Power the many CP devices available, DSTATCOM

Power quality has become an
important topic to electricity consumers at all levels of usage. Nowadays the
demand for receiving the high quality electrical energy is being increasing as
consumer wants not only reliable but also quality power. Power quality can be
improved in distributed system by using shunt compensation device known as
Distribution Static Compensator (DSTATCOM). In this paper, three different
system topologies & control strategies for Distribution Static Compensators
(DSTATCOMs) are modeled and tested using Simulink’ SimPowerSystem Toolbox for
power system quality studies. Simulation tests on a distribution system,
equipped with the unbalanced and non-linear load. The modeled DSTATCOM
topologies can be used to develop and test different, control strategies and
methods for the DSTATCOM.  This
is intended to present a broad perspective on the status of DSTATCOM technology
to researchers dealing with compensation of PQ problems in distribution systems

 

Introduction

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The development of technology and
consequent up gradation of the loads of power system has brought about a
paradigm change in the customer’s outlook for the electrical power he is
willing to receive. Adding to the problem of reactive power compensation, the
proliferation of nonlinear loads is causing a higher level of harmonics in the
received voltage. An alert customer now asks for a power supply that is voltage
regulated, balanced, flickers free, without harmonics and without any outages.
Of the many CP devices available, DSTATCOM can solve most of the customer’s
load related power quality problems. The concept of compensation has its
genesis to reactive power compensation, which initially was conceived with
fixed or passive capacitors. STATCOM systems are used in distribution and
transmission systems for different purposes. D-STATCOMs have faster response
when compared with transmission STATCOMs 11.

 

Literature Survey

The
concept of D-STATCOM was disclosed by Gyugyi 3 in 1976. Instead of directly
deriving reactive power from the energy-storage components, the D-STATCOM
basically circulates power with the connected network. The reactive components
used in the D-STATCOM, therefore, can be much smaller than those in the SVC.

A
vast literature (Ghosh and Ledwich, 2002; Mathur, 1984; Miller, 1982 4) has
appeared on electric power quality problems and many solutions have been
suggested to improve power quality. A number of compensators have been reported
for load balancing using lossless passive elements (L and C) and active
elements (solid state CSI and VSI). Simulation work on some FACTS devices has
been performed using PSCAD, EMTDC (Lara and Acha, 2002), MATLAB (Giroux et al.,
2001) 5, SABER (Muni et al., 2003), etc. Some experimental work (Clouston and
Gurney, 1999; Ekanayake et al., 1995; Jung et al., 2002 6; Muni et al., 2003)
is reported on prototype D-STATCOM models for line voltage regulation, voltage
flicker mitigation and power quality improvement.

Operation
and control of a D-STATCOM in the presence of non-integer harmonics is also
reported (Ghosh et al., 2003) 7. This paper attempts to highlight the behavior
of D-STATCOM connected to balance unbalanced linear as well as non-linear
loads.

Various
possible configuration of DSTATCOM (depending upon the particular application)
is also reported in literature bhimsingh , mahela et al ranges from 3 phase 3
wire or 3 phase 4 wire system, isolated or non-isolated VSC, with transformer
or without transformer connection to the grid, type of transformer used i.e
star/delta, zigzag, star/hexagon, T connected transformer etc. Many control
techniques are reported such as instantaneous reactive power theory (Akagi et
al., 1984), power balance theory, etc. an indirect current control technique
(Singh et al., 2000), hysteresis current controller (Bhim Singh et al ,1998) is
employed to obtain gating signals for the Insulated Gate Bipolar Transistor
(IGBT) devices used in current controlled voltage source inverter (CC-VSI)
working as a D-STATCOM. Some authors have considered biologically inspired
methods of tuning controllers, such as Safari et al., who considered using honey
bee mating optimization (HBMO) in order to ?nd an optimal controller gains and
compared results for controllers tuned using a genetic algorithm (GA).
Kumaravel and Kumar use bats echolocation algorithm to tune STATCOM PI
controllers. The authors note the importance of system impedance, particularly
the variable load impedance, on the controller gains. As with the GA and HBMO
controller tuning methods, the echolocation method requires evaluation of a
large number of possible controllers which is a computationally intensive
process. A paper by Wang presents a model of a power system with a STATCOM
installed based on the 8Phillips-Heffron model, originally proposed for
modeling of synchronous generators. Wang’s paper is of interest for three
reasons. First, the model presented shows that the STATCOM and power system
behavior are both non-linear and highly interdependent. Second, it can be seen
that the interactions between the STATCOM and power system are highly dependent
on the system operating point and that the modeling relies on linearization of
equations about an assumed operating point.

The
reviewed literature has revealed that there is no standard way in which to
determine the controller gains for a STATCOM. The high level of interdependency
between multiple system variables and the variability of performance at
different operating points makes the design of a generic STATCOM voltage
controller difficult. The use of particle swarm optimization and similar
methods has been proposed because of their usefulness in ?nding working
solutions from a large initial set of options; however, these methods are
computationally intensive and therefore may take a very long time to complete
and this may also make them unsuitable for use in an embedded system. C. Rose et.
al9 in his paper explores the impedance sensitive STATCOM control for systems
supported by renewable generation. There are many possible reasons for changes
in the system impedance. Impedance estimation has proved useful in identifying
such changes in system con?guration. The use of on-line impedance estimation in
order to tune the STATCOM controller has therefore been considered as an
alternative to using ?xed controller gains based on the limited information
provided by one-off measurements.

An
increasing the level of penetration of SPV (solar photo voltaic) systems in EPS
(electrical power system) applies additional amount of stress on utility
devices and systems arising complications such as difficulty in voltage
regulation, hampering the standard of power delivered, malfunction operation of
protective equipment, variation in reactive power transfer and posing threat to
security, safety and reliability of the grid 3. To integrate distributed
generation systems (such as wind, PV, etc) with reactive power compensation and
harmonics elimination capabilities to grid at PCC, various converter topologies
11-12 and advanced control algorithms 13 have been reported in the
literature. To attain an efficient SPV conversion system, sometimes additional
stages of converter are introduced before a converter for extracting maximum
power from a SPV system as well as to maintain required voltage level at DC
link of the converter. A comparative study is reported in between a single
stage and dual-stage converter topologies, which confirms that a single-stage
topology is better for a three-phase network in terms of reduced power losses,
cost, system size and efficiency. Moreover, Varma et. al. have demonstrated a
PV-STATCOM system which improves the transmission transferring capabilities
using STATCOM while transferring active power to grid from same converter.
Singh et. al. have demonstrated and implemented a SPV grid tied system with
DSTATCOM capabilities and advanced control schemes thereby improving power
quality issues in the distribution system. Mishra et. al. have implemented PV
fed DSTATCOM using optimized control algorithm. 

Modeling of controller is to be
done using MATLAB software and performing the simulations helps to achieve a
better understanding of the overall system. Much research confirms several
advantages of DSTATCOM compared to other custom power devices. These advantages
include Size, weight, and cost reduction. – Equality of lagging and leading
output. – Precise and continuous reactive power control with fast response. –
Possible active harmonic filter capability. Distribution Static Compensator
(DSTATCOM) has the ability to overcome the problem of limited bandwidth, higher
passive element count which causes increased size and losses, and slower response
of Static Var Compensators (SVC) and it is done by precise control and fast
response during transient and steady state, with lower foot print and weight.
The DSTATCOM has emerged as a promising device to provide solution for voltage
related issues and also serving a host of other current related power quality
problem’s solutions such as voltage regulation, load balancing, reactive power
compensation, power factor correction & improvement and current harmonic
control 2. In this paper, various topologies and different control techniques
of DSTATCOM is demonstrated for voltage regulation or power factor correction
by reactive power compensation along with harmonics elimination and load
balancing.

 

1 . CLASSIFICATION OF
DSTATCOM TOPOLOGY

The DSTATCOM can be classified on
the bases of different topologies, number of switching devices and on the bases
of neutral current compensation etc. These DSTATCOMs are developed to meet the
requirements of different applications in distribution system. Converter Based
Classification . DSTATCOM utilizes either a voltage-source inverter (VSI) or a
current-source inverter (CSI). Voltage source inverter use capacitive energy
storage, while Current source inverter use inductive energy storage in their
respective dc links for voltage and current. However, the voltage source
inverters are broadly used because of the less heat dissipated, smaller size,
and the less cost of the capacitor compared to the inductor, used in the CSI,
for the same power rating 4. The VSI connected in shunt with the AC system
provides multifunctional topology which can be used for different aims such as
voltage regulation and compensation of reactive power, correction of power
factor, and elimination of current harmonics. Voltage source inverter (VSI)
topology is popular because it can be expandable to multilevel, multistep &
multi chain topology to enhance the performance with lower switching frequency
and increased power handling capacity. Various multilevel topologies are Diode
clamp multilevel inverter, Cascaded H-bridge & Flying capacitor multilevel
inverter.                                                                

1.1 Cascade H -bridge
multilevel inverter: In Cascaded H-bridge inverters
separate dc sources (SDC’s) are introduced. This new converter can avoid extra
clamping diodes or voltage balancing capacitors. Fig1 Shows the basic
arrangement of the 5-level cascaded-inverters with SDC’s, shown in a single
phase configuration. Each SDC is related with a single phase full-bridge
inverter. To synthesize a multilevel waveform, the ac output of each of the
different level H- bridge cells is connected in series. The number of output
phase voltage levels in a cascaded inverter is defined by m = 2H +1, where H =
no. of H-bridges; while the relation between phase voltage and line voltages is
same as diode –clamped inverter.

 

                                               

                                                                Fig
1

 

1.2. Flying capacitor
multilevel inverter: A quite well-known topology of
multilevel inverter is Flying Capacitor Multilevel Inverter. It is quite
similar to diode clamped multilevel inverter. The capacitor has to be pre-charged
in this type of multilevel inverter. The topology consists of diodes,
capacitors and switching devices as shown in fig. This has been designed only
up to six levels of voltage because of the practical restrictions. Each leg
consists of switching devices which are in general transistors. Every inverter
limb consists of cells connected in inward nested series. Every cell has two
power switches and a single capacitor. Power switch is a combination of a
transistor connected with an anti-parallel diode. Unlike diode clamped
inverter, this topology uses capacitors for clamping. An inverter with N cell
will have 2N switches and N+1 different voltage levels including zero. We can
also have negative voltage levels, and so all in all we can say that N cell
multilevel inverter can give 2N+1 voltage levels. The voltage level is
decreased as we move towards the load. The number of level depends upon the
number of conducting switches in each limb. It is also known as Imprecated Cell
Inverter. Since the capacitors floats with respect to earth’s potential, they
are called Flying Capacitor

                                               

                                                                Fig
2

 

1.3. Three phase four
leg VSI topology : 
VSI with four legs that are used and requires only one dc storage unit.
Three of its legs are used for phase connection while the fourth leg is
connected to the load neutral and the supply neutral, if available, through a
reactance 6-8. The reference current for the fourth leg is the negative sum
of three phase load currents. This nullifies the effect of dc component of load
current. To maintain the adequate charge on dc-side capacitor a PI regulator is
used to control the flow of real power from ac side towards dc side of the
converter. When the compensator is working, zero sequence current is routed to
path n-n’ containing switching frequency harmonics. Using fourth leg of
inverter, the negative of zero sequence current – iois is tracked. Certainly it
needs a higher bandwidth VSI to track negative of neutral current (- io) as io
contains harmonics due to non-linear loads. This increases the switching
losses. If this current is not tracked properly, it will leave high switching
frequency current components in the N-n path, which is not desirable. The
advantage of the topology is that it requires one less capacitor.

 

 

1.4. Neutral-Clamped
Three-Phase VSI topology: This topology consist of a chopper
circuit which is represented by the switches Schl and Sch2 , a diode Dch1 and
Dch2 in parallel and inductance and resistance which are denoted by Lp and Rp.
The purpose of this chopper circuit is to balance the voltages in the two
capacitors as shown in figure3(c). Let the voltage across Cdc1 be Vdc1 and
voltage across Cdc2 is Vdc2. Normally the two switches are left open and thus
two voltages Vdc1 and Vdc2 are equal. Now suppose there is a voltage drop in
Vdcl due to this there is rise of voltage in V dc2. Current is built up in the
inductor Lp due to closing of switch Sch2 and once the current reaches a
definite level, the switch Sch2 is opened, hence the inductor current get discharged
through the diode Dchl to bring up the voltage Vdcl to the desired level.

 

2 . CLASSIFICATION OF
DSTATCOM CONTROL STRATEGY

 The reactive power needed by the load is
provided by the DSTATCOM and only real power is supplied by the source such
that source current remains at unity PF. Load balancing is achieved by making
reference source current balanced. It has real fundamental frequency component
of the load current and used to decide switching of the VSC and being extracted
by control techniques. Different control strategies are reported in the
literature such as IRP theory, SRF theory, Adaline-based control algorithm, PI
controller for maintaining dc bus voltage.

 

 

2.1  Synchronous reference frame (SRF) based
control strategy :

SRF control technique is based on
transformation of current in synchronously rotating d-q frame. The voltage
signal is sensed and processed by phased lock loop (PLL) to generate sine and
cosine signals as shown in figure 9. The sensed current signal then are
transformed to d-q frame and filtered. After filtering the filtered currents
are back transformed to abc frame and fed to hysteresis current
controller for switching plus generation. The mathematical transformation
equations. The currents generated in ?–? coordinates are transformed tod-q
frame with the help of park’s transformation using ? as transformation angle

 

2.2
Instantaneous p–q Theory :

The control of DSTATCOM is
implemented on the basis of instantaneous reactive power theory (IRPT) or p-q
theory to calculate the desired compensation current. The block diagram for the
control using IRPT is shown in Figure-10. In this method, the sensed
three-phase PCC voltages and load currents are transformed into ?-?-o axis
using Clark’s transformation. In addition, the source must deliver no
zero-sequence active power (so that the zero-sequence component of the voltage
at the PCC does not contribute to the source power). The reference source
currents in the reference ?-?-o frame are converted to the abc frame
using the reverse Clark’s transformation

 

2.3  Neural Network Based Control System: In this control algorithm, there is
requirement of unit vector template corresponding to fundamental positive
sequence component of current in phase with the phase voltage waveform. For
proper estimation of components of load current, a undistorted unit voltage
templates can be represented by:

Vp (t)
=Usin?t (4)

To generate
sinusoid (sincot) vector template, synchronized with ac mains, the zero
crossing of phase voltage is detected in case of voltage being distorted.

ip(t) =
Wpvp(t) (5)

where weight
(Wp) is estimated using Adaline. The weight can be represented in terms of
voltage and current given as:

Wp=I1cos?/U
(6)

To maintain
minimum error, the scheme for estimating weights corresponding to fundamental
frequency real component of current (for three-phase system), based on LMS
algorithm tuned Adaline tracks the unit vector templates.

 

RESULTS:

                                Table
1: Comparison of different control techniques

 

                               

 

CONCLUSIONS AND FUTURE
WORK:
The DSTATCOM is very effective for improvement of power quality (PQ) (Molinas, Suul and Undeland, Low voltage ride
through of wind farms with cage generators: STATCOM versus SVC 2008) problems related to
both current and voltage such as load balancing, Harmonics elimination, power
factor correction, voltage regulation and neutral current compensation in
distribution system. This paper presents a detailed survey on various
topologies and control strategies of DSTATCOM used in both 3phase 3 wire and
3phase 4 wire distribution systems. Comparative studies of various control
schemes are also presented and from the Table-1 we analyzed that IRP is much
complex then the other schemes but excellent in harmonics mitigation. For the
load balancing, IRP and NN control schemes are preferred and for reactive power
compensation, SMC control scheme can be implemented. This comparative study
will helps the users in selecting the particular topology and control technique
of DSTATCOM that suits for specific application. Currently research is going on
to reduce the cost of DSTATCOM without affecting the efficiency and
effectiveness in PQ improvement capability.. The reviewed literature has
revealed that there is no standard way in which to determine the controller
gains for a STATCOM. The high level of interdependency between multiple system
variables and the variability of performance at different operating points
makes the design of a generic STATCOM voltage controller difficult. This
research work mainly focuses on the SPV generation systems connected in the
distribution system. Multifunctional PV-DSTATCOM system is proposed in this
which are classified depending on their connection to AC distribution system
(single-phase or three-phase) and number of power conversion stages
(single-stage or two-stage). Renewable energy (RE) penetration into the
electrical utility grid is increasing day by day and affects the quality of
supplied power. The weather conditions such as variable solar insolation and
wind speed variations affect the power output of RE sources. The implementation
of DSTATCOM in RE based power system is required to be explored as the DSTATCOM
may be an effective solution for these problems