Читать книгу Power Flow Control Solutions for a Modern Grid Using SMART Power Flow Controllers - Kalyan K. Sen - Страница 19

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,

(1‐2)

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 Impedance).

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.