The present invention relates to an electronic earth leakage current
device for a low-voltage power line, having improved functionality.
In particular, the present invention relates to an electronic earth
leakage current device, for single- or three-phase low-voltage power lines (therefore
for voltage values below 1 KV).
Many examples of electronic earth leakage current devices for a low-voltage
power line are known.
The main function of an electronic earth leakage current devices is
to interrupt the power line if an earth leakage current (or residual earth fault
current or imbalance current), above a certain preset threshold, is detected. The
power line, on which a tripping event occurs, can be the power supply line (single-
or three-phase) related to a specific load or to a local electrical user, for example
of the industrial or domestic type.
The expression "earth leakage current" is to be understood as describing,
for example, an earth fault current generated by any malfunction of the power line
or of the electric loads to which the power line is connected.
For this reason, known electronic devices generally comprise the following
functional elements (Figure 1):
- a current sensor 1 for detecting an earth leakage current (also known as imbalance
current or residual fault current) in a phase conductor of a power line 5;
- an electronic circuit 2 for processing a signal, which is indicative of the
earth leakage current, sent by the current sensor 1.
A known device for protection against earth leakage currents, having
the features of the preamble of Claim 1, is disclosed in patent US 4,331,999. Tripping
events, which cause the interruption of the power line, occur according to universally
acknowledged technical standards, which set the technical requirements to be met
in order to consider the electronic earth leakage current device safe for the user.
In common practice, a tripping characteristic is determined for each
electronic earth leakage current device. This tripping characteristic depends on
the constructive characteristies of the device and ultimately on the characteristics
of the electronic circuit 2 of Figure 1.
It is known that the electronic earth leakage current devices of the
state of the art have drawbacks.
First of all, known electronic earth leakage current devices are generally
not provided with monitoring systems, which allow to easily check the operating
condition of the device and to interrupt the power line, if necessary. This can
be a source of many problems for the user. For example, a high operating temperature,
caused by environmental factors or internal faults, can cause a malfunction of the
device, which can compromise its safety.
The absence of monitoring of the voltage level of the power line also
can lead to malfunctions of the device and to damage to the electric devices powered
by the power line. without having a tripping event.
The operating condition of the device in relation to its connection
to the conductors of the power line to which it is connected is also very important.
If monitoring of the connection status of the conductors (neutral
or ground or phase) of the power line is not provided, the device may fail to operate
due to the interruption of one of said conductors or due to their incorrect connection.
In practice, in such situations an earth leakage current in the power line would
not trip the earth leakage current device. Accordingly, no protection of any kind
would be provided for a user who happened to interact in some way with the power
line and/or with the load supplied by said line. This fact entails considerable
safety problems, which, at present, can be solved by coupling an electromechanical
protection device to the electronic earth leakage current device, with a considerable
increase in installation costs.
Furthermore, known earth leakage current devices generally comprise
electronic circuits for setting the minimum tripping current and/or the maximum
time interval for tripping. These circuits are generally constituted by electronic
networks of the RC type, which process the electric signal, which is indicative
of the value of the earth leakage current, that arrives from the current sensor.
For example, the maximum time interval is commonly set by means of introducing a
time delay proportional to the detected earth leakage current value. Although the
technical solutions that are commonly adopted allow to preset some important parameters
that characterize the tripping criteria of the electronic earth leakage current
device, they do not allow to define in advance its overall behavior and therefore
the trend of its tripping characteristic. This fact entails a high degree of uncertainty
as to the behavior of the device for high earth leakage current values, for which
the proportional delay introduced by commonly adopted technical solutions tends
to drop to negligible values.
In this manner, there is no longer any control over the tripping times
of the electronic earth leakage current device, which substantially depend on the
level of wear of the mechanical or electromechanical components of the electronic
earth leakage current device. This fact can lead, for example, to unexpected interruptions
of the power line that are a severe annoyance for the user. Furthermore, considerable
problems can occur if an electric system includes electronic earth leakage current
devices in a cascade configuration (for example a device of a general type, connected
to a network with selective-type devices). In this case, the uncertainty, related
to the minimum tripping time, causes problems in managing the tripping events of
the whole network of devices. In addition to this drawback, there is the fact that,
since the tripping characteristic is difficult to predetermine, many devices can
fail to comply with the standards during testing and inspection. This fact entails,
in addition to a high degree of uncertainty as to the compliance of the earth leakage
current device to the applicable standards, many difficulties, if it is necessary
to meet particular requirements of the electrical user for which the device is intended.
For example, it is difficult to determine in advance whether a device must be of
the general or selective type or must have a more complex tripping characteristic.
Accordingly, complex calibration operations are necessary, in order to achieve a
tripping characteristic, which approximates as closely as possible the intended
one.
Another drawback arises from the fact that known electronic earth
leakage current devices are generally designed to use very specific types of actuator.
In practice, the electronic circuits that generate the tripping signal are capable
of driving only a certain type of actuator. If one decides to change the type of
actuator, then the entire device must be redesigned in order to ensure satisfactory
performance.
Another drawback arises from the fact that, in the known electronic
earth leakage current devices, the design of the electronic circuits for generating
the signal very often takes marginally into account the operating condition of the
sensor devices, suitable to detect the presence of an earth leakage current. In
particular, technical solutions for monitoring, with diagnostics of the ON/OFF type,
the operating condition of the sensor means are very often all that is provided.
Since said sensor means have to interface with electronics designed for other specific
purposes (the generation of a tripping signal if a certain level of earth leakage
current is exceeded), they can find themselves in non-optimum operating conditions,
negatively affecting the performance of the entire electronic protection device.
Therefore, the aim of the present invention is to provide an electronic
leakage current device for a low-voltage power line, which allows interrupting the
power line, if an operating condition that compromises the correct operation of
said electronic leakage current device occurs.
Within the scope of this aim, an object of the present invention is
to provide an electronic leakage current device, which allows interrupting the power
line if its operating temperature exceeds a preset threshold.
Another object of the present invention is to provide an electronic
leakage current device, which allows interrupting the power line if the voltage
of the power line exceeds a preset threshold.
Another object of the present invention is to provide an electronic
leakage current device, which allows interrupting the power line, if an interruption
of the neutral and/or ground conductors of the power line occurs or if an incorrect
connection of the phase and neutral conductors occurs.
Another object of the present invention is to provide an electronic
leakage current device, which allows to easily preset the trend of the tripping
characteristic of said device.
Another object of the present invention is to provide an electronic
leakage current device, which allows to use several actuation devices without substantial
modifications to the electronic circuits suitable to generate the tripping signal
for said actuation devices.
Another object of the present invention is to provide an electronic
leakage current device, which allows to optimize the operation of the sensors suitable
for detecting the earth leakage current.
Another object of the present invention is to provide an electronic
leakage current device, which allows to interrupt the power line if the sensors
suitable to detect an earth leakage current malfunction.
Another object of the present invention is to provide an electronic
leakage current device, which is easy to manufacture and at modest costs.
Thus, the present invention provides an electronic leakage current
device for a low-voltage power line, comprising:
- a moving contact and a fixed contact which can be mutually coupled/uncoupled;
and
- first sensor means for detecting a leakage current and for generating an electric
signal which is indicative of the value of said earth leakage current; and
- first electronic means which are electrically connected to said first sensor
means in order to generate, on the basis of said electric signal, which is indicative
of the value of said earth leakage current, an electric tripping signal; and
- actuation means. operatively connected to said moving contact, in order to perform,
in response to command electric signals, the separation of said moving contact from
said fixed contact;
The electronic leakage current device, according to the present invention, is characterized
in that it comprises:
- second sensor means for detecting one or more physical values, which are indicative
of the operating condition of said electronic device for protection against earth
leakage current; and
- second electronic means, electrically connected to said second sensor means,
for receiving from said second sensor means electric signals, which are indicative
of the operating condition of said electronic leakage current device and for generating
said electric tripping signal depending on said electric signals.
The electronic leakage current device, according to the present invention, allows
achieving the intended aim and objects. In fact, it ensures, by virtue of the presence
of the second sensor means and of the second electronic means, the possibility to
detect abnormal operating conditions, which might compromise the safety of the device.
In practice, an abnormal operating condition is considered in the same manner as
an earth leakage current and produces the tripping of the electronic leakage current
device. This forces the user to intervene in safety on the power line or on the
electronic device itself in order to restore normal operating conditions.
Further characteristics and advantages of the invention will become
apparent from the detailed description of a preferred embodiment of the electronic
leakage current device, according to the present invention, illustrated only by
way of non-limitative example in the accompanying drawings, wherein:
- Figure 1 is a block diagram of an electronic leakage current device of a known
type;
- Figure 2 is a schematic view of an electronic leakage current device, according
to the invention;
- Figure 3 is a schematic view of a constructive detail of the electronic leakage
current device, according to the invention;
- Figure 4 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 5 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 6 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 7 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 8 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 9 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention;
- Figure 10 is a schematic view of another constructive detail of the electronic
leakage current device, according to the invention.
The structure of an electronic leakage current device (reference 30),
according to the invention is illustrated schematically with reference to Figure
2.
The device 30, comprises a moving contact and a fixed contact which
can be mutually coupled/uncoupled (reference 31). The separation of the moving contact
from the corresponding fixed contact obviously produces the interruption of a power
line 32.
The power line 32 can be of the single-phase or three-phase type,
according to the requirements of the electric user. The device 30 comprises first
sensor means 33 for detecting an earth leakage current in a phase conductor of the
power line 32. The sensor means 33 therefore generate an electric signal indicative
of the value of the detected earth leakage current.
According to a preferred embodiment, the first sensor means 33 comprise
at least one current transformer provided with a primary winding which comprises
the conductors of the power line 32. In the case of a three-phase power line, the
primary winding of the current transformer can comprise the conductors of the power
line 32. Accordingly, an electric current signal flows in the secondary winding
of the transformer in response to the presence of an earth leakage current in the
phase conductor that constitutes the primary winding. This electric current signal
is therefore indicative of the value of the earth leakage current.
The device 30, according to the invention, furthermore comprises first
electronic means 34, electrically connected to the first sensor means 33. The first
electronic means 34 generate an electric tripping signal depending on the electric
signal indicative of the value of the earth leakage current (such as for example
the one sent by the sensor means 33). The device 30 furthermore comprises actuation
means 35, operatively connected to the moving contact of electric contacts 31. The
actuation means 35 produce, in response to an command electric signals 750, the
separation of the moving contact from the corresponding fixed contact and, therefore,
cause the interruption of the power line 32.
The actuation means 35 can be, for example, of the electromagnetic
type (solenoid).
Again with reference to Figure 2, the device 30 comprises second sensor
means 15 for detecting one or more physical values, indicative of the operating
condition of the electronic protection device and second electronic means 16. The
electronic means 16 are electrically connected to the sensor means 15, in order
to receive signals 17 indicative of the operating condition of the electronic protection
device. Furthermore, the second electronic means 16 generate an electric tripping
signal 18, depending on the electric signals 17. Advantageously, the first and second
electronic means 34 and 16 comprise a first circuit network 37 for generating an
electric tripping signal following the reception in input of an electric current
signal. The circuit network 37 preferably comprises (Figure 3) a first capacitive
element 42, which is connected to earth. The capacitive element 42 is charged by
an electric current signal 40 in input, so as to generate a charging voltage. A
first comparator element 43, electrically connected to the capacitive element 42,
receives in input said charging voltage and generates the electric tripping signal
39, if said charging voltage exceeds a preset reference voltage VRIF1.
Advantageously, furthermore, the electronic means 16 comprise (Figures
2 and 3) a second circuit network 20 for generating an electric tripping signal
18, following the reception in input of an electric voltage signal 17. The electric
signal 17 is indicative of the detected operating condition. The circuit network
20 (Figure 4) comprises a second comparator element 24 for generating a first enable
signal 23, if said electric voltage signal 17 exceeds a preset reference voltage
VRIF2. The circuit network 20 furthermore comprises a second circuit
block 25 for generating an electric current signal 11 following the reception of
said first enable signal 23. The signal Il is applied in input to a second capacitive
element 26 which is connected to the earth. The second capacitive element 26 is
charged by the electric current signal I1, so as to generate a charging voltage.
A third comparator element 27 is electrically connected to the capacitive element
26. The comparator element 27 receives in input the charging voltage and generates
an electric tripping signal 18 if said charging voltage exceeds a preset reference
voltage VRIF3·
With reference now to Figure 5, the second sensor means 15 comprise
at least one sensor 151, which is arranged inside and/or outside the device 30.
The sensor 151 detects the operating temperature of the device 30 and generates
an electric signal 160, which is indicative of the value of said detected temperature
and is meant to be sent to the second electronic means 16. The sensor 151 can be
provided, for example, by using an appropriately configured transistor, according
to one of the known electronic design techniques.
The sensor 151 is electrically connected to a fourth comparator element
161 in order to generate a second enable signal 162 if the electric signal 160 exceeds
a preset reference voltage VT.
The comparator element 161 is electrically connected to a first circuit
block 163 in order to generate, following the reception of the enable signal 162,
an electric current signal 40, sent in input to the first circuit network 37, which
generates an electric tripping signal 39.
In this manner, as soon as the operating temperature of the device
30 exceeds a preset threshold, the power line is interrupted. Accordingly, any condition
of potential danger for the user is prevented.
With reference now to Figure 6, the second sensor means 15 preferably
comprise a sensor 152 for detecting the voltage of a phase F of said low-voltage
power line. The sensor 152 can, for example, be constituted by an ordinary voltage
divider. The sensor 152 generates an electric signal 171, indicative of the value
of said detected voltage. The electric signal 171 is to be sent to the second electronic
means 15 (alternatively the electric signal 171 can be used directly as the voltage
input signal 17 of Figure 4).
The sensor 152 is electrically connected to the second circuit network
20, which generates an electric tripping signal 18.
Again with reference to Figure 6, the second sensor means 15 can comprise
a sensor 153 for detecting the electrical continuity condition of the neutral conductor.
The sensor 153 can advantageously comprise a circuit network 156, which is connected
to a phase conductor F1, to the neutral conductor N and to the second circuit network
20. The circuit network 156 comprises a resistive element and diodes arranged so
as to increase the voltage of the point designated by B once the interruption of
the neutral conductor in point A has occurred.
In case of interruption, the sensor 153 generates an electric voltage
signal, indicative of the electrical continuity condition of the neutral conductor,
to be sent to said second electronic means 15. The sensor 153 is electrically connected
to the second circuit network 20, which generates an electric tripping signal 18.
Likewise, the sensor means 15 comprise a sensor 154 for detecting
the electrical continuity condition of the earth conductor of the power line and
a sensor 155 for detecting the correct connection condition of the phase and neutral
conductors. Like the sensor 153, the sensors 154 and 155 comprise a circuit network,
which may be similar, in terms of structure and function, to the electric network
156 and generate, respectively, signals 173 and 174.
For example, in the case of the sensor 154, the corresponding circuit
network is connected between a phase conductor and the ground conductor of the power
line and the second circuit network 20.
In a preferred embodiment (Figure 2), the first electronic means 34
comprise first circuit means 36 for generating an electric tripping signal after
a preset minimum time interval. It is also possible to include second circuit means
46 for defining a minimum value of said earth leakage current, above which an electric
tripping signal is to be generated. Also, third circuit means 45 for generating
a tripping signal within a maximum time interval, calculated according to a substantial
relation of inverse proportionality with respect to an earth leakage current higher
than said minimum value, may be included.
Advantageously (Figure 7), the first electronic means 34 comprise
the circuit network 37 for generating an electric tripping signal 39 following the
reception in input of an electric current signal 40, and a fourth circuit block
41 for sending, following the reception of third and fourth enable signals (references
42 and 43), an electric current signal 40 in input to the circuit network 37. The
circuit block 41 is also meant to perform, following the failed reception of the
enable signals 420 and 43, a pre-discharge of said circuit network 37. This can
occur by sending in input to the circuit network 37 a current signal 40 of appropriate
polarity.
Advantageously, the second circuit means 46 comprise a resistive calibration
element 620 for generating (figure 7), by virtue of the reception of an electric
current signal 62, a first reference voltage 48 which is proportional to said minimum
value of the earth leakage current. The third circuit means 45 advantageously comprise
a third capacitive element 39 connected to earth and a third circuit block 50. The
circuit block 50 is meant to send an electric current signal 51 to the capacitive
element 49, depending on the second reference voltage 48, so as to generate a charging
voltage across the capacitive element 49 if an earth leakage current higher than
said minimum value is present. If an earth leakage current higher than said minimum
value is not present, then the signal 51 is sent so as to discharge the capacitive
element 49. Advantageously, the circuit block 50 provides, depending on the second
reference voltage 48, a first logic signal which is used as enable signal 42.
The third circuit means 45 can furthermore comprise a fifth comparator
element 52 which is coupled to the capacitive element 49. The comparator element
52 receives in input the charging voltage across the capacitive element 49 and generates
a second logic signal, which is used as enable signal 43, if said charging voltage
exceeds a preset reference voltage (VR).
Again with reference to Figure 7, the operation of the first, second
and third circuit means comprised in the electronic protection device according
to the invention is described in greater detail.
The calibration resistor 620 generates a reference voltage, depending
on which the circuit block charges the capacitive element 49. In particular, if
the reference voltage indicates the presence of a value of the earth leakage current
which is higher than the minimum value, the block 50 starts the charging process
so that the charging time is inversely proportional to the detected earth leakage
current. Once charging has been completed (enable signal 43), and if a sufficiently
high earth leakage current is still present (enable signal 420), the block 41 charges
the capacitive element 42. In order to ensure a constant charging time, charging
occurs in a linear manner which is not proportional to the detected earth leakage
current. In this manner, a minimum time interval after which the electric tripping
signal 39 is generated is set. The described constructive solutions are particularly
advantageous, since they allow presetting during design the entire tripping characteristic
of the electronic protection device.
The maximum time interval for tripping is in fact defined by the sum
of the charging times of the capacitive elements 49 and 42. On the other hand, for
earth leakage current values relatively higher than the nominal value, the charging
time of the capacitive element 49 becomes negligible, while the charging time of
the capacitive element 42 remains constant. Accordingly, a minimum tripping time
is set. Finally, the calibration resistor allows determining the minimum tripping
current by generating the reference voltage 48. The tripping characteristic of the
electronic protection device can therefore be fully defined during design. Accordingly,
it is possible to decide in advance the tripping characteristics of the electronic
protection device, allowing to better meeting the requirements of the electrical
user.
It is also very important that pre-discharge of the capacitive elements
49 and 42 is performed if the conditions for generating an electric tripping signal
do not occur, according to the described criteria.
This in fact allows to avoid unwanted tripping events (untimely trippings)
caused by accumulations of charge in the capacitive elements 49 and 42. The presence
of accumulated charge would in fact reduce the charging times of the capacitive
elements 42 and 49 and it would therefore no longer be possible to ensure a preset
minimum time interval after which tripping is to occur.
In another advantageous embodiment, the first electronic means 34
comprise fourth circuit means 60 for interfacing with the first sensor means 33.
With reference to Figure 8, the fourth circuit means 60 receive from said first
sensor means 33 an electric signal 61 which is indicative of the value of the earth
leakage current and generate an electric current signal 62 which is indicative of
the absolute value of the earth leakage current.
Advantageously, the circuit means 60 comprise a fifth low-impedance
circuit block 63, which is electrically connected to the first sensor means 33 so
as to ensure their linear operation. A sixth circuit block 64 is furthermore comprised
and is electrically connected to the circuit block 63 in order to receive the electric
signal 61, indicative of the value of the earth leakage current and in order to
generate an electric current signal 62 indicative of the absolute value of the detected
earth leakage current. Preferably, the current signal 62 is sent in input to the
calibration resistor 620, so as to generate the reference voltage 48.
Advantageously, a seventh filtering circuit block 65 is comprised
which is provided with means for eliminating high-intensity noise (protection diodes)
and/or high-frequency noise (an RC network). This embodiment is particularly advantageous,
since it allows to optimize the performance of the sensor means 33 especially if,
as commonly occurs in practice, they comprise a current transformer. In this case,
the secondary winding of the current transformer can operate in substantially ideal
conditions (i.e. close to the short-circuit condition).
In another preferred embodiment, the first electronic means 34 comprise
sixth circuit means 70 for generating an electric tripping signal if said first
sensor means 33 malfunction. With reference to Figure 9, the circuit means 70 advantageously
comprise a ninth circuit block 71 for measuring predefined physical parameters which
are indicative of the operating condition of said first sensor means. For example,
if the sensor means comprise a current transformer, it is possible to measure the
equivalent resistance of the secondary winding of the transformer. If the values
of said physical parameters are not within a preset range, the circuit block 71
generates a third enable signal 72 for a seventh circuit block 73, which is electrically
connected to the first circuit network 37. The circuit block 73 generates, following
the reception of the enable signal 72, an electric current signal 40 to be sent
in input to the circuit network 37, so as to generate an electric tripping signal
39.
This constructive embodiment is particularly advantageous, since it
allows to avoid the possibility of failure of the electronic protection device to
operate, for example due to possible damage suffered during the assembly of the
sensor means 33.
Again with reference to Figure 2, the actuation means are advantageously
connected to fifth circuit means 75 which are suitable to receive an electric tripping
signal (39 and/or 18) and to generate one or more command electric signals 750 for
activating said actuation means. The fifth circuit means 75 preferably comprise
an eighth circuit block (not shown), provided with an output stage of the latch
type, which has two stable operating states. The fifth circuit means 75 furthermore
advantageously comprise one or more electronic circuits (for example an appropriately
configured transistor stage) which ensure a relatively high output current. In this
manner, it is possible to drive actuation means of a different type, without substantially
modifying the first electronic means 36.
Advantageously, the electronic protection device according to the
invention comprises (Figure 2) third electronic means 130, which are electrically
connected to the power line 32 in order to provide a supply voltage VCC
to the first and/or second electronic means 34 and 16.
With reference to Figure 10, the third electronic means can comprise,
for example. a rectifier stage 131 (for example a diode bridge) which is connected
to a filtering stage 132 (for example an RC network), which is in turn connected
to a regulator stage 133, suitable to provide the various voltage levels (reference
134) used to supply the various components or for the voltage references (such as
for example the reference voltages VR, VRIF1, et cetera).
In practice, it has been found that the electronic earth leakage current
device 30, according to the invention, fully achieves the intended aim and objects.
Furthermore, it has been ascertained that the electronic earth leakage
current device 30 is easy to manufacture at low cost. In particular, the first and/or
second and/or third electronic means can be integrated in a microelectronic semiconductor
circuit, particularly in a circuit of the ASIC (Application-Specific Integrated
Circuit) type or in a microcontroller.
This allows to considerably reducing room occupation while reducing
manufacturing costs and improving operating reliability.
