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

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Figure 1-2 Power flow along a controlled path.

      The demand for electrical energy around the world increases continuously; so does the use of various sources of energy from traditional synchronous generators, used at coal/natural gas/nuclear/hydro power stations to modern IBRs that convert renewable wind and solar energy into usable AC electricity. Often, the available sources of energy generation are far away from the load centers. The ever‐growing need for transmitting more electricity can be met either by installing new transmission lines, characterized by a lengthy and costly process and/or by harnessing the dormant capacity of the underutilized transmission lines with a quicker and much less‐costly option. The challenge is how to harness this dormant capacity in the most cost‐effective way. Any investment alternative to harness the dormant capacity of the underutilized transmission lines should be supported by comparing the investment relative to other competing options with all cost/benefit considerations being evaluated for all tangible and non‐tangible factors over the total life cycle.

      The free flow of electricity from one particular point to another might not take the shortest path. Any unwanted path along the way causes extra power loss, loop flow of power, and reduced stability with increased voltage variation in the line. The power industry constantly searches for the most economical ways to transfer bulk power along a desired path. Before considering new transmission lines, it is desirable to explore all the options to increase the loadability of existing transmission lines. The free flow of power through unwanted longer paths, which causes extra losses in the lines can be mitigated with the use of a PFC. The optimum power flow through the lines will enhance the loadability of the lines in the most efficient way. This will reduce the carbon‐based generation that is equal to the unwanted losses in the lines due to free flow of power, which will reduce GHG emissions and contribute to a reduction in global warming.

      Traditional regulators, such as VR, PAR, and RR, regulate one of the three power flow control parameters (line voltage magnitude, its phase angle, and line reactance) and, in turn, control active and reactive power flows (P and Q) simultaneously, meaning both P and Q either increase or decrease. Since the effect of line reactance regulation is equivalent to essentially the combined effects of voltage regulation (using a VR) and phase angle regulation (using a PAR), the two main power flow control parameters are the line voltage magnitude and its phase angle. An IR is functionally equivalent to the combined effects of a VR and a PAR.

      The optimization of P and Q flows in the transmission line requires an independent control of P and Q flows, which requires a simultaneous regulation of the line voltage magnitude and its phase angle. This is functionally equivalent to regulating the effective four‐quadrant impedance (discussed in Section 1.2 ) of the line between its two ends. Early PFCs employed basic technologies, such as transformers, capacitors, and reactors for the compensating voltage injection in the line. Modern‐day PFCs emulate a reactor or a capacitor by creating a compensating voltage whose phase angle is either leading or lagging the current that is passing through the compensating voltage. The advantage of using an emulated series capacitor as compared to an actual series‐connected capacitor is the avoidance of creating any type of resonance with the inductance in the line and the synchronous machine in the form of sub‐synchronous resonance. The voltage across the emulator is limited by the rating of the PFC whereas the voltage across a series‐connected capacitor may be excessively high during resonance. In addition, just by changing the control algorithm, the same power hardware may be used to emulate a series‐compensating capacitor as well as a reactor, instead of using a separate reactor.

      An IR creates a series‐connected virtual impedance that modifies the effective impedance of a line between its two ends. As a result, it is possible to increase power flow in an underloaded line, decrease power flow in an overloaded line, and control the flows of P and Q independently as desired. If deployed in critical locations, the IR will maximize the P flow that will generate the most revenue and minimize the Q flow, reducing unwanted power losses in transmission lines. This will increase the efficiency of the power grid, increase voltage stability margins, and may avoid a cascaded blackout as described in the “Final Report on the August 14, 2003 Blackout in the United States and Canada: Causes and Recommendations” by U.S.‐Canada Power System Outage Task Force, April 2004.

      The consequence of free flow of electricity following the Hanna‐Juniper Line Loss on August 14 2003 contributed to a massive blackout of the North‐Eastern USA and Canada. When a tree and the 345‐kV line touched each other, the line tripped; as a result, the load got redistributed among the available transmission paths, which overloaded some lines that then tripped. These events normally would set off alarms in a local utility’s control room and alert operators to activate controllers in neighboring regions to reroute power flows around the affected site. However, the alarm software failed, and thus the local operators were unaware of the problem. Transmission lines surrounding the failure spot were forced to shoulder more than their safe quota of electricity. Also, at this time, the reactive power supply was at a minimum and when plant operators tried to increase the reactive power flow, the generating plant shut down. This further destabilized the system’s equilibrium, leading to additional lines and generators dropping out of the grid as the cascade continued. Within 8 minutes, 50 million people were experiencing a blackout.

      The final report on the blackout stated that more high‐voltage lines must be built and perhaps even more important, the power grid must be made SMARTER. A self‐healing SMART grid is needed to be able to recognize the problem and then reconfigure the power grid. If IRs were strategically located, these overloaded lines would have their power flows controlled to be within their ATC limits and would neither trip the line nor contribute to the blackout. Hence, the IR improves grid reliability and resiliency.

1.2 Traditional Power Flow Control Concepts

The flow of AC power, irrespective of its source, has two components: active power and reactive power. A transmission line consists of electrical conductors that may be composed of many sections. Each section consists of a resistance (R′), inductive reactance (X′_L), and line‐to‐ground (shunt) capacitive reactance (X′_C) as shown in Figure 1-3. Since there is no significant storage of electric power at the utility scale, the active power, except for the loss in the resistance of the conductor, reaches from one end of the line to the other end. This active power can be used for lighting, heating, cooling, motion force in electric motors, and so on. The distributed series inductive and shunt capacitive reactances of the line absorb and generate reactive power, respectively. This reactive power flow causes an extra loss in the resistance of the line.