Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers. Kalyan K. Sen
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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 (Vs′s) 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).
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