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

Автор: Kalyan K. Sen

Издательство: John Wiley & Sons Limited

Жанр: Физика

Серия:

isbn: 9781119824381

isbn:

СКАЧАТЬ that is variable in magnitude and phase angle to modify the sending‐end voltage (Vs) of the line to the modified sending‐end voltage (Vs′) 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 (Vs′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 (Zse = Rse − jXse) that consists of a resistance (Rse = +R or − R) and a reactance (Xse = XC or − XL) 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 (Plink and Qlink).

Schematic illustration of 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.

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