"Protection system for a power converter connected to a doubly - fed induction generator, and method for operating the same"

TECHNICAL FIELD

The present invention relates to a protection system for power converters connected to doubly - fed induction generator, and to a method for operating said protection system.

PRIOR ART

The patent document EP1965075A1 discloses a system comprising a doubly - fed inductor generator, a grid - side converter, a grid - side converter GSC, a machine- side-converter MSC, a DC link between both converters, a crowbar connected to rotor of the generator, a first control unit for controlling the crowbar depending on the DC link voltage, and a second control unit for controlling both converters GSC and MSC. The first and second controllers are independent from each other.

The patent document US8373293B2 discloses a system comprising a doubly - fed inductor generator, a grid - side converter, a grid - ide converter GSC, a machine - side converter MSC, a DC link between both converters, a chopper connected to the DC link, a crowbar connected to rotor of the generator, a converter controller for controlling both converters GSC and MSC, and a separate protection device for controlling both the chopper and the crowbar.

DISCLOSURE OF THE INVENTION

The object of the invention is to provide a protection system for power converters connected to doubly - fed induction generator, and a method for operating said protection system, as defined in the claims. A first aspect of the invention refers to a protection system comprising a doubly - fed inductor generator comprising a rotor and a stator, which is part of a wind turbine; a grid - side converter; a machine - side converter; a DC link between both converters; a chopper connected to the DC link; a Final Hardware Protection System (FHPS) connected to the rotor of the generator; a first control unit for controlling the FHPS depending on the voltage in the DC link (it continuously reads the DC link voltage); a second control unit for controlling the chopper depending on the DC link voltage (it continuously reads the DC link voltage); and a third control unit for controlling both converters.

The chopper is responsible for absorbing the undesirable energy increase in the DC link, and the FHPS is responsible for disconnecting the generator if the DC link voltage increases to exceed a threshold value. When the FHPS is activated (it is a hardware controlled system that short circuits the rotor terminals without resistances) the wind turbine is disconnected from the grid and passes to an emergency state, unlike the crowbars described in prior art cited in the background field that include resistive elements and that are disconnected once the transient has passed. The three control units are independent from each other, each control unit having its own processor, and with said independence at least the following advantages can be obtained:

As the control units operate independently from each other, the response time to any event is very quick as there is no interference between said control units.

If the first control unit fails, the control over the chopper is not lost as the second control unit remains operative, allowing the system to remain connected to the grid and coping with FRT (Fault-Ride-Through) events.

If the second control unit fails, the control over the FHPS is not lost as the first control unit remains operative. The system can remain connected to the grid increasing energy production, if it is so required.

The same first and second control unit can also been used in other systems, for example: the first control unit can be used in other systems without a chopper; and the second control unit can be used in other systems without a FHPS, such as wind turbines of full converter topology. Therefore, said configuration of control units allows having more flexible solutions (control units adapted to be used in different systems). The control over the FHPS and the chopper are only lost if both control units fail simultaneously.

A second aspect of the invention refers to a method for controlling a protection system as the one of the first aspect of the invention. The method is adapted for being implemented in any embodiment or configuration of said protection system, it being adapted for the corresponding embodiment and/or configuration if needed.

In the method of the invention, the Final hardware protection system (FHPS) is controlled by means of the first control unit, the chopper is controlled by means of the second control unit, and the machine and grid - side converters are controlled by means of a third control unit. The three control units act independently with respect to each other, so that if any of said control units fails, the other two remains still operative.

These and other advantages and characteristics will be made evident in the light of the drawings and the detailed description thereof.

DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic view of an embodiment of a protection system.

Figure 2 shows a block representation of the control acted on a chopper of the protection system of Figure 1.

Figure 3 is an axis of ordinate defining several threshold values of the DC link voltage according to the protection system of Figure 1.

Figure 4 shows a block representation of the control acted on a FHPS of the protection system of Figure 1. DETAILED DISCLOSURE OF THE INVENTION

A first aspect of the invention refers to a protection system comprising a doubly - fed inductor generator 100 with a rotor and a stator, which is part of a wind turbine; a grid - side converter 101 ; a machine - side converter 102; a DC link 103 between both converters; a chopper 104 connected to the DC link 103; a Final Hardware Protection System (FHPS) 105 connected to the rotor of the generator 100; a first control unit 1 for controlling the FHPS 105 depending on the voltage Vbus in the DC link 103 (it continuously reads the DC link voltage Vbus); a second control unit 2 for controlling the chopper 104 depending on the DC link voltage Vbus (it continuously reads the DC link voltage Vbus); and a third control unit 3 for controlling both converters 101 and 102.

The chopper 104 is responsible for absorbing the undesirable energy increase in the DC link 103, and the FHPS 105 is responsible for disconnecting the generator 100 if the DC link voltage Vbus increases to exceed a threshold value. When the FHPS 105 is activated (it is a hardware controlled system that short circuits the rotor terminals without resistances) the wind turbine is disconnected from the grid and passes to an emergency state, unlike the crowbars described in prior art cited in the background field that include resistive elements and that are disconnected once the transient has passed.

The three control units 1 , 2 and 3 are independent from each other, each control unit 1 , 2 and 3 having its own processor. Therefore, the three control unit 1 , 2 and 3 can run in parallel. Each processor can be a PLD ('Programmable Logic Device"), a CPLD 'Complex Programmable Logic Device"), a FPGA 'Field Programmable Gate Array") or any other equivalent device. In Figure 1 a first communication link 13 between the first control unit 1 and the third control unit 3 and a second communication link 23 between the second control unit 2 and the third control unit 3 is shown, but said communication links 13 and 23 are adapted for communication between a corresponding one of the control units 1 , 2 and the third control unit 3. Additionally, this communication is a one way communication in only one direction, said direction being from the corresponding control unit 1 , 2 to the third control unit 3. Thus, said communication links 13 and 23 serve to inform the third control unit 3 about the state (operative or non-operative) of the control units 1 and 2. Additionally, the communications are limited to status updates and do not provide any direct control purpose. Thus, if these communications are interrupted, for example, the control units 1 and 2 continue controlling the FHPS 105 and the chopper 104 respectively, and the third control unit 3 continues controlling both converters 101 and 102. Due to said communications protocol, the first control unit 1 sends a corresponding state signal to the third control unit 3 when operative and the second control unit 2 sends a corresponding state signal to the third control unit 3 when operative, the third control unit 3 determining that control unit 1 , 2 is operative upon receiving the corresponding signal, and that a control unit 1 , 2 is not operative upon not receiving the corresponding state signal. The control units 1 and 2 can send the corresponding state signal continuously, or they can send said state signals periodically (with a known period). There is no communication between control units 1 and 2. Alternatively, instead of a first communication link 13 between the first control unit 1 and the third control unit 3 and a second communication link 23 between the second control unit 2 and the third control unit 3 as shown in Figure 1 , a first communication link outputting the first control unit 1 and a second communication link outputting the second control unit 2 can be serially connected by means of at least one switch, the common link being connected to the third control unit 3. Therefore, via the common link the status of the first control unit 1 and of the second control unit 2 can also reach the third control unit 3.

With the independence between the three control units 1 , 2 and 3 at least the following advantages are obtained:

As the control units 1 , 2 and 3 operate independently from each other, the response time to any event is very quick as there is no interference between said control units 1 , 2 and 3.

If the first control unit 1 fails, the control over the chopper 104 is not lost as the second control unit 2 remains operative, allowing the system to remain connected to the grid and coping with FRT (Fault-Ride-Through) events.

If the second control unit 2 fails, the control over the FHPS 105 is not lost as the first control unit 1 remains operative. The system can remain connected to the grid increasing energy production, if it is so required. The same first and second control unit 1 and 2 can also be used in other systems, for example: the first control unit 1 can be used in other systems without a chopper 104; and the second control unit 2 can be used in other systems without a FHPS 105, such as wind turbines of full converter topology. Therefore, said configuration of control units allows having more flexible solutions (control units adapted to be used in different systems). The control over the FHPS 105 and the chopper 104 are only lost if both control units 1 and 2 fail simultaneously. Each control unit 1 and 2 can also comprise a specific power supply associated, so that both control units 1 and 2 can also be autonomous. The control units 1 and 2 are normally supplied by the grid (via DC-bus) or by an UPS ("Uninterruptible Power System") comprised in the system, and if the voltage of the grid drops below a predetermined value and the UPS fails, thanks to the specific power supply, the control units 1 and 2 are operative during a predetermined time interval in order to control the FHPS 105 and the chopper 104 respectively during said time. Each specific power supply can comprise a bank or capacitors, a battery or other equivalent arrangement, designed for supplying power to the corresponding control unit 1 or 2 during said time interval.

The control over the chopper 104 works as follows:

The chopper 104 is formed by n number of -branches, each n-branch being formed by at least one semiconductor switch and a resistor connected in series (not shown in figures). Preferably, all the switches are operated simultaneously.

The second control unit 2 implements a conventional Pulse Width Modulation (PWM) control and a hysteresis control, as shown in Figure 2. If the DC link 103 voltage Vbus exceeds a threshold value previously fixed (Chopper Lower PWM Threshold CLPT), then the second control unit 2 acts on the chopper 104, generating a signal modulated by the PWM control for actuating over the switches of the chopper 104. The PWM control acts on the chopper 104 when the DC link voltage Vbus is between the CLPT threshold and a Chopper Upper PWM Threshold CUPT. In an exemplary embodiment it comprises eight regions with associated to eight modulation rates, each one defining the time interval during which the semiconductor switches of the chopper 104 are ON during a determined period T: if the modulation rate is 1 the switches are closed along all the period T; if the modulation rate is 0 the switches are opened along all the period. Each region is associated to the DC link voltage Vbus as shown in the following table:

The greater the DC link voltage Vbus the greater the energy that must be absorbed by the chopper 104, and thus the greater is the time that the switches of the chopper 104 must remain ON (closed) to absorb this energy.

If the value of the DC link voltage Vbus exceeds the Chopper Upper PWM Threshold CUPT (region 8 = 1285,50V) of the last modulation rate, it is assumed that the PWM control is not able to control the DC link voltage Vbus and the hysteresis control is activated. This hysteresis control is then activated when a Chopper Hysteresis Activation Threshold CHAT is reached, which can be equal to or, for safety purposes, lower than the Chopper Upper PWM Threshold CUPT. This means that the semiconductor switches of the chopper 104 remain closed (ON) along the period T, until the DC link voltage Vbus drops down to a Chopper Hysteresis Deactivation Threshold CHDT value (for example 997,6V, as shown in Figure 3), the PWM control being ignored until this situation occurs.

If the hysteresis control is not able to reduce the DC link voltage Vbus, the first control unit 1 causes the FHPS 105 to be activated when the DC link voltage Vbus reaches or exceeds an FHPS Activation Threshold FAT (shown in Figures 3) over the CUPT threshold, the generator 100 being disconnected from the grid. Figure 3 shows different operation areas of the control over the chopper 104:

- Area between CHDT and CLPT thresholds: normal operation of the system. The chopper 104 is OFF (opened) and the FHPS 105 is also OFF (opened). The wind turbine could operate in this area without having the control units 1 and 2 operative. In said conditions, the system is able to work at DC link voltage Vbus up to the CLPT threshold.

- Area between CLPT and CUPT thresholds: The second control unit 2 acts on the chopper 104, and the FHPS 105 is OFF. The second control unit 2 controls the chopper 104 implementing the PWM control. The wind turbine could operate in this area without having the first control unit 1 operative. The third control unit 3 is communicated with the other two control units 1 and 2, so, if the first control unit 1 fails, it is able to detect that said first control unit 1 has failed and passes the system to an emergency state (and decouples the system from the grid for example) if the Chopper Hysteresis Activation Threshold CHAT is reached in order to protect the system components.

- Area between CHAT and FAT thresholds: The second control unit 2 acts on the chopper 104, and the FHPS 105 is OFF. The second control unit 2 controls the chopper 104 implementing the hysteresis control. The wind turbine could operate in this area without having the first control unit 1 operative, in this case, the third control unit 3 detects said situation and if the DC link voltage Vbus reaches the FHPS Activation Threshold FAT, it passes the system to an emergency state (and decouples the system from the grid for example).

When working in areas between the CLPT and FAT thresholds, if the first control unit 1 is not operative, the system is also able to work. - DC link voltage Vbus higher than FHPS Activation Threshold FAT threshold: The chopper 104 is OFF (opened) and the first control unit 1 causes the FHPS 105 to be closed (ON). The wind turbine could operate in this area even without having the second control unit 2 operative. The second control unit 2 informs the third control unit 3 about its state continuously, in particular if it is operative or not, if the switches of the chopper 104 are ordered to be ON or OFF, and if there is an overload on any switch or any resistor of the chopper 104. To detect an overload, the time that the switch is being fired is taken into account, taking also into account the time that it is going to be off (opened) during each period due to that the cooling during the off state is slower than the heating during ON state. Said time affects to the switch itself and also to the associated resistor, but due to the different characteristic of a switch and of a resistor, the switches and the resistors are preferably treated independently (one can be overloaded before the other one for example).

For example:

Overload of a switch:

o TON MAX: 50ms (max. time for overload)

o TOFF MAX: 250ms (cooling total time)

o THYST: TOFF MAX / 2 = 125ms (cooling time to deactivate the error) Overload of a resistor:

o TON MAX: 270ms (max. time for overload)

o TOFF MAX: 600s (cooling total time)

o THYST: TOFF MAX / 2 = 300s (cooling time to deactivate the error)

During an overload the firings over the switches are preferably ordered to be OFF, unless a higher-level priority reason occurs. The control over the FHPS 105 works as follows: As noted before, the system comprises a Final Hardware Protection System (FHPS) 105 comprising a thyristor, for the situations where the DC link voltage Vbus exceeds the FHPS Activation Threshold FAT value, controlled by a first control unit 1. The generator 100 is then stopped and decoupled from the grid. If said situation occurs, the first control unit 1 sends an appropriate signal to the thyristor of the FHPS 105 to short-circuit the rotor until said DC link voltage Vbus drops to the Chopper Hysteresis Deactivation Threshold CHDT (thus also defines as FHPS Deactivation Threshold FDT), as shown in Figure 4.

The first control unit 1 informs the third control unit 3 about its state continuously, in particular if it is operative or not, and if the FHPS 105 is ON or OFF. A second aspect of the invention refers to a method for controlling a protection system as the one of the first aspect of the invention. The method is adapted for being implemented in any embodiment or configuration of said protection system, it being adapted for the corresponding embodiment and/or configuration if needed. In the method of the invention, the FHPS 105 is controlled by means of the first control unit 1 , the chopper 104 is controlled by means of the second control unit 2, and the machine and grid - side converters 102 y 101 are controlled by means of a third control unit 3. The three control units 1 , 2 and 3 act independently with respect to each other, and, therefore, they can run in parallel.

In the method, in addition, a state signal is sent, from each of the first control unit 1 and the second control unit 2 to the third control unit 3, when the first control unit 1 and the second control unit 2 are operative. Using said third control unit 3, it is determined that the first control unit 1 and the second control unit 2 are operative in response to receiving the corresponding state signal from the first control unit 1 and the second control unit 2, and that the first control unit 1 and/or the second control unit 2 are not operative in response to not receiving the corresponding state signal.

If the third control unit 3 determines that the first control unit 1 is not operative, the second control unit 2 controls the chopper 104 if the voltage Vbus in the DC link 103 is between a chopper lower threshold CLPT value and a chopper hysteresis activation threshold CHAT value, and the third control unit 3 decouples the system from the grid if the DC link voltage Vbus reaches the FHPS activation threshold FAT, which is higher than the chopper hysteresis activation threshold CHAT.

If the third control unit 3 determines that the second control unit 2 is not operative, the chopper 104 is not controlled and the first control unit 1 controls the FHPS 105 if the DC link voltage Vbus reaches the FHPS activation threshold FAT, which is higher than the chopper hysteresis activation threshold CHAT.

The explanations given for the operations of the control units 1 , 2 and 3 when referring to the first aspect of the invention are also valid for the operation of said control units 1 , 2 and 3 when implementing the method of the second aspect of the invention.