/ ABSTRACT

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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. 1

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. 2

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. 3

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 4

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 5

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. 6

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 7

8 an inphase sequence detector and an inphase instant detector as shown in fig. (1). 8

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11 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

12 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 8085microprocessor 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 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 12

13 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. 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

14 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 14

15 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 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

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19 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

20 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 20

21 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 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 21

22 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 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

23 14BIBLIOGRAPHY 1 JOURNAL OF INSTITUTION OF ENGINEERS (INDIA) VOLUME-80, NOVEMBER THEORY OF ALTERNATING CURRENT AND MACHINERY ALEXANDER.S.LANGSDORF 3 FUNDAMENTALS OF MICROPROCESSORS AND MICROCONTROLLERS B. RAM 23