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

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СКАЧАТЬ 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 СКАЧАТЬ