STATIC RELAYS FOR THE PROTECTION OF 25 kV SINGLE-PHASE TRACTION OVERHEAD EQUIPMENT

Abstract

The electromechanical relays, presently used for the pro tection of the overhead equipment of the 25-kV single-phase 5C~Hz traction system of the Indian Railways, lack good selectivity and fast operation. Unnecessary interruptions in the normal working of the system and lack of definite indication of the type of fault result from the poor selectivity and unavoidable delays in clearing faults from the slow relay operation. The problem has been investigated by the author and modification of the operating characteristics and use of static relays suggested as an effective remedy. It is shown that only a quadrilateral characteristic in place of the present admittance characteristic of the earth fault distance relay can ensure the desired selectivity of the relay to various operating conditions of the system. It is further observed that for the wrong phase coupling (phase-to-phase fault) protection, the non-offset admittance characteristic, wherever in use, should be discarded because of its unwanted operations and the offset admittance characteristic,being used more commonly, may also be advantageously replaced by a restricted directional characteristic. For a faster backup protection on earth faults, a definite time overcurrent relay is recommended in place of the existing inverse definite minimum time overcurrent relay. The problem of providing d.c. supplies for the operation of solid-state relay-circuits, which pertains to the design and application of static relays in general, has been investigated. A novel method of obtaining d.c. supplies for distance and directional relays from the current and voltage transformer outputs jointly, without incorporating a storage battery with in the relay, has been devised. Arrangements for deriving d.c. supplies from both single phase and three phase currents and voltages have been proposed. Laboratory testing and successful application of the new method in static-relay prototypes establish its soundness. Circuits for the relays with simple measuring functions, to work without a d.c. supply, have also been proposed. Static relays, with the aforesaid modified characteristics and constituting a complete protection for the overhead equip ment, have been successfully developed. The restricted directional characteristic of the wrong phase coupling relay has been realised through the block-spike coincidence technique of phase-comparison. The transient overreach and tendency to maloperate on spurious spikes the troubles generally associated with this tech nique, have been overcome through the application of dualcomparator, judicial selection of replica impedance and provi sion of surge suppressors and diverters. D.C. supplies for the relay circuits have been obtained from the a.c. current and volt age within the relay using the aforementioned method. The relay has been tested in laboratory for its performance under steady-state and transient conditions. The results are gratifying. The quadrilateral distance relay, required specifically for the earth-fault protection of the traction overhead equip ment and having a large application potential in the power-line protection, has been developed using sampling principles. Improved -vmeasuring circuits for the purpose have been evolved and application, therein, of integrated circuit operational ampli fiers introduced. The circuits without operational amplifiers require conversion of the amplitude-samples into proportional pulse-widths for comparison. Analysis of the relay operation on unfavourable system conditions and laboratory tests under steady-state and transient conditions carried out on the relay prototype reveal a satisfactory performance. Application of the operational amplifier allows comparison of the instant aneous amplitudes of the relaying signals without an intermediate conversion to pulse-widths. Consequently, the relay circuitry is simplified, accuracy of measurement is bettered and several other advantages ensue. The relay prototype built with operational amplifier circuits has been thoroughly tested and found to work very satisfactorily. In both the prototypes of quadrilateral distance relay, the d.c. supplies are obtained internally from CT and VT outputs. Methods are suggested to realise other dis tance and directional relays also from the versatile comparator circuits applying operation; 1 amplifiers used here in the quadri lateral distance relay. Inverse definite minimum time, definite time and instant aneous overcurrent relays have been developed for the protection of the traction overiiead equipment in particular and that of distribution-networks in general. The method evolved for obtain ing the inverse definite minimum time overcurrent relay has the novelty that no nonlinear resistors are employed for getting the nonlinear time-current characteristic. The measuring circuits for the other two overcurrent relays are quite simple. In both the time overcurrent relays, d.c. supply is derived from the relay current using an auxiliary saturable CT. They have been tested for time-current characteristic, reset ratio, overshoot, frequency effect, etc. The instantaneous overcurrent relay circuit has been designed to work without a d.c. supply. It has been tested for transient overreach, operating time, frequency effect, etc. Performance of all the three relays is satisfactory. A new technique of realising inverse time-current charac teristics of overcurrent relays has been introduced. The method, based on piecemeal realization of a given curve, has the merits of the relay time-current characteristic being dictated by the values of certain linear components only and the feasibility of realizing any inverse time-current relay curve required to match the characteristic of protected equipment. A prototype built on this principle has been tested very satisfactorily. Lastly, a new approach to distance protection has been proposed. It consists in measuring the resistance and inductance, rather than the impedance, of the fault loop. A correct measure ment of the resistance and inductance, unlike that of the impedance, being obtainable under transient conditions, the principle can make the transient overreach of the relay theoretically zero. Details of the measuring scheme and circuits for obtaining qudrilateral distance relay with this principle are given. Use of integrated circuit operational amplifiers to simplify the relay circuitry and improve its accuracy is also dealt with at length. The prototypes of the restricted directional wrong phase coupling relay, quadrilateral distance earth-fault relay (applying loperational amplifiers), inverse definite minimum time backup overcurrent relay and instantaneous overcurrent (high-set) relay have been put on field tests in a 25 kV traction sub station of the Indian Railways, where,in general they are working satisfactorily. In conclusion, the work forms a contribution on two fronts explicitly. Development of static relays with improved characteristics and performance has solved the problem in hand, on one side. On the other side, the state of art of static protective relays has been advanced by the evolvement of new techniques for distance, directional and inverse time overcurrent relays, novel method for obtaining d.c. supplies in relays and improved electronic circuits for various, parti cularly the sampling type distance and directional, relays.