Voltage and var solutions
The voltage/var problems associated with serving urban areas that import large shares of power during peak load hours have slowly been gaining recognition by the international electric power community. Affected urban areas have often been geographically distant; hence, they may not have been aware of similar problems faced by other areas and thought their situation was unique. There is a clear need for broader awareness of this voltage/var issue and possible solutions.
DNV GL Energy & Sustainability has observed common practices among urban areas in developed countries. To find a quantifiable common measure, we developed an indicator—referred to as the compensation ratio—based on total required transmission reactive compensation levels to support imports and total import amounts.
In a 2006 KEMA study, we compared important metropolitan areas around the world that had high levels of compensation: Mexico City, San Diego, New York City, South Florida, El Paso, Cape Town, Auckland, and Albuquerque. We selected these areas because they had these three characteristics in common:
- They are important population and commercial centers in their region.
- They are at the end of the electrical network with a peninsular type of transmission connection to the rest of the larger grid.
- Power imports are an important factor in serving the local load.
While every situation is unique, as a rule of thumb, the transmission reactive compensation ratio is a matter of concern once it exceeds 25%. A level above 50% is generally considered to be unacceptably high. Ratios for these eight metropolitan areas fell within the 15%-30% range as shown in the following chart.
Compensation ratios of selected major urban power importing areas
The most direct way to lower the compensation ratio—without reducing imports—is to build a new transmission line or upgrade the existing network. These are often capital-intensive projects, which may require new rights-of-way and involve time-consuming licensing processes. Increasing reactive compensation is often an economically attractive option to transmission expansion. Reactive devices, such as switched capacitors, SVCS, and STATCOMS, do not require extensive land, and they are not especially visible when compared with major new transmission lines. These characteristics make them much more acceptable to the public and, thus, to transmission system operators responsible for providing reliable electric delivery service.
In most cases, reactive devices are much less expensive to build than new transmission lines. In moderation, such devices are useful additions to the options system planners and operators can use to relieve voltage/var problems and to provide improved operating flexibility. However, excessive dependence on these devices can increase the complexity of system design and operation and lead to increased risk of uncontrolled system collapse as discussed in part 2 of this series.
There are two general types of solutions: increase the local real power (MW) supply (including demand management) or increase the import capability of the transmission system.
Increasing the local real power (MW) supply
New local-area generation
Perhaps the most obvious solution is to add generating capacity to the importing area. Local generation will reduce imports into the area and add to the local var supply. It will also decrease the possibility of voltage collapse and improve dynamic stability of the area.
Increased local demand management
Considering the opposition to new power plants and transmission lines in most urban areas, increased demand management programs can effectively reduce imports in areas nearing peak supply limits. It is important to recognize that this option only defers capital investment for future grid or generation expansion. Furthermore, the costs of incentives that encourage customers to adopt new demand management programs may not be trivial. As a result, system operators must be able to use the program’s features effectively and reliably.
Local-area energy storage
Installing energy storage devices in the load area can also be an effective solution. Modern storage provides both real and reactive power to both reduce imports and provide reactive support. They are especially effective in supporting the system following a contingency—where the stored energy is only needed for a limited time. See the September 2011 issue of TECH Notes for additional insight into the energy storage value proposition.
Increasing the power import capability
Adding transmission at exiting voltage levels
The most obvious transmission solution is to increase the number of transmission lines that serve the area. Adding a new line decreases the impedance and reduces reactive losses. Adding a line to an area supplied through five major transmission lines will reduce the impedance by about 17%. To have a significant impact on reactive losses, however, multiple new lines are required.
The cost of new transmission lines is directly related to the length and number of lines. A new line parallel to the most heavily loaded lines is usually more effective than others. Local opposition could require a portion of the new line to be placed underground, which would significantly increase costs.
Modifying existing transmission
An alternate solution is to modify the configuration of existing lines. Recent research has shown that changes in tower configurations and conductor spacing can reduce line impedances by up to 20%. To have a significant impact on reactive losses, multiple lines have to be modified. Also, the transmission has to be removed from service to make the changes. For heavily loaded import areas, it can be difficult to schedule the necessary outages.
The impedance of these transmission lines can also be reduced by adding series compensation. This involves inserting transmission capacitors in series with the lines. Series compensation is widely used in the Western United States where line lengths are rather long. To have a significant impact on reactive losses, multiple lines have to be modified. Care must be used in applying series compensation, since too much can cause other types of problems, e.g., resonant circuits that introduce new oscillation modes in the power system causes problems for system operation and can potentially damage equipment.
Adding transmission at higher voltage
Often, the most effective solution is to add a new higher voltage level of transmission—usually 500/765 kV. These lines have about half of the effective impedance of 230/345 kV lines. Such a line—with half the impedance—will have only one quarter of the reactive losses of a 230/345 kV line. The higher voltage lines require wider rights-of-way and taller towers, and they often encounter significant opposition because of routing and environmental concerns.
Adding HVDC transmission
A more expensive option is to add high-voltage direct current (HVDC) transmission to deliver real power (MW) into an area. HVDC transmission can reduce imports through the parallel AC transmission system and, thus, reduce the overall reactive losses associated with imports.
An advantage of the HVDC line is that it requires slightly lower and narrower towers than comparable AC lines. While the cost of the tower and conductors are lower for HVDC, they have very high terminal costs, which require expensive electronics to convert between AC and DC. HVDC lines are usually used only when distances are long and power levels are high. In special situations, such as long submarine cables and connecting asynchronous systems, HVDC is also better than AC.
The best solution for any situation depends on the specific characteristics of the power system involved. By understanding the needs, simulating possible solutions, and incorporating local guidelines and constraints, one can develop an effective solution for every situation.
David Korinek also contributed to this article, which is the final part in the three-part TECH Notes series, Generation retirements and var planning.
Read part 2: “Problems with highlight compensated power systems.”
Read part 1: “Reactive power: what it is; why it is important.”
[Image sources: DNV GL]