Doubly fed generator variable speed generation control system

• US 4,994,684 A
• Filed: 01/30/1989
• Issued: 02/19/1991
• Est. Priority Date: 01/30/1989
• Status: Expired due to Fees

• Current Assignee: Oregon State University (Oregon University System)
• Original Assignee: Oregon State University (Oregon University System)
• Inventors: Alexander, Gerald C., Lauw, Hian K.
• Primary Examiner(s): PASCHALL, MARK H
• Application Number:
  US07/304,044
• Time in Patent Office: 750 Days
• Field of Search: 290/43, 290/44, 290/54, 290/55, 290/40 C, 290/52, 322/14-17, 322/29, 322/32, 322/4, 318/800, 318/807, 318/821
• US Class Current: 290/52
• CPC Class Codes: H02P 2101/15 for wind-driven turbines
• CPC Class Codes: H02P 9/42 to obtain desired frequency without varying speed of the generator

Abstract

Disclosed is a device and method to control the conversion of an arbitrary resource energy into mechanical energy and subsequently into electric energy with a variable-speed generation system utilizing a turbine and a doubly-fed generator, to be connected directly to a power grid of a certain grid frequency. Via electronic control of the rotor speed by controlling the rotor winding excitation frequency, the device has the capability of providing an optimum tradeoff in maximum turbine efficiency and minimal generator losses to secure maximum efficiency of the total conversion process, irrespective of varying resource and electric load conditions. The excitation requirements of the generator are confined to moderate levels and yet a wide rotor-speed variation margin, starting from zero speed, is permitted. An integrated strategy is implemented which effectively coordinates the execution of these tasks and the independent control of the generator terminal voltage as well as the active and reactive electric power output. All signal processing as required correspondingly is carried out without the need for cumbersome measurements of turbine input and output power or generator losses.

Claims

1. An energy conversion generation system for receiving resource energy from a resource and converting the resource energy into an electrical power output for supply to a polyphase electric power grid operating at a system frequency, the energy generation conversion system comprising:

a variable speed generation system including turbine means for converting a resource energy input from the resource into mechanical energy at a rotor speed;

energy converter means for producing excitation power;

a doubly fed generator coupled to the turbine means and having a rotor with excitation rotor windings and with means for applying the excitation power from the energy converter means to the rotor windings, the rotor being driven by the mechanical energy from the turbine means, and a stator with stator windings to supply the electrical power output to the power grid; and

control means for varying the rotor speed in response to the power output and resource energy input to increase the ratio of the electrical output power to the resource energy input received from the resource.

2. A system according to claim 1 in which the control means also includes means for controlling the resource energy input from the resource.

3. An energy conversion generation system for receiving resource energy from a resource and converting the resource energy into an electrical power output for supply to a polyphase electric power grid operating at a system frequency, the energy conversion system comprising:

a variable speed generation system including turbine means for converting a resource energy input from the resource into mechanical energy at a rotor speed;

energy converter means for producing excitation power;

a doubly fed generator coupled to the turbine means and having a rotor with excitation rotor windings and with means for applying the excitation power from the energy converter means to the rotor windings, the rotor being driven by the mechanical energy from the turbine means, and a stator with stator windings to supply the electrical power output to the power grid;

input sensor means for sensing at least one parameter of the resource energy received by the turbine means, and for producing a turbine input sensor signal;

generator output sensor means for sensing the output power from the doubly fed generator which is supplied to the power grid, and for producing a generator output sensor signal;

converter signal processing means for receiving and processing the generator output sensor signal and the turbine input sensor signal, the converter signal processing means comprising means for producing a converter control signal in response to the generator output sensor signal and the turbine input sensor signal; and

the energy converter means comprising means for controlling the excitation power in response to the converter control signal to vary the rotor speed to reduce power losses in the generator.

4. An energy conversion generation system for receiving resource energy from a resource and converting the resource energy into an electrical power output for supply to a polyphase electric power grid operating at a system frequency, the energy conversion system comprising:

a variable speed generation system including turbine means for converting a resource energy input from the resource into mechanical energy at a rotor speed, the variable speed generation system further including energy converter means for producing excitation power, and a doubly fed generator coupled to the turbine means and having a rotor with excitation rotor windings and with means for applying the excitation power from the energy converter means to the rotor windings, the rotor being driven by the mechanical energy from the turbine means, and a stator with stator windings to supply the electrical power output to the power grid;

turbine input sensor means for sensing at least one parameter of the resource energy input received by the turbine means, and for producing a turbine input sensor signal;

generator output sensor means for sensing the output from the doubly fed generator supplied to the power grid, and for producing a generator output sensor signal;

a variable speed generation controller including turbine signal processing means for receiving and processing the generator output sensor signal to produce a turbine control signal, and converter signal processing means for receiving and processing the generator output sensor signal, the turbine input sensor signal, and the turbine control signal to produce the converter control signal;

resource energy adjustment means coupled to the variable speed generation controller for receiving the turbine control signal and for controlling the energy delivered to the turbine means, and thereby the resource energy converted into rotational mechanical energy, in response to the turbine control signal;

the energy converter means comprising means for controlling the excitation power in response to the converter control signal; and

the variable speed generation controller comprising means for controlling the excitation power and the resource input energy delivered to the turbine means to control the efficiency of the variable speed generation system, irrespective of variations in the resource energy.

5. An energy conversion generation system according to claim 4 further including means for producing a power demand signal, the turbine signal processing means also receiving and processing the power demand signal to produce the turbine control signal, the system also including means for producing a generator reference demand signal, the converter signal processing means also receiving and processing the generator reference demand signal to produce the converter control signal.

6. An energy conversion generation system according to claim 5 wherein the generator output sensor means comprises generator terminal voltage sensor means for sensing the generator terminal voltage and for producing a terminal voltage sensor signal, and generator output power sensor means for sensing generator power output and for producing a power output sensor signal.

7. An energy conversion generation system according to claim 6 including means for producing a generator voltage reference demand signal and wherein the converter signal processing means includes voltage subcontroller means for subtracting the terminal voltage sensor signal from the generator voltage reference demand signal to produce a difference signal, the converter signal processing means also including means responsive to the difference signal for producing a maximum rotor excitation signal, the converter signal processing means also including rotor speed subcontroller means for receiving and processing the generator output power sensor signal, the turbine input sensor signal and the turbine control signal, and for producing an excitation frequency signal, and the converter signal processing means further including signal generator means for receiving and processing the maximum rotor excitation signal and the excitation frequency signal, and for producing the converter control signal.

8. An energy conversion generation system according to claim 7 wherein the maximum rotor excitation signal represents a maximum rotor winding current amplitude component of the excitation power applied to the rotor windings.

9. An energy conversion generation system according to claim 4 wherein the energy converter means comprises means operable in a controllable current source mode, and the converter control signal represents a rotor winding excitation current signal.

10. An energy conversion generation system according to claim 4 wherein the resource is hydro having a pressure head and a flow rate, and the turbine input sensor means including pressure head sensor means for sensing the parameter of the hydro pressure head and for producing a first turbine input signal comprising a head signal, the turbine input sensor means also including change in flow rate sensor means for sensing the parameter of a change in the flow rate with respect to a previous flow rate and for producing a second turbine input signal comprising a change in flow rate signal, the generator output sensor means comprises power sensor means for sensing generator power output and for producing a power output sensor signal, and the converter signal processing means including hydro turbine rotor speed controller means for receiving and processing the power output sensor signal, the turbine control signal, the head signal and the change in flow rate signal and for producing an excitation frequency signal in response to the received signals, the converter signal processing means also comprising means for producing the converter control signal in response to the excitation frequency signal.

11. An energy conversion generation system according to claim 10 wherein the resource energy adjustment means comprises gate means for varying the hydro flow through the turbine means, and wherein the turbine control signal comprises a gate position control signal, the gate means being responsive to the gate position control signal to vary the hydro flow through the turbine means.

12. An energy conversion generation system according to claim 4 wherein the resource is wind, and wherein the turbine means comprises wind turbine means having blade means for converting the wind into mechanical energy, and wherein the resource energy adjustment means comprises turbine blades with an adjustable pitch angle, and wherein the turbine control signal comprises a blade pitch adjustment signal, the blade means being responsive to the blade pitch adjustment signal to vary the blade pitch and thereby the amount of energy input to the turbine means from the resource.

13. An energy conversion generation system according to claim 4 further including tapping means for tapping a portion of the electrical power output from the stator windings and supplying the tapped portion of the electrical power output to the energy converter means, thereby establishing a tapped excitation system wherein the doubly fed generator output power comprises an excitation power source for the energy converter means.

14. An energy conversion generation system according to claim 4 further including means for supplying the energy converter means with power from one of the group of external excitation power sources comprising the power grid, a battery source, and a separate generator.

15. A method for converting resource energy from a resource into electrical power for supply to a polyphase electric power grid operating at a system frequency, using a doubly fed generator coupled to a turbine having an adjustable turbine setting, the method comprising the steps of:

monitoring the resource energy received by the turbine and producing a turbine input sensor signal;

sensing the doubly fed generator output power supplied to the power grid and producing a generator output power sensor signal;

sensing the generator output terminal voltage and producing a generator output terminal voltage signal;

setting a power demand signal;

setting a generator voltage reference demand signal;

processing the generator output power sensor signal and the power demand signal to produce a turbine control signal;

processing the generator output power sensor signal, the turbine input sensor signal, the turbine control signal, a generator voltage reference demand signal, and the generator output terminal voltage signal to produce a converter control signal;

receiving and converting the resource energy into mechanical energy with the turbine;

driving the coupled doubly fed generator at a rotor speed with the mechanical energy supplied by the turbine;

adjusting the resource energy received by the turbine to control the rotor speed;

converting power received from an excitation power source into excitation power for the doubly fed generator in response to the converter control signal, and

exciting rotor windings of the doubly fed generator with the excitation power to produce generator output power at the system frequency for supply to the electric power grid.

16. A method for converting resource energy according to claim 15 wherein the sensing step further comprises sensing the generator terminal voltage and producing a terminal voltage sensor signal.

17. A method for converting resource energy according to claim 15 wherein the processing step which produces the converter control signal comprises the steps of:

subtracting a generator output terminal voltage signal from the generator voltage reference demand signal to produce a voltage difference signal; and

regulating the generator voltage in response to the voltage difference signal to produce a maximum excitation signal corresponding to the maximum amplitude of the current component of the excitation power, the processing step which produces the converter control signal also comprising the steps of receiving and processing the generator output power sensor signal, the turbine input sensor signal and the turbine control signal to produce an excitation frequency signal, and the processing step which produces the converter control signal further comprising the signal generating step of receiving and processing the maximum excitation signal and the excitation frequency signal to produce the converter control signal.

18. A method for converting resource energy according to claim 15 wherein the receiving step comprises receiving hydro resource energy having a pressure head and a flow rate through the turbine;

and

wherein the monitoring step comprises the steps of (a) monitoring the pressure head and producing a first turbine input sensor signal comprising a head signal, and (b) monitoring a change in the flow rate through the turbine with respect to a previous flow rate and producing a second turbine input sensor signal comprising a change in flow rate signal.

19. A method for converting resource energy according to claim 15 further including the step of tapping a portion of the generator output power to serve as the excitation power source.

20. A variable speed generation controller for controlling a variable speed generation system having a turbine which receives resource energy from a resource, a doubly fed generator driven by the turbine at a rotor speed, and a converter responsive to a converter control signal for exciting the doubly fed generator with excitation power, the generation system producing electrical power for supply to a polyphase electrical power grid operating at a system frequency, the system providing signals including a turbine input sensor signal and a generator output power sensor signal, the variable speed generation controller comprising:

rotor speed subcontroller means for receiving and processing the turbine input sensor signal and the generator output power sensor signal, the rotor speed subcontroller means comprising means for producing an excitation frequency signal from the received and processed signals; and

signal generator means for receiving the excitation frequency signal, the signal generator means comprising means for varying the rotor speed in response to the power output and resource energy input to increase the ratio of the electrical power supplied to the grid to the resource energy received from the resource.

21. A variable speed generation controller for controlling a variable speed generation system having a turbine responsive to a turbine control signal which receives resource energy from a resource, a doubly fed generator driven by the turbine at a rotor speed, and a converter responsive to a converter control signal for exciting the doubly fed generator with excitation power converted from an excitation power source, the generation system producing electrical power having real and reactive components for supply to a polyphase electrical power grid operating at a system frequency, the system providing signals including a turbine input sensor signal, a generator terminal voltage sensor signal, an output power sensor signal, a power demand signal and a generator voltage reference demand signal, the variable speed generation controller comprising:

first voltage subcontroller means for receiving and processing the generator terminal voltage sensor signal and the generator voltage reference demand signal, the first voltage subcontroller means comprising means for controlling the reactive component of the electrical power by producing a maximum excitation amplitude signal from the received and processed signals;

second turbine power subcontroller means for receiving and processing the output power sensor signal and the power demand signal, the second turbine power subcontroller means comprising means for controlling the real component of the electrical power by producing a turbine control signal from the received and processed signals, the resource energy received by the turbine being controlled in response to the turbine control signal;

third rotor speed subcontroller means for receiving and processing the turbine input sensor signal, the output power sensor signal and the turbine control signal, the third rotor speed subcontroller means comprises means for producing an excitation frequency signal from the received and processed signals; and

signal generator means for receiving the maximum excitation amplitude signal and the excitation frequency signal, the signal generator means comprising means for controlling the converter and the generator excitation by producing the converter control signal from the received and combined signals.

22. A variable speed generation system controller according to claim 21 wherein the rotor speed subcontroller means has a response speed which is faster than the response speed of the first and second subcontroller means and faster than changes in conditions of the resource energy.

23. A variable speed generation controller according to claim 21 wherein the rotor speed subcontroller means includes:

turbine efficiency maximizer means for receiving and processing the turbine control signal and the turbine input sensor signal, the turbine efficiency maximizer means comprising means for producing a turbine maximum efficiency excitation frequency signal;

variable speed generation system efficiency maximizer means for receiving and processing the output power sensor signal, the variable speed generation system efficiency maximizer means comprising means for producing a step change in the excitation frequency signal; and

summer means for receiving and combining the turbine maximum efficiency excitation frequency signal and the step change in the excitation frequency signal, the summer means comprising means for producing a summer output excitation frequency signal corresponding to the excitation frequency signal.

24. A variable speed generation controller according to claim 23, wherein the rotor speed subcontroller means further includes oversynchronous excitation protector means for receiving and limiting the summer output excitation frequency signal and for producing the excitation frequency signal.

25. A variable speed generation controller according to claim 23, wherein the turbine efficiency maximizer means includes turbine characteristics storage means for storing the input/output characteristics of the turbine;

matching means for matching the turbine control signal with the input/output turbine characteristics to produce a turbine characteristics output signal; and

means for receiving and processing the turbine characteristics output signal and the turbine input sensor signal, and for producing the turbine maximum efficiency excitation frequency signal.

26. A variable speed generation controller according to claim 25, wherein the variable speed generation system efficiency maximizer means further includes:

monitoring means for monitoring or checking for changes in the turbine maximum efficiency excitation frequency signal, the power demand signal, the generator voltage reference demand signal, and the reactive component of the electrical power, the monitoring means comprising means for selectively activating the variable speed generation system efficiency maximizer if such changes are discovered.

27. A variable speed generation controller according to claim 26, wherein the variable speed generation system efficiency maximizer means further includes:

29. A variable speed generation controller according to claim 21, wherein the turbine power subcontroller means includes:

subtracting means for subtracting the generator output power sensor signal from the power demand signal to produce a power difference signal;

oscillation limiter means for receiving the power difference signal, the deadband limiting means comprising means for limiting undesirable oscillations of the turbine power subcontroller means, the oscillation limiter comprising means for limiting the power difference signal to a value within deadband limits by producing a deadband output signal which is received by the turbine power subcontroller means.

28. new initial value establishing means for establishing an initial step change in the excitation frequency signal at a negative initial frequency step change value, the initial value establishing means further comprising means for measuring an initial value of the output power sensor signal, and means for setting an initial iteration step equal to an initial iteration step value;

measuring means for measuring a new value of the output power sensor signal;

comparator means for comparing the new value of the output power sensor signal with the initial value of the output power sensor signal, the comparator means comprising means for stopping the variable speed generation system efficiency maximizer if the new value is less than or equal to the initial value, the comparator means further comprising means for producing a comparator output signal if the new value is greater than the initial value of the output power sensor signal;

output means for receiving the comparator output signal, the output means comprising means for multiplying the iteration step value by the negative initial frequency step change value to produce an iterated frequency step change signal;

frequency summer means for receiving and summing the initial step change in the excitation frequency signal with the iterated frequency step change signal to produce the step change in the excitation frequency signal; and

speed settling delay means for establishing a delay between successive steps.

30. An energy conversion generation system for receiving resource energy from a resource and converting the resource energy into an electrical power output for supply to a polyphase electrical power grid operating at a system frequency, the energy conversion generation system comprising:

a variable speed generation system including a turbine of a type which converts a resource energy input from the resource into mechanical energy at a rotor speed;

an energy converter of a type which produces excitation power;

a doubly-fed generator coupled to the turbine and having a rotor with excitation rotor windings which receive the excitation power from the energy converter, the rotor being driven by the mechanical energy from the turbine, with the doubly-fed generator also having a stator with stator windings to supply the electrical power output to the power grid; and

a controller for varying the rotor speed in response to the power output and resource energy input to increase the ratio of the electrical output power to the resource energy input received from the resource.

31. A variable speed generation controller for controlling a variable speed generation system having a turbine which receives resource energy from a resource, a doubly-fed generator driven by the turbine at a rotor speed, and a converter responsive to a converter control signal for exciting the doubly-fed generator with the excitation power, the generation system producing electrical power for supply to a polyphase electric power grid operating at a system frequency, the system providing signals including a turbine input sensor signal and a generator output power sensor signal, the variable speed generation controller comprising:

a rotor speed subcontroller of a type which receives and processes the turbine input sensor signal and the generator output power sensor signal, with the rotor speed subcontroller producing an excitation frequency signal in response to the turbine input sensor signal and the generator output power sensor signal; and

a signal generator which receives the excitation frequency signal and produces the converter control signal in response to the excitation frequency signal so as to vary the rotor speed in response to the electrical power produced by the generation system and the resource energy received by the turbine so as to increase the ratio of the electrical power supplied to the grid to the resource energy received from the resource.