1.1 Why is a Power Flow Controller Needed?
The electricity generation is typically located a distance away from the industrial and population, i.e. load centers. Therefore, electricity is transmitted from the points of generation to the points of utilization using transmission and distribution lines. One of the major achievements of the late 19th century was to implement long‐distance alternating current (AC) transmission of electric power. For a given amount of transmitted power, a higher transmission voltage provides less footprint and less current in the line. Since loss in the line is proportional to the square of the current, it is most desirable to transmit electricity from generation to load centers through the transmission lines at higher voltage and lower current. Over the next century, various types of generation, such as coal, natural gas, nuclear, hydro, renewables, and so on, and loads were integrated through interconnected transmission and distribution lines to form the grid as shown in Figure 1-1.
Figure 1-1 Part of a large interconnected transmission system supplying electric power from the generating point to the loads.
Electricity flows freely from a higher potential to a lower potential. If two or more lines with different impedances are connected in parallel, the current in each line is, in some form, inversely proportional to the respective line impedance. This free flow of electricity might take longer paths to reach its destination and cause unwanted power losses in the lines. Additionally, this free flow may cause some transmission lines to be overloaded or underloaded. If the impedance of a line is larger compared to that of the lines connected in parallel, the current and the resulting power flow through the higher‐impedance line is lower compared to that in the neighboring lines, and vice versa. Therefore, power flow in a line is, in some form, inversely proportional to the impedance of the line.
Transmission Reliability Margin (TRM) – the amount of Transmission Transfer Capability (TTC) needed to ensure that the interconnected transmission network is secure under a reasonable range of uncertainties in system conditions. These uncertainties may result from the following:
1 Simultaneous limitations with a parallel path.
2 Reservations for unscheduled flow, i.e. loop flow.
3 Reservations for unplanned transmission outages, i.e. for paths in which contingencies have not already been included in the calculation of TTC.
TRM does not include reservations for planned outages and other known transmission conditions, which have been included in the calculation of TTC (Minimum of {Thermal Limit, Voltage Limit, and Stability Limit}).
Capacity Benefit Margin (CBM) – the amount of TTC reserved by the load‐serving entities to ensure access to generation from interconnected systems to meet generation reliability requirements. These reservations may include the following:
1 Transmission reserved by the Control Area Operator to accommodate operating reserves (spinning and supplemental). Such operating reserves may not exceed NERC and WECC applicable pool requirements or individual members’ reliability requirements.
2 Transmission reserved for the import of ancillary services (such as spinning reserves) from another control area.
3 Transmission reserved for generation patterns and generation contingencies. These patterns and contingencies must be based upon reasonable, publicly available assumptions subject to evaluation in applicable dispute resolution proceedings.
According to “Available Transfer Capability Definitions and Determination,” North American Electric Reliability Council, June 1996, TTC is defined as
(1‐1)
Available Transfer Capability (ATC) is a measure of the transfer capability remaining in the physical transmission network for further commercial activity over and above already committed uses. Therefore,
It is often desirable to utilize the ATC to increase the power flow through a line as much as possible according to Equation (1-2), which results in a higher line utilization, meets greater customer needs, integrates new sources of energy, and avoids or defers building of new transmission lines, at least in the short term. Sometimes, it is the opposite when it may be desirable to lower the power flow of the line so that the power flow can be redirected to a desired transmission line that may include high‐voltage and low‐current and thus, low‐loss lines. This is particularly important when a line becomes overloaded with a level of current that can trip the line or a fault current that must be limited. If an overloaded line trips, its current will be redirected in the available lines inversely proportional, depending on the lines’ impedances. This may cause a previously underloaded line to become overloaded and tripped, which may create a possible cascaded failure of the grid, resulting in a blackout. This type of cascaded failure is 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.
Inadequate management of reactive power and line voltage caused the blackout. Most of the recent power network blackouts have been related to line voltage collapses, which tend to occur from lack of reactive power supports in heavily stressed conditions, usually triggered by system faults. One of the ways to improve the reliability of the line when faced with similar occurrences would be to regulate the effective impedance of a line between its two ends with an Impedance Regulator (IR), which is also known as a Power Flow Controller (PFC).
It is also desirable to use a PFC to enhance the power flow in the line to maximize the revenue‐generating active power flow and minimize the reactive power flow within a permissible limit, while maintaining the voltage stability. The flow of electricity in a particular line of a transmission system can be controlled by using a PFC as shown in Figure 1-2. Since the power flow in a line is inversely proportional to the impedance of the line and assuming the line to be an inductive impedance, the power flow can be decreased by inserting an additional inductive impedance in series with the line, thereby increasing the effective impedance of the line between its two ends. Also, the power flow can be increased by inserting an additional capacitive impedance in series with the line, thereby decreasing the effective impedance of the line between its two ends. Therefore, the regulation of the effective line impedance is the key to maintaining desired power flow in a line. Note that each of a Voltage Regulator (VR) and a Phase Angle Regulator (PAR) also regulates the effective line impedance indirectly. However, the Reactance Regulator (RR) regulates only the effective line reactance directly as discussed in Chapter 2, Section 2.2.2.6 (Representation of a Series‐Compensating Voltage as a Series‐Compensating