The present invention relates to a system for distributing electric
power, particularly systems for low and medium voltage which comprise automatic
protection circuit breakers.
More specifically, the invention relates to a distribution system
which comprises automatic protection circuit breakers which are distributed hierarchically
on various levels. The invention further comprises a method for controlling the
intervention sequence of the automatic circuit breakers, which are mutually interlocked
so as to act selectively in order to cut out the faulty parts of the system.
These distribution systems are currently protected by means of automatic
circuit breakers, in which protection against a short-circuit fault with current
values close to the breaking capacity is provided by means of classical electromechanical
devices based on the electrodynamic effect of fault currents, which have a reaction
time of the order of milliseconds.
Electric power distribution networks have a plurality of said circuit
breakers, which are distributed radially on various levels in order to selectively
limit the power outage to the part or region of the network affected by the fault.
This selectivity among the various circuit breakers is currently achieved
by utilizing the different values of the current and of the intervention times
of the protection devices and their different mechanical inertia, which depends
on their size (and therefore on the masses involved in the opening movement).
In other words, a circuit breaker to which several other circuit breakers
supplying an equal number of loads or subsections of the system are connected has
higher settings for the protection device, is larger and has greater inertia than
the circuit breaker that it supplies, and therefore does not open in case of a
fault downstream of the smaller circuit breakers that it supplies. This of course
occurs for all the higher levels to which the fault current can propagate.
The size of the circuit breakers is in turn a function of the nominal
current of the circuit breaker, and therefore the size allocation of the system,
by having to ensure the selectivity requirements, is based not only on the values
of the working currents that are present in each node but also on the selectivity
requirements, which often require to keep the short-circuit current in the system
for longer than actually necessary to interrupt the fault current.
While the need to minimize the fault energy, i.e. damage, requires
the fastest possible intervention, the need to select among the circuit breakers
the one that must intervene in fact entails slowing the protective intervention.
Accordingly, the conventional systems of the prior art do not allow to optimize
the size allocation of an electric power distribution system as regards the protection
An example of a system of the prior art is described in the article
"Zone selective interlocking reduces fault stress (circuit breakers)"- Michalak
T.A., ELECTRICAL CONSTRUCTION AND MAINTENANCE, JAN. 1989, USA, VOL.88, NO. 1, which
discloses an interlocked system comprising electronic trip circuit breakers provided
with communication capability so as the location of a short circuit or ground fault
is isolated and cleared by the nearest upstream breaker without any intentional
The aim of the present invention is to overcome the drawbacks of the
prior art and particularly to provide an electric power distribution system which
has optimum size allocation of the circuit breakers and ensures both characteristics,
namely quick intervention and assured selectivity.
This aim is achieved by means of an electric power distribution system,
comprising a plurality of automatic protection circuit breakers distributed on
at least two levels, each one of the circuit breakers comprising an electronic
protection unit which opens the circuit breaker depending on the values of a current
(i(t)) flowing in the circuit breaker and of its derivative (di(t)/dt), and means
for mutually connecting the circuit breakers in order to exchange information concerning
the fault conditions in addition to their state characterized in that each one
of the circuit breakers is divided into two sections, respectively a "father" section
and a "child" section, and in that the connection means mutually connect the "child"
section of the circuit breakers of a same level and connect them to the "father"
section of a circuit breaker of the next higher level, and connect the "father"
section of a circuit breaker to the "child" section of a circuit breaker of the
next lower level.
Advantageously, fault detection is based both on the instantaneous
value of the circulating current and on its derivative.
Further characteristics and advantages of the present invention are
described in the appended claims.
The present invention is now described with reference to the accompanying
drawings, which relate to two preferred but non-limitative embodiments of the invention.
In the drawings:
figure 1 is a schematic view of part of an electric power distribution system
according to the present invention;
figure 2 is a view of the structure for acquiring the current values (i(t)),
digitally processing said values and actuating the opening kinematic system, provided
in a circuit breaker used in the present invention;
figure 3 plots an example of a set of acceptable operating conditions;
figure 4 is a view of the interlock structure (divided into two sections) of
a single circuit breaker;
figure 5 is a view of a configuration of circuit breakers interlocked with a
short connection; and
figure 6 is a view of a configuration of circuit breakers interlocked by means
of long connections.
In the various figures, the same reference numerals are used to designate
identical or substantially equivalent components.
Figure 1 schematically illustrates part of an electric power distribution
system according to the present invention, which comprises an electrical switchboard
I which contains a circuit breaker 10 and four circuit breakers 11, 12, 13 and
14, cascade-connected to the first circuit breaker; each one of said four circuit
breakers 11-14 supplies a corresponding load. The system can be applied to three-phase
distribution (with three or four wires), two-phase distribution and phase- neutral
distribution, although for the sake of simplicity in illustration it is shown with
a single wire in figure 1.
Moreover, the figure shows only the load constituted by a switchboard
II, which in turn contains a circuit breaker 20 to which four circuit breakers
21,22,23 and 24 are cascade-connected; each one of said four circuit breakers is
in turn connected to a further load (not shown).
Figure 1, in addition to the connections between the circuit breakers
which allow the flow of electric power (shown in thick lines), also shows connections
17, 18, 19, 27, 28, 29 over which information travels; said information is exchanged
unidirectionally or bidirectionally among the various circuit breakers.
Figure 1 clearly shows that the circuit breakers are distributed on
various levels (four levels are shown in the figure) and that other circuit breakers
of the same level can be provided in parallel to the "main" circuit breakers 10
and 20. Finally, the star configuration of the system shown by way of example in
figure 1 is non-limitative.
In case of a fault that affects, for example, the circuit breaker
24 (short circuit or ground fault current in the circuit downstream of the circuit
breaker 24), the system according to the present invention provides an interlock
condition which ensures not only fast opening of the circuit breaker 24 but also
ensures that the higher-level circuit breakers 20, 14 and 10 remain closed.
With reference also to figure 4, the base element of the system is
constituted by the single circuit breaker 10, which is divided into two sections,
designated respectively as "father" section 10A and "child" section 10B.
Each circuit breaker is capable of communicating with the upstream
and downstream circuit breakers that belong both to the same switchboard and to
different switchboards. According to the invention, two kinds of connection, termed
respectively "upward connection" and "downward connection", are defined for each
The "upward connection" identifies a connection to the so-called "child"
section 10B of the circuit breaker, while the "downward connection" identifies
a connection to the so-called "father" section 10A of the circuit breaker. Accordingly,
a circuit breaker is simultaneously father with respect to the circuit breakers
at the next lower hierarchical level and child with respect to the upstream circuit
breaker to which it is connected.
When a circuit breaker detects a fault condition, it reports it to
the "father" section of the circuit breaker at the next higher hierarchical level,
thus blocking its opening action. If the circuit breaker that receives the information
is in turn a "child" with respect to an upstream circuit breaker and if it, too,
detects the same fault condition, it reports it to its "father" circuit breaker,
and so forth. This communication process ends in one of the following two ways:
either when the highest hierarchical level of the structure is reached, or when
a circuit breaker of the communication network does not detect a fault. It should
be noted that a plurality of "children" can report simultaneously.
Figure 2 illustrates in greater detail the structure of the elements
that compose the part for acquiring and processing the values of the current (i(t))
that is present in a circuit breaker used in this invention.
Said structure comprises a primary current sensor 1, which is capable
of detecting the current that circulates in the power supply conductor that is
connected to the circuit breaker. The sensor 1 is connected to an electronic protection
part 15 which comprises, in a mutual series arrangement, a converter 2, an A/D
converter 3 and a filter 4. The filter output 4 is connected to a processing unit
6 both directly and by means of a block 5 which is capable of computing the derivative
of the input signal.
The processing unit 6 has a memory 7 and its output is connected to
a tripping coil 8 of the circuit breaker, which actuates a kinematic system 9 for
opening the circuit breaker.
Operation of the above-described circuit breaker is as follows. The
sensor 1 detects the currents that circulate in the primary circuit and the resulting
analog signal is converted into a voltage signal by the converter 2. The voltage
signal is then converted into a digital form (for digital processing) by the analog-digital
converter 3 and then filtered in the filter 4, which is of the antispike type and
introduces a minimal delay.
Preferably, the filter 4 is a third-order elliptical digital filter,
with cutoff frequency at 1 kHz. Since fault detection is based on the instantaneous
value of the current (converted into a voltage) and on the values of the current
derivative, filtering has the purpose of preventing signals having relatively low
current values (which cannot therefore be ascribed to a short circuit) but with
a high value of their first derivative from compromising the overall performance
of the protection.
The block 5 computes the instantaneous values of the derivative di(t)/dt
of the signal that represents the circulating current i(t). Preferably, the block
5 uses a second-order derivative (digital) filter which has a passband of kHz,
which is sufficient to precisely detect the derivatives of the currents of a short-circuit
The processing unit 6 thus receives in input a signal which represents
the current and a signal which represents its derivative, and the two values define
a point or vector on the plane i(t)-di(t)/dt, as shown in figure 3.
Figure 3 illustrates an acceptable operating region A; i.e., it defines
the locus of the points that belong to normal operating states, including those
representing overload but not a short circuit. The points outside the area A represent
abnormal operating conditions or conditions which in any case require protective
A time-variable current defines in this plane, a path such as those
designated by T1 and T2 in the figure. As long as said path, for example T2, remains
within the area A (or non-short-circuit region), the situation is considered "normal".
If the path, T1 in the figure, leaves this region, the situation is
considered as a short circuit. A comparison is thus performed in the block 6, moment
by moment, between the pairs of variables i(t) and di(t)/dt that arrive from the
primary circuit and the pairs that form the non-short circuit region.
When a fault condition is reported, the device emits a tripping signal
for the solenoid of the coil 8, which opens the circuit breaker by acting on its
In order to ensure the necessary selectivity, according to the present
invention an exchange of information is ensured among the circuit breakers that
are present in the various nodes of the system and which may have detected the
Communication is particularly fast in order to ensure the exchange
of information among the circuit breakers in a time which on the one hand ensures
the intervention of the circuit breaker directly involved before the current reaches
excessively high values and on the other hand allows to block other circuit breakers
upstream of the fault. Typically, communication with the next level is achieved
in less than 100 microseconds.
Reliability of the communications system is achieved by duplicating
the messages exchanged among the various circuit breakers and by means of a self-monitoring
(performed by the electronic protection itself) of the efficiency and correct functionality
of the transmission medium and of the circuit components that handle transmission.
As mentioned, the aim of the system is to avoid opening circuit breakers
at a higher hierarchical level when the fault has already been recognized by a
circuit breaker located downstream. The system handles both short circuit faults
and ground faults.
The communications system is capable of communicating several fault
conditions. In particular, faults associated with messages designated by the codes
E, G, and SOS are considered. The E message indicates a fault of the EFDP (Early
Fault Detection Prevention, i.e. a short circuit fault type) detected downstream.
The G message indicates a Ground Fault and the SOS message indicates an opening
command due to mechanical problems downstream.
The interlock communication pattern or configuration is of the kind
shown in Table 1.
where the level of the signal can assume a low value (0) or a high value (1). The
first three bits of the message constitute a fixed synchronization signal, while
the rest of the message contains data. The first three bits always have a 0 1 0
configuration, and this allows to compensate for any synchronization losses. The
following Table 2 lists all possible message configurations and shows that the
"010" combination always occurs in the same position, i.e., at the beginning of
the synchronization configuration.
The next six bits are dedicated to the three possible "simple" states
E, G and S, and a state is repeated for two bits (EE, GG, SS) in order to increase
immunity to noise. The level 1 of these bits indicates the presence of the corresponding
fault, while the level 0 indicates that there is no fault.
During communications there is a startup step that also includes synchronization.
Depending on the allocation of the circuit breaker (within a same
switchboard or in different switchboards), two different kinds of connection are
defined. A "short" connection is provided when the circuit breakers belong to the
same switchboard and therefore the communication path is not longer than approximately
10 meters. In this case, there is a father circuit breaker connected to a plurality
of child circuit breakers, as shown by way of example in figure 5, which illustrates
a typical configuration of interlocked circuit breakers with a short connection
18. Only the information-carrying connections have been shown in the figure for
the sake of simplicity.
A "long" connection is provided when the circuit breakers belong to
different switchboards and the communication path reaches lengths of up to 1 km.
In this case there is a single connection between a father circuit breaker connected
to a single child circuit breaker.
In long connections, owing to the length of the communication path,
which is considerably longer than the previous one, galvanic insulation is usually
provided, as well as impedance matching of the communication line from single-ended
to balanced or differential, and filtering of the noise impinging on the transmission
line. The long connection can also provide for the use of optical fibers with the
corresponding hardware modifications (the adapter and the transmission medium change).
In some particular configurations, impedance matching (both with an isolating transformer
and with optical fibers) may be omitted.
Figure 6 illustrates a typical configuration of circuit breakers interlocked
by means of long connections, in which an external module (respectively 32 and
33 in the figure) is interposed between the line 34 and the communication modules
for interlock 30 and 31. As shown schematically, each one of said modules 32, 33
comprises an isolating transformer with a winding which is balanced toward the
line 34 and a noise filter block. In practice, the need to use a "short" or "long"
connection depends not only on the "length" of said connection but also on the
need to have more than one child or on the need, in particular architectures, to
have or not have unidirectional connections. Accordingly, the connections, together
with the associated hardware and algorithm, are both available for the various
Where technical features mentioned in any claim are followed by reference
signs, those reference signs have been included for the sole purpose of increasing
the intelligibility of the claims and accordingly, such reference signs do not
have any limiting effect on the interpretation of each element identified by way
of example by such reference signs.
Elektrisches Energieverteilungssystem mit mehreren automatischen Schutzschaltern
(10, 11; 20, 21), die auf zumindest zwei Ebenen verteilt sind, wobei jeder Schutzschalter
eine elektronische Schutzeinheit (15) enthält, die den Schutzschalter in Abhängigkeit
der Werte eines in dem Schutzschalter fließenden Stroms (i(t)) und dessen Ableitung
(di(t)/dt) öffnet, und mit Mitteln (17, 18, 19; 27, 28, 29) zum Verbinden der Schutzschalter
miteinander zwecks Austausch von Informationen betreffend ihren Zustand sowie die
Fehlerbedingungen, dadurch gekennzeichnet, daß jeder Schutzschalter in zwei
Bereiche, nämlich einen "Vater"-Bereich (10A, 11A) und einen "Kind"-Bereich (10B,
11B) unterteilt ist und daß die Verbindungsmittel den "Kind"-Bereich (11B, 12B,
21B, 22B) der Schutzschalter einer gleichen Ebene (11, 12 ... 21, 22) miteinander
verbinden (18, 28) und diese mit dem "Vater"-Bereich (10A, 20A) eines Schutzschalters
(10) der nächsthöheren Ebene verbinden sowie den "Vater"-Bereich (14A) eines Schutzschalters
(14) mit dem "Kind"-Bereich (20B) eines Schutzschalters (20) der nächstniedrigeren
Ebene verbinden (19).
System nach Anspruch 1, dadurch gekennzeichnet, daß das System zumindest
einen Hauptschutzschalter (10; 20) und mehrere Schutzschalter (11, 12, 13, 14;
21, 22, 23, 24) hat, die bezüglich des Hauptschutzschalters (10; 20) in Kaskadenschaltung
System nach Anspruch 2, dadurch gekennzeichnet, daß die Schutzschalter
durch eine bidirektionale oder "kurze" Verbindung verbunden sind, wenn sie zu der
gleichen Ebene gehören.
System nach Anspruch 2, dadurch gekennzeichnet, daß die Schutzschalter
durch eine unidirektionale oder "lange" Verbindung verbunden sind, wenn sie zu
unterschiedlichen Ebenen gehören.
System nach Anspruch 4, dadurch gekennzeichnet, daß die "lange" Verbindung
externe Module (32, 33) verwenden kann, von denen jedes einen Trenntransformator
mit symmetrischer Wicklung und einen Rauschfilterblock enthält.
System nach Anspruch 1, dadurch gekennzeichnet, daß die elektronische
Schutzeinheit einen Wandler (2) enthält, der in Reihenschaltung mit einem A/D-Wandler
(3) und einem Filter (4) verbunden ist, daß der Ausgang des Filters (4) mit einer
Verarbeitungseinheit (6) verbunden ist, und zwar sowohl direkt als auch über einen
Block (5), der in der Lage ist, die Ableitung des Eingangssignals zu berechnen,
und daß ein Speicher (7) mit der Verarbeitungseinheit (6) verbunden ist.
System nach Anspruch 6, dadurch gekennzeichnet, daß der Ausgang der
Verarbeitungseinheit (6) mit einer Vorrichtung (8) zum Auslösen des Schutzschalters
System nach Anspruch 7, dadurch gekennzeichnet, daß es einen Hauptstromsensor
(1) hat, der mit der elektronischen Schutzeinheit (15) verbunden ist.
System nach Anspruch 7, dadurch gekennzeichnet, daß es ein System für
Nieder- und Mittelspannung ist.
An electric power distribution system, comprising a plurality of automatic protection
circuit breakers (10, 11; 20, 21) distributed on at least two levels, each one
of the circuit breakers comprising an electronic protection unit (15) which opens
the circuit breaker depending on the values of a current (i(t)) flowing in the
circuit breaker and of its derivative (di(t)/dt), and means (17, 18, 19; 27, 28,
29) for mutually connecting the circuit breakers in order to exchange information
concerning the fault conditions in addition to their state characterized in
that each one of the circuit breakers is divided into two sections, respectively
a "father" section (10A, 11A) and a "child" section (10B, 11B), and in that
the connection means mutually connect (18, 28) the "child" section (11B, 12B, 21B,
22B) of the circuit breakers of a same level (11, 12...21, 22) and connect them
to the "father" section (10A, 20A) of a circuit breaker (10) of the next higher
level, and connect (19) the "father" section (14A) of a circuit breaker (14) to
the "child" section (20B) of a circuit breaker (20) of the next lower level.
A system according to claim 1, characterized in that the system has at
least one primary circuit breaker (10; 20) and a plurality of circuit breakers
(11, 12, 13, 14; 21, 22, 23, 24) which are cascade-connected with respect to the
primary circuit breaker (10; 20).
A system according to claim 2, characterized in that when the circuit
breakers belong to the same level, they are connected by a bidirectional or "short"
A system according to claim 2, characterized in that when the circuit
breakers belong to different levels, they are connected by a unidirectional or
A system according to claim 4, characterized in that the "long" connection
can use external modules (32, 33), each whereof comprises an isolating transformer
with balanced winding and a noise filter block.
A system according to claim 1, characterized in that the electronic protection
unit (15) comprises a converter (2) which is series-connected to an AID converter
(3) and to a filter (4), in that the output of the filter (4) is connected
to a processing unit (6), both directly and by means of a block (5) capable of
computing the derivative of the input signal, and in that a memory (7) is
connected to the processing unit (6).
A system according to claim 6, characterized in that the output of the
processing unit (6) is connected to a device for tripping (8) the circuit breaker.
A system according to claim 7, characterized in that it has a primary
current sensor (1) connected to the electronic protection unit (15).
A system according to claim 7, characterized in that it is a low- and
medium voltage system.
Système de distribution d'énergie, comportant une pluralité de disjoncteurs
de protection automatiques (10, 11; 20, 21) répartis sur au moins deux niveaux,
chacun des disjoncteurs comportant une unité de protection électronique (15) qui
ouvre le disjoncteur en fonction des valeurs d'un courant (i(t)) circulant dans
le disjoncteur et de sa dérivée (di(t)/dt), et des moyens (17, 18, 19 ; 27, 28,
29) pour connecter mutuellement les disjoncteurs afin d'échanger des informations
concernant les conditions de défaut en plus de leur état, caractérisé en ce
que chacun des disjoncteurs est divisé en deux sections, une section "parent"
(10A, 11A) et une section "enfant" (10B, 11B) respectivement, et en ce que
les moyens de connexion connectent mutuellement (18, 28) la section "enfant" (11B,
12B, 21B, 22B) des disjoncteurs d'un même niveau (11, 12 ... 21, 22), et les connectent
à la section "parent" (10A, 20A) d'un disjoncteur (10) du niveau supérieur suivant,
et connectent (19) la section "parent" (14A) d'un disjoncteur (14) à la section
"enfant" (20B) d'un disjoncteur (20) du niveau inférieur suivant.
Système selon la revendication 1, caractérisé en ce que le système a
au moins un disjoncteur primaire (10 ; 20) et une pluralité de disjoncteurs (11,
12, 13, 14 ; 21, 22, 23, 24) qui sont connectés en cascade par rapport au disjoncteur
primaire (10 ; 20).
Système selon la revendication 2, caractérisé en ce que, lorsque les
disjoncteurs appartiennent au même niveau, ils sont connectés par une connexion
bidirectionnelle ou "courte".
Système selon la revendication 2, caractérisé en ce que, lorsque les
disjoncteurs appartiennent à des niveaux différents, ils sont connectés par une
connexion unidirectionnelle ou "longue".
Système selon la revendication 4, caractérisé en ce que la connexion
"longue" peut utiliser des modules externes (32, 33), chacun de ceux-ci comportant
un transformateur isolant ayant un enroulement équilibré et un bloc de filtre de
Système selon la revendication 1, caractérisé par le fait que l'unité
de protection électronique (15) comporte un convertisseur (2) connecté en série
à un convertisseur analogique-numérique (3) et à un filtre (4), par le fait
que la sortie du filtre (4) est connectée à une unité de traitement (6) à la
fois directement et par l'intermédiaire d'un bloc (5) qui est capable de calculer
la dérivée du signal d'entrée, et par le fait qu'une mémoire (7) est connectée
à l'unité de traitement (6).
Système selon la revendication 6, caractérisé par le fait que la sortie
de l'unité de traitement (6) est connectée à un dispositif de déclenchement (8)
Système selon la revendication 7, caractérisé par le fait qu'il comporte
un capteur de courant primaire (1) qui est connecté à l'unité de protection électronique
Système selon la revendication 7, caractérisé par le fait qu'il est à
faible et moyenne tension.