Method for suppressing circulating current in modular multilevel converter for high voltage direct-current transmission

Patent metadata

Field	Value
Publication number	US-9444322-B2
Application number	US-201314063698-A
Country	US
Kind code	B2
Filing date	Oct 25, 2013
Priority date	Feb 13, 2013
Publication date	Sep 13, 2016
Grant date	Sep 13, 2016

Abstract

Provided is a method for suppressing a circulating current in a modular multi-level converter for a high voltage direction-current (HVDC) transmission system. The HVDC transmission system converts an alternating current (AC) into a direct current (DC) and vice versa, transmits energy using a DC cable, and including a modular multilevel converter generating a high voltage source by stacking a plurality of sub-modules in series. In the circulating current suppression method, a circulating current (i diffj ; j=a,b,c) of a,b,c phase in an abc 3-phase stationary reference frame, a DC current (i dc ) flowing in a DC cable, a current reference value (i* dc ) of a DC component that needs to flow in the DC cable are inputted. The circulating current (i diffj ) of the a,b,c phase is controlled to become zero. A compensation value (V* diffj ) for suppressing a harmonic component of the circulating current is outputted.

First claim

What is claimed is:

1. A method for suppressing circulating currents of a, b, c phases in a modular multi-level converter for a high voltage direction-current (HVDC) transmission system converting an alternating current (AC) into a direct current (DC) and vice versa, transmitting energy using a DC cable, and having the modular multilevel converter generating a high voltage source by stacking a plurality of sub-modules in series, the method comprising:
receiving circulating currents in each of a phase, b phase, and c phase in an 3 phase stationary reference frame, a DC current i dc flowing in a DC cable, a current reference value i* dc of a DC component that needs to flow in the DC cable;
controlling AC components of the circulating currents in each of a phase, b phase, and c phase to become zero; and
outputting a compensation value V* diffj for suppressing a harmonic component of the circulating current,
wherein the controlling of AC components of the circulating currents comprises:
calculating the current reference value i* dc of the DC component that needs to flow in the DC cable by equation of i* dc =P acpower /V dc (with P acpower :known);
extracting a current i* dc -i dc of an AC harmonic component flowing in a DC link terminal by eliminating the DC current i dc flowing in the DC link terminal from the current reference value i* dc of the DC component; and
controlling the current i * dc -i dc of an AC harmonic component flowing in a DC link terminal to become zero,
wherein the controlling of AC components of the circulating currents further comprises:
calculating AC components i t of the circulating currents i diffj by eliminating an ⅓ of the DC current i dc from the circulating currents i diffj ; and
removing the AC components i zj of the circulating currents i diffj ,
wherein the removing of the AC components i zj is performed in the 3-phase stationary reference frame by Equation V j _ diffcrtripple _ rej _ ref =PIR[i* zj −i zj ] (with i* zj =0, j=a, b, c) by applying a Proportional-Integral and Resonant (PIR) controller having an infinite gain at a frequency of ±2ω 0 when an angular frequency ω 0 equals to 2πf 0 at a power system frequency of f 0 ,
here,

PIR
⁡

[
error
]

=

{

K
P

+

K
i

s

+

2
⁢

K
r

⁢
s

s
2

+

(

2
⁢

⁢

ω
0

)

2

}

×
error

a, b, c: division factors for dividing 3-phase system voltage into a-phase, b-phase, c-phase
P acpower : converted AC power by modular multi-level converter
V dc : DC cable voltage (DC_link voltage) of HVDC,
i* zj : AC component reference value among components of circulating current
V j _ diffcrtripple _ rej _ ref : output value after controller for suppressing AC component of circulating current
K p , K i , K r : proportional gain, integral gain, proportional gain of resonant controller error: signal input including error information
s: d( )/dt in Laplace transform.

2. The method of claim 1
wherein the current i * dc -i dc of the AC harmonic component becomes zero by allowing the extracted current i* dc -i dc of the AC harmonic components to be an input value of the Proportional-Integral and Resonant controller and controlling the Proportional-Integral and Resonant controller having a resonant point at a frequency of +2ω 0 .

3. The method of claim 1 , wherein the outputting of the compensation value V* diffj for suppressing the harmonic component of the circulating current comprises generating voltage reference values V* diffa , V* diffb , V* diffc of the circulating current suppression component by adding up an output value V j _ diffcrtripple _ rej _ ref of the circulating current of an AC component suppression controller and an output value V dccrtripple _ rej _ ref of the DC cable current of the AC component suppression controller.

4. The method of claim 3 , wherein the outputting of the compensation value V* diffj for suppressing the harmonic component of the circulating currents comprises calculating voltage reference values V pj , V nj of an upper arm and a lower arm of the modular multilevel converter by Equations

V
pj

=

V
dc

2

-

V

j
⁢

-

⁢
ref

-

V
diffj
*

⁢

⁢

(

with
⁢

⁢
j

=
a

,
b
,
c

)

Assignees

• Keri Korea Electrotechnology Res Inst

Inventors

• Park Jung Woo
• Kang Dae Wook
• Kim Jong Hyun
• Yoo Dong Wook
• Moon Ji Woo
• Kwon Jin Soo
• Kim Chun Sung
• Bae Deuk Woo

Classifications

• Y02E60/60

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

• H02M5/4585

  having a rectifier with controlled elements · CPC title

• H02M2007/4835

  Electricity · mapped topic

• H02M1/12 Primary

  Arrangements for reducing harmonics from AC input or output · CPC title

• H02M1/15

  using active elements · CPC title