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National University of Computer & Emerging Sciences
Department of Electrical Engineering
Feasibility of a Variable Frequency Based
Solar Mini Grid in Rural Areas
Hafiz Waqas Moazzam
Masters of Electrical Engineering
2018
Certificate of Approval
It is certified that the research work presented in this thesis, entitled “Feasibility of a
Variable Frequency Based Solar Mini Grid” was conducted by Hafiz Waqas Moazzam under
the supervision of Dr. Syed Aun Abbas.
No part of this thesis has been submitted anywhere else for any other degree.
This thesis is submitted to the Department of Electrical Engineering in partial fulfillment of the
requirements for the degree of Master of Science in Electrical Engineering
at the
National University of Computer & Emerging Sciences
Islamabad, PAKISTAN
Aug 2018
Candidate Name: Hafiz Waqas Moazzam Signature: ______________________
Evaluation Committee:
Dr. Syed Aun Abbas (supervisor)
Professor, National University of Computer & Emerging Sciences
Professor, National University of Computer & Emerging Sciences
Assistant Professor, National University of Computer & Emerging Sciences
Author’s Declaration
I, Hafiz Waqas Moazzam, Roll No. 16L-5120, hereby declare that I am the sole author of this
thesis. To the best of my knowledge this thesis contains no material previously published by
any other person except where due acknowledgement has been made. This thesis contains no
material which has been accepted as part of the requirements of any other academic degree or
non-degree program, in English or in any other language.
This is a true copy of the thesis, including final revisions.
Hafiz Waqas Moazzam
Roll Number: 16L-5120
Signature: _______________
Date: ___________________
Plagiarism Undertaking
I solemnly declare that research work presented in the thesis titled “Feasibility of a Variable
Frequency Based Solar Mini Grid in Rural Area” is solely my research work with no
significant contribution from any other person. Small contribution/help wherever taken has
been duly acknowledged and that complete thesis has been written by me.
I understand the zero tolerance policy of the HEC and National University of Computer &
Emerging Sciences towards plagiarism. Therefore I as the Author of the above titled thesis
declare that no portion of my thesis has been plagiarized and any material used as reference is
properly referred / cited.
Hafiz Waqas Moazzam
Roll Number: 16L-5120
Signature: ______________
Date: _________________
Abstract
Water pumping systems are used to irrigate farms in rural areas around the world. In rural
areas, there are many issues with the grid supply such as voltage fluctuation and load
shedding. Diesel generators or Tractors are used to overcome the issue of unavailability.
These issues mean less profit for farmers due to high cost of crop water. A trend is now
becoming prevalent where PV based solar water pumping systems are being used to pump the
water from ground. However, due to high capital costs, this solution is not workable for all
farmers. Having one solar power solution per water pump is inefficient form a cost and
utilization standpoint. In this thesis we have investigated a new architecture to solve these
inefficiencies. Rural distribution grid is split into smaller sections whereby many water pump
connections can be served from a single node. One large sized solar inverter based on a
Variable Frequency Drive is used to run many pump motors at lower frequencies to in the
event of low voltages by running the motors at lesser rpm. Two scenarios are used to simulate
and validate the feasibility of the concept. Issue with regards to distortion due to the use of
PWM based and multi-level based Variable Frequency Drive is investigated with different
possibilities of output filters. Finally, simulation results show that the proposed scheme is
feasible.
Acknowledgements
I would like to start by thanking my supervisor Dr. Syed Aun Abbas for the guidance and
encouragement he has provided me during the courses and for this thesis. I am sincerely
grateful to him for sharing his views on a number of issues related to the thesis work. His
constant feedback and learned insight has been a guiding force throughout my thesis work. I
would also like to thank all of my other colleagues at FAST, friends and family for their
generous support.
Chapter#1
Introduction
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Chapter#2 3
Background and Literature Survey 3
Chapter#3
System Simulations Model
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Simulations and Results
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Simulation Results and Discussion
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Chapter#6
Conclusions
References
Appendix: MATLAB Codes8
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Chapter#1
Introduction
PV Solar pumps have replaced the current pumping systems which result in both social and
economic benefits as well as climate related benefits. The installation of solar based PV
system is easy and portable. But, these types of systems have some drawbacks, for example
many site visits are required for maintenance. Additionally, fuel is being costly day by day
also not easily accessible in rural areas especially in developing countries. The consumption
of coal and fuels also have swear climate effect. The main agenda of greenhouse effect and
global warming is to minimize the combustion of carbon dioxide. The combustion of carbon
dioxide could easily reduce by using the renewable energy resources which are already
inexpensive comparatively than other combustion engines or generators. Large scale PV and
wind turbine are the good examples in case of renewables [1].
Therefore solar water pumping techniques have replaced the traditionally or current pumping
systems. The water provided by the solar water pumps can be used to irrigate crops.
Somehow the water is also stored for animals. In PV pumping systems electrical energy is
produced by the solar panels connected in parallel and series depending on current and
voltage requirements. Typical solar pumping systems consist of a solar panel array that
powers an electric motor directly or indirectly depending on the type of motor. Electric motor
could be an AC motor which is connected to solar panel via inverter or it could be DC motor
directly connected to solar panel. These motors then derive the water pumps. The power
provided by the solar panels must cover the power demand of the pump sufficiently in rural
areas where the less power issues are common [2].
;
Mini grid can be defined as the set of generators, distributed energy resources and somehow
the large battery bank are interconnected to a distribution system. And this distribution
system is being supply electricity to certain group of users within that area. Generally a mini
grid is able to deliver a capacity of electricity generation up to 10000 watts to 10000 kilo
watts. They provide the electricity to a limited users connected via distribution grid. A mini
grid can operate in isolation mode from utility grid or in grid connected mode depending
upon its type of functionality.
Mini grids can supply the electricity in islanded mood, grid connected mood and somehow in
hybrid mood with a control synchronized mechanism. When the mini grids are interconnected
with each other then they give a feasibility of operation without a large storage system of
battery bank, because in this case the grid has enough power available for support. Mini grids
can support to such loads which are higher in magnitude such as micro enterprises and
irrigation pumping systems. They also overcome the losses for distribution system as they
inject the power to grid by having a control of localized generation and consumption of
power [3].
& -#
Industrial drive based water pumping systems contain VFD, PV module, three phase
induction motor and water pump. General figure is shown in figure 1.
& -/!
The VFD derive takes in a fixed voltage and frequency and gives out a variable voltage and
frequency to the motor. In VFD first step involves rectification of unregulated AC voltage,
means AC to DC conversion to form a DC bus then after again DC to AC control inversion
using the PWM inverter shown in figure 1. Three phase induction motors have the most
widely used for agricultural and industrial control automation. They are often known as the
workhorse of the motion industries due to their robustness, reliability, less maintenance and
of high durability [4]. Control techniques of induction motor can be divided into two major
types, i) scalar control and ii) vector control methods. Scalar control could be open loop or
feedback control. In this thesis we used open loop V/f control mechanism. The structure of
open loop speed control is very simple and without speed feedback. Nevertheless, this
controller does not achieve a good accuracy in both speed and torque responses, because
torque and stator flux are not directly controlled. Pabitra Kumar Behera et.al, 2014 [5]
presented a design of scalar control of induction motor. This method principals to regulate the
speed of three phase induction motor by controlling frequency and voltage of induction
motor, keeping the V/f ratio constant. It also presents a comparative study of open and close
loop V/f control of induction motor.
It is often required to control the output voltage of inverter for the constant V/f control of an
induction motor. The pulse width modulation based inverter provides best constant V/f
control of induction motor. Open loop V/f control is cheap and easily construct able way. But
this method gives the slow transient response. It is because the V/f constant method controls
the magnitude of voltages and frequency instead of controlling the phase and magnitude of
currents. Open loop V/f has a low performance, but it has a stable control system. This
control method controls the currents therefore it can operate with fast responses. Open loop
strategy fulfils the requirements of dynamic drives, where fast response is need to be
necessary. Also the use of open loop V/f is a reliable control method to handle transients. The
disadvantage of closed loop method is complexity, and the high price of the driver circuit.
Nevertheless, it is a high performance control technique. Thus, the both techniques could be
used depending upon the conditions.
If we talk about the motor inner structure then it consists of main parts, i) stator and ii) rotor.
Stator is responsible to rotate the rotor with the help of mutual induction mechanism. As,
when three phase balance ac voltage is applied to the stator winding of the induction motor it
induces the voltage in the rotor winding because of the transformer action. Here the
transformation of electrical energy into mechanical energy occurs.
The constant speed of an induction motor could be controlled by the well-known equation of
synchronous speed of induction [5]. As the Synchronous speed of the induction motor is
given as in eq. 1.
NS=120. f
P
9
It is clear that the Synchronous speed has direct relation to rated frequency 50Hz or 60Hz,
and it is inversely proportional No. of poles. It is also known as constant speed control
strategy. While in variable speed control strategy, Ac voltage is applied to the variable speed
drive which converts the AC voltage into DC than after controlled insulated gate bipolar
transistors (IGBTs) are used to convert the DC voltage into controlled AC voltage. Basically
IGBTs are act like switches. The switching frequency of the IGBTs is controlled by pulse
width modulation generator which also controlled the output of the VFD. The frequency
control method is depends on stability of electromagnetic flux, the stability of V/f ratio could
be presented by eq. 2 [6].
V
f=4.44 N Φmax
;
V
f
α T (2)
The equation 2 shows that V/f is directly proportional to the magnetic flux, where V and f are
the rated voltage and frequency. If we increase the voltage above the rated voltage by keeping
the frequency constant than magnetic flux will be disturb therefore the torque will not be
constant too. [7]. Therefore by keeping the torque constant V/f ratio should be maintain in
this manner so that the torque will remain constant.
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A variable frequency drive is a device which is used to control the speed of the induction
motor by varying the output frequency. It consists of four basic units,
1) Rectifier Unit
2) DC Bus Link
3) Inverter Unit
4) Control Stage
The block diagram of variable frequency drive is shown in figure 1.
&* >(
The rectifiers can be controlled or uncontrolled voltage source or current source derived
based on DC power stage to either buck or boost. For the highly qualified rectification active
front ends filter (line reactance) could be used which overcome the harmonics. The full wave
rectifier H-bridge converts the AC voltage into DC.
Full wave diode bridge shown in figure 2. The one diodes diagonal of rectifier allows to pass
through the positive cycle while the other diagonal of diodes only allows to pass through the
negative cycle. To rectify the each phase two diagonal pairs are required, each has two
diodes. Thus, to rectify the three phase AC voltage into DC we need six diodes. This is done
in order to vary frequency of induction motor as it is easier to convert AC supply to DC
supply as AC can be easily rectified to DC. DC supply has no hard or soft frequencies
generated as DC supply is a continuous flow of current hence can be easily controlled to be
able to generate different frequencies as compared to Ac supply which has fixed frequency.
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&% ?
This comprises of a single link inductor or shunt capacitor or a combination of the two. At
this stage, the voltage of photovoltaic PV panels could be tied using a maximum power point
tracker (MPPT) or without MPPT. The inductor or capacitor or a combination of both also
reduces ripple current and voltage. This is done through volt-second balance in inductors and
charge amp-second balance in capacitors.
&) /$@
Basically the inverter unit consists of transistor to convert the DC voltage into AC. In VDFs
there are the special type of transistors are used having the high switching frequency and
rated power. Most common switching transistor which are used known insulated gate bipolar
transistors (IGBTs). The on and off switching speed of the IGBTs is several thousand times
per unit second. These IGBTs controlled by the pulse width modulation generator. The motor
speed is dependent on frequency and number of poles. By varying the frequency, output of
VFD controls motor speed. As Speed (rpm) = Frequency (hertz) x 120 / No. of poles.
The switches are the combination of power diodes, MOSFET and IGBT transistors, generally
arranged in three basic combinations, i) single quadrant, ii) two quadrants and iii) four
quadrants. The two quadrant switches can be current bidirectional or voltage bidirectional.
The four quadrant switches are developed using two quadrant types. MOSFET transistors
tend to be used for high current, bidirectional switch applications, and IGBT transistors tend
to be used for high voltage bidirectional applications. Also, all switching is done in such a
way that the device is only operated in its linear active region for a very short time. It is either
turned on or turned off. To get these devices to switch fast (in the kHz range), it is necessary
to use driver circuits and snubbers to optimize the switching speed and minimize losses. Also,
faster switching translates directly to smaller energy storage elements. The inverter of VFD
can be a voltage inverter source (VSI) or current inverter source (CSI).
&4
The industries of power electronic devices are gradually growing. The advancement in power
electronic technologies have rapidly improved thus, the performance of VFDs is also
improved in terms of PWM control as well as the hardware and software based drive systems
[8]. Good dynamics of possible energy savings and flexibility are some of the most common
features that run the VFD market. They are becoming an integral part industrial and
commercial loads.
In order to build a PWM (pulse width modulation) signal we compare a low power reference
sine wave with a triangular wave shown in figure 3a. When these two signal will be given at
input of a comparator, we get a PWM signal at the output of the comparator which is shown
in figure 3b. The PWM controls the on off mechanism of IGBT switches of the inverter. The
frequency of triangular wave is the switching frequency of the inverter. It is also known as
carrier frequency of the inverter. Hence, we can say that we have a control mechanism to vary
the frequency of the output signal. So there is also a consideration in VFD that is to control
the amplitude of the output wave of the inverter. The output of the inverter is controlled by
the Modulation Index of the PWM based inverter. Modulation index is the ratio of amplitude
of sinewave reference signal and the amplitude of the triangular signal. Its value lies in
between 0 to 1. It is denoted by
ma
.
ma=Am , sin
Am , tri
9
If the maximum value of the message signal is equal to the maximum value the triangular
carrier then the PWM generator will generate the highest value of the voltage. The figure of
the PWM procedure is shown in figure 3.
*+, --+, --
The frequency modulation index is the ratio of triangular wave signal to the frequency of
reference sine wave. It is denoted by
mf
.
mf=ftri
fsin
9
For each phase the reference sine wave is compared with triangular wave to generate the
output signal of the PWM generator. Than this output generated signal will become the gate
pulses of the IGBTs bridge. Our designed simulation model is discussed in chapter 3.
&. 19 -#
Pump is a mechanical device that applies energy to move liquids from one place to another at
increased pressure, flow rate and to an elevated height. In any pumping system, the
role of the pump is to provide sufficient pressure to overcome the operating pressure of the
system to move fluid at a required flow rate. The solar PV pumps are generally categorized
into two main types [9],
1) Positive displacement pumps
2) Centrifugal pumps
8
The centrifugal pumps are commonly used in irrigation. They have simple construction, they
only consist of moving parts shaft and impeller.
Usually technical documentation covers the main operational characteristics of the centrifugal
pumps. For instance, energy head-flow rate curves, efficiency flow rate curves and power-
flow curves [10]. Because of complicated and non-linear relations between main pump
characteristics, it is quite inconvenient to use analytical equations to describe these relations.
Therefore, the technical data given by the producer helps to examine the properties of the
centrifugal pump system and predict the value of system characteristics during different
control operations. The requirement of the power for water pumps depends how much the
flow rate is required and head or pressure. [11]. The required power is also dependent on
motor efficiency which drives the pumps.
1) Hydraulic Power
It is the output power of pump which is required to lift up the water, under a certain
pressure and flow rate through a given head.
PH
(
w
)
=ρgHQ
3600
9
where,
ρ
is fluid density in (kg/m³) and g is the gravity acceleration in (m/s²).
2) Shaft Power
The shaft power (
Psh
) is the power which is supplied by the motor into the pump’s
shaft. It depends on the pump efficiency (
ηP
),
Psh
(
w
)
=PH
(
w
)
ηP
9
3) Affinity Law
The equation 7 relates performance parameters of centrifugal pump in terms of flow,
head and power absorbed, to speed are known as the Affinity Laws. It is defined for a
given pump, the flow rate will be directly proportional to the speed, the head will be
directly proportional to the square of the speed, and the required power will be
directly proportional to the cube of the speed.
Q1
Q2=N1
N2,H1
H2=
(
N1
N2
)
2
,P1
P2=
(
N1
N2
)
3
9
where,
Q = Flow Rate
H = Head (pressure)
N = Rotational Shaft Speed (rpm)
3
P = BHP = Break Horse Power (hp)
There are many other pumps are being used to pull out the water from the surface for
irrigation purpose. The submersible pumps and turbine pumps are the most common
examples for the water pumping depending upon the conditions and environmental
circumstances. If the water level is very low than the surface of the earth that the turbine or
vertical turbine pumps are used to pull out the water. Centrifugal pumps can’t do the same job
in an efficient way if the water level is low. Submersible pumps are also more use full than
centrifugal pumps where the water level is low. It uses the directly pressure through the pipe
alinement to pull out the water instead of suction methods. The efficiency of vertical turbine
and submersible pumps is greater than the centrifugal pumps in term of distances. But the
flowrate are less than centrifugal pumps. Generally the turbine and submersible pumps are
being used for commercial levels. In our thesis we are using the centrifugal pumps derived by
induction motors. General scenario which we used in our simulations is shown in figure 4.
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Chapter#2
Background and Literature Survey
In developing and modern industrial society with the population growth, the world energy
demand is increasing day by day. This subject causes to make a great deal of investments in
different energy solutions to increase the energy performance and power quality problems.
One of these energy resources which has the considered cases is PV energy resources. PVs
are substitute and friendly to environment energy resources. Photovoltaic energy resources
6
provide a good solution to supply remote area with sustainable and clean energy during the
day time.
Before using the common used of VFD some other mechanisms are used to control the flow
of the AC loads. But in these systems efficiency disturbs and the output power requirements
fluctuate while using an external means of adjustment. Commonly used methods for these
types of flow control include throttling or restrictive devices such as valves, outlet dampers,
inlet vanes and diffusers shown in figure 7 [12]. Sometime gearboxes are used to have a
control of induction motor speed. As in gearbox there are two metallic disks having the saw
tooth edges connected with each other to rotate them. If the primary disk is greater than the
secondary than speed will increase and vice versa. Typically figures of flow control of pump
are shown in figure 5 [13], with and without gearbox. However all these strategies to control
the flow rate or pressure waste the energy because of the friction losses, and degrade the
efficacy of the overall system. The motor has a fixed speed for rated horsepower, therefore
we do not have a control on speed of induction motor to control the flow rate. Hence, we
cannot minimize the energy with respect of demand. No doubt, we can control the flow rate
using throttled valves or gearbox but in this case we do not have a control of input energy.
Because motor is running at its rated energy [14].
There is also a way to increase the speed the of the induction motor to oversized it, but in this
way the efficiency compromises. Let say, if there is a motor having the rated speed of 1800
rpm. It is clearly shown in figure 6 load vs efficiency graph that it has the maximum efficacy
with its rated pole and rpm. If we decreases the number of pole to two than the speed will
increase to 3600 rpm but the efficiency will decrease. Similarly if we increases the number of
poles than also the efficiency will compromise. Thus, the 1800 rpm motor is more efficient
and the pumps derived by the 1800 rpm motors will have lower NPSHR and suction energy.
The common use of variable frequency drives are not only to optimized the flow control
systems. Somehow is also be used to control the torque of the heavy machinery. To clean the
harmonic distortion VFD is one of the most promising drive as it converts the fluctuated AC
in to DC and then into clean control AC [13]. There are mostly papers published related to
single pump-motor configuration. In these paper flow rate is managed using VFD and
simulated results are presented in terms of flow rate, speed and motor characteristics i.e,
speed-torque curve, rotor and stator currents, and phase voltage [16].
But in the proposed system the main focus is to simulate the multi pumps over the
distribution network using single VFD. Also here in this place renewable energy resources
could be integrated to overcome the less power issue [17]. In this way we are being analyzing
and simulating the scenarios proposed in next section.
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The solar systems are being popular as they present a good and feasible solution for rural
areas. The photovoltaic industry is growing day by day in the field of power electronics, as it
presents the attractive technologies for different applications. Electricity produced by PV
systems is not cost effective when it is compared by grid. Many standalone PV system are
running based on individual identities in developed countries. The hybrid systems are also
being used in developing countries as they work with utility grid to fulfill the power demand
of end users [18].
There are many different techniques which is discussed in [18-19]. These technologies is
based on PV solar directly connected to the motor pump systems. Somewhere these PV
systems directly connected to the motor pump systems via MPPT algorithms and in different
manner the PV system is also being used directly connected to DC motors, known as BLDC
motor pump systems [20]. But these solutions are generally being used in isolated modes.
In this paper [21] speed control mechanism is used with the stand alone PV solar system to
control the flow rate of the water pump. Directly PV output is connected to voltage source
controlled inverter with a boost converter and MPPT. A review of comparison of
conventional PV water pumping systems, VSI controlled inverter and VFD is discussed in
this paper [22]. In all this discussion a single motor pump system is used directly connected
to PV solar system.
Here is a little discussion based of grid connected or hybrid PV solar systems. For the
isolated or stand-alone PV system a large PV array system is required. Therefore, in rural area
hybrid system is also being used for PV solar pumping systems [23] which reduce the size of
PV panels. There are different concerns related to grid connected inverters i.e, voltage
regulation and frequency synchronization. Total harmonic is also a big concern with it.
So to overcome the issues of total harmonic and voltage and frequency regulation at the point
of common coupling (PCC) the authors are discussing various algorithms and techniques in
this paper [24]. In this paper for the installation of water pump system, voltage source
converter based of V/f control is used, vector control based on virtual phase-locked loop
(PLL), droop control strategy based on adaptive virtual impedance loop and virtual
frequency-impedance loop methods are discussed. So based on all these discussion in our
research we are using a single VFD which is based on open loop V/f control method to
control and regulate the voltage and frequency issues. Also we are using the transmission
lines to run the multiple motor pump systems at different distances. To overcome the issue of
THD passive filtering process is adopted in different topologies discussed in section 4.
The use of solar power for agricultural purposes is driven by the need to integrate renewable
energy sources into the existing distribution system [25]. Further, regular outages in rural
areas and the ever increasing agricultural demand of farmers, there is a requirement for
alternative power sources. These outages are caused due to aging distribution infrastructure
[26]. To solve these problems, AC or DC microgrids are proposed in the literature [27-29].
This will enable the low voltage (LV) network to operate both in grid connected and islanded
mode.
There is a hybrid architecture for solar-powered pumping system is discussed in [30]. The
system makes use of a switchable control from the main grid to PV power, for an outage
scenario. The single inverter takes the load for pumping water into the tank. This architecture
cannot be used in multi-induction machine load environment. The single inverter will not be
sufficient to take the load of all the motors, unless a higher power rating of PV panel and
inverter is installed at the site.
A practical implementation of PV based solution for water pumping is proposed in [31]. The
configuration makes use of PV and battery storage system (BESS) in a hybrid structure to
control the common DC link voltage, with the induction motor connected through an inverter
based solution. The dual inverter stages in the proposed architecture will introduce odd-
harmonics and will require an additional control stage and/or filter stages to ensure power
quality to the connected loads.
A three-phase grid connected solar water pumping system is introduced in [32]. The proposed
control scheme works in three modes. In mode 1, the PV power not only supports local load
but also sends excess power to the grid. Under this mode, the available power varies with
solar irradiance. Under mode 2, only the grid supports the load. Therefore, the motors will be
running at rated speed. Finally in mode 3, both PV and grid support the loads, while the
motor runs at rated speed.
For the solar water pumping people are using the different types of pumping schemes
depending upon the conditions in rural areas. Some are using isolated PV solar inverters and
some are using grid connected inverters. For the purpose of energy cost saving and to control
the flow rate accordingly multiple VFDs are being used for multiple pump systems. There are
also some inefficient methods used just like gearbox and valves. On the other side the use of
numbers of VFDs (having the non-linear mechanism of power converts) cause the harmonic
distortion for the distribution system.
In this thesis a single VFD is used over a distribution network using different topologies. The
use of a single VFD will be providing a clean and well-regulated three phase controlled AC
voltage which will be able to run the motor pump systems at different distances. The use of
single large VFD also provides us to supply DC from PV array directly into the DC bus
inside the VFD.
Chapter#3
System Simulations Model
* # @
In this thesis study, many issues of rural power grid feeding a number of water pumps is
studied through the use of a double conversion device, i.e, Variable Frequency Drives or VFD
as shown below.
8
Part of a distribution network that only feeds water pumps is segmented in a way that a VFD
is used to address the power system parameter issues to ensure continuous operation of Tube
Wells even in the pressure of unacceptable i.e, a VFD, transmission network, VFD output
filters and induction motors deriving pumps loads. In the following sections, simulation
model of these system components is described.
*& 91/$
The design process of PWM control signal had already discussed in chapter 1 belongs to
figure 3. The comprehensive switching concept to PWM generator is described in paper [33].
As in our case we used PWM based inverter to convert the DC voltage into three phase AC
voltage based on PWM generator, shown in figure 9. The switching states of the PWM
inverter is shown in table 1. Each pair of two switches is responsible to generate the PWM
sine wave for each phase shown in figure 11 which was filtered later.
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This switching state table is also described in terms of PWM generated signals shown in
figure 10. These six gate pulses are responsible to derive the IGBTs circuit which is shown in
figure 9. Switches (1,3,5) are responsible to generate the positive peak of phase A, phase B
and phase C, while switches (4,6,2) generate the negative peak of the phase A, phase B and
phase C. These figures are shown by running the MATLAB simulations for 0.05 sec.
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In this thesis work modified sine wave based PWM inverter is also used. For this purpose
three-level bridge block is used. It consists of a three phase IGBT based voltage sourced
converter (VSC). The output of this multi-level PWM inverter is shown in figure 13.
*2-2-$91/$
**
Generally transmission lines could be categorized into three main types, i) short transmission
line, ii) medium transmission line, and long transmission line. The main feature of
transmission line is to transfer the power from one end to the other end.
8
There are many transmission line model exist which could be used depending upon the
system requirements and the distance between the sending and receiving end. Every
transmission line has three basic electrical parameters. The conductor of the transmission line
will have resistance, inductance and capacitance. All these parameters are dependent on the
physical conditions, temperature and placement of the transmission lines. Inductance of the
inductor has an inverse relation with the radius of the conductor while it has direct relation
with spacing between the phases of transmission lines. Similarly the capacitance will have the
direct relation with radius of the conductor and inverse relation with spacing between the
phases of transmission lines.
Nominal pi model representation is used in out thesis, pi model of the transmission line has
series impedance placed in middle of the circuit, while shunt admittances lie at both receiving
and sending sides of the circuit shown in figure 14. In pi model of transmission line the total
shunt admittance is divided into two equal halves. The physical appearance of the pi model
transmission line resembles of symbol Π. That why it is known as pi model for medium line
transmission.
% - -
The transferring of power from generation side to the distribution can be done through
transmission lines. As the power transmit from one end to the other end, many losses occurs
which degrading the efficiency of the power. To analyze and improving the proficiency if
transmission, two port networks are used.
Using the figure 14,
Vab=Vr
, where
Zab =1/Yab
Iab=Vab /Zab
=
YV r/2
At Node a,
I=Ir+Iab
I=Ir+YV r/2
Voltage at sending end,
3
Vcd=Vab +ZI
Vs=Vr+Z(Ir+YV r/2)
Vs=(1+ZY /2)Vr
+
ZIr
(A)
By the ohm’s law,
Icd=Vcd /Zcd
=
Y/2[(1+ZY /2)Vr+ZIr]
Now at node c,
Is=I+¿
Icd
Is=Ir+YV r/2
+
Y/2[(1+ZY /2)Vr+ZIr]
Is=Y(1+ZY /4)Vr+(1+ZY /2)Ir
(B)
Using the eq. A) and B) the transmission matrix for pi model is given as,
[
Vs
Is
]
=
[
A B
C D
]
[
Vr
Ir
]
(C)
The transmission parameters matrix describes the network in terms of both voltage and
current waves. The matrix C is the ABCD parameters of transmission line, where A =
Vs/Vr
at
Ir=0A
, B =
Vs/Ir
at
Vr=0V
, C =
Is/Vr
at
Ir=0A
and D =
Is/Ir
at
Vr=0V
are the coefficients defined using superposition theorem.
The transmission matrix in terms of series impedance and shunt admittance is given as,
A =
(1+ZY /2)
unitless
B =
Z
ohm
C =
Y(1+ZY /4)
moh
D =
(1+ZY /2)
unitless
To find the values of A, B, C, and D MATLAB code is gives in appendix. To find the value of
these coefficients we need to find the value of resistance, inductance, capacitance and
frequency for the transmission lines. Therefore, R = r.
lsec
, L = l.
lsec
and C = c.
lsec
.
Here in our case the parameter section of these values is shown in figure 15,
6
)- -
lsec
= length of line section in (km)
r = resistance per unit length in (Ω/km)
l = inductance per unit length in (H/km)
c = capacitance per unit length in (F/km)
Hence, by knowing these values one can easily find the values of R, C and L and conversely
the value of ABCD matrix.
*%
The LC filter is used which consists of an inductor series with transmission and a shunt
capacitor. Three filters are used for each phase. The mathematical expression of cutoff
frequency in case of LC filter is given below.
fC=1
2π
√
LC
98
4
By using the equation No. 8 one can select the suitable cutoff frequency for LC filtering to
overcome the higher order harmonics. The Q factor for the LC filter shows the quality of the
filter lower the value of Q more losses and higher the Q value means losses are less. It could
be represented as,
Q=1
ωoRC =ωoL
R=1
R
√
L
C
93
The phase and magnitude response of the LC filter shown is figure 17. It could be calculated
by the transfer function of the LC filter in frequency domain. The MATLAB code for this
calculation is given in appendix.
.92 -(
There are many types of active and passive filtering process being used. We used a LC
filtering process to filter out the output of PWM inverter. The passive filtering process are
discussed in [34-35].
Chapter#4
Simulations and Results
Three simulation scenarios have been conceived to test the viability of the proposed rural
mini grids and are described in the following along with the detail simulation system and the
results.
%
The scenario 1 is simple one which is shown in figure 18. In this case a variable speed drive
is used at 400V lines. The flow rate is controlled by the 5-HP motor pump system. The rated
parameters of the system are, rpm=1460, V=400V and f=50Hz. The flow rate of the pump is
16.88gpm. The flow rate of the centrifugal pump is controllable by varying the frequency and
voltage of the variable frequency drive which is shown in figure 19. In start the flow rate at
the rated parameter is 17.8gpm, alter by adjusting the V/F ratio the flow rate is controlled
which is 14.02gpm at V=320V and f=45Hz. Voltage is controlled by the modulation index of
the PWM inverter. The inverter output wave form of voltage source PWM based inverter is
shown in figure 20.
8
It was the general simulation which is later implemented on high transmission lines and low
transmission lines by using the setup of step-up and step-down transformers, output filters of
inverter and compensating capacitors.
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Now for the validation we run this model for 30 values by decreasing the frequency from 50
Hz to 20 Hz with adjusting the modulation index to control the output voltage keeping the V/f
ratio constant. The extracted the data is shown in table 2. It is clear that by decreasing the
output of the VFD, output power also reduced with the provided grid parameters. In start at
50 Hz and 395.7 volts the input power is 3414.9 watts and VFD controlled output power is
2925.2 watts. Similarly at 30 Hz and 396.9 volts the input power is 2135.3 watts and output
power is 1970.4 watts. It clearly shown that the VFD is controlling the output power. In this
way at less power we will be able to run the motor pump systems with the low rpms and flow
rates. Over all relation of motor speed vs flow rate is shown in figure 21.
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To analyze the impact of the PV solar on DC bus of VFD as shown in figure 22, we dropped
15% grid side voltage manually. Without decreasing the grid side voltage we have the flow
rate 9.41 gpm at 30 Hz and after the drop of 15% in voltage flow rate also decreased to 7.27
gpm. Now, here the impact of the PV involved and it was observed that the flow rate
increased again back to approx. 9.35 gpm. No doubt the grid side voltages has dropped 15%
but we are still getting the same flow rate 9.35 gpm after tying the PV solar on DC bus of
VFD. Measured values are shown in table 3.
Grid Side Parameters VFD Output
Power at 30Hz
Motor Speed
(rpm)
Flow Rate
(gpm)
At 400V and 50Hz 2135.3 watt 814 9.41
At 340V and 50Hz 2219.6 watt 642 7.24
At 340V and 50Hz with PV 2348.7 watt 810 9.35
*%!%!#!;7!!".%
%& &
In scenario 2 multiple motor pump users are connected at a certain distances over a
distribution network. It is understood that as the distances increases the voltages drop and
motor will take the more current to run the pump. But in this model VFD could produce the
controlled output power which will be less according the demand but all motor pump systems
will run below the rated speed. No doubt the speed and flow rate will be less collectively but
they are still useful for the end used. Because in this way overall system is not being
overloaded. This scenario feed the users in small range area wiz. In this scenario the input
voltage is 11 kV then after it is step down to 400 V using 100 kVA step down transformer.
After the step down transformer the variable speed drive is used to control and regulate the
voltage. At 400 V the low transmission lines are used.
&*&
The GNAT conductor is used for LT lines having the 1.311 resistance/km and 0.421
reactance/km. But in case when we used PWM inverter we used the 0.95 resistance/km in our
transmission lines. The three motor pump systems are used at different distances as shown in
figure 24. One is very close to VSD, second is at 400 m and finally third is at the distance of
800 m. At these distances the THD (Total Harmonic Distortion) and flow rates are calculated.
Basically there are three different sub-scenarios in scenario 2 to improve the THD,
8
1) Without Output Filter
2) With Output Filter (LC)
3) With Compensating Capacitor
In each sub-scenario the calculations are performed to compare the results.
%& 12-
In this case no output filter is used. The filtering effect of transmission lines is analyzed at
400 m and 800 m for both type of inverters PWM and multilevel. Resultant wave forms are
shown in figure 25a and 25b. The right sided values are based of multilevel inverter and left
sides for PWM inverter in table 4.
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Results show that in the case of multilevel inverter the wave form is being more filter than
the PWM case. As the total harmonic increases the flow rate decreases.
%&& 12-+,
In this case the LC filter is used after the VFD output and then it is being transmitting through
transmission lines of 400V as shown in figure 27. The filter aspects have already described in
section 3.4. The results are shown in figure 28a and 28b which shows that in the case of
PWM the voltage drops are slightly greater than multilevel inverter. Also the total harmonic
distortion is greater and gradually increased as the distances increases. Table 5 shown the
significant affect in improvement of total harmonic distortion if we compare it with table 4.
In this scenario all the components having the multiple of the fundamental component 50 Hz
which are greater than cutoff frequency is being blocked by the low pass LC filter. The fast
fourier transform (FFT) analysis is shown in figure 29.
6
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Capacitors are normally used to control the power factor losses. It could be used to decrease
the inductance of the power lines by connecting them in series. The compensation of
transmission lines imply a modification in the electrical characteristic of the power.
Compensating capacitors have the main objective to increase the power transfer capability by
correcting the power factor. When we use the series compensation then the main purpose is to
overcome the transfer reactance of the transmission lines. Which results the enhancement of
system stability [36]. For the voltage support capacitors are placed in parallel known as shunt
capacitors. The use of capacitor is usually done to achieve to get the following goals,
1) To Reduce the losses due to reactive load current
2) Decreases kVA demand
3) Reduction of customer energy consumption
4) Improve voltage profile
5) Rise the growth revenue
Hence, we can say that, indirectly the use of compensating capacitors also provide good
result in longer equipment’s lifetime because they reduce equipment stresses. [37].
In this scenario 2 which is shown in figure 30, compensating capacitors are used after the
transmission lines for second and third motor pump system to improve the power factor. The
overall inductive behavior with the shunt capacitors make an operation just like as LC
filtering. Finally we got the sine wave at the terminal levels of 400 m and 800 m. The
simulation results are shown in table 6. In this scenario the first motor pump system is
running directly with VFD output wave form which is neither filtered nor compensated so
that in this case the THD is same as was in first scenario shown in table 4.
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In the scenario 2 with compensating capacitors, we maintained the V/f ratio constant to
control the output power of the VFD. Thus we are still getting the flow rate by decreasing the
output voltage of VFD at different intervals. The voltage drop and currents drawn by the
motors are given in table 7.
.%!%!#!;7!""!
%* *
In this scenario 3 the input voltage is 11 kV then after it is step down to 400 V using 100 kVA
step down transformer to deploy the variable speed drive. Than after it is again step up to 11
kV using the 100 kVA transformer. In this scenario there are more components are involved
than the scenario 2. It covered the larger area because 11 kV transmission lines has less
voltage drops than the 400 V. In this case every end user has to need a step down transformer
to take the service from the 11 kV transmission line.
***
Gopher conductor is used for high transmission lines having the resistance/km is 0.95 and
reactance/km is 0.415. Multiple motor pump systems are used at different distances as shown
in figure 33. One is very close to speed drive, second is at the distance of 400 m and final one
is at 800 m using the step down transformers from 11 kV to 400 V having the rating of 25
kVA. Similarly in this scenario THD (Total Harmonic Distortion) and flow rates are
calculated at 400 m and 800 m. Also in this scenario there are three different sub-scenarios,
1) Without Output Filter
2) With Output Filter (LC)
3) With Compensating Capacitor
In each sub-scenario the calculations are performed to compare the results.
%* 12-
In this scenario three motor pump systems are deployed at 11 kV lines by using the step down
transformer of 25 kVA at different distances 400 m and 800 m. In this scenario the total
harmonic is significantly less as compare to scenario 2 with no filtering. Also the PWM wave
form is more filtered and just like sine wave. In this case there is no output filter used but still
the PWM output wave is filtering. The phase voltages of PWM based inverter and multilevel
inverter are shown in figure 35.
Filtering is the significant part in renewable energy systems. There are many types of filter
which generally use in grid tied inverters or for VFDs. First order passive L type filter,
second order LC filter and third order LCL filter are the types of filter. The L filters and LC
filters are large in size, while LCL filters have the lower in cost and smaller in size. These all
types of filter are used to remove the harmonic. As the transformers have the inductance and
resistance. So in many harmonics mitigation method they have the significant roll to
overcome the harmonic. Today, there are many methods which are being used to reduce
harmonics, i) Delta-Delta and Delta-Wye transformers, ii) isolation transformer and iii) line
reactors. In Delta-Wye and Delta-Delta transformers technique two separate transformers are
used to distribute the nonlinear load. Phase shift procedure is used in this technique. In
isolation transformer technique an individual transformer is used before the VFD which is
also some time refer for active front end filtering to overcome the effect of harmonic
distortion. Isolation transformer is a unique type of transformer which is specially designed
for VFD. It has equal the primary and secondary voltages. It only steps up or down its voltage
by using the tap changing procedure [38]. Finally the use of line reactors is cost effective and
modest way to reduce the total harmonic distortion from the running systems which have the
nonlinear components or loads. This is comparatively better solution than the use of
transformer methods. Different line reactor are available in market with respect to their
reactance to overcome the total harmonic distortion.
There are two different types of line reactors one is used for AC lines known as AC line
reactors, others are known as DC bus reactors. DC line reactors are used in DC bus of
variable frequency drive to filter out the ripple during the AC to DC. As AC line reactors are
used as active front end filtering, therefore in this scenario 3 transformers are being an
important role to filter the harmonic distortion. Because transformers have the reactance. But
the disadvantage of line reactance is that they have losses issue which cause the voltage
reduction [39-40].
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In this scenario the LC filters are used after the 25 kVA transformer shown in figure 37. As
the filters are designed for 400 V transmission lines therefore these filters are being used after
step down the voltages for motor pump systems. So the phase voltages of PWM and
multilevel inverter will become more filtered as compare to previous case. Simulation results
are given in table 9. Similarly FFT analysis is shown in figure 34 for both inverters at 800 m.
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%** 12 --
In this scenario compensating capacitors are used after the step down transformer at
secondary side for second and third motor pump system to improve the power factor. The
overall inductive behavior of transformer with the shunt capacitors make a sense of LC
filtering. The simulation is shown in figure 40. Finally we got the filtered sine wave at 400 m
and 800 m. The simulation results are shown in table 10. In this scenario the first motor pump
system is running directly with VFD output wave form which is neither filtered nor
compensated. The FFT analysis is given in figure 42.
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In the scenario 3 with compensating capacitor, four different values are calculated by
adjusting the output of the VFD in order to show the output flow rate of the motor pump
systems at low voltages or power. Extracted data is shown in table 11.
6
%!%!#!;7!""!
Chapter#5
Simulation Results and Discussion
The simulation results show that the flow rate decreases when the distance increases and the
total harmonic distortion considerably increases in case of without output filtering in scenario
2 when we used PWM inverter but in multilevel inverter it becomes half. But in scenario 3
which belongs to high voltage transmission lines, the total harmonics are less.
Scenario 2 could be make useful to reduce the THD by using the output filter and
compensation capacitors. In both scenario the THD could be reduced. But the problem in this
situation is that when we use the output filter in sub-scenario 2 of scenario 2 and sub-scenario
2 of scenario 3, the output voltage become decreases which is not enough to run the motor
pump systems at nominal voltages in transient state, when the motor takes the large current.
So to overcome this problem we have two solutions, i) Increase the input voltage during that
interval of time when the motor starts, and ii) Increase the transformer secondary voltage by
increasing the transformer secondary turns. So these two methods could be implemented but
they are difficult. Therefore the use compensating capacitor is best option for the
implementation because in the way there is no voltage drop problem occur as well as it also
filter the PWM output pulsating output waveform in multilevel inverter. Hence the THD also
reduces at the distances where the second and third motor pump systems are used. Rest of the
comprehensive comparisons are given in table 12 and 13.
) -2 -
This operation of transforming is used when the voltage fluctuate at secondary side of the
transformer. It is the self-sensing transformer, it vary its voltage at secondary side varying the
number of turns in secondary side. A MATALB simulation model is discussed in [41], in
which they are using the tap changing transformer in three different ways tap1, tap2 and tap3.
By changing the tap transformer they analyzed the effect of the harmonic depending upon the
wind speed. The IGBTs based voltage source inverter, controlled with sinusoidal pulse width
modulation (SPWM) is used to convert the DC voltage to the desired three phase 50Hz AC
voltage. The inverter output is filtered using the LCL filter than in is entered into the tap
changing transformer whose secondary side is connected to the load.
The procedure of L, LC and LCL filtering has been discussed in many papers [42-43]. But
the LCL has the most cost effective solution with batter filtering properties. The
comprehensive study to evaluate the design parameters of LCL filter has been discussed in
[44].
&%!%!#!
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In the whole thesis the inexpensive solutions are discussed and simulated. These simulation
are based of passive filtering process. There are many others advance techniques based of
active filtering or tune filtering in which the active filters inject anti phase harmonic into load
supply to cancel the existing harmonics. There are also some other techniques to mitigate the
THD which are used before the VFD known as active front end filtering,
1) 12 pulse rectification
2) 24 pulse rectification
3) Controlled rectification
4) External active filters
5) Passive parallel tune filters
6) Passive series tune filters
Chapter#6
Conclusions
The purpose of this research was to develop an understanding of rural based solar water
pumping system using a single variable frequency drive (VFD) which drives multi water
pumps at certain distances over a distribution network. The output of the PV panels can be
tied into the DC bus of VFD which converts the VFD to a hybrid PV solar pumping system.
To overcome the total harmonic distortion due to the introduction of VFD various filtering
approaches are used to filter the output of VFD. In addition, a detailed comparison of solar
water pumping system is discussed between the PWM based variable frequency drive and
three level cascaded variable frequency drive. Results show that the proposed scenario
number three gives the batter performance than scenario number two. Also it is seen that
three level cascaded VFD reduces the THD more than as compare to PWM based VFD.
References
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8
Appendix: MATLAB Codes
(2 9
clc
clear all
fprintf('****** This program is written by Hafiz Waqas Moazzam ******\n');
fprintf('* Calculation for ABCD Parameter for PI Transmission Lines *\n');
R = input('Enter the value of Resistance (in ohm) = ');
L = input('Enter the value of Inductance (in H) = ');
C = input('Enter the value of Capacitance (in F) = ');
f = input('Enter the value of Frequency (in Hz) = ');
x1 = (2*pi*f*L);
x2 = (1/(2*pi*f*C));
Z = sqrt((R*R)+(x1*x1));
Y = 1/x2;
A = (1+(Y*Z)/2);
B = Z;
C = (Y*(1+(Y*Z)/4));
D = (1+(Y*Z)/2);
fprintf('Parameter of transmission line are:\n A=%f\n B=%f ohm\n C=%f moh\n
D=%f\n',A,B,C,D);
(2>-
3
s = tf('s');
L = 2.97e-3; % resistance of resistor in
RC circuit
C = 5.33e-5; % capacitance of capacitor
in RC circuit
H = 1/(1+s^2*L*C); % RC circuit transfer
function
figure(1)
hold
bode(G)
title('LC Circuit Frequency Response (L = 2.97e-3 H, C = 5.33e-5 F)')
s = tf('s');
L = 2.97e-3; % resistance of resistor in
RC circuit
C = 5.33e-5; % capacitance of capacitor
in RC circuit
H = 1/(1+s^2*L*C); % RC circuit transfer
function
options = bodeoptions;
options.FreqUnits = 'Hz'; % or 'rad/second', 'rpm',
etc.
figure(2)
hold
bode(H,options);
title('LC Circuit Frequency Response (L = 2.97e-3 H, C = 5.33e-5 F)')
?99/
2
?$
6
?9 -