Fault detection system for a generator

A fault detection system is provided, and includes a generator, a power converter, a breaker, a current monitoring device, and a control module. The power converter is selectively connected to the generator and is selectively activated to produce a test voltage. The breaker is located between the generator and the power converter for selectively connecting the generator to the power converter. The breaker includes an open position and a closed position. The current monitoring device is located between the generator and the power converter. The current monitoring device measures a line current between the generator and the power converter. The control module is in communication with the current monitoring device and the power converter. The control module has a memory with a threshold current value.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to a fault detection system system, and more specifically to a fault detection system including a generator and a power converter that is selectively connected to the generator.

Wind turbines are one alternative to conventional sources of energy. A wind turbine includes multiple blades that are connected to a rotor, where spinning of the blades by wind spins a shaft of the rotor. The rotor is connected to a generator that generates electricity. Some wind turbines include a gearbox that couples the rotor to the generator. Various types of generators are employed in wind turbines such as, for example, induction machines and separately excited synchronous machines. Specifically, one type of generator that may be employed is a permanent magnet generator. The permanent magnet generator allows for higher efficiency but, because the permanent magnet generator is excited by permanent magnets, the permanent magnet generator is almost always capable of producing voltage when spinning Thus, as long as the permanent magnet generator is spinning, voltage will typically be produced. However, a fault may occur on the output of the permanent magnet generator while voltage is being produced. Thus, a breaker may be provided to the output of the generator to generally prevent the current flow in the event of a fault.

A power converter system may also be employed to convert the variable frequency electrical output power of the generator to an AC electrical power that matches the frequency of an electrical grid. Sometimes the power converter is not activated. For example, the power converter may not be activated if there is not adequate wind, maintenance work is being performed on the wind turbine, or in the event the wind turbine is nonoperational.

A fault may occur between the cables that connect the generator to the power converter. For example, the fault may be a short or an open circuit. In the event the power converter is activated, detection of a fault between the generator and the power converter is typically relatively simple to detect. While operating, both the power converter and the breakers an internal trip relay will detect the fault. The power converter will shut down and the breaker will open. However, in the event the power converter is not activated, this fault may go undetected. When a generator breaker is in a closed position, the power converter may or may not be activated. One approach for detecting a fault between the generator and the power converter when the power converter is not activated but the breaker is closed involves employing an internal trip relay in the generator main circuit breaker. However, the internal trip relay may not always be reliable at lower frequencies, and may also be prone to overheating while the power converter is activated due to the current harmonics produced by the power converter.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a fault detection system is provided, and includes a generator, a power converter, a breaker, a current monitoring device, and a control module to detect a fault when the breaker is in the open position. The power converter is selectively connected to the generator and is selectively activated to produce a test voltage. The breaker is located between the generator and the power converter for selectively connecting the generator to the power converter. The breaker includes an open position and a closed position. The current monitoring device is located between the generator and the power converter. The current monitoring device measures a line current between the generator and the power converter. The control module is in communication with the current monitoring device and the power converter. The control module has a memory with a threshold current value. The control module includes control logic for monitoring the breaker to determine if the breaker is in one of the open position and the closed position. The control module includes control logic for activating the power converter to produce the test voltage if the breaker is in the open position. The control module includes control logic for monitoring the current monitoring device for the line current. The control module includes control logic for determining if the line current is greater than the threshold current during a period of time while the test voltage is being produced. The control module includes control logic for determining that a fault is present if the breaker is in the open position and if the line current is greater than the threshold current. In one embodiment, the control module includes control logic for maintaining the breakers in the open position if the fault is detected. In another embodiment, a voltage monitoring device is provided between the generator and the power converter for measuring a line voltage, and the control module is in communication with the voltage monitoring device.

According to another aspect of the invention, a wind power plant having a fault detection system is provided having a wind turbine, a gearbox connected to the wind turbine through a shaft, a generator connected to the shaft, a power converter, a breaker, a current monitoring device, a voltage monitoring device, and a control module. The power converter is selectively connected to the generator for being selectively activated to produce a test voltage. The breaker is located between the generator and the power converter for selectively connecting the generator to the power converter. The breaker includes an open position and a closed position. The current monitoring device is located between the generator and the power converter. The current monitoring device measures a line current. The voltage monitoring device is provided between the generator and the power converter for measuring a line voltage. The control module is in communication with the current monitoring device, the voltage monitoring device, the generator, the breaker and the power converter. The control module has a memory with a threshold current, a threshold voltage, and an expected voltage stored therein.

The control module includes control logic for monitoring the breaker to determine if the breaker is in one of the open position and the closed position. The control module includes control logic for activating the power converter to produce the test voltage if the breaker is in the open position. The control module includes control logic for monitoring the current monitoring device for the line current. The control module includes control logic for determining if the line current is greater than the threshold current during a period of time while the test voltage is being produced. The control module includes control logic for determining that a fault is present if the breaker is in the open position and if the line current is greater than the threshold current or if the line voltage differs from the expected voltage by a specified voltage value. The control module includes control logic for calculating a measured voltage of the generator. The measured voltage of the generator is based on the line voltage, the line current, a cable resistance value, a cable inductance value, an internal inductance of the power converter and at least one parameter of the generator. The control module includes control logic for calculating an estimated voltage of the generator. The estimated voltage of the generator is based on the rotational speed of the generator, the line current, and the at least one parameter of the generator. The control module includes control logic for calculating a magnitude of the measured voltage of the generator and a magnitude of the estimated voltage of the generator. The control module includes control logic for determining the difference between the magnitude of the measured voltage of the generator and the magnitude of the estimated voltage of the generator. The control module includes control logic for determining a generator fault is present if the difference between the magnitude of the estimated voltage of the generator and the magnitude of the measured voltage of the generator is greater than the threshold voltage, and if the breaker is in the closed position.

DETAILED DESCRIPTION OF THE INVENTION

As used herein the terms module and sub-module refer to an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.

Referring now toFIG. 1, a schematic exemplary wind power plant10is illustrated. The wind power plant10includes a wind turbine20, a gearbox22, a generator24, a main power convertor26, a converter controller28, and a main transformer30. The wind turbine20includes a hub40and a plurality of rotor blades42that are connected to the hub40. A shaft44transfers energy from the wind turbine20to the generator24through the gearbox22. The rotational speed of the shaft44is dependent on wind speed. The generator24is typically any type of machine that converts the mechanical energy from the shaft44into electrical energy such as, for example, a permanent magnet generator. Typically, a set of three connections46of the generator24are connected to the main power converter26, and are labeled as ‘A’, ‘B’ and ‘C’. Each of the connections46also include a breaker48for selectively interrupting the flow of current between the generator24and the main power converter26. In one embodiment, the breaker48may be a three-phase breaker. Referring now toFIGS. 1-2, a fault detection system50is employed for detecting faults between the generator24and the main power converter26. For example, the fault detection system50may be able to detect a short or open circuit condition between the generator24and the main power converter26. The fault detection system50may also determine if a tachometer66(shown inFIG. 2) measuring the rotational speed of the generator24is not functioning. AlthoughFIG. 1illustrates a wind power plant10, it is to be understood that the generator24and the main power convertor26may be used in other applications as well.

The main power convertor26generally includes circuitry for converting the variable frequency AC voltage from the generator24into a voltage that may be supplied to an AC grid (not shown). Specifically, the main power converter26is selectively activated to produce an output voltage, which is the AC voltage supplied to the AC grid. The main power converter26may include various power switching devices such as, for example, insulated gate bipolar transistors (IGBTs) or integrated gate-commutated thyristors (IGCTs). The power converter26includes a generator side converter52and a grid or line side converter54. The generator side converter52receives an AC input voltage from the generator24and provides for conversion of the AC input voltage into a DC voltage. The generator side converter52provides the DC voltage to the line side converter54through a DC bus that includes a DC link capacitor60(shown inFIG. 2). The grid side converter54converts the DC voltage to an AC output voltage that is fed to the AC grid (not shown).

Breaker48includes a closed position as well as an open position. In the closed position, current may flow in the connections46between the generator24and the main power converter26. In the open position, current is generally unable to flow in the connections46between the generator24and the main power converter26. Thus, the breaker48selectively connects the generator24to the main power converter26.

FIG. 2is a single line diagram of a portion of the wind turbine10including the fault detection system50. A current monitoring device62and a voltage monitoring device64are each located between the generator24and the main power converter26. The current monitoring device62measures a line current of the connection46between the generator24and the main power converter26. Specifically, referring toFIG. 3, an illustration of three current monitoring devices62measuring the specific line currents associated with connections ‘A’, ‘B’ and ‘C’ is shown. Referring back toFIG. 2, the voltage monitoring device64measures a line voltage of the connection46between the generator24and the main power converter26. An illustration of two voltage monitoring devices64is also shown in detail inFIG. 3. In one embodiment, the current monitoring device62and the voltage monitoring device64may be located at the terminals of the main power converter26to measure the current and voltage at the main power converter26.

The control module28is in communication with the generator side converter52, the line side converter54, the current monitoring device62, the voltage monitoring device64, the breaker48and an encoder or tachometer66of the generator24. The tachometer66is used to monitor the rotational speed of the generator24. The control module28includes control logic for determining if a fault exists in the connection46between the generator24and the main power converter26, within the main power converter26, with the tachometer66, a fault in the generator24or between the generator24and the breaker48when the breaker48is closed. The control module28includes a memory. The memory of the control module stores a threshold current, a threshold voltage, and an expected voltage VExpected, which are used to determine if a fault exists. In one embodiment, the threshold current value may range from about zero to about 300 Amps, and the threshold voltage value may range from about 150 to about 600 Volts.

In one approach, the fault detection system50determines if a fault exists in the event the breaker48is in the open position. In this approach, the control module28includes control logic for determining if there is a short circuit condition either within the main power converter26or between the main power converter26and the breaker48. In another approach, the control module28also includes control logic for determining if a fault condition exists if the breaker48is in the closed position, and where the main power converter26may or may not be supplying output voltage to the generator24.

Turning now toFIG. 3, an approach for connecting the current monitoring devices62and the voltage monitoring devices64to the connections46between the generator24and the main power converter26is illustrated. Specifically, each of the connections ‘A’, ‘B’ and ‘C’ include a current monitoring device62and at least one voltage monitoring device64. The output of the current monitoring devices62include IA, IB, and IC, where IAis the measured current in line ‘A’, IBis the current in line ‘B’, and ICis the current in line ‘C’. The output of the voltage monitoring devices64include VAB, and VBC, where VABis the measured voltage that appears between lines ‘A’ and ‘B’, and VBCis the voltage that appears between lines ‘B’ and ‘C’. The values IA, IB, IC, VAB, and VBCare sent to the control module28.

FIG. 4is a block diagram of an approach for determining if a fault condition exists if the breakers48are in the open position. Specifically, a test sequencer67is provided. The test sequencer67may be part of the control module28(shown inFIG. 2). Alternatively, in another embodiment the test sequencer67is a stand-alone unit. Referring now to bothFIGS. 2 and 4, the control module28includes control logic for monitoring the current monitoring device62for the line current and the voltage monitoring device64for the line voltage. The control module also includes control logic for monitoring the breaker48to determine if the breaker48is in the open position or the closed position. The control module28includes control logic for determining a DC link voltage by monitoring the DC link capacitor60. The control module also includes a line inductance value that is specific to the system and is saved in the memory of the control module28. The control module28includes control logic for sending signals to the test sequencer67indicating a breaker position, the DC link voltage, and the line inductance.

The test sequencer67includes control logic for calculating a Gating Command signal for a specific duration of time. The specific duration of time is indicated inFIG. 4as a Gating Duration T. Specifically, the Gating Command signal is applied to each of the connections46that are denoted as ‘A’, ‘B’ and ‘C’ in a sequencing fashion. In one embodiment the Gating Command signal is first sent to the connection ‘A’. Then, the Gating Command signal is sent the connection ‘B’. Then, the Gating Command signal is sent to the connection ‘C’. Voltage pulses are also produced by simultaneously gating two phases, for example a voltage pulse produced by connections ‘A’ and ‘B’, connections ‘B’ and ‘C’, and connections ‘C’ and ‘A’. A block68labeled Gating Control receives the Gating Command from the test sequencer67, and sends a command to the Power Bridge69to create a test voltage that is sent through the connections46. In the embodiment as shown inFIG. 4, the test voltage is a voltage pulse V. The Power Bridge69is the part of the control module28that contains various power switching devices (not shown) such as, for example IGBTs or IGCTs, which create the voltage pulse V. The voltage pulse V is then impressed on one or more of the connections46labeled ‘A’, ‘B’, or ‘C’. The voltage pulse V is a voltage pulse that is configured for lasting for a period of time that is the Gating Duration T (sent from the test sequencer67). The magnitude of the voltage pulse V is the expected voltage VExpectedthat is saved in the memory of the control module28but with some difference due to the tolerances of the electronics, and is denoted as VCommand.

Continuing to refer toFIGS. 2 and 4, the current monitoring device62and the voltage monitoring device64are in communication with the connections46and measure the line current and the line voltage to determine if a fault condition is present during the Gating Duration T. Specifically, the control module28includes control logic for determining if the line current is greater than the threshold current saved in the memory of the control module28when the voltage pulse V is sent. In the event that the line current is greater than the threshold current, the control module28includes control logic for indicating that a fault condition exists. That is, a short circuit is present either in the main power converter26or between the main power converter26and the breaker48. The control module28also includes control logic for determining if the line voltage is a different value than the expected voltage VExpectedwhen the voltage pulse V is sent. Specifically, if the line voltage varies by more than a specified voltage value from the expected voltage VExpected, this is an indication that a fault condition exists. For example, in one embodiment, if the line voltage varies by more than about 25% of the expected voltage VExpected, this is an indication that a fault condition exists, however it is to be understood that other variations may occur as well. In the event a fault condition is detected, the control module28includes control logic for maintaining the breakers48in the open position.

FIG. 5is a block diagram of an approach for determining if a fault condition exists that is executed by the control module28(shown inFIG. 2). In the approach as shown inFIG. 5, the breakers48are in the closed position and the main power converter26may or may not be supplying output voltage to the generator24. The values IA, IB, IC, VAB, VBCand the rotational speed of the generator24(shown inFIGS. 1-2), which is denoted as SpeedGenTach, are used in the approach. Referring toFIG. 2, the tachometer66is used to monitor the rotational speed of the generator24. Turning back toFIG. 5, the control module28includes control logic for receiving the measured voltages VABand VBC. The measured voltages VABand VBCrepresent the voltages that are either measured between the generator24and the main power converter26at the connections46(shown inFIG. 2), or alternatively at the terminals of the main power converter26. The control module28includes control logic for providing the measured voltages VABand VBCto a demodulator70. The demodulator70includes control logic for converting the measured voltages VABand VBCinto a DC reference frame. Specifically, the control module28includes control logic for converting the measured voltages VABand VBCthat are in a stationary three-phase frame into a rotating dq0 frame, where variables and ‘q’ are DC quantities. Specifically, in one embodiment the control module28may use Park's Transformation (also referred to as direct-quadrature-zero transformation) to convert the measured voltages VABand VBCinto DC reference measured voltages VDCONVand VQCONV.

The control module28further includes control logic for calculating measured voltages of the generator24(shown inFIGS. 1-2) in block72. The measured voltages of the generator24are denoted as VDgenMeasand VQgenMeas, and are based on the DC reference measured voltages VDCONVand VQCONV, the measured current values IA, IB, IC, a cable resistance of the connections46, an inductance of the connections46, an internal inductance of the power converter26, and certain parameters of the generator24. Specifically, for example, some of the parameters of the generator24that may be used to calculate the measured voltage of the generator24include, but are not limited to, a d-axis synchronous reactance, a q-axis synchronous reactance, a stator resistance, and a generator flux. A block74is then provided to calculate a magnitude of the measured voltages of the generator VDgenMeasand VQgenMeas. The magnitude of the measured voltages of the generator is denoted by VMagGenMeas.

The control module28includes control logic for calculating an electrical frequency ω based on the rotational speed of the generator SpeedGenTach. Calculation of the electrical frequency ω is performed in block80. The control module28includes control logic for estimating voltage at the generator24based on the electrical frequency ω of the generator24, the measured current values IA, IB, IC, generator flux, and the specific parameters of the generator24in block82. The estimated voltages of the generator24are denoted as VDgenEstand VQgenEst. A block84is then provided to calculate the magnitude of the estimated voltages of the generator VDgenEstand VQgenEst. The magnitude of the estimated voltages of the generator is denoted by VMagGenEst.

The magnitude of the estimated voltages of the generator VMagGenEstand the magnitude of the measured voltages of the generator VMagGenMeasare then both sent to block88. The control module28includes control logic for determining the difference between the magnitude of the estimated voltages of the generator VMagGenEstand the magnitude of the measured voltages of the generator VMagGenMeas. The control module28further includes control logic for comparing the difference between the magnitude of the estimated voltages VMagGenEstand the magnitude of the measured voltages of the generator VMagGenMeaswith the threshold voltage that is saved in the memory of the control module. If the difference between the magnitude of the estimated voltages of the generator VMagGenEstand the magnitude of the measured voltages of the generator VMagGenMeasis greater than the threshold voltage, this is an indication that there is either a short circuit or an open circuit present in the generator24(shown inFIG. 2), or if the tachometer66(shown inFIG. 2) measuring the rotational speed of the generator24is not functioning. In the event a fault condition is detected, the control module28includes control logic for switching the breaker48(shown inFIGS. 1-2) into the open position.

The fault detection system50as illustrated inFIGS. 1-5determines if a fault exists in the event the breakers48are in the open position. Moreover, the fault detection system50as described will provide protection to the various components of the wind turbine10in the event that a fault condition occurs and the breakers48are in the closed position.