Differential protective circuit

SWEDEN

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PUBLISHING WRITING No. 359 409 mtci H 02 h 3/26

Patent Application. no 15158/70 Received on 10X11970

PATENT AND REGISTERED GOVERNMENT

Validity Day on

Ans. widely available on

XI 1970 12 V 1971

Ans. published and published · written on 27 VIII 1973

Priority requested from 11 XI 1969 (Federal Republic of Germany DT, 19 56 527)

SIEMENS AG, BERLIN AND MÖNCHEN, FEDERAL REPUBLIC OF GERMANY DT Inventor: A Fendt, Erlangen and E Haenicke, Berlin Representative: N Larfeldt

The differential protection device

The present invention relates to a differential protection device by means of which one can protect electrical means and wires when current transformers are connected to the electrical outputs of these means and wires. In the differential protection device, based on the secondary currents of the current transformers, it is tested whether the sum current of the currents flowing into the plant to be protected in accordance with Kirchhoff's law is equal to zero. Theoretically, this applies in the flawless state only when the capacitance of the plant to be protected is negligible and when the current transformers used for the purpose convert the current flowing in the primary circuit in a correct way in terms of current and phase position.

Since these conditions are not met in practice, known differential protection devices are provided with a so-called stabilization coupling to avoid failure tripping. In a two-pole differential protection device, the stabilizing circuit delivers a current that counteracts the trigger current acting on the differential protection relay. This stabilization current is proportional to the current flowing through the system to be protected in two-pole differential protection devices. The differential protection device thus becomes insensitive to an external short circuit with increasing short circuit current. In the case of more than 2 pole differential protection devices (eg transformers with three windings, multi-branch busbars), the stabilizing current is formed by the sum of the amounts of the currents flowing into the system to be protected.

5th When the triggering and stabilizing current of these known differential protection devices are rectified and in opposite directions act on a differential protection relay and a contact-free working indicator, it is necessary to obtain the average measurement of the rectangular and rectifier in order to obtain a correct measurement of the fault location. the currents took. This means that the triggering time cannot be made as small as possible. Admittedly, by using fast contact-free indicators, it is possible to achieve that the differential protection device responds to the instantaneous value of the tripping current and the blocking current. Due to current current transformer errors, for example due to saturation, is

15th however, it is fully conceivable that at a certain point the instantaneous value of the blocking current is small and that the instantaneous value of the triggering current is relatively large, so that, as a result of the comparison coupling, a contact-free indicator would react even if the fault is outside the protection area.

20th The object of the invention is to provide a differential protection device which permits a measurement dependent on the instantaneous values ​​of the sum current formed in the differential protection device, or, where applicable, the blocking current.

The invention therefore relates to a differential protection device 25. understanding the outputs of a plant (line, machine, transformer, busbar) to be protected, connected current transformers and an additional connection, which is connected to the secondary windings of the transformers, and which forms the geometric sum of the currents flowing into the plant to be protected, as well

30 °. which supplies an electrical quantity derived from this sum to an indicator connected in series (threshold step, relay winding) whose output affects a circuit breaker for switching off the system to be protected. The novelty of the invention means that an indicator is connected in series with a switch, which via an OR-

35th circuit is connected to the outputs of the threshold step. whose outputs via each coupling for sensing the current transformer saturation are connected to the secondary winding of a saturable current transformer, whereby the switch connected to the indicator is opened when a current transformer comes to saturation.

40th Thus, the operation of the device according to the invention is based on the fact that even in the case of overloaded current transformers, which operate in the saturation area, the flow sometimes decreases to values ​​below the saturation point. In particular, the benefit is derived from the fact that all current transformers are unsaturated, and consequently turn over correctly,

5th before an error occurs. Thus, in addition to the triggering current of the device according to the invention, which is in accordance with known differential protection devices, a blocking current affects the indicator, then the influence of the blocking current can be made much less than in known differential protection devices. Thus, the differential protection device is

10th even in the case of the current and the trigger current, the reverse current is significantly more sensitive than a differential protection device which is based on the integration of the instantaneous values ​​of the trigger and the blocking current over several periods.

For registration of the current transformer saturation, for example

15th a harmonic filter is plugged into the secondary circuit of a current transformer. When, after this harmonic filter, a limit value step is switched on, which responds when the harmonic content in the secondary winding of the current transformer exceeds a certain definite limit, then the high harmonic level occurring at the beginning of the current transformer can be utilized20. in the secondary winding of the current transformer to determine the time when the current transformer starts to saturate.

The invention is explained in more detail below with reference to the drawings, in which: Fig. 1 shows an exemplary embodiment of a coupling for. sensing of saturation in the transformer, which coupling can emit one

25th signal already when the transformer starts saturating; Fig. 2 shows a coupling which allows quick detection of a beginning short circuit; and where Fig. 3 shows the block diagram of an embodiment of the differential protection device according to the invention.

The registration coupling shown in Fig. 1 comprises a boundary 30. value step and a formerly integrated integrating circuit. To this, a mirror resistor Z is connected in the secondary winding of a current transformer W1 and in series with the other consumers N.This mirror resistor Z must be proportional to the total resistance charged to the current transformer W1. If so, the mirror resistance Z appears

35th a voltage proportional to the voltage across the secondary winding of the current transformer W1. Parallel to the mirror resistor Z lies the series connection of an ohmic resistor R1 and a capacitor C1. Via a Graetz-connected rectifier G1, the behavior of the capacitor C1 is rectified and against the integral of the voltage across the current transformer.

40th Wl secondary winding proportional voltage drop. Parallel to the rectifier G1's DC connections is a threshold value step GS1, which in known manner consists of two transistors T1, T2 and at its output terminals K1 gives a voltage when a certain specific input voltage value is exceeded.

5th The input of the threshold step GS1 is parallel to that of the rectifier

Gl DC connections are provided with a zener diode ZD. The control voltage of transistor TI is taken from the zener diode ZD.

In Fig. 2 a connection is shown which allows quick registration of the start to a short circuit. For this purpose, current change 10 is utilized. the speed of a current transformer W2. In the secondary circuit of the current transformer W1, a resistor R3 is coupled and in parallel there are coupled alternating current terminals of a rectifier G2 in series with a capacitor C3. Thereby, on the DC terminals of the rectifier G2, a current is proportional to the derivative of the current through current15. transformer W1, which current, depending on the internal resistance of the threshold value stage GS2 connected thereto, results in an input voltage to the threshold circuit which, when exceeding a certain threshold value, causes an output voltage at the threshold value stage GS2 output terminal K2. For smoothing the voltage peaks, it is at the threshold value20. in step GS2, the zener diode ZD provided an RC circuit which includes a resistor R2 and a capacitor C2. The capacitance of capacitor C2 is thus so small that no significant delay of the input signal can occur. Incidentally, the threshold circuit GS2 is constructed in exactly the same way as the threshold circuit GS1 in Fig. 1. The components of

25th the threshold value step GS2 therefore has the same designations as those in the threshold value step GS1.

Fig. 3 shows a block diagram of the differential protection device according to the invention. The fault detection couplings according to Fig. 2 are there designated as derivative circuits with the letter D and cup 30. the wires for recording the voltage integrals on the secondary windings of the current transformers are designated as integration circuits with the letter I. In the same way as with known differential protection devices, secondary winding of the current transformers W1, W2 and W3 are connected to the intermediate current transformers ZW1, ZW2 and ZW3, whose secondary 35. windings are closed over rectifiers G4, G5 and G6. The rectifiers G4,

G5 and G6 DC terminals are mutually parallel and connected antiparallel to the DC terminals of a further rectifier G7. The rectifier G7 is connected to the secondary winding of an intermediate current transformer ZW4, whose primary winding is part of a current circuit ge40. by which flows the sum current of the secondary currents in the current transformers W1, W2 and W3 ·

The secondary windings of the current transformers W1, W2 and W3 and the primary windings of the intermediate current transformers ZW1, ZW2 and ZW3 are connected to the inputs of the derivation circuits D1, D2 and D3, which, as shown in Figure 2, each includes a derivation coupling and a

5th in series with this coupled threshold value step. In addition, in series with each derivation circuit there is an integration circuit II, 12 and I3_S each of which, as shown in Fig. 1, comprises an integration coupling and a threshold value step coupled to it. The outputs of the derivation circuits D1-D3 and the integration circuits 11-13 are parallel cup10. laid with each other via relaxation diodes without designation in Fig. 3. The output signals of the derivative circuits D1-D3 thereby affect switches S2, which are connected to the output of the parallel-connected and anti-parallel-connected rectifiers G4-G7, and connect this output with an indicator ID when one or more of the derivation circuits D1-D3 emit

15th an output signal.

Via further diodes not shown in Fig. 3, the outputs of the integration stage are always connected in parallel and connected to a switch S1, which is connected between the output ID of the indicator and the bistable flip-flop KS. This switch closes the connection between

20th the indicator ID and the bistable flip-flop when at least one of the integration circuits 11-13 emits an output signal. The output of the bistable rocker KS is not connected to the circuit-breaker inputs for breaking the circuit-breakers, which initiates disconnection of the system to be protected from the other network.

25th The bistable rocker KS is provided with an additional input, which via a timing circuit t2 which is connected to the output of the derivative circuits D1-D3 connected in parallel. The delay time in this timing t2 amounts to slightly more than one period of the AC voltage, which means that the bistable rocker after disconnection30. one due to a previously occurring short circuit, ie. when none of the derivation circuits D1-D3 no longer output any output, it is reset after a period of time, and the differential protection device thereby becomes operational again.

When a short circuit occurs inside or outside the protection area '

35th Thus, at least one of the derivation circuits D1-D3 will output an output signal and connect the indicator ID to the output of the rectifiers G4-G7 connected in parallel. Depending on the instantaneous value of the sum current that will be received from the parallel connection, the indicator ID will respond or not. In the first moment after that

40th that a fault occurring inside or outside the system to be protected is not yet saturated by the current transformers W1-W3, ie. the voltage integral, which is formed in the integration connections in the integration steps 11-13, has not yet reached the value required for the threshold values ​​steps contained therein to react. Because nothing

5 of the integration steps 11-13 emit some output, switch S1 is closed. If the fault lies within the protection area, then the trigger current in rectifier G7 will consider and allow the indicator to react. This causes the bistable flip-flop KS to be switched via the closed switch S1, which in turn gives an output signal for

10th breaking of the circuit breakers belonging to the system which 'shall be protected. As soon as one of the three current transformers W1, W2 or W3 comes close to the saturation limit, the associated integration stage II, 12 or 13 will output an output which switches the switch S1 so that the bistable flip-flop KS independent of the indicator

15th The ID output remains in its set position.

This also applies to cases where the fault is outside the protection area. In this case, at the beginning of the fault triggering current via rectifier G7 will assume a value in the vicinity of zero due to the not yet saturated current transformers W1, W2 and W3. Indicator ID

20th can therefore not supply any output signal. As soon as one of the three current transformers W1, W2 and W3 is saturated, the switch disconnects the indicator ID from the subsequent bistable flip-flop KS and a resultant saturation of one of the current transformers can affect the indicator ID, but does not cause any

25th disconnection order for the associated circuit breakers.

One particular advantage is obtained in that the indicator ID provided by switch S2 at each current zero passage (= maximum current change) is coupled to the output of the parallel connected rectifiers G4-G7. This starts during each period of the alternating voltage

30 °. a new measurement time, so that an error in the protection area, which occurs only after an error has occurred outside the protection area, can also be detected.

When connecting a differential protection device for transformers, known differential protection devices have built-in time delay35. delay means for avoiding error triggers. This is no longer possible with a quick differential protection according to the invention. However, when using this differential protection device for transformers, one may proceed such that a separate differential protection device is provided for each transformer winding and to both ends40. the dummy of each transformer winding connects current transformers,

Λ which supplies the input sizes of the differential protection device.