CA1159899A - Automatic device for synchronization of prime mover with electrical grid - Google Patents

Abstract

AUTOMATIC DEVICE FOR SYNCHRONIZATION
OF PRIME MOVER WITH ELECTRICAL GRID
ABSTRACT OF THE DISCLOSURE
A control circuit is disclosed for synchronizing a prime mover or electrical generator with an electrical grid or power line such that when the prime mover is connected to the grid;
a) the instantaneous phase and slip are within allowable limits;
b) the phase and slip expected at breaker closure, as contrasted to the instantaneous measured phase and slip, will be within given trajectory, i.e when they are closest to their most desired values during the given transition. The circuit includes a clock for measuring the phase difference between the power line voltage and the generator voltage and means for calculating the rate of change of the phase difference which is a measure of difference of the respective frequencies known as the slip. The trends of phase and slip are then extrapolated by the amount of time it takes the breaker to close and a command for breaker closure is issued when the above three conditions are simultaneously satisfied.
speed matching can be caused by having the circuit produce a signal to raise generator speed when the slip is less than a given lower limit and by producing a signal to reduce generator speed if the slip exceeds a set upper limit.

Description

8 9 ~

- 1 - 51 DV 2~83 AVTOMATIC DEVICE FOR SYNCHRONIZATION
OF PRIME MOVER WIT~ ELECTRICAL G~ID
This invention relates to synchronization circuits for synchronizing an electrical generator with an electrical power system such that the generator can be connected into the electrical system with the phases and 5 frequencies of the generator and line matched as well as possible.
It is well known that if the phases and frequen-cies of a generator and a power line are not closely matched at the time a breaker closes to connect the 10 generator to the electrical grid or line, very large transfers of energy may occur between the electrical grid and the generator. The resulting high electric current may damage the stator windings of the generator. Moreover, the accompanying transient torque on the generator shaft 15 may reach values up to twenty times the design torque and could cause failure of the shaft. It is therefore necessary that the generator and line be synchronized before the generator is connected to the line and that the synchronizer be extremely accurate and fast.

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1 ~9~99 ~I DV2~83

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The above problem is well recognized and is described, for example, in Vnited Sttes Letters Patent 3,801,796, patented Apr;1 2, 1~74 in the nome of Charles E. Konrad, The synchronization scheme described in the above patent senses differences in frequency and phase 5 of a power generator and the line to which it is to be connected and produces control signals for changing the speed of the power generator until the frequency and phase full within defined tolerable limits Thereafter, a control signal is produced to effect the connection of the power generator to the operating power line. This arrangement works 10 very well but does not consistently assure a very small phase difference, preferably less than about 10 electrical degrees, between the line voltage and generator voltage at the instant the breaker closes fo connect the generator to the power line, Another prior art scheme which discloses the synchronization 15 between a generator and a power system is that contained in United States Letters Patent 3,892,978, patented July 1, 1975 in the name of Paul H. Haley. That patenf discloses a control system for synchronizing a gas turbine driven generator to an external power line and controls the fuel flow to the generator to obtain synchronization The system 20 determines the trajectory of turbine velocity and angular position so that synchronous speed and permissible phase angle difference reauired for synchronizotion are simultaneously obtained, The system of the afore-mentioned patent, however, will not ensure phase differences between line voltage and generator voltage at the instant of breaker closure 25 which are very small and preferably less than about 10 electrical degre~s.
BRIEF SUMMARY OF THE PRESENT INVENTION
In accordance with the present invention, a control system is provided wherein the phase difference between line voltaye and generator 1 159~953 voltage i5 exceptionlly small and preferably less than abouf 10 electrical degrees. At the instant of rlosing~ the frequency of the generator is close to and preferably slightly higher than that of the line frequency.
The present invention takes intc~ a~count the mechanica5 inertio of the circuit breaker used to electr;cally connect the power generafor to the power line and recognizes the delay from the time between the command to close and the actual connection between the generator and line on the order of 0.1 second. Consequently, and in accordance with the present invention, the novel synchronizer circuit reliably anticipates 10 the phase and slip between generator voltage and line voltage at a time in the future, related to the delay time in the closing of the circùit breaker from the time of the command fo close until the contacts actually close and the generator is connected to the line. To this end, the phuse of the power generator voltage and the phase of the power line voltage 15 are very accuratel y measured so that as many of their derivatives as possible can be calculated with hi~h accuracy, The circu;t then produces signals related to the phase difference and the rate of change of phase difference between power line voltuge and the generator ~oltage and the trends of phase and slip are extrapolated ;nto the future by the amoun~
20 of time which it takes the breaker to close. A command for breaker closure is then issued when the following three conditions are simultaneously satisfied:
I) the instantaneous phase and slip are within allowable pre-determined limits;
2) the phase and slip expected at the instant of breaker closure are within given limits;

3) the phase and slip at the instant of breaker closure are op-timal for the given trajectory; that is, when phase and slip trajectories .. , .. . _ .. . . .. . .. .

~ 159~g show that they will be the closest to their most desirable values during the ~iven transition.
The novel circuit produces speed matching outputs ~hereby a signal is produced to raise or lower the generator speed when the slip is measured to be less than or greater than, respecti~ely, gi~en set lower and upper limits.
Figure 1 is a block diagram of the novel synchronizing circuit o~ the present in~ention.
Figure 2 shows the measured generator ~olta~e and measured line voltage after squaring as a function o~ time.
Figure 3 shows a plot of a permissable area of closing in a phase plane o~ phase difference and slip with two particular trajectories related to phase and slip superimposed on the phase plane.
Figure 4 illustrates an index of optimization 1 as a function of time in connection with the plot shown in Figure 3.
Figure 5 schematically illustrates lines of constant 1 (optimization~ for particular synchronization requirements.
Figure 6 i5 a detailed circuit diagram of a synchronization circuit which can be used with the present invention.
Figure 7 is a block diagram of the system of the in~ention.
Referring first to Fi~ure 1, the synchxonizing circuit o~ the in~ention schematically illustrated therein includes a terminal 10 which is connected to the power line bus and a terminal 11 which is connected to a generator bus. The voltage applied to the power line bus terminal 10 is connected through a suitable positive going zero crossing detector 1 ~989g circuit 12 which produces a squared output shown in Figure 2 as "line Yoltage V~ a~ter s~uar;ng". Similcrly, the generator bus termina5 11 ;s connected to a positive going zero crossing detector circvit 13 which produces a square wave output shown tn Figure 2 as "generator vo~tage ~; Vg aher squaring".
These fwo signals, Vg and VL wh7ch each contain information regarding the phse of the generator voltoge and line voltage, are applied to an appropriate rnicrocomputer 14 which may be an Intel microcomputer ô748. The microcompùter 1~ will appropriately process the input signals VL and Vg as will be descri6ed horeinafter. The microcomputer 1~ is coupled to a timer circuit 1~ which may be an Intel timer 8253. The circuit o~ Figure I also contains three input lerminals 20, 21 and 22 which respectively rece7ve signals related to start/stop for the synchronizer circuit; a breaker closed signal; and a signal indicating a voltage match.
1~ These signal terminals are connected to an appropriate input buffer 23 which is in turn connected to the microcomputer 14 to pr~vide data for the microcomputer dealing with the start or stop order, a breaker closed signal and a voltage match signal, The microcomputer 14 is next coupled to an output buffer 30 which has three oufput terminals 31, 32 and 33,, Output terrninal 31 is an ovtput terminal connected to the generator speed control circuit (nc~t shown) and causes a decrease in the generator speed and thus a decrease in the generator frequency. Output terminal 32 produces an ou,put signal when it is required to increase the speed of fhe generator in ord2r to cause its frequency to increase to some preset synchron7zing lev~l. Output ferminal 33 has a signal appearing thereon at the instant it is desired to initiate closing the circuit breaker since the synchronfzing conditions which ~59~39(3 are required have been attained. The oufput buffer 30 is also provided with a suitable external communication link 34 as shown.
The operation of the circl~it of Fig~-re 1 ;s as ~llows and can be obtained ~y appropriate programm7ng o~ the microcomputer in a rrlanner wh7ch 5 will be obvious to any ordin~rily skille~ programmer:
The synchronizer is fumished with 1wa period;c signals, the generator voltage Vg(t) and the line voltage VL(t). The squaring circuits 12 and 13 operate to produce square wave signals which rise at the instants t 0, t 1' t 2' ~ . . t; for the gener(Jtor voltage and at instants tLo, tLl, tL2, 10 . . . tLj for the line voltage. These are the instants at which the genera.o~ voltage V and line voltage VL cross zero going from negative to positive, At these instants, interrupts are provided to the compuf2r 14 and can, for example, cause it to read the state of a pulse counter which is driven by a high resolution crystal oscillator (not shown). The difference 1~; of the teadings is proportional to the time:

aj = tgj - tL; (1) The time aj is a measure of the phose difference between VL (t) and V (t). Note ,hat the rate of change of a; is a measure of slip~
The valve of Qj is calculated once per period, immediately after 20 the interrupt from the voltage which is not leading. A voltage transition VL is defined as leading if the latest interrupt from V precedes the interrupt from VL by more than half a cycle of the 60 Hz wave, For example, in Figure 2, at time tL2 the voltoge VL is leading, and at time t 0 the line voltage VL is not leading. (81erly between t 0 and 25 tL2 the lead has changed. The calculation of the parameters of synchro-nization follows the computation of a; thus assuring no interrupts coming during the subsequent half cycle, that is, for more than 8msec. in a 60 H system.

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When the lead changes, the value of ~'a" changes sign, as seen in Figure 2. FDr that to happen, there are two consecut;ve interrupts coming from the same source. The computer 14 is appropriately programmed to detect that event by counting passes through branches pertinenf to a S given ;nterrupt. The lead will also change wherl the phase difference "a;" exceeds one-half cyc1e, The computer 14 does the compcrison and switches lead when appropriate.
To defermine the current valve of phose Aj, the algebraic average of the last four consecutive values of "a" is computed. Algebraic 10 averaging is used since the srrors in reading of "a" tend to compensate on subsequent cycles. In this way, the effect of noisa with frequency higher than 30 H~ is reduced by at least 75%.
When the lead has changed due to the phase difference exceeding 1/2 cycle, the pr~ious values of "a" are cancelled and a set of four 15 new values must be accumulated before the new value of A is calculated.
Therefore, fhe Yalue of A is updated once a cycle until again the phase crosses 180o The slip (Sj) is calculated as a change of the average phase "A"
averaged over four consecutive cycles. Thus:
A. - A.
i 4 (2) By arithmetic averaging, all high frequency errors cre cancelled or greatly reduced.
The rate of change of slip (D) is calculated in an analogous way, as a change of slip S averaged over four consecutive cyclesO Thus-~ ~L59899 51 DV~583 S. - S; 4 D j 4 ~3) Speed matching function ;s achieved by comparing the sl;p 5 with two predetemmined limits, lower limit DL and upper limit Dh. If S
is less than DL, a command is issued at terminal 32 in Figure I to raise 5 the speed of the generator. Similarly, if S exceeds Dh then a command is issued at terminal 31 in Figure I to lower the generator speed If S
is between DL and Dh then no command i5 issued at e;ther af terrninals 31 or 32.
In accordance with the invention and for the synchronization 10 process, the va1ues of phase and slip at the instant the breaker will close, i.e. when the electrical connection is made, rather than their instanfaneous values, are included in the control process. Dependin_ on the mechanical inertia of the breaker and on other factors, the circuit breaker closing time ~ varies with breaker designO The IS expected values of phase and slip at the breaker closure instant:
A ( ~) and S ( 'r), respectively, can be calculated from the instantaneous values of phase and slip A and S with the help of Taylor's series expansion:
S (?') = S + D~ (4) A (~) = A + Sr+ D ~ (5) A* and S* denote the most- desirable, optimal values of A (~) and S (~). Usually S* has a small positive value, on the order of 0.002 which assures that upon breaker closing the prime mover will deliver power into the line rather than being driven by the line. The 25 valu~ of A* is usually zero. A measure of "goodness" of synchroni-ing 1 159~9~
~I DV2583 process is how "close" the parameters A and S are to the optimal pair A* and S*, An index of optimization I tA,S) is defined asO
l(A,S) = ¦A-A*¦ + K ¦S-S*¦ (6) where the constant "K" weights the relative importance of ~ and S.
- 5 The smaller the value of I at 6reaker closure time I (~), the better the synchronization between the generator and the line.
During synchronization the volues of A and S vary with time;
they can be considered state variables and follow a tralectory in Q
phase plane as shown in Figure 3. Two trajectories, Tl (~) and T2 (L) 10 are shown in Figure 3, The lines or constant I in the phase plane (~,S) have the shape of a diamond.
Assume that the trajectory T2 (t) in Figure 3 is followed and that the synchronizer issues a "permission-to-close-breakerl' signal at terminal 33 in Fiaure I when the value of I (t) falls below a given 15 value I, e.g. within the shaded area in Figure 3. The breaker would thus be commanded to close at the time when the trajectory reached point to and ~he actual closing, after, ~, would be accomplished at tl where tl to + ~
It is clear from Figure 3 that it would~have been better if the 20 synchronizer held back fhe command untii time t2 which is ~ before t~ when the index I reaches the minimum on the whole trajectory T2(t).
The time t2 iS the best time to command the breaker to close since t*
is the best time for the electrical connection to be made.
The determination of the best time t2 to initiate the breaker-25 close command can be understood from Figure 4. In order to find thetime t2, the computer is programmed to calculate I (t + ~ ) which is ~ ~59899 the expected value of the index 1 (t~ into the future by ~ seconds. As long as 1 (t + ~r ) is progressively decreasing (from to to t in Figure 4~ no command to close the breaker is issued. As soon as 1 (t + ~ ) begins increasing, the breaker is ordered to close and the actual closing is effected r seconds later, presumably at time ,* when 1 (t) reaches a local mlnimum.
The function 1 (t ~ ~ ) reaching a minimum at t-t2 i5 not enough for synchronizatlon to be permitted. The value of the index at the instant of closing, l(t*), must also be small enough and less than a given constant B2.
Since A (t*~ and S (t*) are calculated by means o~ se~eral di~ferentiations, as in Equation (4) and Equation (5), they are subject to error. To assure that such error does not result in the breaker closing out of phase, the value of 1 at the time t2 must also be small enough and less than a given constant Bl. Moreover, to guard against errors resulting from deformation of a single cycle of a voltage sinusoid it is also required that the value of 1 at one cycle before t2 be less than sl. Therefore, for issuing at time t2 a command to close the breaker, the following four conditions must be fulfilled simultaneously:
1) 1 (t2) < Bl 2~ 1 (t2- ~t~<Bl where~ t ~17ms is a period of 60 Hz wave.
3~ 1(t2~ <B2

4) dl(t~
dt -t t- 2 It is not necessary that the index 1 be formulated by Equation (6). In a conventional representation the lines of constant 1 form a rectangle as shown in Figure 5 and the expression for 1 is:

l 159~99 51 DV 2583 1 (A,S) ~ Max ( S - S* , K ~ - A*
A common re~uirement for synchronization is that the phase A be ~ithin (-15) and + 15 and slip S
within (-0.1%) and ~0.3%. In such a case:
S* - 0.001, A* - 0, and K - 2.~8.
These lines of constant 1 are shown in ~igure 5.
The thic~ line surrounds the permitted area.
Re~errin~ next to ~i~ure 6, there is shown therein a detailed circuit diayram of the no~eI
synchronizer and speed matcher circuit of the present in~ention. The arrangement o$ Figure 6 has been used for the synchronization of a ~as turbine driven power generator with apower line operable at 60 Hz. The power line is connected between two terminals 50 and 51 to a signal transformer 52 connected as shown. In a similar manner, the generator voltage output is connected between two terminals 53 and 54 to a transformer 55, where the pickup circuits for the line voltage and generator voltage are identical.
One term~nal of the center-tapped secondary winding of transformer 52 is connected to an input terminal of amplifier 60 while the other terminal o the secondary winding is connected to the input of an amplifier 61. Note that amplifiers 60 and 61 may be each 1~4 of an integrated circuit type LM 339. The other input terminal of each of amplifiers 60 and 61 are connected to ground while their outputs are connected to the OR ~ate 62.

l 1~9~99 51 DV 2583 The circuit described above will produce the line voltage VL after s~uaring as shown in Figure 2. An identical circuit is provided ~or producin~ the generator voltage Vg after squaring and is connected to the seeondary winding of trans~ormer 55. This circuit includes amplifier sections 70 and 71 which are each 1/4 o~ the integrated circuit ~ 339 in common with amplifier seetions 60 and 61. The outputs of amplifier sections 70 and 71 are connected to the OR gate 72. Note that OR
10 gates 62 and 72 are each 1~4 of the circuit on an integrated cireuit type 74 LS 32.
The outputs of the OR gates 62 and 72 are then eonneeted to mieroeomputer 80 which is a type 8748 mierocomputer manu$aetured by Intel Corporation.
Also conneeted to the mieroeomputer 80 are terminals 81 through 84 which receive signals related to the starting of the synehronizing circuit, a synehronization eaneel signal, a voltage matehed signal and a breaker elosed si~nal. These signals are all applied through inyerters 85 to 88, respeetively, and are eonneeted to the microeomputer 80.
Next connected to the microcomputer 80 is a power supply terminal P5 and a reset terminal 90. A6 MHz crystal 91 is applied to mieroeomputer 80 as shown.
A timer 92 ~or the microcomputer 80 consists o~ an integrated eircuit timer manufactured by Intel Corporation, type 8253. The timer 92 is connected to the mierocomputer 80 as illustr~ted.
A eloek terminal output of the timer 92 is 30 eonneeted to the inverter 100, the mieroeomputer 80 and output terminal 101. Terminals Plo, Pll, P12 and To of mieroeo~puter 80 are respeetively eonneeted 1 lS9~9g throu~h inverters 102, 103 and 104 with their oufpufs respectively connected to terminols 105, 106 and 107, respectively, related ,o IowerTng generator speed, raising generator speed and a signal indicating synchronization, respectively, A further oufput of timer ~2 is connected to OR gate 110 which may also be 1/4 of the same integrated circuit ContQining OR gates 62 and 72 The output of OR gate 110 is connected to a baud rate output terminal 111. Ot~er chips including, for example, interface ahips, memory chips and the like can also be connected to tlie microcompufer 80 in any desired manner as determined by the programmer and circuit designer.
The microcomputer 80 and timer ~2 will then be prog~amr~ed to parmit the calculations described previously. Those skilled in th~ art can appreciate that various programs can be used to perform the calculations descri b~d, The foregoing shows fhe system of the invention implemented with-microprocessor technology. Clearly, however, any measurement and signal processing circuits could be used as demonstrated in Figure 7, Thus, in Figure 7, there is illustrated a single phase power line 200 which is energized by other prime movers, not shown, and a generator 201 is to be connected to the line 200. Note that-a multiphase circuit could also - have been shown. Generator 201 is connected to line 200 by closing the conventional clrcuit breaker 202. Circuit breaker 202 is controlled by a conventional operating mechanism 203-which in ~urn is releasad to closed breaker 202 by a trip signal from circuit 204. There will be the time delay ~ from the instant the trip signal is produced until the con-tacts are closed.

~ 159899 51 DV2~8 Phase measurir)g circuits 20~ und 206 clre connected to generator 201 and line 200, respect;vely, and these prodvce output signals to the slip measuring means such as a circuit 207. The slip measuring circuit 207 can be an analog c;rru;t and produce output signals to generator 201

5 to increase or decrease its speed until the measured slip is within given I imits.
Sl;p measuring circuit 207 is also connected to circuit 208 which produces a signal related to anticipated slip at some time ~ tn the fùture, depending on the rate of change of the instcntaneous slip.
10 Circuit 207 is also connected to phase and slip trajecto~y measuring cir-cuits 209 which determine their trajectory and produce an output when their trajectory is optimum Note that circuits 207, 208 and 209 can be analog or their functions can be carried out by a microcomputer, as in Figures 1 and 6.
1~ Circuits 207, 208 and 20~ then control the trip signal generator 204 such that a trtp signal cannot be applied to mechanism 203 unless their outputs are within predetermined bounds.
From the foregoing description, those skilled tn the art can appreciate that a novel control circuit is described for synchronizing a 20 prime mover or electrical generator to an electrical grid or line such that connection between the prime mover and the grid occurs when the following three conditions are simultaneously satisfied:
1) the instantaneous phase and 51ip are within allowable predetermined limits, 2) the phase and slip expected at the instant of breaker closure are within given limits and .... ... , , .. ... . ___. _ ... _ .. _ . _.__ __ . .. .. . ... . . .. .

~i ~5989~

3) the phase and slip at the instant of breaker closure are optimal for the given trajectory, thclt is, when phase and slip trcjectories show thwt they will be the closest to the;r most desirable valves during the given transition.
Although the present 7nvention has been described in connection with a preferred embodiment thereof, many variations and modifications will now become apparent to those skiiled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.

Claims (8)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An automatic device for ensuring synchronization between the output voltage and an electrical generator and the voltage of a line to which the generator is to be connected at the instant of closure of a circuit breaker which connects said electrical generator to said power line; said circuit breaker having an energizable closing circuit which causes the closing of the circuit breaker contacts after some delay time;
said automatic device comprising:
first phase sensor means for sensing the instantaneous phase of the voltage of said generator;
second phase sensor means for sensing the instantaneous phase of the voltage of said power line;
slip determining means connected to said first and second phase sensor means for developing signals related to the slip between the instantaneous phases of said power line and said generator output voltages;
trajectory determining means for determining the trajectory of slip and phase to anticipate the expected slip and phase at times in the future;
signal output means responsive to said first and second phase sensor means, said slip determining means and said trajectory determining means for delivering a signal to said circuit breaker energizable closing circuit to initiate closing of said circuit breaker upon the simultaneous occurrence of the following three conditions:
(1) at an instant the instantaneous phase and slip are within given limits, (2) the phase and slip expected at the instant of breaker closure are with given limits and (3) when the phase and slip are optimal for their particular trajectory.

2. The device of claim 1 which further includes generator speed control means; said slip determining means connected to said generator speed control means to respectively increase or decrease generator speed to change the slip and to bring the slip into a predetermined range of values.

3. The device of claim 1 wherein said first and second phase sensor means include identical respective positive going zero crossing detectors.

4. The device of claim 2 wherein said first and second phase sensor means include identical respective positive going zero crossing detectors.

5. The device of claim 1 which further includes generator speed control means; said slip determining means connected to said generator speed control means to respectively increase or decrease the generator speed to change the slip and to bring the slip into a predeter-mined range of values.

6. The device of claim 5 wherein said first and second phase sensor means include identical respective positive going zero crossing detectors.

7. The device of claim 1 which further includes microcomputer means for determining the slip between the phases of said generator and power line and for determining said trajectory of slip and phase.
8. The process of connecting a power generator to an energized electric line when the two are synchronized in phase to within less than about 10 electrical degrees comprising, in combination;
measuring the instantaneous phase of the output voltage of said generator;
measuring the instantaneous phase of the voltage of said electric line;
determining the slip between said generator and electric line;
determining the trajectory of the instantaneous phase and slip;
and producing a signal to initiate closing a

Claim 8 continued:
circuit breaker to connect said generating to said line when;
(a) the instantaneous phase and slip are within given limits;
(b) the phase and slip which are expected at the instant -the circuit breaker contacts close at some given delay time following said signal to close said circuit breaker are within given limits; and (c) the phase and slip are optimal for a given trajectory during a given transition.