NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN …ieejournal.com/Vol_4_No_1/Neuro-Fuzzy Based Power Quality... · neuro-fuzzy based power quality improvements in a three phase four

International Electrical Engineering Journal (IEEJ) Vol. 4 (2013) No. 1, pp. 953-961 ISSN 2078-2365

953

NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN A THREE PHASE FOUR WIRE DISTRIBUTION SYSTEM USING DSTATCOM

NEURO-FUZZY BASED POWER

QUALITY IMPROVEMENTS IN A THREE

PHASE FOUR WIRE DISTRIBUTION

SYSTEM USING DSTATCOM

E.Babu1,R.Subramanian

2

1 , Department of Electrical and electronics engg 2 Department of Electrical and electronics engg

[email protected],

[email protected]

Abstract- This project presents reactive power

compensation in a distribution power system.

DSTATCOM (distribution static compensator) is a

reactive power compensator for power quality

improvement in a three phase four wire distribution

system. A voltage source converter (VSC) based

DSTATCOM with a zig-zag transformer is used for

the compensation of reactive power for voltage

regulation along with load balancing, elimination of

harmonic currents, and also the neutral current

compensation at the point of common coupling. The

neuro-fuzzy based system is used to control scheme of

the VSC. The zig-zag transformer is used for

providing a path to the zero sequence current in a

three-phase four-wire distribution system. The

performance of the proposed DSTATCOM system is

verified through simulations using MATLAB software

with its Simulink.

Keywords- Distribution static compensator

(DSTATCOM), voltage-source converter (VSC), zig-

zag transformer and neutral current compensation.

I. INTRODUCTION

The present day ac power distribution systems are

suffering from severe power quality problems. These power

quality problems include high reactive power burden, harmonics

currents, load unbalance and excessive neutral current etc. The

power quality at the point of common coupling (PCC) with the

utility grid is governed by the various standards and the IEEE-519 standard is widely accepted. Some remedies to these power

quality problems are reported in the literature. A group of

controllers together called Custom Power Devices, which

includes the DSTATCOM (distribution static compensator),

DVR (dynamic voltage restorer) and UPQC (unified power

quality conditioner).

The DSTATCOM is a shunt connected device, which takes the care of the power quality problems in the currents.

Three-phase four-wire distribution systems are used to supply

single-phase low voltage loads such as computer loads, lighting

mailto:[email protected]

mailto:[email protected]


954


ballasts, small rating adjustable speeds drives (ASD) in air

conditioners, fans, refrigerators and other domestic,

commercial appliances etc. The most of the connected loads are

generally nonlinear loads. The harmonics in currents and

unbalance in load increases the neutral line currents. The

excessive neutral current which has both harmonic and

fundamental component of load currents may over load the

neutral conductor of three-phase four-wire distribution system.

A simple three phase four wire system circuit is shown in fig. 1.

There are many topologies reported in the literature for three-phase four-wire DSTATCOM such as a three single-phase

VSC, VSC with four leg, three leg VSC with capacitors and

three leg VSC with a zig-zag transformer and three-leg VSC

with neutral terminal at the positive or negative terminal of dc

bus. The application of a zig-zag transformer for reduction of

the neutral current is having an advantage due to passive

compensation, rugged and less complex over the active

compensation techniques.

Fig. 1. Three phase four wire system circuit

There are many theories available for the generation of

reference source currents for the control of VSC of

DSTATCOM for a three-phase four-wire system. There are

instantaneous reactive power theory (p q theory), synchronous

reference frame theory, power balance theory, etc. A fuzzy based

control of an active filter is proposed in the literature for

harmonic elimination in the supply currents. In this paper, a

Fuzzy logic control of the three-phase four-wire DSTATCOM is

proposed with a new method control for the voltage regulation at

the PCC.

II. PROPOSED DSTATCOM

The three-phase four-wire DSTATCOM is used for

reactive power compensation and harmonic current compensation along with load balancing and neutral current

compensation. Fig. 2. Shows the power circuit of proposed VSC

based DSTATCOM along with a zig-zag transformer

connected in the three-phase four-wire distribution system. This

Zig-Zag transformer can supply the path for the zero-sequence

current. Fig. 3. Shows the phasor diagram of zig-zag

transformer.


955


Fig. 2. Schematics of three phase four wire systems with DSTATCOM.

From phasor diagram, it can be found that the voltage across

the transformer’s winding is of the phase voltage of the three-

phase four-wire distribution power system.

Fig. 3. Zig-Zag transformer phasor diagram.


956


The linear and non-linear, balanced and unbalanced

loads are connected at the point of common coupling. The

DSTATCOM consists of a 3 leg pulse width modulated (PWM)

voltage source converter (VSC) using six insulated gate bipolar

transistors (IGBTs), and dc capacitors. The T- connected

transformer connected at the load terminal provides a

circulating path for zero sequence harmonic and fundamental

currents. The DSTATCOM provides harmonics elimination,

neutral current compensation and load balancing along with power factor correction or line voltage regulation. The

compensator current is used to compensate the reactive power

component of the load current. The DSTATCOM injects a

current Ic, such that the load current, Is and source voltage, Vs.

III.CONTROL OF DSTATCOM

The block diagram of the control scheme is shown in

Fig. 4. The fundamental real and reactive power components of

load current are extracted in each phase, and they are

considered as the reference source currents (iSa, iSb, iSc). The

load currents (iLa, iLb, iLc) and the PCC voltages (vSa, vSb, vSc) are extracted.

The extraction of the weights corresponding to the

fundamental active components of the load current (wpa, wpb,

wpc) and the weights corresponding to the fundamental reactive

components of the load current (wqa, wqb, wqc). The averages

of the respective weights are Wp and Wq are updated at each

sample time.

For an ac system, the source voltages and load currents

have harmonic components along with fundamental components

where V1 and Vn are the peaks of the fundamental and

harmonic components of the voltage. Similarly, I1 and In are the

peaks of the fundamental and harmonic components of the

current.

The unit template for three phases can be represented as

ua = U sin ῳt ub = U sin (ῳt-120) uc = U sin (ῳt-240)

xa = U cos ῳt xb = U cos (ῳt-120) xc = U cos (ῳt-240)

Fig. 4. Extraction of fundamental real and reactive current in a three-phase system.


957


where U = 1. The fundamental load current can be

decomposed as

I1 = Ip + Iq

where Ip and Iq are the active and reactive power

components of the load current, respectively.

The estimates of the fundamental active power and

fundamental reactive power components of the load current for

a single phase are obtained by estimating the respective weights

corresponding to the fundamental active components of the

load current (wpa, wpb, wpc) and the weights corresponding to the fundamental reactive components of the load current (wqa,

wqb, wqc).

The average weight corresponding to the active and

reactive components of the load is shown as

wp =(wpa + wpb + wpc)/3

wq =(wqa + wqb + wqc)/3

Two proportional–integral (PI) controllers are used for

controlling the dc bus voltage and ac terminal voltage. A self

supporting dc bus is realized using a PI controller over the

sensed (vdc) and reference values (v∗dc) of the dc bus voltages of DSTATCOM. This PI controller estimates the loss

component of the source current (wloss), and hence, this is

added with the wp. The second PI controller is used over the

amplitude of PCC voltage (VS) and reference values (V ∗S ),

and it estimates the reactive component of the DSTATCOM

current (wqr), and hence, this is added with the wq.

Now, the real components of the reference source

currents are computed as

ipa = wp*ua ipb = wp*ub ipc = wp*uc.

Similarly, the reactive components of the reference source currents are obtained as

iqa = wq*xa iqb = wq*xb iqc = wq*xc.

The reference source currents are obtained as the sum

of active and reactive power currents as

i*Sa = ipa + iqa i*Sb = ipa + iqa i*Sc = ipa + iqa.

The reference source currents in three phases are used

for the control of the three-leg VSC. The sensed and reference

source currents are compared, and the error is used to generate

the gating signals.

IV.FUZZY LOGIC SYSTEMS

Fuzzy logic systems are one of the main developments

and successes of fuzzy sets and fuzzy logic. It is a rule-base

system that implements a nonlinear mapping between its inputs and outputs. A fuzzy logic system is characterized by four

modules.

Fuzzifier

Defuzzifier

Inference engine

Rule base

A schematic representation of a FLS is presented in Fig.

5. The operation of a FLS is based on the rules contained in the

rule base.

Fuzzification

Measure the values of input variables

Perform a scale mapping that transforms the range of

values of input variables into corresponding universe of

discourse.

Function of the fuzzification that converts input into

suitable linguistic values.

Fig.5. Structure of fuzzy based system

Knowledge Base


958


It consists of the data base and linguistic control rule

base.

The database provides a necessary definitions, which

are used to define linguistic control rules and fuzzy

data, manipulation in an, FLC.

The rule base characterizes of the control goals and

control policy of the domain experts by means of set

of linguistic control rules.

Decision Making Logic

It has a capability of simulating human decision

making based on fuzzy concepts and of inferring fuzzy

control actions employing fuzzy implication and the rules of inference in fuzzy logic.

Defuzzification

A scale mapping which converts a range of values of

input variables into corresponding universe of

discourse.

The defuzzification which yields a non-fuzzy, control

action from an inferred fuzzy control action.

V. MATLAB-BASED MODELING OF THE SYSTEM

The three leg VSC and the zig-zag transformer based

DSTATCOM for a three-phase four wire system are modeled

and simulated using MATLAB and its Simulink and SimPower

System toolboxes. The DSTATCOM system shown in Fig. 2 is modeled in MATLAB, and its developed model is shown in

Fig.6. The multi winding transformer model available in the SPS

is used for modeling the zig-zag transformer. The ripple filter

has connected to the VSC of the DSTATCOM for filtering the

ripple in the PCC voltage.

The control algorithm for the DSTATCOM is also

modeled in MATLAB/SIMULINK. The reference source

currents are derived from the sensed PCC voltages (vSa, vSb,

vSc), load currents (iLa, iLb, iLc), and the dc bus voltage of

DSTATCOM (vdc). Fig. 7 shows the Simulink model of control

model of distribution system.

Fig. 6. Simulink model for three phase four wire system with DSTATCOM


959


Fig. 7. Simulink model of control model of distribution system

A pulse width modulation (PWM) current controller is

used over the reference and sensed compensator currents to

generate the gating signals for the IGBTs of the VSC of the

DSTATCOM.

VI. SIMULATION RESULTS AND DISCUSSION

The performance of the three phase VSC based

DSTATCOM and the zig-zag transformer for PCC voltage

regulations, along with the neutral current compensation and

load balancing of a three phase four wire linear load, is shown

in Fig. 8. At 0.10 s, a three phase linear load is

changed to two phase load and again to single phase load at

0.15 s. The PCC voltages (vs), balanced source currents (is),

load currents (iLa, iLb, iLc), compensator currents (iC), load

neutral current (iLn), transformer neutral current (iCn) and dc

bus voltage (vdc) are demonstrated under change of load

conditions. It’s observed that the amplitude of PCC voltage

(VS) is regulated to the reference amplitude by the required

reactive power compensation and that the source neutral current (isn) is maintained at nearly zero because of the zig-zag

transformer. The DC bus voltage of the capacitor (vdc) of the

VSC of DSTATCOM is regulated by the controller, and the dc

voltage has maintained near the reference dc voltage under

varying load disturbances.

The performance of DSTATCOM for PCC voltage regulation

and harmonic eliminations, along with neutral current compensation, is shown in Fig. 9. At 0.10 s, a three-phase

nonlinear load is changed to two phase load and again to single

phase load at 0.15 s. These loads are applied again. The PCC

voltages (vs), balanced source currents (is), load currents (iLa,

iLb, iLc), compensator currents (iC), load neutral current (iLn),

transformer neutral current (iCn) and dc bus voltage (vdc) are

demonstrated under varying nonlinear loads. The PCC voltage


960


Fig. 8. Performance of three-phase three-leg VSC and a zig-zag

transformer for neutral current compensation, load balancing, and voltage regulations.

is regulated to the reference amplitude value. It’s observed that

the load is drawing a non sinusoidal and unbalanced current, but the source currents are sinusoidal and balanced due to the

proper compensation currents injected by DSTATCOM.

Fig. 9. Performance of three-phase three-leg VSC and a zig-zag

transformer for neutral current compensation, harmonics compensation, and voltage regulation.

CONCLUSION

The analysis and simulation of a new three phase four

wire DSTATCOM consisting of a three leg VSC with a zig-zag

transformer controlled with fuzzy logic have been carried out,

and its performance have been demonstrated for neutral current elimination along with reactive power compensation, harmonic

eliminations, and load balancing. The VSC is controlled using

the fuzzy logic controller. The voltage regulation and power

factor correction modes of operation of the DSTATCOM have

been observed. The dc bus voltage of the DSTATCOM has been

regulated to the reference dc bus voltage under wide varying

loads.


961


The zig-zag transformer has been found to be effective

for compensating the zero-sequence fundamental and harmonic

neutral currents. The kilovolt ampere rating of transformers in

the zig-zag configuration is much less compared to that of a

similar star–delta transformer. It has been found that the control

technique is simple to implement, is fast in response, and gives

nearly zero phase shift in the estimated reference currents.

REFERENCES 1. R. C. Dugan, M. F. McGranaghan, and H. W. Beaty,

‘Electric Power Systems Quality’, 2nd ed. New York,

Mc Graw Hill, 2006.

2. K. R. Padiyar, ‘FACTS Controllers in Transmission

and Distribution’. New Delhi, India, New Age Int., 2007.

3. J. C. Montano and P. S. Revuelta, ‘Strategies of

instantaneous compensation for three phase four wire

circuits’, IEEE Power Eng. Rev., vol. 22, no. 6, p. 63,

Jun. 2002.

4. H. L. Jou, J. C. Wu, K.. D. Wu, W. J. Chiang, and Y.

H. Chen, ’Analysis of zig-zag transformer applying in

the three-phase four-wire distribution power system’,

IEEE Trans., Power Del., vol. 20, no. 2, pp., 1168–

1173, Apr. 2005.

5. H.-L. Jou, K.-D. Wu, J.-C. Wu, and W.-J. Chiang, ‘A

three phase four wire power filter comprising a three phase three wire active filter and a zig-zag

transformer’, IEEE Trans., Power Electron., vol. 23,

no. 1, pp., 252–259, Jan. 2008.

6. M. I. Milanés, E. R. Cadaval, and F. B. González,

‘Comparison of control strategies for shunt active

power filters in three phase four wire system’, IEEE

Trans., Power Electron., vol. 22, no. 1, pp. 229–236,

Jan. 2007.

7. B. Singh, V. Verma, and J. Solanki, ‘Neural networks

based selective compensation current quality problems

in distribution system’, IEEE Trans., Ind. Electron, vol.

54, no. 1, pp. 53–60, Feb. 2007.

8. B. Singh, P. Jayaprakash, S. Kumar and D. P. Kothari,

‘Implementation of neural network controlled three leg

VSC and a transformer as three phase four wire

DSTATCOM’, IEEE Trans. Ind. Electron., vol. 47, no.

4, Jul-Aug.. 2011

Documents

NEURO-FUZZY BASED POWER QUALITY IMPROVEMENTS IN …ieejournal.com/Vol_4_No_1/Neuro-Fuzzy Based Power Quality... · neuro-fuzzy based power quality improvements in a three phase four