microprocessor based system of automatic synchronizer report
Post on 22-Oct-2014
98 Views
Preview:
TRANSCRIPT
1
ABSTRACT
The manual method of synchronization demands a
skilled operator and the method is suitable for no load operation or normal
frequency condition. under emergency condition such as lowering of
frequency or synchronizing of large machines a very fast action is needed,
which may not be possible for a human operator. Thus there is a need of
autosynchroniser in a power station or in an industrial establishment where
generator are employed. This paper describes a microprocessor based set up
for synchronizing a three phase alternator to a busbar. Also existing methods
of synchronization are mentioned.
2
1. INTRODUCTION
It is well known that electrical load on a power system or
an industrial establishment, is never constant but it varies. To meet the
requirement of variable load , economically and also for assuring continuity
of supply the number of generating units connected to a system busbar are
varied suitably . The connection of an incoming alternator to system bus, ie;
synchronization requires fulfillment of the condition like the same phase
sequence equality of voltages and frequency between the incoming machine
and frequency between the in coming machine and busbar. In order to order
to overcome the 9 technical drawbacks of the conventional synchronization
methods we can introduce a microprocessor based system.
3
2. EXISTING METHODS OF SYNCHRONIZATION AND PRINCIPLE
a Synchronizing Lamp
The operation of connecting an alternator parallel with
another alternator or with a common busbar is known as synchronizing for
proper synchronization of alternators the following three conditions must be
satisfied
1.The terminal voltage of incoming machine must be the same as the busbar
voltage.
2.the speed of the incoming machine must be same such that the frequency
is equal to the busbar frequency.
3. The phase of the alternator voltage must be identical to the busbar voltage.
It means that the switch must be closed at the instant the two
voltages are in correct phase.
Condition 1 can be checked with the help of voltmeter, frequency
is adjusted by varying the prime mover speed. In the dark lamp method the
lamps are connected across the alternator and busbar terminal. If the phase
sequence is different, the lamps will brighten in a cyclic manner correct
phase sequence is indicated by simultaneous darkening brightening of
lamps. The switch is closed in the middle of the dark period.once
synchronized properly, the two alternators continues to run in synchronism.
4
b sychroscope
The armature of the sychroscope will align itself so that the
axis of windings are R and F are inclined at an angle equal to phase
displacement between V and V’. If there any difference between the
frequencies of V and V’ a pointer attached to the armature shaft will rotate at
slip speed, and the direction of of its rotation will indicate whether the
incoming machine is running above or below synchronism. At synchronism,
the pointer will remain stationary, but it must be brought to the particular
position which indicates zero phase displacement between V and V’ before
the main switch of the incoming generator is closed.
3 AUTOMATIC SYCHRONIZATION
Synchronization by means of manually operated switching
served well enough when the individual generators were relatively small, but
with the growth of system capacity, it becomes necessary to use automatic
devices to ensure the closing of the main switch of the incoming machine at
the proper instant.
The scheme introduced here is for the complete automation of
synchronization i.e.; the adjustment of magnitude of voltage and frequency
of incoming alternator is done automatically. When all the requirements of
synchronization are satisfied, closing of the main switch of the incoming
machine is done by the automatic synchronizer
5
4 CRITERIA OF DESIGN
The auto synchronizer has been developed to carry out the
following tasks related to the synchronization such as
I To check if the phase sequence of incoming machine is correct
or otherwise, in case of wrong phase sequence, to terminate the further steps
in the process and also to indicate corrective action.
II To check if frequency of incoming machine is equal to that of
busbar and to adjust it to a value nearly equal to the busbar frequency.
III To check machine voltage is equal to that of busbar and to adjust
it to a value nearly equal to the busbar voltage and
IV After ascertaining the fulfillment of the above condition, to give
closing signal to the circuit breaker so that the breaker will close the exact
inphase instant.
In addition, the auto synchronizer has been designed so that the
alternator is started with in minimum voltage and minimum frequency
conditions
6
5 HARDWARE DETAILS
The hardware has been designed to fulfill all the requirements
of the synchronizing process.
Block diagram of auto synchronizer setup is shown in fig (1)
A microprocessor trainer kit is used as a controller for the setup. Also the
figure showing the auto synchronizer setup consist of
a Frequency control unit
b Voltage control unit
c Potential transformer unit
d Signal conditioning card
e Display card and
f Circuit breaker with the switching circuit.
5.1 Frequency Controlling Unit
The frequency of an alternator can be changed by varying the
speed of the prime mover which is a DC shunt motor in this case .A rheostat
is provided in the field circuit of the motor for this purpose The frequency
controlling unit is a lead screw arrangement driven by a stepper motor
attached to the variable point on the rheostat the stepper motor (SM1) is
controlled by an 8085 microprocessor system through a driver circuit.
7
5.2 Voltage Controlling Unit
Once frequency of alternator is fixed ,or adjusted ,its voltage
is controlled by variation of excitation current. This excitation current is
varied by providing a rheostat in the field circuit of the alternator. The
automatic variation of excitation current is obtained by lead screw and
stepper motor(SM2) arrangement similar to the one used for frequency
control.
5.3 Potential Transformer Unit
This unit consist of a bank of four shell type
transformer(P.Ts).Fig.(2) shows the connection diagram. Out of the four
transformers thee are used for stepping down three phase voltages of
alternator and the remaining one is used for stepping down the voltage of the
phase R of the bus bar. The potential transformers connected to the phase
R of the bus bar and the phase R of the alternator are having two
secondaries. Hence one secondary is used for voltage measurement and the
other is used for frequency measurement .The potential transformers
connected to the Y and B phases have only one secondary each
5.3 Signal Conditioning Card
It is subdivided into (i) signal conditioning card and (ii) ADC subunit.
The signal conditioning subunit consists of for identical
circuits each of which comprises of a zero crossing detector (ZSD)(for
ralt,yalt,balt and rbus) two rectifier and filter circuits for ralt2 and rbus2 and
an inphase sequence detector and an inphase instant detector as shown in
fig.(1).
8
9
10
The ZSD converts sinusoidal output of potential transformer secondary to
rectangular signal Fig.3 shows the ZSD output waveforms these square
waves are fed to microprocessor system for measurement of frequency and
phase sequence detection using developed software
11
The rectifier and filter circuits converts the AC signal of ralt2 and
rbus2 to DC signal compatible for ADC 0809.These are used for the voltage
measurements of the alternator and the bus.
Inphase instant detector circuit is used for detecting the
inphase instant of signals ralt1and rbus1 which is the correct instant for
synchronization.
The ADC subunit consists of ADC0809 interfaced with 8085-
microprocessor system. The clock required for this ADC is derived from a
frequency divider circuit made up of three 7490 counter ICs. The clock
available on microprocessor kit of 1.7 MHz, which is divided by further
factors 5,10,10. Therefore out of three available outputs, 340KHz and
3.4KHz outputs are used respectively for the ADC 8255. The digital output
corresponding to the alternator and busbar voltages are obtained using
separate channels for alternator and busbar voltages
5.5 Display Card
Display card has been provided for indication of messages
during alternator synchronization process It uses four seven –segment LED
displays to represent the three inphase synchronization conditions and circuit
breaker position. Also the kit display is used for displaying messages such as
‘HALT’,’DONE’etc.
5.6 Circuit Breaker With Switching Circuit
The circuit breaker used as a synchronizing switch is in the
form of a direct on line starter .In order to operate the circuit breaker, its
12
operating coil is connected to 230 V d.c Supply through electromagnetic
relay. The relay is activated at proper instant by the microprocessor so that
the circuit breaker is closed at the correct inphase instant.
6 PROGRAM STRUCTURE
The main program performs the following functions.
1. Phase sequence detection
2. Alternator frequency measurement and its adjustment
3. Alternator voltage measurement and its adjustment, and
4. Synchronizing at zero phase difference condition
13
The following subroutines are developed and called in the main program
1 IN PHASE : The subroutines checks the in phase instant of Ralt
and Rbus
where Ralt refers to the phase R of the incoming
alternator and
Rbus1 refers that of the bus bar.
2 LSW : This subroutines checks if the limit switch is
closed or not
3 SM : Rotates the stepper motor either in clockwise or
anticlockwise
direction.
4 KCLOSE : Checks the closure of the key to be handled by the
operator
5 PSEQ : Checks the phase sequence of the alternator
6 FRQ : Measures the frequency of the alternator or bus
bar
7 VOLM : Measures the voltage of the alternator or bus bar
8 CMPHD : Compares the contents of HL register pair with the
contents of DE pair
9 SUBDH : Subtract the contents of DE pair from contents of
HL pair
10 In addition the following monitor subroutines are used whenever
required :
a. CRLF clears the display
b OUT MSG displays the given message on the display
c delay provides delay in the program
d DONE Displays the message ‘DONE’
14
Fig (4) shows flowchart of the main program for autosynchronising
setup. The status of the limit switches LS1and LS2 are checked .
These are provided with the field circuit rheostats of exciter and
driving motor. Accordingly the stepper motor are rotated in
appropriate directions to obtain initial positions respectively of field
rheostat (Rf) and exciter rheostat (Rex) . ht emessage ‘START’ is
displayed indicating operator to start the DC motor (prime mover).
When the operator sees the prompt, he switches ‘ON’ the DC motor
of the alternator. Once the alternator is started, it develops some
voltage at some frequency, following sequence of events will take
place automatically.
1 Detection of phase sequence
2 Frequency measurement and control
3 Voltage measurement and control
4 Synchronizing
15
16
17
18
8 DETECTION OF PHASE SEQUENCE
Before alternator is connected to the busbar first of all we have to
ensure that the phase sequence of the incoming alternator is the same as that
of the busbar. The program checks the ZCD outputs corresponding to Ralt
and Yalt phases for their low to high transitions and count corresponding to
time T1 as shown in fig (8) is obtained using subroutine ‘PSEQ’. Similarly
the ZCD outputs corresponding to Ralt and Balt are measured or checked for
their zero to one transition and count corresponding to time T2 is obtained.
To check the phase sequence ,T1 and T2 are compared . When T1 is greater
than T2, the phase sequence is not correct. This condition is indicated by ‘N’
and the display of message ‘HALT’ will be there and the program execution
19
is stopped on the other hand, if T1 is less than T2, the phase sequence is
‘OK’ or correct and is indicated by ‘O’. There after the program control is
transferred to frequency measurement and control part.
9 FREQUENCY MEASUREMENT AND CONTROL
The subroutine FRQ written for frequency measurement of bus 0or
alternator checks their respective ZCD outputs for low to high transitions
In software, the register HL(for busbar signal) or DE (for alternator
signal) initialized with zero components are incremented till the ZCD
outputs are in a high to low transition . This count in HL is equivalent to the
time period corresponding to the half cycle of alternator signal . The counts
obtained inHL and DE pairs are compared. If the count in HL is less than
that of DE , it indicates that alternator frequency is less than the busbar
frequency. The difference in frequency is checked and if the difference is
greater than allowed difference (0.1Hz), then the stepper motor (SM2) is
rotated to bring the difference with in the limit, and ‘FE’ is displayed when
this condition is achieved.
On the other hand, if the count in the HL pair is greater than that in
DE, alternator frequency is high and is indicated by ‘FH’. The stepper motor
(SM2) is rotated in reverse direction to bring the difference in frequency
within limit till ‘FE’ is displayed.
10 VOLTAGE MEASUREMENT AND CONTROL
The digital output corresponding to the alternator and bus voltages
are obtained by the following method. The busbar output and the incoming
alternator output are first stepped down in the same ratio using P.T unit .
20
These step-down transformer signals are fed to the rectifier and filter
circuits. The output from it is given to ADC through separate channels. ADC
output ie; the digital outputs are compared and the difference of these is
obtained. When the difference is less than the allowed difference,(1%) the
‘VE’ is displayed and the program execution is continued.
When the difference is greater than allowed difference, either
‘VH’or ‘VL’ is displayed to indicate high or low voltage of alternator
respectively. The stepper motor (SM1) is rotated in appropriate direction to
bring the difference with in the limit till ‘VE’ is displayed.
11 SYNCHRONIZING
After satisfying all these condition, the time (Ti) between
consecutive inphase instants of Rus and Ralt (obtained from inphase instant
detector ) is measured using 8253 in mode ‘0’. The time interval (Ti-To)
where T0 is operating time of switching circuits , is obtained.
The closure of circuit breaker is achieved by sensing next
inphase instant with delay of (Ti-T0) wich will enable to switch on the
circuit exactly at the next inphase instant.
12 RESULT The phase sequence has been checked by using developed prototype. When
phase sequence is R.Y.B the auto ‘synchroniser’ gives a prompt to the
operator by displaying ‘O’ (ie inphase sequence OK). For the improper
21
phase sequence, ie R.B.Y., the auto synchronizer displays ‘n’(NOT OK)and
‘Halt instruction gets executed to stop entire operation.
The frequency of incoming machine which depends on the speed of the
alternator, ie prime mover(dc shunt motor)is measured and adjusted to bring
the difference in frequency with in the tolerance limit.
To achieve the equality of voltages, the exciter voltage or circuit resistance
was adjusted by auto synchronizer. After obtaining proper phase sequence,
equality of frequency and voltage, the auto synchronizer has to carry out
synchronization
13 CONCLUSION
The microprocessor based system of automatic
synchronizer can be used more effectively compared to conventional
methods of synchronization such as dark lamp method, bright lamp
method and synchronization using synchronoscope this because of the
fact that the conventional, method calls for of the operator and accuracy
is less and it depends on the sense of correct judgment of the operator.
Moreover the microprocessor based alternator synchronizer is user
friendly and requires less maintenance. It also exploits the advantage of
superior performance of the microprocessor like accuracy speed and
reliability.
22
14BIBLIOGRAPHY
1 JOURNAL OF INSTITUTION OF ENGINEERS (INDIA)
VOLUME-80, NOVEMBER1999
2 THEORY OF ALTERNATING CURRENT AND
MACHINERY
ALEXANDER.S.LANGSDORF
3 FUNDAMENTALS OF MICROPROCESSORS AND
MICROCONTROLLERS B. RAM
top related