The large variability in wind output power can adversely impact local loads that are sensitive to poor power quality. To mitigate large swings in power, the wind turbine output power can be conditioned by using a small energy buffer. A power conditioner is developed to smooth the wind power output by utilizing the energy of an electrochemical capacitor, or ultracapacitor. The conditioner is based on a single phase voltage source inverter connected between the grid inter-connection point and the ultracapacitor. The VSI shunt inverter injects or absorbs active power from the line to smooth the wind power output by utilizing the short term storage capabilities of the ultracapacitor. The ultracapacitor is connected to the DC link through a bidirectional DC-DC converter. The bidirectional DC-DC converter and VSI are constructed and field tested on a Skystream 3.7 wind turbine installed at the Missouri University of Science & Technology.
Due to the price volatility and carbon impact of fossil fuels, wind power generation is rapidly growing as an alternative energy source in many parts of the world. Due to the intermittency of wind speed, wind turbine output power can be highly variable. Power fluctuations from the wind turbine may cause severe power quality problems when connected to the grid. The large variability in wind turbine output power can adversely impact local loads that are sensitive to pulsating power, posing a challenge to the use of wind power extensively. The rapid growth of the wind power and its immense potential as a future energy source encourage us to find a way to smooth the intermittent wind power. Energy storage technologies can be used to improve the quality of the wind power [1,2]. In this paper, a power quality conditioner with the ultracapacitor is proposed to smooth the variable wind turbine output power. The short term storage capabilities of the ultracapacitor can be effectively used to smooth the wind power to minimize rapid power excursions that may damage sensitive local loads.
Ultracapacitors (also known as supercapacitors) have been proposed as a short term energy storage mechanism to provide improved active power output performance for wind turbines [3-7]. All of these articles address the application of ultracapacitors to three-phase wind turbine systems and none of them present actual experimental results for their designs. In [
The power conditioner mainly consists of power converters to shape the injected current at the point of common coupling [
Control of the injected active power via the shunt inverter is presented in this paper. The VSI controller calculates the compensating active power, which is then synthesized by using the bipolar pulse width modulation (PWM) switching sequence. The reference signal to the shunt inverter controller is obtained from a low pass filter, which has a large time constant. The fluctuating wind power is passed through the low pass filter to get the smoothed reference value. The conditioner ensures the smooth power is available at the grid interconnection point. The simulation results are presented to show the efficiency of the conditioner in smoothing the variable wind turbine output power.
The power conditioner design and control will be validated on the Skystream3.7 wind turbine installed at the Missouri University of Science and Technology. The installed wind turbine is shown in
The neutral and ground terminals of the wind turbine are tied together and connected to the neutral of the grid. The power quality conditioner is connected in shunt between the output of the wind turbine and the grid interconnection point.
As shown in
The primary objective of the conditioner is to inject a current Iinv at the point of common coupling (PCC) such
that the current supplied to the grid (IPCC) is relatively smooth. The smoothed current is obtained by passing the (measured and scaled) wind turbine current through a low pass filter that is tuned to provide the appropriate high-frequency cutoff. The ultracapacitor is charged and discharged rapidly to supply the required current while holding the DC link constant. Note that the reference current is not a constant, but rather a slowly varying current. If the reference current were held constant, this would imply that the electrochemical capacitor would have infinite ability to charge and discharge. By allowing the reference current to slowly vary, the energy supplied to the grid will track the energy supplied by the wind turbine.
The three primary components of the proposed power conditioner are the shunt inverter to control the injected current, the ultracapacitor, and the DC-DC converter to regulate the DC link voltage and control the ultracapacitor injected current. The topology of the bidirectional DC-DC converter is shown in
The DC-DC converter operating modes can be divided into four modes:
• Mode 1: The DC-DC converter acts in buck mode when the DC link voltage is greater than the reference value. In this mode, the DC-DC converter controls the current to charge the ultracapacitor.
• Mode 2: The DC-DC converter acts in boost mode when the DC link voltage falls below the reference value. In this mode, the ultracapacitor discharges.
• Mode 3: When the ultracapacitor is fully charged, the DC-DC converter shuts down to avoid damaging the ultracapacitor.
• Mode 4: When the ultracapacitor is fully discharged, the conditioner shuts down until the wind turbine produces sufficient current to resume charging of the ultracapacitor.
The bidirectional DC-DC converter shown in
Ultracapacitors are electrochemical double layer capacitors that have unique characteristics when compared to other energy storage devices. Ultracapacitors (UCAPS) have high energy density and large time constants as well. Although multiple time-scale models of UCAPs have been developed, a simple UCAP model such as the one in
capacitor (C) [
The benefits of using ultracapacitors are quite extensive. Ultracapacitors have low losses while charging and discharging. They have a very low ESR, allowing them to deliver and absorb very high currents and to be charged very quickly, making them well suited for energy buffer applications. Ultracapacitors are highly efficient components even at very high currents. The characteristics of the UCAP allow it to be charged and discharged at the same rates, something most batteries cannot tolerate. Ultracapacitors have a wide voltage window and can be deeply discharged. The energy storage mechanism of an UCAP is a highly reversible process. The process moves charge and ions only. It does not make or break chemical bonds like batteries; therefore it is capable of millions of cycles with minimal change in performance. It is therefore capable of many years of continuous duty with minimal change in performance. These advantages make ultracapacitors well suited for power quality conditioning applications.
The power conditioner was constructed using two series-connected Maxwell BMOD0165 ultracapacitor modules of 165F nominal capacitance and rated voltage 48.6 V. This ultracapacitor has a maximum continuous current of 77 A at 15˚C [
A full-bridge IGBT based inverter topology is used in this application. The full-bridge inverter consists of four switching devices, which are connected to form the fullbridge inverter circuit shown in
The VSI operates in the following two modes:
• Mode 1: When the wind turbine power is greater than the reference value, the converter acts like a rectifier drawing active power from the line and charging the DC link capacitor.
• Mode 2: When the wind turbine power is less than the reference value, the converter acts like a VSI injecting active power into the line by discharging the DC link capacitor.
The variable wind power is passed through a low pass filter to get a smoothing reference signal for the inverter controller. The output of the low pass filter is given to the shunt inverter controller as the reference value [
where is the time constant of the filter and is the output of the wind turbine. The smoothing performance of the wind turbine output power depends on the time constant of a low pass filter. The time constant of the low pass filter is in the range of several seconds and is tuned to provide the desired smoothing. The power fluctuation is smoothed by drawing or injecting the difference of the reference power and the variable wind power.
The experimental setup and different components in the construction of the single phase shunt inverter are shown in
the shunt inverter. A TI TMDSDOCK28335 DSP is used in the closed loop control of the shunt inverter. The current and voltage sensors measures and sends signals to the DSP through the signal shaping circuit. DSP drives the IGBTS with the ACNW3190 drivers.