A) TECHNICAL FIELD
[0001] The present invention generally relates to electrical power converters and more particularly to a system and method to control the three phase active rectifier type power converters.
B) BACKGROUND OF THE INVENTION
[0002] The power converters are generally used in applications where the electrical power is to be converted from DC to AC or vice versa. Such conversion apparatus is commonly referred to as inverters for converting DC to AC, or alternatively as a rectifier if the conversion is from AC to DC power.
[0003] An active rectifier is a type of power converter used in power systems. It is usually connected between a three phase power supply system and a load. It is designed to convert the input AC power to intermediate DC power. During this conversion, the rectifier injects harmonics. As a result, the output loads will be nonlinear. It also results in low power factor thereby reducing the system utilization factor. The injected harmonics spoils the process and quality of the loads. This was the major issue with the rectifier connected to the DC loads. Hence, there is a need to minimize the harmonics in the line current.
[0004] Currently implemented techniques utilize scalar control to minimize the harmonics in the line current. The major drawbacks with the scalar control are high switching losses, low dynamic performance and low power quality. The high switching loss is due to the asymmetric pulse width modulation signals. In scalar

control technique, the parameters are controlled in offline thereby yielding a poor transit time response and low dynamic performance. Hence, there is a need to implement a new system and method to minimize the harmonics, to improve the power factor and dynamic performance of the three phase active rectifiers.
C) OBJECTS OF THE INVENTION
[0005] The primary object of the present invention is to develop a system and method to control the three phase active rectifier type power converters to minimize the harmonics present in the line current.
[0006] Another object of the present invention is to provide a three phase active rectifier controller with embedded algorithms to allow bi-directional power transfer and to operate three phase active rectifier in rectifier and inverter modes.
[0007] Yet another object of the present invention is to control the three phase active rectifier using Space vector modulated pulse width modulated (PWM) signals to minimize inverter switching loss and to achieve higher DC bus utilization.
[0008] Yet another object of the present invention is to provide a three phase active rectifier with a vector control technique for online/real-time control of parameters and to provide good dynamic performance in varying loads.
[0009] Yet another object of the present invention is to develop a three phase active rectifier with space vector modulation technique to modulate switching signals sinusoidally to provide three phase voltage feedback readings.

[0010] Yet another object of the present invention is to develop a three phase active rectifier with space vector modulation technique to control the dc link voltage to predetermined voltage.
[0011] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control the input AC phase currents to have a near sinusoidal shape in phase with the AC phase voltages.
[0012] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control the magnitude of the AC phase current to match the load on the DC bus.
[0013] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control the IGBT converter module.
[0014] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control bi directional flow to enable power regeneration.
[0015] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control three phase line currents for correcting power factor.
[0016] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to control current in real time.

[0017] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to improve the transient response.
[0018] Yet another object of the present invention is to develop a three phase active rectifier system with space vector modulation technique to improve the switching frequency.
[0019] These and other objects and advantages of the present invention will become readily apparent from the following detailed description taken in conjunction with the accompanying drawings.
D) SUMMARY OF THE INVENTION
[0020] The above mentioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.
[0021] The various embodiments of the present invention provide a system and method to regulate the output of a converter using space vector modulation technique. According to one embodiment of the invention, a controller is connected to the converter arranged between a three phase AC power supply and an end DC load device. An inductor capacitor filter is connected between the AC power supply and the converter. The controller executes an algorithm to implement a space vector modulation technique to regulate the output of the converter and to allow bi directional power transfer to operate the converter in rectifier and inverter modes.

[0022] The converter is an Insulated Gate Bipolar Transistor (IGBT) converter. The three phase active rectifier controller is designed to control the iGBT converter output and to minimize the harmonics in the DC load. The controller has a system transformation circuit to transform the three phase voltages into two phase time varying system quantities using Clarke transformation. The system transformation circuit further converts the time varying two phase system quantities into steady state DC quantities using Park transformation. The controller regulates the modulation of power converter and phase of modulation using the algorithm to control the input current to be equal to unity power factor. The controller regulates the converter to regenerate power, when the DC bus voltage is more than the desired DC output voltage.
[0023]The three phase active rectifier controller includes an embedded algorithm to control the feedback voltage and current values. The algorithm is designed to control the direct current (DC) link voltage to a predetermined voltage. It controls the input AC phase currents to be sinusoidal and in phase with the AC phase voltages. It controls the magnitude of the AC phase current to match the load on the DC bus. The switching signals are modulated sinusoidally through space vector modulation technique. The depth of modulation (m) and the phase of modulation (f) are simultaneously controlled to control the input current to be in unity power factor. The value of inductor connected between the AC main power supply and the converter is selected from the rated input voltage desired DC link voltage. A MATLAB simulink model is developed to simulate the controller for specified voltages and frequencies. A control strategy for unit power factor operation is developed using the developed MATLAB simulink. The system model and control strategy is combined to prepare an

overall controlled system model. The steady state and dynamic results of normal operation of the front end converter is obtained from this model.
[0024] The controller is programmed to control the three phase IGBT converter. It utilizes space vector modulated PWM generator for controlling the IGBT (Insulated-Gate Bipolar Transistor) converter module to reduce the switching losses and to improve converter efficiency. The current controller is implemented in d-q (direct-quadruple) rotating frame, which makes the control easy and compact. All the feedback voltages and the currents are converted in to d-q rotation frame by using Clarke and Parks transformations for controlling parameters. This control will make power factor near unity and minimize harmonic in line current.
[0025] The three phase active rectifier controller in the present invention receives two feedback signals Vabc (voltage feedback) and labc (current feedback). The feedback signals will undergo transformation called as Clarke and Parks transformation. For fast updating of the parameters during system transformations, a space vector modulation technique is used. The online/real-time updating of parameters is possible using space vector modulation PWM (pulse width modulation). The space vector modulation depends on time. It checks for each and every sampling time of the wave form for updating to perform an overall control process. Using the space vector modulated PWM signals, the over all system performance is improved.
[0026]The Algorithm embedded in the controller enables bi-directional power transfer, to operate the converter in rectifier and inverter modes. The regeneration algorithm in the present invention makes the converter to synchronize with tine voltage and continue to switch in inverter mode to continuously supply the feedback

power to line. The power converter will regenerate, when the DC bus voltage VDC is more than the desired DC output voltage, VDCset. During regeneration, the DC power will be fed back to three phase power supply system.
[0027] The vector control technique according to the present invention provides on¬line/real-time control for fast controlling of parameters and to provide good dynamic performance in varying load system. It provides space vector modulated PWM to improve inverter switching loss and to achieve higher DC bus utilization. The system transformation is made easy using vector control technique. It helps in real-time fast updating of the parameters. The algorithm in the present invention calculates the difference between the voltage and the current waveforms. As a result, G, the current waveform is adjusted to match with the input voltage waveform to minimize the error. Thus the output current waveform will be made equal to the input voltage waveform thereby reducing the harmonics.
E) BRIEF SESCRIPTION OF THE DRAWINGS
[0028] The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
[0029] FIG. 1 shows a block diagram illustrating a three phase active rectifier system according to one embodiment of the present invention.
[0030] FIG. 2 shows a block diagram illustrating a basic control structure of the
controller used in the three phase active rectifier system according to one embodiment of the present invention.

[0031] FIG. 3 illustrates frame orientation transformations in a co-ordinate system according to one embodiment of the present invention.
[0032] FIG. 4A illustrates Space Vector Modulation scheme with space vector rotating through six sectors used in the three phase active rectifier according to one embodiment of the present invention.
[0033] FIG. 4B illustrates a time series chart for space vector modulation scheme used in the three phase active rectifier according to one embodiment of the present invention.
[0034] Although specific features of the present invention are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the present invention.
F) DETAILED DESCRIPTION OF THE INVENTION
[0035] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.

[0036] The various embodiments of the present invention provide a system and method to control the converter output by implementing a vector control technique. In accordance with the present invention, the feedback output from the three phase source and the active rectifier is controlled by using a three phase active rectifier controller.
[0037] According to one embodiment of the invention, a controller is connected to the converter arranged between a three phase AC power supply and an end DC load device. An inductor capacitor filter is connected between the AC power supply and the converter. The controller executes an algorithm to implement a space vector modulation technique to regulate the output of the converter and to allow bi directional power transfer to operate the converter in rectifier and inverter modes.
[0038] The converter is an Insulated Gate Bipolar Transistor (IGBT) converter. The three phase active rectifier controller is designed to control the IGBT converter output and to minimize the harmonics in the DC load. The controller has a system transformation circuit to transform the three phase voltages into two phase time varying system quantities using Clarke transformation. The system transformation circuit further converts the time varying two phase system quantities into steady state DC quantities using Park transformation- The controller regulates the modulation of power converter and phase of modulation using the algorithm to control the input current to be equal to unity power factor. The controller regulates the converter to regenerate power, when the DC bus voltage is more than the desired DC output voltage.

IWJ7JUIC mice pnasc active recnner controller includes an embedded algorithm to control the feedback voltage and current values. The algorithm is designed to control the direct current (DC) link voltage to a predetermined voltage. It controls the input AC phase currents to be sinusoidal and in phase with the AC phase voltages. It controls the magnitude of the AC phase current to match the load on the DC bus. The switching signals are modulated sinusoidally through Space vector modulation technique. The depth of modulation (m) and the phase of modulation (f) are simultaneously controlled to control the input current to be in unity power factor. Trie value of inductor connected between the AC main power supply and the converter is selected from the rated input voltage desired DC link voltage. A MATLAB simulink model is developed to simulate the controller for specified voltages and frequencies. A control strategy for unit power factor operation is developed using the developed MATLAB simulink. The system model and control strategy is combined to prepare an overall controlled system model. The steady state and dynamic results of normal operation of the front end converter is obtained from this model.
[0040] The controller is programmed to control the three phase 1GBT converter. It utilizes space vector modulated PWM generator for controlling the IGBT (Insulated-gate bipolar transistor) converter module to reduce the switching losses and to improve converter efficiency. The current controller is implemented in d-q (direct-quadruple) rotating frame, which makes the control easy and compact. All the feedback voltages and the currents are converted in to d-q rotation frame by using Clarke and Parks transformations for controlling parameters. This control will make power factor near unity and minimize harmonic in line current.

[0041] The three phase active rectifier controller in the present invention receives two feedback signals Vabc (voltage feedback) and Iabc (current feedback). The feedback signals will undergo transformation called as Clarke and Parks transformation. For fast updating of the parameters during system transformations, a space vector modulation technique is used. The online/real-time updating of parameters is possible using space vector modulation PW'M (pulse width modulation). The space vector modulation depends on time. It checks for each and every sampling time of the wave form for updating to perform an overall control process. Using the "space vector modulated PWM signals, the over all system performance is improved.
[0042]The algorithm embedded in the controller enables bi-directional power transfer, to operate the converter in rectifier and inverter modes. The regeneration algorithm in the present invention makes the converter to synchronize with line voltage and continue to switch in inverter mode to continuously supply the feedback power to line. The power converter will regenerate, when the DC bus voltage VDC is more than the desired DC output voltage, VDCset. During regeneration, the DC power will be fed back to three phase power supply system.
[0043] The vector control technique according to the present invention provides On¬line/real-time control for fast controlling of parameters and to provide good dynamic performance in varying load system. It provides space vector modulated PWM to improve inverter switching loss and to achieve higher DC Bus Utilization. The system transformation is made easy using vector control technique. It helps in real-time fast updating of the parameters. The algorithm in the present invention calculates the difference between the voltage and the current waveforms. As a result, the current waveform is adjusted to match with the input voltage waveform to minimize the error.

Thus the output current waveform will be made equal to the input voltage waveform thereby reducing the harmonics.
[0044] FIG. 1 show the block diagram illustrating the overall structure of active rectifier system connected to a grid. A three phase IGBT converter 12 is connected between three phase power grid 11 and load 13. Three phase active rectifier controller 14 is connected to converter 12. Controller 14 is designed to control the converter output to minimize the harmonics in the load.
[0045] A three phase LC (Inductance Capacitance) filter is connected between AC grid 11 and IGBT converter 12 to filter the differential mode inverter switching noise from mains and to minimize the losses. The front end converter draws power from balanced three phase AC power source 11. The three phase voltages Va, Vb and Vc are converted to two phase system parameters for convenience. The DC link is a DC bus voltage maintained by a bank of electrolytic capacitors. Controller 14 executes an algorithm to implement a space vector modulation technique to regulate the output of the converter and to allow bi directional power transfer to operate the converter in rectifier and inverter modes.
[0046] The system also has provision to regenerate when DC bus voltage is more than VDCset. When the DC bus is more than VDCset value, the system works in reverse power flow from DC bus to three phase line. The regeneration algorithm will make the converter to synchronize with line voltage and continue to switch in inverter mode and to continue to feed back power to line.

[0047] The grid voltage feedback, line current feedback and the bus voltage feedback is applied to the three phase active rectifier controller block 14. A reference input voltage is input to controller 14. Three phase active rectifier controller 14 is programmed to control output of IGBT converter 12. U utilizes space vector modulated PWM generator for controlling the IGBT converter module to reduce the switching losses and to improve converter efficiency. The current controller is implemented in d-q rotating frame, which will make control easy and compact. All the feedback voltages and the currents are converted in to d-q rotation by using Clarke and Parks transformation or system transformation for controlling parameters. This control will make power factor near unity and minimize harmonic in line current.
[0048] Once the feed back voltages are transformed, the out put of the Clarke and park transformations is applied to the current controller and DC bus controller (PI). Here the current is controlled proportional to voltage waveform to get the sinusoidal waveform. To make the current in sinusoidal with the voltage waveform, Vadc is taken as reference and the current is controlled with respect to voltage. The algorithm calculates the phase difference between the voltage and the current waveform and accordingly the current waveform is adjusted to match with the input voltage waveform so that error is minimized. As a result the harmonics is reduced.
[0049] FIG. 2 shows a block diagram illustrating a basic control structure involving three phase active rectifier controller according to the present invention. The feedback inputs Vabc, labc and Vdc is passed to the analog to digital converter. ADC converts all the input analog signals to digital signals. Once it is digitally converted it is very easy for system transformation. All the feedback voltages and the currents are converted in to d-q rotation by using Clarke and Parks transformation or system

transformation for controlling parameters. This control will make power factor near unity and minimize harmonic in line current.
[0050] The input to the system transformation is Va, Vb and Vc, that is three phases with 120 degree phase shift. It is converted to Vd and Vq. The two phases 'd' and 'q' is orthogonal that is 90 degrees. Where 'd' represents a direct axis and ;q' represents a quadruple axis The 120 degree phase shift is converted to 90 degree phase shift. It is not just conversion form 3 phase to 2 phase, it represents the frame orientation. It is called stationary frame rotating parameters, here the parameters will be rotating and frame will be stationary. As a result in real-time we can obtain a continuously varying sin function. So in real-time it will be continuously time varying quantity or time dependent and the frame or the reference will be stationary. By rotating the parameters difference in the phase is controlled.
[0051 ] During system transformation, the three phase quantities "a, b, c' are converted in to two phase quantities 'd, q\ This transformation is called system transformation. We will estimate omega (ai) from the transformed feedback voltages. Omega («) is a real-time quantity. It represents the current position of the voltage and current. The voltage and the current waveforms will be sampled with respect to time.
[0052] If there is a power factor lag between the voltage and the current waveform, then it results in a phase difference V. It will generate a reactive power which can be controlled by 'q'. The three phase current parameters are converted to two phase current parameters such as d and q form. The Iq value will reflect the reactive power ultimately it makes reactive power in to zero. As a result the reactive power will be

zero. So power factor will be unity. As a result one object of unity power factor is achieved.
[0053] Once the feed back voltages are transformed, the out put of the Clarke and park transformations is applied to the current controller and DC bus controller (PI). Here the current is controlled proportional to voltage waveform to get the sinusoidal waveform. To make the current in sinusoidal with the voltage waveform, Vadc is taken as reference and the current is controlled with respect to voltage. The algorithm calculates the phase difference between the voltage and the current waveform and accordingly the current waveform is adjusted to match with the input voltage waveform so that error is minimized. As a result the harmonics is reduced. The output from the current controller is fed to the reverse Clarke and parks transformation and finally through the power circuit hardware, the feedback voltage and current will be feedback to the ADC.
[0054] FIG. 3 illustrates frame orientation transformations in a rectangular co¬ordinate system. It shows the magnitude and the direction of the three phase AC source. The a„, pu in the fixed coordinate stator phase are transformed to the d„ and q„ currents components in the d,q rotating frame with the Park transform. The Clarke transform uses three-phase currents ia, ib and ic to calculate currents in the two-phase orthogonal stator axis: ia and ib. These two currents in the fixed coordinate stator phase are transformed to the isd and isq currents components in the d, q frame with the Park transform.


[0056] The three stator current components can be represented by a single equivalent vector called resultant line current space phasor is which is given by;


[0059] When i. superposed with i„ and ia + ib + ic is zero, then the quantities ifl, ib and h are transformed to i , and i . with the following mathematical transformation:

[0060] The transformed orthogonal two phase system quantities in a stationary frame are converted in to that in a rotating frame as explained below. The two phases . frame representation calculated with the Clarke transform is then fed to a vector rotation block where it is rotated over an angle . to follow the frame d,q attached to the rotor flux.

[0062] FIG. 4 illustrates Space Vector Modulation scheme with space vector rotating through six sectors and with the explanation of the calculation of time periods, used in the three phase active rectifier according to one embodiment of the present invention. In order to generate the PWM signals that produce the rotating vector, the equations are required to determine the PWM time intervals for each sector.
[0063] With respect to the sector I bounded by vectors 100, 110 and the null vectors 000 and 111 in FIG.4, the vector Vx within this sector can be resolved as


[0064] Based on the values of ta and tb, a symmetrical switching pattern constructed for two consecutive periods of TO to satisfy the equation A is shown in the time series chart in FIG. 4B.
G) ADVANTAGES OF THE INVENTION
[0065] The three phase active rectifier controller according to present invention includes an embedded algorithm to control the feedback voltage and current values. The present invention utilizes a vector control method to minimize the harmonics. The online control of parameters is possible using vector control. Vector control provides very good dynamic performance in varying load system. It provides fast

. it provides improved system efficiency with power regeneration, reduces switching loss in IGBT and power regeneration is possible. The algorithm is designed to control the direct current (DC) link voltage to a predetermined voltage. It controls the input AC phase currents to have a nearly sinusoidal shape and in phase with the ac phase voltages. It Controls the magnitude of the ac phase current to match the load on the dc bus.
[0066] Although the invention is described with various specific embodiments, it will be obvious for a person skilled in the art to practice the invention with modifications. However, all such modifications are deemed to be within the scope of the claims.
[0067] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the present invention described herein and all the

CLAIMS
What is claimed is:
1. A three phase active rectifier system comprising:
a converter connected to a three phase main power supply and to an end load
device; and
a controller connected to the converter;
wherein the controller executes an algorithm to implement a space vector
modulation technique to regulate the output of the converter and to allow bi
directional power transfer to operate the converter in rectifier and inverter
modes.
2. The system according to claim 1, wherein the converter is an insulated gate bipolar transistor.
3. The system according to claim I. wherein the controller has a system transformation circuit to transform the three phase voltages into two phase time varying system quantities using Clarke transformation.
4. The system according to claim I, wherein the system transformation circuit converts the time varying two phase system quantities into steady state DC quantities using Park transformation.
5. The system according to claim 1, wherein the controller regulates the modulation of power converter and phase of modulation using the algorithm to control the input current to be equal to unity power factor.

6. The system according to claim ]. wherein the controller regulates the
converter to regenerate power, when the DC bus voltage is more than the
desired DC output voltage.
7. The system according to claim 1, wherein the end load device is a DC load
device.