Download Reactive Power Compensation by Using Microprocessor

REACTIVE POWER COMPENSATION – AN EFFECTIVE APPROACH
MICROPROSESSOR CONTROLLED TRIAC ARRANGEMENT
Abstract: The main idea of paper is to achieve a
flat and better voltage profile i.e. to maintain the
constant voltage as equal to the supply voltage at
given load. A power system is to be well designed if
it gives a good quality of reliable supply. Good
quality is meant the voltage levels with in
reasonable limits. When power is supplied to a
load through a transmission line, the receiving end
voltage under goes variations depending upon the
magnitude of load and power factor of load. The
voltage variation at a node is an indication
between the reactive power generated and
consumed by that node.Practically all the
equipment on the power system are designed to
operate satisfactorily, only when the voltage levels
on the system correspond to their rated voltages or
at the most the variations are within say 5% . If
the voltage variation is more than the prescribed
value, the performance of the equipment suffers
and the life of most of equipment is sacrificed. The
picture on the television set starts rolling if the
voltage is below a certified value, the fluorescent
tube refuses to glow if the voltage is below the
certain level. The torque of an induction motor
(which forms about 70% of the load the system)
varies as the square of the terminal voltage and so
on Thus the necessity of the voltage on the system
is very much strong. It is to be known that voltage
variation is due to in balanced in the generation
and consumption of the reactive power in the
system with generated reactive power is more than
what is being consumed voltage level goes up and
vice versa. To keep network voltages with in
permissible limits, mean must be provided to
control the voltage i.e., to increase the voltage
when it is too low and to reduce when it is too high.
This can be achieved by means of a reactive power
compensation that is by generation of suitable
quantity of reactive power using a fixed capacitor,
TRIAC and microprocessor arrangement .The
disadvantage involved in reactive power control
using a capacitor banks is stepped control. This
method provides smooth and step less control of
voltage. This method of control can be modified in
such a way so as to applicable for any type of loads
and industrial purposes.
Index Terms:—Micro Processor , Capacitor,
Traic ,Inductive loads ,A/D converter
USING A
I.INTRODUCTION
The main idea of paper is to achieve a flat and better
voltage profile i.e. to maintain the constant voltage as
equal to the supply voltage at given load .A power
system is to be well designed if it gives a good quality
of reliable supply. Good quality is meant the voltage
levels with in reasonable limits. When power is
supplied to a load through a transmission line, the
receiving end voltage under goes variations
depending upon the magnitude of load and power
factor of load. The voltage variation at a node is an
indication between the reactive power generated and
consumed by that node. There fore voltage variations
are directly proportional to reactive power Q 
2-V1Here XL is the
inductive reactance and as it is a reactive power
consumer, so it certainly consumes some amount of
reactive power that this is possible only when we
provide that reduced VARs i.e. we give leading VARs
to the transmission line by means of a shunt capacitor
connected across the load. The leading VARs
supplied by the capacitor is IC2XC where IC is current
thorough capacitor and XC is the capacitive reactance.
Therefore VR will be constant and equal to is given as
Q= I2XL, where ‘I’ is current through inductor and XL
is inductive reactance. Due to reduction in reactive
power, voltage at receiving end also reduced. But the
aim is to maintain it constant. Vs only when
I2XL=IC2XC
There fore in IC2XC only possibility of variation is Ic.
This can be varied according to our requirements only
by means of varying the firing angle of the triac. Thus
by varying the firing angle RMS value of thyristor
current also varied .The main cause for the voltage
variation in the power system, which supplies load
through transmission line keeping sending end
voltage constant, is the magnitude and the power
factor of the load. With the increase in load on the
supplying system the voltage at the consumer
premises falls due to increases in voltage drop. The
causesare1.Alternatorsynchronous
impedanc.2.Transformerimpedance.3.Feeders.
4.Distributors.5.Transmission lines. The reverse
would also happen should be load on the power
system fall. There are other reasons, which also cause
the voltage variations they are unbalance in power
system loads. Different types of loads cause
unbalance are 1.Arc furnaces. 2) Inductive furnaces.3)
Steel rolling mills. Very large induction motors
Generally arc and an induction furnace acts at the low
power factor under full load conditions. So load
conditions operate at the higher power factor and take
less kvar. If sudden on and off of load furnaces takes
places it will lead to variation in kvar demand and
causes variation in voltages .In steel rolling mills
generally induction motors are used as drives. These
induction motors operate at low power factor under
no load conditions; hence it takes high kvar. Similarly
at full loads it takes high power factor and hence less
kvar. Due to sudden switching of induction motors
unbalance in power system takes place, the same will
happen in case of large induction motors.
Thus because of above loads, characteristics of
power system and equally of supply in detoriates.
Thus it is desirable to 3-phase current and voltages as
balance as possible so that undue heating various
rotating machines can be avoided.
To understand the voltage control problem ref to the
figure.
Thus this relation shows that the reactive power Q is
proportionate to the magnitude of voltage in the line.
Thus voltage control and reactive power control are
inter related. The voltage at the consumer terminals
must be maintained constant with in prescribed
limits irrespective of the type and load of the
magnitude. In power system there are several
sources and sinks of reactive power they are
1.Transmissionlines.2.Transformers.3.Cables4.
SynchronousachineInductivestatic
loads.5.Inductionmotors.6.Staticcapacitor.
Transmission lines Lines under normal conditions
that is in loaded cases acts as sink for reactive
power, where as under light load conditions acts as
source of reactive power. Let the line be loaded such
that the load current is I amp, and load voltage is V
volt. Assuming the transmission line to be loss less
the reactive power absorbed by the line will be I2WL
Where
the line.
W=Angular frequency. L=Inductance of
Due to the shunt capacitance of the line reactive
VARs supplied by the lines are
V2WC where
C=shunt capacitance of the line.
In case the reactive vars supplied by the line are equal
to the reactive vars absorbed.
Node 1 is generator node with reference voltage v1
and node 2 is a load node with voltage V2. A short
line interconnects these two bus bars. Assuming the
interconnected must be loss less and the voltage V1 is
constant (by adjusting the excitation of generator) the
following relations are hold good.
I2WL=V2WC
V/I = L/C= Z
V2=V1-IX V1*=P-JQ (assuming inductive load)
I=P-JQ/V1* V1*=V1, being the reference vector.
Substitute I in the above equation we get
V2=V1-J (P-JQ/V1) X
V1-QX/V1-JPX/V1
Where Z is the natural impedance of the line. The
loading condition at which the vars absorbed are
equal to vars generated by the line and is called the
surge impedance loading. In case if
From the above it is clear that the load
voltage is not effected much due to x component of
the load as it is normal to vector V1, where as the drop
due reactive component of the load is directly
subtracted from the voltage V1. Assuming the voltage
drop due to real power is negligible, the voltage drop
is directly proportional to the reactive power Q. The
relation is given by V2=V1-QX/V1
Where the voltage variationV=QX/V1
Thus in order to keep the receiving end voltage V2 is
fixed for a particular sending voltage V1, the drop
(X/V1) must remain constant, since in this q is only
variable quantity.
I2WL>V2WC
the voltage will sag
I2WL<V2WC the voltage will raise
Thus if the line is terminated by a load corresponding
to surge impedance the voltage at the load is constant.
Transformers act as a sink for reactive power. It
always absorbs the reactive power
Let X t be the per unit reactance of a transformer
KVA is the volt amp rating KV is the voltage rating
Since by definition
P.u reactance =actual reactance i/v;
Actual reactance X= (Xt.V)/I;
I=KVA/ (1.44KV); Reactive power loss=3 I2X
= (3.KVA2.1.4.Xt.KV2.1000)/3.KV2.KVA
=1.4.KVA.Xt.KVARs
Thus transformer absorbs high value of reactive
power under full load condition and less value of
reactive power under light load condition. Cables
have very small inductance and relatively very large
capacitance, because of nearness of conductors largest
size of the conductor and dielectric metrical is used
has a relative permittivity greater than unity. They are
therefore generators of the reactive power under all
conditions of loading. A synchronous machine which
acts as a generator under which it is known that
power transmitted from a generator from a generator
bus to an infinite bus bar is given by
/X
Similar relation for the reactive power for a round
rotor machine is given by Q=V.ECOS 2/X The
above formula tellsIf E COS When Q>0
and the generator provides reactive power that is acts
as a capacitor. Thus an over excited a synch machine
(gen or motor) provides reactive power and acts as a
shunt capacitor. Similarly under excited synch
machine consumes reactive power and act as a shunt
inductance. Generally inductive static loads will cause
power of load to be decreasing this in turn causes the
reduction of voltage at the load. Static capacitor
generates reactive power and may be connected in
series or parallel to the system..
VOLTAGE CONTROL. METHODS
To keep network voltages with in permissible limits,
mean must be provided to control the voltage i.e., to
increase the voltage when it is too low and to reduce
when it is too high.
The following are the method used to provide the
reactive power compensation i.e., either generation or
absorption as per the requirement. So that flat voltage
profile at the load can be achieved.1.Excitation
control 2.Tap changing Transformers 3. Shunt
reactors 4.Synchronous phase modifiers 5.Shunt
capacitors & Series capacitors 6 Induction
regulators,7.Booster transformers8.Static var systems.
Static var system has got certain advantages when
compared to other methods. Shunt capacitor is the
capacitor connected in parallel with the lines. They
are installed near the load terminals in receiving end
substations, distribution and in switching substations.
They are arranged in three phase banks. It suffers
from some problems .Since Q is proportional to the
square of the terminal voltage for a given capacitors
bank their effectiveness tends to decrease as the
voltage sags under full load conditions.2.If the system
voltage contain appreciable harmonics, the fifth being
the trouble some, the capacitors may be over loaded
considerably.3.With shunt capacitors at no load or
light load the receiving end voltage may considerably
exceed the sending end voltage .Series capacitors are
connected in series with the line at suitable location
Series capacitors reduces the inductive reactance
between the load and the supply point. So that they
can improve the power transmission capacity of line,
system stability, control voltage regulation and ensure
proper load division among parallel feeders. Series
capacitor improves the power Transfer capability &
stability of line.
The power transfer over line is given by P1= (Vs Vr
sinδ) /XL;WhereP1= power transferred per
phase (W) Vs= sending end phase voltage (V) Vr=
receiving end phase voltage (V),XL= series
inductance of the line per phase Phase angle
between Vs and Vr
If a capacitor having capacitive reactance XC is
connected in series with the line. The reactance of the
line is reduced from XL to (XL--XC). The power is
given by P2=VsVrsinδ/(XL-XC)P2/P1=XL/(XL-XC)
=1/(1-k) The factor k is known as degree of
compensation or compensation factor Where
k=XC/XL %of compensation=XC/XL*100;Where XL is
the total series inductance per phase. XC is the
capacitive reactance of the capacitor bank per phase.
In practice k lies between 0.4 and 0.7,For k=0.5
P2/P1=1/1-k=1/1-0.5=2;Thus the power transfer is
double by 50% compensation. It suffers some
problems.The draw back of series capacitors
is the high over voltages produced across the
capacitor terminals under short circuit conditions the
drop across the capacitor is If XC.Where If is the fault
current which is of order of 20 times the full load
current under certain conditions a spark gap with a
high speed contractor is used to protect the capacitor
under these condition. Series compensated lines have
a tendency to produce series resonance at frequencies
lower than the power frequencies. This is known as
sub synchronous resonance (SSR). The SSR current
produce mechanical resonance in turbo generator
shafts, which may result in high torsional stresses in
rotor shafts.
STATIC VAR SYSTEMS (S V S): In E H V lines,
when the voltage at a bus falls below the reference
value, capacitive vars are to be injected. When the bus
voltage becomes higher than the reference value,
inductive vars are supplied to lower the bus voltage.
Shunt compensation, shunt reactors are connected
during low loads and shunt capacitors connected
during heavy loads or low lagging power factor loads.
Such switching operations are very slow because of
the greater time. Moreover these are not suitable for
frequent operations; these limitations have been over
come by STATIC VAR SYSTEMS. In this system
thyristors are used for switching operations. For
compensation we use many SVS schemes. Some of
the most common schemes are
1. Thyristor control reactor (TCR) 2. Thyristor
switched capacitors (TSC) 3. Fixed capacitor,
thyristor control reactor (FC –TCR) 4. Thyristor
switched capacitor, thyristor control reactor (TSCTCR).
A single-phase control reactor X is shown in figure.
The current through reactor can be varied by
controlling the firing angles of back-to-back pair of
SCRs connected in series with the reactor. This
scheme is used in EHV lines for providing lagging
VARS during low loads or at load rejections.
TSC: A single-phase thyristor switched capacitor
X is a shown in fig. The current through the capacitor
can be carried by controlling the firing angles of
back-to-back connected SCRs in series with the
capacitors. This scheme is used in EHV lines for
providing leading VARs during high load.
FC – TCR:This type of VAR compensation is
as shown in fig. This arrangement provides discrete
leading VARs from the capacitors and continuously
lagging vars from the thyristor switched reactors.
Two or more capacitor banks supply leading VARs.
The current through the reactor can be varied by
controlling the firing angles of back to back pairs of
SCR connected in series with the reactor.Small
reactors are usually connected in the fixed capacitor
branches, in order to tune these branches as filter for
the 5th and 7th harmonics. The TCR and secondary
winding of the coupling transformer are connected in
delta as shown in fig. This delta connection eliminates
3rd harmonics.
TSC-TCR
This scheme is as shown in fig. It consists of number
of TSC-TCR banks. The voltage level determines
their number. By controlling the firing angle for backto-back pairs of SCRs connected in series with each
reactor can vary the current through the reactors.
Similarly the current through the capacitor is changed
by controlling the firing angle of back-to-back pairs
of SCRs connected in series. During heavy loads the
thyristors of TSC are made to conduct for longer
duration of each cycle. Thus leading VARs are
provided by TSC during heavy loads. Like wise
during low loads TCR is made to conduct.The TSCTCR is capable for producing both lag and lead VARs
rapidly, continuously and each independently. Some
fixed capacitors are also connected in the circuit, in
addition to switched capacitor. These serves as a
filter for harmonics when only reactors are switched.
It is necessary to connect inductor in series with each
fixed capacitor to limit the current in the SCR
switched and to reduce the risk of resonance with Ac
impedance.
APPLICATIONS
The following are the important applications of SVS
in EHV circuits.A SVS provides fast, smooth and step
less variation of reactive power the line. Thus it
ensures accurate voltage control of buses over wide
range of loads.The SVS is used to control reactive
power demand for large fluctuations of loads.Using
SVS, Ferranti effect is compensated and the SVS
provides control of dynamic over voltages caused by
stability is increased. The most typical functions of
SVS in OPERATIONAL AMPLIFIER
The operational amplifier (commonly referred to as
op-amp) is a multi terminal device which internally
quite complex, described by its terminal
characteristics and those of external compounds
connected to it.
Zero crossing detector:
Zero crossing detector is a circuit that
detects zero crossing of input sine wave and produces
square wave as output. One of the important
applications of operational amplifier is zero crossing
detection The ZCD produces square wave output
when sine wave as input with zero reference voltage.
MONOSTABLE MULTIVIBRATOR:
The monostable multivibrator, as the name itself
indicates that it has only one stable state i.e. it has one
stable state and one quasi state. The time period for
quasi-stable state is more than the stable state.
Generally monostable multivibrator is used as delay
timer and a pulse generator. Basically rectangular
pulses are produced during its operation. necessary.
Therefore ZCD is used as a triggering input to
monostable multivibrator. voltages.
TRIAC
An SCR is a unidirectional device as it can conduct
from anode to cathode only and not from cathode to
anode. Where as a Triac can conduct in both the
directions, thus a Triac is a bi-directional thyristor with
three terminals. It is used extensively for the control of
power in a.c circuits. Triac is the word derived from
the combination of the words Triode and A.c. In.
OPTO COUPLER:
The combination of a miniature light source and a
photo conductor in the same package is a family of
devices is commonly known as opto isolators. These
are available with virtually every possible
combination of light source and a photoconductor, as
well as combination with integrated circuit amplifier
in detector output. The main application of opto
isolator involves electronic circuit isolation, where the
isolator eliminated common ground connection which
prevents ground loops and substantially reduces
common mode noise.
less maintenance. Unlike the conventional methods it
offers smooth control rather than stepped control.
INTERFACING
The A/D converter is used to convert analog signals
to digital quantity. The digital output is fed to
microprocessor for process data. The most popular
method of analog to digital converter is successive
approximation. The clock frequency required for
A/D conversion is lies in the range of 50 kHz to 800
kHz. MICROPROCESSOR:
The microprocessor is the central processing unit of a
microcomputer. It is the heart of the microcomputer
Intel 8086 is a 16 bit microprocessor. It is a 40 pin I.C
it uses a 5Vd.c supply for its operation 8086 uses
twenty line address bus. It can directly address up to 2
power 20 bytes of address .It uses 16 line data bus 16
bit data word is subdivided into a low order byte and
a higher order byte. The 20 lines of address bus lines
are multiplexed with data and 4 high order address
bus lines are multiplexed with status signals..
The main difference of comparison between
microprocessor and micro controller can be
highlighted from the following fact that most
microprocessors have many operational codes for
moving data from external memory to CPU, where as
the micro controller may have one or two,
microprocessors may have one or two
types of bit handling instructions and
micro controllers will have many.
HARDWARE AND SOFTWARE The hardware
consists of micro Processor, Traic,A/D converter,
opto coupler, Transformer ,bridge rectifier, Zero
crossing detector and inductive load.
RESULTS & CONCLUSIONS
The Hard ware of the circuit has been tested .The
simulated results have been compared.
CALUCLATION OF FIRING PULSES
The microprocessor based reactive power control
scheme has more flexibility than the other
conventional compensating schemes due its
programmable approach.In the same manner
compensation can be done for different loads Hence it
can be concluded that the microprocessor based
compensating scheme is inevitable due to moderate
cost, high efficiency, high stability, high speed and
REFERENCES BOOKS
[1] Modern Power System Analysis by I.J.Nagrath &
G.K.Kothari
[2] An Introduction to 8086 Micro Processor by
Douglas V Hall