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In such stations it is often very essential also to have supervision over the current flowing in each circuit leading out from the station. A means of providing this current supervision and also relay protection for each circuit without the necessity of supplying and providing room for mounting both ammeters and relays, is provided by the COA relay. This relay consists of the CO relay with the addition of a current indicator which serves to show at all times the value of the current flowing in the circuit.
The current indicator consists of a thin copper segment mounted on a separate shaft directly above the main disc of the relay in such a way that it is actuated by the same flux that passes through the relay disc. As the relay winding is connected directly in the circuit to be protected by means of a current transformer, the current indicating element shows at all times the quantity of current flowing in the line.
The current indicating element also gives a double protective assurance, inasmuch as any accident which occurs either within or without the relay to prevent the main electro-magnet from operating the relay disc will also keep the indicating disc from operating. Certain relay applications are such that only very little energy is available for operating the relay. This is especially the case on high-voltage lines where low-ratio bushing-type current transformers are the only convenient means for supplying energy for the operation of relays or instruments.
A modification of the CO relay has been designed to fit such conditions. This low-energy CO relay requires approximately 2 volt-amperes for tripping. In construction and operation it is very similar to the standard CO relay, except that the contacts are not closed directly by the action of the disc but through a train of gears.
This train of gears also supplies the definite-minimum time characteristics, thus eliminating the torque compensator. Due to the construction of the relay it is somewhat more sensitive than the other CO relays and is, therefore, suitable for use on applications requiring great sensitivity.
The differential protection of generators is an example of this type of application. All relays dependent on current alone for their operation function when sufficient current is flowing in the winding regardless of the direction of the flow.
In protecting transmission lines and in making possible the sectionalizing of such lines on the occurrence of trouble, it is often necessary to have protective relays which will close their contacts only when current is flowing in one certain direction. The CR relay has been developed to fill these requirements. The CR directional relay is a combination of the CO overcurrent relay and a watt element. The current element is the same as that of the standard CO overcurrent relay and has identical operating characteristics.
The watt element is similar to the CW power relay except that it operates on a very small percentage of normal voltage. The contacts of the overcurrent element are connected in series with the contacts of the watt element and as this element will close its contacts only when the power flows in one direction, the relay as a unit operates only when current is flowing in any given direction. As the relay has all the characteristics of the CO relay, the following conditions must exist before the tripping circuit is completed: 1 Excess current must be flowing; 2 current must be flowing away from the bus bars or in the direction for which the relay is connected; 3 current must be flowing for a sufficient length of time.
Thus the directional element may close its contacts upon momentary surges of current in the reverse direction, or the overcurrent element may close its contacts on the flow of excess current in the normal direction, but the tripping circuit is completed only when both elements close their contacts. The low-energy CR directional relay has been developed to be used in the same way as the standard CR relay, except that it is suitable where only a small quantity of energy is available for the operation of the relay.
The low-energy characteristic of the relay has been obtained by substituting the low-energy CO overcurrent element for the standard CO overcurrent element.
Thus the low-energy CR relay has overcurrent characteristics similar to those of the low-energy CO relay, and directional element characteristics similar to those of the standard CR relay.
In cases where it is desired to have an indication of the current flowing in the circuit, the COA overcurrent element has been used in the CR relay in place of the standard CO element.
This makes a compact, convenient arrangement serving the purpose of an ammeter without adding greatly to the expense of installation or detracting at all from the operating characteristics of the relay. On large complicated transmission systems the method of sectionalizing by means of overcurrent and directional relays is often difficult to apply and sometimes even impossible.
This is due to the necessity of employing such a large number of relays that it is impossible to keep the timing sequence within safe limits. At times it is impossible to secure proper discrimination at all, especially in the case of a complicated loop which is fed at several points, and where the points of feed change with load conditions. The Westinghouse Company has recently developed a relay which overcomes all such difficulties experienced on large and complicated systems.
Its application does not supplant that of the standard CO and CR relays, but makes protection possible where otherwise it would be difficult or even impossible. The only requisite is that, with a short at the far end of the line, the impedance drop in the line should be at least 3. The CZ relay consists of an overcurrent element and a voltage element. The overcurrent element is similar to that of the CO relay. The special feature of this relay is the construction of the voltage element.
The voltage element is so constructed that its action in relation to the closing of the relay contacts depends on the distance the relay is away from the fault. In other words, the voltage element acts in opposition to the overcurrent element, and when the relay is a considerable distance from the fault, the voltage on the element is comparatively high, thus lengthening the time required for the relay to close its contacts. On the other hand; the relays near the fault will have a very low voltage imposed on the voltage element, thus causing the relay to operate in much less time, due to less opposition being offered to the action of the overload element by the voltage element.
Thus the relay that is nearest to the fault operates first, and at no time does the time exceed 0. For some applications it is necessary to restrain the relay from operating unless the power is flowing in a given direction. To fill such needs a directional element similar to that of the CR relay is added to the CZ relay. The type CA ratio differential relay, Fig. As the name implies, it operates differentially, but it differs from the relays usually used in that the tripping current varies with the load.
The current required to trip the relay increases in proportion to the line current instead of remaining constant for all load conditions.
The CA relay is designed to avoid this trouble, and will trip out generators and motors on a trouble current as low as 2. For protection of star-delta transformer banks, Scott connected banks, or in any case where the currents in the circuits to be balanced are unequal, the relay is provided with taps, so that balancing auto transformers are unnecessary.
A saving in expense and in switch-board space is made possible by the provision of these taps. This relay is less sensitive than that used for generator protection. In order to get the ratio feature of the relay, both of the secondary currents to be balanced, as well as the differential current, are used in the relay. A field proportional to the sum of the currents reacts with a field proportional to their difference, and the resultant field acts to produce rotation.
The quadrature flux is supplied to the upper poles by transformer windings on each of the lower poles, as shown in the wiring diagram, Fig. The arrows show the relative instantaneous directions of current flow under normal conditions. If the currents supplied by each transformer are equal, there will be no current flow in Coil A. If the current transformers supply unequal currents, the difference flows through coil A. The direction of current in coil A with respect to coil B does not affect the tripping direction of the relay.
The flux produced by coil B tends to hold the relay open, and that due to coil A tends to trip the relay. As the line currents increase, the torque holding the relay open increases, and more torque and consequently more current is required to close it. The sensitivity cannot be varied on this relay. The energy required to operate the relays, particularly the transformer relay, is very small. This, combined with the fact that the operating current increases in proportion to the line currents, makes the relays very adaptable to conditions requiring the use of bushing-type current transformers.
When greater sensitivity is required, as in the case of the generator relay, the burden of the relay is somewhat greater, but under these conditions the current transformers used are of higher grade, and the load is well within the capacity of the current transformers.
The relay is very rugged, there are no adjustments to get out of order, and no delicate mechanical parts to give trouble. The contacts are set well out in front, where they can be seen through the glass cover, and where they are easily accessible for adjustments or repair.
All the relays are provided with two trip circuits and with an operation indicator and contactor switch, as in the CO relay. The cost of the current transformers used with relays often forms a considerable part of the entire cost. Where the current transformers must be connected to high-voltage circuits, they must be insulated for the high voltages; and their cost is high.
The CB relay has been designed for use on high-voltage installations. It is a combination of the CO overcurrent relay and the BT transfer relay with the addition of a special contactor switch. A standard transformer is used without regard to insulation for the high-voltage line, and the CO relay element is connected in the usual manner. The CO and BT relays, however, with the current transformer, are mounted on a slate base which is mounted on an insulator.
The contactor switch is separate from this assembly and is connected to the BT relay by means of a micarta chain. Thus the tripping circuit is insulated from the high-voltage circuit which is to be protected. When the CO relay contacts close they serve to operate the BT transfer relay. This relay, in turn, by means of the micarta chain, closes the tripping-circuit contactor switch. The operating characteristics of the CB relay are the same as those of the standard CO relay.
Balanced-current protection is very often desired where two or more similar transmission lines are used to connect two stations or two different points in a system. Balance-current protection on such lines means that each line normally carries approximately the same current, and if this condition does not exist the line carrying the greatest current must be in fault.
The CD relay furnishes a simple method of securing such protection. It avoids the necessity for any special apparatus, such as split conductor cable, and as it operates on current alone, potential connections, or potential transformers are not required. The relay operates on the induction principle and is in reality two overload elements so located that they act in opposition upon a common disc through a common magnetic circuit.
Each element is connected separately to its own current transformer and to corresponding phases of the two lines that are to be protected. These two elements are electrically opposed, and under conditions of balanced line load, which will give approximately the same current in each element, the fluxes in the magnetic circuit of the relay are equal and opposite, giving a resultant zero torque in the relay disc.
When a fault occurs in either line, however, the current in that line will become greater and the unbalance will cause the relay to operate, tripping out the line in which the greater current is flowing. The CD relay has inverse-time characteristics and also a definite-minimum time of operation with excessive overcurrent. The inherent characteristics of the relay are such that when one line of a pair is tripped out double the normal current is required in the other line before it will be tripped out, thus making it possible for one of a pair of protected lines to be open and the other line to carry its load without interference from the relay.
Any number of lines may be balanced against one another, but the tripping out of certain lines in a group will necessarily destroy the balance connection between the remaining lines. The CD relay has the same current adjustment and time adjustment as that of the CO relay. The time adjustment is, however, seldom of importance, inasmuch as the relays usually are desired to operate almost instantaneously.
In many instances a motor is so connected to its load that under certain conditions the load may drive the motor, causing it to act as a generator. A mine hoist driven by an induction motor is a good example of such an installation. In such cases it is sometimes necessary to provide some sort of protection to insure against having the motor return too much power to the line when it is being driven as a generator.
The CW power relay serves this purpose. It is also often used to prevent undesirable or excessive interchange of power between parts of a system, as in the case of a small plant connected in parallel with a larger plant where some means must be provided to prevent the small plant from being subjected to the entire load of the system, should the larger plant be suddenly disconnected.
The CW relay is essentially a contact-making watt-meter. It operates on the induction principle and is similar to the CO relay, except that it requires both current and potential connections. It closes its contacts in but one direction and only after a predetermined time, when a predetermined quantity of power is flowing in a predetermined direction, which direction may be either the normal or the reverse, according to the way the relay is connected.
The relay has a standard time adjustment and also taps for changing the wattage values necessary to trip the relay. The CV relay is designed for general use when it is desired to have circuits or apparatus protected against voltage changes of any predetermined value.
It operates to close a circuit either for the purpose of disconnecting the apparatus or of actuating a signal. The time-delay characteristic of the relay is used also in many applications where a timing-device is desired. In these applications the relay is used to close a circuit after a predetermined interval when the relay winding is energized.
This relay is similar in appearance to the CO relay and operates on the same principle. The chief difference between the two types is that the CV relay operates on voltage, whereas the CO relay operates on current. The latest design of the CV relay is provided with a voltage adjustment, by means of which the operating voltage can be adjusted over a wide range. This adjustment, which is shown in Figure 97, consists of a slide-wire resistor connected in the upper pole circuit of the relay.
The resistor is provided with a calibrated scale so that the relay can be readily set to close its contacts at the desired voltage value. The time adjustment of the relay is identical with that of the CO relay.
One form of the CV relay closes its contacts only when the voltage is raised to the operating-point or above, and another form closes its contacts only when the voltage is lowered to the operating point or below that value.
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