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Document Type and Number:
WIPO Patent Application WO/2010/109262
Kind Code:
A2
Publication Date:
September 30, 2010
Export Citation:
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International Classes:
F03D9/02
Download PDF:
Claims:
C l a i m s

1. A control system for fluid-flow turbines comprising a plurality of control units, wherein at least one control unit is powered exclusively by direct current (DC), the control system having a back-up power supply comprising : a first power supply powered by AC power and supplying DC power to said control unit, the first power supply being rated for the full power requirements of said control unit; a second power supply powered by AC power and supplying DC power to said control unit, the first power supply being rated for the full power requirements of said control unit; a supercapacitor storage dimensioned to store the full power consumed by said control unit for a predetermined period of time, and an OR connection between said supercapacitor storage and said first power supply and said second power supply.

2. The control system in accordance with claim 1 wherein said OR connection is a diode OR-gate.

3. The control system in accordance with claim 1 wherein said OR connection is a bridge rectifier.

4. The control system of any of the preceding claims, wherein the control unit is a generator control unit or a turbine control unit.

Description:
A redundant, supercapacitor , back-up power supply for wind turbine conversion and control systems

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to fluid-flow turbines, such as wind turbines under water current turbines, and to other prime movers, and more particularly to a redundant power supply, with supercapacitor energy storage used to provide DC power and DC Back-Up power for wind turbine conversion and control systems .

DESCRIPTION OF THE PRIOR ART

Because power supplied by a utility grid is not 100% reliable, back up power for wind turbine controls is necessary. There are also utility ride through reguirements that specify how long a turbine must remain connected to the grid in the event of a grid outage. Additionally, capacitor storage systems and especially supercapacitor energy storage systems provide surge current capability far beyond the current-limit rating of the DC power supply itself. This feature is often found useful with high performance contactors and other short-term loads, that have a high current in-rush, but a low current continuous rating.

Supercapacitors are electric double-layer capacitors, also known as electrochemical double layer capacitors (EDLCs) or ultracapacitors . Supercapacitors have a very high energy density as compared to common electrolytic capacitors. Supercapacitors were created to fill the gap between electrolytic capacitors and batteries.

To alleviate the problems of power surges and mechanical loads with a constant speed wind turbine, the wind power industry has been moving towards the use of variable speed wind turbines. A variable speed wind turbine is described in US Patent Number 7,072,110, granted May 9, 2006, and assigned to Clipper Windpower Technologies, Inc. Further, there are various publications concerning the design of wind turbines running with constant or variable speed, e.g. "Wind Energy Handbook", Tony Burton et al, John Wiley & Sons, 2001.

The controls of a Wind Turbine are usually powered by AC power from the utility grid. The conventional approach to power supply back up for the controls of a wind turbine is to employ some type of Uninterruptible Power Supply (UPS) system. These devices contain a power supply, a battery and an inverter to provide Alternating Current (AC) in order to replace the utility AC lost due to a power outage. Most UPS systems employ some type of high-speed switch that connects the UPS to the device being protected when a line outage is detected. Other, more sophisticated, and expensive, designs remain connected at all times eliminating the switching time during a line outage event. In either case, these devices reguire a DC to AC inverter in order to supply the current power necessary for the load.

Koenig patent US 6,737,762 is an example of a prior uninterruptible power supply (UPS) . The load draws power from a utility-provided AC power source until a fault condition appears. When the fault condition appears, the load switches its power draw from the utility-provided AC power source to an inverter AC output of the UPS. An energy storage device maintains a DC voltage at the inverter input until the generator starts and accelerates to a level sufficient to provide the DC voltage. The system may also include switching devices for providing uninterruptible power to a critical load, while permitting a noncritical load to be subjected to a fault-condition on the utility-provided AC power signal for a short period of time, before switching to receive power from the inverter output.

Janssen Patent 6,921,985 is another example of a prior uninterruptible power supply (UPS), used in a wind turbine. Janssen discloses a wind turbine, which includes a blade pitch control system to vary pitch of the blades and a turbine controller coupled with the blade pitch control system. A first power source is coupled with the turbine controller and with the blade pitch control system to provide power during a first mode of operation. An Uninterruptible power supply (UPS) is coupled to the turbine controller and with the blade pitch control system to provide power during a second mode of operation. In the event of a fault condition in the power providing grid the turbine controller detects a transition from the first mode of operation to the second mode of operation and causes the blade pitch control system to vary the pitch of the blades (feather) in response to the transition . Until recently, nearly all UPS systems employed some type of sealed lead acid battery in order to provide a back up power usually lasting for 10 to 60 minutes. Although there is a trend to employing supercapacitors in this same architecture, their use is still limited due to the cost of providing a storage capacitor equivalent to their battery counterpart .

Finally, despite the efficiency of modern inverters and power supplies, the conversion from AC to DC to AC is no greater then 75 or 80% for even the best systems. The complexity of providing a second power supply, a battery, and an inverter make such back up system less reliable than the system they are backing up.

Therefore, there is the continued need for reliable and efficient back up power setups for wind turbines. SUMMARY OF THE INVENTION

The invention adresses the above drawbacks of the state of the art and relates to a back-up power supply system for use with wind turbine controls. According to the invention controls for the wind turbine are used which are powered by direct current (DC) in contrast to the AC controls used in the state of the art. Further, the power supply of the invention comprises a first power supply rated for the full power requirements of said controls, a second power supply rated for the full power requirements of said controls and an OR connection between a supercapacitor storage and said first power supply and said second power supply. The OR connection is either a dual diode OR-gate or a four diode bridge rectifier .

Since back up supply of AC driven pumps, motors, and fans is unnecessary during a ride through event lasting no more then 3 seconds, the back up supply system according to the invention is designed such that only DC power would be required. All contactors, sub-system sensors, external sensors, and internal intelligences such as the Turbine Control Unit or the Converter Control Board, are designed to operate with VDC (e.g. 24 VDC) . By providing this arrangement, back up power may be in the form of capacitor storage, eliminating the inverter and the batteries, and their associated chargers. With DC powering all of the control components and sensor, multiple supplies for DC power are used in a diode gated fashion so that a failure in one would not be noticeable to the operation of the wind turbine. Each supply is rated for the full power requirements of the control system.

An advantage of this back up power supply system is that it provides not only energy storage, but also a redundant fault tolerant design since it utilizes two separate power supplies. Dual power supplies are diode OR-gated to the capacitor storage system to provide continuous DC power in the event of a single power supply failure. This system requires that all contactors, sub-system supplies, and critical components with the control system itself, operate on DC, however, such operation eliminates the requirements for an output Uninterruptible Power Supply (USP) with the control system itself. Therefore, it is one aspect of the invention to use a control system, the critical components of which are powered by DC always, regardless of the condition of the utility grid. While known back up supplies use USPs and inverters in order to maintain operation during fault conditions on the utility grid, the invention changes the way in which the components of the control system are powered during normal operation in order to provide an advantageous back up supply during fault conditions on the utility grid and in order to provide surge current capability beyond the current-limit rating of the DC power supply itself.

Finally, the energy storage capabilities of this system are always available within the controller. No switching or active power supply monitoring is reguired to enable such storage as it is always enabled and operates 100% of the time. During a power outage, the storage system is instantly and seamlessly available. During normal operation the supercapacitors are charged in order to be readily available during fault conditions. If the supercapacitors are fully charged there is no need to stop the charging process actively but due to the increasing potential drop at the capacitor the charging process stops by itself. If the supercapacitors are partly or completely discharged due to compensation for surge currents or grid faults, they are charged automatically again as soon as DC power is available again.

The redundant supercapacitor power supply system is a unigue method of energy storage that does not employ the traditional uninterruptible power supply (UPS) system. By employing the combination of supercapacitors and DC supplies, all electrical storage and control can be maintained during a power outage with the exception of AC loads. Since most AC loads, like induction motors, need to be turned off during a power outage, this satisfies the reguirement for a back up power supply without having to employ expensive and complex AC inverters and battery chargers.

The invention has the advantage that storage availability is continuous. No switching or sensing is reguired to engage the storage capacity.

The invention has the advantage that storage capacity allows for large in-rush currents loads without placing DC power supplies into a current-limited situation. Since supercapacitors have lower internal resistance than batteries they are capable of providing very high short term current into a load, well above the current-limited rating of the DC power supplies themselves. The setup according to the invention therefore not only bridges power outages but also provides extra energy reserve for high load conditions.

The invention has the advantage that the redundant supplies of the invention allow for continuous operation without interruption, even during failure of one of the power supplies . The invention has the advantage that storage capacitor charge/discharge cycle life exceeds batteries by a factor of 1,000 for long operational life without problems. Further, since supercapacitors do not require any maintenance, this setup is more reliable and cost effective than design utilizing conventional batteries.

The invention has the advantage that temperature compensation is not required to charge the capacitor storage system, unlike batteries.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its mode of operation will be more fully understood from the following detailed description when taken with the appended drawings in which:

FIGURE 1 is a block diagram of a variable speed wind turbine in which the present invention is embodied;

FIGURE 2 is a circuit diagram of a prior art storage system, which employs standard, computer grade, electrolytic capacitors;

FIGURE 3 is a circuit diagram of a first embodiment of a storage system of the present invention;

FIGURE 4 is a circuit diagram of an overall storage system in which the present invention is embodied; and

FIGURE 5 is a circuit diagram of second embodiment of the present invention. FIGURE 6 is a circuit diagram of third embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

Refer to FIGURE 1, which is a block diagram of a variable- speed wind turbine apparatus in which the present invention is embodied. The basic components of the system are as follows. (1) A turbine drive train including a rotor hub-mounted pitch servo system 40, blade rotor including root blade 42 and extender blade 44, gearbox and a permanent magnet generator (PMG) 48, (2) generator rectifier/inverter unit 50; (3) a control system comprising a turbine control unit (TCU) ; generator control unit (GCU) 62, (4) a pad-mount transformer 52, and (5) SCADA (Supervisory Control And Data Acguisition) interface 64 connecting the system to a utility grid.

The turbine comprises one or more rotor blades 42, 44 connected, via a rotor hub mounted pitch-angle servo, which is powered through slip rings via blade drive signal bus 74. The hub 40 is mechanically connected to a turbine main-shaft 46, which transmits the turbine' s torque to a gearbox 48. There is a sensor for measuring turbine speed 54 on the low speed shaft, the output of which is the shaft speed 56. The turbine shaft is coupled via gearbox 48 and some suitable coupling device to, in this example, a permanent magnet or wound field synchronous generator. The generator electrical output is connected to block 50, which includes a rectifier, which converts the electrical power to DC voltage and current I (wind) on a DC bus. The DC bus is connected to wind turbine generator (WTG) inverter. The inverter regulates the DC current and by doing so, the generator torque is controlled. The inverter regulates this DC current by synchronizing to the grid and by supplying unity power factor current into the grid system. A generator control unit (GCU) 62 controls the inverter within block 50. The GCU takes inputs such as grid voltage, DC bus voltage, grid current load power demand I (demand in) from its own measurements and receives commands such as torque level from a Turbine Control unit (TCU) 60. The AC grid voltage measurement and current measurement are obtained from the output of block 50 and are used by the GCU for purposes of synchronizing the inverter to the AC grid. The converter takes all of its input voltage and current signals and converts these into pulse-width-modulated (PWM) signals, which tell a switch in the inverter 50 when to turn on and off. These switches are controlled in such a way as to maintain regulated AC output current in response to the current command supplied by the TCU. Line filters on the inverter output are used to reduce any harmonics that may have been generated by the inverter before passing power to a pad- mount transformer 52 on the utility grid. As shown in the above-referenced patent 7, 042, 110, the TCU 60 and GCU 62 work together in a multiple generator system to stage the generators, when the turbine is operating at less than full power rating. The controller brings each generator of the plurality of synchronous generators in the turbine online sequentially in the event of low energy conditions of the source of energy (wind, water, etc.) to improve system efficiency at low power. The controller may optionally- alternate the sequence in which the controller shifts the order in which the generators are brought online such that each generator receives substantially similar utilization.

The TCU 60 receives sensor information provided by sensor inputs 58 such as turbine speed, blade pitch angle, tower acceleration (vibration) , nacelle acceleration (nacelle vibration) , wind speed, wind direction, wind turbulence, nacelle position, AC line parameters, DC bus voltage, generator voltage, power output, and other fault related sensors. The TCU 60 has control of the principle actuators on the turbine; the generators via the GCU 62, the pitch unit (PCU) 66 and the Blade Extension Control Unit (ECU) 68. The TCU 60 performs a complicated, coordinated control function for both of these elements, and does so in a way, which maximizes the energy capture of the turbine while minimizing the machine' s mechanical loads. Finally, the TCU 60 also controls a yaw system, which works to keep the turbine always pointed into the wind. The TCU 60 is also in communication with the turbine' s SCADA system 64 in order to provide and receive sensor and status information. The Turbine Control Unit (TCU) sends the proper generator torque required as a signal to the Generator Control Unit (GCU) . This signal is based on the rotor speed and required torque at that speed, based on either a table or an algorithm. The converter modifies the torque command to help with gearbox damping, by employing notch filters within the torque command issued to the insulated gate bipolar transistor (IGBT) switches. In high winds the turbine remains at a constant average output power through a constant torque command from TCU and a constant speed command to the PCU.

The control system governs the variable rotor radius (via blade extension/retraction) , the pitch of the rotor blades, and the rotational rate of said rotor. The TCU 60 determines a pitch angle for the blades by means of an algorithm or lookup tables. A blade pitch command 70 is sent from the TCU 60 to the Blade Pitch Control Unit (PCU) 66 which generates blade rotation drive signals Dl, D2, D3, which pass over bus 74 to each of three servo motors that turn their respective blades. The TCU 60 also determines the desired position of the extendable/ retractable blade extensions 44 by means of an algorithm or lookup tables. An extension command is 72 sent by the TCU 60 to the Blade Extension Control Unit (ECU) 68 which generates blade extension drive signals El, E2, E3, which pass over bus 74 to each of three servo motors that extend/retract their respective blade extensions.

A DC Power Supply and Back-up 55 receives AC power 53 from the grid 52 and converts the AC input 53 to a DC output 57, which powers the Generator Control Unit 62, the Turbine Control Unit 60, Input Sensors 56 and other controls designed to operate with 24 VDC. AC driven subsystems, such as pumps, motors, and fans are supplied by AC power 59.

DC Power Supply Back Up Methodology.

Since back up supply of AC driven pumps, motors, and fans is unnecessary during a ride through event lasting no more then 3 seconds, the back up supply system is such that only DC power is required. According to the invention, all contactors, sub-system sensors, external sensors 58, and internal intelligences such as the Turbine Control Unit 60, Generator Control Unit 62 and the Converter Control Board 50, are designed to operate with 24 VDC. By providing this arrangement, back up power can be in the form of capacitor storage, eliminating an AC inverter and batteries, and their associated chargers.

Although the turbine is required to "ride through" utility disturbances of up to 3 seconds, the Turbine Control Unit and its associated sensors are many times required to maintain their own intelligence for the length of time that is required to feather the turbine blades from an operational pitch angle to a standby or feathered pitch angle position. Most turbines pitch at a rate between 2 and 7.5 degrees per second and therefore the time required would be 90 degrees of blade travel divided by 2 or 45 seconds maximum. For those turbines with a higher pitch rate this time could be as little as 90/7.5 or 12 seconds.

For most applications it is not necessary that he TCU maintain its intelligence for this entire period, but for a period of time required to bring the turbine off line or the time required to pitch the blades to a position that would result in a power output of the turbine that is zero. In this case the turbine would no longer be synchronized to the utility grid. That time is no more than a 30 degree pitch angle away from the fine pitch setting. 30 divided by 2 degrees per second gives a 15 second maximum time span that would be required for Turbine Control Unit back up power and 4 seconds if the pitch rate is as high as 7.5 degrees per second. The design of the super capacitor back up energy storage system requires that it provide 4 to 45 seconds of back-up power for most of the applications today.

With DC now powering all of the control components and sensors, a more reliable power supply is necessary. To achieve this requirement at least two supplies are used, diode gated so that a failure in one will not be noticeable to the operation of the wind turbine. Each supply is rated for the full power requirements of the control system in order to achieve this requirement.

Electrolytic Capacitor Enerqy Storage Nearly all DC power supplies employ some kind of capacitor energy storage, if only on their rectifier output for filtering and smoothing of the rectified AC waveform. Many power supplies also include energy storage on the output of their DC voltage regulators. Linear power supplies usually have larger capacitors than switching supplies but this is changing within the power supply industry as the need for higher levels of energy storage become apparent.

It is necessary to determine the lowest level of DC voltage that each component can allow and then design for a voltage drop near that lowest level. Through testing it was determined that all contactors and intelligence modules and sensors performed properly with as little as 9 to 14 VDC on their supply terminals. Therefore, assuming 15 VDC as the lowest voltage for proper operation, relatively constant discharge current and relatively low ESR (Equivalent Series Resistance) within each capacitor, the operation of the capacitor is characterized by the following formula:

i = C * dV/dT where: i = current in amperes

C = capacitance in farads dV = discharge voltage dt = discharge time Solving for C, C = i/dV * dt

Therefore with a nine-volt delta (24 V minus 9 V) , and a one second window of time and assuming a constant 4 amperes of current flow (the original Turbine Control Unit ODP power draw), the total capacitance required is .44 farads. It is further assumed that this time would actually be greater as the current draw of the system would go down with voltage. A DC storage system designed and tested by the applicant employs standard, computer grade, electrolytic capacitors to achieve these specification with a 22% margin. The schematic of a respective system with prior art components is shown in FIGURE 2. Two 270,000 microfarad, 35 volt capacitors are used connected as shown behind the power supplies diode gates. This gives a total of .54 farad. Tests show that the controller would remain active for periods of about 2 to 3 seconds.

Placing the components on a simple circuit board results in the ability to add LED' s for indication of each power supply and the total output. Output terminal strips provide distribution to external. Finally, the need for high current inrush during contactor pull in was specified and provided for by the capacitors themselves, allowing for power supplies with current ratings below this inrush value. Inrush current or input surge current refers to the maximum, instantaneous input current drawn by an electrical device when it is first turned on .

A complete test set-up was connected and the in-rush current for each contactor measured. Since the Turbine Control Unit is expected to turn on multiple contactors simultaneously, the total current required for the inrush is about 12 amperes. The power supplies in the test set-up would supply 4.8 amperes. The remaining 7.2 amperes is provided by .54 farad capacitors with ease as the in-rush period is only 20 milliseconds.

Supercapacitor Energy Storage System (first embodiment) Utilities sometimes specify ride through times of up to 3 seconds. It is apparent that standard electrolytic capacitors do not have sufficient storage to achieve these requirements.

The specification for the Control Unit (TCU) shows a maximum of 6 amperes of controller current for a period of time that would be 2 to 3 times the longest utility ride through specification or 6 to 9 seconds. This requires 4 to 8 farads of capacitance to achieve these types of numbers. The only way to obtain this kind of storage is to employ supercapacitors .

Unlike electrolytic capacitors, supercapacitors are available only in 2 to 2.5 volt ratings. This means that 12 would have to be placed in series with a reduction in their total capacitance by 12 for that operation. However 50-farad capacitors are available off-the- shelf with at least two manufacturers providing nearly identical parts. The schematic of this new back up board 24 VDC supply is shown in FIGURE 3. The DC supply inputs (TBl) are connected to draw power from the AC utility grid. Each phase (1+ and 2+) is connected to a series diode (Dl, D2 ) . The outputs of the two diodes are connected together and to the 24V outputs (TB2, TB3, TB4). The output of diode Dl is connected to 12 series connected supercapacitors (Cl, C2,.... C12). The output of diode D2 is connected to 12 series connected supercapacitors (C13, C14, ....C24) .

The total capacitance achieved by this design is 50/12 X 2 or 8.3 Farads. With a 6-ampere constant current draw, the total elapsed time to a discharge level of 15 volts (9 volt delta from the fully charged 24 Volt state) is:

dT = dV/I * C

or 12.54 seconds. Of course this is if the capacitance remains at 50 farads and the current draw remains constant. Neither of these occurs during actual operation, however. Testing suggests that this capacitance will vary with ambient temperature. However, since the TCU enclosure is heated this amount of capacitance will give a reasonable ride through time allowing for proper shut down of the turbine even if the ride event lasted 3 seconds during cold weather conditions . Another aspect of the controller design is that the current draw will be reduced as the voltage drops on the power supply. The amount of current reduction depends upon many factors including turbine state, weather conditions, and external control functions at the time of the ride-through event. In the end, testing shows that the 8.3 farad storage system is capable of providing 12 to 20 seconds of ride through capability on the TCU, meeting and exceeding the calculation shown above for most applications.

As with the electrolytic capacitor design shown in FIGURE 2, all of the components are mounted on a single circuit board. When completed this is smaller than the electrolytic capacitor board, yet containing 15 times the capacitance.

Refer to FIGURE 4, which is a wiring diagram of the overall system, including power supplies (PWRl, PWR2 ) and the 24-volt back up supply board of FIGURE 3. The two supplies PWRl and PWR2 are ORed together at the back up 24-volt supply board. The 12 series connected supercapacitors (Cl, C2,.... C12) of FIGURE 3 are illustrated as a single supercapacitor of 4.15 farad. Likewise, the 12 series connected supercapacitors (C13, C14, ....C24) of FIGURE 3 are illustrated as a single supercapacitor of 4.15 farad. Operating with two independent power supplies in parallel requires that they share current and this is easily accomplished by adjusting the output voltage of these supplies to be with the rated voltage (in this case 24.5 VDC) by +/- .1 volt. This gives good current sharing. During operation with the turbine in the Run state, the current draw from each supply is between 2.0 and 3.0 amperes with differences between the two supplies running about 100 to 150 milli-amperes .

Designed as a system, failure of either supply (PWRl or PWR2) is unnoticeable to the operator and to the turbine control unit. One can determine which power supply has failed by observing LED' s on the back up board itself, or by the use of a simple DVM for measurement of output voltage. According to the invention there are the DC powered main components of the turbine connected to the supply board at the connectors A, B, ..., L. The combination of using control components which are always powered by DC power (that is during normal operation and fault condition) and the utilization of supercapacitors in the circuit of the supply board as back up makes the system according to the invention simple and very efficient and reliable at the same time.

Supercapacitor Energy Storage System, 7.5 MW Turbine (second embodiment)

Advances in supercapacitors and the need for high current supplies and longer ride through periods have generated a need for a more advanced version of a redundant energy storage system, especially for use with larger multi-mega wind turbine design now approaching the 10 MW level. In addition, smaller and lighter switching power supplies are desirable instead of linear supplies.

A wiring diagram of this power supply system is shown in FIGURE 5. Again, the main components of the control system of the turbine are DC powered and connected to the DC power supply system shown. Two 10 ampere, switching power supplies (PWRl, PWR2) are tied through a bridge rectifier (BDl) which in turn feeds a 52.5 farad energy storage reservoir. The energy storage reservoir is comprised of two 105 farad, 15 volt supercapacitor modules in series, resulting in 52.5 farad. The actually total capacitance is about 52 farads at a rated voltage of 30 Volts DC. This is over 6 times the storage of the prior system. In addition, due to the advances in this technology, the cost of these modules in guantity is nearly the same as the prior back up board shown in FIGURE 2. Overall the system has increased current performance, increased "inrush" current performance, and longer ride through capability all within the cost envelope of the back up system shown in FIGURE 2.

This system employs a later generation of supercapacitors, with better performance over temperature, small size versus capacity and when purchased as a module, better egualization across each capacitor. The modules chosen (Nippon Chemi-Con) use active egualization for each cell.

The footprint required for two of these modules is less then the footprint required for the prior back up board. The difference in size is all in the vertical direction, which lends itself well to a more compact layout within the TCU enclosure itself. Further, the size of the switch regulators is substantially smaller then the original 7.2 ampere linear supplies.

This new system employs a diode bridge rectifier to "or" gate the two power-supplies to the super capacitor energy- storage system. This bridge rectifier employs four diodes with two that do the "or gate" steering and two that provide reverse voltage transient protection between each of the power supplies and ground or the negative terminal of the power supplies as shown in Figure 5.

A Triple Redundant, three phase AC input, Super Capacitor Back-up Energy Storage System (third embodiment)

Shown in Figure 6, three 10 ampere, switching type, 24 Volt DC supplies are used to provide charging and operation current for the 24 Volt controller distribution system. Within the wind turbine these supplies are fed with 240 VAC taken from three individual phases of the input 400 VAC/3 phase accessory power within the Master Control Unit (MCU) enclosure. Since the load requirements never exceed two of these three individual supplies, this system is capable of providing full voltage and current without interruption during single phase failures on the incoming utility 400 VAC line. Only when a three phase fault occurs will the back up capacitors come into play.

This capability is achieved through the use of a COTS' s three phase bridge rectifier as shown in Figure 6. Additionally a larger capacitor is used for storage, totaling 233 Farads each at 15 volts DC enabling a total of 116 Farads for the power supply output itself. Because all three power supplies work in parallel during the charge cycle, the total charge time is not much greater then the previous 52 Farad version and runs about 60 seconds, depending upon the short circuit current limit characteristics of the DC supplies themselves . Using three supplies allows for higher DC current output for distribution, up to 20 amperes continuous. As with the previous supplies, peak currents are entirely supplied by the super capacitors and do not originate within the DC supplies themselves. In-rush currents required for high performance contactors are easily handled by these large super capacitors. Additionally, distribution is divided by circuit breakers including an individual breaker for the safety system with the wind turbine control unit itself. Finally, each of the Switching DC supplies has an "alarm output" which is shown in this drawing. Each of these is connected to the turbines master or slave controller and allow for monitoring of the health status of each of these supplies. If any single supply fails, due to the redundant nature of this design, only a "warning" status will be issues for the operator, allowing time to enable repair and return to full service. During this interval the system can operate as before and in fact this kind of alarm does not cause interruption of the controller operation at all. The Redundant Super Capacitor Power Supply system according to the invention is a unique method of energy storage that does not employ the traditional Uninterruptible power supply system. By employing super capacitors and DC supplies, all electrical storage and control can be maintained during a power outage with the exception of AC loads. Since most AC loads, like induction motors, need to be turned off during a power outage, this satisfies the requirement for a back up power supply with the necessity of employing expensive and complex AC inverters and battery chargers . The advantages of this type of storage system are:

• Storage availability is continuous. No switching or sensing required too engage the storage capacity.

• Storage capacity allows for large in-rush currents loads without placing DC power supplies into a current limited situation. Super Capacitors have lower internal resistance then batteries and therefore are capable of providing very high short term current. • Redundant supplies allows for continuous operation even during a single power supply failure - without interruption.

• The Storage capacitor charge/discharge cycle life exceeds batteries by a factor of 1,000 for long operational life without problems.

• Temperature compensation is not required to charge the capacitor storage system, unlike batteries.

• Using the three phase approach, and loading only two of the three supplies, full power supply current and voltage capabilities are available during a single phase loss of power event .