC‐based solutions. However, most of the power flow control needs are in synchronous networks and the use of the Shunt–Series configuration of the ST that uses transformer/LTC‐based solutions is sufficient to meet the utilities’ power flow control needs in the most cost‐effective way.

It is shown in Chapter 3, Section 3.4 that the active and reactive power flow control area is virtually the same for the ST and UPFC. While the UPFC is a FACTS Controller that uses power electronics‐based VSC, the ST, in its preferred form, uses only transformers and LTCs. The reference, titled “Comparison of the ‘Sen’ Transformer with the Unified Power Flow Controller,” IEEE Trans. on Power Delivery, vol. 18, no. 4, pp. 1523−1533, Oct. 2003, states that “At the present time, two major drawbacks of all VSC‐based FACTS Controllers are their high installation and operating costs.” Over the decades, the list of drawbacks has expanded to include component obsolescence, impracticability of relocation, and lack of interoperability. Since the commissioning of the first commercial VSC‐based FACTS Controller more than two decades ago, a great deal has been learned about the true needs of a utility for its everyday use and they are

 high reliability, requiring the lowest number of components

 low installation and operating costs

 component non‐obsolescence

 interoperability (components usage from various suppliers), and

 easy relocation to adapt to changing power system’s needs

while providing an independent control of active and reactive power flows. In response to these requirements, the novel impedance regulation method of a UPFC and the proven and reliable transformer/LTC technology that is used in a PAR for almost a century are combined to create the ST. This low‐cost form of an IR can improve the fault level in otherwise weak networks, thereby making it possible to have a widely connected grid‐scale renewable generation in weaker and isolated parts of the transmission networks as outlined in PES‐TR‐77, titled “Stability Definitions and Characterization of Dynamic Behavior in Systems with High Penetration of Power Electronic Interfaced Technologies,” which is available at https://resourcecenter.ieee‐pes.org/technical‐publications/technical‐reports/PES_TP_TR77_PSDP_stability_051320.html .