DE2713587A1 - PROCEDURE AND EQUIPMENT FOR MULTI-PHASE OVERCURRENT MONITORING - Google Patents

Description

Fd / dh 2/24/77

BBC Public Company Brown, Boveri & Cie. , Baden (Switzerland)

Method and device for multiphase overcurrent monitoring

The invention relates to a method for multiphase overcurrent monitoring, in which the phase current amplitudes dependent measurement signals are compared separately with a limit value and exceeding the limit value as a short-circuit indicator is used.

The subject matter of the invention also includes a device for carrying out such a method.

Usual overcurrent relays compare the phase currents with fixed limit values and trigger the associated ones if the limit value is exceeded Protective device. A second delay between pickup and tripping or switch-off enables accordingly staggered setting a locally selective effect of the overcurrent protection. There is a difficulty with this

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Overcurrent protection in that the short-circuit currents may be in the range of the upper operating current limit, especially in the event of short circuits in a low-load operating state. The response limit values for the phase current comparison can therefore only be set relatively reliably from case to case on the basis of empirical values, however either the non-detection of a certain low-load short-circuit area or unnecessary tripping when on permissible limit loads are still to be accepted in fault-free operation.

The object of the invention is therefore to create a method and a device for multiphase overcurrent monitoring, which enables an improved differentiation between short-circuit and limit normal operating states.

The inventive solution to this problem characterizes With regard to the method in connection with the feature mentioned at the beginning, the fact that for the comparison For the measurement signals, a limit value is used that changes in the same direction as the respective smallest measurement signal Share includes.

The device according to the invention for solving the task at hand comprises a measuring signal transmitter for each phase current, which supplies a measurement signal that changes in the same direction as the phase current amplitude, as well as a limit value meter and

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a comparator for each phase current, with a first comparator input to a measuring signal generator and a second comparator input is connected to the limit switch.

This solution is characterized by the fact that the limit value transmitter has corresponding inputs to the measuring signal transmitter is connected and has a minimum value selection circuit, each of which is only the smallest in terms of amount Sets the measuring signal with the output of the limit indicator in operative connection.

Accordingly, the current comparison is not carried out with a fixed response or threshold value, but with one The comparison signal dependent on the operating current, which is derived from the current of the - apart from the case of a three-phase short circuit - from Short circuit not affected phase depends in the same direction.

This is done in an advantageously simple manner through the selection of the smallest value reached under the measurement signals representing the phase current amplitudes. This automatic adaptation of the comparison signal This means that in the event of a short circuit that is superimposed on a low-load operating state, the phase currents can be compared with a lower response threshold than in the case of a short circuit in an operating state with stronger Load. The overcurrent protection can therefore be comparatively insensitive can be adjusted, making the area unnecessary

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Disconnections is restricted while all short circuits are still detected with sufficient certainty.

The limit value transmitter expediently has one of the lowest value selection circuits on the output side additively connected, preferably adjustable constant signal generator. The effective The response threshold is then made up of a component corresponding to the operating current and a constant component together, so that for the phase affected by the short-circuit, the difference between the short-circuit current and the operating current is compared with a predetermined response threshold. So you can use the fixed portion of the comparison signal to directly specify the amount by which a phase current may not exceed the current of the most weakly loaded phase.

The invention is further illustrated by the exemplary embodiment illustrated in the drawing by a basic circuit diagram explained.

The circuit includes three phase current measuring devices on the input side.

signal transmitter MG_, MG ₀ , MG _n , with current transformers Ζ _Ί , to Z _T sott ο X Xj

like measuring rectifiers G ₁ to G, and subsequent smoothing or filter capacitors. The latter resistors R _R , R _g , R _{T connected in} parallel thus lead to voltage drops which, with a certain time delay, correspond to the amplitudes of the

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associated phase currents and which are designated as measurement signals I _n , I ₀ , Im for the sake of simplicity. These measurement signals are connected to corresponding inputs of a limit value transmitter GG, which has one of the illustrated diodes connected together on the anode side at point A for each measurement signal. Point A is connected via a first constant signal generator KG in the form of an adjustable resistor to the output AGG of the smallest value transmitter. This output is in turn connected to the plus of a direct current supply _{via a series resistor R 1.}

The diodes of the limit indicator form a minimum value selection circuit Smin, which in each case only switches through the measurement signal of the smallest amount via the associated diode applied in the forward direction, while the other diodes are blocked and separate the associated measurement signals from point A. The forward current of the respective conducting diode is supplied by plus via R and KG. As a result of the correspondingly high-resistance dimensioning of R _{1, however} , the potential at point A is essentially only determined by the measurement signal that is switched through. _{The voltage drop across the resistor of KG 1} lies between point A and the output AGG of the limit switch, which means that the short-circuit response increase with regard to the respective operating current can be set in the form of an additive limit value component.

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The measurement signals I ". I ₀ and I _m are each connected to the non-inverting input of a comparator VG_ or VG ₃ or VG ₁ -, the inverting inputs of which are connected together to the output AGG of the limit indicator. If one of the measurement signals exceeds the common limit value at the output AGG, ie the lowest phase current plus the additive limit value component at KG, a positive signal occurs at the output of the relevant comparator, which in the example acts as an affirmative dual signal "1" for a subsequent logic circuit, while a negative output signal acts as a negative dual signal "0". The comparators thus act as limit switches that respond to a short circuit to be detected in the associated phase with the output signal "1".

Each one acts via an AND circuit VU _R or VU _g or VU _T and an OR circuit V0 _R or VO ₃ or V0 _T as well as a timing element T _R or T "or Tm and an unspecified amplifier Comparator output to a trigger switch S _R or S or S _m and / or to one of the position indicators L _n or L ₀

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and L, p. In the example, there is simultaneous display and triggering indicated. Each phase overcurrent monitoring can therefore selectively display a corresponding display and / or shutdown trigger. A common input ER is available for resetting the display. In the example, the trip is continue from a common, unselective suggestion

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an input EUS dependent, which is connected on the input side with the comparator outputs to the AND circuits VU _R , VU ₃ , VUrp. This design can be used, for example, for busbar protection, the monitoring circuit shown being arranged at an infeed point of the busbar, while the individual busbar outputs have overcurrent relays that are non-phase-selective but respond earlier due to a corresponding time delay. The outputs of these outgoing relays are then to be connected disjunctive (via an OR circuit) to the input EUS in a manner not shown. This arrangement has the advantage that the outlay for a phase-selective overcurrent detector is only required at the feed point, while in cooperation with the simple outgoing relay a phase-selective display or shutdown is nevertheless ensured. For _{this purpose, the timers T R} , T ₃ , T _T are to be set to a delay which is greater than that of the outgoing relays.

Provision has still been made for the case of the three-phase short circuit, in which generally only minor ones There are differences between the phase currents and therefore no tripping via the comparator despite the excessive current level can occur because the limit value generated is also too high. The triggering takes place with the help of the unselective Suggestion about EUS in conjunction with the non-

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all comparators VG _R , VG_, VGm respond · This logic operation takes place via an OR circuit VO with subsequent inverter Vi and by means of an AND circuit VU in the manner shown. An input EAP of an _{all-phase disconnection or short-circuit indication given by the OR circuits V0 R} , VO, V0 _T is connected to the output of the aforementioned AND circuit VU via an OR circuit VO which serves further triggering purposes.

Furthermore, in the example, a special overcurrent monitoring for high-current short circuits is also provided, for example with regard to short circuits directly on the busbar, the feed point of which is provided with the present monitoring circuit. In this case, the triggering takes place independently of an excitation via EUS by a maximum value selection circuit Smax, which is connected to the measurement signals I _R or I or I_ with a correspondingly polarized diode and the largest measurement signal to a common comparator VG switches. The other input of this comparator is connected to a second constant signal generator KG- which is set to a correspondingly high overcurrent value as a response threshold. This high-air overcurrent protection also responds to three-phase short circuits and thus represents an additional safety element.

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Claims (5)

BBC Baden patent claims 1.J Method for multiphase overcurrent monitoring, in which measurement signals dependent on the phase current amplitudes are compared separately with a limit value and exceeding the limit value is used as a short-circuit indicator, characterized in that a limit value is used to compare the measurement signals (I _n , I_, I_) is used, which comprises a portion that is variable in the same direction as the smallest measurement signal in each case. 2. Device for performing the method according to claim 1, comprising a measuring signal transmitter for each phase current, which supplies a measurement signal that is variable in the same direction as the phase current amplitude, as well as a Limit value transmitter and a comparator for each phase current, each with a first comparator input is connected to a measuring signal transmitter and a second comparator input is connected to the limit value transmitter, characterized in that the limit value transmitter (GG) has corresponding inputs to the measuring signal transmitter ( ^MG R * ^MG s » ^MG T ^ is connected and has a minimum value selection circuit (Smin), each of which only the smallest measurement signal with the 809835 / 0U9 BBC Baden It 1358V . S- ^16/7? Sets the output (AGG) of the limit indicator in operative connection. 3. Device according to claim 2, characterized in that the limit value transmitter (GG) is one of the minimum value selection circuit (Smin) on the output side, preferably adjustable constant signal generator connected additively (KG,). 4. Device according to claim 2 or 3, characterized in that the outputs of the comparators (VG _n , VG ₀ , VG _m ) which are in the affirmative when the limit value is exceeded η ο 1 a negative OR circuit (VO) are merged, the output of which is combined with an unselective excitation channel a short-circuit protection device is combined via an AND circuit (VU), and that the Output of this AND circuit (VU) with the trigger input (EAP) of an all-phase disconnection or short-circuit indicator connected is. 5. Device according to one of claims 2 to 4, characterized in that the outputs of the measuring signal _{generators (MG n} , MG ₀ , MG_) to corresponding inputs of a large ο 1 value selection circuit (Smax) are connected, their Output together with a preferably adjustable constant signal generator (KG_) to corresponding inputs 809835/0449 BBC Baden ο 16/77 • «j ' a common comparator (VG), the output of which is connected to the trigger input (EAP) of an all-phase disconnection or short-circuit indicator. BBC Public Company Brown, Boveri & Cie. 809835/0449