Page 50 - Energize June 2022
P. 50

TECHNICAL



        This contact is burned off and       and other factors can adversely affect the operating time of a circuit breaker which has been
        becomes shorter at each interruption   idle for some time.
        of load current. Dynamic resistance    The “first trip” will typically represent and reveal the circuit breaker’s true operating
        measurements clearly reveal this     characteristics, unlike subsequent operations where mechanism components are loosened
        shortening of the arcing contact. To obtain   or freed up. Comparisons between the “first trip” and second or third breaker operations can
        reliable DRM data a high test current is   also reveal problems and should be used in determining the breaker’s condition.
        required as well as test equipment with   The biggest benefit of using first trip testing is to test “real world” operating conditions. If
        good voltage measurement resolution.  the circuit breaker has not operated for year, first trip testing will show if the circuit breaker
                                             is slower due to problems such as corrosion in the mechanism linkages. With traditional
        Switch operation test (timing        methods, the testing is carried out after the circuit breaker has been taken out of service
        analysis)                            and has been operated once or twice. The interpretation of circuit breaker operating coil
        The trip or operation test is applied to   signatures may provide information about the condition of the latching systems.
        confirm that the switch operates correctly   Referring to Figure 3, when a trip coil is first energised (1), current flows through its
        and to measure the state of the operating   windings. The coil magnetises the iron core of the armature, in effect inducing a force in the
        mechanism. The mechanism operation   armature. The current flowing through the trip coil increases, to the point where the force
        cannot be observed or measured directly,   exerted on the armature is sufficient to overcome combined gravitational and friction forces
        so use is made of the operating coil   which may be exerted, pulling it through the trip coil core (2).
        current as a proxy measurement. By     The magnitude of the initial current (1-2) is proportional to the energy required to move
        analysing the current consumed by the trip   the armature from its initial rest position. The movement of the iron core through the trip coil
        coil during the circuit breaker’s operation,   generates a back emf in the coil, which in turn has an effect on the current flowing through it.
        it is possible to determine whether there   The rate of rise of current depends on the change in the inductance of the coil.
        are mechanical or electrical issues present.   The armature operates the trip latch (3-4), which in turn collapses the trip mechanism
        In many cases, such issues can be localised   (4-5). More energy is required for the armature to resume motion and overcome the
        and help to find the root cause. Optionally,   additional loading of the mechanism. The anomaly at (4) may be caused by degradation of
        monitoring the tripping supply’s voltage   the mechanism bearings, lubrication, changes in temperature, excessive opening spring force
        during the operation can detect issues   or mechanism adjustment. The armature completes its travel (4-5) and hits a stop (5).
        arising with tripping batteries.       Of interest is the curve (4-5). As the armature moves from the point where the trip
                                             mechanism is unlatched (4) to the stop (5) the inductance of the coil changes. The curve is an
        First trip test                      indication of the speed of the armature. The steeper the curve the faster the armature is moving.
        A first trip test is important to determine   After the armature has completed its travel and has hit the stop (5), there is a change in
        the condition of the coil operating   the current pattern. The magnitude of the current (7) is dependent on the DC resistance of
        mechanism and give information how the   the coil. The auxiliary contact opens (8) to de-energise the trip coil and the current decays
        circuit breaker would perform in a real-life   to zero.
        fault situation. Therefore, capturing the
        first trip operation is essential to effective
        circuit breaker condition-monitoring. A
        circuit breaker spends most of its lifetime
        conducting current without operating.
        Once the protection relay detects a
        problem, the circuit breaker, which was
        idle for a long time, has to operate as
        quickly as possible. However, if the circuit
        breaker has not been operated for a long
        time, the latch friction may increase.
        Frequently, this problem is cleared after
        the first operation of the breaker, and thus
        the original cause cannot be detected in
        subsequent testing.
           Information about the latch friction
        can be obtained from the coil current
        waveform recorded during the first trip
        operation. When a fault occurs, it is
        expected of the circuit breaker to operate
        correctly. Unfortunately, environmental
        contaminants, hardened grease, vibration   Figure 3: Operating coil current (Megger)



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