DE19801708C1 - Short circuit current limiter for limiting fault currents in power networks - Google Patents

Abstract

The short circuit current limiter in a main current circuit has a current switch off unit (1) for interrupting the phase current, in the event of fault current and in an auxiliary current circuit has a quenching circuit assigned to the current switch off unit. Each bridge branch of the bridge circuit of the quenching circuit (3) has at least one switchable quenching element (T(QpA)....). A control unit (2,5,6) is connectable with each of the quenching elements. With a detected fault current, two quenching elements arranged in the diagonal lying opposite are controlled in such a manner that the discharge current of the quenching capacitor (C(Q)) superimposes the current (i(Q)) branched from the main circuit in the same direction in the auxiliary current circuit.

Description

The invention relates to a short-circuit current limiter for the limitation before fault currents in an electrical Energy network according to the preamble of claim 1.

Protection of energy systems from short-circuit currents and a large one comes before surge short-circuit currents in particular Meaning too. For example, at less Short-circuit strength of a network section when connecting of electric drives in the kW and MW range or at Energy supply from decentralized Power generation plants in an energy supply network such protection may be necessary to the equipment of the network from additional mechanical stresses to protect.

In "The Critical Switching Parameters of a New Hybrid AC Low-Voltage Circuit Breaker Without and With ZnO Varistor" (Proceedings of the 6 ^th international Symposium on Short Circuit Currents in Power Systems Liège, Belgium, 6-8 September 1994, p. 3.11 .1-3.11.8; Hasan, Zyborski, Lipski) an overcurrent protection (short-circuit current limiter) is described in which a mechanical switch is used as a current cut-off for the interruption in a power supply line in the event of a short-circuit current, in parallel with which an extinguishing circuit is provided. The quenching circuit consists of a series connection of two thyristors connected in opposite directions and an antiparallel to a series connection of two diodes. The two series circuits connected in parallel are additionally connected to one another between the two thyristors or between the two diodes via a series circuit comprising an inductor and a capacitor. In the event of a short-circuit current, one of the thyristors is ignited, depending on the current direction of current, so that the short-circuit current can commutate into the quenching circuit while the mechanical switch is opened.

The invention has for its object the generic Develop short-circuit current limiter so that it improved protection against short circuit currents in electrical power networks.

This object is invented for one Short-circuit current limiter solved according to the preamble by the characteristics of the characteristic part of Claim 1.

The short-circuit current limiter according to the invention ensures that an occurring fault current within from the power cut-off to the The extinguishing circuit is commutated and thus mechanical Current cut-offs minimized to no arc is switched and the necessary Reconsolidation time is observed and at  power electronic power cut-offs the necessary Free time is guaranteed.

With the short-circuit current limiter according to the invention both generator / consumer and grid side Short circuits can be switched off.

By using at least one switchable Extinguishing element in each of the bridge branches of the Bridge circuit can also be a significantly higher Current steepness of the commutation can be achieved as this with one switchable bridge branch per Direction of current is possible, since this is initially a Circular discharge of the quenching capacitor within the Clear circuit is prevented and practically the  total discharge current passed through the cut-off switch becomes.

Finally, there is a possibility that Determine the zero crossing of the current cut-off current and thus at any value of the phase current without problems (with mechanical switches arc-minimized or arc-free) with the same Results for different currents and current steepness switch off. A forecast of the Circuit breaker current zero crossing is not necessary.

In a preferred embodiment it is provided that in addition to the two in diagonally opposite Bridge branches arranged at least one extinguishing elements another extinguishing element outside of these bridge branches is controlled such that within the Extinguishing circuit a freewheeling current path for the Discharge current of the quenching capacitor is formed, as soon as the current through the power switch to zero has become and the current cut-off the current path in the Main circuit has broken. That way avoided that the load current during the interruption of the main circuit due to the rest of the Quenching capacitor stored energy around a capacitive Peak increased.

Advantageously, in addition to the two in arranged diagonally opposite bridge branches delete elements already activated at least two further extinguishing elements in the other two diagonally  opposite bridge branches controlled in such a way that within the clear circuit two in parallel mutually arranged freewheeling current paths for the Discharge current of the quenching capacitor are formed. By activating both switchable extinguishing elements the thermal load on the individual is reduced Extinguishing element to about half.

The timing of the activation of the extinguishing circuit according to the invention is determined by the control unit. The short-circuit detection algorithm used for this purpose by the control unit is advantageously characterized in that

• - Current and / or current steepness within each phase the connection points of the extinguishing circuit measured become,
• - The measured values are low-pass filtered
• - a moving average from the low-pass filtered Values is formed
• - If the averaged current value is below a predetermined current threshold I _min or above a predetermined current threshold I _max , the extinguishing circuit is never activated, and
• - If the averaged current value is in a range Ba for which I _min <B _a <I _max applies, then the quenching circuit is activated if
  • 1, the current I exceeds a predetermined threshold _{trig itrig} for a predetermined time T
  • 2. or the average current _steepness for a given tent T _{didttrig is} above a predetermined current _{steepness threshold} di / dt _trig .

The advantages of such a short-circuit detection are that the parameters for the different Assessment thresholds of the respective network conditions can be adjusted. In addition, one is sufficient rapid release guaranteed, with protection at the same time before false triggering due to incorrect measurements and transients Operations that are not caused by short circuits were such. B. with capacitive inrush currents.

An application of the invention Short-circuit current limiter is in the area of same, AC and three-phase systems can be seen, above all for a surge short-circuit current limitation in electrical Drives in the kW and MW range or in decentralized ones Energy feed systems, as with Combined heat and power plants, wind turbines etc.

Further preferred embodiments are shown in FIGS Sub-claims emerge.

The short-circuit current limiter according to the invention is in following explained with reference to drawings. Here demonstrate:

Fig. 1 is an overview of the basic structure of a device according to the invention,

FIG. 2a, b two examples of Stromabschalter the circuit of Fig. 1,

Fig. 3 shows an embodiment for the quenching circuitry of FIG. 1,

Fig. 4. Comparative diagram of the current through a Stromabschalter with different drive method for the quenching circuitry,

Fig. 5a, b show two embodiments of the current / current rise detection or determination of an apparatus of FIG. 1, and

Fig. 6 is a schematic representation of the inputs and outputs of the central controller in FIG. 1.

A line having a current cut-off switch 1 is shown in FIG. 1 as a main current path. The line can be, for example, a phase of a three-phase voltage system of an energy feed from a wind power plant. The other phases are not shown since the short-circuit current limiters for these are equivalent to that of the phase shown. In parallel to the current cut-off switch 1 , an extinguishing circuit 3 is provided in a secondary circuit and is connected to a charging device 4 . A central control 2 is connected both to the current cut-off switch 1 and to the quenching circuit 3 . In series with the current cut-off switch 1 and within the parallel connection of the current cut-off switch 1 and the quenching circuit 3 , a current measuring device 5 is shown, which is connected to a measured value processing unit 6 . The outputs of the measured value processing unit 6 are connected to inputs of the central control 2 . In the introduction, the entirety of the elements central control ( 2 ), current measuring device ( 5 ) and measured value processing unit ( 6 ) is referred to as a control unit.

In fault-free operation, the load current i _L arriving on the main current path is conducted completely as current i _A via the current cut-off switch 1 ; no current flows through the quenching circuit 3 . The measuring device 5 continuously detects the current i _A flowing through the current cut-off switch 1 and makes the measured value available to the measured value processing unit 6 . In the measured value processing unit 6 , the current current steepness di _A / dt is additionally calculated from the measured current i _A. The two values of i _A and di _A / dt are fed to the central control 2 , possibly multiplied by a predetermined factor in each case.

A decision is then made in the central control 2 as to whether the supplied values of the current i _A and the current current steepness di _A / dt indicate a fault current which must be reacted to.

If such a short-circuit current is determined, the central controller 2 triggers the quenching circuit 3 so that it can take over part of the load current i _L. In the current cut-off switch 1, on the other hand, there is a complete current interruption at the next voltage zero crossing. A voltage memory of the quenching circuit 3 , which has been precharged via the charging device 4 , thereby accelerates the occurrence of a voltage zero crossing.

Optionally, the central control 2 can also be used to control the current cut-off 1 in normal operation.

The individual elements of the circuit from FIG. 1 will now be discussed with reference to FIGS. 2-5.

FIG. 2 shows two possible alternatives for the current cut-off 1 in FIG. 1. In Fig. 2a) a current cut-off switch 1 is provided which has a mechanical switch S _L. If such a mechanical circuit breaker is used in the short-circuit current limiter according to the invention, it is advantageously designed as a hybrid switch. In such a case, the switch is opened via a power electronic circuit (not shown), which can consist, for example, of a series connection of a precharged capacitor, an inductor and a thyristor that can be ignited by the central controller 2 . When a relevant short-circuit current is detected, the thyristor is then fired, whereupon the capacitor discharges via the inductance, which causes the switch S _L designed for this to open by a repulsive force thus generated.

Another embodiment of a current cut-off switch 1 consists of a power electronic current cut-off switch which uses, for example, thyristors and GTOs. Fig. 2b) shows a Stromabschalter 1, the two anti-parallel connected thyristors T _LA and _LB T is formed. Thyristor T _LA carries current i _A in the direction shown in FIG. 1 and thyristor T _LB in the opposite direction. The two thyristors T _LA , T _{LB are activated for} this purpose, for example, by the central controller 2 from FIG. 1.

Basic details of an erase circuit 3 from FIG. 1 are shown in FIG. 3. The _quench circuit 3 provides four thyristors ( _quench thyristors) T _QpB , T _QpA , T _QnA , T _QnB , one of which is arranged in a branch of a bridge circuit. The four thyristors T _QpB , T _QpA , T _QnA , T _{QnB are switched} in their _{forward direction} such that when two thyristors (T _QpA / T _QnA , or T _QpB / T _QnB ) are _fired in bridge _branches , which are diagonal opposite, a current can flow through these bridge branches and a transverse branch of the bridge circuit, but always in the same direction in the transverse branch. A series connection of an inductance (limiting inductor) L _b and a quenching capacitor C _{Q is} provided in the shunt arm of the bridge circuit. In the example shown, the quenching capacitor C _{Q is} additionally connected twice in parallel with a ZnO varistor V ₁ or V ₂ in series with a resistor R ₁ or R ₂ . The series circuit consisting of the inductor L _b and the quenching capacitor C _Q is also connected to a charging device.

The ZnO varistors V ₁ and V _{2 connected in} parallel to the quenching capacitor C _Q are used as voltage-limiting elements (limiters), whereby for a multiple parallel connection, as in this example, a double, appropriately dimensioned series resistors R ₁ , R ₂ must be used to compensate for the production-related spread of the limiters.

The quenching capacitor C _Q is charged via the charging device 4 to a defined voltage _value U _CQ0 such that a voltage corresponding to the direction shown in FIG. 3 for U _CQ0 is present at the quenching capacitor C _Q. Leakage losses are compensated for by continuous charging. The erase circuit 3 is thus ready for short-circuit current limitation in the event of a detected fault current.

According to a first method for activating the quenching circuit 3 , upon detection of a fault current by the central controller 2, an ignition signal is given by the central control 2 to the quenching circuit 3 , so that when the current i _A shown in FIG. 1 is positive, the two _quenching thyristors T _QpA and T _QnA , _switch through the two _quenching thyristors T _QpB and T _QnB in the case of a negative current direction of i _A. In this way, the secondary circuit with the quenching circuit 3 forms a continuously conductive connection connected in parallel with the current cut-off switch 1 , via which part of the load current i _L (ie the fault current) now flows, as a result of which the current i _A through the current cut-off switch 1 is reduced. In addition, the precharged quenching capacitor C _Q can now discharge via the limiting inductor L _b and the parasitic impedances which are still in series. The discharge current of the quenching capacitor C _Q has the same direction as the part of the load current i _L that has been taken over and is superimposed with it to form a total current i _C. The part of the load current i _L taken over by the secondary circuit continues to flow behind the connection point to the main circuit in the same direction as before. In contrast, the major part of the discharge current flows back through the current cut-off switch 1 , where it oppositely overlaps the part of the load current i _L still flowing through the current cut-off switch 1 , so that there is finally a total current of zero through the current cut-off switch 1 . The current i _A through the current cut-off switch 1 thus commutates completely to the secondary circuit and thus the current cut-off switch 1 , which is no longer ignited, can achieve its blocking capability (reconsolidation, removal of load carriers).

Due to the energy stored in the network, the polarity of the capacitor C _{Q is} reversed, so that a counter-voltage builds up on the capacitor C _Q , which lies as a voltage-time area on the network inductances and here the current can become zero. If the current has become zero, the previously conductive _quenching thyristors T _QpA , T _QnA , or T _QpB , T _{QnB block} and the process is ended.

According to a second, alternative control method in turn an ignition signal of the current i _A, the thyristors T and T _{QnA QPA} and negative current direction of the current i _A light through the thyristors T _QPB and _QNB T in the case of the quenching circuitry in a positive current direction. As in the first control method, the quenching capacitor C _{Q is} discharged and the current i _{A is} commutated from the current cut-off switch 1 to the secondary circuit. As soon as the current i _A through the current cut-off switch 1 has become zero, one of the two non-conductive quenching thyristors is additionally ignited as an alternative to the procedure according to the first method, so that the quenching capacitor C _Q discharges in a circuit current within the quenching circuit 3 .

In this way it is avoided that the residual energy still stored in the quenching capacitor C _Q , which was not required for the complete commutation of the load current i _L to the secondary circuit, is impressed into the network.

A third preferred control method initially corresponds exactly to the second control method. In contrast to the second control method, however, it is provided that as soon as the current i _A through the current cut-off switch 1 has become zero, both quenching thyristors that are not yet conductive are ignited, so that the quenching capacitor C _Q is discharged via two parallel circuit currents within the quenching circuit. This additional ignition of these two quenching thyristors reduces the thermal load on the individual quenching thyristors to approximately half.

The quenching thyristors lock as in the first described taxation procedures also in the second and third Control procedure once the current in the cross branch of the Bridge circuit has become zero.

FIGS. 4a and b show examples plotted in seconds on the time the current flow in the load current i _L in the case of a short circuit, with Fig. 4b represents a section of the curves of FIG. 4a stretched in time. In each case a first curve 1 represents the current profile without measures for interrupting the current, a second curve 2 the current profile with a current interruption according to the first control method described and a third curve 3 the current profile with a current interruption according to the two further control methods. It can be seen that in the event of a current interruption according to one of the control methods described, the load current decays in a very short time, "softly" and without reaching the maximum of the short-circuit current. Furthermore, the additional advantage of the second and the third control method becomes clear, since the increase in the load current by a capacitive peak, which occurs when the first control method is used, was avoided in the other two control methods.

FIGS. 5a and 5b it can be seen that, for signal acquisition and processing to obtain the values of current i _A and current gradient di _A / dt, which the central controller 2 are fed, are also several options available.

Fig. 5a shows a measuring device 5, and a measurement value processing unit 6, wherein the measuring device S is composed of a current transformer and the measurement value processing unit 6 further comprises a low-pass filter 10, a multiplier 12 and a multiplier / differentiator 11 includes. The current transformer is connected to the input of the low pass 10 of the measured value processing unit 6 . The output of the low pass 10 is present at the input of the multiplier 12 and at the input of the multiplier / differentiator 11 . The respective output of multiplier 12 and multiplier / differentiator 11 is applied to central controller 2 from FIG. 1.

_A current K _W .i _A proportional to the current i _A through the current cut-off switch 1 is detected with the aid of the current transformer. After a low-pass filtering by the low-pass filter 10 , the current K _W .i _{A is} multiplied by a factor K _P in the multiplier 12 and differentiated in the multiplier / differentiator 11 and multiplied by a factor K _D. The output of the multiplier 12 thus provides a current i _A proportional value K _{W P} .K .i _A and the output of the multiplier / differentiator 11 a of the current of current rise di _A / dt in Stromabschalter 1 proportional value K _{W D} .K .di _A / German The two values K _W .K _P .i _A and K _W .K _D .di _A / dt are then made available to the central controller 2 for evaluation.

In Fig. 5b is an alternative signal acquisition and processing, in turn, a measuring device 5, and a measurement value processing unit 6 is shown. In contrast to the measuring device from FIG. 5a, the measuring device 5 here consists of a Rogowski coil. In this case, the elements of the measured value processing unit 6 are a low-pass filter 13 , a multiplier 14 and a multiplier / integrator 15 . The Rogowski coil is connected to the input of the low pass 13 , the output of the low pass 13 on the one hand to the multiplier 14 and on the other hand to the multiplier / integrator 15 . Both the output of the multiplier 14 and that of the multiplier / integrator 15 are in turn connected to the central controller 2 from FIG. 1.

With the help of the Rogowski coil, a current K _W .di _A / dt proportional to the current steepness di _A / dt is detected. After a low-pass filtering by the low-pass filter 13 , the detected value is multiplied by a factor K _P in the multiplier 14 and integrated in the multiplier / integrator 15 and multiplied by a factor K _I. The output of the multiplier 14 thus provides a current rise di _A / dt value proportional K .K _{W P A} .di / dt and the output of the multiplier / integrator 15, a current i to the current _A proportional value by the Stromabschalter 1 _W K .K _I .i _A. The two values K _W .K _P .di _A / dt and K _W .K _I .i _A are then made available to the central controller 2 for evaluation.

With the Fig. 6, finally, the one function of the central controller 2 of Fig. Will be explained. In this example, the central controller 2 controls the thyristors of the current cut-off switch 1 as well as the quenching circuit in the event of a fault current detected by the central controller 2 . The operation of the central control 2 is essentially based on an algorithm for short-circuit detection, which receives various input values, such as values proportional to current i _A and current steepness di _A / dt, as well as different threshold values P _i , and the control pulses CS _A , CS _Q as output values for control the power switch 1 and the extinguishing circuit 2 outputs.

For short-circuit detection, as described above, either the current i _A or the current steepness di _A / dt are measured in each phase to be protected within the connection points of the quenching circuit 3 . The aim of the algorithm for detecting a short circuit is, in particular, a reliable fault current determination, in particular a surge short-circuit current, while at the same time avoiding false tripping due to transient processes on the line or current harmonics. The values of current i _A and current steepness di _A / dt received by the measured value processing unit 6 are subjected to a moving averaging over several measuring points. If the absolute value of the current is below a minimum current threshold I _min or above a maximum current threshold I _max , the quenching circuit is not activated. For the area B _{A of} possible tripping I _min <B _A <I _max . Within B _A trigger threshold I is _trig, is triggered in which, when it is exceeded _itrig after the low pass filtering and averaging for a defined time T. If the absolute current value is in B _A, it can continue to be triggered if, after low-pass filtering and averaging, the current steepness di _A / dt is above the current _{steepness threshold} di / dt _trig for a defined tent T _{di / dttrig} .

If a short-circuit current reaches values above the maximum current threshold I _max , which is normally prevented by the described short-circuit detection, the current cut-off switch 1 is blocked without the extinguishing circuit being triggered.

All parameters can be changed if necessary Grid conditions are adjusted, which on the one hand a sufficiently fast release is guaranteed and on the other hand through low-pass filtering and averaging protection against false triggering due to incorrect measurements and transient processes that are not caused by short circuits were caused (e.g. capacitive inrush currents), given is. When the trigger criteria are met in one Phase can either be switched to all phases, n-1- phase with an isolated network or with a temporal Graduation.

Depending on the requirements of the current cut-off switch 1, the trigger signal is given to the current cut-off switch 1 and the extinguishing circuits at the same time or in a staggered manner.

Claims (13)

1. short-circuit current limiter for limiting fault currents in an electrical power network,

• 1. which has
  • 1. for at least (n-1) phases in a main circuit, a current cut-off switch ( 1 ) to interrupt the phase current in the event of a fault current and
  • 2. an extinguishing circuit ( 3 ) assigned to the current cut-off switch ( 1 ) in a secondary circuit,
  • 3. the delete circuit ( 3 )
  • 4. can be controlled by a current actual value control unit ( 2 , 5 , 6 ) and
  • 5. comprises a bridge circuit, in the transverse branch of which a series circuit comprising a quenching capacitor (C _Q ) and a limiting choke (L _b ) charged by a charging device ( 4 ) is arranged,

characterized in that

• 1. each of the bridge branches of the bridge circuit of the quenching circuit ( 3 ) comprises at least one switchable quenching element (T _QpA , T _QpB , T _QnA , T _QnB ),
• 2. the control unit ( 2 , 5 , 6 ) is connected to each of the switchable quenching elements (T _QpA , T _QpB , T _QnA , T _QnB ) and
• 3. in the case of a detected fault current, two quenching elements (T _QpA , T _QnA or T _QpB , T _QnB ) arranged in diagonally opposite bridge branches are controlled in such a way that the discharge current of the quenching capacitor (C _Q ) corresponds to the current branched off from the main circuit (i _Q ) superimposed in the same way in the secondary circuit.

2. Short-circuit current limiter according to claim 1, characterized in that

• 1. in addition to the two quenching elements arranged in diagonally opposite bridge branches, at least one further quenching element outside of these bridge branches is controlled in such a way that a freewheeling current path for the discharge current of the quenching capacitor (C _Q ) is formed within the quenching circuit ( 3 ).

3. Short-circuit current limiter according to claim 1, characterized in that

• 1. in addition to the two quenching elements arranged in diagonally opposite bridge branches, at least two further quenching elements outside of these bridge branches are controlled in such a way that two freewheeling current paths arranged parallel to one another for the discharge current of the quenching capacitor (C _Q ) are formed within the quenching circuit ( 3 ).

4. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. the current cut-off switch ( 1 ) are mechanical or power electronic switching elements.

5. Short-circuit current limiter according to claim 4, characterized in that

• 1. the current switch ( 1 ) are formed in each phase by an anti-parallel circuit of thyristors.

6. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. the at least four switchable quenching elements (T _QpA , T _QpB , T _QnA , T _QnB ) are each thyristors.

7. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. the quenching capacitor (C _Q ) at least one voltage-limiting element (V ₁ ), in particular a ZnO varistor, is connected in parallel.

8. Short-circuit current limiter according to claim 7, characterized in that

• 1. when using several voltage-limiting elements (V ₁ , V ₂ ) for these series resistors (R ₁ , R ₂ ) are provided to compensate for scattering of the voltage-limiting elements.

9. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. the control unit ( 2 , 5 , 6 ) has inputs for entering parameters for a differentiated detection of the fault current.

10. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. The actual current values detected by the control unit ( 2 , 5 , 6 ) can be measured as current using a current transformer or as a current steepness using a Rogowski coil
• 2. the control unit ( 2 , 5 , 6 ) has a measured value processing unit ( 6 ) for determining the respective other value.

11. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. The quenching circuits ( 3 ) of the different phases of the energy network are controlled in such a way that a trigger signal is output to all phases simultaneously, at least n-1-phase in an isolated network, or staggered over time.

12. Short-circuit current limiter according to one of the preceding claims, characterized in that

• 1. it is used in electrical drives in the kW and / or MW range or in the case of decentralized energy supply, in particular in combined heat and power plants or wind power plants.

13. Short-circuit current limiter according to claim 10, characterized in that

• 1. for the values of the current or the current steepness measured in each phase within the connection points of the quenching circuit ( 3 ) are low-pass filtered
• 2. a moving average (average current value or average current steepness) is formed from the low-pass filtered values and
• 3. If the averaged current value is below a predefined minimum current threshold I _min or above a predefined maximum current threshold I _max , the quenching circuit ( 3 ) is not activated
• 4. If the averaged current value is in a range Ba for which I _min <B _a <I _max applies, then the quenching circuit ( 3 ) is activated if at the same time
  • 1. The current I _trig a predetermined threshold for a predetermined time T exceeds or _itrig
  • 2. the averaged current _steepness for a predetermined time T _{di / dttrig lies} above a predetermined current _{steepness threshold} di / dt _trig .