Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers. Kalyan K. Sen

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      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, СКАЧАТЬ