Method and configuration for identifying short circuits in low-voltage networks

A method for identifying short circuits in a low-voltage network includes the step of determining a first envelope and a second envelope for a plurality of locus curves of a current steepness as a function of a current in a low voltage network, the first and second envelopes including all switching angles, the first envelope being determined for a lower power factor, the second envelope being determined for an upper power factor. Additionally a third envelope is determined taking into account rated-current switching operations between the lower power factor and the upper power factor. A resultant envelope is formed from a combination of the first, second, and third envelopes by overlaying the first, second, and third envelopes. The resultant envelope defines a tolerant locus curve criterion indicating a short circuit for values outside the tolerant locus curve criterion. A configuration for identifying short circuits is also provided.

BACKGROUND OF THE INVENTION
 Field of the Invention
 The invention relates to a method for identifying short circuits in
 low-voltage networks using selected switching criteria. In addition, the
 invention also relates to an associated configurations for carrying out
 the method.
 Short circuits in low-voltage (LV) systems result in high electrodynamic
 and thermal stresses both on the downstream system parts, such as
 conductor lines, cables, busbar systems or the like, and on the power
 breaker or circuit breaker which carries out the disconnection. The amount
 of stress is governed primarily by the time period from the occurrence of
 the short circuit until it is cut off. A part of this time is required
 purely for detecting the unacceptable operating state, this is referred to
 as the so-called short-circuit identification time. The aim is to find a
 method which is tolerant to various network parameters, in particular the
 power factor, and which allows short circuits to be identified quickly.
 A range of methods for a short-circuit identification have already been
 proposed:
 Conventionally, the magnitude of the current i is assessed by magnetic
 and/or thermal releases or triggers, and a disconnection is triggered
 (i-criterion) if a limit is exceeded. Since the current through the
 network inductances is continuous, a certain time period always passes
 before the current rises above the limit and the short circuit is thus
 identified. A further disadvantage is that it is necessary to set a limit
 well above (in practice by a factor of .gtoreq.3) the rated
 root-mean-square current based on the surge factor .kappa., in order to
 prevent an inadvertent tripping. In consequence, it is possible that
 "weak" short circuits will never be identified. An additional delay occurs
 as a result of the mechanical and, in particular, thermal inertia of the
 releases r trigger devices. In order to compensate for this, numerous
 electronic releases heave been configured on the basis of an exclusive
 current assessment (i-criterion), which compare the actual current to the
 tripping limit with no inertia or with little inertia.
 An algorithm is proposed in the reference etz 112 (1991), pages 718 to 722
 which, in addition to the current i, also uses the current steepness di/dt
 for identifying a short circuit. Tripping takes place when the following
 condition, which is called an extrapolation criterion, is satisfied:
 ##EQU1##
 where
 I.sub.N : is the rated current (root mean square value),
 .phi..sub.N : is the phase shift in the rated current circuit,
 .tau..sub.N : is the time constant of the rated current circuit with
 .tau..sub.N =tan (.phi..sub.N)/(2.pi.f), and
 G.sub.extra : is the tripping limit.
 On a graph with current i as the abscissa and the current steepness di/dt
 as the ordinate, equation (1) defined above represents a straight line.
 In comparison with all other known methods, which use the current i and
 current steepness di/dt for identifying short circuits, the extrapolation
 criterion is optimal for the identification characteristics such as the
 identification time, the current heating integral, and the current at the
 identification time. The extrapolation criterion has a stable reaction
 with regard to switching the current level within the permissible limits.
 A disadvantage of this method is that the power factor in the rated
 operation of the network to be protected must be known for an adaptation
 of the method. This requires that the network conditions are known and do
 not change since, if the load varies, the method will be incorrectly
 matched and may result in an inadvertent or unauthorized tripping.
 A method which is insensitive to different power factors within certain
 interval limits and likewise uses the current and current steepness for a
 short-circuit identification is described in the Geriuan patent DE 36 42
 136 C1. All the possible combinations of currents and current steepnesses
 which can occur when a circuit is switched on at power factors of, for
 example, cos .phi.=0.2 . . . 0.95 are plotted as locus curves in a common
 diagram, that is to say the current i on the abscissa and the current
 steepness di/dt on the ordinate. Since, in the German patent DE 36 42 136
 C2, all the locus curves start on the ordinate where i is equal to zero,
 it must be assumed that no current was flowing before the switching
 operation, and, in consequence, that no initial current is present.
 Consequently, an envelope is produced around the resultant family of
 curves, which is declared as a so-called threshold value function.
 However, no rule for composing the envelope is defined in the German
 patent DE 36 42 136 C1. If an observation point, which is expressed by the
 pair of values current/current steepness, leaves the region bounded by the
 threshold value function, this leads to a tripping.
 The German patent DE 36 42 136 C1 cannot take into account those switching
 processes in which an initial current was flowing, that is to say a
 so-called changeover takes place, wherein the power factor and/or current
 change within permissible limits as a result of the switching operation.
 These operation conditions, which in practice occur with a high
 probability, can, under certain circumstances and at specific switching
 phase angles, cause an unjustifiable tripping of the switching device for
 carrying out the method described in the German patent DE 36 42 136 C1.
 Furthermore, the article in the reference ABB Technik 4/1997, page 41
 proposes that disconnection criteria for low-voltage switches be developed
 further through the use of suitable algorithms in order to detect any
 short circuit which occurs in the microsecond range. The aim is, when a
 fault occurs in electrical distribution networks with low-voltage systems,
 to isolate the fault as quickly as possible and to isolate only the faulty
 part of the system, as well as limiting the down time and the damage to a
 minimum.
 SUMMARY OF THE INVENTION
 It is accordingly an object of the invention to provide a method and a
 configuration for identifying short circuits in low-voltage networks which
 overcome the above-mentioned disadvantages of the heretofore-known methods
 and configurations of this general type and which guarantee a stable
 operation for the identification of short circuits under all conceivable
 operating conditions.
 With the foregoing and other objects in view there is provided, in
 accordance with the invention, a method for identifying short circuits in
 low-voltage networks. The method includes the steps of determining a first
 envelope and a second envelops for a plurality of locus curves of a
 current steepness as a function of a current in a low voltage network, the
 first and second envelopes including all switching angles, the first
 envelope being determined for a lower power factor, the second envelope
 being determined for an upper power factor; additionally determining a
 third envelope taking into account rated-current switching operations
 between the lower power factor and the upper power factor; forming a
 resultant envelope from a combination of the first, second, and third
 envelopes by overlaying the first, second, and third envelopes, the
 resultant envelope defining a tolerant locus curve criterion indicating a
 short circuit for values of at least one of the current steepness and the
 current outside the tolerant locus curve criterion, the tolerant locus
 curve criterion being independent of power factors and independent of an
 initial current; and detecting at least one of an instantaneous current
 value and an instantaneous current steepness value for use as a
 disconnection criterion.
 In accordance with another mode of the invention, the disconnection
 criterion is simplified by expanding extrapolation criteria and the short
 circuit is identified when the extrapolation criteria are satisfied for
 both limits of a power factor interval defined by the lower and upper
 power factors, subject to the following equations:
 ##EQU2##
 and
 ##EQU3##
 where
 i: is the current,
 di/dt: is the current steepness,
 I.sub.N : is a rated current as a root mean square value,
 .phi..sub.N : is a phase shift in a rated current circuit,
 .tau..sub.N : is a time constant of the rated current circuit where
 .tau..sub.N =tan (.phi..sub.N)/(2.pi.f) with f being a network frequency,
 and G.sub.extra : is a tripping limit.
 In accordance with yet another mode of the invention, the disconnection
 criterion is simplified by approximating the resultant envelope with a
 polygon-shaped envelope.
 In accordance with a further mode of the invention, the polygon-shaped
 envelope is formed from tangents having tangent points at significant
 points of the resultant envelope.
 In accordance with yet a further mode of the invention, the significant
 points are a maximum current value and a minimum current value of the
 resultant envelope, an intersection point of extrapolation criteria
 resulting from the lower and the upper power factors, a maximum current
 steepness and a point-symmetrical projection, or alternatively a
 mirror-symmetrical projection, of the intersection point and the maximum
 current steepness.
 In accordance with another mode of the invention, coordinates of the
 maximum and minimum current values are determined from the mathematical
 expression .kappa..multidot.2.multidot.I.sub.rated, where .kappa.
 represents a surge factor related to the lower power factor and
 I.sub.rated is a rated current.
 In accordance with an added mode of the invention, the intersection point
 of the extrapolation criteria resulting from the lower and upper power
 factors are determined from the following relationships:
 ##EQU4##
 and
 ##EQU5##
 where
 (dI/dt).sub.2 : is a current steepness at an intersection of two straight
 extrapolation lines,
 I.sub.2 : is a current at the intersection of the two straight
 extrapolation lines,
 I.sub.N : is a rated current as a root mean square value,
 .omega.: is a network circular frequency,
 .phi..sub.u : is a phase shift between the current and a voltage when using
 the lower power factor, and
 .phi..sub.o : is a phase shift between the current and the voltage when
 using the upper power factor.
 In accordance with another mode of the invention, the maximum current
 steepness is determined with the following relationships:
EQU for 2.multidot.cos.phi..sub.u &lt;cos.phi..sub.o
 ##EQU6##
 and
EQU for 2.multidot.cos.phi..sub.u.gtoreq.cos.phi..sub.o
 ##EQU7##
 where
 (dI/dt).sub.3 : is the maximum current steepness,
 I.sub.3 : is the current at the maximum current steepness, .psi..sub.max :
 is a switching angle, related to the voltage, with a subsequent maximum
 current steepness in accordance with
 ##EQU8##
 where
 .phi..sub.u : is a phase shift between the current and a voltage when using
 the lower power factor, and
 .phi..sub.o : is a phase shift between the current and the voltage when
 using the upper power factor.
 In other words, the object of the invention is achieved with a method for
 identifying short circuits in low-voltage networks using selected
 switching criteria, in which case, in particular, the instantaneous value
 of the current and the current steepness are detected and are used as
 disconnection criteria, for which purpose the current steepness is
 represented as a function of the current and, using this locus curve
 representation, envelopes are derived which enclose all the locus curves
 which are possible in normal operation, in which case a resultant envelope
 is formed by superimposing the envelopes, which, in addition to various
 switching times and power factors, takes account of any required initial
 current within the normal operating range, for which purpose
 first of all, separate envelopes are formed for the lower power factor on
 the one hand, and the upper power factor on the other hand, with all the
 switching angles being enclosed in each case,
 in addition, a further envelope is determined which takes account of the
 rated-current switching operations between two power factor limits,
 the envelopes obtained in this way are combined and are superimposed to
 form the resultant envelope which embodies a "Tolerant Locus Curve
 Criterion" (TLC) which is independent of the power factor and initial
 current, and a short circuit is indicated if the values are outside the
 tolerance locus curve criterion.
 With the objects of the invention in view there is also provided, a
 configuration for identifying a short circuit in a low voltage network,
 the configuration including a sensor for detecting a current steepness; a
 filter connected to the sensor; an analog-digital converter connected to
 the filter; a level matching unit connected to the analog-digital
 converter for generating a first signal representing the current
 steepness; an adder unit connected to the level matching unit, the adder
 unit generating a second signal representing an instantaneous value of a
 current; and a digitally operating evaluation unit connected to the level
 matching unit, the evaluation unit receiving the first and second signals
 and storing a tolerant locus curve criterion, the tolerant locus curve
 criterion indicating a short circuit for values of at least one of the
 first and second signals outside the tolerant locus curve criterion, the
 tolerant locus curve criterion being defined by a resultant envelope
 formed from a combination of a first, a second, and a third envelope, the
 first envelope and the second envelope being determined from a plurality
 of locus curves of the current steepness as a function of the current in a
 low voltage network, the first and second envelopes including all
 switching angles, the first envelope being determined for a lower power
 factor, the second envelope being determined for an upper power factor,
 the third envelope taking into account rated-current switching operations
 between the lower power factor and the upper power factor.
 In accordance with further features of the invention, the sensor for
 measuring the current steepness may be embodied as a uniform-field coil,
 and the filter is an anti-aliasing filter.
 In accordance with another feature of the invention, a release is connected
 to the evaluation unit and is actuated by the evaluation unit.
 The configuration for carrying out the method uses evaluation devices which
 are configured in analog, digital and/or hybrid form.
 The described method provides an algorithm which advantageously operates in
 all conceivable types of operation and which, apart from the power factor
 of the network to be protected, is also independent of previous switching
 operations. This results in considerably better characteristics than those
 obtained with conventional methods.
 Associated configurations having devices for carrying out the method
 according to the invention may alternatively operate using analog, digital
 or hybrid evaluation devices. In particular, one advantageous
 configuration has a sensor for measuring the current steepness, downstream
 from which a filter, an analog/digital converter and a unit for level
 matching or level adaptation are connected, from which a first signal for
 the current steepness and, via an adder unit, a second signal for the
 instantaneous value of the current are passed to a digitally operating
 evaluation unit having a tolerant-locus-curve criterion stored in it. A
 release for the respectively used switch is actuated by the evaluation
 unit.
 Other features which are considered as characteristic for the invention are
 set forth in the appended claims.
 Although the invention is illustrated and described herein as embodied in a
 method and a configuration for identifying short circuits in low-voltage
 networks, it is nevertheless not intended to be limited to the details
 shown, since various modifications and structural changes may be made
 therein without departing from the spirit of the invention and within the
 scope and range of equivalents of the claims.
 The construction and method of operation of the invention, however,
 together with additional objects and advantages thereof will be best
 understood from the following description of specific embodiments when
 read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Referring now to the figures of the drawings in detail and first,
 particularly, to FIGS. 1 and 2, which will partially be described
 together. FIGS. 1 and 2 are in particular used to describe the novel
 method and to indicate the improvement over the prior art. FIG. 3 is used
 to describe a simplified disconnection criterion while FIG. 4 shows a
 practical implementation of the method according to the invention.
 The invention assumes that the power factor of the network to be protected
 remains in a specific interval (for example, cos .phi.=0.1 . . . 0.9)
 during rated operation.
 FIG. 1 shows an illustration of the envelopes 1 and 2, which are initially
 independent of one another, for switching on the rated current circuit,
 with the respective lower power factor (for example cos .phi.=0.1) and an
 upper power factor (for example cos .phi.=0.9). In FIG. 1, 1/I.sub.n is
 plotted on the abscissa, and 1/(.omega..multidot.I.sub.n).multidot.di/dt
 on the ordinate.
 If all possible combinations of current and current steepness (locus curves
 on the i-di/dt graph) are then considered which occur when
 switching on the rated current circuit at various power factors,
 changing the power factor during the rated operation,
 varying the current level in the rated range, i.e. switching and as a
 result of these points being combined, then this includes all the
 operational situations which are possible in practice, and a resultant
 envelope 3 is obtained as the outer boundary of the family of locus
 curves. This resultant envelope is shown in FIG. 2, which, in a common
 envelope, takes into account not only all the locus curves for any
 arbitrary or desired switching angles, but also the envelopes resulting
 from this for various power factors.
 If an i-di/dt operating point during a system operation is located outside
 the envelope formed in this way, then a short circuit must be present.
 Since this decision feature is now independent of the power factor and the
 initial current, within predetermined limits, it is referred to as a
 tolerant locus curve criterion (TLC criterion).
 Detailed investigations have shown that the TLC criterion has the following
 characteristics:
 the TLC criterion Ls better than the i-criterion in all ranges.
 As the short-circuit current level rises, the TLC criterion approaches the
 extrapolation criterion for all power factors.
 The TLC criterion and the extrapolation criterion hardly differ for power
 factors at the upper dimensioning limit of the TLC criterion and
 short-circuit currents where I.sub.K &gt;2.multidot.I.sub.N.
 If the specific network relationships are known, it is possible for the TLC
 criterion to approach the extrapolation criterion by constraining the
 range of possible power factors.
 The influence of harmonics in the network is compensated for by widening
 the envelope, although this results in a deterioration in the
 identification characteristics at the same time. One specific method for
 coping, in particular, with current peaks is not to identify a short
 circuit unless the operating point of the circuit infringes the TLC
 criterion for a specific time or a given number of sample values.
 Expanding the extrapolation criterion results in a simplified variant of
 the TLC criterion. A short circuit is identified only if the extrapolation
 criterion is exceeded for both limits of the power factor interval, for
 example, cos .phi.=0.1 and 0.9. This can be formulated as follows:
 ##EQU9##
 and
 ##EQU10##
 While the normal region of the original TLC criterion is located inside a
 complicated shape, only the two limit lines need be considered for the
 simplified TLC criterion. However, this simplification is associated with
 a minor deterioration in the identification characteristics.
 In order to implement the simplified TLC criterion, the resultant envelope
 3 as shown in FIG. 2 is approximated by a polygon 4. FIG. 3 shows such a
 polygonal approximation for the upper half plane. The polygonal
 approximation is completed by a centrally symmetrical mirroring onto the
 lower half plane.
 The polygon 4 shown in FIG. 3 contains characteristic points. In this case,
 the current level of the point 31 or 34, respectively, related to the rated
 current, is given by the maximum/minimum possible current, and this
 respective current level can be obtained by using the surge factor .kappa.
 related to the lower power factor limit,
 point 32 is the intersection of the lines resulting from the two
 extrapolation criteria; the current level at the point 32 is given by:
 ##EQU11##
 the current steepness at the point 32 is given by:
 ##EQU12##
 where
 (dI/dt).sub.2 : is the current steepness at the point 32,
 I.sub.2 : is the current level at the point 32,
 .phi..sub.u : is the phase shift at the lower power factor limit, and
 .phi..sub.o : is the phase shift at the upper power factor limit,
 point 33 is the position of the maximum current rise, whose current
 steepness is given by
 ##EQU13##
 and
 ##EQU14##
 for 2.multidot.cos.phi..sub.u &lt;cos.phi..sub.o
 and whose current level is given by:
 ##EQU15##
 where
 (dI/dt).sub.3 : is the current steepness at the point 33,
 I.sub.3 : is the current level at the point 33,
 .psi..sub.max :is the switching angle, related to the network voltage, with
 subsequent maximum current steepness in accordance with
 ##EQU16##
 FIG. 4 shows a configuration for early short-circuit identification using
 the TLC criterion for any desired switching device, in detail.
 Conventional switching devices may be used to implement the described
 method. Appropriate configurations may operate fully in analog form,
 purely in digital form, or else in hybrid form, with the latter being
 preferred.
 FIG. 4 shows a sensor 41, for example a uniform-field coil or Rogowski
 coil, for measuring the current steepness di/dt or the current level i.
 FIG. 4 also shows, an A/D converter 43 with an antialiasing filter 42
 connected upstream of the A/D converter 43, a level matching unit 44 for
 simultaneously setting the rated current level, an adder unit 45 for
 calculating the current i from the current steepness di/dt or,
 alternatively, a differentiating unit for calculating the current
 steepness di/dt from the current level i, as well as a digitally operating
 evaluation unit 46 which compares the measured values with the TLC
 criterion and, if a short circuit is identified, actuates a release 47.
 The configuration according to FIG. 4 may be used for rapid identification
 of short circuits in low-voltage networks in conjunction with any desired
 switches.