Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/18—Arrangements for adjusting, eliminating or compensating reactive power in networks
  • H02J3/1821—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators
  • H02J3/1835—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control
  • H02J3/1842—Arrangements for adjusting, eliminating or compensating reactive power in networks using shunt compensators with stepless control wherein at least one reactive element is actively controlled by a bridge converter, e.g. active filters
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
  • H02J3/381—Dispersed generators
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J2300/00—Systems for supplying or distributing electric power characterised by decentralized, dispersed, or local generation
  • H02J2300/20—The dispersed energy generation being of renewable origin
  • H02J2300/28—The renewable source being wind energy
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
  • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
  • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
  • H02J3/40—Synchronising a generator for connection to a network or to another generator
  • H02J3/44—Synchronising a generator for connection to a network or to another generator with means for ensuring correct phase sequence
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E10/00—Energy generation through renewable energy sources
  • Y02E10/70—Wind energy
  • Y02E10/76—Power conversion electric or electronic aspects
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E40/00—Technologies for an efficient electrical power generation, transmission or distribution
  • Y02E40/20—Active power filtering [APF]

Definitions

• Wind energy has emerged as the fastest growing source of energy, offering a clean, renewable, and ecological-friendly alternative to fossil-based energy supplies.
• wind energy conversion is projected to produce more than 117,000 MW by the year of 2009, claiming about 1.25% of the global electricity generation.
• wind turbine generators are now increasingly installed in large-scale (e.g., multi-megawatt) wind farms and integrated into power grids that can deliver electricity to consumers nationwide.
• the performance of a grid-connected WTG can be influenced by many factors, such as voltage fluctuations on the grid. For example, a short circuit on the grid may result in a sudden voltage drop, which reduces the effective drag on the WTG and may cause both the turbine and the generator to accelerate rapidly.
• some WTGs have been designed to trip off-line (i.e., disconnect from the grid and shut down) as soon as grid voltage drops below a prescribed level (e.g., 85% of nominal voltage). After fault clearance, these WTGs enter a restart cycle that can last several minutes before resuming power transmission to the grid.
• the Spanish Grid Code requires WTGs to be able to sustain ("ride-through") line voltage at 20% of rated level for at least 500 ms.
• FIG. IA shows an example of voltage transients when a low- voltage event occurs. In this case, after an initial dip of 500ms, line voltage starts to recover and within 15 seconds has returned to 95% of nominal. During the entire low-voltage period ( ⁇ 15s), the Spanish Grid Code requires WTGs to continue to operate and supply current in controlled amounts to help stabilize the grid.
• FIG. IB shows the required current behavior, measured by the ratio of the magnitude of reactive current to total current (/ re ⁇ cftv Jl ⁇ to tai) as a function of line voltage. Note that other countries may have different regulations on grid-connected WTGs' current and voltage behaviors in response to low voltage disturbances.
• a system for connecting a wind turbine generator to a utility power network.
• a first power converter converts an AC signal from the wind turbine generator to a DC signal and supplies a controlled amount of reactive current to the wind turbine generator.
• a second power converter connected in series with the first converter, converts the DC signal from the first power converter to a line-side AC signal and supplies a controlled amount of current to the utility power network.
• a power dissipation element is coupled to the first and second power converters for dissipating power from the first power converter.
• Embodiments of this aspect of the invention may include one or more of the following features.
• the amount of current supplied to the utility power network satisfies a predetermined criterion associated with a voltage condition of the utility power network.
• the predetermined criterion may include that when a voltage of the utility power network falls below a predetermined threshold, the magnitude of reactive current supplied to the utility power network is at least twice as much as the magnitude of real current supplied to the utility power network.
• the first and second power converters are connected via a DC bus.
• a capacitor is coupled to the DC bus.
• a first and second AC filter reactor may be coupled to the first and second power converter, respectively.
• the power dissipation element may include a resistor.
• the resistor may include a dynamic braking resistor.
• a controllable switching device may be coupled to the resistor for regulating a current passing through the resistor.
• a power factor correction unit may be provided for adjusting a power factor of the electric power supplied to the utility power network.
• the power factor correction unit may include a controllable capacitor that can be switched on and off by electrical signals.
• a control system for controlling an interconnection between a wind turbine generator and a utility power network.
• the control system electrically opens a first path of the interconnection.
• a second path of the interconnection is controlled during the low voltage event to provide a first current suitable for maintaining an operation of the wind turbine generator and a second current having a predetermined characteristic associated with an operation of the utility power network.
• Embodiments of this aspect of the invention may include one or more of the following features.
• the control system may determine the occurrence of a low voltage event based on a voltage condition associated with the utility power network, or alternatively, on a current condition associated with the wind turbine generator, or a combination of both of these methods.
• the first current includes a reactive current component sufficient for maintaining an excitation of the wind turbine generator.
• the second current includes a real current component and a reactive current component. During the low voltage event, the second current is controlled so that the magnitude of the reactive current component is at least twice the magnitude of the real current component.
• the first path includes a switch unit controllable by external signals, and may further include a forced commutation circuit configured to provide a commutation signal to the switch unit.
• the second path includes a first power converter for converting AC power from the wind turbine generator to DC power and for providing the first current.
• a second power converter is connected in series with the first converter for converting the DC power from the first power converter to line-side AC power and for providing the second current.
• a power dissipation element is coupled to the first and second power converter for dissipating power from the first power converter.
• the power dissipation element may include a resistor and a controllable switching device coupled to the resistor configured for regulating a current passing through the resistor.
• a capacitor is coupled to the first and second power converter.
• the control system may further control a power factor correction unit to adjust a power factor of the electric power supplied to the utility power network.
• the power factor correction unit may include a controllable capacitor that can be switched on and off by electrical signals.
• a system for connecting a wind turbine generator to a utility power network is provided.
• electric power generated by the WTG can be delivered to the utility power network with near unity power factor and negligible power loss in the LVRT system (e.g., less than 0.3%).
• the system maintains near nominal voltages at generator terminals and provides sufficient impedance to the generator.
• the WTG continues to operate without experiencing low- voltage impacts (e.g., over-speeding).
• the amounts of real and reactive power delivered to the network can also be controlled based on voltage conditions.
• reactive power can be injected to the grid in sufficient amounts (e.g., at least twice the amount of real power) to help stabilize the utility network in a major low voltage event.
• sufficient amounts e.g., at least twice the amount of real power
• proper selection of power electronics and circuit design can also reduce system response time to faults.
• FIGs. IA and IB illustrate some aspects of LVRT requirements in the Spanish Grid Code.
• FIGs. 2A and 2B provide an overview and an exemplary implementation of a wind power generation system with LVRT capability, respectively.
• FIG. 3 is a flow chart illustrating a control scheme of the wind power generation system.
• FIGs. 4A to 4D are examples of steady-state and transient operations of one implementation of the wind power generation system.
• a wind power generation system 200 with LVRT capability includes a rotor 202 (e.g., a low speed propeller) which drives a wind turbine generator 204 for converting wind power to electric power in the form of alternating current (AC).
• a rotor 202 e.g., a low speed propeller
• AC alternating current
• a transformer 242 which by stepping up the AC voltage, transmits the power to a local grid 244.
• the interconnection system 208 includes a switch unit 210 and a back-to-back conversion unit 220, which provide a first and second paths 211 and 221 respectively, between the generator 204 and the transformer 242.
• switch unit 210 can be electrically turned “ON” (closed) or “OFF” (open) by external signals (e.g., control signals) to allow or block current passage in first path 211.
• Switch unit 210 can be a single power electronic switch (e.g., a thyristor), or a circuit that functions essentially as an electric switch having at least two states of distinct impedance.
• switch unit 210 presents negligible impedance to the current generated by the generator 204, thereby minimizing potential power loss during transmission.
• switch unit 210 When the grid is operating under normal conditions (e.g., voltage fluctuation remains within ⁇ 10% of nominal), switch unit 210 is closed, allowing power from the generator to be transmitted via first path 211 to transformer 236 in full capacity. When a low voltage event occurs (e.g., grid voltage drops below 90% of nominal), switch unit 210 is quickly opened to block first path 211. Subsequently, the full output of the generator is delivered through second path 221 to back-to-back conversion unit 220. When the grid voltage drops significantly to, for example, one- fifth its nominal value (i.e. 20%), five times nominal current will flow for the grid to absorb the pre-sag power generated by the WTG.
• a low voltage event e.g., grid voltage drops below 90% of nominal
• switch unit 210 When the grid voltage drops significantly to, for example, one- fifth its nominal value (i.e. 20%), five times nominal current will flow for the grid to absorb the pre-sag power generated by the WTG.
• back-to-back conversion unit 220 provides power in controlled amounts based on voltage conditions.
• back-to-back conversion unit 220 also provides reactive current necessary to excite generator 204 so that the generator continues to operate and generate power without being affected by the voltage drop.
• Other functions of the back-to-back conversion unit include a means to absorb or dissipate the excess power from the WTG that cannot be absorbed by the grid and, optionally, provide reactive current to the grid to aid in post- fault voltage recovery, which is described in greater detail below.
• a master controller 270 is provided in interconnection system 208 to control power transmission between the generator and the grid.
• master controller 270 is able to detect low voltage faults (as will be described in greater detail below) and act upon these faults to coordinate and control the operations of the switch and conversion units 210 and 220 to provide LVRT features of this power generation system.
• the implementation and logic of master controller 270 will be described in greater detail in the context of an exemplary interconnection system provided below.
• FIG. 2B an exemplary implementation of the interconnection system 208 shown in FIG. 2A is provided.
• switch unit 210 back-to-back conversion unit 220, master controller 270, and an optional power factor correction unit 234 is described in the following sections.
• Switch unit 210 includes a static switch 212 consisting of two controllable semiconductor switching devices, here, thyristors 212a and 212b. When closed, the pair of thyristors conducts AC current in alternative half-cycles, allowing the full output of the generator through the first path 211 with near zero voltage drop.
• thyristors 212a and 212b are selected to be "over-sized" (i.e., current ratings higher than required) to minimize on-state power consumption.
• switch unit 210 has a built- in forced commutation circuit 214 to which the thyristors are connected.
• the forced commutation circuit 214 When a control signal is received by the forced commutation circuit 214, the commutation circuit generates a current pulse of sufficient magnitude with a polarity that generates a zero crossover of current with the thyristors.
• the static switch 212 By forced commutation, the static switch 212 can be quickly turned off to help reduce system response time and improve transient performance.
• the back-to-back converters 222 and 224 can be controlled to also generate a commutation current pulse in the static switch thyristors 212.
• Back-to-back conversion unit 220 includes a generator-side AC/DC converter 222 and a line-side DC/AC converter 224 connected in series via a DC bus 225. Also coupled to DC bus 225 are one or multiple DC bus capacitors 226 which supports a DC bus voltage Vdc, and a power dissipation device 228 capable of dissipating real power. Power dissipation device 228 can include for example, a resistor (e.g., a dynamic braking resistor) to dissipate real power, and a controllable switching device that controls the amount of current passing through the resistor. In some examples, AC filter reactors (not shown) for reducing undesired harmonics and distortion in AC signals are also provided on both the generator and line sides.
• a resistor e.g., a dynamic braking resistor
• line-side converter 224 is configured to provide not only real power but also reactive power in controlled amounts (e.g., reactive power at least twice as much as real power) to grid 244.
• the exact ratio of reactive to real power may be arbitrarily set or imposed by applicable grid interconnection requirements (e.g. the Spanish Grid Code).
• generator-side converter 222 also provides reactive current necessary to keep the generators excited and operating at constant speed during low voltage events while simultaneously absorbing the real power output of the generator.
• generator- side converter 222 also provides reactive current necessary to keep the generators excited and operating at constant speed during low voltage events while simultaneously absorbing the real power output of the generator. In these type of generators, without the reactive current being applied under low voltage conditions, the generator sees reduced torque and begins to accelerate rapidly, which can damage the WTG.
• a power factor correction unit 234 is optionally coupled to line-side terminal 232 for improving the power factor (PF) of the electricity delivered to utility grids.
• PF power factor
• PF is a dimensionless number between 0 and 1, representing the ratio of real power to total power (also referred to as apparent power).
• a power factor of zero indicates that energy flow in the circuit is entirely reactive and stored energy in the load returns to the source on each cycle, whereas a power factor of unity indicates that energy flow is entirely real and thus unidirectional from source to load. Under normal conditions, it is generally desirable to operate power generation systems at near unity power factor to provide high efficiency power to utility grids.
• power factor correction unit 232 includes a group of capacitors that can be individually switched on and off by means of contactors (e.g., electrically controlled switches). During normal operation, these capacitors provide reactive power in adjustable amounts (e.g., depending on the number of capacitors switched on) to help achieve near unity power factor (e.g., above 0.9) at grid connection points.
• This power factor correction unit 232 may be provided as part of an existing wind turbine system, the interconnection system 208, or a combination of both.
• Master controller 270 is coupled to each of static switch 212, forced commutation circuit 214, back-to-back conversion unit 220, and possibly other components in interconnection system 208. Master controller 270 oversees system operation and controls power transmission between the generator and the grid based on various grid conditions.
• the logic and functions of master controller 270 are briefly illustrated in a flow chart 300.
• the master controller uses feedback signals from multiple sensors (e.g., line-side and generator-side current/voltage sensors) to monitor current/voltage dynamics for determining system states. If the system is operating in steady state (step 310) —that is, grid voltage appears within ⁇ 10% of nominal, the master controller functions to maintain the on-state of the switch unit 210 and disable/disconnect the back-to-back conversion unit 220. As a result, power is transmitted to the transformer only through first path 211.
• sensors e.g., line-side and generator-side current/voltage sensors
• Line faults including both unbalanced faults and sudden voltage drops, can be detected by the master controller upon sensing voltage or current anomalies.
• a line fault is often followed by generator current exceeding a preset instantaneous level (e.g., 120% of nominal depending on system configuration), or line voltage falling below a preset threshold (e.g., 90% of nominal).
• a preset instantaneous level e.g. 120% of nominal depending on system configuration
• line voltage falling below a preset threshold e.g. 90% of nominal
• One way to turn off switch unit 210 is to command forced commutation circuit 214 to generate a defined width commutation current pulse to commutate off the thyristor (212a or 212b) that is in its conducting state. Pulse polarity can be determined as a function of generator current polarity.
• An alternative way to turn off the switch unit uses the generator-side and/or line-side converter. Current is injected by the converter in reverse direction to the existing current in thyristors, thereby creating zero current crossover that biases the thyristors off-state. In some systems, having a converter on each side of the switch unit helps offset source impedance effects that often contribute to the delay in thyristors' response time (i.e. line impedance limiting the rate of change in the commutation current). This commutation process can occur simultaneously on all three phases of the LVRT system regardless of how many line phases are faulted.
• master controller 270 controls the operation of back-to-back conversion unit 220 to provide LVRT capability.
• the desired output of conversion unit 220 may vary depending on system design in compliance with specific grid connection standards. For example, to meet the requirements in the Spanish Grid Code, master controller 270 regulates conversion unit 220 so that 1) generator-side converter 222 receives generator power and provides reactive power to keep the generator excited and rotating at constant speed; and 2) line-side converter 224 supplies a safe amount of real power to grid 244 and injects sufficient reactive power to help stabilize the grid.
• Generator power in excess of the amount that can be safely absorbed by grid 244 is dissipated by power dissipation device 228, which consumes power in response to a regulated DC bus voltage, or can be controlled directly by matching the power dissipated to the excess generator power.
• master controller 270 also controls power factor correction unit 234 to provide reactive power in suitable amounts for improving power factor at gird connection points.
• FIGs. 4A to 4D further illustrate how an interconnection system operates to provide satisfactory electric power to utility grids in ways that conform to the Spanish Grid Code. Circuit performance during each of several stages, including a steady state and multiple transient states following a low voltage event, is described in detail below.
• wind power generation system 200 is operating in steady state with line-side voltage at nearly 100% of rated level.
• 706 kW of real power produced by turbine generator 204 is delivered entirely through switch unit 212 to transformer 242, with less than 0.3% of energy loss.
• No power passes through generator-side converter 222, line-side converter 224, or the power dissipation device (e.g., a resistor 227).
• the power factor correction unit e.g., a set of capacitors 233 provides about 250 kVAR of reactive power to excite the wind turbine generator 204. With zero net reactive output at terminal 236, electricity is being provided to the grid at a power factor of unity.
• line-side converter 224 starts to operate to supply both real and reactive current to AC line 232.
• the amount of real and reactive current transferred by line-side converter 224 is controlled such that the reactive power is twice the real power (e.g., 134kVAR and 67k W, respectively) and the total current does not significantly exceed the current rating of the turbine transformer 242. Since only a small portion of the generated power (67kW out of 706k W) is transferred to the AC line 232, energy builds up on DC bus 225.
• the net output of line-side converter 224 includes 280A of real current and 560A of reactive current. Together with the diminished reactive current provided by the power factor correction unit 233 (at 20% of line voltage, the correction unit provides 20% of rated current), the total current supplied to the transformer 242 is 663 A. This amount of total current represents only 106% of transformer rating (well within transformer capability), with an I re act ⁇ ve/I total ratio of 0.907.
• line-side converter 224 may continue to supply reactive current for an extended period (e.g., 150ms) unless line voltage exceeds a predetermined level (e.g., 110% of nominal).
• a predetermined level e.g. 110% of nominal.
• this additional supply of reactive current provides post- fault voltage support that may be desired in some systems following a major low voltage event.
• the approach described above can be generally applied in many power generation systems to provide steady- state and transient fault behaviors that satisfy the requirements of one or multiple grid interconnection standards.
• the interconnection systems described in FIGs. 2A and 2B may also be modified to allow wind turbine generators to continue to operate and supply electricity to grid under other fault conditions.
• the power electronics used in these systems can be conveniently coupled to a wide variety of wind turbine generators (e.g., Squirrel Cage Induction Generators, Doubly Fed Induction Generators, and Synchronous Generators) operating in either constant speed or variable speed modes.
• thyristors can be coupled in use with the master controller that is configured to provide such gate control signals.
• gate control thyristors include Gate Turn-Off thyristors (GTOs) and Integrated Gate-Commutated Thyristors (IGCTs).
• GTOs Gate Turn-Off thyristors
• IGCTs Integrated Gate-Commutated Thyristors
• non-thyristor solid-state devices e.g., transistors
• Line faults may be detected by the master controller upon sensing generator current exceeding a preset instantaneous level, or line voltage falling below a preset threshold.
• the master control may monitor a rate of change of line voltage and/or current together with absolute thresholds as a means of detecting a sag event.
• the line-side converter is controlled to provide power compensation by outputting reactive current that is twice the amplitude of real current.
• line-side converter may instead output zero real current while providing capacitive reactive current of an arbitrary amount (up to the overload limit of the converter).
• both line- side and generator-side converters may operate in an "overload” mode to reduce cost. Operating converters in so-called “overload” mode is described in U.S. 6,577,108, which issued on June 10, 2003 and whose disclosure is incorporated herein by reference.
• one or both of the converters may be turned on to provide additional power-factor correction.
• additional PF correction from converter(s) can potentially boost the PF to 1.0, or even to a leading (capacitive) PF when desired.

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• 2008
  • 2008-07-02 AU AU2008358896A patent/AU2008358896A1/en not_active Abandoned
  • 2008-07-02 WO PCT/US2008/068949 patent/WO2010002402A1/en active Application Filing
  • 2008-07-02 KR KR1020117002437A patent/KR20110026500A/ko active IP Right Grant
  • 2008-07-02 CA CA2728849A patent/CA2728849A1/en not_active Abandoned
  • 2008-07-02 EP EP08781252A patent/EP2311165A1/en not_active Withdrawn

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