Power converters

DESCRIPTION

Technical Field

The present invention relates to power converters, and in particular to power converters that can be used to interface a generator that provides variable voltage at variable frequency to a power grid or supply network.

BackRround Art

A generator can be used to convert mechanical energy into electrical energy. A generator normally includes a rotor and a stator. The rotor is rotated using mechanical energy causing an ac voltage to be developed at the stator (the "stator voltage"). The rotor can be connected to, or driven by, the output shaft of a turbine or prime mover such as a wind turbine, a tidal turbine, a hydro-turbine, a steam turbine engine, a diesel engine or a gas turbine engine, for example.

The frequency of the stator voltage is directly proportional to the speed of rotation of the rotor. It is often desirable for the speed of rotation of the rotor to be strictly controlled so that the frequency of the stator voltage is substantially constant. However, in many cases this is simply not possible. The speed of rotation of the output shaft of a wind turbine, for example, will vary according to the speed of the wind driving the turbine blades. Although the speed of rotation of the output shaft can be controlled to be within certain limits by altering the pitch of the turbine blades, this is not sufficient to produce a constant stator voltage having constant frequency. However, constant voltage and frequency is often an important requirement if the electrical power produced by the generator is going to be fed back to a power grid or supply network. The stator voltage must therefore be conditioned and converted using a power converter.

In a power converter described in European Patent Application EP 0884883, back-to- back pulse width modulated voltage source inverters are used with a common

capactive dc link filter to condition the stator voltage. The terminals of the generator are connected to a first pulse width modulated voltage source inverter whose function is to step-up the dc link voltage to a value that is greater than the peak of the ac line voltage in the power grid or supply network. A second pulse width modulated voltage source inverter then derives the voltage that is fed to the power grid or supply network from the common dc link voltage.

In a different power converter described in United States Patent Application 2004/0201221 the terminals of the generator are connected to a diode rectifier, which is much easier to implement than the pulse width modulated voltage source inverter mentioned above. The output of the diode rectifier is connected to the first stage dc link filter of a dc to dc converter, whose function is to step-up the dc link voltage to a value that is greater then the peak of the ac line voltage in the power grid or supply network, and to feed this voltage to a second stage dc link filter. A pulse width modulated voltage source inverter derives the voltage that is fed to the power grid or supply network from the second stage dc link voltage.

Summary of the invention

The present invention provides a power converter that can be used to interface a generator that provides variable voltage at variable frequency to a power grid or supply network operating at nominally fixed voltage and nominally fixed frequency, the power converter comprising, the power converter comprising: a machine rectifier electrically connected to the stator of the generator; and a step-up inverter electrically connected to the machine rectifier.

The term "machine rectifier" as used herein refers to any rectifier that does not require any form of control (such as a conventional diode rectifier, for example) or to a rectifier where only minimal control is needed (such as a conventional thyristor rectifier that can be phase controlled, for example). More particularly, the machine rectifier can be a conventional diode bridge rectifier of industry- standard topology, which is an uncontrolled rectifier where the dc output voltage is directly related to the ac input voltage (in this case, the stator voltage). The machine rectifier can also be a

thyristor bridge rectifier of industry-standard topology (such as a Graetz bridge), which is a phase controlled rectifier. Phase control can be achieved by point in cycle control of the thyristor turn on so that the dc output voltage can be adjusted below that produced by an uncontrolled rectifier, and in fact to zero. In the present invention, a thyristor bridge rectifier has the advantage of being able to regulate current in the dc link filter in the event that the load impedance is very low (see below). A thyristor bridge rectifier also has the ability to reverse the dc output voltage, a feature that may be employed to rapidly force current out of the dc link filter for protective purposes or to provide electronic isolation of the generator output. It will be readily appreciated that other suitable naturally commutated topologies (including brushless electronic equivalents) can also be used.

The term "nominally fixed" as used herein refers to the fact that the voltage and frequency of the power grid or supply network may vary slightly between predetermined limits during normal operation. The voltage and frequency may also vary during network fault and transient conditions.

The power converter of the present invention is much simpler to implement than the known power converters mentioned above. For example, in the case of the power converter that uses back-to-back pulse width modulated voltage source inverters, the generator-side inverter is much more complicated than a naturally commutated machine rectifier. This is because the generator-side inverter must be controlled using pulse width modulation and this requires the use of several power semiconductors and control circuits. The use of the machine rectifier in place of the generator-side inverter therefore means that the power converter according to the present invention will experience less power dissipation and improved reliability.

In the case of the power converter described in United States Patent Application 2004/0201221, both the step-up dc to dc converter and the pulse width modulated voltage source inverter must be controlled using pulse width modulation. In the power converter according to the present invention, the step-up dc to dc converter, the first and second stage dc link filters, and the pulse width modulated voltage source

inverter are effectively replaced by a single step-up inverter so that only one fully controlled stage is required. The reduction in the number of power semiconductors and control circuits means that the power converter according to the present invention will experience less power dissipation and improved reliability.

The use of a machine rectifier as the interface with stator of the generator provides a number of other benefits.

(i) It has the simplest possible power converter topology and requires no (or in the case of a thyristor rectifier the most minimal) control input.

(ii) It has the maximum possible efficiency and reliability, gravimetric and volumetric power density.

(iii) It is the least expensive form of rectifier.

(iv) It allows the use of novel air gap flux distributions and generator output maximisation. For example, the circumferential distribution of air gap flux may be trapezoidal as opposed to the conventional sinusoidal distribution, in order to maximise the mean air gap shear stress that is achievable, using a given maximum air gap flux density.

(v) It facilitates the physical integration of the rectifier within the generator.

These benefits are achieved without the need for a step-up dc to dc converter and two capacitive dc link filters, as described in United States Patent Application 2004/0201221.

The step-up inverter is preferably a current source inverter, having comparable performance characteristics to the voltage source inverters used in the known power converters described above. Such a step-up inverter may be implemented using ALSPA CDM8000 previously available from ALSTOM Power Conversion Limited, Boughton Road, Rugby, CV21 IBU, United Kingdom or the Powerflex 7000 available from Rockwell Automation UK at Pitfield Farm, Milton Keynes, MKI l 3DR, United Kingdom. The step-up inverter will condition the variable rectified voltage output from the generator to meet the operating requirements of the power

grid or supply network. The output from the step-up inverter may be a low distortion single or multiphase voltage, with regulated voltage, frequency and current, and may additionally source and sink regulated quantities of reactive power. The step-up inverter may supply power to a dedicated or localised supply network or to an existing energised supply network such as the national power grid, for example.

The power converter preferably further comprises a dc link filter electrically connected between the machine rectifier and the step-up inverter, and an ac line filter electrically connected to the output of the step-up inverter. At least one protective switchgear can be provided to isolate the power converter and the generator from the power grid or supply network. If necessary, an output transformer can be connected to the output of the step-up inverter.

The operation of the step-up inverter is preferably controlled by a converter controller. The converter controller may receive one or more reference signals indicative of the operating requirements of the power network and outputs a series of control signals to the step-up inverter. The converter controller may also receive input signals indicative of the ac voltage and current that are supplied from the step- up inverter and the dc link current that is supplied to the step-up inverter.

The converter controller preferably controls the operation of the step-up inverter using a pulse width modulation strategy.

The power converter may include a surge arrester circuit that can be operated in the event of a short circuit in the power grid or supply network, or during other operating circumstances when the generator output is in excess of the capacity of the supply network to absorb power, for example, when the supply network frequency is higher than the required level.

The present invention further provides a generator system comprising: a generator that provides variable voltage at variable frequency; and

a power converter as described above for interfacing the generator to a voltage power grid or supply network operating at nominally fixed voltage and nominally fixed frequency.

The generator includes a rotor and a stator, and the rotor can be connected to, or driven by, the output shaft of a turbine or prime mover such as a wind turbine, a tidal turbine, a hydro-turbine, a steam turbine engine, a diesel engine or a gas turbine engine, for example. The generator can be exited by any suitable means. Possible exciter means include permanent magnets or superconducting field windings but it is generally preferred that the form of excitation is capable of large scale, rapid adjustments that are necessary to stabilise the output voltage of the generator under conditions of fluctuating rotor speed.

In the case where the rotor is driven by a wind turbine, the wind turbine preferably includes at least one turbine blade mounted on a shaft. The main shaft of the wind turbine can be connected to the rotor of the generator through a gearbox, although this is not essential. The relative pitch of the at least one turbine blade can be controlled by a pitch actuator.

The present invention further provides a power conversion method for converting a variable voltage at variable frequency supplied by a generator to a nominally fixed voltage at nominally fixed frequency, the method comprising the steps of: rectifying the variable voltage and variable frequency output of the generator to provide a variable dc link voltage; and supplying the variable dc link voltage to a step-up inverter.

The method may further comprise the step of filtering the variable dc link voltage before it is supplied to the step-up inverter.

The step of rectifying the variable voltage and variable frequency output of the generator is preferably carried out using a machine rectifier and the step-up inverter is preferably a current source inverter, both as described above.

The method may further comprise the step of supplying the output of the step-up inverter to a power grid or supply network operating at a nominally fixed voltage and nominally fixed frequency, and wherein the step-up inverter is regulated to provide the nominally fixed voltage and nominally fixed frequency of the power grip or supply network, whilst simultaneously providing stabilising influences to the voltage and frequency of the power grid or supply network.

Drawings

Figure 1 is a schematic drawing showing how a power converter according to the present invention is used to interface between a wind turbine driving a variable speed generator and a fixed frequency power network;

Figures 2a and 2b are schematic drawings showing the operation of the current source inverter that forms part of the power converter of Figure 1 ;

Figure 3 is a schematic drawing showing the space vectors for the current source inverter of Figures 2a and 2b;

Figure 4 is a schematic diagram showing a first control loop hierarchy of the power converter of Figure 1, without a power reference;

Figure 5 is a schematic diagram showing a second control loop hierarchy of the power converter of Figure 1, with a power reference;

Figure 6 is a schematic diagram showing the protective circuits for the current source inverter of Figures 2a and 2b; and

Figure 7 is a schematic diagram showing fault current limitation using a phase controlled thyristor rectifier.

With reference to Figure 1, a power converter is used to interface between a wind turbine driving a variable speed generator 4, and a fixed frequency power network 14. The wind turbine includes a pair of turbine blades 1 (three or more turbine blades are also possible) mounted on an output shaft and whose collective and relative pitch can be controlled by means of a pitch actuator 2 in order to optimise the conversion of wind energy to output shaft power. A gearbox 3 is used to connect the output shaft to the rotor of the variable speed generator 4. In some cases, the output shaft can be

connected directly to the rotor of the variable speed generator. It will therefore be readily appreciated that the speed of rotation of the rotor varies as a function of the wind speed and that the frequency of the voltage developed at the stator of the generator 4 (the "stator voltage") may therefore vary over wide ranges.

The terminals of the generator 4 are connected to a diode rectifier 5. The dc output voltage of the diode rectifier 5 is also variable and is fed, via a dc link filter inductor 6, to a current source inverter 7.

The ac output voltage of the current source inverter 7 is fed to an ac line filter capacitor 8. When the stator voltage is low then the dc output voltage from the diode rectifier 5 will be much lower than the peak of the sinusoidal voltage that must be output from the current source inverter 7 and applied to the ac line filter capacitor 8. As a result, the current source inverter 7, in combination with the dc link filter inductor 6 and the ac line filter capacitor 8, must have a voltage step-up capability. However, unlike in the known power converters described above, this step-up capability is not achieved using back-to-back pulse width modulated voltage source inverters or a step-up dc to dc converter.

If the power network 14 requires particularly low voltage distortion then additional ac line filters can be used. For example, the power converter may include an ac line filter inductor 9 and capacitor 10.

The filtered ac voltage from the current source inverter 7 passes through a first protective switchgear 11, an output transformer 12 and a second protective switchgear 13. These provide a reliable connection to the power network 14. The second switchgear 13 isolates the generator system from the power network 14 in the event that the generator system experiences a major equipment failure.

A pitch controller 21 is used to regulate the pitch actuator 2. The pitch controller 21 responds to a set of position references 18 that are generated by an external system

and controls the pitch actuator 2 to alter the pitch of the turbine blades 1 in order to maximise the aerodynamic power conversion efficiency of the generator system.

The current source inverter 7 is regulated by a converter controller 22 (described in more detail below) so that the operating requirements of the power network 14 and the generator 4 can be satisfied. The converter controller 22 receives input signals 15 indicative of the voltage and current that is being fed to the power network 14, and an input signal 16 indicative of the dc link current supplied to the current source inverter 7 by the diode rectifier 5. The pitch controller 21 also provides a power reference 25 so that the output of the generator 4 can be coordinated with the operating conditions of the wind turbine. An external system generates a set of power distribution references 19 (which may be constant or varying) according to the operating requirements of the power network 14. The power distribution references 19 may include the ac line voltage, real power, reactive power magnitude and polarity, ac line current limit, ac line current-dependent voltage drop and other variables necessary to make sure that the ac output voltage stipplied by the power converter will integrate properly with the power network 14.

The converter controller 22 satisfies the requirements set by the power reference 25 and the power distribution references 19 by regulating the current flowing in the dc link filter inductor 6 and the ac line filter capacitor 8 according to a pulse width modulation strategy. This is described in more detail below.

The converter controller 22 is also able to manage without the power reference 25 by delivering all of the ac output voltage from the current source inventor 7 to the power network 14 in accordance with the power distribution references 19.

Figures 2a and 2b show the two fundamental modes of a pulse width modulation strategy that can be used to control and regulate the current source inverter 7. The current source inverter 7 consists of three-phase bridges where three output lines A ₅ B and C can be connected to two input lines in a predetermined configuration through an array of switches that are opened and closed in accordance with the pulse width

modulation strategy. For convenience, each switch is shown as a series-connected switch (preferably a semiconductor device such as an Insulated Gate Bipolar Transistor (IGBT) ₃ Metal Oxide Semiconductor Field Effect Transistor (MOSFET) ₅ Integrated Gate Commutated Thyristor (IGCT), MOS-Controlled Thyristor (MCT) and Gate Turn Off Thyristor (GTO), for example) and a diode. However, each switch can also be a semiconductor device having reverse blocking capabilities such as a Reverse Blocking-Insulated Gate Bipolar Transistor (RB-IGBT) or Reverse Blocking- Gate Turn Off Thyristor (RB-GTO) or Reverse-Blocking Integrated Gate Commutated Thyristor (RB-IGCT), for example. The RB-GTO and its derivatives are particularly preferred because they can be implemented in series connected strings of pressure contact format (press pack) devices, in order to provide a high current rating and high voltage capability and with n+1 series redundancy. This facilitates the manufacture of a power converter with high power ratings (for example, greater than 2 MW) and high voltage ratings (for example, greater than 3.3 IcV line).

Figure 2a shows two different states of the so-called "boost mode". In state 1, the dc output voltage from the diode rectifier 5 is short circuited by closing both of the switches associated with any of the three output lines (i.e. in one phase). The dc link current increases and energy is stored in the dc link filter inductor 6 (the current flow is represented by the continuous arrow labelled 1). In state 2, the dc link current is directed around any two of the output lines by closing the appropriate switches and the energy that has been stored in the dc link filter inductor 6 is transferred to the ac line capacitor 8 (the current flow is represented by the dashed arrow labelled 2). This facilitates a voltage step-up capability according to the well known charge pumping principle, where a proportion of the energy that is stored in an inductor is transferred to a capacitor in the form of a current pulse train, each pulse of current representing an increment in the charge stored in the capacitor. There is some similarity here with a step-up converter of the boost type, but in particular, it is well known that boost converters are incapable of voltage step-down operation. However, in the present case, the energy that is trapped in the dc link filter inductance may be directed into and out of multiple phase and line permutations. State 1, can be achieved in three phase permutations and state 2 (the line output state) can be achieved in six line

permutations. Thus., in the present case, the energy that is stored in the dc link filter inductor 6 and the charge that is stored in the filter capacitor 8 may be sequentially incremented and decremented, providing the load impedance presented by the supply network 14 is above a critical value and the filter capacitor therefore periodically has sufficient terminal voltage to exceed the output voltage of the diode rectifier 5. The case when the load impedance presented by the supply network 14 is below a critical value such that surge arrester means must be employed to limit the dc link filter current is described in more detail below.

Figure 2b shows two different states of the so-called "normal current source inverter mode" where the dc link current is directed around any two of the output lines by closing the appropriate switches. In state 2, the dc link current is directed around output lines A and C (the current flow is represented by the continuous arrow labelled 2). In state 3, the dc link current is directed around output lines A and B (the current flow is represented by the dashed arrow labelled 3). Although not shown, it will be readily appreciated that the dc link current can be directed around any two of the output lines in any of the six possible permutations.

It is generally preferred that the two modes described above are merged according to an industry standard space vector modulation strategy in order to minimise power semiconductor switching losses, to minimise harmonic distortion of the power network 14 and maximise the efficiency of the generator system, for a given pulse width modulation carrier frequency. Figure 3 shows such a space vector modulation strategy where the ac output line current vector of the current source inverter 7 switches between seven possible states and where the amplitude of the current vector is the dc link current. The states are labelled SVO to SV6 in Figure 3. It will be readily appreciated that the operation of the surge arrester components, described below, provide an enhancement to the industry-standard space vector SVO. Thus, as described above with reference to Figures 2a and 2b, state 1 can be achieved in three phase permutations in order to provide industry-standard space vector SVO and states 2 and 3 (the line output states) can be achieved in the six line permutations to provide industry-standard space vectors SVl to SV6. The enhanced space vector SVO can be

provided by turning off all the power semiconductor devices in the current source inverter 7 whilst the surge arrester components operate.

Control loops with a carefully defined hierarchy are used to control the pulse width modulation strategy and determine when the dc link current is transferred to the ac line filter capacitor 8. Figure 4 shows a generator system that does not receive a power reference 25 from a pitch controller 21 (see Figure 1). In this case, the converter controller 22 includes a dc link current regulator "Idc reg" and a power factor regulator "Theta reg". They rank equally as the lowest regulators in the control hierarchy. An ac line voltage regulator "V reg" is the highest regulator in the control hierarchy and provides a control reference to the dc link current regulator "Idc reg". The dc link current regulator "Idc reg" and the power factor regulator "Theta reg" provide references to a space vector modulator.

The voltage regulator "V reg" responds to the error between an externally generated voltage reference 19a and the feedback voltage 15a and outputs a current reference. The dc link current regulator "Idc reg" responds to the error between the current reference from the voltage regulator "V reg" and the dc link current feedback 16 and outputs a modulator amplitude demand to the space vector modulator. The power factor regulator "Theta reg" responds to the error between an externally generated power factor reference 19b and the power factor feedback "PF fb" that is calculated using the feedback voltage 15a and the feedback current 15b and outputs a modulator angle demand to the space vector modulator. The space vector modulator therefore responds to the modulator amplitude and angle demands and outputs control references 17 to the power semiconductor switching devices in the current source inverter 7. Note that such a modulation strategy does not have complete independence of control of dc link current, output current and output power factor.

The space vector modulator assigns an increasing proportion of the pulse width modulation carrier period to SVO to increase the dc link current. The space vector modulator assigns a variable proportion of the pulse width modulation carrier period to whichever is the most appropriate of SVl to SV6 at a given time to adjust the

power factor of the output that is supplied to the power network 14 by the current source inverter 7.

Figure 5 shows a generator system that does receive a power reference 25 from the pitch controller 21. In this case, the converter controller 22 includes a power regulator "P reg" which replaces the voltage regulator "V reg" in providing the current reference to the dc link current regulator "Idc reg". The voltage regulator retains the highest position in the control hierarchy but becomes a non-linear system whose function is to provide an overriding reference to the power regulator "P reg" in the event that the power network 14 is unable to accept the generated power because of low demand. The power regulator "P reg" adopts the middle level in the control hierarchy.

The dc link current regulator "Idc reg" and the power factor regulator "Theta reg" still provide references to a space vector modulator. However, in this case the voltage regulator "V reg" responds to the error between an externally generated voltage reference 19a and the feedback voltage 15a and outputs an overriding power reference to the power regulator "P reg". The power regulator "P reg" responds to the error between the overriding power reference from the voltage regulator "V reg", the power reference 25 and the power feedback that is calculated using the feedback voltage 15a and the feedback current 15b and outputs a current reference to the dc current regulator "Idc reg". The remaining operation of the converter controller 22 is the same as that described above.

Other overriding regulator systems such as an ac line output current limit, for example, may also be incorporated in the control loop to provide protection for the power converter. Protective regulator systems may take the form of additional nonlinear, high sensitivity feedback inputs to existing regulator systems, or they may take additional levels in the control loop hierarchy.

An objective of the generator system is to optimise the generator 4 and its immediate power converter interface. When a machine rectifier such as the diode rectifier 5 is

used it has no control capability and is unable to contribute to the fault current control of the power network 14. For example, if a short circuit is applied to the power network 14 then the current source inverter 7 is unable to limit the fault current because energy that is already trapped in the dc link filter inductor 6 and the stator leakage reactance of the generator 4 will cause an over voltage failure of the power semiconductor devices in the current source inverter 7 if they attempt to interrupt the fault current. This problem can be addressed in two ways.

Referring first to Figure 6, additional means can be provided to absorb the trapped energy and a suitable surge arrestor circuit is shown. (If Figure 6 is compared to Figures 2a and 2b then it can be seen that the switches can be implemented using IGBTs and series connected diodes but it will be readily appreciated that any other suitable power semiconductor device can be used instead.) In the event of a short circuit across the supply network 14, the switch G is closed so that the resistor R is connected across the terminals of the current source inverter 7 and absorbs generated power when the current source inverter 7 is pulse suppressed in order to provide the enhance space vector SVO as mentioned above. This has the effect of limiting the dc link voltage. The diode D and the capacitor C provide transient voltage suppression in order to overcome the limitations that may possibly be posed by any stray inductance in the resistor R and the turn on delay of the switch G. It would also be acceptable to stipulate a time limit within which the current source inverter 7 would regulate the short circuit current and the surge arrester circuit would dissipate the full power of the generator 4. This time limit should be sufficient to permit any over current protective switchgear (not shown) associated with the power network 14 to operate without excessive disruption to the wind turbine 1. Once the over current protective switchgear is cleared, the generator system would resume normal operation. In the event that the over current protective switch gear is not cleared within the time limit, the first protective switchgear 11 would be opened and the generator system would be placed "offline".

With reference to Figure 7, the diodes in the diode rectifier 5 may be replaced by thrysitors 23. The thyristors 23 can be phase controlled by reference 24 to permit the

stator voltage to be regulated and limit fault current (see the description of diode and thyristor rectifier bridges set out above). When the fault current is limited by this means, the current source inverter 7 may continue to operate according to the pulse width modulation strategy described above so that the power factor can be regulated and limit the harmonic distortion in the fault current. However, when phase control is employed, the generator load power is reduced to a level that is similar to the real power component of the fault current and its shaft speed may therefore increase if its source of driving torque has a poor speed regulation response. If the fault is prolonged, the source of driving torque may have to be reduced to the extent that it is impossible to rapidly re-establish voltage regulation of the power network 14 when the fault is eventually cleared. The use of the diode rectifier 5 and the surge arrester components G, R, D and C described above is superior in this respect because the source of the driving torque will suffer little disruption during a fault.