Controller for performing a decoupling control of a transformerless reactive series compensator

• US 6,225,791 B1
• Filed: 06/02/2000
• Issued: 05/01/2001
• Est. Priority Date: 12/15/1999
• Status: Expired due to Fees

• Current Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Original Assignee: Mitsubishi Denki Kabushiki Kaisha (Mitsubishi Electric Corporation)
• Inventors: Beer, Andreas, Fujii, Toshiyuki
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US09/585,551
• Time in Patent Office: 333 Days
• Field of Search: 323/205, 323/207, 363/41, 363/43
• US Class Current: 323/207
• CPC Class Codes: H02J 3/1814 wherein al least one reactive element is actively controlled by a bridge converter, e.g. unified power flow controllers [UPFC]
• CPC Class Codes: Y02E 40/10 Flexible AC transmission systems [FACTS]

Abstract

A controller for generating a modulation signal m applied to a transformerless reactive series compensator connected serially in a power transmission line. A modulation signal generator generates a modulation signal m in the form of m=m_d cos (ωt)−m_q sin (wt). The controller includes a current control loop with a current controller outputting the modulation index m_q of the modulation signal m. A voltage control loop including a DC voltage controller outputs the modulation index m_d, of the modulation signal m. The current control loop and the voltage control loop are respectively provided for outputting indices m_q′, m_d′ in order to control a line current and a DC capacitor voltage of a capacitor of the compensator to reference values. The controller includes a decoupler so that the line controller and the capacitor voltage are independent of the output of the current controller. Therefore, the amplitude of the line current and the magnitude of the DC capacitor voltage can be controlled completely independently from each other, i.e., the control loops are decoupled.

Claims

1. A controller for controlling a reactive series compensator inserted into a power transmission line comprising:

line current detection means for detecting a line current flowing in a power transmission line;

DC voltage detection means for detecting a DC voltage of a capacitor connected to a modulator of an inverter of a reactive series compensator;

modulation signal generation means for generating an inverter modulation signal m=m_d cos (ω

t)−

m_q sin (wt), where ω

is a line frequency to be supplied as a modulation signal to the modulator of the compensator;

a current loop for controlling the line current to a reference current, a current controller of the current control loop outputting a modulation index m_q′

for the modulation signal;

a voltage control loop for controlling the DC voltage of the capacitor to a reference voltage, a DC voltage controller of the voltage control loop outputting a modulation index m_d′

or the modulation signal; and

decoupling control means receiving the modulation index m_q′

from the current controller and the modulation index m_d′

of the DC voltage controller and outputting new modulation indices m_q and m_d to the modulation signal generation means so that the line current is independent of m_d′ and

the DC voltage of the capacitor is independent of m_q′

, whereby an AC current amplitude and the DC voltage of the capacitor are controlled independently.

2. The controller according to claim 1, wherein the current control loop comprises a current subtractor for subtracting an active current amplitude of the line current from the reference current to produce a difference current and for outputting the difference current to the current controller.

3. The controller according to claim 2, wherein the voltage control loop comprises a current subtractor for subtracting the DC voltage of the capacitor from the reference voltage to produce a difference voltage and for outputting the difference voltage to the DC voltage controller.

4. The controller according to claim 3, wherein the modulation signal generation means comprises coordinate transformation means receiving a first reference signal (sin (ω

t)), a second reference signal (cos (ω

t)), and the modulation indexes m_q′ and

m_d′

, and for outputting the modulation signal.

5. The controller according to claim 4, wherein the power transmission line is a three-phase system having three phases, and the compensator, the DC voltage controller, the decoupling means, and the modulation signal m are provided for each of the three phases and a single current controller is provided.

6. The controller according to claim 5, comprising:

a voltage detector for detecting the line voltage of each phase;

a voltage phased locked loop (PLL) detector unit for receiving the line voltage from the voltage detector and for outputting a third reference signal sin ω

t and a fourth reference signal cos ω

t, each of the third and fourth reference signals being synchronized to the phase of the line voltage and the line frequency;

phase rotation means for receiving the third and fourth reference signals and a phase signal and for generating the first and second reference signals to be supplied to the coordinate transformation means; and

a three-phase polar transformation unit receiving a line current for each phase and for outputting an active current amplitude to the current subtractor and the decoupling means, and the phase signal to the phase rotation means.

7. The controller according to claim 5, comprising:

a current phased locked loop (PLL) detector unit receiving the line current (i) from the line current detection means and for outputting as the first reference signal sin ω

t, and as the second reference signal cos ω

t, each of the first and second reference signals being synchronized to the phase of the line current and the line frequency, to the decoupling means; and

a component detector for receiving the line current and for outputting an active current amplitude to the current subtractor and the decoupling means, and a reactive current amplitude to the decoupling means.

8. The controller according to claim 4, wherein the power transmission line is a single-phase system, and consisting of a single compensator, the DC voltage controller, the decoupling means, the coordinate transformation means, and the current controller.

9. The controller according to claim 8, comprising:

a current phased locked loop (PLL) detector unit receiving the line current (i) from the line current detection means and for outputting as the first reference signal sin ω

t, and as the second reference signal cos ω

t, each of the first and second reference signals being synchronized to the phase of the line current and the line frequency, to the decoupling means; and

a component detector for receiving the line current and for outputting an active current amplitude to the current subtractor and the decoupling means, and a reactive current amplitude to the decoupling means.

10. The controller according to claim 9, wherein the decoupling control means comprises:

a first multiplier for multiplying the DC voltage of the capacitor and the line frequency output by the current PLL detector unit to produce a first product;

a first divider for dividing from the current by the first product output by the first multiplier to produce a first dividend;

a second multiplier for multiplying the reactive current amplitude output by the component detector by the first dividend produced by the first divider to produce a second product;

a first adder for subtracting the second product from m_q′

*K_d to produce a first sum;

a second divider for dividing the first sum by the active component amplitude output by the component detector to produce a second dividend;

derivation means comprising a second adder receiving the second dividend and an output of an integrator to produce a second sum, a time lag unit receiving the second sum and feeding the second sum to an input of the integrator, wherein the integrator produces a modified modulation index, m_d;

a third divider dividing the input of the integrator by the line frequency output by the current PLL detector unit to produce a third dividend; and

a third adder for subtracting the third dividend from the first dividend, and the third adder (25i) outputting as a third sum a modified modulation index, m_q.

11. The controller according to claim 9, wherein the component detector comprises first and second multipliers for respectively multiplying the line current and the first and second reference signals (sin (ω

t), cos (ω

t)) and first and second filters for respectively passing a fundamental frequency output by the first and second multipliers, outputs of the first and second filters constituting the reactive current amplitude and the active current amplitude of the line current.

12. The controller according to claim 6, wherein each of the decoupling control means comprises:

a first multiplier for multiplying the DC voltage of the capacitor by the line frequency output by the current PLL detector unit to produce a first product;

a first divider for dividing the output (m_q′

) of the current controller (20) by the first product to produce a first dividend;

a second divider for dividing the output (m_d′

) of the DC voltage controller by the active current amplitude output by the three-phase-polar transformation unit to produce a second dividend;

derivation means comprising a first adder, a time lag unit, and an integrator, the first adder subtracting an output of the integrator from the second dividend to produce a first sum and outputting the first sum to the time lag unit, the time lag unit outputting a result as an input to the integrator, and the integrator outputting the modified modulation index, m_d;

a third divider for dividing the input to the integrator by the line frequency output by the phase detector to produce a third dividend; and

a second adder for subtracting the third dividend from first dividend, the second adder outputting as the modified modulation index m_q as a second sum.

13. The controller according to claim 4, wherein the modulation signal generation means further comprises voltage fluctuation compensation means receiving the modulation signal from the coordinate transformation means and outputting the modulation signal m to the inverter of the compensator wherein the voltage fluctuation compensation means reduces fluctuations in the modulation signal due to voltage fluctuations of the DC voltage of the capacitor.

14. The controller according to claim 4, wherein the coordinate transformation means comprises a first multiplier and a second multiplier respectively multiplying the output of the current m_q′

and the first reference signal (sin (ω

t)) and the output m_d′

of the DC voltage controller and the second reference signal (cos (ω

t)), and a subtractor for subtracting a multiplication result produced by the first multiplier from a multiplication result produced by the second multiplier.

15. The controller according to claim 13, wherein the voltage control loop further comprises a filter for filtering the DC voltage of the capacitor and the voltage fluctuation compensation means comprises a divider dividing an output of the filter by an input of the filter and a multiplier multiplying an output of the coordinate transformation means and an output of the divider, a multiplication result output by the multiplier being the modulation signal m fed to the inverter of the compensator.

16. The controller according to claim 5, including the filter and the voltage fluctuation compensation means for each of the phases.

17. The controller according to claim 8, consisting of one filter and one voltage fluctuation compensation means.

18. The controller according to claim 1, wherein the controller controls a transformerless reactive series capacitor.

19. The controller according to claim 1, wherein the controller controls a reactive series compensator having a transformer.

20. The controller according to claim 7, wherein the component detector comprises first and second multipliers for respectively multiplying the line current and the first and second reference signals (sin (ω

t), cos (ω

t)) and first and second filters for respectively passing a fundamental frequency output by the first and second multipliers, outputs of the first and second filters constituting the reactive current amplitude and the active current amplitude of the line current.