Improvement of Power Quality by using Super Capacitors ... · PDF fileconditions cause voltage sag or swell in the system and ... The mitigation of these on the source and load

SSRG International Journal of Electrical and Electronics Engineering– (ICEEMST’17) - Special Issue- March 2017

ISSN: 2348-8379 www.internationaljournalssrg.org Page 34

Improvement of Power Quality by using Super

Capacitors Based UPQC

S.V.SATYANARAYANA1

Assistant Professor in EEE

RISE Krishnasai Prakasam group of institutions

Ongole, AP, India

V.PRASANTHI 2

UG Student Scholar,


Ongole, AP, India

M.G.MAHALAKSHMI 3

UG Student Scholar,


Ongole, AP, India

Y.PRAVALIKA4

UG Student Scholar

RISE Krishnasai Prakasam i group of institutions,

Ongole, AP, India

Abstract - This paper introduces the integration of

unified power quality conditioner with the

supercapacitor for improving the power quality at the

supply mains. This work described the unified power

quality conditioner principles and power restoration for

balanced or unbalanced voltage sags or swells in a

distribution system. This method proposes a typical

configuration of unified power quality conditioner for

compensating sag or swells conditions for three phase

system that consists of a DC/DC converter supplied by

a supercapacitor at the DC link. A suitable series-shunt

controller is employed for controlling the unified power

quality conditioner under balanced or unbalanced load

currents is also presented. The operation of the

proposed system is modeled and simulated in MATLAB

environment using Simulink and Simpower System

toolboxes.

Key words — Power quality, unified power quality

conditioner (UPQC), supercapacitor, DC/DC

converter,third harmonic distortion (THD), phase

locked loop (PLL), synchronous reference frame (SRF).

I. INTRODUCTION

The modern equipment’s that are used in home are very

sensitive and prone to harmonics as well as voltage

disturbances with poor power factor. The power quality

problem is also due to the different faults conditions

occurring on the power system network. These

conditions cause voltage sag or swell in the system and

malfunctioning of devices which damages the sensitive

loads. The mitigation of these on the source and load

sides is most important for improving the reliability as

well as performance on the system. Unified Power

Quality Conditioner (UPQC) is expected to be one of

the most powerful solutions to large capacity loads that

are sensitive to the changes in supply voltage, flicker or

imbalance. The UPQC has a single topology that

combines series active power filter and shunt active

power filter with a common DC link. These two are

connected in a back to back configuration. Shunt active

power filter compensates all current related distortions

and series active power filter compensates all voltage

related distortions. The compensation can be done

effectively, if there is an effective DC link. The

operation of both series active power filter and shunt

active power filter are based on voltage source

converter technique. The shunt compensator takes care

of reactive power compensation, current harmonic

compensation, load unbalance compensation and power

factor improvement. The series compensator acts for

voltage harmonics, voltage sag or swells, flickering etc.,

with the harmonic isolation between load and supply.

The supercapacitor is used as a battery storage device

across the DC link for short time duration.

The energy can be stored in the form of batteries,

flywheels, compressed air, hydraulic systems and super

conducting energy storage systems [1]. A configuration

with STATCOM-super capacitor energy storage system

is used to enhance power system stability and quality

[2]. Super capacitors are also find applications in metro

vehicles and hybrid electric vehicles [3], also in traction

[4]. The battery has a high storage capacity but

unreliable and flywheels requires a lot of maintenance.

The discharge rate is slower in batteries because of

slower chemical process. But now the future is turned to

higher rate of charging and discharging the energy

which is possible with the super capacitors. The super

capacitors stores less energy however the power transfer

capability is high compared to the conventional

batteries. The rate of discharge while compensation is

fast and it takes only a small current for charging [1].

Use of super capacitor is proposed in UPQC scheme as

it is characterized by less weight, faster

charge/discharge cycle time, higher power density,

higher efficiency and almost maintenance free.

The paper [5] explains the power circuit modeled as a

3-phase 4-wire system with a non-linear load that is

composed of 3- phase diode-bridge rectifier with RC

load in the DC side. The Third Harmonic Distortion

(THD) of more than 40% is observed. In paper [6]

described the application of Bi-directional full bridge

DC-DC converters in UPQC. In the papers concentrated

only on voltage sag or swell utilizing the series inverter.

The most important parameters of a super capacitor



include the capacitance(C), equivalent series resistance

and equivalent parallel resistance which is also called

leakage resistance. UPQC is a custom power device and

consists of combined series active power filter

compensates voltage harmonics, voltage sag, voltage

swell, flicker etc. and shunt active power filter

compensates current related problems such as reactive

power compensation, reactive power compensation and

power factor improvement etc. In all the operating

conditions the THD of source current has been observed

within an IEEE 519-1992 standard limit of 5% . Using

UPQC how to minimize the power losses and improve

voltage profile to determine the best location and the

size of UPQC while the load constraints, network

constraints and operational constraints are satisfied and

compensation of harmonics. This Integrated unit is

developed to enhance the capability of the UPQC and to

maintain a high quality voltage, current and power

factor at the point of common coupling. This paper

concentrates on both series and shunt inverters

functioning on voltage related issues like sag, swell and

flickering etc. and reduction of harmonics and reactive

power control.

II. SUPERCAPACITOR BASED UPQC

The block diagram representation for the proposed

system is shown in Fig 1. The dc link of both of these

active filters is connected to a supercapacitor through a

common dc link capacitor. The three-leg voltage source

inverter based shunt active filter is capable of

suppressing the harmonics in the source currents, load

balancing and power factor correction. The series filter

is connected between the supply and load terminals

using a three phase transformer. The main aim of the

series active power filter is to obtain harmonic isolation

between the load and supply. In addition to injecting the

voltage, these transformers are also used to filter the

switching ripple content in the series active filter. A

small capacity rated R-C filter is connected in parallel

withthe secondary of each series transformer to

eliminate the high switching ripple content in the series

active filter injected voltage. The voltage source

inverters for both the series and shunt active power

filters are implemented with insulated gate bipolar

transistors. The load under consideration is a

combination of linear and non-linear type.

Fig 1. Proposed Block Diagram of UPQC with Super

capacitor

The super capacitor bank consists of number of series

and parallel capacitors to increase the current and

voltage at the DC link and the DC/DC converter is used

to maintain constant voltage at the DC link irrespective

of the voltage at the super capacitor bank. It boosts the

voltage level when sag appears in the line, and

consumes energy when there is a swell in the line.

UPQC can be utilized to solve power quality problems

simultaneously [9]. Proposed method is about the usage

of super capacitor instead of a conventional energy

storage for a UPQC. The UPQC for harmonic

elimination and simultaneous compensation of voltage

and current, which improve the power quality, offered

for other harmonic sensitive loads at the point of

common coupling. UPQC has the capability of voltage

imbalance compensation as well as voltage regulation

and harmonic compensation at the consumer end. The

shunt active power filter is used to absorb current

harmonics and to regulate the dc-link voltage between

both active power filters. The UPQC, therefore, is

expected to be one of the most powerful solutions to

large capacity loads sensitive to supply-voltage-

imbalance distortions. In this paper, the proposed

synchronous reference frame based control method for

the UPQC system with a DC/DC converter to control

voltage at the super capacitor end is used and the

system performance is improved. In the proposed

control method, load voltage, source voltage, and

source current are measured, evaluated, and tested

under unbalanced and distorted load conditions using

MATLAB/Simulink software.

III. POWER QUALITY PROBLEMS IN

DISTRIBUTION NETWORK WITH HIGH

PENETRATION OF DGS

In distribution network with high penetration of DGs,

enough power support is used to restraint output power

fluctuation. The power could be supplied by energy

storage technology, which includes two aspects: one is

high efficient mass storage, and the other is fast and

efficient energy conversion. Energy storage technology

applied in power system can realize peak load shifting

and system reserve demand reduction. Meanwhile, it

would provide technical support for reducing network

power loss and improving power quality. Super

capacitor storage is normally used for smoothing the

power of short duration, high power load or used in

high peak power situation such as high power DC motor

starting and dynamic voltage restorer. When it comes to

voltage sags or instantaneous disturbance, Super

capacitor storage technology is able to improve the

power supply and quality. Thus, this technology is

suitable for solving power quality problems in

distribution network with high penetration of DGs

Custom power technology, based on power electronic

technology, could provide power supply up to reliability

and stability level which users required in MV/LV



distribution network system. UPQC, with feature of

series compensation and parallel compensation being

integrated together, has been considered as the most full

featured and effective one of all DFACTS technologies

so far. To improve power quality of distribution

network with the high penetration of DGs, developing

custom power technology based on UPQC, which can

inject active power during the voltage regulation and

integrate to reactive compensation, is a feasible

strategy.

Fig.2. Structure scheme of UPQC

Traditional UPQC used in power distribution system,

integrating series compensation voltage principle and

parallel compensation voltage principle in one device,

can compensate three-phase asymmetric and

harmonicon both mains supply voltage and nonlinear

loads. UPQC is composed of the main circuit shown in

Fig. 4.1, including series and parallel PWM converter,

and the control circuit. There are two basic control

strategies, i.e. direct control scheme and indirect control

scheme. Direct control scheme means series converter

is controlled as sinusoidal current source to isolate

voltage disturbance comes from grid and load. And

parallel converter is controlled as sinusoidal voltage

source to avoid load reactive power, load harmonic

current and unbalance from being injected into grid. On

the other side indirect control scheme means series

converter works as a non-sinusoidal voltage source,

outputting compensation voltage which offsets grid

voltage distortion and fundamental deviation,

accordingly it ensures load voltage being rated

sinusoidal voltage.

Meanwhile, parallel converter works as an non-

sinusoidal current source, outputting reactive power and

harmonic current which offset reactive load power and

load harmonic current, accordingly it could make the

injected current be sinusoidal and running under unit

power factor by compensating reactive power and

harmonic current[13]. Indirect control scheme by

researched more common is mainly discussed in this

paper. With the series and parallel PWM converter

topology, three phase four-leg circuit structure

implements both three-phase and single phase

structure, as a result, it is more flexible and versatility.

And three-phase control systems can drive unbalanced

loads as a result of three phases being mutually

independent. Therefore, it chooses the three-phase four-

leg circuit structure as the topology of power quality

improving device.

In view of the above, this paper presents a kind of three

phase four-wire power quality conditioning device

based on fast energy storage named Energy-storage

UPQC (EUPQC) aiming for power quality problems in

distribution network with high penetration of DGs.

IV. STRUCTURE OF EUPQC

As shown in Fig. 4.2, the main circuit system structure

of EUPQC includes series converter, parallel converter,

booster and discharge unit which consisting of super

capacitor energy storage and DC/DC converter,

outputting power transformer TsA~TsCof series

converter, output filters Ls and Cs of series converter

and inductance Lp of parallel converter [5]. The

electric interfaces A1, B1, C1, and N1 connect

distribution network source and the A2, B2, C2, and N2

connect various loads. Two sets of three-phase four-leg

converter respectively compose the series and parallel

converters of the EUPQC. The series converter output

enters into distribution network via LC filter and

transformer in series, while the parallel device output

enters into distribution network with filter inductance in

parallel.

The switching sequence could be shown in Fig. 4.2.

When EUPQC accesses to distribution network and sets

to work, the DC bus voltage equals to that of the super

capacitor bank. Then close contactors KMp2, 380V AC

power supply charges to the dc side via pre-charge

resistance R1 and parallel converter. When charging

completes, close KMp1, and break KMp2 and DC/DC

converter starts to work. Adjust the DC side voltage to

nominal reference level 690V. Detect unbalanced

degree and harmonic content of mains supply voltage

and load current in load side, in order that parallel

converter could be put into operation when over ranging

problem happens. And when voltage problems like

voltage sag and swell happen to mains supply, series

converter will be put into operation and output

compensation voltage until the problems are solved.

Then series converter quits working and the SCRA,

SCRB and SCRC bypass.



Fig.3. Main circuit system structure of EUPQC

The single phase structure schematic diagram of

EUPQC is illustrated in Fig. 4.3. Series converter output

voltage vector to compensate voltage unbalance and

harmonic of power supply side. Parallel converter is

used to solve power quality problems in load side, such

as unbalance and harmonic of nonlinear load including

reactive compensating and current harmonic. Super

capacitor energy storage and DC /DC converter buffer

reactive power, exchange and provide energy for

voltage compensation. As a result, decoupling series

converter and parallel converter is implemented.

Moreover, voltage quality problems of power

interruption, which beyond the reach of traditional

UPQC, can be resolved successfully.

Fig. 4.The single phase structure schematic of EUPQC

Fig.5.Control schematic of EUPQC

The ultimate purpose of EUPQC control is to keep load

voltage on a constant level and be sinusoidal feature,

compensate load reactive power and harmonic and

ensure power supply has unity power factor

characteristic in all circumstances. As is the control

schematic of EUPQC shown in Fig. 4.4, series

converter works as a non-sinusoidal voltage source,

outputting compensation voltage uc which offsets grid

voltage distortion and fundamental deviation,

accordingly it ensures load voltage uL being rated

sinusoidal voltage.

Meanwhile, shunt converter works as a non-sinusoidal

current source, outputting reactive power and harmonic

current Ic which offset reactive load power and load

harmonic current, accordingly it could make the

injected current Is be sinusoidal by compensating

reactive power and harmonic current. And the angle

between the injected voltage us and the injected current

is is zero at the moment, namely the power factor in

grid side is unity.

V.THE CONTROL STRATEGY OF EUPQC

The control of EUPQC mainly includes three aspects:

the control of series converter, the control of parallel

converter and the control of DC bus voltage. In control

strategy diagram of series converter shown in Fig. 4.5,

usa, usb, usc are distribution network three-phase

voltage respectively. Through software phase-locked

loop, we could get t ω sin andt ω cos, which is essential

to dqrotary transformation. And then we perform dq

transform and dq inverse transform on three phase

standard voltage to make it in-phase with mains supply

voltage. Then subtract the distribution network

unbalance voltage from this standard voltage to get

three phase reference compensation voltage Compare

reference voltages with three phase actual compensation

voltage uca , ucb , ucc, and constitute closed loop

control by using a PI regulator. Specifically, in SPWM

mode three phase driving signal of series converter is

generated, consequently series converter is controlled to

output corresponding voltage vector to compensate.

The control of the forth leg of series converter is

aiming to keep load zero sequence voltage to zero,

which function is implemented through closed loop

control with feed-forward control for voltage



constituted by a PI regulator. Symbols uLa, uLb, uLcin

Fig.4.5 represent three-phase load voltage respectively.

Fig. 6. Series converter control strategy diagram

As parallel converter control strategy diagram shown In

Fig. 4.6, perform dq transform on three phase load

current iLa, iLb, I Lc.

Then let the transformed current pass low-pass filter to

generate active component id and reactive component

iq. Perform dq inverse transform on these two

components to get fundamental component of three

phase load current. Subtract load current from this

standard current to get three phase reference

compensation current. Compare the reference currents

with three phase actual compensation current ica, icb,

icc, and constitute closed loop control by using a PI

regulator. The same as the series converter control

mode, in SPWM mode three phase driving pulse signal

of parallel converter is generated, consequently parallel

converter is controlled to output corresponding current

vector to compensate. The control of the forth leg of

shunt converter is aiming to keep load zero sequence

current to zero, which function is implemented through

closed loop control constituted by a PI regulator.

Symbols isa, isb, iscin Fig. 4.6 represent three-phase

power supply current respectively.

Parallel converter can realize reactive compensation by

controlling reactive component iq. If iq=0, then all

reactive power of the load is provided by parallel

converter.

Fig.7.Parallel converter control strategy diagram

DC side of EUPQC, consisting of bi-directional DC-DC

converter based on super capacitor fast energy storage,

is able to solve problems of deeper voltage sag and

voltage instantaneous interruption. Fig. 4.7 illustrates

control strategy of DC/DC converter. After comparing

reference voltage Udef with DC bus voltage Ud, the

two voltages pass through closed loop PI control and

then compared by limited driver to generate PWM

signal. They could drive IGBT3 and IGBT4 in Fig. 4.2

respectively to implement the control of DC/DC

converter. And then use the output to maintain Ud at a

stable level. The function of discharge circuit

comprising IGBT1 and IGBT2 could avoid over tension

happens to DC bus voltage Ud

Fig.8.DC/DC converter control strategy diagram

VI. MATLAB/SIMULINK RESULTS

Fig.9. Matlab/Simulink Model of UPQC Based on fast

energy storage

Fig.10.shows load voltage, DVR voltage and source

voltage



Fig.11.shows source current, load current and

compensating current

Case.1: balanced load


voltage for balanced load


compensating current for balanced load

Case.2: Unbalanced load


voltage for unbalanced load


compensating current for unbalanced load

Case.3: Non linear load


voltage for non-linear load


compensating current for non-linear load



VII. CONCLUSION

This paper proposes a new configuration of UPQC that

consists of the DC/DC converter and the super

capacitor. The proposed UPQC compensated the

reactive power, harmonic currents, voltage sag and

swell, voltage unbalance, and the voltage interruption.

In all the operating conditions the THD of source

current has been observed within an IEEE 519- 1992

standard limit of 5%. The operation of proposed system

was demonstrated through simulation with

MATLAB/SIMULINK software. The proposed UPQC

has the ultimate capability of improving the power

quality at the installation point in the distribution

system.

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AUTHOR’S PROFILE

S.V.Satyanarayana born in Ongole ,A.P.,

India in the year 1989.He Received B.Tech

degree in "EEE" from QIS College Of

Engineering & Technology ., affiliated to

JNTUK, in 2010. He Received M.Tech

degree in "EEE" with specilization in Power

System (High Voltage) from Godavari

Institute Of Engineering & Technology ,Rajahmundry

affiliated to JNTUK in 2014.His Area of Interests are

POWER SYSTEMS,HIGH VOLTAGE. He is a Member in

IAENG ,ISRD .He is Presently working as an Asst.Prof. in

EEE in RISE Krishnasai Prakasm Group of Institutions

,Ongole.

V.PRASANTHI, UG Scholar, currently

studying B.Tech in the stream of EEE in

RISE Krishnasai Prakasm Group of

Institutions, Ongole. Her Area of Interests

are POWER SYSTEMS, FACTS.

M.GAYATHRI.MAHALAKSHMI,

UG Scholar, currently studying

B.Tech in the stream of EEE in

RISE Krishnasai Prakasm Group of

Institutions, Ongole. Her Area of

Interests are POWER SYSTEMS,

POWER QUALITY

Y.PRAVALIKA, UG Scholar,

currently studying B.Tech in the

stream of EEE in RISE Krishnasai

Prakasm Group of Institutions,

Ongole. Her Area of Interests are

Distribution Systems, Measurements.

Documents

Improvement of Power Quality by using Super Capacitors ... · PDF fileconditions cause voltage sag or swell in the system and ... The mitigation of these on the source and load