Pulse width modulation power transmission system

• US 4,941,079 A
• Filed: 02/13/1989
• Issued: 07/10/1990
• Est. Priority Date: 10/07/1988
• Status: Expired due to Term

• Current Assignee: Royal Institution For The Advancement of Learning (McGill University)
• Original Assignee: Royal Institution For The Advancement of Learning (McGill University)
• Inventors: Ooi, Boon Teck
• Primary Examiner(s): Salce, Patrick R.
• Assistant Examiner(s): STERRETT, JEFFREY L
• Application Number:
  US07/309,154
• Time in Patent Office: 512 Days
• Field of Search: 363/35, 363/37, 363/95, 363/96, 363/98, 363/132, 363/137
• US Class Current: 363/132
• CPC Class Codes: H02J 3/36 Arrangements for transfer of electric power between ac networks via a high-tension dc link
• CPC Class Codes: H02M 7/7575 for high voltage direct transmission link
• CPC Class Codes: Y02E 60/60 Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD]

Abstract

A converter for interconnecting two electric networks to transmit electric power from one network to the other, each network being coupled to a respective power generator station. The converter, having an AC side and a DC side, includes a bridge of semiconductor switches with gate turn-off capability coupled to a control system to produce a bridge voltage waveform having a fundamental Fourier component at the frequency of the electric network coupled to the AC side of the converter. The control system includes three inputs for receiving reference signals allowing to control the frequency, the amplitude and the phase angle of the fundamental Fourier component with respect to the alternating voltage of the network coupled to the AC side of the converter. Through appropriate feedback loops, the converter may be used to maintain at a predetermined level the power flowing therethrough or to keep at a preset value the voltage across the DC terminals of the converter and, in both cases, to maintain the frequency synchronism between the fundamental Fourier component and the alternating voltage of the network coupled to the DC side of the converter.

Claims

1. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, there being a DC voltage having a certain amplitude across the DC link terminals, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said alternating voltage having an amplitude which is always lower than the amplitude of said DC voltage, said converter comprising:

a bridge of valves each associated with an antiparallel diode, each valve of said bridge being capable of assuming two different states, namely an ON staten and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current, each diode being normally reverse-biased between said DC and alternating voltage, and said bridge being capable of operating either as an inverter to enable transmission of electric power from the first network to the second one, or as a rectifier to enable transmission of electric power from the second network to the first one;

inductor means interposed between said bridge and said AC link terminals for producing a boost voltage across said inductor means, said boost voltage forward-biasing said diodes to allow the same to conduct electric current to thereby enable operation of said bridge as an inverter and rectifier;

capacitor means across said DC link terminals, said bridge being interposed between the inductor and capacitor means;

valve control means coupled to said bridge for commanding the valves thereof to switch state to operate said bridge as an inverter or rectifier, said valve control means including;

signal generating means for outputting a control signal representative of a desired valve state switching sequence to obtain a bridge voltage waveform having a fundamental Fourier component at a frequency corresponding substantially to the frequency of said alternating voltage; and

frequency control means coupled to said signal generating means to adjust said control signal in accordance with adrift of the frequency of said alternating voltage to cause the frequency of said fundamental Fourier component to track the frequency of said alternating voltage.

2. A converter as defined in claim 1, wherein said bridge is a three-phase bridge.

3. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

capacitor means across said DC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means including;

(a) signal generating means for outputting a control signal representative of a desired valve state switching sequence to obtain a bridge voltage waveform having a fundamental Fourier component at a frequency corresponding substantially to the frequency of said alternating voltage; and

(b) frequency control means coupled to said signal generating means to adjust said control signal in accordance with adrift of the frequency of said alternating voltage to cause the frequency of said fundamental Fourier component to track the frequency of said alternating voltage, said frequency control mens comprising a feedback circuit to generate an error signal representative of a drift between the frequency of said alternating voltage and the frequency of said fundamental Fourier component, wherein said frequency control means adjusts said control signal in accordance with said error signal to keep the frequency of said fundamental Fourier component in synchronism with the frequency of said alternating voltage.

4. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

capacitor means across said DC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch sate, said valve control means including;

(a) signal generating means for outputting a control signal representative of a desired valve state switching sequence to obtain a bridge voltage waveform having a fundamental Fourier component at a frequency corresponding substantially to the frequency of said alternative voltage; and

(b) frequency control means coupled to said signal generating means to adjust said control signal in accordance with adrift of the frequency of said alternating voltage to cause the frequency of said fundamental Fourier component to track the frequency of said alternating voltage, wherein said valve control means further includes memory means in which is stored state switching sequence data for the valves of said bridge, and first input means coupled to said memory means for receiving a signal representative of a desired frequency of said fundamental Fourier component, according to the signal received through said first input means, said memory means outputting state switching sequence data allowing to obtain a bridge voltage waveform with a fundamental Fourier component at said desired frequency.

5. A converter as defined in claim 4, wherein said valve control means further comprises a second input means coupled to said memory means for receiving a signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, according to the signal received through said second input means, said memory means outputting state switching sequence data to obtain a bridge voltage waveform with a fundamental Fourier component at said desired phase angle.

6. A converter as defined in claim 5, wherein said valve control means further comprises a third input means for receiving a signal representative of a desired amplitude of said fundamental Fourier component, according to the signal received through said third input means, said valve control means outputting a control signal to obtain a bridge voltage waveform with a fundamental Fourier component with said desired amplitude.

7. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means including;

(a) signal generating means for producing a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency, with a certain amplitude and at a certain phase angle with said alternating voltage;

(b) memory means coupled to said signal generating means, in said memory means being stored control data allowing to generate different valve state switching signals, each allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a different frequency;

(c) a voltage controlled oscillator for receiving an analog signal representative of the desired frequency of said fundamental Fourier component and generating in response to said analog signal an oscillatory signal representative of said desired frequency;

(d) counter means coupled to said voltage controlled oscillator and to said memory means, said counter means counting the oscillations of said oscillatory signal and generating an output signal allowing said memory means to retrieve and output control data allowing said signal generating means to generate a valve state switching signal to obtain a bridge voltage waveform with a fundamental Fourier component at said desired frequency.

8. A converter as defined in claim 7, wherein said valve control means further comprises:

analog-to-digital converter means for receiving an analog signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage and for converting the last mentioned analog signal in a digital signal representative of said desired phase angle;

adder means coupled to said analog-to-digital converter means, to said counter means and to said memory means, said memory means containing control data allowing to generate different valve state switching signals, each corresponding to a bridge voltage waveform with a fundamental Fourier component at a different phase angle with said alternating voltage, said adder means adding said digital signal and the output signal of said counter means and outputting a signal allowing said memory means to retrieve and output control data allowing said signal generating means to generate a valve state switching signal to obtain a bridge voltage waveform with a fundamental Fourier component at said desired phase angle.

9. A converter as defined in claim 8, wherein said valve control means further comprises a digital-to-analog converter means coupled to said memory means for converting control data to a control analog signal.

10. A converter as defined in claim 9, wherein said valve control means further comprises a signal processing means coupled to said digital-to-analog converter means, said signal processing means receiving a signal representative of a desired amplitude of said fundamental Fourier component, said signal processing means combining the last mentioned signal with an output signal from said digital-to-analog converter means to produce a signal allowing said signal generating means to generate a valve state switching signal for obtaining a bridge voltage waveform with a fundamental Fourier component having said desired amplitude.

11. A converter as defined in claim 10, wherein said signal processing means is a multiplier.

12. A converter as defined in claim 11, further comprising an adder including a first input means for receiving a signal representative of a nominal amplitude of said fundamental Fourier component, and a second input means for receiving a signal representative of the desired deviation of the amplitude of said fundamental Fourier component with respect to said nominal amplitude, an output of said adder being coupled to said multiplier.

13. A converter as defined in claim 12, comprising a voltage amplitude feedback means for generating an error signal representative of a difference between the amplitude of said fundamental Fourier component and said nominal amplitude, said voltage amplitude feedback means being coupled to said adder for applying to said error signal to said second input means.

14. A converter as defined in claim 8, wherein said valve control means further comprises:

signal processing means coupled to said analog-to-digital converter means, said signal processing means including;

(a) first input means for receiving an analog signal representative of a nominal phase angle between said fundamental Fourier component and said alternating voltage; and

(b) second input means for receiving a signal representative of a desired deviation of the phase angle between said fundamental Fourier component and said alternating voltage with respect to said nominal phase angle, said signal processing means processing the signals from said first and second input means and outputting said analog signal representative of the desired phase angle between said fundamental Fourier component and said alternating voltage.

15. A converter as defined in claim 14, comprising a phase angle feedback means for generating an error signal representative of a difference between said nominal phase angle and the phase angle between said alternating voltage and said fundamental Fourier component, said phase angle feedback means being coupled to said signal processing means for applying said error signal to said second input means.

16. A converter as defined in claim 8, wherein said bridge is a three-phase bridge.

17. A converter as defined in claim 16 wherein said memory means comprises:

a modulating wave memory circuit for each phase of said bridge, the memory circuit for each phase of said bridge being coupled to a respective digital-to-analog converter, each digital to analog converter outputting a modulating wave;

a carrier wave memory circuit coupled to a digital-to-analog converter outputting a carrier wave, said signal generating means comprising modulating means coupled to said memory circuits for separately modulating said carrier wave by the modulating wave for each phase of said bridge, said modulating means outputting a signal for each phase of said bridge representative of a desired valve state switching sequence.

18. A converter as defined in claim 17, wherein said carrier wave is a triangular wave and said modulating wave being a sine wave.

19. A converter as defined in claim 7, wherein said valve control means further comprises:

signal processing means coupled to said voltage controlled oscillator, said signal processing means including;

(a) first input means for receiving a signal representative of a nominal frequency of said fundamental Fourier component;

(b) second input means for receiving a signal representative of a desired deviation of the frequency of said fundamental Fourier component with respect to said nominal frequency, said signal processing means processing the signals from said first and second input means and outputting said analog signal.

20. A converter as defined in claim 19, wherein said signal processing means is an adder.

21. A converter as defined in claim 19, further comprising a frequency feedback means for generating an error signal representative of a drift between the frequency of said alternating voltage and the frequency of said fundamental Fourier component, said frequency feedback means being coupled to said signal processing means for applying said error signal to said second input means.

22. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means including;

(a) signal generating means for outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency and at a certain phase angle with said alternating voltage; and

(b) frequency and phase angle control means coupled to said signal generating means, said frequency and phase angle control means including;

(i) a first signal processing circuit for receiving a signal representative of a desired frequency of said fundamental Fourier component and generating in response an output signal whose instantaneous value is representative of the voltage angle of said fundamental Fourier component with respect to a certain reference;

(ii) a second signal processing circuit having first and second inputs and an output, said first input being coupled to said first signal processing circuit and said output to said signal generating means, said signal processing circuit receiving at said second input a signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage and outputting a signal allowing said signal generating means to produce a valve state switching signal to obtain a bridge output waveform having a fundamental Fourier component at said desired frequency and at said desired phase angle with said alternating voltage.

23. A converter as defined in claim 22, wherein said frequency and phase angle control means further comprises:

an adder having two inputs and an output coupled to said first signal processing circuit, at one of the inputs of said adder being applied a signal representative of a nominal frequency of said fundamental Fourier component and at the other input of said adder being applied a signal representative of a desired deviation of the frequency of said fundamental Fourier component with respect to said nominal frequency.

24. A converter interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means including;

(a) signal generating means for outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency and at a certain phase angle with said alternating voltage; and

(b) frequency and phase angle control means coupled to said signal generating means, said frequency and phase angle control means including;

(i) a first signal processing circuit for receiving a signal representative of a desired frequency of said fundamental Fourier component and generating in response an output signal whose instantaneous value is representative of the phase angle of said fundamental Fourier component with respect to a certain reference;

(ii) a second signal processing circuit having first and second inputs and an output, said first input being coupled to said first signal processing circuit and said output to said signal generating means, said second signal processing circuit receiving at said second input a signal represntative of a desired phase angle between said fundamental Fourier component and said alternating voltage and outputting a signal allowing said signal generating means to produce a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired frequency and at said desired phase angle with said alternating voltage;

wherein said frequency and phase angle control means further comprises;

an adder having two inputs and an output coupled to said first signal processing circuit, at one of the inputs of said adder being applied a signal representative of a nominal frequency of said fundamental Fourier component and at the other input of said adder being applied a signal representative of a desired deviation of the frequency of said fundamental Fourier component with respect to said nominal frequency; and

wherein said first signal processing circuit is a voltage controlled oscillator.

25. A converter interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means including;

(a) signal generating means for outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency and at a certain phase angle with said alternating voltage; and

(b) frequency and phase angle control means coupled to said signal generating means, said frequency and phase angle control means including;

(i) a first signal processing circuit for receiving a signal representative of a desired frequency of said fundamental Fourier component and generating in response an output signal whose instantaneous value is representative of the phase angle of said fundamental Fourier component with respect to a certain reference;

(ii) a second signal processing circuit having first and second inputs and an output, said first input being coupled to said first signal processing circuit and said output to said signal generating means, said second signal processing circuit receiving at said input a signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage and outputting a signal allowing said signal generating means to produce a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired frequency and at said desired phase angle with said alternating voltage wherein said frequency and phase angle control means further comprises;

an adder with two inputs and an output; and

an analog-to-digital converter having an input coupled to the output of said adder and also having an input coupled to said second input, at one of the inputs of said adder being applied an analog signal representative of a nominal phase angle between said alternating voltage and said fundamental Fourier component, at the other input of said adder being applied a signal representative of a desired deviation of the phase angle between said fundamental Fourier component and said alternating voltage with respect to said nominal phase angle.

26. A converter as defined in claim 25, wherein said second signal processing circuit is an adder.

27. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given substantially fixed frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge for commanding the valves thereof to switch state, said valve control means generating a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency, at a certain phase angle with said alternating voltage and having a certain amplitude, said valve control means including;

(a) a frequency input means for receiving a signal representative of a desired frequency of said fundamental Fourier component;

(b) a phase angle input means for receiving a signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage; and

(c) an amplitude input means for receiving a signal representative of a desired amplitude of said fundamental Fourier component, said valve control means processing the signals received at said frequency input, phase angle input and amplitude input means and outputting a valve state switching signal for obtaining a bridge voltage waveform with a fundamental Fourier component at said desired frequency at said desired phase angle with said alternating voltage and having said desired amplitude.

28. A converter as defined in claim 27, wherein said frequency input means includes a first input for receiving a signal representative of a nominal frequency of said fundamental Fourier component and a second input for receiving a signal representative of a desired deviation of the frequency of said fundamental Fourier component with respect to said nominal frequency.

29. A converter as defined in claim 27, wherein said phase angle input means includes a first input for receiving a signal representative of a nominal phase angle between said fundamental Fourier component and said alternating voltage and a second input for receiving a signal representative of a desired deviation of the phase angle with respect to said nominal phase angle value.

30. A converter as defined in claim 27, wherein said amplitude input means includes a first input for receiving a signal representative of a nominal amplitude of said fundamental Fourier component and a second input for receiving a signal representative of a desired deviation of the amplitude of said fundamental Fourier component with respect to said nominal amplitude.

31. A converter for interconnecting a first electric network and a second electrical network to maintain the flow of real power from one network to other at a predetermined value, each of said networks being coupled to the respective active power source, said first network including DC link terminals for coupling said first network to said converter, there being a DC voltage having a certain amplitude across the DC link terminals, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said alternating voltage having an amplitude which is always lower than the amplitude of said DC voltage, said converter comprising:

a bridge of valves each associated with an antiparallel diode, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current, each diode being normally reverse-biased between said DC and alternating voltage, and said bridge being capable of operating either as an inverter to enable transmission of electric power from the first network to the second one, or as a rectifier to enable transmission of electric power from the second network to the first one;

inductor means interposed between said bridge and said AC link terminals for producing a boost voltage across said inductor means, said boost voltage forward-biasing said diodes to allow the same to conduct electric current to thereby enable operation of said bridge as an inverter and rectifier;

capacitor means across said DC link terminals, said bridge interposed between the inductor and capacitor means, the capacitor means for (1) maintaining a DC voltage across said bridge greater than an absolute value of said alternating voltage, and (2) filtering out switching ripples occurring at said DC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to operate said bridge as an inverter or rectifier, and to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding converter, there being a DC voltage having a certain amplitude across the DC link terminals, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said alternating voltage having an amplitude which is always lower than the amplitude of said DC voltage, said converter comprising;

a bridge of valves, each associated with an antiparallel diode, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current, each diode being normally reverse-biased between said DC and alternating voltage, and said bridge being capable of operating either as an inverter to enable transmission of electric power from the first network to the second one, or as a rectifier to enable transmission of electric power from the second network to the first one;

inductor means interposed between said bridge and said AC link terminal for producing a boost voltage across said inductor means, said boost voltage forward-biasing said diodes to allow the same to conduct electric current to thereby enable operation of said bridge as an inverter and rectifier;

capacitor means across said DC link terminals, said bridge interposed between the inductor and capacitor means, the capacitor means for (1) maintaining a DC voltage across said bridge greater than an absolute value of said alternating voltage, and (2) filtering out switching ripples occurring at said DC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to operate said bridge as an inverter of rectifier and, to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage, substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage;

signal processing circuit coupled to said phase angle input means, said signal processing circuit outputting said phase angle signal;

feedback means coupled to said signal processing circuit, said feedback means producing an error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value, said signal processing means receiving said error signal and altering said phase angle signal in accordance with said error signal allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a real power flow through said converter at said predetermined value.

32. A converter as defined in claim 31, wherein said feedback means includes:

first means for producing a signal representative of the amount of real power flowing through said converter;

second means for producing a signal representative of said predetermined value;

third means coupled to said first and second means for processing said signal representative of the amount of real power flowing through said converter and said signal representative of said predetermined value, and outputting said error signal.

33. A converter for interconnecting a first electric network and a second electric network to maintain the flow of real power from one network to the other at a predetermined value, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage;

a signal processing circuit coupled to said phase angle input means, said signal proceeding circuit outputting said phase angle signal;

feedback means coupled to said signal proceeding circuit, said feedback means producing an error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value, said signal processing means receiving said error signal and altering said phase angle signal in accordance with said error signal allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a real power flow through said converter at said predetermined value;

wherein said signal proceeding circuit includes;

(a) means responsive to said error signal to generate a frequency drift error signal indicative of a drift between the frequency of said fundamental Fourier component and the frequency of said alternating voltage; and

(b) means responsive to said frequency drift error signal to alter said phase angle signal for maintaining the frequency of said fundamental fourier component in synchronism with the frequency of said alternating voltage.

34. A converter for interconnecting a first electric network and a second electric network to maintain the flow of real power from one network to the other at a predetermined value, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage;

a signal processing circuit coupled to said phase angle input means, said signal processing circuit outputting said phase angle signal,

feedback means coupled to said signal processing circuit, said feedback means producing an error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value, said signal processing means receiving said error signal and altering said phase angle signal in accordance with said error signal allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a real power flow through said converter at said predetermined value wherein said feedback means includes;

first means for producing a signal representative of the amount of real power flowing through said converter;

second means for producing a signal representative of said predetermined value;

third means coupled to said first and second means for processing said signal representative of the amount of real power flowing through said converter and said signal representative of said predetermined value, and outputting said error signal; and

wherein said signal processing circuit includes;

fourth means for generating a signal representative of a nominal frequency of said fundamental Fourier component;

fifth means coupled to said fourth means and to said third means, said fifth means processing the signal representative of a nominal frequency of said fundamental Fourier component and said error signal, and outputting said frequency drift error signal.

35. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter maintaining a voltage across said DC link terminals at a predetermined value, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage;

signal processing circuit coupled to said phase angle input means, said signal processing circuit outputting said phase angle signal;

feedback means coupled to said signal processing circuit, said feedback means producing an error signal representative of a difference between the voltage across said DC link terminals and said predetermined value, said signal processing means receiving said error signal and altering said phase angle signal in accordance with said error signal allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a voltage across said DC link terminals at said predetermined value.

36. A converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter maintaining a voltage across said DC link terminals at a predetermined value, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage;

signal processing circuit coupled to said phase angle input means, said signal processing circuit outputting said phase angle signal;

feedback means coupled to said signal processing circuit, said feedback means producing an error signal representative of a difference between the voltage across said DC link terminals and said predetermined value, said signal processing means receiving said error signal and altering said phase angle signal in accordance with said error signal allowing to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a voltage across said DC link terminals at said predetermined value wherein said signal processing circuit includes;

means responsive to said error signal to generate a frequency drift error signal indicative of a drift between the frequency for said fundamental Fourier component and the frequency of said alternating voltage;

means responsive to said frequency drift error signal to alter said phase angle signal for maintaining the frequency of said fundamental Fourier component in synchronism with the frequency of said alternating voltage.

37. A converter as defined in claim 36, wherein said feedback means includes:

first means for producing a signal representative of the voltage across said DC link terminals;

second means for producing a signal representative of said predetermined value;

third means coupled to said first and second means for processing said signal representative of the voltage across said DC link terminals and said signal representative of said predetermined value, and outputting said error signal.

38. A converter as defined in claim 37, wherein said signal processing circuit includes:

fourth means for generating a signal representative of a nominal frequency of said fundamental Fourier component;

fifth means coupled to said fourth means and to said third means, said fifth means processing the signal representative of a nominal frequency of said fundamental Fourier component and said error signal, and outputting said frequency drift error signal.

39. In a converter for interconnecting a first electric network and a second elecric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage,

a process for controlling the amount of real power flowing through said converter, said process comprising the step of;

varying said phase angle signal to bring the amount of real power flowing through said converter at a desired value.

40. A process as defined in claim 39, comprising the step of varying the signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage to keep the amount of real power flowing through said converter at a predetermined value.

41. A process as defined in claim 40, comprising the following steps:

generating an error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value;

altering said phase angle signal in accordance with said error signal to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a real power flow through said converter at said predetermined value.

42. In a converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage, a process for controlling the amount of real power flowing through said converter, said process comprising the steps of;

varying said phase angle signal to bring the amount of real power flowing through said converter at a desired value;

varying the signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage to keep the amount of real power flowing through said converter at a predetermined value, the last mentioned signal varying step comprising;

generating an error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value, and

altering said phase angle signal in accordance with said error signal to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a real power flow through said converter at said predetermined value;

deriving from said error signal a frequency drift error signal representative of a drift between the frequency of said fundamental Fourier component and the frequency of said alternating voltage;

altering said phase angle signal in accordance with said frequency drift error signal for maintaining the frequency of said fundamental Fourier component in synchronism with the frequency of said alternating voltage.

43. A process as defined in claim 42, further comprising the steps of:

generating a signal representative of the amount of real power flowing through said converter;

generating a signal representative of said predetermined value;

adding the signal representative of the amount of real power flowing through said converter and the signal representative of said predetermined value to obtain the error signal representative of a difference between the amount of real power flowing through said converter and said predetermined value.

44. In a converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage,

a process for controlling the voltage across said DC link terminals, said process comprising the step of;

varying said phase angle signal to bring the voltage across said DC link terminals to a desired value.

45. A process as defined in claim 44, comprising the step of varying said phase angle signal to keep the voltage across said DC link terminals at a predetermined value.

46. A process as defined in claim 45, comprising the following steps:

generating an error signal representative of a difference between the voltage across said DC link terminals and said predetermined value;

altering said phase angle signal in accordance with said error signal to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a voltage across said DC link terminals at said predetermined value.

47. In a converter for interconnecting a first electric network and a second electric network to transmit electric power from one network to the other, each of said networks being coupled to a respective active power source, said first network including DC link terminals for coupling said first network to said converter, said second network including AC link terminals for coupling said second network to said converter, there being an alternating voltage at a given frequency across said AC link terminals, said converter comprising:

a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

inductor means between said bridge and said AC link terminals;

valve control means coupled to said bridge to command the valves thereof to switch state to obtain a bridge voltage waveform with a fundamental Fourier component having a frequency corresponding substantially to the frequency of said alternating voltage, said valve control means including a phase angle input means for receiving a phase angle signal representative of a desired phase angle between said fundamental Fourier component and said alternating voltage, in response to said phase angle signal said valve control means outputting a valve state switching signal to obtain a bridge voltage waveform having a fundamental Fourier component at said desired phase angle with said alternating voltage,

a process for controlling the voltage across said DC link terminals, said process comprising the step of;

varying said phase angle signal to bring the voltage across said DC link terminals to a desired value

varying said phase angle signal to keep the voltage across said DC link terminals at a predetermined value, the latter signal varying step comprising the steps of

(a) generating an error signal representative of a difference between the voltage across said DC link terminals and said predetermined value, and

(b) altering said phase angle signal in accordance with said error signal to obtain a bridge voltage waveform with a fundamental Fourier component at a phase angle with said alternating voltage corresponding to a voltage across said DC link terminals at said predetermined value;

deriving from said error signal a frequency drift error signal representative of a drift between the frequency of said fundamental Fourier component and the frequency of said alternating voltage; and

altering said phase angle signal in accordance with said frequency drift error signal for maintaining the frequency of said fundamental Fourier component in synchronism with the frequency of said alternating voltage.

48. A process as defined in claim 47, further comprising the steps of:

generating a signal representative of a voltage across said DC link terminals;

generating a signal representative of said predetermined value;

adding the signal representative of the voltage across said DC link terminals and the signal representative of said predetermined value to obtain the error signal representative of a difference between the voltage across said DC link terminals and said predetermined value.

49. In combination:

a positive DC bus;

a negative DC bus;

a first electric network coupled to an active power source, said first electric network including AC link terminals, there being an alternating voltage across said AC link terminals;

a second electric network coupled to an active power source, said second electric network including AC link terminals, there being an alternating voltage across the AC link terminals of the second electric network;

a first converter coupled to the AC link terminals of said first network, said first converter including DC link terminals coupled to said buses;

a second converter coupled to the AC link terminals of said second network, said second converter including DC link terminals coupled to said buses,

each converter including;

(a) a bridge of valves, each valve of said bridge being capable of assuming two different states, namely an ON state and an OFF state, in said ON state the valve allowing current to pass therethrough, in said OFF state the valve blocking the passage of current;

(b) inductor means between said bridge and the AC link terminals of the network coupled to the converter;

(c) valve control means coupled to said bridge for commanding the valves thereof to switch state to obtain a bridge voltage waveform having a fundamental Fourier component at a certain frequency and at a certain phase angle with the alternating voltage at the AC link terminals of the network coupled to the converter, said valve control means including a phase angle input means for receiving a signal representative of a desired phase angle between said fundamental Fourier component and the alternating voltage at the AC link terminals of the network coupled to the converter, in response to the signal received through said phase angle input means said valve control means generating a valve state switching signal to obtain a fundamental Fourier component at the desired phase angle with the alternating voltage at the AC link terminals of the network coupled to the converter,

first signal processing circuit coupled to the phase angle input means of the valve control means of said first converter, said first signal processing circuit generating a signal to control the phase angle between the fundamental Fourier component of said first converter and the alternating voltage at the AC link terminals of said first network to maintain the voltage across the DC link terminals of said first converter generally constant; and

second signal processing circuit coupled to the phase angle input means of the valve control means of said second converter, said second signal processing circuit generating a signal to control the phase angle between the fundamental Fourier component of the second converter and the alternating voltage at the AC link terminals of said second network to maintain the amount of real power flowing through said second converter at a predetermined level.