[Doi 10.1109%2FICEMS.2014.7013909] Tai, Bingyong; Gao, Congzhe; Liu, Xiangdong; Lv, Jingliang -- [IEEE 2014 17th International Conference on Electrical Machines and Systems (ICEMS)

8/18/2019 [Doi 10.1109%2FICEMS.2014.7013909] Tai, Bingyong; Gao, Congzhe; Liu, Xiangdong; Lv, Jingliang -- [IEEE 2014 1…

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2014 17th International Conference on Electrical Machines and Systems (ICEMS),Oct. 22-25, 2014, Hangzhou, China

Abstract — Neutral-point-clamped (NPC) multilevel-converter

(MLC) is very popular in high voltage converters with multilevel

topology. However, the dc-link capacitor voltage unbalance of

this topology is a key issue in the NPC applications, and which

will cause distortion and asymmetry of the output voltages, and

also increase the voltage stress of power switches. In this paper,

the three-level (3L) NPC with unbalance capacitor voltages is

analyzed. A capacitor voltage balancing control strategy based on

fuzzy logic controller (FLC) for the 3L-NPC-MLC is studied andpresented. A fuzzy membership function is ratiocinated and

determined, and based of which a capacitor voltage control

strategy to suppress the capacitor voltage imbalance is proposed.

Finally, simulation results show that both the balancing strategy

and the FLC are effective.

I.

I NTRODUCTION

Multilevel power converters are a fast developing

technology and potentially useful for wide range of

applications, such as active power filtering, static VAR

compensators, wind power, photovoltaic power, marine drivers,

steel rolling mills, and other adjustable speed motor drivers, to

name a few. Many of these processed have been continuouslyincreasing their demand for power which can be dealt through

the following two different ways:

1) Maintaining traditional converter topologies and through

developing semiconductor technology to reach higher nominal

voltages and currents [1]-[2].

2) Developing new converter topologies with traditional

semiconductor technology such as multilevel converters [3]-

[4].

The first approach inherits the benefit of well-known

circuit structures and control methods. However, the newer

semiconductors are more expensive and need power filters to

fulfill power quality requirements. While the second approach

uses the well-known and cheaper semiconductors, but needsmore complex circuit structures.

Three-level neutral-point-clamped multilevel-converter

(3L-NPC-MLC) was initiated by invention of NPC inverter by

the authors in [5], and it is now proven technology for

medium-/high-voltage high-power applications [6]-[7].

However, the 3L-NPC-MLC has an inherent problem of

unbalanced voltages across dc-link capacitors due to various

internal and external factors such as load unbalance,

nonuniform distribution of charges, nonidentical properties of

dc-link capacitors and so on [3][8].

Neutral-point (NP) voltage balancing issue in 3L-NPC-

MLC has been widely discussed in the past [9]-[11]. Many

balancing control strategies have been proposed to solve this

problem in the last decades. Reference [12] discusses a

transformerless hybrid active filter which consists of an active

filter using a three-level diode-clamped converter and a passive filter tuned to seventh harmonic frequency, and

voltage balancing control characterized by superimposing a

sixth harmonic zero-sequence voltage on the three-level

converter with triangle carrier modulation to keep the two dc

capacitors voltages well-balanced. A novel NP controller with

full power-factor range and low distortion has been proposed

in [13], which has the minimum common voltage injection at

no unbalance or slight unbalance conditions. In [14], the

proposed modulation has a strong balancing ability at all

regions, yet it will increase the switching events and output

total harmonic distortion.

A capacitor voltage balance control strategy based on FLC

with little effect on switch events and output voltage is presented in this paper.

II. A NALYZE U NBALANCED CAUSES OF NPC-MLC

The topology of three-phase 3L-NPC-MLC is shown in Fig.

1, and the relationship between the phase-voltage of 3L-NPC-

MLC and the state of switches is listed in Table I which takes

phase A for an example.

Fig. 1. Topology of three-phase 3L-NPC-MLC

A Voltage Balancing Controller with Fuzzy Logic

Strategy for Neutral Point Clamped Multilevel

Converter

Bingyong Tai,

Congzhe Gao*

, Xiangdong Liu, Jingliang LvScholl of Automation, Beijing Institute of Technology, China

E-mail: [email protected]

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Table I

Relationship between phase-voltage and switch state of phase A

1 xS

SwitchState

out V

11S 12S 13S 14S

( )2dc

V p+ on on off off

0(o) off on on off

( )2dcV n− off off on on

Under the condition of adopting fundamental frequency as

the control signals of switches, the phase-voltage changes

between2

dcV − , 0 and2

dcV + , at the same time, the line-

voltage can be synthesized as five-level staircase waveform

without unbalance of NP voltage. The output voltage

waveform of three-phase 3L-NPC-MLC where 800VdcV = is

shown as Fig. 2, which has less harmonic component and

much more close to sine waveform.

Fig. 2. Waveform of phase-voltage and line-voltage of 3L-NPC-MLC with

balanced NP voltage

Fig. 3. Waveform of phase-voltage and line-voltage of 3L-NPC-MLC with

unbalanced NP voltage

However, the synthesized five-level staircase waveforms

have produced distortion when the NP voltage is unbalanced,as shown in Fig. 3. And the voltage distortion will be moreserious with the increase of load current.

As can be seen from Fig.1, the relationship between upper

capacitor current1C

i and lower capacitor current2C

i under the

condition of 1 2C C C = = can be expressed as:

1 22

m

C C

ii i= = (1)

where mi is the NP current.

In dynamic state, mi can be expressed as:

2 22 22 2 2

C C m C

du dui i C C

dt dt = = − = − (2)

The relationship between NP voltage NP V and NP current

mi is determined as in (3) when the NP voltage is referenced to

the negative terminal of DC bus.2 2

2 2

NP C C mdu du i i

dt dt C C = = − = − (3)

According to expression (3), it can be seen that the

unbalanced voltage in NP is caused by the NP current mi , and

which is also related with the capacitance of DC bus. However,

as shown in Table I, each phase has three switch states “p”,“o”, and “n”, and the neutral point connects with load via

clamped-diode in state “o”. NP balanced voltage also depends

on whether the load is balance and the power factor of the load.

The following design of NP balance controller and the fuzzy

logic strategy is based on balance load and adjusts the switches

of state “o” to keep the NP current mi a constant which is

closed to zero.

III. CONTROLLER DESIGNED WITH FUZZY LOGIC STRATEGY

Fuzzy logic has rapidly become one of the most successful

of today’s technologies for developing sophisticated control

systems. With the aid of fuzzy control, complex requirementsmaybe implemented in amazingly simple, easily maintained,

and inexpensive controllers. The same fuzzy technology, in the

form of approximate reasoning, is also resurfacing ininformation technology, where it provides decision-support

and expert systems with powerful reasoning capabilities bound

by a minimum of rules [15]. The structure of FLC which

consists of fuzzification, fuzzy reasoning, fuzzy knowledge base and defuzzification is shown in Fig. 4.

Fig. 4. Structure of FLC

The designed NP voltage balancing controller with fuzzylogic strategy is based on Mamdani Fuzzy System which

requires two input variables (e, ec) to control the outputvariable (u), and where “e” is the voltage deviation of down

capacitor, “ec” is the variation of “e” and “u” is the controlled

parameter that adjusts switch state of state “o”.

The domain of discourse of “e”, “ec” and “u” is defined as

follows:

e = {-6, -5, -4, -3, -2, -1, -0, +0, +1, +2, +3, +4, +5, +6};ec = {-6, -5, -4, -3, -2, -1, 0, +1, +2, +3, +4, +5, +6};

u = {-6, -5, -4, -3, -2, -1, 0, +1, +2, +3, +4, +5, +6};

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Table II

Valuation of the fuzzy variable “e”

e

( ) A x µ

E

-6 -5 -4 -3 -2 -1 -0 +0 +1 +2 +3 +4 +5 +6

NB 1.0 0.8 0.4 0.1 0 0 0 0 0 0 0 0 0 0

NM 0.2 0.7 1.0 0.7 0.2 0 0 0 0 0 0 0 0 0

NS 0 0 0.1 0.5 1.0 0.8 0.3 0 0 0 0 0 0 0

NO 0 0 0 0 0.1 0.6 1.0 0 0 0 0 0 0 0

PO 0 0 0 0 0 0 0 1.0 0.6 0.1 0 0 0 0

PS 0 0 0 0 0 0 0 0.3 0.8 1.0 0.5 0.1 0 0PM 0 0 0 0 0 0 0 0 0 0.2 0.7 1.0 0.7 0.2

PB 0 0 0 0 0 0 0 0 0 0 0.1 0.4 0.8 1.0

Correspondingly, the fuzzy set of “e”, “ec” and “u” isdefined as follows:

E = {NB, NM, NS, NO, PO, PS, PM, PB};

EC = {NB, NM, NS, O, PS, PM, PB};

U = {NB, NM, NS, O, PS, PM, PB};2( )

( ) x a

b A

x e µ −

−

= (4)

Normal function such as (4) is adopted to determine the

membership functions, where

( , 4, 2, 0, 0, 2, 4, ), 0a b b b∈ − − − − + + + + >

and “b” is also a positive number.

The values of all the three fuzzy variables “e”, “ec” and “u”

have been ratiocinated and calculated in this paper, but onlythe value of “e” is listed in Table II. However, membership

functions of all three variables which are determined finally

and used in simulations are illustrated in Fig. 5, Fig. 6 and

Fig.7 separately.

Fig. 5. Membership function of input variable “e”

Fig. 6. Membership function of input variable “ec”

Fig. 7. Membership function of output variable “u”

And the behavior of the control surfaces is defined asfollows, which is listed in Table III. Using MATLAB/Fuzzy

Logic Toolbox, the 3D visualization of the inference rules is

shown in Fig. 8.

Table III

Fuzzy rulesE

UEC

NB NM NS NO PO PS PM PB

NB PB PB PB PM PM PM NM NB

NM PB PB PS PS PM PM NM NB

NS PB PM PS PS PS PS NM NB

O PB PM PS O O NS NM NB

PS PB PM NS NS NS NS NM NBPM PB PM NM NM NM NS NS NB

PB PB PM NM NM NM NB NB NB

Fig. 8. 3D visualization of the inference rules

IV. SIMULATION R ESULTS

Structure of NP voltage balancing controller with fuzzy

logic strategy is studied and designed, and based of which the

simulation model using MATLAB/Simulink Toolbox is

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detailed in Fig. 9. The NP voltage balancing control unit is

formed by fuzzy calibration unit and FLC which consists offuzziness of inputs, setting-up of fuzzy control rules and

certainty of output.The effectiveness of the voltage balancing strategy and the

FLC was verified by simulations, and the simulation results

were shown in Fig. 13, Fig. 14, Fig. 15 and Fig. 16. In order to

contrast, the simulation results of 3L-NPC-MLC without NP

voltage balancing strategy as shown in Fig. 10, Fig. 11 and Fig.12.

In the simulation, the dc-link voltage was set to 800V , and

1 2800V =400V

2 2dc

dc dc

V V V = = = . The Three-Phase Series

RL Branch was taken as three-phase load where R 5= Ω and

L 5mH= .

Fig. 9. Simulink model of 3L-NPC-MLC with NP voltage balancing FLC

The NP voltage unbalance of 3L-NPC-MLC is caused byvarious internal and external factors. The unbalance voltage

state which has been simulated in this paper was illustrated in

Fig. 10, where the unbalance voltage offset compared with thereference voltage

2dc

V has been up to 100V± .

Fig.10. Unbalancing voltages of equalizing capacitors

Under the unbalancing condition as shown in Fig. 10, output

voltages of the 3L-NPC-MLC without NP voltage balancing

strategy and FLC have produced serious distortion. Line-

voltage abU , phase-voltage aU and U was shown in Fig. 11

and Fig. 12.

Fig. 11. Line-voltageabU without NP voltage balancing FLC

Fig. 12. Output voltages without NP voltage balancing FLC

Under the same unbalancing condition which is shown in

Fig. 10, but the NP voltage balancing strategy and FLC has

been adopted, the NP voltage has been controlled balance as

shown in Fig. 13. At the same time, output voltages have got

good waveform and the load current has low total harmonicdistortion. Fig. 14, Fig. 15 and Fig. 16 show the simulation

waveforms separately.

Fig. 13. Balancing voltages of equalizing capacitors

Fig. 14. Line-voltageabU with NP voltage balancing FLC

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Fig. 15. Output voltages with NP voltage balancing FLC

Fig. 16. Three-phase load current

V. CONCLUSION

The NP voltage balancing FLC for 3L-NPC-MLC presented

in this paper has good capacitor voltage imbalance suppressionability. The fluctuation of output voltages can be controlled

nearly at zero and the NP voltage has low ripple as well. At thesame time, with three-phase load, the waveforms of three-

phase load current are more close to the sinusoidal waveform

and also have much lower total harmonic distortion.

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Documents

[Doi 10.1109%2FICEMS.2014.7013909] Tai, Bingyong; Gao, Congzhe; Liu, Xiangdong; Lv, Jingliang -- [IEEE 2014 17th International Conference on Electrical Machines and Systems (ICEMS)