development of simulation model of the...
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Chapter 4 Development of Simulation Model of the proposed SHAF for 100
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
CHAPTER 4
DEVELOPMENT OF SIMULATION MODEL OF THE
PROPOSED SHAF FOR HARMONIC MITIGATION IN
LV POWER DISTRIBUTION SYSTEM
4.1 INTRODUCTION
This chapter is dedicated to the development of MATLAB/Simulink
simulation model of the proposed three-phase shunt hybrid active power filter
(SHAPF) for harmonic mitigation in low voltage power distribution system. To begin
with, a low voltage test system model is developed using MATLAB/Simulink
environment, followed by the development of low voltage test system with proposed
shunt hybrid active filter compensation for harmonic mitigation. The development
details of all the blocks of the overall model are discussed part by part, starting with
voltage source inverter, VSI control strategy and tuned passive filter (TPF). In
addition, the development of basic shunt APF simulation model is elaborated. It is to
become the benchmark comparison for the proposed shunt hybrid APF topology.
4.2. MATLAB/SIMULINK SIMULATION MODEL OF LV TEST
SYSTEM WITHOUT ANY COMPENSATION
A low voltage test system with a three phase AC source connected to a three
phase nonlinear load through a distribution line is developed using
MATLAB/Simulink which is shown in Fig. 4.1. The development details of the
blocks of the LV test system are discussed in the following sections.
Chapter 4 Development of Simulation Model of the proposed SHAF for 101
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
Fig. 4.1 Simulation model of LV test system.
4.2.1 Three Phase Distribution Source Model
The power distribution source developed in the simulation model is a 3-phase,
2000 V (rms), 50 Hz sinusoidal AC voltage source. It is developed using three single
phase „AC source‟ blocks from “SimPowerSystems/Electrical Source” library and
connected in star configuration as shown in Fig. 4.2. An inductance (Ls) is connected
in series with each phase to limit the inrush current and to represent combined
inductance of source and distribution line per phase. The Ls is constructed using
“Series RLC Branch” block set.
Chapter 4 Development of Simulation Model of the proposed SHAF for 102
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
Fig. 4.2 Simulink model of “Three phase distribution source”
The selected value of Ls is 15 mH/phase. The current and voltage signals are
sensed using “Current Measurement” and “Voltage Measurement” block sets
respectively from “SimPowerSystems/ Measurements” library. Three phase AC
source is connected to a three phase diode bridge rectifier load through a distribution
line.
4.2.2 Three Phase Nonlinear Load model
Fig. 4.3 Simulink model of three phase nonlinear load block.
Chapter 4 Development of Simulation Model of the proposed SHAF for 103
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
The simulink model details of the “Three phase nonlinear load” block are
shown in Fig. 4.3. It consists of a three-phase full-bridge diode rectifier constructed
using „Diode‟ blocks from Simulink library. A R-L load (R=20 ohm and L=0.1 mH)
is connected on DC side of the rectifier using „series RLC element‟ from
Simulink/elements library. This nonlinear load can produce harmonic current as most
of the power electronic equipment. Hence it is considered as nonlinear load in this
thesis.
4.3 MATLAB/SIMULINK MODEL OF LV TEST SYSTEM WITH
PROPOSED SHAF COMPENSATION
The complete MATLAB/Simulink simulation model of the LV test system
with the proposed shunt hybrid active filter compensation is depicted in Fig. 4.4. The
model consists of a low voltage test system connected to the „voltage source inverter
(VSI) block‟, „overall control system for VSI‟ block and „tuned passive filters‟ block
connected in parallel with the load. The development of simulation models of all the
blocks are explained in detail in the following sections.
Chapter 4 Development of Simulation Model of the proposed SHAF for 104
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
Fig. 4.4 Simulation model of LV test system with proposed SHAF.
Chapter 4 Development of Simulation Model of the proposed SHAF for 105
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
4.3.1 Development of VSI model with Interfacing Elements and DC
Bus Capacitor
The details of “Voltage Source Inverter ” block is illustrated in Fig. 4.5. The
VSI block consists of an “Universal Bridge” block set in which the MOSFETs with
anti-parallel diodes are configured in full bridge, interfacing inductor (Lf) and
capacitor (Cf) and a DC-bus capacitor (Cdc) are connected as shown in Fig. 4.5. The
design expression described in Chapter 3, Eqn.(3.8) is used to calculate the value of
Lf.
Fig. 4.5 Simulink model of “VSI” block.
The DC-bus reference voltage (Vdc) taken in the simulation is set to 4700V,
which is approximately one and a half times higher than the amplitude of source
voltage. The maximum switching frequency of the switching ripple (fsw,max) and peak-
to-peak switching ripple(∆Isw,p−p) of the compensation current are selected to be 2kHz
and 400A respectively.
Lf,min =Vdc
2. ΔIsw ,p−p . fsw ,max
Chapter 4 Development of Simulation Model of the proposed SHAF for 106
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
=4700
2 ∗ 400 ∗ 2000= 2.93𝑚𝐻
Therefore, L f is chosen as 3 mH.
The DC-bus capacitor value is calculated using Eqn.(3.10), Chapter 3. The DC
bus capacitor design parameters are given by
Vs = 2000 V(rms), ΔIL= 280 A, T= 20 ms, Vdc,ref = 4700V
where Vs is the r.m.s value of the source voltage, ΔIL is the peak r.m.s value of the
reactive and harmonic load currents and T is the period of source voltage and ΔVDC is
the maximum or minimum DC-bus voltage.
Cdc ≥Vs .ΔIL . T
Vdcmax 2 − (Vdc ,ref )2
≥ 2 ∗ 2000 ∗ 260 ∗ 0.02
4794 2 − (4760)2
≥ 4527.6 µF
Therefore, the selected value for Cdc is 4600µ F.
4.3.2 Simulink Model of Overall Control System of SAF
Fig. 4.6 Details of “Overall Control System of SAF” block.
Chapter 4 Development of Simulation Model of the proposed SHAF for 107
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
The “Overall Control System of SAF” block of the proposed scheme is
presented in Fig. 4.6. The task of the control system is to produce appropriate gating
signals for the switching transistors (MOSFETs) of SAF. It consists of three blocks
namely d-q-0 theory based “Compensation Current Reference Estimator”, “Hysteresis
Current Controller” based gating signal generator and fuzzy logic based “DC-bus
voltage controller”.
Simulink Model of Compensation Current Reference
Estimator Using d-q-0 Theory
In this thesis synchronous reference frame theory is employed to obtain
compensation current reference signal, since it deals mainly with DC quantities and
computation is instantaneous. The component diagram of synchronous reference
frame theory model is depicted in Fig. 4.7.
Fig. 4.7 Simulink model of d-q-0 theory based reference current estimator.
This control strategy uses „discrete PLL‟ block for generating sinusoidal
reference currents, „a-b-c to d-q-0 transformation‟ block for Park‟s transformation,
Chapter 4 Development of Simulation Model of the proposed SHAF for 108
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
and d-q-0 to a-b-c transformation block for Inverse Park‟s transformation. Two
„second order digital low pass filter‟ blocks are used for extracting fundamental
component from load currents. From simulink block set “Discrete Phase-Lock Loop”
block is selected from “SimPowerSystems\Discrete Control Blocks” library and its
parameters are set as: Frequency = 50 Hz, Phase angle=0, Sampling frequency=
20 kHz. The “a-b-c to d-q transformation block” converts three phase a-b-c-reference
frame currents in to stationary d-q reference frame currents and applied to a “Discrete
2nd-Order LPF” block from “SimPowerSystems/Discrete Control Blocks” library for
noise filtering.
For the simulation model of Butterworth LPF, the design parameters are
selected as ζ = 0.707, fLPF= 75 Hz, fs= 40 kHz, where ζ is the damping ratio, fLPF is the
cut-off frequency and fs is the sampling frequency of the digital Butterworth LPF. The
low frequency fundamental components obtained from LPF are subtracted from non-
filtered signal and added to current signal obtained from DC bus voltage controller to
obtain compensation reference currents in d-q reference frame. By applying these
currents to “d-q-0 to a-b-c transformation block” the compensation reference currents
in a-b-c reference frame are obtained.
Simulink Model of Hysteresis Current Controlled Switching
Signal Generator
In this thesis Hysteresis band Current Controller model is used for generating
switching signals for the transistors of VSI, and is illustrated in Fig. 4.8.This current
control technique imposes a bang-bang type instantaneous control that forces the
compensation current to follow its estimated reference. The actual compensation
current is subtracted from its estimated reference. The resulting error is sent through a
Chapter 4 Development of Simulation Model of the proposed SHAF for 109
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
hysteresis controller to determine the appropriate gating signals. In the simulation
model, the hysteresis band ( H ) is chosen as 0.1 A with 0.05 as upper limit and -0.05
as lower limit. The hysteresis controller is constructed using “Relay” block set from
“Simulink\Discontinuities” library as shown in Fig. 4.8.
Fig. 4.8 Simulink model of Hysteresis current controller for phase-a.
Simulink Model of Fuzzy Logic based DC-Bus Voltage
Controller
For maintaining DC bus voltage constant at a reference value, Fuzzy Logic
Controller (FLC) is employed in this simulation work. The details of the “Fuzzy logic
controller” block is shown in Fig. 4.9.
Fig. 4.9 Simulink model of Fuzzy logic controller.
Chapter 4 Development of Simulation Model of the proposed SHAF for 110
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
The DC-bus voltage is first sensed and compared with DC reference voltage
and error signal is generated. The error signal and its derivative are applied to fuzzy
logic controller. Error signal is applied to “Memory” block and its output is subtracted
from the error signal to obtain derivative of error signal as shown in the Fig. 4.9.
The two inputs and the output use seven triangular membership functions
namely Negative Big (NB), Negative Medium(NM), Negative Small(NS), Zero(ZE),
Positive Small (PS), Positive Medium(PM), Positive Big(PB). The type and number
of membership functions (MFs) decides the computational efficiency of a FLC. The
shape of fuzzy set affects how well a fuzzy system of If–then rules approximate a
function. Triangles have been the most popular for approximating non-linear function
because the parametric functional description of triangular membership function is
most economic one. Also these are preferred because of their striking simplicity, solid
theoretical basis and ease of computation, since they are symmetrical and have zero
value at some point away from their center. Hence the triangular MFs are chosen in
this work.In this controller seven membership functions are considered which will
give precisely accurate results. Reducing the number of MFs will produce improper
results at some band, while increasing the number of MFs will produce a delay due to
more computational steps required. The linguistic variables are defined by M = (a, b,
c), where a, b, c are starting, middle point with unity membership grade, and end
points, respectively.
The membership values of input and output variables are shown in the
Fig. 4.10. Each input has seven linguistic variables, therefore there are 49 input label
pairs. A rule table relating each one of 49 input label pairs to respective output label
is given in Table 4.1. The type of fuzzy inference engine used is mamdani and the
Chapter 4 Development of Simulation Model of the proposed SHAF for 111
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
centroid method is used for defuzzification.
Table.4.1 Fuzzy rule representation
4.10 (a)
e
de NB NM NS ZE PS PM PB
NB NB NB NB NB NM NS ZE
NM NB NM NM NM NS ZE PS
NS NB NM NS NS ZE PS PM
ZE NB NM NS ZE PS PM PB
PS NM NS ZE PS PS PM PB
PM NS ZE PS PM PM PM PB
PB ZE PS PM PB PB PB PB
Chapter 4 Development of Simulation Model of the proposed SHAF for 112
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
4.10(b)
4.10(c)
Fig. 4.10 The degree of membership functions for (a) The error (b) The derivative of
error and (c) The output.
4.3.3 Simulink Model of Tuned Passive Filter
The details of Simulink model of “Tuned Passive Filter”(TPF) is depicted in
Fig. 4.11. The TPF consists two parallel connected single tuned passive filter
branches tuned to absorb 5th
and 7th
harmonic currents of the load current. The 5th
order filter consists a series R-L-C branch with a capacitor (C5), inductance (L5) and
resistance (R5) and 7th
order filter consists R7, L7 and C7 in series as shown in
Fig.4.11. The TPF is connected in parallel with the load.
Chapter 4 Development of Simulation Model of the proposed SHAF for 113
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
Fig. 4.11 Simulink model of tuned passive filter block.
The design procedure of the TPF is described in Chapter 3, Eqn.(3.18). For 5th
harmonic filter the resonance frequency is 250Hz and for 7th
harmonic it is 350Hz.
Quality factor of the filter is selected as 75[4] and the filter capacitance value is fixed
at 30µF. The calculated values of the TPF parameters are R5=0.2829Ω, L5=
13.504 mH, C5 =30µF, R7= 0.2021Ω, L7= 6.892 mH, C7 =30µF.
4.4 MATLAB/SIMULINK MODEL OF LV TEST SYSTEM WITH
BASIC SAF COMPENSATION
A basic shunt APF simulation model constructed under MATLAB/Simulink
environment is illustrated in Fig. 4.12. It is used as a benchmark to investigate the
improvement in harmonic mitigation by the proposed Shunt hybrid APF. The SAF
consists of distribution source, nonlinear load, VSI with DC side capacitor and overall
control system. The simulation model of the basic SAF is similar to the model of the
proposed SHAF topology presented in Fig. 4.4, except for the removal of “TPF”
Chapter 4 Development of Simulation Model of the proposed SHAF for 114
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
block. Therefore, the descriptions given in Section 4.3 are also applicable for the basic
shunt APF except the simulation model of TPF. The basic SAF is configured to
generate compensation current equal to the reactive and harmonic load current.
Fig. 4.12 Complete simulation model of the basic SAPF.
Chapter 4 Development of Simulation Model of the proposed SHAF for 115
Harmonic Mitigation in LV Power Distribution System
Ph.D Thesis submitted to Jawaharlal Nehru Technological University Anantapur, Anantapur.
4.5 CONCLUSION
The complete MATLAB/Simulink simulation model of the proposed shunt
hybrid active filter is presented. The model is discussed part by part, starting with the
development of distribution source, nonlinear load, VSI, tuned passive filter (TPF)
until the development of the overall control system. Furthermore, a basic shunt APF
simulation model is developed as a benchmark. The simulation results of test system
with SAF and with SHAF compensations are analyzed and compared in Chapter 6,
section 6.2.