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

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СКАЧАТЬ a capacitor. A TCSC or SSSC modifies the magnitude and phase angle of the line voltage, which are the combined functionalities of a VR and a PAR as shown in Figure 2‐28. Since, the series reactance compensation technique does not change the effective line resistance, it cannot control the active and reactive power flows in the line independently as shown in Figure 1‐30.

      In a lightly loaded line, the reactive power absorbed by the series reactance of the line may be much less in comparison to the reactive power generated by the line‐to‐ground, shunt capacitance of the line. The resulting voltage increase in the line may reach or exceed the allowable limits for other loads that are connected to the grid. In a heavily loaded transmission line, the reactive power needed by the series reactance of the line may be much more in comparison to the reactive power generated by the shunt capacitance of the line. The resulting voltage along the line may decrease to a point that is below an acceptable limit when the full performance of the load is not possible. If the voltage along the transmission line is increased to be regulated at its nominal value by using a VR, the active power flow increases over the natural flow as discussed in Chapter 2, Section 2.6.1 (Shunt‐Compensating Reactance). If the phase angle between the voltages at the two ends of the transmission line is increased by using a PAR, the active power flow also increases. The unintended consequence of increasing active power flow by voltage regulation or phase angle regulation is that the reactive power flow in the line is also affected. When the line reactance is regulated, both the active and reactive power flows in the transmission line are varied simultaneously.

      If the reactive power along the line is reduced, the freed‐up capacity of the line can be used to increase the revenue‐generating active power flow. As a result, the connected‐generators will be required to supply less reactive power. Furthermore, the efficiencies of the generators and step‐up transformers under a reduced reactive power condition will also increase. Since the loss in the line decreases due to less reactive power flow, the grid becomes more efficient. Therefore, it is desirable to compensate the lines to operate under independent, not simultaneous, control of the active and reactive power flows so that the line can facilitate the delivery of active power with the greatest value. The active and reactive power flows in a line can be regulated independently with a UPFC or an ST that controls the effective impedance of the line between its two ends, which is functionally equivalent to regulating both the magnitude and phase angle of the line voltage simultaneously.

1.3 Modern Power Flow Control Concepts

In the early 1990s, there was a renewed interest to experiment with novel power electronics VSCs‐based PFCs due to the availability of high‐power, forced‐commutated, semiconductor switches, such as 4500 V, 4000 A‐rated gate‐turn‐off (GTO) thyristors. A new definition, namely Flexible Alternating Current Transmission Systems (FACTS), was proposed as “alternating current transmission systems incorporating power electronic‐based and other static controllers to enhance controllability and increased power transfer capability.” The purpose of the FACTS solution was to completely overhaul and improve the power delivery techniques that were developed, since the introduction of free flow of electricity in the late 19^th century. Under this program, Westinghouse installed a ±100‐MVA‐rated STATCON (STATic CONdenser as discussed in the literature) at the Tennessee Valley Authority’s (TVA) Sullivan substation in Tennessee, USA. This new equipment was fundamentally different from the conventional thyristor‐controlled SVC. This equipment was called STATCON because its steady‐state output characteristics are similar to those of the rotating synchronous condenser. This was the world’s first commercial installation of a STATCON, which was later renamed as STATCOM. This project was based on a previously demonstrated project that was documented in a report, titled 1 MVAR Advanced Static VAR Generator Development Program, ESEERCO Report, EP 84‐30, April 1987. The TVA‐STATCOM demonstrated (i) realization of a ±100 MVA, 161 kV‐rated VSC, based on a harmonic‐neutralization technique that did not require the use of any filter at the output of the VSC, (ii) viability of GTO‐based VSCs at high power for transmission applications and (iii) fast control response of a VSC‐based compensator as shown in Figure 1-13. This STATCOM was built using MSDOS (Microsoft Disk Operating System), which became obsolete before its installation in 1995 due to the arrival of MS Windows operating system in 1993. The STATCOM was retired from service due to component obsolescence. This demonstrates the consequential risks associated with the new power electronics technologies in regard to the life expectancy under utility conventional standards. Rapid advances in semiconductor technology and computer operating systems do not align with the utility business model and quickly outpace the utility adoption standards. This is a prime example of technology obsolescence that should be considered when making a technical evaluation.

Figure 1-13 Response time of the first commercial STATCON for 100 Mvar capacitive step (left) and 100 Mvar inductive step (right) (field performance) (Westinghouse).

The capability of providing a 100‐Mvar step change in 2 ms by a VSC‐based STATCOM, using forced‐commutated GTOs, was a major improvement in response time than what was obtained in a naturally commutated, thyristor‐based SVC. The SVC uses thyristors, which have a natural transport delay of half of a power cycle, meaning when a thyristor turns on in a positive half cycle, it turns off naturally in the next negative half cycle following the zero‐crossing of the voltage. Figure 1-14 shows a two‐month voltage profile without an SVC (left) and with an SVC (right). This is a significant improvement over no compensation. The experience of the last five decades has shown that the response time of several cycles using an SVC is quite adequate in most utility applications. This is primarily why the higher cost of the VSC technology with component obsolescence issues have prevented its widespread use/adoption in utility applications.

Figure 1-14 Voltage profile without an SVC (left) and with an SVC (right) (field performance) (Barot et al. 2014).

      However, the fast response from a VSC may be just the right solution to address various issues in addition to var compensation, such as

       Unbalanced voltage

       Harmonic voltage and current

       Voltage spikes

       Voltage flicker.

One such application is shown in Figure 1-15 where a voltage source (V_source) supplies power to an electric arc furnace load through a network whose impedance is represented as Thèvenin impedance (Z_Th). The bus voltage (V_plant) at the input of the plant is stepped down twice through, first, the Main Transformer and, then the Arc Furnace Transformer.

Figure 1-15 A single‐line diagram of an electric arc furnace.

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