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A CONTROL STRATEGY FOR UNIFIED POWER QUALITY CONDITIONER BASED ON INSTANTANEOUS ACTIVE AND REACTIVE POWERS
Document By
SANTOSH BHARADWAJ REDDY
Email:help@matlabcodes.com
Engineeringpapers.blogspot.comMore Papers and Presentations available on above site
Abstract
: - One of the serious problems in electrical systems is the increasing number of electronic- components of devices that are used by industry as well as residences. Thesedevices, which need high-quality energy to work properly, at the same time, are the mostresponsible ones for injections of harmonics in the distribution system. Therefore, devices thatsoften this drawback have been developed. One of them is the unified power qualityconditioner (UPQC). This paper presents a control strategy for a Unified Power QualityConditioner. This control strategy is used in three-phase three-wire systems. The UPQC devicecombines a shunt-active filter together with a series-active filter in a back -to- back configuration, to simultaneously compensate the supply voltage and the load current. Some of the other control strategy for shunt-active filter that guarantees sinusoidal balanced andminimized source currents even if under unbalanced and / or distorted system voltages, alsoknown as “Sinusoidal Fryze Currents”. Then, this control strategy was extended to develop adual control strategy for series-active filter. Now, this paper deals about the integration principles of shunt current compensation and series voltages compensation, both based oninstantaneous active and non-active powers, directly calculated from a-b-c phase voltages andline currents. Literature-simulated results are presented to validate the proposed
Index Terms
: Active Filters, Active Power Line Conditioners, Instantaneous Active andReactive Power, Sinusoidal Fryze Currents, Sinusoidal Fryze Voltages.
I. Introduction
ONE of the serious problems in electrical systems is the increasing number of electronic- components of devices that are used by industry as well as residences. Thesedevices, which need high-quality energy to work properly, at the same time, are the mostresponsible ones for injections of harmonics in the distribution system. Therefore, devices that
soften this drawback have been developed. One of them is the unified power qualityconditioner (UPQC), as shown in Fig.1. It consists of a shunt- active filter together with aseries-active filter. This combination allows a simultaneous compensation of the load currentsand the supply voltages, so that compensated current drawn from the network and thecompensated supply voltage delivered to the load are sinusoidal, balanced and minimized. Theseries and shunt-active filters are connected in a back-to-back configuration, in which the shuntconverter is responsible for regulating the common DC-link voltage.Fig1. General Configuration of UPQCIn the 30’s of the last century, Fryze [1] proposed a set of active and non-active(reactive) power definitions in the time domain. From these concepts, Tenti
et al
[2] developeda control strategy for shunt-active filters that guarantees compensated currents in the network that are sinusoidal even if the system voltage at the point of common coupling (PCC) alreadycontains harmonics. However, this control strategy does not guarantee balanced compensatedcurrents if the system voltage itself is unbalanced (i.e. it contains a fundamental negative-sequence component). In [3], this drawback was overcome, by the addition of a positivesequence voltage detector in the shunt-active filter controller. This control circuit determinesthe phase angle, frequency and magnitude of the fundamental positive sequence voltagecomponent. This new control strategy has been denominated as the
“Sinusoidal FryzeCurrents”
control strategy.
This work exploits the use of that positive-sequence voltage detector to develop a newcontrol strategy for series- active filter. It is based on a dual minimization method for voltagecompensation, together with a synchronizing circuit (PLL circuit). The synchronizing circuit isresponsible to detect the fundamental frequency, as well as the phase angle of the positive-sequence voltage component. The dual minimization method is responsible to accuratelydetermine the magnitude of this voltage component. This control strategy is denominated hereas the
“Sinusoidal Fryze Voltages”
control strategy. Further, this paper presents the integrationthe
“Sinusoidal Fryze Currents”
and the
“Sinusoidal Fryze Voltages”
control strategies into anUPQC controller. Additionally, the UPQC controller includes an algorithm that providesdamping in harmonic voltage propagation and hinders load harmonic currents to flow into thenetwork.
II. The UPQC Controller
Fig. 2 shows the complete functional block diagram of the UPQC controller. The part shown in Fig. 2(a) is responsible to determine the compensating current references for PWM control of the UPQC shunt converter, whereas the other part shown in Fig. 2(b)generates the compensating voltage references for PWM series converter. Next, each functional block of Fig. 2 will be detailed. Fig 2: The functional diagram of the UPQC Controller (a). Shunt UPQC Converter, (b). Series UPQC Converter
A. The positive sequence voltage Detector:
A positive-sequence voltage detector is [V
+1
voltage detector block in Fig. 2(a)] interms of minimized voltages . A dual principle for voltage minimization together with a phase-locked loop circuit (PLL circuit), as shown in Fig. 3. The used PLL circuit is detailed inthe next section. In fact, this dual principle of voltage minimization is used here for extracting
instantaneously the fundamental positive-sequence component (
V
+1
in phasor notation, or
v
a1
,
v
b1
,
v
c1
as instantaneous values, as the outputs of Fig. 3) from a generic three-phase voltage.The distorted and unbalanced voltages
v
as
,
v
bs
,
v
cs
of the power supply are measured and givenas inputs to the PLL circuit.As shown in the next section, it determines the signals
i
a1
,
i
b1
,
i
c1
, which are in
phasewith the fundamental positive-sequence component (
V
+1
) contained in
v
as
,
v
bs
,
v
cs
. Thus, onlythe magnitude of
V
+1
is missing. The fundamental characteristic of the used PLL allows the useof a dual expression for determining active voltages in the form
)1(
111212121111
−−−−−
++++=
cbacbacc sbb saa s
c pb pa p
iiiiiiiviviv
vvv
As an artifice to extract the
V
+1
component from
v
as
,
v
bs
,
v
cs
. The reason is that thesignals
i
a1
,
i
b1
,
i
c1
are three symmetric sinus functions with unity amplitude, which correspond toan auxiliary fundamental positive-sequence current
I
+1
that is
in phase
with
V
+1
.
Hence, theaverage value of the three-phase instantaneous power , 3
V
+1
I
+1
cosØ
,
is maximum (would bezero if
V
+1
and
I
+1
are orthogonal), and the average signal
R
bar
in Fig. 3 comprises the totalamplitude of
V
+1
. Therefore, it is possible to guarantee that the signals
v
a1
,
v
b1
,
v
c1
aresinusoidal and have the same magnitude and phase angle of the fundamental positive-sequencecomponent of the measured system voltage.

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