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

The intermittency of energy sources, need for bidirectional flows, remote locations of solar and wind projects, and so on, are challenging grid planners and operators to integrate more renewable energy into the grid. Even before the meaningful penetration of renewables is reached, forecasters are factoring renewable curtailment as a major strategy to balance supply and demand.

      An SPFC, such as an IR with a proper dynamic response capability, can greatly aid in fulfilling these new requirements. The SPFC creates a variable virtual impedance that can be connected in series with the line, to maintain steady power flows to the load centers. The SPFC can limit the power flow in congested lines to be within their ATC so that the renewable generation does not need to be curtailed.

As discussed in great detail in Chapter 2, any PFC that connects a compensating voltage in series with the line is actually an IR, except an SSSC, which is a RR. The PAR acts as an IR in an indirect way with no capability to select the compensating resistance (R_se) and reactance (X_se) independently as discussed in Chapter 2, Section 2.2.2.6. Therefore, a true IR that 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 is really needed. Additionally, increasing the installation of roof‐top solar as generation and its integration into a low voltage distribution network alters the traditional feeder voltage profile in the distribution networks, which can be mitigated, using an IR.

Figure 1-35 Today’s grid with traditional generation and integrated renewable generation.

      Advances in power electronics have made it possible to develop the UPFC, which is an IR. The VSC‐based UPFC is capable of providing responses in the range of ms as shown in Figure 1-13 in the demonstration of the first commercial STATCOM at TVA Sullivan substation in 1995. However, the experiences from the last three decades show that the needed response time is in seconds in most utility applications as shown in Figures 1-29 and 1-30 in the demonstration of world’s first UPFC at AEP Inez substation. Nevertheless, the cost of a UPFC is about the same, whether it is used in slow‐response or fast‐response applications. Therefore, it is desirable to explore the alternate designs of an IR that meet 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 VSC‐based FACTS Controllers. This was the motivation to develop an SPFC whose objectives are as follows:

       S – specific (design a power flow controller that meets utilities’ needs)

       M – measurable (high reliability, high efficiency, cost‐effectiveness, component non‐obsolescence, and ease of relocation)

       A – attainable (realistic expectation about the outcome)

       R – relevant (efficient power grid)

       T – time‐bound (delivery milestones).

The SPFC can control a bidirectional and independent active and reactive power flows dynamically as shown in Figure 1-36.

      1.6.1 Example of an SPFC

      In a particular application, the functional requirement of an SPFC may be written as follows:

The SPFC shall provide voltage regulation, phase angle regulation, positive and negative resistance regulation, capacitive and inductive reactance regulation, four‐quadrant impedance regulation, and independent regulation of active and reactive power flows in a line by connecting in series with the line a compensating voltage that has the following characteristics:

Figure 1-36 Interconnected transmission system, integrated with a SMART Power Flow Controller (SPFC).

      1 variable in magnitude within its design limit

      2 variable in phase angle with respect to the line voltage or the prevailing line current

      3 response time in the range of operation in less than 30 seconds

      4 availability of 99.9999% of the time.

      1.6.2 Justification

      The natural power flow in an AC transmission line depends on (i) line voltage magnitude, (ii) its phase angle, and (iii) line impedance. The power flow in a line may be controlled by regulating any of these three parameters to optimize the voltage profile and the power flow in the line while maintaining the voltage stability and minimum power loss in the line.

      1.6.3 Additional Information

      The desired features of an SPFC are as follows:

       High reliability with the lowest number of components used

       Impedance control feature using a Shunt–Series configuration

       Lowest installation cost

       Lowest operating cost with minimum maintenance and losses

       Practically relocatable when the system needs change

       Free from component obsolescence for at least three decades, and

       Interoperability so that components from various suppliers can be used, resulting in a global manufacturing standard, ease of maintenance, and ultimately lower cost to consumers.

To meet the above‐mentioned functional requirements, thyristor‐based SVC and transformer/LTCs‐based ST may be a suitable solution as shown in Figure 1-37. Since the proposed solution is based on functional requirements and the lowest cost, it is considered to be an SPFC.

Figure 1-37 Voltage regulation with an SVC and independent power flow regulation with an ST.

1.7 Discussion

Various compensators for utility applications are summarized in Table 1-3. The features, advantages, and benefits of various solutions are listed in Table 1-4. The objective of using any of these solutions is to increase utility asset utilization.

      It is recognized that the superior response capability of a power electronics inverter‐based solution may be beneficial in applications