Method for controlling a converter

• US 20040246746A1
• Filed: 07/12/2004
• Published: 12/09/2004
• Est. Priority Date: 07/16/2001
• Status: Abandoned Application

• Current Assignee: ABB AB (ABB Limited)
• Original Assignee: ABB AB (ABB Limited)
• Inventors: Norrga, Staffan, Jonsson, Tomas
• Application Number:
  US10/484,091
• Publication Number:
  US 20040246746A1
• Time in Patent Office: Days
• US Class Current: 363/16
• CPC Class Codes: H02M 7/4811 having auxiliary actively switched resonant commutation circuits connected to intermediate DC voltage or between two push-pull branches
• CPC Class Codes: H02M 7/4826 operating from a resonant DC source, i.e. the DC input voltage varies periodically, e.g. resonant DC-link inverters
• CPC Class Codes: H02M 7/487 Neutral point clamped inverters

Abstract

The invention relates to a method for controlling a VSC-converter provided with a resonant circuit (16). In connection with the effectuation of an intended commutation process ordered by the modulator (30), the control device (24) is made to send control signals to the current valves (2, 3) and auxiliary valve (18) that are taking part in the commutation process, for turning on or turning off thereof, at instants (t₁) determined on the basis of a desired commutation instant (t_tr) given by the modulator (30) and a calculation algorithm, which is based upon values of the phase current and the intermediate link voltage and knowledge about the influence the components included in the converter have on the intended commutation process, said calculation algorithm being elaborated in consideration of the condition that the desired commutation instant (t*_tr) is to coincide with an equivalent transition instant (t_tr) for the phase voltage, which equivalent transition instant (t_tr) is estimaged with the aid of knowledge about the influence the components included in the convernter have on the transition of the phase voltage (u_ph(t)) during the intended commutation process.

Claims

1. A method for controlling a VSC converter, which converter comprises

a series connection of at least two current valves arranged between two poles a positive and a negative, of a direct voltage side of the converter, each of which current valves comprising a semiconductor element of turn-off type and a rectifying member connected in anti-parallel therewith, an alternating voltage phase line being connected to a midpoint denominated phase output, of the series connection between two current valves while dividing the series connection into two equal parts,

an intermediate link arranged on the direct voltage side of the converter, which intermediate link comprises at least one intermediate link capacitor,

a resonant circuit comprising at least a capacitive member, an inductor and an auxiliary valve provided with semiconductor components of turn-off type for recharging said capacitive member in connection with commutation of the phase current,

a control device for controlling turn-on and turn-off of the semiconductor elements of turn-off type of the current valves and the semiconductor components of turn-off type of the auxiliary valve, and

means for measuring the phase current,

said turn-on and turn-off being controlled by the control device in dependence on control signals received from a modulator, which signals indicate desired instants for commutation,

wherein the control device, in connection with the effectuation of an intended commutation process ordered by the modulator, sends control signals to the current valves and auxiliary valve that are taking part in the commutation process, for turning on or turning off thereof, at instants determined on the basis of a desired commutation instant given by the modulator and a calculation algorithm, which is based upon knowledge about the influence the components included in the converter have on the intended commutation process, said calculation algorithm being elaborated in consideration of the condition that the desired commutation instant is to coincide with an equivalent transition instant for the phase voltage, which equivalent transition instant is given by the condition

${\int }_{}^{}$

ttr-tcttr=tc

up

 



h

(t)



t=0

where u_ph(t) is the phase voltage, t_tr is the equivalent transition instant and t_c is so chosen that the changing of the phase voltage (u_ph(t)) has not began at the instant t_tr−

t_c and has been completed at the instant t_tr+t_c.

2. The method according to claim 1, wherein the value of the intermediate link voltage, the value of the phase current, the value of the snubber capacitance and the value of the inductance of the resonant circuit is taken into consideration in said calculation algorithm.

3. The method according to claim 2, wherein the value of the inductance of the resonant circuit and/or the value of the snubber capacitance is determined by measurings performed on the converter.

4. The method according to claim 3, wherein the measurings for determining the value of the inductance of the resonant circuit and/or the value of the snubber capacitance are performed repeatedly during the service life of the converter in order to compensate for changes of these values.

5. The method according to claim 3, wherein the value of the inductance of the resonant circuit and/or the value of the snubber capacitance is continuously updated and adjusted based upon measurings performed during previous commutation processes.

6. The method according to claim 2, wherein the value of the voltages across the intermediate link capacitors is determined by continuous measurings.

7. The method according to claim 2, wherein the variation of the phase current during the intended commutation process is taken into consideration in said calculation algorithm.

8. The method according to claim 7, wherein a linear model of the load connected to the phase output is used in order to describe the variation, of the phase current during the intended commutation process.

9. The method according to in that claim 1, wherein parameter values included in the calculation algorithm are continuously updated and adjusted based upon measurings performed during previous commutation processes.

10. The method according to claim 1, wherein the converter is an ARCP-converter, the intermediate link comprising a series connection of at least two intermediate link capacitors arranged between the two poles of the direct voltage side of the converter, and the resonant circuit comprising a series connection of an inductor and an auxiliary valve arranged between the phase output and a midpoint of said series connection of intermediate link capacitors said auxiliary valve comprising at least two semiconductor components of turn-off type arranged in opposite polarity in relation to each other, the resonant circuit further comprising capacitive members, each of which being connected in series with said inductor and auxiliary valve and in parallel with one of said current valves.

11. The method according to claim 10, wherein at a commutation process implying a commutation, not assisted by the resonant circuit, of the phase current from a semiconductor element of turn-off type of a first current valve to a rectifying member of a second current valve, the control device is made to send:

a turn-off signal to the first current valve at a first instant given by the formula

(t_II)=t*_tr−

½

T_I−

t_d1

where t_II is said first instant, t*_tr the desired commutation instant given by the modulator, T_I the estimated duration of the commutation process, and t_d1 the estimated time delay from the instant the turn-off signal is sent from the control device to the instant the turn-off of the semiconductor element of the first current valve is effectuated, and

a turn-on signal to the second current valve at or after a second instant given by the formula

(t_I2)=t*_tr−

½

T_I−

t_d2

where t_I2 is said second instant and t_d2 the estimated time delay from the instant the turn-on signal is sent from the control device to the instant the turn-on of the semiconductor element of the second current valve is effectuated.

12. The method according to claim 11, wherein T_I is given by the formula

13. The method according to claim 12, wherein the formula for calculating T_I is adjusted in consideration of the reverse recovery of the semiconductor element of the first current valve.

14. The method according to claim 10, wherein at a commutation process implying a commutation, assisted by the resonant circuit, of the phase current from a semiconductor element of turn-off type of a first current valve to a rectifying member of a second current valve the control device is made to send:

a turn-on signal to the auxiliary valve at a first instant (t_II1) given by the formula

(t_II1)=t*_tr−

½

T_II−

t_d3

where t_II1 is said first instant, t*_tr the desired commutation instant given by the modulator, T_II the estimated duration of the commutation process, and t_d3 the estimated time delay from the instant the turn-on signal is sent from the control device to the instant the turn-on of the intended semiconductor component of the auxiliary valve is effectuated,

a turn-off signal to the first current valve at or before a second instant given by the formula

(t_II2)=t*_tr−

½

T_II−

t_d4

where t_II2 is said second instant and t_d4 is the estimated time delay from the instant the turn-off signal is sent from the control device to the instant the turn-off of the semiconductor element of the first current valve is effectuated,

a turn-on signal to the second current valve at a third instant given by the formula

(t_II3)=t*_tr−

1/2T_II−

t_d5

where t_II3 is said third instant and t_d5 is the estimated time delay from the instant the turn-on signal is sent from the control device to the instant the turn-on of the semiconductor element of the second current valve is effectuated, and

a turn-off signal to the auxiliary valve at or after a fourth instant given by the formula

(t_II4)=t*_tr−

½

T_II−

t_d6

where t_II4 is said fourth instant and t_d6 is the estimated time delay from the instant the turn-off signal is sent from the control device to the instant the turn-off of the intended semiconductor component of the auxiliary valve is effectuated.

15. The method according to claim 14, wherein T_II is given by the formula

16. The method according to claim 15, wherein the formula for calculating T_II is adjusted in consideration of the reverse recovery of the semiconductor element of the first current valve and/or the power dissipations in the resonant circuit and/or unequalities in the voltage distribution between the intermediate link capacitors.

17. The method according to claim 10, wherein at a commutation process implying a commutation, assisted by the resonant circuit, of the phase current from a rectifying member of a first current valve to a semiconductor element of turn-off type of a second current valve the control device is made to send:

a turn-on signal to the auxiliary valve at a first instant given by the formula

(t_III1)=t*_tr−

T_ru−

½

T_res−

t_d7

where t_III1 is said first instant, t*_tr the desired commutation instant given by the modulator, T_res the estimated duration of the resonance period, T_ru the estimated duration of the ramp-up period and t_d7 the estimated time delay from the instant the turn-on signal is sent from the control device to the instant the turn-on of the intended semiconductor component of the auxiliary valve is effectuated,

a turn-off signal to the first current valve at or before a second instant given by the formula

(t_III2)=t*_tr−

T_ru−

½

T_res−

t_d8

where t_III2 is said second instant and t_d8 is the estimated time delay from the instant the turn-off signal is sent from the control device to the instant the turn-off of the semiconductor element of the first current valve is effectuated,

a turn-on signal to the second current valve at a third instant given by the formula

(t_III3)=t*_tr+½

T_res−

t_d9

where t_III3 is said third instant and t_d9 the estimated time delay from the instant the turn-on signal is sent from the control device to the instant the turn-on of the semiconductor element of the second current valve is effectuated, and

a turn-off signal to the auxiliary valve at or after a fourth instant given by the formula

(t_III4)=t*_tr+½

T_res+T_rd−

t_d10

where t_III4 is said fourth instant, T_rd the estimated duration of the ramp-down period and t_d10 the estimated time delay from the instant the turn-off signal is sent from the control device to the instant the turn-off of the intended semiconductor component of the auxiliary valve is effectuated.

18. The method according to claim 17, wherein:

T_ru is given by the formula

$T_{\mathrm{ru}}=\frac{L_{\mathrm{res}}·<br><br><br><br>i_{p<br><br>\text{\hspace{1em}<br><br>}<br><br>h}<br><br>}{u_{\mathrm{di}}}$

where L_res is the inductance of the resonant circuit, i_ph the phase current, u_di the voltage between a first one of the poles and the midpoint of the intermediate link when the phase current is commutated from a current valve arranged between said first pole and the phase output to a current valve arranged between the second pole and the phase output whereas u_di is the voltage between the second pole and the midpoint of the intermediate link when the phase current is commutated from a current valve arranged between the second pole and the phase output to a current valve arranged between the first pole and the phase output,

T_rd is given by the formula

$T_{r<br><br>\text{\hspace{1em}<br><br>}<br><br>d}=\frac{L_{\mathrm{res}}·<br><br><br><br>i_{p<br><br>\text{\hspace{1em}<br><br>}<br><br>h}<br><br>}{u_{\mathrm{dj}}}$

where u_dj is the voltage between the second pole and the midpoint of the intermediate link when the phase current is commutated from a current valve arranged between the first pole and the phase output to a current valve arranged between the second pole and the phase output, whereas u_dj is the voltage between the first pole and the midpoint of the intermediate link when the phase current is commutated from a current valve arranged between the second pole and the phase output to a current valve arranged between the first pole and the phase output, and

T_res is given by the formula

$T_{\mathrm{res}}=\frac{\mathrm{\pi <br><br>}}{{\mathrm{\omega <br><br>}}_0}$$\mathrm{where}<br><br>\text{\hspace{1em}<br><br>}<br><br>{\mathrm{\omega <br><br>}}_0=\frac{1}{\sqrt{L_{\mathrm{res}}·<br><br>C_s}}.$

19. The method according to claim 18, wherein the formula for calculating T_res is adjusted in consideration of the reverse recovery of the rectifying members of the current valves and/or the power, dissipations in the resonant circuit and/or unequalities in the voltage distribution between the intermediate link capacitors.

20. The method according claim 11, wherein the respective time delay is estimated with the aid of a measured value of the phase current.

21. The method according to claim 1, wherein the control device receives the control signals that are indicating the desired commutation instants from a PWM-modulator.