Operating method of full-bridge modular multilevel converter boosting AC voltages for high voltage direct current transmission

Patent metadata

Field	Value
Publication number	US-9735702-B2
Application number	US-201615064246-A
Country	US
Kind code	B2
Filing date	Mar 8, 2016
Priority date	Dec 11, 2015
Publication date	Aug 15, 2017
Grant date	Aug 15, 2017

Abstract

Provided is an operating method of a full-bridge sub-module (FBSM)-based modular multilevel converter for HVDC transmission with AC-side voltage boosting. The peak value of the AC-side voltage is increased under a constant DC-link voltage by using FBSM's negative output voltage under steady state, wherein keeping the semiconductor's current rating constant during AC-side voltage boosting is in favor of reducing converter cost by decreasing energy interaction between the upper and lower arms in a leg, and further capacitance value of FBSM's capacitor under a constant capacitor voltage ripple, keeping the RMS value of AC-side current constant during AC-side voltage boosting can effectively improve transmission capacity of the converter while reducing converter cost, and keeping converter transmission capacity constant during AC-side voltage boosting can reduce RMS value of arm currents while reducing converter cost, thereby reducing power loss of FBSMs and improving converter efficiency.

First claim

What is claimed is:

1. An operating method of a full-bridge sub-module (FBSM)-based modular multilevel converter with AC-side voltage boosting, comprising:
(1) obtaining a rated DC-link voltage U dc and a rated capacitor voltage of FBSMs U c ;
(2) calculating the number of FBSMs in each arm N satisfying AC-side voltage boosting requirements without considering the redundancy of FBSMs using a converter modulation index m after AC-side voltage boosting, the rated DC-link voltage U dc and the rated capacitor voltage of FBSMs U c ;
(3) calculating a reference value of converter's output active power P according to different control objectives, and calculating AC-side output voltages u j according to the reference value of the active power P and a converter output active power p′ detected in real time, further making the converter output active power p′ track the reference value P , where j=a, b and c, representing phase a , phase b and phase c, respectively;
(4) calculating a reference value of output voltage of each arm according to the AC-side output voltages u j and a detected DC-link voltage u dc ;
(5) calculating the number of the inserted FBSMs in each arm according to the reference value of output voltage of each arm and the rated capacitor voltage of FBSMs U c ;
(6) determining the inserted FBSMs of each arm in the principle of balancing capacitor voltages of FBSMs in each arm according to the detected capacitor voltages of FBSMs in each arm and polarity of the arm currents; and
(7) making FBSMs of each arm required to output positive voltage output positive voltage and those required to output negative voltage output negative voltage, and bypassing other FBSMs, thereby boosting AC-side voltages of the full-bridge modular multilevel converter.

2. The method of claim 1 , wherein in step (3), keeping RMS value of AC-side currents constant during AC-side voltage boosting, the reference value of converter output active power P is expressed as P=mP o , where P o is a rated output active power before AC-side voltage boosting.

3. The method of claim 1 , wherein in step (3), keeping semiconductor's current rating constant after AC-side voltage boosting, the reference value P of converter output active power is expressed as

P
=

m
0
2

⁢

m
2

⁢

⁢

cos
2

⁢

⁢
φ

+

2
⁢

⁢

m
2

m
0
2

⁢

m
2

⁢

⁢

cos
2

⁢

⁢
φ

+

2
⁢

⁢

m
0
2

⁢

P
0

,

where m o is a converter modulation index before AC-side voltage boosting, ø is a power factor angle, and P o is a converter output active power before AC-side voltage boosting.

4. The method of claim 1 , wherein in step (3), keeping converter transmission capacity constant after AC-side voltage boosting, the reference value P of converter output active power is expressed as P=P o , where P o is a converter output active power before AC-side voltage boosting.

5. The method of claim 1 , wherein in step (2), the number of FBSMs in each arm N satisfying AC-side voltage boosting requirements is expressed as:

N
=

{

U
dc

U
c

,

0
<
m
≤
1

(

1
+
m

)

⁢

U
dc

2
⁢

⁢

U
c

,

Assignees

• Univ Huazhong Science Tech

Inventors

• Hu Jiabing
• Lu Maozeng
• Xiang Muchao
• Lin Lei

Classifications

• H02M2007/4835

  Electricity · mapped topic

• H02J3/36

  Arrangements for transfer of electric power between AC networks via high-voltage DC [HVDC] links; Arrangements for transfer of electric power between generators and networks via HVDC links · CPC title

• H02M7/483 Primary

  Converters with outputs that each can have more than two voltages levels · CPC title

• H02M7/4835

  comprising two or more cells, each including a switchable capacitor, the capacitors having a nominal charge voltage which corresponds to a given fraction of the input voltage, and the capacitors being selectively connected in series to determine the instantaneous output voltage · CPC title

• Y02E60/60

  Arrangements for transfer of electric power between AC networks or generators via a high voltage DC link [HVCD] · CPC title