OPTIMAL PLACEMENT OF DSTATCOM IN AN INDIAN POWER SYSTEM FOR LOAD AND

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OPTIMAL PLACEMENT OF DSTATCOM IN AN INDIAN POWER SYSTEM FOR LOAD AND Powered By Docstoc
					 International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
INTERNATIONAL JOURNAL OF ELECTRICAL ENGINEERING
 6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME
                           & TECHNOLOGY (IJEET)

ISSN 0976 – 6545(Print)
ISSN 0976 – 6553(Online)                                                     IJEET
Volume 4, Issue 3, May - June (2013), pp. 63-74
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     OPTIMAL PLACEMENT OF DSTATCOM IN AN INDIAN POWER
          SYSTEM FOR LOAD AND VOLTAGE BALANCING

                     D.K. Tanti1, M.K. Verma2, Brijesh Singh3, O.N. Mehrotra4
                    1,4
                      Department of Electrical Engineering, B.I.T., Sindri (INDIA)
            2,3
                  Department of Electrical Engineering, I.I.T.( BHU), Varanasi (INDIA)



  ABSTRACT

          The present paper deals with the problem of unbalanced voltages arising due to
  unbalanced loads in an electrical power system network. In this paper, placement of
  Distribution Static Compensator (DSTATCOM) in an Indian power system network has been
  considered to balance load voltages and currents against switching of unbalanced loads.
  Impact of DSTATCOM has also been observed in balancing voltage at all other buses which
  get affected due to connection of unbalanced load in the system. A feed forward neural
  network with back propagation algorithm has been trained with unbalanced bus voltages with
  targets defined as balanced bus voltages prior to connection of unbalanced load in the system.
  The DSTATCOM has been placed at the bus having maximum squared deviation of three
  phase unbalanced bus voltage from its target value. Simulations have been carried out in
  standard MATLAB environment using SIMULINK and power system block-set toolboxes.
  Simulation results establish effectiveness of DSTATCOM placement in load and voltage
  balancing in the Indian power system considered.

  Keywords: Power quality, Load balancing, Voltage balancing, DSTATCOM, Optimal
  placement, ANN

  1. INTRODUCTION

          Power quality is of increasing importance in worldwide distribution. The present
  distribution systems are facing severe power quality problems such as poor voltage
  regulation, high reactive power demand, harmonics in supply voltage and current, and load
  unbalancing [1]. Therefore, maintenance of power quality is becoming of increasing

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importance in worldwide distribution systems. Industrial consumers with more automated
processes require high quality power supply else equipments such as microcontrollers,
computers and motor drives may get damaged. High quality power delivery includes
balanced voltage supply to consumers. Connection of unbalanced load at a bus may cause
unbalanced voltage and current drawn by other loads connected at that bus. Switching of
unbalanced load at a bus may also result in unbalanced voltage at some other buses.
Unbalanced voltages contain negative and zero sequence components which may cause
additional losses in motors and generators, oscillating torques in Alternating Current (AC)
machines, increased ripples in rectifiers, saturation of transformers, excessive neutral currents
and malfunctioning of several type of equipments. With the advancement in power
electronics, new controllers known as Flexible AC Transmission System (FACTS) have been
developed [2]. These controllers have been proved to be quite effective in power flow
control, reactive power compensation and enhancement of stability margin in AC networks
[3].
        Power electronics based controllers used in distribution systems are called custom
power devices. Custom power devices have been proved to be quite effective in power
quality enhancement [1]. The custom power devices may be series, shunt, and series-shunt or
series-series type depending upon their connection in the circuit. Most prominent custom
power devices include Distribution Static Compensator (DSTATCOM), Dynamic Voltage
Restorer (DVR) and Unified Power Quality Conditioner (UPQC) [1]. There are several
papers reported in literature on placement of custom power devices in balancing of
unbalanced load in radial distribution systems. Load voltage balancing using DVR against
unbalanced supply voltage in radial distribution system has been considered [4], [5].
Placement of DSTATCOM in weak AC radial distribution system for load voltage and
current balancing has been considered in [6]. Balancing of source currents using
DSTATCOM in radial distribution system has been considered in [7]. In [7], unbalancing has
been caused by connection of unbalanced and non-linear load. Load compensation using
DSTATCOM against unbalancing caused by opening of one of the phase of the load in radial
distribution system has been considered in [8]. Balancing of supply across an unbalanced 4-
phase load along with power factor improvement using DSTATCOM has been suggested in
[9]. A Voltage Source Converter (VSC) based controller has been proposed in [10] to balance
terminal voltage of an isolated standalone asynchronous generator driven by constant speed
prime mover. A non-linear and unbalanced load has been connected at the generator
terminals in [10] to create unbalance in supply voltages. The paper [11] uses three phase four
wire four leg VSC topology for a DSTATCOM application. The four leg inverter is operated
in a current controlled mode by a suitable control strategy to inject compensator currents in
order to achieve harmonic compensation, load balancing and power factor correction. The
control of DSTATCOM for reactive power, harmonics and unbalanced load current
compensation has been presented [12] for a diesel generator set for an isolated system. The
paper [13] proposes a method to use SVCs (Static VAR Compensators) with four wire three
phase loads for load balancing and reactive power compensation. A DVR/APF (Active Power
Filter) based on Proportional Resonant (PR) controller has been proposed in [14] to protect
sensitive industrial loads at the point of common coupling, against voltage harmonics,
imbalances and sags.
        Most of the work on placement of custom power devices in load balancing has
concentrated on radial distribution systems. Very limited attempt seems to be made in load
balancing in interconnected power systems using custom power devices. Unbalanced load

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connected at a particular bus may cause voltage unbalances at several other buses in an
interconnected power system network. An Artificial Neural Network (ANN) based approach
has been applied for optimal placement of custom power devices in IEEE 14- bus system
considering it as an interconnected distribution system, for balancing bus voltages at all the
buses caused by unbalanced load connected at a particular bus [15]. However, IEEE 14-bus
system may be considered as a small and well behaved system. The methodology suitable for
optimal placement of custom power devices for this system may not be suitable for a large
and practical system. In this paper, Artificial Neural Network based approach suggested in
[15] has been considered for optimal placement of DSTATCOM to balance unbalanced
voltages in a practical 75-bus Indian system representing earlier Uttar Pradesh and
Uttarakhand Power Corporation Network. The ANN has been trained with Levenberg
Marquardth back-propagation algorithm (trainlm).

2. DSTATCOM MODEL

        In the present work, DSTATCOM has been represented as three independently
controllable single phase current sources injecting reactive current in the three phases at the
point of coupling. The proposed DSTATCOM model has been shown in Figure-1. The
control scheme consists of three control switches which can be set on/off as per compensation
requirement.




                          Figure-1. Proposed DSTATCOM model

3. METHODOLOGY

       The simulation model of the power system network under study is developed using
MATLAB/SIMULINK software [16]. The developed plant model was used to find three
phase balanced bus voltages prior to switching of unbalanced load, unbalanced three phase
voltage and current at the bus where unbalanced load is switched on, and unbalanced three

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phase voltages at other buses in the system. The voltage data base so prepared has been used
for training of ANN for finding the optimal location of DSTATCOM. A feed forward
Artificial Neural Network with back propagation algorithm has been used. The neural
network has been trained to give a desired pattern at the output, when the corresponding
input data set is applied. The training process is carried out with a large number of input and
output target data. The system has been made unbalanced by connection of highly unbalanced
load at different load buses. The three phase balanced per unit (p.u.) voltages of buses prior to
connection of unbalanced load, have been taken as output target data. The three phase p.u.
voltages of buses under unbalanced loading conditions have been considered as input data to
train the neural network. Once the network is trained, some data are used to test the network.
The testing results provide information about the optimal location for the placement of
DSTATCOM controller. Mean Square Error (MSE) has been computed for all the buses. The
load bus corresponding to highest mean Mean Square Error value has been selected as the
optimal bus for the placement of DSTATCOM controller.

4. CASE STUDIES

        Case studies were performed on a practical 75-bus Indian system [17]. The 75 bus
Indian system is a reduced representation of earlier Uttar Pradesh and Uttarakhand Power
Corporation Network. It consists of 75 buses including 15 synchronous generators and 97
transmission lines. There are 42 load buses in the system having a net real and reactive power
demand of 6573.5 MW and 1002.37 MVAR, respectively. The single-line-diagram of the
system has been shown in Figure-2. Simulation model of 75-bus system Indian system was
developed using software package MATLAB/SIMULINK [16]. The simulation block
diagram of the system has been shown in Figure-3. The developed plant model shown in
Figure-3 was used to find three phase balanced bus voltages prior to switching of unbalanced
load, unbalanced three phase voltage and current at the bus where unbalanced load is
switched on, and unbalanced three phase voltages at other buses in the system. In order to
create unbalance loading condition, an additional Y- connected highly unbalanced load ;
Phase A [P=1MW, Q=100MVAR] , Phase B [ P=25KW, Q=200KVAR] , Phase C [ P=1KW,
Q=0.1KVAR] was connected at each bus considered at a time, with all other buses having
balanced base case loadings. A feed forward neural network was trained with three phase
unbalanced bus voltages. The balanced three phase voltages of different buses prior to
connection of unbalanced load at a bus were considered as target data for the neural network.
The Mean Square Errors (MSE) were calculated for all the load buses using training data and
target data. The MSE of all the buses have been shown in Figure-4. It is observed from
Figure-4 that bus-47 has maximum MSE value. Therefore, bus-47 was selected as the optimal
location for the placement of DSTATCOM controller.
        Three phase voltage at all the buses and three phase current at the bus with
unbalanced load were found with DSTATCOM placed at bus-47 for all the unbalanced
loading cases. The variation of three phase voltage with respect to time for all the buses and
variation of three phase current with respect to time at the bus with unbalanced load were
plotted using MATLAB software [16]. Three phase voltage and current at bus-30 with
unbalanced load connected at bus-30 have been shown in Figure-5. Three phase voltage at
buses 16, 49, 62 and 74 with unbalanced load connected at bus-30 have been shown in
Figure-6. Three phase voltage and current at bus-39 with unbalanced load connected at bus-
39 have been shown in Figure-7. Three phase voltage at buses 28, 57, 65 and 73 with

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unbalanced load connected at bus-39 have been shown in Figure-8. Three phase voltage at
buses 20, 32, 52 and 69 with unbalanced load connected at bus-47 have been shown in
Figure-9. Three phase voltage and current at bus-47 with unbalanced load connected at bus-
47 have been shown in Figure-10. Three phase voltage and current at bus-50 with unbalanced
load connected at bus-50 have been shown in Figure-11. Three phase voltage at buses 25, 34,
53 and 64 with unbalanced load connected at bus-50 have been shown in Figure-12. Three
phase voltage and current at bus-54 with unbalanced load connected at bus-54 have been
shown in Figure-13. Three phase voltage at buses 24, 42, 60 and 66 with unbalanced load
connected at bus-54 have been shown in Figure-14. It is observed from figures 5, 7, 10, 11
and 13 that placement of DSTATCOM at bus-47 results in considerable balancing of load
voltage and current at the bus with unbalanced load. It is observed from figures 6, 8, 9, 12
and 14 that placement of DSTATCOM at bus-47 is also able to produce considerable voltage
balancing at other buses.




                  Figure-2. Single line diagram of the 75-bus Indian system




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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
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              Figure-3. 75-bus Indian system (MATLAB/SIMULINK) model




        Figure-4. Mean Square Error for different load buses (75-bus Indian System)


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Bu               Without DSTATCOM                            With DSTATCOM at Bus 47
 s
No.
30




30



Figure-5. Three phase voltage and current at bus-30 with unbalanced load connected at        bus-30


Bus                  Without DSTATCOM                          With DSTATCOM at Bus 47
No.
16



49



62




74



 Figure-6. Three phase voltage at buses 16, 49, 62 and 74 with unbalanced load connected at bus-30


 Bus                 Without DSTATCOM                          With DSTATCOM at Bus 47
 No.
 39



  39



      Figure-7. Three phase voltage and current at bus-39 with unbalanced load connected at bus-39


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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
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Bus               Without DSTATCOM                       With DSTATCOM at Bus 47
No.
  28




  57




  65




  73




  Figure-8. Three phase voltage at buses 28, 57, 65 and 73 with unbalanced load connected
                                        at bus-39


 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 no.
 20




  32




  52




  69




       Figure-9. Three phase voltage at buses 20, 32, 52 and 69 with unbalanced load at
                                   connected at bus-47

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 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 No.
 47



  47



   Figure-10. Three phase voltage and current at bus-47 with unbalanced load connected at
                                         bus-47

 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 no.
 50



  50




   Figure-11. Three phase voltage and current at bus-50 with unbalanced load connected at
                                         bus-50

 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 no.
 25



  34



  53


  64


       Figure-12. Three phase voltage at buses 25, 34, 53 and 64 with unbalanced load at
                                    connected at bus-50


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 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 no.
 54



  54



   Figure-13. Three phase voltage and current at bus-54 with unbalanced load connected at
                                         bus-54

 Bus              Without DSTATCOM                       With DSTATCOM at Bus 47
 no.
 24


  42


  60




  66




       Figure-14. Three phase voltage at buses 24, 42, 60 and 66 with unbalanced load at
                                    connected at bus-54


5. CONCLUSION

        Custom power devices have shown to be quite effective in power quality
enhancement. However, due to high cost and for most effective utilization, these controllers
are to be placed optimally in the system. In the present work, optimal placement of
DSTATCOM has been considered in a practical Indian power system based on ANN
methodology to balance voltages and currents caused by switching of unbalanced loads.
Simulation results show that DSTATCOM is capable of enhancing not only voltage and
current unbalances at unbalanced load location, but also voltages at other locations.



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International Journal of Electrical Engineering and Technology (IJEET), ISSN 0976 –
6545(Print), ISSN 0976 – 6553(Online) Volume 4, Issue 3, May - June (2013), © IAEME

 [15] D. K. Tanti, M.K.Verma, Brijesh Singh and O.N.Mehrotra, “Optimal Placement of
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