Tim Poor and Bud Kehrli, American Superconductor
The development of wind energy has expanded very rapidly in the past five years. According to the American Wind Energy Association (www.AWEA.org), the amount of wind generation installed in the U.S. more than doubled between 1998 and 2001. With the recent extension of the production tax credit, U.S. production of wind energy is expected to experience continued strong growth over the next several years.
Developers have begun concentrating large numbers of wind generators at a single location commonly called a wind farm. Because of the remote locations of most large wind farms, the power they generate must often be delivered a long distance to the ultimate customer on a relatively weak transmission grid. The need to interconnect large amounts of wind power on relatively weak utility transmission grids is causing various types of problems and is straining the relationship between wind farm operators/developers and the utilities.
Technical challenges
The problems seen by the wind farm operators/developers (and by the utilities) come in many forms. For utilities, the most pervasive problem is the large amount of reactive power (VARs) consumed by the typical wind farm. This reactive demand associated with induction wind generators can cause an unacceptably large voltage drop at or near the wind farm interconnection point. These voltage variations can create alternating high and low voltage for the wind farm and for other utility Dynamic reactive power requirements for wind farms customers served near the wind farm on the same transmission system.
The traditional method of compensating for the VAR demand at the wind farm site is to add shunt capacitors either on the transmission system itself or on the medium voltage wind farm collector bus. If the interconnection contract does not require the wind farm developer to compensate for the VAR demands of the wind farm, then the obligation generally will fall back on the utility to solve the problem. When a utility independently solves the problem, it normally will take the least-cost option, which is to install a small number of large capacitor banks on the transmission system.
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When large capacitor banks switch on or off, the voltage at the wind farm (and at the wind generators themselves) is often subjected to a step-voltage change between 3 and 6 percent. Repeated capacitor bank switching events and their associated step-voltage changes cause excess gearbox wear and tear and can eventually result in premature gearbox failure.
Alternatively, when the interconnection contract specifies that the wind farm developer must compensate for the wind farm VAR demand, it is normally done with a large number of smaller capacitor banks installed on the medium voltage collector system at the wind farm substation. Several problems are associated with this method of compensation. First, the need to respond to rapidly changing generation and VAR demands requires a complex control scheme which results in very large numbers of capacitor bank switching events to maintain the required voltage profile or to meet utility VAR demand or power factor criterion. Second, although the individual capacitor banks are smaller, each switching event still results in a step-voltage change to the wind generators, which causes excess gearbox torque and contributes to gearbox failure.
It is best to address these issues during the planning stage of a wind farm project. Addressing the voltage and VAR compensation issues during the project development stage allows both parties to choose the least-cost and most effective mitigation plan.
New FACTS-based solution
A better method of solving these problems involves the installation of a small dynamic reactive power source such as a flexible AC transmission system (FACTS) device that can vary its output to solve a variety of problems associated with wind farms. FACTS devices can be integrated with low cost capacitor banks to provide an extremely cost effective solution for large wind farms. For instance, a small (8 MVA) dynamic VAR device combined with a number of medium voltage capacitor banks is sufficient to solve all of the above mentioned problems as well as several other typical wind farm problems. (The figure shows a one-line diagram of a typical dynamic VAR device and capacitors connected to a wind farm.)
The dynamic VAR system shown in the figure continuously monitors the 34.5 kV bus voltage to ensure that the voltage remains within the customer's predetermined (but variable) deadband such as 0.98-1.02 pu. Continuous voltage regulation is accomplished by a combination of VAR injection or absorption from the dynamic VAR system and by the controlled switching of the 34.5 kV capacitor banks. The objective is to prevent the voltage from deviating outside a preset voltage bandwidth. In addition, the system prevents step-voltage changes at the 34.5 kV collector bus caused by switching of large external transmission banks. Finally, the dynamic VAR system minimizes the number of 34.5 kV capacitor bank switching events needed to maintain voltage within the specified range.
To eliminate the large step-voltage changes caused by switching of large transmission capacitor banks located remotely on the utility grid, the same sensing and control scheme can be used with the dynamic VAR system. The three phase 34.5 kV voltage is continuously monitored. When all three phases rise or fall by a preset percentage (such as 1.0-1.5 percent), the dynamic VAR system responds instantaneously and injects (or absorbs) sufficient VARs to fully offset the step-change in voltage that would otherwise have occurred. The average three-phase voltage is used to avoid false responses to remote non-three phase events.
As mentioned above, the dynamic VAR system's output is utilized first to regulate voltage. Only if the dynamic VAR system output reaches 5-8 MVAR, or a level at which it equals the size of an available capacitor bank, will a capacitor bank switch on (or off). By utilizing the capacitive and inductive capability of the dynamic reactive device, unnecessary capacitor bank switching operations are avoided and total annual capacitor bank switching operations are minimized. This results in less maintenance time and lower costs for the wind farm owner/operator.
Benefits of installing a small dynamic VAR device
Installing a small dynamic reactive device at the utility/wind farm interface can resolve many issues and can benefit both the wind farm and the utility. The benefits to the wind farm developer/operator include the following:
- The collector bus voltage is regulated continuously within a narrow bandwidth. This keeps the wind farm on-line and avoids the need to trip-off due to either steady-state or transient low or high voltage conditions. It also maximizes KWH output opportunities and increases revenues.
- Step-voltage changes due to local and remote capacitor bank switching are eliminated, preventing excess gearbox torque and premature gearbox failure.
- Capacitor bank switching events are minimized which reduces switch maintenance costs.
- Wind farm revenues are maximized because of the ability to remain on-line.
- Overall interconnection and impact mitigation costs are minimized.
Utilities derive benefits by:
- Eliminating large VAR demands, and their resulting voltage swings caused by uncompensated wind farm operation.
- Mitigating or eliminating the need to install capacitor banks on the transmission system to control voltage.
- Minimizing their costs, if transmission capacitor banks are installed for any reason, since they can be larger in size and still won't cause problems for the wind farm operator.
Bud Kehrli is manager of network solutions for American Superconductor located in Middleton, WI. Bud has 26 years of utility transmission planning experience and is credited with a U.S. patent for voltage control systems that utilize a combination of dynamic VARs and capacitors. He can be contacted at: (608) 828-9178 or bkehrli@amsuper.com.
Tim Poor is manager of business development for the power electronic systems division of American Superconductor, also located in Middleton, WI. He has 13 years of broad electric industry experience. He can be reached at: (608) 828-9126 or tpoor@amsuper.com.
American Superconductor is a manufacturer of dynamic reactive compensation systems known as D-VAR, which provide voltage support to utility transmission and distribution systems. In addition to wind farms, D-VAR devices are also being utilized world-wide to address a variety of grid-related problems such as voltage instability, power transfer constraints, and steady state voltage regulation. (D-VAR is a trademark of American Superconductor, Inc.)
