Page 1 of 8

Journal for Studies in Management and Planning

Available at http://edupediapublications.org/journals/index.php/JSMaP/

e-ISSN: 2395-0463

Volume 02 Issue 9

September 2016

Available online: http://edupediapublications.org/journals/index.php/JSMaP/ P a g e | 78

Power Quality Improvement by Using IUPQC in

Transmission lines

P.Uday & Mr.V. Balu

Abstract —This paper presents an improved controller

for the dual topology of the unified power quality condi

tioner (iUPQC) extending its applicability in power-quality

compensation, as well as in microgrid applications. By

using this controller, beyond the conventional UPQC power

quality features, including voltage sag/swell compensation,

the iUPQC will also provide reactive power support to

regulate not only the load-bus voltage but also the volt- age at the grid-side bus. In other words, the iUPQC will

work as a static synchronous compensator (STATCOM) at

the grid side, while providing also the conventional UPQC

compensations at the load or microgrid side. Experimental

results are provided to verify the new functionality of the

equipment.

Index Terms—iUPQC, microgrids, power quality, static

synchronous compensator (STATCOM), unified power qual- ity conditioner (UPQC).

I. INTRODUCTION

CERTAINLY, power-electronics devices have brought

about great technological improvements. However, the

increasing number of power-electronics-driven loads used

gen- erally in the industry has brought about uncommon

power- quality problems. In contrast, power-electronics- driven loads generally require ideal sinusoidal supply voltage

in order to function properly, whereas they are the most

responsible ones for abnormal harmonic currents level in the

distribution system. In this scenario, devices that can mitigate

these drawbacks have been developed over the years. Some of

the solutions involve a flexible compensator, known as the

unified power quality conditioner (UPQC) [1]–[7] and the

static synchronous compensator (STATCOM) [8]–[13].

The power circuit of a UPQC consists of a combination of a

shunt active filter and a series active filter connected in a

back-to-back configuration. This combination allows the

simultaneous compensation of the load current and the

supply voltage, so that the compensated current drawn from

the grid and the compensated supply voltage delivered to the

load are kept balanced and sinusoidal. The dual topology of

the UPQC, i.e., the iUPQC, was presented in [14]–[19],

where the shunt active filter behaves as an ac-voltage source

and the series one as an ac-current source, both at the

fundamental frequency. This is a key point to better design

the control gains, as well as to optimize the LCL filter of the

power converters, which allows improving significantly the

overall performance of the compensator [20].

The STATCOM has been used widely in transmission net- works to regulate the voltage by means of dynamic reactive- power compensation. Nowadays, the STATCOM islargely used

for voltage regulation [9], whereas the UPQC and the iUPQC

have been selected as solution for more specific applications

[21]. Moreover, these last ones are used only in particular

cases, where their relatively high costs are justified by the

power quality improvement it can provide, which would be

unfeasible by using conventional solutions. By joining the extra

functionality like a STATCOM in the iUPQC device, a wider

scenario of applications can be reached, particularly in case of

distributed generation in smart grids and as the coupling device

in grid-tied microgrids.

In [16], the performance of the iUPQC and the UPQC was

compared when working as UPQCs. The main difference be- tween these compensators is the sort of source emulated by the

series and shunt power converters. In the UPQC approach, the

series converter is controlled as a nonsinusoidal voltage source

and the shunt one as a nonsinusoidal current source. Hence, in

real time, the UPQC controller has to determine and synthesize

accurately the harmonic voltage and current to be compensated.

On the other hand, in the iUPQC approach, the series converter

behaves as a controlled sinusoidal current source and the shunt

converter as a controlled sinusoidal voltage source. This means

that it is not necessary to determine the harmonic voltage and

current to be compensated, since the harmonic voltages appear

naturally across the series current source and the harmonic

currents flow naturally into the shunt voltage source.

In actual power converters, as the switching frequency in- creases, the power rate capability is reduced. Therefore, the

iUPQC offers better solutions if compared with the UPQC in

case of high-power applications, since the iUPQC compensat- ing references are pure sinusoidal waveforms at the fundamen- tal frequency. Moreover, the UPQC has higher switching losses

due to its higher switchingfrequency.

Page 2 of 8

Journal for Studies in Management and Planning

Available at http://edupediapublications.org/journals/index.php/JSMaP/

e-ISSN: 2395-0463

Volume 02 Issue 9

September 2016

Available online: http://edupediapublications.org/journals/index.php/JSMaP/ P a g e | 79

Fig. 1. Example of applicability of iUPQC.

This paper proposes an improved controller, which expands

the iUPQC functionalities. This improved version of iUPQC

controller includes all functionalities of those previous ones,

including the voltage regulation at the load-side bus, and now

providing also voltage regulation at the grid-side bus, like a

STATCOM to the grid. Experimental results are provided to

validate the new controller design.

This paper is organized in five sections. After this introduc- tion, in Section II, the iUPQC applicability is explained, as

well as the novel feature of the proposed controller. Section III

presents the proposed controller and an analysis of the power

flow in steady state. Finally, Sections IV and V provide the

experimental results and the conclusions, respectively.

II. EQUIPMENT APPLICABILITY

In order to clarify the applicability of the improved iUPQC

controller, Fig. 1 depicts an electrical system with two buses

in spotlight, i.e., bus A and bus B. Bus A is a critical bus of the

powersystem thatsuppliessensitive loads and serves as point of

coupling of a microgrid. Bus B is a bus of the microgrid, where

nonlinear loads are connected, which requires premium-quality

power supply. The voltages at buses A and B must be regulated,

in order to properly supply the sensitive loads and the nonlinear

loads. The effects caused by the harmonic currents drawn by the

nonlinear loads should be mitigated, avoiding harmonic voltage

propagation to busA.

The use of a STATCOM to guarantee the voltage regulation

at bus A is not enough because the harmonic currents drawn

by the nonlinear loads are not mitigated. On the other hand, a

UPQC or an iUPQC between bus A and bus B can compensate

the harmonic currents of the nonlinear loads and compensate

the voltage at bus B, in terms of voltage harmonics, unbalance,

and sag/swell. Nevertheless, this is still not enough to guarantee

the voltage regulation at bus A. Hence, to achieve all the desired

goals, a STATCOM at bus A and a UPQC (or an iUPQC)

between buses A and B should be employed. However, the costs

of this solution would be unreasonably high.

An attractive solution would be the use of a modified iUPQC

controller to provide also reactive power support to bus A,

in addition to all those functionalities of this equipment, as

presented in [16] and [18]. Note that the modified iUPQC serves

as an intertie between buses A and B. Moreover, the microgrid

connected to the bus B could be a complex system comprising

distributed generation, energy management system, and other

Fig. 2. Modified iUPQC configuration.

control systems involving microgrid, as well as smart grid

concepts [22]. In summary, the modified iUPQC can provide

the following functionalities:

a) ―smart‖ circuit breaker as an intertie between the grid and

the microgrid;

b) energy and power flow control between the grid and the

microgrid (imposed by a tertiary control layer for the

microgrid);

c) reactive power support at bus A of the power system;

d) voltage/frequency support at bus B of the microgrid;

e) harmonic voltage and current isolation between bus A and

bus B (simultaneous grid-voltage and load-current active- filtering capability);

f) voltage and current imbalance compensation.

The functionalities (d)–(f) previously listed were extensively

explained and verified through simulations and experimental

analysis [14]–[18], whereas the functionality (c) comprises

the original contribution of the present work. Fig. 2 depicts,

in detail, the connections and measurements of the iUPQC

between bus A and busB.

According to the conventional iUPQC controller, the shunt

converter imposes a controlled sinusoidal voltage at bus B,

which corresponds to the aforementioned functionality (d). As

a result, the shunt converter has no further degree of freedom

in terms of compensating active- or reactive-power variables to

expand its functionality. On the other hand, the series converter

of a conventional iUPQC uses only an active-power control

variable p, in order to synthesize a fundamental sinusoidal

current drawn from bus A, corresponding to the active power

demanded by bus B. If the dc link of the iUPQC has no large

energy storage system or even no energy source, the control

variable p also serves as an additional active-power reference to

the series converter to keep the energy inside the dc link of the

iUPQC balanced. In this case, the losses in the iUPQC and the

active power supplied by the shunt converter must be quickly

compensated in the form of an additional active power injected

by the series converter into the bus B.

The iUPQC can serve as: a) ―smart‖ circuit breaker and as

b) power flow controller between the grid and the microgrid

only if the compensating active- and reactive-power references

Page 3 of 8

Journal for Studies in Management and Planning

Available at http://edupediapublications.org/journals/index.php/JSMaP/

e-ISSN: 2395-0463

Volume 02 Issue 9

September 2016

Available online: http://edupediapublications.org/journals/index.php/JSMaP/ P a g e | 80

Fig. 3. Novel iUPQC controller.

of the series converter can be set arbitrarily. In this case, it is

necessary to provide an energy source (or large energy storage)

associated to the dc link of the iUPQC.

The last degree of freedom isrepresented by a reactive-power

control variable q for the series converter of the iUPQC. In

this way, the iUPQC will provide reactive-power compensation

like a STATCOM to the bus A of the grid. As it will be

confirmed, this functionality can be added into the controller

without degrading all other functionalities of the iUPQC.

III. IMPROVED IUPQC CONTROLLER

A. Main Controller

tude and frequency. Consequently, the signals sent to the

PWM controller are the phase-locked loop (PLL) outputs

with ampli- tude equal to 1 p.u. There are many possible

PLL algorithms, which could be used in this case, as verified

in [29]–[33].

In the original iUPQC approach as presented in [14], the

shunt-converter voltage reference can be either the PLL

outputs or the fundamental positive-sequence component

VA+1 of the grid voltage (bus A in Fig. 2). The use of VA+1 in

the con- troller is useful to minimize the circulating power

through the series and shunt converters, under normal

operation, while the amplitude of the grid voltage is within an

acceptable range of magnitude. However, this is not the case

here, in the modified iUPQC controller, since now the grid

voltage will be also regulated by the modified iUPQC. In other

words, both buses will be regulated independently to track their

reference values. The series converter synthesizes the current

drawn from the grid bus (bus A). In the original approach of

iUPQC, this current is calculated through the average active

power required by the loads P L plus the power P Loss. The load

active power

can be estimated by

PL = V+1_α · iL_α + V+1_β · iL_β (2)

where iL_α, iL_β are the load currents, and V+1_α, V+1_β are

the voltage referencesforthe shunt converter.Alow-passfilter

is used to obtain the average active power (PL).

The losses in the power converters and the circulating power

to provide energy balance inside the iUPQC are calculated

indirectly from the measurement of the dc-link voltage. In other

words, the power signal PLoss is determined by a proportional–

integral (PI) controller (PI block in Fig. 3), by comparing the

measured dc voltage VDC with its reference value.

The additional control loop to provide voltage regulation like

aSTATCOMatthe grid busisrepresented by the controlsignal

QSTATCOM in Fig. 3. This control signal is obtained through a

PI controller, in which the input variable isthe error between

the reference value and the actual aggregate voltage of the grid

bus, given by

Vcol =

.

V 2 2

Fig.2depictstheiUPQChardware andthemeasuredunitsof

a three-phase three-wire system that are used in the controller.

Fig. 3 shows the proposed controller. The controller inputs

are the voltages at buses A and B, the current demanded by

bus B (iL), and the voltage vDC of the common dc link. The

outputs are the shunt-voltage reference and the series-current

reference to the pulsewidth modulation (PWM) controllers. The

A+1_α + VA+1_β

. (3)

The sum of the power signals P L and P Loss composes

the active-power control variable forthe series converterofthe

iUPQC (p) described in Section II. Likewise, QSTATCOM is the

reactive-power control variable q. Thus, the current references

i+1α and i+1β of the series converter are determined by

voltage and current PWM controllers can be as simple as those

employed in [18], or be improved further to better deal with

.

i+1_α

.

i+1_β = V

2

1

.

VA+1_α VA+1_β

.

2 VA+1_β −VA+1_α

voltage and current imbalance and harmonics [23]–[28].

First, the simplified Clark transformation is applied to the

measured variables. As example of this transformation, the grid

voltage in the αβ-reference frame can be calculated as

A+1_α +VA+1_β

.

P L + P Loss

.

× . (4)

QSTATCOM

.

VA_α

.

=

.

1 1/2

..

VA_ab

.

. (1)

VA_β 0

√

3/2 VA_bc B. Power Flow in Steady State

The shunt converter imposes the voltage at bus B. Thus, it is

necessary to synthesize sinusoidal voltages with nominal ampli- The following procedure, based on the average power flow,

is useful for estimating the power ratings of the iUPQC

nient to define the following sag/swell factor. Considering VN

as the nominal voltageksag/swell =

|

|V ̇N| VN

From (5) and considering that the voltage at bus B is kept

regulated, i.e., VB = VN , it follows that Fig. 4. iUPQC power flow in steady-state.