Technical Articles
Nowadays, developing countries are looking forward to renewable energy sources to suffice the increasing power demands in a sustainable way. This has led to a significant growth in power generation through wind farms in recent years. Out of various types of generators used in wind farms, the most popular design is with induction generator (asynchronous fixed speed machine), as it is more economical and simpler in electrical as well as mechanical design and in its operation. They are also more rugged, requiring no brushes or commutator.
A drawback of the inductive wind electric generator is the consumption of reactive power under all load conditions, as well as during generator start-up. The consumption of reactive power increases with increase in power output (kW). Overall efficiency of the wind electric generator is significantly impacted by the reactive power compensation system. Hence the usage of power factor correction capacitors is mandatory in wind mills.
The below figure is a typical block diagram of constant speed wind electric generating station, which depicts wind turbine, gear box to maintain same rotor speed, induction generator, power capacitors and step-up transformers.
An induction generator is a type of electrical generator that is mechanically and electrically similar to an induction motor. Induction generator produces electrical power when their shaft is rotated faster than the synchronous frequency of the equivalent induction motor. Induction generators are not self-exciting, i.e., they require an external supply to produce a rotating magnetic flux and the power required for this is called reactive power. This is in contrast with synchronous generators, where permanent magnets or dedicated DC supply is given for excitation of the rotor. Hence synchronous generators do not consume reactive power and no capacitors are required
During operation, if this reactive power is drawn from the grid, it would lead to increased transmission losses, poor voltage profile and over loading / blocked capacity of the transmission & distribution devices. Hence localized reactive power compensation through power capacitor is mandatory for Wind Electric Generators.
Following are some technical challenges in wind farms, in relation to the reactive power management and their appropriate solutions.
Majority of wind farms are based on asynchronous generation, consuming reactive power instead of contributing, unlike synchronous generators. Furthermore, the consumption of reactive power of asynchronous wind generators fluctuates randomly with wind velocity which imposes still more disturbance on the grid voltage.
The reactive power consumption of an induction generator is a function of its loading and increases as the active power output increases. The power factor at rated load is usually in the range of 0.85 to 0.90, which means that the typical consumption of reactive power is little more the half of the active power generation. At partial no load conditions, the power factor can drop to 0.60 or below. In this case the reactive power requirement is almost equal to or greater than that of the active power generation.
Following are the typical wind mill ratings and the power ratings of capacitor required them:
| Rated active power of the wind mill | Approx reactive power required to achieve UPF*** |
|---|---|
| 250kW | 150kVAr |
| 600kW | 375kVAr |
| 1000kW | 625kVAr |
| 1200kW | 750kVAr |
| 1500kW | 950kVAr |
| 2000kW | 1250kVAr |
***These are typical values. Actual values may vary with respect to wind mill manufacturers.
Typical generating voltage is 690 V and hence all the capacitor and switchgear should be rated for the same.
The reactive power requirement fluctuates widely in a very short duration, due to rapid wind speed variations and hence faster correction by frequent switching is required. Conventional systems with electromechanically (contactor) switched capacitor steps are limited by their inherently slow reaction to load fluctuation. This is due to the fact that capacitor discharge time has to be taken into account before step reconnection can take place. Hence it is impossible to maintain unity power factor with the time delay (60 seconds) in switching. When someone tries to override the time delay criterion, life of the capacitors and the contactors will comedown. Also the contactor’s contacts would be welded, which is the most common failure that happens in this application.
Additionally, conventional method of utilizing multiple power contactors result in generation of disturbances (peak inrush current of > 100 x In) on the electrical system during connection and disconnection. Hence they are prone to early failure due to the very frequent switching of capacitive loads. The usage of capacitor duty contactors that are normally fitted with in-rush limiting resistors, can invariably limit the in-rush peak to 20 x In. But still, even with capacitor duty contactors, the time delay is necessary, else it will result in contacts welding.
The solution for both the challenges (switching time and transients) would be Thyristor Switching, which can repeatedly switch the capacitors in less than 20ms. The zero-voltage switching method employed in thyristor switching modules will take care of inrush current too. In order to prevent the premature failure of thyristors because of transients spikes, the Peak Inverse Voltage (PIV) of the semiconductor should be at least 2200V.
Most wind electric generators employ induction generators as electromechanical energy coverters. Sudden disconnection of the generators from the grid may result in over compensation or self-excitation of the induction generators because of the capacitors connected across them. This sudden islanding would also result in over voltages, which could be dangerous.
Hence the capacitance connected across the induction generators must be immediately disconnected as soon as the grid drops. Thyristor switches can disconnect the capacitors in less than 20ms on disconnection of the generator from the grid; thus reducing the capacitance to a value lower than the critical value, whereas the contactor switched capacitors require at least 100ms to isolate the capacitors and thereby it cannot prevent any over voltage.
All the wind mills with induction generators should be equipped with power factor correction capacitors for efficient power generation. The switching of capacitors should be done through thyristor switching modules for dynamic compensation and proper voltage regulation. As the switching is very frequent, the capacitor should also be rated for higher switching operations, such as LTXL capacitors.
Abhinav L Purkar,
Manager Product Management & Marketing (PMM)Abhinav Purkar is a Product Management, Marketing, and Application Engineering professional with over 10 years of experience in the Power Quality domain. He has extensive expertise in power quality analysis, harmonic mitigation, reactive power compensation, energy efficiency, and electrical system optimization, helping customers improve system reliability and operational performance. With proven experience in product lifecycle management, market development, portfolio growth, and go-to-market strategy, Abhinav has successfully driven initiatives that enhance customer value and strengthen market presence. His strong technical foundation, combined with strategic business acumen, enables him to bridge the gap between customer requirements and innovative product offerings.
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