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    Seminar 2010 Automatic alternator synchronisation

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    S.B.C.E Department of EEE

    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 generators 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.

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    CONTENTS

    Page no:

    1. INTRODUCTION 32. EXISTING METHODS OF SYNCHRONISATION 43. CRITERIA OF DESIGN 64. HARDWARE DETAILS 75. PROGRAM STRUCTURE 126. FLOWCHART 147. SYNCHRONISING 218. ADVANTAGES 229. RESULT 2310. CONCLUSION 2411. REFERENCE 25

    INTRODUCTION

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    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.

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    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 busbarvoltage.

    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.

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    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 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.

    AUTOMATIC SYCHRONI SATI ON

    Synchronisation 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

    synchronisation are satisfied, closing of the main switch of the incoming machine is

    done by the automatic synchronizer

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    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 theprocess 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

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    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 carde Display card and

    f Circuit breaker with the switching circuit.

    1 Frequency Controll ing 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 inthe 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.

    2 Voltage Controll ing Uni t

    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

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    excitation current is obtained by lead screw and stepper motor (SM2) arrangement

    similar to the one used for frequency control.

    3 Potential Transformer Unit

    This unit consists of a bank of four shell type transformer (P.Ts). 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

    4 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).

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    BLOCK DIAGRAM OF MICROPROCESSOR BASED

    ALTERNATOR SYNCHRONISATION

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    ZCD OUTPUT WAVEFORM

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    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

    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, 340 KHz and 3.4 KHz 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 Display Card

    Display card has been provided for indication of messages during

    alternator synchronization process it uses four sevensegment 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,DONEetc.

    6 Cir cuit Breaker With Switching Cir cui t

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    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 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.

    PROGRAM STRUCTURE

    The main program performs the following functions.1. Phase sequence detection2. Alternator frequency measurement and its adjustment3. Alternator voltage measurement and its adjustment, and4. Synchronizing at zero phase difference condition

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    The following subroutines are developed and called in the main program

    1 IN PHASE : The subroutine checks the in phase instant of Ralt and Rbus

    Where Ralt refers to the phase R of the incoming alternator

    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 in either direction

    4 KCLOSE : Checks the closure of the key 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 and DE register 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

    Fig (4) shows flowchart of the main program for autosynchronising setup.

    The status of the limit switches LS1and LS2 are checked. These are providedwith the field circuit rheostats of exciter and driving motor. Accordingly the

    stepper motors 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 sequence2.Frequency measurement and control

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    3.Voltage measurement and control4.Synchronizing

    FLOWCHART

    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

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    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 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.

    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) isrotated in reverse direction to bring the difference in frequency within limit till FE

    is displayed.

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    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. 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 alloweddifference,(1%) the VE is displayed and the program execution is continued.

    When the difference is greater than allowed difference, either VHor 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.

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    SYNCHRONISING

    After satisfying all these condition, the time (Ti) between consecutive inphase

    instants of Rbus 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) which will enable to switch on the circuit exactly at the next inphase

    instant.

    ADVANTAGES OF MICROPROCESSOR

    BASED ALTERNATOR SYNCHRONISATION

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    1. Anticipatory close signal provides smooth synchronizing with minimum

    system impact.

    2. Patented real-time adaptive proportional speed control algorithm provides

    fast, reliable frequency matching while eliminating overshoots and

    hunting. Includes smart target pulse to prevent hung scope condition.

    (Patent #5,761,073)

    3. Highly flexible design can be configured for optimum performance over

    a wide range of system characteristics from sensitive, asymmetrical low

    inertia to high inertia hydro systems.

    4. One unit can control multiple systems with up to six different sets of

    breaker closing parameters.

    5. Test module facilitates testing via front panel terminals.

    6. Standard 19 inch rack-mounted case with front cover.

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    RESULT

    The phase sequence has been checked by using developed prototype. When phase

    sequence is R.Y.B the auto synchronizer gives a prompt to the operator by

    displaying O (ie inphase sequence OK). For the improper 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

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    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 sychroscope 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.

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    REFERENCES

    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