Wind Power Generation: The Journey to a Cleaner Future
Climate change is a pressing global issue – and with unequivocal scientific evidence of humanity’s direct role in global warming, adoption of clean energy sources has taken off. Wind energy alone is projected to produce 113.43 gigawatts (GW) of electric power in 2020—over double what it produced in 2013. That number is only expected to grow, reaching 404.25 GW by 2050.
While clean energy is a very attractive option, there are issues associated with wind power generation. Information related to these issues and solutions will help the developers, designers and operators of wind power plants.
1. Harmonics
Harmonics has been a concern for some time, beginning with the grid’s introduction to high-voltage direct current (HVDC) systems in the 1980s. Harmonic injections distort normal currents in the grid, creating higher voltage levels and dangerous temperatures within transformers that can negatively impact performance. As with erratic loading and vibration, the heat associated with harmonics often creates gassing issues that cause severe damage.
In the 1980s, adjustable speed drives were installed at industrial customer facilities to replace conventional induction motors, which caused high levels of harmonics to flow within the system. The introduction of renewable power worsened the situation. To prevent excessive harmonic injections, utilities adopted a very strict regimen to make sure industrial customers applied appropriate harmonic filters within their distribution systems. IEEE produced Guide IEEE 519 to create harmonics limits even before clean energy sources gained momentum in the United States. To this day, the guide is a good tool for utilities and industrial customers looking to contain harmonics through application of harmonic filters at appropriate locations within their facilities.
IEC also provides guidance on this topic, taking a different approach than IEEE 519 for restricting the flow of harmonic current. IEC imposes restrictions on equipment that produces harmonic currents making it a responsibility of the equipment manufacturer to limit the generation of harmonics.
2. Ferroresonance
Wind farms inherently take up large areas – about three quarters of an acre for each megawatt (MW) – because wind turbines need to be separated by a safe operating distance. Each generating unit is typically rated for 500kW to 2MW, with some as high as 9.5MW. Many units are required for the total output capacity of the farm to be in a reasonable range, which means long cables need to connect the units together to a common collector bus, which attaches to the grid via a step-up transformer.
Under certain conditions, a harmonic resonance can occur, which consists of magnetizing reactance of the transformer and the cable capacitance. This type of resonance is called ferroresonance and can cause the system voltage to rise to dangerous levels. This situation can be avoided through proper considerations during system design and plant operation.
3. Transformer impact
The risk of overheating is especially pertinent to small transformers applied at each turbine generator, and the large main step-up transformer between the collector bus and the grid.
Standards such as the IEEE C57-110 can help determine whether the transformer rating is large enough to handle harmonics. Based on the results of the calculations – which can usually be performed by existing system study software such as ASPEN, CAPE, SKM, ETAP and more – transformers may require derating. The calculations can be very helpful when making purchasing decisions, as an appropriate rating can be determined at the outset.
Small, 480V-rated transformers should be capable of handling the effects of harmonic currents. Transformers rated for 600V or less are marketed with a UL-certified, K-factor rating. These K-factors determine the level of harmonic currents these transformers can withstand. A K-factor of zero implies the transformer is not suited to carry any harmonic current. The same software that can calculate large transformer capabilities can likely determine the necessary K-factor for each of these transformers as part of the simulation results.
In addition to harmonics, transformers at wind farms are subjected to other conditions such as erratic loading, and in some instances vibration. All these factors can cause major issues.
4. Relay protection
The main issue with relay design is the low fault current contributions from the converters. For faults within a plant, there is adequate short circuit current contribution from the grid. However, the contribution from converter-based wind generators is 3-4 times the rated current of the converter for 4-6 cycles. This drops down to 1.1-1.4 times the rated current of the converter after 4-6 cycles.
This level and duration of short circuit current is normally adequate for instantaneous operation of relays such as Zone 1 distance relays, transformer differentials, line current differential and instantaneous overcurrent elements. These levels of short circuit current cause difficulties in achieving a time-based coordinated protection scheme. To trip the generators in the event of a fault, normally there is a need to implement direct transfer trips due to a lack of adequate fault current levels past 3-6 cycles. Some manufacturers have recommended protective schemes that utilize GOOSE messaging per IEC 61850 standard instead of utilizing hard-wired direct tripping.
One other issue is that the negative sequence component is absent in the fault current contribution from the converters. This causes additional protection related issues.
5. Inertia
Inertia in a power system comes from heavy equipment such as steam or gas turbines. A grid that contains power plants with little or no inertia exhibits instability, power quality issues and is very susceptible to out of step conditions.
For example, when a conventional generator is operating in synchronization with the host grid, the frequency of the generator and the grid are the same. If there is a sudden demand of load the frequency of the grid tends to decrease very rapidly. However, if there is significant rotating mass, such as conventional generators, rate of change slows and system stability is maintained.
For wind power plants that contain converters between the induction generator and the grid, the wind generator is decoupled from the grid. This means there is no inertia to help reduce the rate of change of frequency.
When several conventional generators are replaced by wind power generators, grids face reduced system inertia. One of the solutions is to install synchronous condensers between the grid and the converters associated with the wind power generators. In addition, research is being conducted on several other solutions to solve this problem. Most of these solutions focus on emulating the inertial response in the control loops of the converters.
The time is now for cleaner energy
The U.S. has finally begun a clean energy revolution, but it’s critical for the power industry to address these wind power plant performance concerns through appropriate planning, training and resource allocation.
Interested in learning more about how to integrate renewables for a greener future? Join us for a session on harmonics, protection or other topics.
