METHOD AND APPARATUS FOR THE DETERMINATION OF THE SHORT-CIRCUIT CURRENT THAT WOULD FLOW UPON OCCURENCE OF A SHORT CIRCUIT IN THREE-PHASE ELECTRICAL SUPPLY NETWORKS

摘要

1,268,546. Measuring characteristics of polyphase networks. ASSOCIATION LYONNAISE DES PROPRIETAIRES D'APPAREILS A VAPEUR ET ELECTRIQUES. June 2, 1969 [Oct. 24, 1968], No. 27801/69. Heading G1U. To determine the short circuit current that would flow on occurrence of a short circuit between any two or all three phases of a three-phase network 2, an unbalancing load 2 is connected between two of the phases and measurements are made of the current in this load and of the negative phase sequence component of the network voltage, from which measurements the required short circuit current may be evaluated according to formulae given in the Specification. To determine the current that would flow on occurrence of a short circuit between one phase and neutral 3, an unbalancing load is connected between one phase and neutral and measurements are made of the load current and negative sequence voltage component, as above, and also of the zero sequence component, from which measurements again the required short-circuit current may be evaluated. Preferably in the latter case, the sum of the negative sequence component plus half the zero sequence component is determined directly, since the appropriate formula requires the evaluation of this sum. Apparatus for carrying out the above measurements comprises an unbalancing resistive load Z in series with ammeter 5 and connectible, by means of switches 4, 14, either between two phases or between one phase and neutral. Detectors 6, 7, transformer-coupled to the network provide outputs proportional to the negative phase sequence and zero phase sequence components respectively. A switch 16, ganged to switch 4, selects either the output of detector 6 alone or the combination of the outputs of detectors 6 and 7, according to the measurements to be made, the output of switch 16 being passed via harmonic suppressor 8 and amplifier 9 to a meter 11. A voltage divider 12 is connected between two phases of the network to provide a known calibrating voltage, which may be selected by switch 13 in order to calibrate meter 11, the calibration being altered according to the type of short circuit to be investigated. To compensate for any unbalance in an existing load of the network before switch 14 is closed, a correcting device 15 is provided, which produces an output adjustable in amplitude and phase, enabling the meter reading to be brought to zero initially.