IRJET- UPQC System with an Improved Control Method Under Distorted and Unbalanced Load Condition

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 06 Issue: 04 | Apr 2019 www.irjet.net p-ISSN: 2395-0072
© 2019, IRJET | Impact Factor value: 7.211 | ISO 9001:2008 Certified Journal | Page 2561
UPQC System With an Improved Control
Method Under Distorted and Unbalanced Load
Condition
Kismat Patel1,Prof. R.T. Patel2 PG Student1 Asst. Professor2
Electrical Engineering Department12,
Government Engineering College, Bhuj.
----------------------------------------------------------------
Abstract:-In order to improve the power quality at the distribution side the unified power quality conditioner has become
the most attractive solution. The unified power quality conditioner(UPQC) is consisting of both the series and shunt active
power filters such that it can improve power quality both on the source side as well as on the load side. In this paper
simplified control techniques have been described for controlling the shunt and series active power filters. For controlling
the shunt active power filter two methods are analysed and the series active power filter is controlled by synchronous
reference frame theory using three phase locked loop (PLL). The model for the control techniques of both the shunt and
series active power filters have been presented.
Keywords:-Unified Power Quality Conditioner (UPQC), Active Power Filter(APF), Phase Locked Loop(PLL), Power
Quality(PQ).
[I] INTRODUCTION
The unified power quality conditioner (UPQC) is one of the most preferable custom power device which can mitigate both
voltage and current related power quality issues in the distribution system. The voltage related issues are mitigated by
using series active power filter while the current related issues are cleared by using the shunt APF. As due to the nonlinear
and distorted unbalanced load at the distribution side the power quality at the distribution side is getting poor day by day.
Hence, in order to mitigate the harmonics introduced from the load side of the distribution system these active power
filters plays a significant role. The voltage sags ,swells, harmonics, flickers all these voltage related problems are mitigated
by using series APF with the three phase locked loop. While to eliminate current harmonics and improve power factor at
the load side shunt APF is used. Hence, UPQC is one of the most commonly used custom power device which is capable to
improve the power quality not only on the load side but also on the source side simultaneously with the reactive power
compensation.
There are many control strategies to control the active power filters. This paper is presenting simplified method for
controlling the shunt APF using the Constant Power Control Technique and the Synchronous Reference Frame Theory.
While the series APF is being controlled by using simple Synchronous Reference Frame Theory with the help of three
phase locked loop (PLL).
[II] Power Quality issues with their causes and effects
Power Quality is surely an important concern in most industries today, because of the increase in the number of load
sensitive to power disturbances. The power quality is an index to quality of current and voltage available to industrial,
commercial and household consumers of electricity. The problem regards both the utilities and customers. For the utilities,
to provide adequate power quality is a moving objective because of changes in user equipment and requirements.
Power quality is a topic embracing a large field. On one side, several different events are involved in power quality: spikes
or surges, sags, swells, outages, under and over voltages, harmonics, flicker, frequency deviations, and electrical noise.

Voltage sag in electrical grid are not always possible to avoid because of finite clearing time of faults that cause the voltage
sag. Voltage sag are the common reason for interruption in production plants and for end user equipment malfunctions in
general. In particular, tripping of equipment in a production line can cause production interruption and significant costs
due to loss of production.
Table 1. Summary of Disturbances with Solutions
Disturbanc
e
Category
Wave Form Effects
Possible
Causes
Possible
Solutions
1.Transients
Impulsive
Loss of data,
possible
damage, system
halts
Lightning, ESD,
switching
impulses,
utility fault
clearing
TVSS, maintain
humidity
between 35-
50%
Oscillatory Loss of data,
possible
damage
Switching
off
inductive/
Capacitive
Loads
TVSS, UPS,
Reactors/Choke
s, Zero Crossing
Switch
2.Interruptions
Loss of data,
possible
damage,
shutdown
Switching,
utility
faults, CB
tripping,
component
failures
UPS, DVR
3. Sag / Under voltage
Sag
System
halts, loss of
data,
shutdown
Startup
loads, faults
UPS, DVR
Under
Voltage
System
halts, loss of
data,
shutdown
Utility
faults, load
changes
UPS, DVR

4. Swell / Overvoltage
Swell
Nuisance
tripping,
Equipment
damage/
reduced
Life
Load changes,
utility faults
DVR , UPS,
Ferro resonant
control
Transformers
Overvoltage equipment
damage/reduced
life
Load changes,
utility faults
DVR ,UPS,
Ferro resonant
control
Transformers
5. Waveform Distortion
DC Offset
Transformers
heated,
ground fault
current,
nuisance tripping
Faulty
rectifiers,
power supplies
Troubleshoot
and replace
defective
equipmen
t
Harmonics
Transformer
s heated,
System halts
Electronic
loads (non-
linear loads)
Reconfigure
distribution
, install
k-factor
transformers
Inter
harmonics
light flicker,
heating,
communicatio
n interference
faulty
equipment
Cyclo converter
freq.
converters,
induction
motor arcing
devices
Power
Conditioner,
Filters,
UPS
Notching
System halts,
data loss
Variable
speed drives,
arc welders,
light dimmers
Reconfigure
distribution,
relocate
sensitive
loads, install
filters, UPS
Noise System halts,
data loss
Transmitters
(radio), faulty
equipment,
ineffective
grounding
Remove
transmitters,
reconfigure
grounding,
increase
shielding, filters

6.Voltage Fluctuations
System
halts, light
flicker
Intermittent
operation of
load
equipment
Reconfigure
distribution,
Relocate
sensitive
loads,
UPS
7. Power Frequency Variations
Synchronous
equipment
failure, No
effect
on IT equipment
Standby
generators
ineffectivel
y
governed
Upgrade
generato
rgovern
or
[III]Control Strategies for Active Power Filters
A.Shunt Active Power Filter
The SAPF technology is now a mature technology for providing harmonic current compensation, reactive power
compensation, and neutral current compensation in AC distribution networks. It has evolved in the past quarter century
with development in terms of varying configurations, control strategies, and solid-state devices. Shunt active power filters
are also used to regulate the terminal voltage and suppress voltage flicker in three-phase systems. These objectives are
achieved either individually or in combination depending upon the requirements, control strategy, and configuration that
need to be selected appropriately. Shunt active power filter is considered as an ideal device for mitigating power quality
problems. The shunt active power filters are basically categorized into three types, namely, single-phase two-wire, three-
phase three-wire, and three-phase four-wire configurations, to meet the requirements of the three types of nonlinear loads
on supply systems. Some single-phase loads such as domestic lights, ovens, TVs, computer power supplies, air
conditioners, laser printers, and Xerox machines behave as nonlinear loads and cause power quality problems. Single-
phase two-wire active power filters of varying configurations and control strategies have been investigated to meet the
needs of single-phase nonlinear loads. Starting from 1971, many configurations of shunt active power filters have been
developed and commercialized in UPS applications. Both current source converter with inductive energy storage and
voltage source converter with capacitive energy storage are used to develop single-phase SAPFs. A major amount of AC
power is consumed by three-phase loads such as ASDs with solid-state control and lately in the front-end design of many
other electrical loads the active power filters have been incorporated. A substantial work has been reported on three-
phase, three-wire APFs. Starting from 1976, many control strategies such as instantaneous reactive power theory (IRPT),
synchronous frame d–q theory, synchronous detection method, notch filter method are used in the development of three-
phase APFs. The problem of excessive neutral current is observed in three-phase four-wire systems mainly due to
nonlinear unbalanced loads such as computer power supplies, fluorescent lighting, and so on. These problems of neutral
current and unbalanced load currents in four-wire systems have been attempted to resolve through elimination/
reduction of neutral current, harmonic compensation, load balancing, and reactive power compensation.
Principle of Operation of Shunt Active Power Filters :-The main objective of shunt active power filters is to mitigate
multiple power quality problems in a distribution system. SAPF mitigates most of the current quality problems, such as
reactive power, unbalanced currents, neutral current, harmonics, and fluctuations, present in the consumer loads or
otherwise in the system and provides sinusoidal balanced currents in the supply along with its DC bus voltage control. In
general, a SAPF has a VSC connected to a DC bus and its AC side is connected in shunt normally across the consumer loads
or across the PCC, as shown in Figure 1. The VSC uses PWM current control; therefore, it requires small ripple filters to
mitigate switching ripples. It requires Hall effect voltage and current sensors for feedback signals and normally a digital

signal processor (DSP) is used to implement the required control algorithm to generate gating signals for the solid-state
devices of the VSC of the SAPF. The VSC used as SAPF is normally controlled in PWM current control mode to inject
appropriate currents into the system. The SAPF also needs many passive elements such as a DC bus capacitor, AC
interacting inductors, and small passive filters.
Fig.1:-Shunt Active Power Filter
Control of Shunt Active Power Filter:- Reference current signals for the control of SAPF have to be derived accordingly
and these signals may be estimated using a number of control algorithms. There are many control algorithms available for
the control of SAPFs. In this paper two control strategies are discussed.
i)Constant Power Control Technique
ii)Synchronous Reference Frame Theory
Constant Power Control Technique
Clark Transformation : abc to αβ
Inverse Clark Transformation : αβ to abc
Based on instantaneous p-q theory

Instantaneous Active Power,
Instantaneous Reactive Power
Reference Current
Fig.2:-SAPF using Constant Power Control Technique
Synchronous Reference Frame Theory
p v i v i
q v i v i
   
   
 
 
p p p
q q q
 
 
*
* 2 2
1c loss
c
v vi p p
v vi qv v
 
   
     
           
Power loss in VSC and Required power to Charge Capacitorlossp 

Fig.3:-SAPF using Synchronous Reference Frame Theory
B. Series Active Power Filter
Among the power quality problems (sags, swells, harmonics…) voltage sags are the most severe disturbances. In order to
overcome these problems the concept of custom power devices is introduced recently. One of those devices is the series
APF, which is the most efficient and effective modern custom power device used in power distribution networks. It is
generally connected where sensitive loads are present at the load side.
A power electronic converter based series compensator that can protect critical loads from all supply side disturbances
other than outages is called a series APF. It injects a set of three phase AC output voltages in series and synchronism with
the distribution feeder voltages. The amplitude and phase angle of the injected voltages are variable thereby allowing
control of the real and reactive power exchange between the device and the distribution system.
Fig.4:-Series Active Power Filter

BASIC Series APF OPERATING PRINCIPLE
The series APF is operated by injecting three phase AC voltages in series with the three phase incoming network voltages
during a sag, compensating for the difference between faulty and nominal voltages. All three phases of the injected
voltages are of controllable amplitude and phase.
Voltage source inverters (VSI) fed from a DC link and supply the active and reactive power. Harmonics produced by the
operation of VSI must be reduced to an acceptable limit fined by proper filtering scheme. VSI switches an impact on the
harmonics produced. The required energy during sags has to be supplied by an energy source. The necessary amount of
energy that must be delivered by the energy source depends on load MVA requirement, control strategy applied, deepest
sag to be protected.
Control of Series APF:-The control of series active power filter is achieved by using simple Synchronous reference frame
theory with the battery aided support system as shown in the fig.5 given below.
Fig.5:-Series APF using Synchronous Reference Frame Theory
Conclusion:-The proposed control techniques have been used to control both the shunt and series APF’S with the
minimum measurements like load current and source voltages. The paper describes the basic principle of the control
strategies. With these control strategies both the active power filters are able to mitigate voltage distortion and current
harmonics on both the load as well as source side simultaneously with the reactive power compensation.
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