US1977384A - Automatic synchronizer - Google Patents

Description

15, 1934. F. H. GULLIKSEN AUTOMATIC SYNCHRONIZER Filed Juhe so INVENTOR /?7m//. Gull/K6120.

AT'i'ORNEY fF'eQuen cg D/X/erance WITNESSES I- Patented Oct. 16, 1934 mm AUTOMATIC SYNCHRONIZEB Finn ll. Gnllihen, Wllkinsburg, 2a., alsignor to Westinghouse Electric It Manufacturing Company, a corporation of Pennsylvania Application June 30, 1931, Serial No. 547,895

"Claims.

My invention relates, in general, to automatic 'synchronizers and more particularly to automatic synchronizers of the thermionimtube type, designed to control the paralleling of two alternatlug-current sources, under predetermined conditions.

Various designs of automatic synchronizers, of

l the mechanical type, have been built according to the teachings of the prior art and are now being extensively utilized in generating stations and sub-stations to improve the paralleling service and to relieve the operators in these stations of the responsibility involved in paralleling large ,power systems.

However, automatic synchronizers of the mechanical type are complicated in construction and require expensive and elaborate calibration and adjustment. Because of the friction and inertia of the moving parts of a mechanical synchronizer 29 a relatively large amount oi energy is required to operate them, which imposes a high volt-ampere burden upon the instrument transformers which connect the synchronizer to the power systems. Mechanical synchronizers, therefore, cannot be operated from standard condenser bushing potential devices, which have a relatively low voltampere capacity.

An object of my invention, generally stated, is to provide an automatic synchronizer, of the thermionic-tube type, which shall be simple and eilicient in operation, and which may be readily and economically manufactured and installed.

A more speciflc'object of my invention is to provide for initiating the closing of a switch for 5 paralleling two alternating-current sources at an advanced phase angle, proportional to the instantaneous frequency difference between the current sources, to permit the current sources to be paralleled at substantially zero phase-angle 0 displacement.

, Another object of my invention is to prevent fully-hereinafter, or will be apparent to those skilled in the art.

Inaccordance with my invention, a switch for paralleling two alternating-current sources is controlled by relays which are energized by electronic tubes connected to the current sources.

' f the closing of a switch for paralleling two alter- The relays are sointerlocked that the paralleling of the two sources is effected only when the phase and frequency relations are suitable.

For a fuller understanding of the nature and scope of my invention, reference may be had to the following detailed description, taken in conjunction with the accompanying drawing, in which:

Figure l is a diagrammatic view of an automatic synchronizer embodying my invention, together with the circuits necessary for a proper functioning thereof and Fig. 2 is a graphical representation illustrating the operating characteristics of the automatic synchronizer shown in Fig. 1.

Referring to the drawing, the reference character 10 designates a paralleling switch that is disposed to parallel two sources of alternating current (not shown) which are cormected to the two groups of power conductors designated byreference characters 11 and 12. The switch 10 may be ofa standard type having a closing solenoid mechanism 13 and a tripping mechanism 14.

As shown, the energization of the closing solenoid 13 is controlled by an automatic synchronizer, which will be described in detail hereinafter, and the tripping mechanism 14 may be controlled by a master relay 15. The operation of the relay 15 may be controlled by a push-button switch 16, which establishes an energizing circuit for actuating coil of the relay 15, and a second pushbutton switch 17, which interrupts the circuit for the actuating coil. A holding circuit is established by means of a contact member 18, on the relay 15, after the relay is actuated to the closed position. When the relay 15 is in the open or deenergized position, the tripping mechanism 14 is energized by means of a contact member 19 on the relay 15. An auxiliary relay 21 is provided for disconnecting the synchronizing apparatus from the power sources after the paralleling switch 10 s As illustrated, the thermionic synchronizer comprises two standard vacuum tubes 22 and 23, of the three electrode type, which are" connected in series with their respective relays 22' and 23'.

The synchronizer is connected to the two power systems 11 and 12 by means including transformers 24 and 25, each of which comprises a primary winding and three secondary windings. The primary windings of the transformers 24 and 25 are connected to potential transformers 26 and 27, respectively, through contact members of the relays 15 and 21. The potential transformers 26 and 27 are energized from the respective power systems 12 and 11, which are to be paralleled.

The secondary windings 28 and 29 of the transformers 24 and 25, are so connected to the alternating-current terminals of a rectifier 31, which is preferably of the well-known copper oxide type comprising a Wheatstone bridge arrangement of copper oxide'discs, that the voltages of the two systems are directly opposed when the phase angle displacement between the systems is zero,

thereby applying a beat voltage to the rectifierproportional tothe frequency difference between the systems. .The direct-current 'v-oltage out: put of the rectifier 31 is therefore essentially a pulsating voltage which varies according to a composite sine wave between zero and maximum, once per second for each cycle frequency difference between the two systems to be paralleled. It will be understood that the voltage impressed on the rectifier is a maximumand consequently the output voltage is highest, when the phase angle displacement between the system voltages is 180 degrees. A condenser 32 is connected across the direct-current terminals of the rectifier 31 to smooth out the alternating-current components of the rectified voltage. Resistors 66 and 67 and a condenser 68 are also connected across the terminals of the rectifier 31. These are utilized to apply. a grid bias to the tube 22, as will be explained in detail hereinafter.

Another multiple rectifier 33 is connected to the secondary windings 34 and 35, of the transformers 24 and 25, in the same manner as the rectifier 31. A condenser 36 and'a resistor 59 are also connected across the direct-current terminals of the rectifier. Accordingly, the output voltage of the rectifier '33 also varies from zero to maximum, once per second for each cycle frequency difference between the 'two systems.

In addition to the foregoing apparatus, a breaker closing relay3'7 and an intermediate relay 38 are provided. One set of contact members of therelay 37 controls the energization of the closing solenoid 13, and the other set is utilized to establish a holding circuit for the relay. The

' energization of the actuating coil of the relay 37 is controlled by the relays .22 and 23'. The function of relay 38 is to interpose a time delay, corresponding to on phase rotation, .when the synchronizer is first given control thereby ensuring that synchronizing will not talreplace any reliablesource of direct-current energy. As

previously explained, the synchronizing appa-' ratus is automatically disconnected from both the alternating .and the direct-current sources of energy when the circuittbreaker 10 is closed,

in order that the life of the apparatus will be increased.

As previously stated, there are two require-- ments which should be fulfilled by an automatic synchronizer: first, the synchronizer should operate to energize the paralleling-switch closing solenoid only if the instantaneous frequency difference between the two systems to be paralleled is less than a predetermined lock-out frequency difference, in order to prevent tying together generators which are operating at too great a difierence in speed. Secondly, the paralleling switch closing mechanism should be energized at an advance phase angle proportional to the instantaneous frequency difference, to make allowance for the time required for relay operation and circuit breaker closing, in order.that the two systems will be paralleled at substantially the exact instant of zero phase angle displacement between the voltages of the two systems.

It will be evident from the following description of operation that the automatic synchronizer, herein disclosed, meets the foregoing requirements. the paralleling switch 10 under the control of the automatic synchronizer, the push-button switch 16 may be closed to energize the actuating coil of the relay 15, from the direct-current supply circuit 39, thereby causing the relay to be actuated to its uppermost position. When the'relay 15 is actuated, the conductors 41 and 42 are connected to the direct-current control bus 39, and the primary windings of the transformers 24 and 25 are energized from the potential transformers 26 and 2'1, respectively, provided relay 21 remains in its normal or lower position.

The thermionic tube 23 is adapted to be con- Assuming that it is desired to put nected in series with the actuating coil of relay 23 across the direct- current conductors 41 and 42. It will be seen that the filament 43 of the tube 23 is supplied with heating current by the secondary winding 44 of the transformer 25, also that the filament 45 of the tube 22 is heated by the secondary winding 46 of the transformer 24. Mid-tap connections are provided to the windings 44 and 46, as shown on the drawing, for a purpose to be set forth below. The energizing circuit for the actuating coil of the relay 23', which is established through the tube 23 provided the contact members of the relay 38 are closed, as will be explained hereinafter, extends from the positive conductor 41, through conductor 47, the contact members 48 of the relay 22' in its lower position, conductors 49 and 51, the contact members 52 of the relay 38 in its upper position, conductor 53, the coil of the relay 23', conductor 54, the plate 55 of the tube 23, the filament 43, thence to the mid-tap conductor 56 and a portion of the potentiometer resistor 5'7, to the negative conductor 42.

By means of the potentiometer 57, the grid 58 of the tube 23 is supplied with a negative bias which may be varied by changing the potentiometer setting. In addition to this-bias, a negative bias equal to one half the voltage across the rectifier 33 is also applied to the grid 58.

degrees. Therefore, the negative bias applied to the grid 58 is also a maximum when the system voltages are 180 degrees out of phase. When the phase angle is reduced, the negative bias on grid 58 is reduced until a point is reached where the total negative bias on the grid is low enough to permit suflicient current to how through the tube to operate relay 23', thereby closing its contact members. The phase angle displacement at which the relay 23' is permitted to operate may be varied by adjusting the potentiometer 57. The relay 23' will, therefore, operate and.

close its contacts at a fixed point in advance of synchronism, the closing characteristic being independent of the frequency difierence between the two systems, as illustrated by curve b of i 2. I

As shown, the actuating coil of the relay 22' is connected in series with the thermionic tube 22, through a circuit which extends from the positive conductor 41, through conductor 47, the coil of relay 22', conductor 62, the plate 63, the filament 45, thence to the mid-tap conductor 64, and a portion of the potentiometer resistor 65, to the negative conductor 42. Accordingly, the operation of the relay 22' is controlled by the tube 22 ,in somewhat the same manner as the tube 23 controls the relay 23.

However, the relay 22' will operate and close it contacts at a point in advance of synchronism proportional to the frequency difierence, as illustrated by curve "a of Fig. 2. The proportional advance characteristic of relay 22 is produced by the combined action of the rectifier 31, resistors 66 and 67, and the condenser 68.

I As previously explained, the direct-current output voltage of the-rectifier 31 pulsates at a rate proportional to the frequency diilerence between the two systems to be paralleled. The voltage across the condenser 68, when the frequency difference is zero, is equal to the output voltage of the rectifier 31. If there is a definite frequency difference between the two systems and the phase angle displacement is changing from 180 degrees towards zero, then the voltage across the terminals of the rectifier 31 and the voltage across the condenser 68 will be decreasing, but the voltage across the condenser 68 will be higher than the rectifier voltage by an amount dependent upon the frequency difference. The discharge current from the condenser, which flows in the direction indicated by the arrow 69, will produce a voltage drop across the resistor 67 with polarity as indicated. When the phase angle displacement between the system voltages is increasing from zero to 180 degrees the polarity of the voltage drop across the resistor will be reversed. The magnitude of the voltage drop across the resistor 67 is proportional to the frequency difference.

The resistor 67 is so connected in the grid circuit of the tube 22 that the voltage drop across the resistor will give a positive bias to the grid 71 when the phase angle displacement is decreasing from 180 degrees toward zero and the condenser 68 is discharging through the resistor 67.

A A constant negative bias is'applied to the grid 71 by means of the potentiometer 65 and a varying negative bias, which is proportional to the phase angle displacement, is obtained from the rectifier 33. The grid circuit for the tube 22 may be traced from the potentiometer 65, through the mid-tap conductor 64, the filament 45, the grid 71, conductor 72, the resistor 67, conductor 73, the rectimi- 33, and conductors 74 and 61. to the negative conductor 42.

The potentiometer 65 is so adjusted that the negative grid bias obtained from the potentioometer is just low enough to permit sufiicient current to flow through the tube 22 to operate the relay 22' at zero phase angle displacement when the frequency diiference between the two systems is zero. For any definite frequency difference, a definite positive bias is applied to the grid of tube 22 which will reduce the total negative bias on the grid, and relay 22' will therefore operate at an advanced phase angle position, the amount of advance being proportional to the instantaneous frequency difference. The operating characteristics of relay 22' are illustrated by curve a of Fig. 2.

As shown in Fig. 2, the relay 23 will operate and close its contact members at a fixed point in advance of synchronism, the closing characteristic being independent of the frequency difference, as illustrated by line b, while the relay 22' will close its contact members at a pointin advance of synchronism proportional to the frequency difierence, according to line a of Fig. 2. Therefore, it will be seen that if the frequency diiference is less than the value represented by ",f, the point of intersection of curves a and b, the relay 23' will operate at a larger phase angle displacement than the relay 22', but if the frequency difference is above the value of f the relay 22' will operate at a larger phase angle than the relay 23.

As shown in Fig. 1, the contact members of the relays 22 and 23' are so interlocked that the breaker closing relay 37 is energized only if the relay 23' is operated before the relay 22 is closed. It will be seen that both of the relays 22' and 23' must be closed before the relay 37 can operate because the actuating coil of the relay 37 is connected in series with the normally open contact members of the relays 22 and 23. The relay 23' must operate ahead of the relay 22', since the energizing circuit for the actuating coil of the relay 23 extends'through the normally closed contact member 48, of the relay 22, as was previously ergizing the breaker closing coil at an advanced 7.

phase angle proportional to the instantaneous frequency difference. The amount of advance at which the relay 22' will operate, may be varied by adjusting the potentiometer 66 to provide for the time required to close the particular breaker being controlled, thereby causing the breaker to be closed at substantially exactly zero phase angle displacement.

In addition to the relays 22' and 23, and the breaker closing relay 37,an intermediate relay 38 is also provided. The function of this relay is to interpose a time delay corresponding to one phase rotation when the synchronizer is first given control of the circuit breaker 10, as will be explained hereinafter, and thus prevent synchronizing until the condenser 68 has assumed the charge corresponding to the instantaneous phase angle displacement and frequency difierence.

When the synchronizer is given control by actuating the push-button switch 16 to energize the master relay 15, the, sequence of relay operation is as follows: The relay 22' is actuated t its uppermost position the first time that the phase angle displacement between the system voltages the relay 3'7 to operate.

established through the actuating coil of the relay 38, which extends from the positive conductor 41, through conductor 4'7, the bridging contact members '75 of relay 22, conductor '76, the coil of the relay 38, and a resistor 7'7, to the negative conductor 42.

When the contact members of the relay 38 are closed a holding circuit is established which keeps the coil 38 energized after the relay 22' has dropped to its lowermost position. The holding circuit may be traced from the positive conductor 41, through conductors 4'7 and '78, the bridging contact members 79 and the coil of the relay 38, and resistor '77, to'the negative conductor 42.

If the frequency difference, when the phase angle during the next beat is decreasing from 180 degrees towards zero, is higher than the selected lockout frequency difference, the relay 22' will again operate at a point in advance of synchronism and close its contact members before the relay 23' operates. Since the normally closed contact members 48 of the relay 22' are connected in series with the actuating coil of relay 23', the relay 23' cannot operate until the relay 22', drops to its lowermost position at zero phase angle displacement. Therefore, synchronizing will not take place if the frequency difference is higher than the selected lockout frequency.

However, when the frequency difference between the two systems becomes less than the selected lockout frequency difference, the relay 23' will operate ahead of the relay 22', as previously explained. When the relay 22 finally operates at the proper phase advance,relay 23 will, because of the action of 'a. condenser 81 connected across the coil of relay 23, remain closed for a time interval of sufiicient length to permit The circuit for the actuating coil of relay 3'7 may be traced from the positive conductor 41, through conductor 4'7, the bridging contact members 82 of the relay 22', conductor 83, the bridging contact members 84 of the relay 23, conductor 85, the coil of the relay 3'7, and conductor 86, to the negative conductor 42.

A holding circuit for the actuating coil of relay 3'7 is established when its contact members are closed. The holding circuit may be traced from the positive conductor 41, through conductor 87, bridging contact members 88 and the coilof the relay 3'7, and conductor 86, to the negative conductor 42.

When'the relay 3'7 is operated, an energizing circuit is established for the actuating coil of the breaker closing mechanism 13, thereby closing theparalleling switch 10 and completing the synchronizing operation. The circuit for the coil of the closing mechanism may be traced from the positive conductor 41, through bridging contact members 89 of the relay 3'7, conductor 91, the

.actuating coil of the closing mechanism 13- and conductor 92, to the negative conductor 42.

It will be observed that the actuating coil of the relay 21 is energizediby means of an interlock 20 on the circuit breaker 10 when the circuitbreaker is closed. Accordingly, the relay 21 is actuated to its upper position and the conductor '42 is disconnected from the control bus 39. The

potential transformers 26 and 27 are also disconnected from the transformers 24 and 25 when the relay 21 is operated. In this manner, the automatic synchronizer is dee'nergized while the two systems are paralleled which, as previously exis zero. When the relay 22 operates, a circuit is plained, greatly increases the life of the thermionic tubes 22 and 23.

Since the paralleling switch 10 cannot be closed unless both of the relays 22' and 23' are operated, the synchronizer is made inoperative by the failure'of one of the tubes 22 or 23. In this manner, the two systems-are prevented from being paralleled unless the previously mentioned pre-requisite conditions are fulfilled. Therefore, faulty synchronizing because of tube failure is prevented.

When the synchronizer is utilized for controlling a paralleling switch which may be the last switch to be closed in a loop or ring power system, there will be nophase rotation between the voltages of the power conductors which are to be paralleled although there may be a phase angle displacement. Under these conditions, the relays 22' and 23' would not operate in the proper sequence, as previously explained, to effect the closing of the paralleling switch.

In order that the synchronizer may be utilized in a loop system,.the actuating coil of a voltage relay 93 of the well-known revolving .disc type, having a time delay action, is connected through conductors 97 and 98 to the primary windings of the transformers 24 and 25, which in turn are connected across the potential transformers 26 and 27, as shown in the drawing. The contact members 94 and 95, which are disposed to be bridged by a contact member 96 of the relay 93, are connected in parallel-circuit relation to the normally closed contact members 48 of the relay 22'. Therefore, when the relay 93 actuates the contact member 96 to bridge the contact members 94 and 95, the relay 23' may operate after the relay 22' to effect the closing of the paralleling switch 10. However, the time delay feature of the relay 93 may be so adjusted that the relay has no eife'ct when the synchronizer is utilized to control the paralleling switches of power systems of the usual type.

It will be evident from the foregoing description that I have provided an automatic'synchronizer which will effect the operation of a paralleling switch to connect two power systems at substantially the exact instant of zero phase angle displacement between the voltages of the two systems, provided the frequency difference between the two systems to be paralleled is less than a predetermined value.

It will also be evident that the automatic synchronizer herein described has the following advantages over synchronizers of previously known types:

(a) Low power consumption.

(b) No friction between moving mechanical parts.

(c) May be readily adjusted, when installed, to meet difierent operating conditions. a

(d) Inexpensive in construction and efilcient and reliable in operation.

In conclusion, I desire to say that I am not to be restricted to the specific embodiment of my invention herein shown and described, since it .is evident that it may be changed or modified without departing from the spirit and scope of my invention as defined in the appended claims.

I claim as my invention:

1. In a system for paralleling alternating-cur-- rent sources, the combination of a paralleling switch, switching means for controlling the 0105- ing of said paralleling switch, and means for effecting the operation of said switching means at an advanced phase angle proportional to the frequency difference between the two current sources comprising means for controlling the switching means and an electronic tube responsive to control potentials proportional to the inthe operation of said switching means at an ad vanced phase angle proportional to the frequency difierence between the two current sources comprising a relay for controlling the switching means and an electronic tube responsive to control potentials proportional to the instantaneous frequency diilerence between the current sources for controlling the energization of said relay, and means for varying the advanced angle at which said relay is energized.

3. In a system for paralleling alternating-current sources at substantially the instant of phase coincidence, the combination of a paralleling switch, switching means for controlling the operation 015 said paralleling switch, electronic tube means responsive to control potentials proportional to the phase angle displacement between the voltages of the sources for controlling said switching means, electronic tube means responsive to control potentials proportional to the frequency diflerence between the current sources for effecting the operation of said switching means at a phase displacement in advance of phase coincidence proportional to the frequency diilerence, and means disposed to prevent said switching means from closing the paralleling switch so long as the frequency diilerence between the current sources to be paralleled is above a predetermined value.

4. In a system for paralleling altematingcurrent sources at substantially the instant of phase coincidence, the combination of a paralleling switch, switching means for controlling the operation of said paralleling switch, electronic tube means responsive to control potentials proportional to the phase angle displacement between the voltages of the sources for controlling said switching means, electronic tube means responsive to control potentials proportional to the frequency difference between the current sources for effecting the operation of said switching means at a phase displacement in advance of phase coincidence proportional to the frequency diiference, relay means disposed to prevent said switching means from closing the paralleling switch so long as the frequency dinerence between the current sources to be paralleled is above a predetermined value, and means for varying the frequency diil'erence at which the paralleling switch may be operated.

5. Ina system for paralleling altematingcurrent sources at substantially the instant of phase coincidence, the combination of a paralleling switch, switching means for controlling the operation of said paralleling switch, means responsive to the phase angle displacement between the voltages of the sources for controlling said switching means comprising a relay and an electronic tube for controlling the energization of the-relay, andmeans responsive to the frequency diiference between the current sources for eflecting the operation of said switching means at a phase displacement in advance of phase coincidence proportional to the frequency diflerence comprising a relay and an electronic tube for controlling said relay, means for interconnecting the contact members of said relays to control the operation oi said switching means to prevent the closing of the paralleling switch so long as the frequency difference between the current sources to be paralleled is above a predetermined value.

6. In a system for paralleling alternatingcurrent sources at substantially the instant of phase coincidence, the combination of a paralleling switch, switching means for controlling the operation of said paralleling switch, means responsive to the phase angle displacement between the voltages of the sources for controlling said switching means comprising a relay ,and an electronic tube for controlling the energization of the relay, means responsive to the frequency diiIerence between the current sources for effecting the operation of said switching means at a phase displacement in advance of phase coincidence proportional to the frequency difierence comprising a relay andan electronic tube for controlling said relay, means for interconnecting the contact members of said relays to control the operation of said switching means to prevent the closing of the paralleling switch so long as the frequency difierence between the current sources to be paralleled is above a predetermined value, and means for varying the frequency difierence at which the paralleling switch may be operated. I. In a system for paralleling alternatingcurrent sources, the combination of a paralleling switch, switching means for controlling the closing of said paralleling switch, relay means actuated in accordance with the frequency difference between said sources for energizing said switching means when the frequency difference between the sources to be paralleled is below a predetermined value and at a time in advance of the occurrence of zero phase difference between said sources which is proportional to the frequency difference, electronic tube means for controlling the energization of said relay means, and means for deferring the energization of said swltching'means after the first occurrence of zero phase difierence at a frequency difference below said predetermined value for a time interval substantially'corresponding to one phase rotation of the voltages of the current sources.

8. In a system for paralleling alternatingcurrent sources, the combination of a paralleling switch, switching means for controlling the closing of said paralleling switch, relay means actuated in accordance with the frequency difierence between said sources for energizing said switching means when the frequency difl'erence between the sources to be paralleled is below apredetermined value and at a time in advance of the occurrence of zero phase difference between said sources which is proportional to the frequency diiference, electronic tube means for controlling the energization of said relay means, means for deferring the energization of said switching means after the first occurrence of zero phase difference at a frequency difl'erence below said predetermined value for a time interval substantially corresponding to one phase rotation of the voltages of the current sources, and means for varying the value of frequency diiference and the time in advance of the occurrence of zero phase diiference between the sources at which said switching means may be energized.

9. A system for paralleling alternating-current sources including a paralleling switch, switching 15o means for controlling the closing of said switch, and means for effecting the operation of said switching means at a time, in advance of the occurrence of exact phase coincidence proportional to the frequency difference between the sources to be paralleled when the frequency difference is below a predetermined value, comprising av plurality of relays and a plurality of electronic tubes responsive to potentials proportional to the frequency difference between the current sources for controlling the operation of said relays.

10. A system for paralleling alternating-current sources including a paralleling switch, switching means for controlling the closing of said switch, means for effecting the operation of said switching means at a time in advance of the occurrence of exact phase coincidence proportional to the frequency difference between the sources to be paralleled when the frequency difference is below a predetermined value, comprising a plurality of relays and a plurality of electronic tubes responsive to potentials proportional to the frequency difference between the current sources for controlling the operation of said relays, means for applying a biasing potential to said electronic tubes, and means for varying the biasing potential to vary the value of frequency difference and the advanced phase angle at which said switching means may be operated.

11. An automatic synchronizer for paralleling altemating-current sources comprising a relay disposed to operate at a definite point in advance of synchronism, a second relay disposed to operate at an advanced phase angle displacement proportional to the frequency difference between the sources to be paralleled, and electronic tubes disposed to be energized by the beat voltage between the sources and responsive to potentials proportional to the frequency difference between the current sources for controllingthe relays, the contact members of said relays being so connected as to effect the paralleling of the sources only under predetermined'conditions.

12. An automatic synchronizer for paralleling alternating-current sources comprising a relay disposed to operate at a definite point in advance of synchronism, a second relay disposed to operate at an advanced phase angle displacement proportional to the frequency difference between the sources to be paralleled, the contact members of said. relays being so connected as to effect the paralleling of the sources only under predetermined conditions, electronic tubes disposed to be energized by the beat voltage between the sources and responsive to potentials proportional to the frequency difference between the sources for controlling the relays, means for applying a biasing.

potential to the electronic tubes, and means for varying the biasin potential, whereby the paralleling conditions may be varied.

13. In a system for paralleling .altematingcurrent sources, the combination of a paralleling switch, and means for initiating the closing of said switch at an advanced phase angle proportional to the frequency difference between the current sources comprising an electronic tube responsive to control potentials proportional to the instantaneous frequency difference between the current sources for controlling the closing of the paralleling switch.

14. In a system for paralleling alternating-current sources, the. combination of a paralleling switch, means for closing said switch, and means for effecting the energization of 'said closing means at an advanced phase angle proportional to the frequency difference between the two current sources comprising an electronic tube responsive to control potentials-proportional to the instantaneous frequency difference between the current sources for controlling the energization of the means for closing the paralleling switch.

15. In a system for paralleling alternating-current sources, the combination of a paralleling switch, means for closing said switch, means for effecting the energization of the closing means at an advanced phase angle proportional to the frequency difference between the. two current sources comprising a relay for controlling the 7 operation .of the means for closing the switch and an electronic tube-responsive to control potentials proportional to the instantaneous frequency difference between the current sources for controlling the energization of said relay, and means for varying the advanced angle at which said relay is energized.

16. In a system for controlling the operation of a switch for paralleling alternating-current sources, in combination, relay means for controlling .the closing of said paralleling switch,

and means for effecting the operation of said relay means at an advanced phase angle proportional to the frequency difference between the current sources comprising means for obtaining a potential proportional to the frequency diifer-' ence between the current sources and an electronic tube responsive to said potential forcontrolling the operation of said relay means.

. 17. In a system for controlling the operation of a switch for paralleling alternating-current sources, in combination, relay means for controlling the closing of said paralleling switch, and means for effecting the operation. of said relay means at an advanced phase angle proportional to the frequency difference between the current sources comprising means for obtaining a potential proportional to the frequency difference between the current sources, means for measuring the rate of change of potential and an electronic tube responsive to said potential for controllin the operation of said relay means.

18. In a system for controlling the operation of a switch for paralleling alternating-current sources, in combination, relay means for control ling the closing of said paralleling switch, and means for effecting the operation of said relay means at an advanced phase angle proportional to the frequency difference between the current sources comprising means for obtaining a rectifled potential proportional to the frequency difference between the current sources, means for measuring the rate of change of said potential and an electronic tube responsive to said potential for controlling the operation of said relay means.

FINN H. GULLIKSEN.

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