Comparison Of Star/Delta and Zig-zag/Star Transformer Based

DSTATCOM For Power Quality Improvement

Guided BY- Submitted By-Monika Jain Anurag Jain Asst. Professor (0105EE13MT02)EX Department, OIST Bhopal

Content • Introduction• Literature Review• Single line diagram of Proposed system for Star/Delta transformer• Control algorithm • MATLAB model• Simulink Results• Single line diagram of Proposed system for Zig/Zag transformer • Simulink Results• Comparison • Conclusion• Appendix• Paper Published• References

Introduction

There are many factors both internal and external that affect the quality and quantity

of power that is being delivered. In present day power distribution systems are

facing severe Power Quality problems, such as poor voltage regulation, high

reactive power, harmonics, excessive neutral current, etc.

The Custom Power Devices(CPDs) are used to resolve these problems.

Power Quality

“Continuity” of supply and the “Quality” of voltage.

Power Quality is the interaction of electrical power with electrical equipment.

If electrical equipment operates correctly and reliably without being damaged or

stressed, we would say that the electrical power is of good quality.

On the other hand, if the electrical equipment malfunctions, is unreliable, or is

damaged during normal usage, we would suspect that the power quality is poor.

Power Quality Problems

Voltage sag

Voltage swell

Transient

Power Quality Problems

Harmonics

Voltage unbalance

Under voltage

Power Quality Problems

OVERVOLTAGE

VOLTAGE FLICKER

INTERRUPTION

Custom Power Devices

To improve the reliability and power quality in a distribution, a group of devices,

called Custom Power Devices (CPD) are in use now days.

DSTATCOM

DVR

UPQC

DSTATCOM

• It is a shunt connected CPD based on a power electronics Voltage Source Converter

• The distribution static compensator (DSTATCOM) is used for compensating PQ

problems in the current.

VSC• A voltage-source converter is a power electronic device, which can generate a

sinusoidal voltage with any required magnitude, frequency and phase angle.

• The VSC is used to either completely replace the voltage or to inject the

“missing voltage‟. The “missing voltage” is the difference between the

nominal voltage and the actual.

Literature Review• Bhim Singh, et.al. [2008] A new topology of three-phase four-wire DSTATCOM

consisting of a H-bridge VSC (voltage source converter) with a star/delta

transformer is proposed for power quality improvement in the distribution

systems.

• T R Somayajulu, et.al. [2008] In this paper, a new topology of DSTATCOM

(Distribution Static Compensator) is proposed for power quality improvement in a

three-phase four-wire distribution system. A three-leg VSC (Voltage Source

Converter) based DSTATCOM is integrated with a zig-zag transformer

• D.P. Kothari et.al. [2008] In this paper, a new three-phase four-wire distribution

static compensator (DSTATCOM) based on a T-connected transformer and a

three-leg voltage source converter (VSC) is proposed for power quality

improvement.

• P Jayaprakash, et.al. [2012] In this paper, a new topology of DSTATCOM using

three single-phase VSCs (voltage source converters) integrated with a zig-zag

transformer is employed for the power quality improvement in three-phase four-

wire distribution systems. The zig-zag transformer is used to providing a path to

the zero sequence fundamental as well as harmonics currents.

• Bhim singh et.al [2008] A new topology of DSTATCOM (distribution static

compensator) consisting of a H-bridge VSC (voltage source converter) and a

star/delta transformer is proposed for power quality improvement in the three-

phase four wire distribution system.

• P. Jayaprakash et.al [2008] A detailed investigation is made into the causes,

standards, and remedies of the excessive neutral current. A reduced rating voltage-

source converter with a zig-zag transformer as a distribution static compensator is

proposed for power quality improvement in the three-phase four wire distribution

system.

• Bhattacharya Sourabh, [2012] D-STATCOM (distribution static compensator) is a

shunt device which is generally used to solve power quality problems in

distribution systems. D-STATCOM is a shunt device used in correcting power

factor, maintaining constant distribution voltage and mitigating harmonics in a

distribution network.

• D.P. Kothari, et.al. [2009] A three-phase four-wire DSTATCOM (distribution

static compensator) based on three-leg VSC (voltage source converter) isolated

with a star/hexagon transformer is demonstrated for power quality enhancement in

a three-phase four-wire distribution system.

Single line diagram of Proposed system for Star/Delta transformer

Design of Star/Delta TransformerThe kVA rating of the transformer is given as

kVASD = 0.5ΣVrmsIrms

kVASD = 0.5(ΣVYrmsIYrms +ΣV∆rmsI∆rms)

For the integrated DSTATCOM configuration using a Star/Delta transformer, the rating is obtained as,Rms value is given as

√3VLIL= (kW*1000/CosФ)

kVASD = 0.5[{3*(INm/3)*(VL/√3)+3*(INm/3)*(VL/√3)}]

The rating of transformer 3.4 kVA and ratio 240/140 V are selected for the operation

Star/Delta Transformer

Such connections are used

principally where the voltage is to

be stepped down.

Star/Delta transformer connection and its Phasor diagram

Design of DSTATCOMDC Bus Voltage: The dc bus voltage is defined as,

Vdc = 2√2 VLL/(√3m)

Where m is the modulation index and is considered as 1. Thus, one may obtain the

value of Vdc as 677 V for VLL of 415 V. Thus, Vdc of the value of 680 V is selected.

DC Bus Capacitor: Dc capacitor is governed by the reduction in the dc bus voltage

Cdc[(dc) dc1)] = 3V(aI)t

where a is the over loading factor, and t is the response time of the DSTATCOM and t

is considered as 350µs .

Considering Vdc= 680 V, Vdc1= 670 V, V=415/3, a=1.2 , the calculated value of Cdc

is 2600 µf. So Cdc is chosen to be 3200 µf.

AC Inductor: The selection of the ac inductance depends on the current ripple

icr,p-p. The ac inductance is given as,

Lf = [(√3mVdc) /(12afs icr(p-p))

The value of AC Inductance is defined 5mH.

Ripple Filter: A high pass first-order filter tuned at half the switching frequency is

used to filter out the noise from the voltage at the PCC. The time constant of the

filter should be very small compared to the fundamental time period (T).

Rf Cf << T/10

Control Algorithm

Control StrategyA. Active Component of Reference Source Currents

• Three-phase voltages at the PCC are considered sinusoidal and hence their

amplitude is computed as,

Vt = {(2/3) (vSa2 +vSb

2 +vSc2)} 1/2 (1)

• The unit vector in phase with vsa, vsb and vsc are derived as,

ua = vSa/Vt; ub = vSb/Vt; uc = vSc/Vt (2)

• The error in dc bus voltage of VSC (Vdcer (n)) of an DSTATCOM at nth sampling

instant is as,

Vdcer(n) = Vdcref(n) – Vdc(n) (3)

• The output of the PI controller for maintaining dc bus voltage of the VSC at the

nth sampling instant is expressed as,

iloss(n) = iloss(n-1) + Kpd { Vdcer(n) – Vdcer(n-1)} + KidVdcer(n) (4)

• In-phase components of reference source currents are estimated as:

isad* = ua iloss ; isbd* = ub iloss ; iscd* = uc iloss (5)

B. Quadrature Component of Reference Source Currents

• The unit vectors (wa, wb and wc) in quadrature with vsa, vsb and vsc may be derived

using a quadrature transformation of the in-phase unit vectors ua, ub and uc as,

wa = -ub / √3 + uc / √3 (6)

wb = √3 ua / 2 + (ub – uc) / 2√3 (7)

wc= -√3 ua / 2 + (ub – uc) / 2√3 (8)

• The error in PCC voltage of VSC (Vser (n)) of an DSTATCOM at nth sampling

instant is as,

Vter(n) = Vtref(n) – Vt(n) (9)

• The output of the PI controller for maintaining amplitude of PCC voltage at the

nth sampling instant is expressed as,

iqr(n) = iqr(n-1) + Kpq { Vter(n) – Vter(n-1)} + KiqVter(n) (10)

• The quadrature components of reference source currents are estimated as:

isaq* = waiqr ; isbq* = wbiqr ; iscq* = wciqr (11)

• Total reference source currents are sum of in-phase and quadrature components of

the reference source currents as,

isa* = isad* +isaq* (12)

isb* = isbd* +isbq* (13)

isc* = iscd* +iscq* (14)

These reference source currents (isa*, isb* and isc*) are compared with the sensed

source currents (isa, isb and isc) in PWM current controller of VSC of DSTATCOM.

These current errors controlled using a current controlled PWM to generate gating

pulse of the switches.

MATLAB Model

Subsystem of firing algorithm

DSTATCOM consisting Star/Delta Transformer with Nonlinear Load

Source Voltage THDTHD 1.61%

Source Current THDTHD 4.69%

Load Current THDTHD 49.15%

Single line diagram of Proposed system for Zig/Zag transformer

Design of Zig/Zag TransformerConsidering a turn ratio of 1:1:1 and voltage across each winding VX volts and

line voltage is VL then,

VX = (VL / √3)

Similar to Star/Delta transformer, kVA rating of Zig/Zag transformer is given as

kVAZZ = 0.5 {ƩVzrmsIzrms)

For the integrated DSTATCOM configuration using a zig-zag / star transformer,

the rating is obtained as,

(kVA)ZZ/S = 0.5[{6*(IL)*(VL/3)} + {3*(IL)*(VL/3)}]

(kVA)ZZ/S = 0.5[{6*(15)*(140)} + {3*(15)*(140)}]

The transformer of rating 3kVA and ratio 140/140/140 V are selected.

Zig/Zag Transformer

Zig/Zag Transformer connection and its Phasor diagram

DSTATCOM consisting Zig/Zag Transformer with Nonlinear Load

Source Voltage THDTHD 1.68%

Source Current THDTHD 4.29%

Load Current THDTHD 49.33%

ComparisonS.No.

 Parameter DSTATCOM Zig/Zag Star/Delta

1. kVA 

12 3 3.4

2. Input Voltage 415 v, 50Hz 140 v 240 v

3. DC Voltage 

700 340 340

4. DC Bus Capacitance

 

4000 µf 3200 µf 3200 µf

5. Transformer Required

 

- 140v/140v/140v 240v/140v

6. VSC 

4 leg 3 leg 3 leg

7. No. of switches 

8 6 6

Conclusion• The performance of system consisting DSTATCOM with Star/Delta and Zig/Zag

transformer has been demonstrated for Harmonic elimination, neutral current

compensation, and power factor correction for three-phase balanced system.

• The Proposed systems has been reduced the rating of VSC and found effective for

compensating to solve power quality issues.

• The rating of Zig/Zag transformer is lesser than the rating of Star/Delta transformer.

Appendices• AC line voltage: 415 V, 50 Hz

• Line Impedance: Rs=0.01ohm, Ls=1mH

• Load: Non-linear: Three single phase bridge rectifier with an R-L-C load with, R

= 25ohm and C = 100μF and L = 8mH

DSTATCOM Parameters:

• VSC: Six IGBTs connected as three-leg VSC

For Star/Delta transformer

• DC bus voltage: 340V

• DC bus capacitance: C= 3200μF.

• AC Inductance: Lf= 5 mH

• Three numbers of single-phase transformers of rating 3.4kVA with 240V/140V for

the system.

For Zig/Zag transformer

• DC bus voltage: 340V

• DC bus capacitance: C= 3200μF.

• AC Inductance: Lf= 5 mH

• Three numbers of single-phase transformers of 3kVA with 140 V/140 V/140V.

Paper Published • Anurag Jain and Monika Jain “Comparison of Star/Delta and Zig/Zag Transformer Based

DSTATCOM for Power Quality Improvement” 3rd IEEE International Student’s Conference on

Electrical, Electronics and Computer Science 5-6 march 2016, accepted (SCEECS-2016),

Maulana Azad National Institute of Technology (MANIT) Bhopal, India.

• Anurag Jain and Monika Jain “Isolated operation of Star/Delta and Zig/Zag transformer Based

DSTATCOM for Power Quality Improvement” National conference of Recent Advancement in

Futuristic Technologies 17-18 October 2015.

• Anurag Jain and Monika Jain “Comparative Analysis of Star/Delta and Zig/Zag Transformer

Based DSTATCOM for Power Quality improvement “International Journal of Innovative Trends

in Engineering (IJITE), ISSN: 2395-2946 IJITE_12_01403 Volume 12 No-1 December 2015.

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