Patent GB-583489-A (GB)
Title: Improvements in or relating to the measurement or indication of electric currents or potentials and to the protection of circuits carrying such currents
IPC: H02H3/26
Filed: 1944-02-28
Abstract: 583,489. Protective cut-out systems. CASSON, W., and LAST, F. H. Feb. 28, 1944, No. 3688. [Class 38 (v)] [Also in Groups XXXVI and XXXVIII] For measuring the mean value of an alternating current or voltage, and the protection of the circuit carrying such current, an electromagnetic relay which operates in a time that is short compared with a half-cycle of the alternating current is arranged to control the completion of a secondary circuit that has a constant voltage applied across it so as to produce a pulsating direct current therein which operates measuring or controlling means. As shown in Fig. 1, one winding 30 of a high-speed relay has an alternating voltage applied across it. The duration of each half-cycle of the current is large compared with the operating time of the relay, so that the relay closes and reopens its contacts 31 during each half-cycle. The line 35, Fig. 2, indicates the minimum current that will operate the relay, so that a current 32 having a peak value which only just reaches this line will not operate the relay. An alternating current 33 having a peak value greater than the minimum operating current closes the contacts 31 at the point 36 and reopens them at the point 37, so that the contacts are closed for the portion X of the half-cycle. A current 34 of greater magnitude closes the contacts for a period Y. During each half-cycle, therefore, the relay contacts are closed for a period proportional to the magnitude of the alternating current. The contacts 31 are in series with an ammeter or relay 38 and a constant voltage is applied across the circuit. The mean value of the D.C. impulses in this circuit is a measure of the A.C. in the winding 30. The high-speed relay may have a second winding 39 supplied with a constant direct current less than the minimum operating current. During one half-cycle the relay will be operated by an alternating current of smaller magnitude, thereby increasing the sensitivity. The mean D.C. in the device 38 may be made an inverse function of the alternating current by arranging the contacts 31 so that they are normally closed. Alternatively an alternating voltage of the same frequency as the current through the coil 30 may be applied to the circuit of the contacts 31, the coil 30 being polarised by rectifiers. In this case the indication of the device 38 varies according to the magnitude of the current through the winding 30, the current through the contacts 31, and also to the phase angle between them. Fig. 5 shows an arrangement for measuring the phase difference between two alternating voltages V1, V2. The windings 30a, 30b of two high-speed relays are connected one to each supply and are each polarised by rectifiers 40a, 40b, 41a, 41b, so that the winding 30a carries positive half-cycles only, and the winding 30b carries negative half-cycles only. If the voltages V1, V2 are in phase, the contacts 31a, 31b close alternately and there is no current through the instrument 38. For a phase difference of 180‹ the contacts 31a, 31b close and open simultaneously and transmit pulses of D.C. to the instrument 38, which indicates the mean value of the pulsating D.C. For other phase differences the closures of the contacts 31a, 31b overlap proportionally to the phase difference, and the mean D.C. through the instrument 38 is proportional to the phase difference. By reversing the rectifiers 40b, 41b the instrument 38 gives a maximum reading when the voltages V1, V2 are in phase and zero reading when they are 180‹ out of phase. Fig. 14 shows an instrument for measuring power. The coil 30 of a high-speed relay is energised from a voltage supply V through a rectifier 40 and its contacts 31 are connected in series with the secondary winding of a current transformer 150, the primary winding of which is connected in the load circuit. A moving coil instrument 38 indicates true power. At unity power factor the full half-cycle of current passes through the instrument, but at zero power factor one half of the positive half-cycle is followed by one half of the negative half-cycle, and the instrument 38 gives a zero reading. Polyphase units on the two or three wattmeter principle may be constructed using a single moving-coil instrument. Volt amperes may be measured by inserting a rectifier in series with the contact 31. For communication or signalling purposes, current or voltage magnitudes or phase differences may be transmitted through a pilot wire connected in series with the contacts 31, Fig. 1, or 31a, 31b, Fig. 5. Alternatively the current impulses through these contacts may be employed to modulate a carrier wave which is radiated by a transmitter or transmitted over a power or other circuit. Fig. 16 shows an arrangement for giving measurements more nearly linear than the arrangement of Fig. 1. The coils 30a, 30b are connected through rectifiers 40a, 40b to secondary windings on summation transformers 50a, 50b. A phase shifting device 53 is connected in the circuit of the relay 30b. Primary windings 85a, 85b are connected in parallel to a constant voltage device 51. Primary windings 84a, 84b are connected in series opposition to the current supply. The normally open contacts 31a, 31b are connected in series to a D.C. supply, a transmitter 52, and a local instrument 38. The phase shifting device 53 is adjusted until the instrument 38 indicates zero with the voltage supply alone connected. When the current supply is applied it produces an opposite effect on the flux of the transformers 50a, 50b and thereby causes a phase shift between the secondary voltages applied to the relays 30a, 30b proportional to the current being measured. Power factor may be measured by operating one relay from the voltage supply only and the other by the corresponding current only. Fig. 20 shows an overcurrent protective device for feeders, transformers, reactors, generators, and the busbars of switch gear of a power supply system. The coil of a high-speed relay 30 is provided with tappings and connected in the current supply circuit either directly or through a current transformer. The relay contacts 31 connect a D.C. supply to a relay 54, the operating time of which is proportional to its mean coil current. The contacts 55 are adjustable to vary the time setting and complete the circuit of circuit-breaker trip coils 56. For a wattmetric directional relay the coil of a highspeed relay is polarised by rectifiers and connected to one phase of the current circuit. The relay contacts connect an alternating voltage to the coil of a polarised relay, the contacts of which are in the control circuit of protective devices. Fig. 22 shows a circuit for controlling protective gear which operates circuit-breakers at the supply points of a protected zone when there is a fault which causes a large difference in phase between currents at points in the protected zone or an overcurrent when the protected zone is fed from one point only. Two highspeed relays 30a, 30b are polarised by rectifiers 40a, 41a, 40b, 41b and the contacts 31a, 31b are connected as shown. A D.C. is fed through the contacts to the coil of an instrument or relay 62. This arrangement may be duplicated, the coils of the second relays being shunted by resistances to lower their sensitivity. The operation of this apparatus is independent of the relative magnitudes of the two currents within limits determined by the setting of the two pairs of relays. In a modification, Fig. 24, a pilot circuit or a carrier current channel is provided between the supply points A, B on the protected zone. The coils of the high-speed discriminating relays 30a, 30c, 30d, 30b at the two points are polarised and connected to the secondaries of the current transformers 63a, 63b. The coils of high-speed repeat relays 64a, 64b are connected to the ends of the pilot wires as shown, or to the high-frequency receiver if a carrier current channel is employed. The protected zone is healthy when current enters at one point, passes through the zone, and out at the other point. For this condition, on one half-cycle, the relay 30a acts and also the. relay 30d which operates the high-speed repeat relay 64a so that both contacts 65, 66 operate and the tripping relay 62a is not energised. Neither of the relays 30b, 64b operates, so that the tripping relay 62b is not energised. For the next half-cycle the relays 30b, 30c, 64b operate but 30a, 64a do not, so that the relays 62a, 62b are not energised. If there is a fault in the protected zone and fault current is fed from the end A only, during one half-cycle the relay 30a operates but the relay 64a does not. Thus .the contact 65 closes but 66 remains unoperated, consequently a tripping impulse is given to the relay 62a at each alternate half-cycle. When fault current is fed from both ends A, B during one half-cycle relays 30a, 30b only operate, contacts 65, 68 closing and the relays 62a, 62b being tripped. Fig. 25 shows an arrangement by which the tripping relays at all points are operated if the feed into the fault at one point is below the minimum necessary to operate the discriminating relay. The relay coil 30d is polarised oppositely to the relay coils 30b, 30f. When the relay 30d operates it causes the pilots 124 to be energised from a D.C. supply at the point B and the coils of the high-speed repeat relays 64b, 64d to be disconnected from the pilots. Since there is insufficient fault current fed from the point A the relays 30a, 30c, 30e do not operate and the operation of the relay 30d causes the repeat relays 64a, 64c to operate. Charging current then flows through a condenser 70a and the coil of a tripping relay 72a, which tends to operate. During the next half-cycle the relays 64a, 64c reset and the condenser 70a discharges slowly through the resistance 71a, taking the half-cycle to do so. The tripping relay 72a is designed to operate after two or three half-cycles of charg