Switching Surges and Transient Over-Voltages in Wind Farms
Category: TransformersThe long cable runs and frequent switching operations found in multi-tower wind farms puts the wind turbine step-up transformer at greater risk than a conventional distribution or power transformer installations. Carefully locating the wind farm, along with using transformers with fault ride through capability, grounding transformers, surge arrestors, and properly rated transformer bushings, are among the strategies that can be used to counter the risks from switching surges and over-voltages at wind farms.
Locating Transformers at Wind Farms
The general rule of thumb for locating a transformer is to reduce the costs of large copper cables by placing the transformer in a manner that reduces the length of low voltage, high current cables. When this consideration is applied to wind farms, it follows that the wind generator and its associated transformer should be as close together as possible.
For land based sites, the turbine step-up transformer can either be located as near to the tower base as possible, or alternately, within the tower or nacelle. For off shore sites, the latter is the only realistic choice available. It should be noted that while liquid filled pad-mounted transformers are the normal for location adjacent to the turbine tower, dry-type transformers are normally used for nacelle mounted transformers. Of course, the placement of the transformer is decided by the construction of the wind generator manufacturer.
The problem with large sprawling turbine arrays is that the need for connecting the individual turbine step-up transformers to the “collector” bus results in very long cable runs. This in turn results in increased voltage drops, cable related resistive and capacitive losses, and the increased potential for cable ground faults. The extensive use of cable in wind farms coupled with their “daisy chain” connection pattern leads to two primary systems problems: cable faults and voltage stress caused by single or double line-to-ground fault.
Cable Faults
A radial wind farm configuration typically connects ten to twelve transformers in a daisy chain fashion. The pad-mount transformers are configured with a loop-feed bushing arrangement, in which the transformer at the end of the radial line is connected to the next transformer in line. The second transformer from the end is connected to the third and so forth until the first transformer in line is connected to the collector bus. Since a cable fault can happen anywhere along this radial line, the transformer must be able to handle fault currents from a fault at any location The fault ride through requirement becomes critical, since clearing a fault would require disconnecting a complete radial line, approximately 20 to 30 MVA of generation.
Single or Double Line-To-Ground Fault Voltage Stress
In addition to the fault currents that occur during a fault, a single or double line-to-ground fault causes voltage stress on the transformer. A single phase cable fault, on the Delta-connected HV winding, causes one phase to ground, putting phase-to-phase voltage between the other two phases and ground. The resulting voltages overstress the transformer’s insulation system. Finding and clearing the cable fault is exacerbated by the longer cable runs and the wind farm layout.
The use of grounding transformers at critical locations within the wind farm helps alleviate this type of dielectric stress. The grounding transformer provides a zero sequence impedance to support the voltage on a faulted leg during a single line-to-ground fault. By holding the faulted phase above ground, this impedance acts to limit the resulting overvoltage on the un-faulted phases. Most grounding transformers have a thermal fault rating in the range of 10 seconds to one minute. This may give an indication of the expected length of a fault. Fault duty due to either voltage or current are serious concerns that need to be addressed by both the wind farm developer and the transformer manufacturer.
Increased Lightning Exposure and Voltage Surges from Repeated Switching
Further concerns for the reliability of the turbine step-up transformer center on the increased exposure to lightning strikes and transient voltage surges resulting from the repeated switching.
Wind farms are located in remote areas, typically at higher elevations or exposed plains, where wind patterns are unobstructed by surrounding terrain and man-made or natural obstacles. Unfortunately, this increases the transformer exposure to storm events and lightning strikes, especially considering the geometry of the turbine and tower combination. Surge arresters should figure prominently in the wind turbine step-up transformer protective equipment list.
Perhaps an even greater concern is transient over-voltages cause by switching. Changes in wind strength are directly translated into a varying load profile for the step-up transformer. As the wind speed increases, the turbines are brought on line. When the wind strength begins to wane, the loading drops and ultimately the turbine is switched off line.
This can happen multiple times within a 24-hour period as surface wind is affected by diurnal heating cycles or incoming weather patterns. It can also happen when a feeder breaker opens and disconnects the turbine step-up radial line from the collector circuit. Breaker operations introduce a transient recovery voltage (TRV) wave into the wind turbine step-up transformer voltage circuit. This phenomenon is exacerbated by the present day extensive use of vacuum breakers and their extremely rapid switching times.
The TRV surges associated with breaker switching operations on either the HV or LV side of the transformer can combine with cable capacitance and produce standing waves and ringing that are many times the original voltage levels. These extreme voltages can lead to transformer dielectric failures. When the frequency content of the high voltage, fast rise-time, TRV surges coincide with the internal resonant frequencies of the winding, the circuit can resonate and elevate the electric stress in the windings beyond the dielectric withstand strength of the windings. A recent IEEE Standard addressed the interaction between breaker switching and transformer response.
A Final Thought - The Importance of Padmount Transformer Bushings
One of the most common specification errors occurs on padmount transformers. On 34.5 kV rated windings, the highest rated dead front bushing is rated at 150 kV BIL. Thus most specifications also specify 150 kV BIL as the insulation level of the associated winding. Instead, a good practice would be to specify a full 200 kV BIL for the winding, while leaving the bushing at 150 kV BIL. It is much easier to replace a failed bushing than a failed transformer. |
