According to the American Wind Energy Association (AWEA), wind energy development in the United States is expected to grow at a remarkable 25 percent in 2003. Driven by local incentives and the federal production tax credit, the forecast for new wind energy development through 2006 is quite robust. Typically wind farms tend to be located at remote locations, driven by wind and weather patterns with little up-front analysis performed regarding the existing power grid in that location. Such locations tend to be weaker points in the transmission system, which means they require special consideration in connecting wind generation to the grid. In particular, four common problems must often be addressed:
Steady-State Low Voltage
One of the most common technologies used to convert wind power to electricity is the induction generator. Induction machines, however, tend to drain large amounts of reactive power from the grid, which can cause low voltage and/or voltage stability problems for the transmission owner. The harder the wind blows, the more reactive power is consumed, which can drag transmission voltages to unacceptably low levels. Conventional solutions would call for capacitor banks to provide the VAR (voltage ampere reactive) support required to offset the reactive power demands of wind farms. Capacitor banks, however, must be switched in “blocks” of VARs. Because the fault power in these remote locations tends to be lower, wind farm developers must use many small capacitor banks in combination with elaborate switching schemes to minimize step voltage changes and offset ever-varying VAR demands. On larger wind farms, the use of capacitor banks alone for VAR control can be very difficult to implement, and can result in hundreds of capacitor bank operations per day. This can greatly accelerate the wear and tear on the capacitor bank switches. In addition, many transmission owners now require wind power generation facilities to meet standards similar to those of conventional generators. A common requirement for wind power interconnection calls for generation facilities to have an infinitely controllable power factor, for example from .90 leading to .95 lagging. Achieving such a power factor on a wind farm is nearly impossible with capacitor banks alone.
Gear Box Damage from Capacitor Switching
Using capacitor banks alone, both at the wind farm distribution bus and on the transmission system, can also cause other headaches for wind farm owners. Each capacitor switching event — either on or off — causes a near instantaneous step change in voltage. The step change in voltage, in turn, correlates to a step change in torque on the gear boxes of the wind turbines. Depending on the frequency and magnitude of these switching events, this can accelerate wear and tear on the wind turbines’ sensitive gear boxes and can ultimately lead to premature mechanical failure. According to European wind energy associations, gear boxes are already the number one source of mechanical failure on wind farms (see Figure 1).
Wind Generators Tripping Offline
Another common problem with wind power is that most wind generators are very sensitive to brief voltage sags or swells, which occur normally on all transmission systems. Depending on the depth and duration of the sags, the wind generators can trip offline. This unpredictability makes grid planning and maintenance more difficult for transmission owners. Most transmission owners, therefore, specify voltage sag “ride-through” requirements as a pre-condition for interconnecting a wind farm to the power grid. Achieving this “ride-through” capability requires dynamic voltage support, capable of instantaneously and precisely correcting voltage transients — effectively making them invisible to the wind turbines. This type of “ride-through” is not possible using only capacitor banks.
Wind Generators Causing Voltage Sags on Local Distribution Network
Depending on proximity to the local load and the distribution network configuration, wind generators can cause voltage transients or flicker on a local distribution grid. This occurs due to the inrush associated with the energization of the stators. Depending on the strength of the local grid, an undesirable voltage dip can result every time the wind turbines are started. For this reason, local utilities often require that wind generators limit the magnitude of their voltage disturbances to a maximum of 1–2%. Mitigating these voltage disturbances requires dynamic reactive compensation such as a FACTS (Flexible AC Transmission System) device that can instantaneously and infinitely vary its output to precisely counteract the reactive current being drawn by the stators.
Solution
The most effective and economic solution to these common interconnection problems is to install a dynamic reactive power source (a FACTS device) at the wind farm collector bus. Such a device automatically maintains precise system voltages at the wind farm and provides critical protection from voltage sags. New developments in this technology have driven prices to affordable levels and lead times to just a few months. In addition, recent advances have proven that some FACTS devices can be integrated with low-cost capacitor banks to provide large-scale wind farm solutions without the problems usually associated with capacitor bank switching.
The most effective FACTS devices utilize power electronics (voltage source converters) to generate reactive power. Because they are strictly a current source, power electronic-based FACTS devices have considerably more output at reduced voltages than capacitor-based FACTS technology, whose maximum output declines by the square of the voltage. These characteristics combined with their dynamic capabilities make power electronic-based FACTS systems superior solutions for protecting against voltage sags.
Figure 2 depicts a typical wind farm solution utilizing D-VAR™ technology (power electronics). In this example, the D-VAR system constantly monitors the 34.5kV wind farm bus and is capable of injecting precise amounts of reactive power within 1 line cycle (0.017 seconds). This near instantaneous response is also dynamically variable to exactly match and offset the varying amounts of reactive power consumed by the turbine generators. This system also completely controls the shunt capacitor banks — switching them in or out depending on wind farm voltage, VAR demand, or power factor. The capacitors are “soft-switched” by the D-VAR system, which also acts as a “shock absorber” to eliminate the step voltage changes that would otherwise occur. Because of the low cost of capacitors, this hybrid FACTS/capacitor bank solution is capable of providing all required voltage and power factor control for a very large wind farm — at substantially lower cost than with a FACTS device alone.
The solution depicted in Figure 2 is also capable of going into temporary “overload” mode, supplying up to 3.0 times its rated output for 1 full second. This extra dynamic response is an important feature which provides extra short-term reactive power for additional protection from voltage sags, allowing the wind farm to "ride-through" voltage transients that naturally occur on the transmission system.
Summary
Wind farms today are far from “plug and play.” Several important grid interconnection issues must be addressed, and these issues vary among transmission owners. As the megawatt capacity of wind farms grows ever larger, interconnection challenges are becoming even more significant. Fortunately, power electronic-based FACTs technology can now provide proven, reliable and economic solutions for the wind power challenges of today and tomorrow.
About the Author
Tim Poor is Director of Business Development for American Superconductor’s Power Electronic Systems Business Unit, which is based in Madison, Wisconsin. He has worked closely with various transmission owners, wind farm developers, and wind turbine manufacturers to reliably and cost-effectively integrate wind power on the grid. Tim can be reached at tpoor@amsuper.com.
- Wind farms can cause steadystate low voltage on regional transmission networks.
- Wind turbine gear boxes are sensitive to capacitor bank switching, which can cause damage and premature failure of the gear boxes.
- Wind generators are sensitive to voltage transients, and can suddenly trip offline if a voltage sag of a specific depth and duration occurs on the transmission system.
- Wind turbines can cause momentary voltage sags on the local distribution system, which, in turn, can cause problems for the locally served load. This article addresses these four areas of concern.
Steady-State Low Voltage
One of the most common technologies used to convert wind power to electricity is the induction generator. Induction machines, however, tend to drain large amounts of reactive power from the grid, which can cause low voltage and/or voltage stability problems for the transmission owner. The harder the wind blows, the more reactive power is consumed, which can drag transmission voltages to unacceptably low levels. Conventional solutions would call for capacitor banks to provide the VAR (voltage ampere reactive) support required to offset the reactive power demands of wind farms. Capacitor banks, however, must be switched in “blocks” of VARs. Because the fault power in these remote locations tends to be lower, wind farm developers must use many small capacitor banks in combination with elaborate switching schemes to minimize step voltage changes and offset ever-varying VAR demands. On larger wind farms, the use of capacitor banks alone for VAR control can be very difficult to implement, and can result in hundreds of capacitor bank operations per day. This can greatly accelerate the wear and tear on the capacitor bank switches. In addition, many transmission owners now require wind power generation facilities to meet standards similar to those of conventional generators. A common requirement for wind power interconnection calls for generation facilities to have an infinitely controllable power factor, for example from .90 leading to .95 lagging. Achieving such a power factor on a wind farm is nearly impossible with capacitor banks alone.
Gear Box Damage from Capacitor Switching
Using capacitor banks alone, both at the wind farm distribution bus and on the transmission system, can also cause other headaches for wind farm owners. Each capacitor switching event — either on or off — causes a near instantaneous step change in voltage. The step change in voltage, in turn, correlates to a step change in torque on the gear boxes of the wind turbines. Depending on the frequency and magnitude of these switching events, this can accelerate wear and tear on the wind turbines’ sensitive gear boxes and can ultimately lead to premature mechanical failure. According to European wind energy associations, gear boxes are already the number one source of mechanical failure on wind farms (see Figure 1).
Figure 1: Frequency of component failures
in wind generation systems
Wind Generators Tripping Offline
Another common problem with wind power is that most wind generators are very sensitive to brief voltage sags or swells, which occur normally on all transmission systems. Depending on the depth and duration of the sags, the wind generators can trip offline. This unpredictability makes grid planning and maintenance more difficult for transmission owners. Most transmission owners, therefore, specify voltage sag “ride-through” requirements as a pre-condition for interconnecting a wind farm to the power grid. Achieving this “ride-through” capability requires dynamic voltage support, capable of instantaneously and precisely correcting voltage transients — effectively making them invisible to the wind turbines. This type of “ride-through” is not possible using only capacitor banks.
Wind Generators Causing Voltage Sags on Local Distribution Network
Depending on proximity to the local load and the distribution network configuration, wind generators can cause voltage transients or flicker on a local distribution grid. This occurs due to the inrush associated with the energization of the stators. Depending on the strength of the local grid, an undesirable voltage dip can result every time the wind turbines are started. For this reason, local utilities often require that wind generators limit the magnitude of their voltage disturbances to a maximum of 1–2%. Mitigating these voltage disturbances requires dynamic reactive compensation such as a FACTS (Flexible AC Transmission System) device that can instantaneously and infinitely vary its output to precisely counteract the reactive current being drawn by the stators.
Solution
The most effective and economic solution to these common interconnection problems is to install a dynamic reactive power source (a FACTS device) at the wind farm collector bus. Such a device automatically maintains precise system voltages at the wind farm and provides critical protection from voltage sags. New developments in this technology have driven prices to affordable levels and lead times to just a few months. In addition, recent advances have proven that some FACTS devices can be integrated with low-cost capacitor banks to provide large-scale wind farm solutions without the problems usually associated with capacitor bank switching.
The most effective FACTS devices utilize power electronics (voltage source converters) to generate reactive power. Because they are strictly a current source, power electronic-based FACTS devices have considerably more output at reduced voltages than capacitor-based FACTS technology, whose maximum output declines by the square of the voltage. These characteristics combined with their dynamic capabilities make power electronic-based FACTS systems superior solutions for protecting against voltage sags.
Figure 2 depicts a typical wind farm solution utilizing D-VAR™ technology (power electronics). In this example, the D-VAR system constantly monitors the 34.5kV wind farm bus and is capable of injecting precise amounts of reactive power within 1 line cycle (0.017 seconds). This near instantaneous response is also dynamically variable to exactly match and offset the varying amounts of reactive power consumed by the turbine generators. This system also completely controls the shunt capacitor banks — switching them in or out depending on wind farm voltage, VAR demand, or power factor. The capacitors are “soft-switched” by the D-VAR system, which also acts as a “shock absorber” to eliminate the step voltage changes that would otherwise occur. Because of the low cost of capacitors, this hybrid FACTS/capacitor bank solution is capable of providing all required voltage and power factor control for a very large wind farm — at substantially lower cost than with a FACTS device alone.
Figure 2
The solution depicted in Figure 2 is also capable of going into temporary “overload” mode, supplying up to 3.0 times its rated output for 1 full second. This extra dynamic response is an important feature which provides extra short-term reactive power for additional protection from voltage sags, allowing the wind farm to "ride-through" voltage transients that naturally occur on the transmission system.
Summary
Wind farms today are far from “plug and play.” Several important grid interconnection issues must be addressed, and these issues vary among transmission owners. As the megawatt capacity of wind farms grows ever larger, interconnection challenges are becoming even more significant. Fortunately, power electronic-based FACTs technology can now provide proven, reliable and economic solutions for the wind power challenges of today and tomorrow.
About the Author
Tim Poor is Director of Business Development for American Superconductor’s Power Electronic Systems Business Unit, which is based in Madison, Wisconsin. He has worked closely with various transmission owners, wind farm developers, and wind turbine manufacturers to reliably and cost-effectively integrate wind power on the grid. Tim can be reached at tpoor@amsuper.com.
