PC based tap changing system-A case study
Rupali Vibhute" ,R.M.Holmukhe2 Mr.P.S. Chaudhari", Mr.T.S.Hasarmani4
1.4post Graduate Student, Electrical Engineering Department,
2Assistant Professor, Electrical Engineering Department,
Bharati Vidyapeeth Deemed University,
College of Engineering, Pune, India
3Scientist ,DRDO, Pune
2rajeshmholmukhe@hotmail.com
Abstract: Tap changers have not changed radically
since their invention in the early part of this century.
They are slow in operation, typically traversing
19
taps in around 100 seconds and require frequent
maintenance.
A new TRIAC assisted tap changer is introduced
here which addresses
the problem
of previous
schemes. It eliminates excessive conduction losses,
which are inherent in solid-state tap changers and at
the same time provides a fast response speed by
means of fast acting vacuum switches.
Calculations show that it will be possible to traverse
a 19-tap range in around 0.5s. a unit for a 240 MVA
transformer
will require minimum maintenance
as
the vacuum switches used in the selector diverter
never or braje more than few amperes.
I. INTRODUCTION
1.1
Need oftap changing system
The excitation control method is satisfactory only for
relatively short lines. However, it is not suitable for long
lines as the voltage at the alternator terminals will have
to be varied too much in order that the voltage at the far
end of the line may be constant. Under such situations,
the problem of voltage control can be solved by
employing other methods. One important method is to
use tap changing transformer
and is commonly
employed where main transformer is necessary. In this
method, numbers of tapping are provided on the
secondary of transformer. The voltage drop in the line is
supplied by changing the secondary e.m.f. of the
transformer through the adjustment of its number of
turns.
Taps are usually provided on the light voltage winding to
enable a fire control of voltage; generally the tap
changing is done only when the transformer is in the deenergized state. However in some cases tap changing is
also possible when transformer is energized and such
transformers are known as tap changing under load or
"On load tap changing transformer".
1.2
Types of tap changing transformer
1. Off load tap changing transformer.
2. On load tap changing transformer.
1.2.1
Off load tap changing transformer
In this method of tap changing tapping are provided on
the secondary. As the position of tap is varied, the
effective number of secondary turns is varied and hence
the output voltage of the secondary can be changed.
When the movable arm makes contact with first stud
then secondary voltage is minimum and when it makes
contact with last stud the secondary voltage is
maximum. During the period of light load, the voltage
across the primary is not much below the alternator
voltage and the movable arm is placed on first stud.
When the load increase the voltage across the primary
drops but secondary voltage can be kept at previous
value by placing the movable arm on higher stud.
Whenever tapping's are to be changed in this type of
transformer, the load is kept off and hence the named
"Off load tap-changing transformer."
Off load designs
In low power, low voltage transformer, the point can take
the form of a connection terminal, requiring a power lead
to be disconnected by had and connected to the new
terminal. Alternatively, the process may be assisted by
means of rotary or slider switch. Because the different
tap points are at different voltages, the two connections
should not be made simultaneously, as this short circuit
number of turns in the winding and would result in an
excessive circulating current. This therefore demands
that the power to the load be physically interrupted
during the switch over time. Off load tap changing is also
employed in high voltage transformer design, though it is
only applicable to installations in which loss of supply
can be tolerated.
1.2.2
On load tap changing transformer
The on load tap changer (OL TC) is used the tapping's
connection
of the transformer
winding while the
transformer is energized. The tap changer can be
designed as a single unit for a single and three
applications with one common neutral point. Depending
on the three phase rating, it might require three separate
units, each having its own insulated phases. Tap
changers can be located either inside the transformer,
main tank or outside in its own compartment. Switching
from one position to another has to be performed
through an impedance can be either resistor or reactor.
In supply system tap changing has normally to be
performed on load so that there is no interruption to
supply. The secondary consist of two equal parallel
winding which have similar tapping. Under normal
working condition each secondary winding carried one
half of total current. This way tapping position is changed
without interrupting the supply this method has the
following disadvantages.
1. During switching the impedance is increased
and there will be a voltage surge.
2. There are twice as many tapping as the
voltage steps.
A transformer tap is connection point a transformer winding
that allows the number of turns to be selected. By this
means, a transformer with a variable turn's ratio is
produced, enabling voltage regulation of the secondary
side. Selection of the tap in use is made via a tap
changer mechanism.
Voltage considerations
Tap points are usually made on the high voltage, or low
current, side of the winding in order to minimize the
current handling requirements of the contacts. To
minimize the number of windings and thus reduce the
physical size of a transformer, use may be made of a
'reversing' winding, which is a portion of the main
winding able to be connected in its opposite direction
and thus oppose the voltage. Insulation requirements
place the tap points at the low voltage end of the
winding. This is near the star point in a star connected
winding. In delta connected windings, the tapping's are
usually at the centre
of the winding.
In an
autotransformer, the taps are usually made between the
series and common windings, or as a series 'buck-boost'
section of the common winding.
A mechanical on load tap changer design, changing
back and front between tap positions 2 and 3. Because
interrupting the supply is usually unacceptable for a
power transformer, there are often fitted with a more
expensive
and compels
on load tap changing
mechanism. On load tap changers may be generally
classified as mechanical, or as electronic, which in turn
may be either assisted or solid state.
1.2.3 Mechanical tap changers
A mechanical tap changer physically makes the new
connection before the old, but avoids the high current
from the short-circuited turns by temporarily placing a
large diverter resistor (sometimes an inductor) in series
with the short -circuited turns before breaking the
original connection. This technique overcomes the
problems with open or short circuit taps. The changeover
nevertheless must be made rapidly to avoid overheating
of the diverter. Powerful springs are wound up, usually
by a low power motor, and then rapidly released to affect
the tap changing operation. To avoid arcing at the
contacts, the tap changer is filled with insulating
transformer oil. Tapping normally takes place in separate
compartment to the main transformer tank to prevent
contamination of its oil.
One possible design of on-load mechanical tap changer
is shown to the right. It commences operation at tap
position 2, with load supplied directly via the right hand
connection. Diverter resistor A is short circuited; diverter
B is unused.
In moving to tap 3, the following sequence occurs:
1. Switch 3 closes, an off -load operation.
2. Rotary
switch
turns,
breaking
one
connection
and supplying
load current
through diverter resistor A.
3. Rotary switch continues to turn, connecting
between contacts A and B. Load now
supplied via diverter resistors A and B,
winding turns bridged via A and B.
4. Rotary switch continues to turn, breaking
contact with diverter A. Load now supplied
via diverter B alone, winding turns no longer
bridged.
5. Rotary switch continues to turn, shorting
diverter B. Load now supplied directly via left
hand connection. Diverter A is unused.
6. Switch 2 opens, an off load operation.
The sequence is then carried out in reserve to return to
tap position 2.
1.2.4 Thyristor assisted tap changers
Thyristor assisted tap changers use thyristors to take the
on load current whilst the main contacts change over
from one tap to the next. This prevents arcing on the
main contacts and can lead to a longer service life
between maintenance activities. The disadvantages is
that these tap changers are more complex and require a
low voltage power supply for the thyristor circuitry. They
also can be more costly.
1.2.5 Solid state (thyristor) tap changers
These are relatively recent developments which use
thyristors both to switch the load current and to pass the
load current in the steady state. Their disadvantage is
that all of the non conducting thyristors connected to the
unselected taps still dissipate power due to their leakage
current. This power can add up to a few kilowatts which
has to be removed as heat and leads to a reduction in
the overall efficiency of the transformer. They are
therefore only employed on smaller power transformers.
1.3
Block diagram:
Internal block diagram of the pc based tap changer
consist of:
1. Voltage sensing unit
2. Main transformer unit
3. Zero crossing detector
4. Reference and logic generation
5. Relay / Static Switched, isolation ~nd dri.ver
6. Add-on card interface chip decoding logic
7. Pc
8. Load
Voltage sensing unit:
In this unit a 230 V ac supply is given as input and
regulated too 12V supply and it is given to. voltage
sensing unit which comprises of two amplifiers I.n
differential mode and it is given to reference and logic
generation unit and to the main transformer.
Main transformer unit:
It is most important part of tap changing system. Its
rating is 500W but it can withstand lo~d up to.1 KW.
Tapping's
are provided on the primary side of
transformer so that we can get constant voltage
irrespective of changes in load. Output of main
transformer is given to load. It gets input from voltag.e
sensing unit and feedback is given to relay/static
switches unit.
Zero crossing detectors:ZCD gets input from a main that is 230 V ac supply. It
detects the zero point of ac sine wave because at the
point both current and voltage is zero (in case of
resistive load) Its output is given to the ADDON card.
Reference and logic generation:This unit gets input from the voltage sensing unit. It
consists of circuitry which generates binary logic as (00,
01, 10, 11). This binary signal is given to the ADDON
card, which is then feed to the computer.
Relay/Static Switches, isolation and driver:This unit consists of relays and static switches such as
TRIAC. TRIAC is a bidirectional device that means it can
conduct in both directions. Isolation is provided to reduce
the unwanted noise signal. This unit is connected with
the tapping of main transformer. This unit is also
connected with the ADDON card.
ADDON card interface chip decoding logic:ADDON card plays very important role in connecting our
hardware with software. It consist of four ic's namely
magnitude comparator, buffer, peripheral interface 8~55
ic, AND gate ic. This unit gets signals from zero crossing
detector reference and logic generation. It gives signal
to the r~lay and static switches. This ADDON card is
inserted in the PC.
PC:It is any personal computer having ISA slot in it, so th~t
we can insert ADDON card in that computer generally It
is P1 or P2 pc.
Load:- It may be any load bank, from where we can very
the load.
II. MODELLING AND DEVELOPMENT
2.1 Wiring diagram:
.
Wiring diagram provides assembly of all of tap c~an~lng
system. In our wiring diagram a 230 V ac supply ISgiven
to a step down transformer whose secondary rating is .12
V. This secondary voltage is applied to Voltage sensing
unit.
In Voltage sensing unit the change in Voltage rating
depends on tap changer. The Voltaqe sen~ing .u~it
provides negative or positive error signal. This u.nlt IS
connected to port A of 8255 supply voltage to primary
side of load tap changing transformer, tapping are also
provided on this side of transformer. Secondary. si~e of
load tap changing transformers connected to resistive to
resistive load.
Lower tap of transformer is connected to NO ~f rel~y
land its Pole is connected to NO of relay 3. The higher IS
connected to NO of relay 2 with its pole to NO of relay 4.
NO of triac 3 is connected to NO of relay of triac 4. The
common terminals of two triacs are connected to primary
side of current transformer.
The main use of triac is that when triac is on secondary
is fully loaded at the same time primary ~ide have the
minimum opposition to flow of current which decreases
amount of current flow at the time of tap changing.
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.
......."
NO
NO
VA 0S1
P
OSlVB
P
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CHANGING SV-STEM
III. SYSTEM ANALYSIS
3.1 Digital input - output ADDON card:As we are aware that PC has a motherboard on which
the main CPU and the RAM/ROM memory chips are
located. All other functionalities like disk controllers,
parallel and series ports and the display adapter are
located on the ADDON cards which plug into the
expansion slots on the motherboard. This general
purpose ADDON card is useful in many applications like
EPROM programmer and copier, process controller,
logic monitor etc. It consists of four ics namely:
1. 74HCT08 Quad 2 - input AND gate
2. 74HCT688 Magnitude comparator
3. 74HCT245 8-bit bidirectional bus driver
4.
8255A
programmable
peripheral
interface.
For designing this card we need a PCB edge connector
with 62 pins which are arranged as two rows of 31 pins
on each side of expansion slot, named side A and side
B. The circuit is based on IC8255 which is a
programmable digital I/O (input/output) device with three
external 8-bit ports. That means we get 24 general
purpose input and / or output pins.
As shown in the circuit diagram A 1 to A8 bits of the IC3
are connected with the 07 to DO bits of the A side of
PCB edge connector pin no. 20 is connected to VCC
and pin no. 10 to ground. Chip select of IC2, IC3 and IC4
is connected together. B1 to B8 bits of IC3 are given to
07 to DO bit of IC4. Two AND gates are used (N1, N2)
and connected to the IC2.
IC2 (74HCT688) is an 8 bit magnitude comparator. The
address bus lines A2 through A9 are monitored by it,
and compared with the address set by the resistor and
the DIP switches on the of IC2. The AEN signal is also
invovled in the address selection as can seen from
connection to pin no 12 of IC2. This is done in order to
disable our circuit during DMA operations. When the two
address (the one on the address bus of IBM pc as
extended to left side of IC2 and the other set with the
help of Dip switches on the right hand side of IC2)
match, IC2 issues an active low chip select signal to IC3
and IC4.
Here was we have specified an address of 300 hex
which is a vacant slot on the IBM pc. This address has
been reserved by IBM for experimental work. If you want
to change this address be sure that no other device with
the intended address exists on the pc, to avoid clash .
The bi-directional bus transceiver IC3 (74HCLT245)
connects the data bus of PC to the data lines of IC4
(8255A). The direction in which data is transmitted is
determined by the DIR pin no. 1 of the transceiver IC3.
This pin is controlled by active low READ signal. This
ensures proper direction data flow.
IC4 8255A is versatile clip which can be programmed in
three modes. Mode 0 is a basic input - output mode.
Mode 1 and 2 involve advanced I/O operations. The clip
has three 8-bit ports named A,B and C and a control port
(internal) which determine how the clip is programmed.
The two pins marked AO and A 1 (pin 9 and 8
respectively) determine which of these four ports are
accessed. This is why the two least significant address
bits of the Pc bus are directly connected to these pins.
With our address assignment of 300 (hex) as base
address, the addresses of these 4 ports will be given
below:
Address (hex)
300
301
302
303
Port accessed
portA
port B
port C
control
The three 8-bit ports A, Band C together with +5V and
ground connection are brought out on a 26-pin FRC type
male connector for external use. Incidentally, this circuit
does not need an external power supply. A positive 5 Volt supply is readily available on pins B3 and B29 of the
62-pin PC edge connectors itself. This is to be
connected to the Vcc pins of all the four ICs. Similarly
the ground pins of all four ICs are tied to the common
ground point of the PC edge connector (B1, B10, B31).
ADD ON
CARD
,
!
:
i
ADDON
CARD (8255)
3.2
8255:
The 8255 Programmable Interface (PPI) is a
very popular and versatile input/output chip that is easily
configured to function in several different configurations.
The 8255 is used on several of our range of cards that
plug into an available slot in your IBM PC. This clip
allows you to do both digital input and output (010) with
your PC. For example, you may want to have your PC
turn on a switch, or have a switch electronically activate
your PC to execute a program. Each 8255 has 3 off 8 bit TTL compatible I/O ports which will allows the control
of up to 24 individual outputs or to read 24 individual
inputs, or indeed a mixture of both input or output. For
example, you could attach this to a robotic device to
control movement by use of motors to control motion
and switches to detect position etc.
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Ci--~
Our range of cards plug into any available 8 or 16 - bit
slot (also known as an AT or ISA slot) on your PC's
motherboard, just like a sound card or disk drive
controller card does. Your CPU (Central Processing
Unit) communicates with cards by knowing the card's
address and sending data to it. By physically using
jumpers on the card, we can assign a set of addresses
to the card, then in software, we can tell the CPU what
these addresses
are (more about this in the
Programming Section).
Table 2: DC-0600 Address
Option I: default Option (JP2 Open)
(JP2 Linked)
8255 Port
Address (Hex Dee) Address (Hex dee)
Port l A
300 H (768)
360 H (864)
Port IB
301 H (769)
361 H (865)
Port l C
302 H (770)
362 H (866)
Port I Control 303 H (771)
363 H (867)
Reg.
Port 2A
304 H (772)
364 H (868)
Port 2B
305 H (773)
365 H (869)
Port 2C
306 H (774)
366 H (870)
Port 2 Control 307 H (775)
367 H (871)
Reg.
3.2.1
8255 Configuration
The first thing that must be done, before the chip can be
used, is to tell it which configuration is required. The
configuration tells the 8255 whether ports are input or
output and even some strange arrangements called bidirectional and strobed, but these 'funny' modges go a
little beyond the scope of this tutorial. The 8255 allows
for three distinct operating modes (Modes 0, 1 and 2) as
follows:
•
Mode 0: Ports A and B operate as either inputs or
outputs and Port C is divided into two 4-bit groups
either of which can be operated as inputs or
outputs.
•
Mode 1: Same as Mode 0 but Port C is used for
handshaking and control
•
Mode 2: Port A is bidirectional (both input and
output) and port C is used for handshaking. Port B
is not used.
For most applications using this range of cards Mode 0
will be used.
Each of the 3 ports has 8 bits, each of these bits can be
individually set on or off, its a bit like having 3 banks of
8 light switches. These bits are configured in groups to
be inputs or outputs allowing their function to either
read data into the computer or control data out of the
computer. The various modes can be set by sending a
value to the control port. The control port is Base
Address + 3 (i.e. 768 + 3 = 771 Decimal). The table
below shows the different arrangements that can be
configured
and the values to be sent to the
configuration port.
Table 3 : 8255 Control Re
Control
Word
(Hex(Dee)
80H (128)
82H(l30)
85H(133)
87H(l35)
88H(l36)
8AH(138)
8CH(l40)
8FH(l43)
ister Conti uration (Mode 0:)
Port A
Port B
Port C
OUT
OUT
OUT
OUT
IN
IN
IN
IN
OUT
IN
OUT
IN
OUT
IN
OUT
IN
OUT
OUT
IN
IN
OUT
OUT
IN
IN
As mentioned the control port is Base Address + 3. Port
A is always at Base Address; Port B is Base Address +
1; Port C is Base Address +2. Thus in our example
Ports A, Band C are at 768, 769 and 770 (Decimal)
respectively. By writing say, 128 to the Control port will
then configure the 8255 to have all three Port set for
output. This can be done using Quick Basic's OUT
Statement, for example:
3.3 Buffer:
This class of amplifier is designed to follow low level
stages; one example is a crystal oscillator. An oscillator
for optimum performance can NOT be loaded down, it
needs an intermediate stage following. This will then
present a sufficiently high enough input impedance so it
is not considered a significant load to the oscillator. The
intermediate or buffer stage, while not representing a
load must then have a sufficiently low output impedance
to drive successive stage.
If these concepts of impedance confuse or worry you
then look at my other page it's a rough and ready
explanation, followed by a more technical explanation of
impedance matching but hopeful you will come back
with a considerably better understanding.
From what I said above, two described traits of a buffer
amplifier are high input impedance and low output
impedance. Obliviously succeeding stages should also
be linear because we don't want to introduce distortion.
As a matter of interest a buffer amplifier is not solely
limited to following an oscillator. You could have a
requirement of wanting to sample the output of the first
mixer for what is called a "Pandaptor", a means of
visually seeing adjacent signals on an oscilloscope.
Also a high quality agc system should be derived from
the last Lf. stage, it should be buffered before going to
the agc amplifier and then on to the agc detector.
Other examples are low level outputs of audio amplifier
stages such as microphone
inputs where the
microphone is a high impedance type/ Although not so
common now, phono inputs to audio am pliers alsc
needed buffering.
IV. ADVANTAGES
Advantage of pc based tap changing transformer.
1. These are easily checked and reset by the
operator from the front panel or through digital
communication.
2. Contractor life is more due to absence of
resister type OL TC.
3. Voltage Fluctuation is minimum during a tap
change.
4. Low operating temperature.
5. True "current zero crossing" switches.
6. High over load capacity
7. Operation and coordinated control of fixed and
variable speed wind.
8. This technology is accurate and very efficient.
9. On the following phase current zero relay A is
turned on and relay C is turned off.
10. It eliminates excessive conduction losses.
11. In this design scheme inter tap circulating
current does not flow not during a tap change
thus it is possible to make multiple tap.
12. An approximate solution to lower tap changing
or capacitor times in reactive power control in
distribution system.
13. Voltage
control
integrated
in distribution
management system.
14. Tap changing transformer
role in voltage
stability.
15. Avoiding sustained oscillation in power system
with tap changing transformer.
16. Combined use of tap changing transformer and
static VAR compensation for enhancement of
steady state voltage stability.
17. Voltage regulations via automatic load tap
changing transformer
evolution of voltage
stability condition.
18. A mixed integer program for optimal static
capacitor system.
19. Highest winding current and highest, winding
temperature can also read through the digital
and analog output.
20. To control the timing of relay operation, each
enhanced alarm includes settable pickup and
dropout delay This feature reduces momentary
equipment activation or false alarm due to
transient conduction.
21. The relay as operated sensor on fault condition.
22. The software is written in C and is designed for
PC use, the advantage of using Care
•
Direct Oil Temperature
•
Simulated Winding Temperature
•
Calculated Winding Temperature(CT Models)
•
LTC Temperature Difference (LTC Models)
•
Single, Dual and Three RTD Units
•
Multi-Stage Fan/Pump Control
•
Analog and Digital Inputs
•
Weatherproof NEMA4X Metal Case
•
SCADA Ready - Analog, Digital, DNP3
V. DISADVANTAGES
Disadvantage of pc base tap changing transformer:
1. Increased computerization of the Substation
costs.
2. Pc base limitation which signals analog or
digital
3. A disadvantage of the binary coded method is
section describes a populated diversity.
4. Voltage control simulated annealing that it
Borland c+= and implemented on pc Pentium
computer.
5. Normally does not provided isolation has
moving part.
6. Normally do not provide short circuit protection
slower response time
7.
rr
~
It is slower produces arching each time the tap
setting
is changed
and
need
regular
maintenance.
8. A pc based man machine interface work station
was installed There are large space required.
9. In spite of the operating the inductance to
observe the change in voltage during transient
the we can use a steady the analysis based on
the system.
10. The software is written in C is designed for pc
use calculation method.
11. High capital cost.
VI. COMPARISON BETWEEN RESISTIVE TYPE
ONLOAD TAP CHANGER AND NEW PC BASED TAP
CHANGER
Resistor type OLTC
PC Based OL TC
1) Mechanical
operated
Here mechanical operated
mechanism is required to mechanism is not required
change taps many time to change taps. Taps get
such system may fail
changed automatically with
help of programmed
in
computer.
2) It uses solid state Here fast acting switches
switches
therefore
fast are used due to which a
response speed cannot be fast response speed is
obtained.
obtained.
3)
It
has
excessive
It eliminates
excessive
conduction losses
conduction losses
4) They are slow in They are fast in operation.
operation
5)
here
frequent
In this there is no need of
maintenance is required
frequent maintenance
6) In this solid state There
is no need as
swithes are permanently
protection as solid state
connected in a circuit they switches are not used.
require
protection
from
high voltage surges.
7) In this full scale tap
In this full scale tap
changing
can
not
be changing is obtained
obtained
8) Here sparking may take Here sparking does not
place at time of tap take place at time of tap
chanqinq
changing
9) Here deterioration
as Here contactor has long
contact
take
place life as there is no detritions
therefore contactor life is to contact
less
CONCLUSION
The operating of new scheme has been shown to work
and tap changer combination. The tap changer has a
fast response speed. Since the inter tap circulating
current is small, so multiple tap change are feasible. A
pc base operated mechanism is required to change the
taps many times such system more accurate as
compare to mechanical operated.
On the following phase current zero relay A is turned on
and relay C is off. So to minimize switching losses. It
eliminates excessive conduction losses. In this way, loss
of transformer due to tap changer failure will be avoided
A patent is pending upon the techniques out lined above.
Additional analogI digital inputs are available on the
enchanted version of any transformer advantage these
inputs handle AC or DC from 5 to 300 V.
REFERENCES
1) http://en.wikipedia.org
2) "Electronics for you" January 99 edition
3) IEEE paper presented by R. Shuttle worth, X. Tian, C. Fan, A.
Power on
New tap changing Scheme.
4) Paper published by Douglas M. Getson on "On load tap changer"