U.S. patent application number 10/520810 was filed with the patent office on 2006-07-27 for intermediate circuit capacitor short-circuit monitoring.
Invention is credited to Dierk Gress, Harald Kramer.
Application Number | 20060164102 10/520810 |
Document ID | / |
Family ID | 29796386 |
Filed Date | 2006-07-27 |
United States Patent
Application |
20060164102 |
Kind Code |
A1 |
Kramer; Harald ; et
al. |
July 27, 2006 |
INTERMEDIATE CIRCUIT CAPACITOR SHORT-CIRCUIT MONITORING
Abstract
The electronic monitoring circuit for monitoring one of a group
of series-connected capacitor units in an intermediate circuit in
order to detect short-circuiting of one of the capacitor units
includes a device for deriving a reference voltage (14) from an
intermediate-circuit voltage (L(+), L(-)) applied across the
series-connected capacitor units (1, 2), a device for generating a
control signal consisting of a voltage difference between the
reference voltage (14) and a junction voltage at a junction (13)
between two capacitor units (1, 2) and a device for generating an
error signal when said voltage difference falls below or exceeds an
activation threshold voltage thus indicating that one of the
capacitor units (1,2) has been short-circuited. The activation
threshold voltage corresponds to a breakdown voltage of a zener
diode provided in the monitoring circuit.
Inventors: |
Kramer; Harald;
(Vorstadtstrasse, DE) ; Gress; Dierk;
(Grenzstrasse, DE) |
Correspondence
Address: |
Striker Striker & Stenby
103 East Neck Road
Huntington
NY
11743
US
|
Family ID: |
29796386 |
Appl. No.: |
10/520810 |
Filed: |
July 14, 2003 |
PCT Filed: |
July 14, 2003 |
PCT NO: |
PCT/DE03/02358 |
371 Date: |
August 1, 2005 |
Current U.S.
Class: |
324/548 |
Current CPC
Class: |
H02H 7/16 20130101 |
Class at
Publication: |
324/548 |
International
Class: |
G01R 31/12 20060101
G01R031/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 13, 2002 |
DE |
102 32 145.0 |
Claims
1-13. (canceled)
14. An electronic monitoring circuit for monitoring one of at least
two series-connected capacitor units in an intermediate circuit in
order to detect short-circuiting of said one of said capacitor
units, said monitoring circuit (16) comprising means for deriving a
reference voltage (14) from an intermediate-circuit voltage
(L(+),L(-)) applied across said at least two series-connected
capacitor units (1, 2); means for generating a control signal
consisting of a voltage difference between said reference voltage
(14) and a junction voltage at a junction (13) between two of said
capacitor units (1, 2); and means for generating an error signal
when said control signal falls below or exceeds an activation
threshold voltage thus indicating that said one of said capacitor
units (1,2) has been short-circuited.
15. The monitoring circuit as defined in claim 14, wherein said
means for deriving said reference voltage (14) comprises a
plurality of series connected resistors (3,4) and wherein said
plurality of said series-connected resistors (3,4) is connected in
parallel with said at least two series-connected capacitor units
(1,2).
16. The monitoring circuit as defined in claim 14, wherein said
means for generating said error signal when said voltage difference
falls below or exceeds said activation threshold voltage comprises
a zener diode (15) and said activation threshold voltage
corresponds to a breakdown voltage of said zener diode.
17. The monitoring circuit as defined in claim 14, wherein said
error signal corresponds to an error signal voltage and said error
signal voltage is based on a freely selectable ground
potential.
18. The monitoring circuit as defined in claim 14, wherein said
means for generating the error signal comprises a light-emitting
diode (17) and a light-sensitive transistor (18) and said
light-emitting diode (17) and light-sensitive transistor (18) are
arranged to generate said error signal from said voltage difference
when said voltage difference is applied to said light-emitting
diode.
19. The monitoring circuit as defined in claim 14, wherein said
means for generating said error signal comprises a current-voltage
converter and said current-voltage converter generates an error
signal voltage directly from a current flowing due to voltage
asymmetry produced with an error occurs.
20. The monitoring circuit as defined in claim 17, wherein said
current flowing due to said voltage asymmetry when said error
occurs is limited by a resistor chain.
21. The monitoring circuit as defined in claim 20, wherein
respective intermediate-circuit capacitor units correspond to
corresponding parts of said resistor chain, each of said parts
consisting of at least one chain resistor, and respective ratios of
capacitance of said respective intermediate-circuit capacitor units
to resistance of said corresponding parts of said resistor chain
are substantially the same for all resistor-chain-part-capacitor
pairs.
22. The monitoring circuit as defined in claim 20, wherein each of
said parts of said resistor chain consists of a single chain
resistor.
23. The monitoring circuit as defined in claim 14, wherein each of
said capacitor units consists of a capacitor.
24. The monitoring circuit as defined in claim 14, wherein each of
said capacitor units consists of a plurality of capacitors
connected in series and/or parallel with each other.
25. The monitoring circuit as defined in claim 14, wherein each of
said capacitor units have the same capacitance.
Description
[0001] The present invention relates to an electronic circuit for
short-circuit monitoring of one of at least two series-connected
intermediate-circuit capacitor units, according to Claim 1.
[0002] Monitoring of this type is required when a plurality of
capacitors is connected in series, to achieve a certain voltage
sustaining capability. Converters that are connected directly to a
3-phase network operate with an intermediate-circuit voltage of
approximately 750 V. Economical capacitors usually have a maximum
voltage sustaining capability of 450 V, however. As a result, to
obtain the necessary voltage sustaining capability, at least two
capacitors of this type must be connected in series. The
series-connected capacitors are always connected in parallel with
the intermediate circuit.
[0003] Modern converters in the low to moderate performance class
function according to the PWM (pulse-width modulation) principle,
which results in high switching current frequencies and alternating
current frequencies in the intermediate circuit. The currents which
are therefore produced subject the capacitors to a high thermal
load. In the extreme case, this load can result in an at-first
undetected short circuit in one of the series-connected elements.
The missing capacitance results in an overloading of the remaining
capacitors and, possibly, a risk of fire.
[0004] The object of the present invention is to provide a simple
monitoring system that is capable of quickly detecting a short
circuit in one of a plurality of series-connected capacitor units,
whereby a unit can be composed of one or more capacitor(s)
connected in any fashion. The system should be able to signal a
short circuit to a higher-order control system, and it must be
robust enough to withstand high voltages, high temperatures and
strong electromagnetic interferences.
[0005] This object is attained by the features of Claim 1. An
advantage of the wiring according to the present invention is that
the monitoring is carried out using a simple voltage comparison,
whereby the difference between the voltage present at the junction
between two of the capacitor units to be monitored and a reference
voltage that is relevant for the monitoring and is shunted from the
intermediate-circuit voltage is used as the control signal, which,
if the capacitor short circuits, falls below or exceeds a response
threshold and generates an error signal. The state of the error
signal is monitored by the drive computer or a higher-order control
unit. If an error occurs, the relevant error reaction is carried
out. As an alternative or in addition, the error can also be
displayed using a display means, e.g., a light-emitting diode on
the drive.
[0006] Preferred embodiments of the present invention are indicated
in the dependent claims.
[0007] An advantage of the present invention is that the capacitors
are either monitored individually, or they can be connected in
parallel or in series to form units that can be treated and
monitored as individual capacitors. The units can then be adapted
exactly to the requirements of the converter.
[0008] In a preferred embodiment, the required reference voltage is
generated from a chain of series-connected resistors, the chain
being connected in parallel with the units to be monitored. The
voltage divider created in this manner ensures that voltage
fluctuations, which are common in an intermediate circuit, are
automatically mirrored by the reference signal and are therefore
compensated for. The capacitor monitoring is indifferent with
regard for voltage fluctuations of this type.
[0009] In a further preferred embodiment, the response threshold
which is relevant for the system is established by the breakdown
voltage of a zener diode. The zener diode ensures that electronic
disruptions do not result in an undesired triggering of the error
signal.
[0010] To insulate the error signal voltage from the zero volt
potential of the intermediate-circuit voltage, an error signal
voltage is generated using a current-voltage converter directly
from the current that flows due to the voltage asymmetry produced
when an error occurs.
[0011] A further advantage of the present invention lies in the
fact that the error signal-generating current is limited by the
chain of resistors required to generate the reference voltage. The
resistors have two separate functions for this reason, and are
optimized for both of them simultaneously. This results in a
reduction in the number of components.
[0012] In a further preferred embodiment, an element corresponding
to each capacitor unit is provided in the chain of resistors,
whereby an element is composed of one or more resistors. The
individual capacitor units are monitored in this manner.
[0013] In a further preferred embodiment, the ratio of capacitor
capacitance (in farads) to the corresponding part of the chain of
resistors is essentially the same for all pairs of corresponding
resistor parts and capacitors. This ensures that the electrical
potential difference which exists in the normal state between the
junctions between two of the capacitors to be monitored, and that
which exists in the normal state at the junction between the two
corresponding parts of the chain of resistors does not exceed a
predefined threshold.
[0014] In a further preferred embodiment, the error signal voltage
is free of ground potential. The advantage of this is that the
error signal voltage can be allotted to any ground potential.
[0015] In a further preferred embodiment, the error signal voltage
is generated using a light-emitting diode-insulated transistor. For
this reason, the error signal voltage is galvanically separated
from the high voltage to be monitored. Opto-insulating components
are reliable and easy to install, in contrast to magnetic
components.
[0016] In a preferred embodiment, all capacitor units to be
monitored have the same capacitance. This simplifies the circuit
and the selection of resistors corresponding to the capacitors. To
further reduce the fabrication cost and simplify the circuit, every
part of the chain of resistors is composed of a resistor.
[0017] In a further preferred embodiment, the number of capacitor
units to be monitored is equal to two. The advantage of this
embodiment is that the complexity of the circuit is minimized.
[0018] In a further preferred embodiment, every capacitor unit is
composed of one capacitor, which results in further simplification
of the circuit.
[0019] An exemplary embodiment of the present invention is shown in
FIG. 1 and will be described in greater detail below.
[0020] FIG. 1 shows a schematic illustration of the
intermediate-circuit bus (10, 11) of a frequency converter,
including two series-connected capacitors (1, 2) and the monitoring
circuit (16) according to the present invention. FIG. 2 shows an
example of an intermediate-circuit bus with a plurality of
series-connected capacitors (1) and their associated monitoring
units (19). In FIG. 1, the capacitors (1, 2)--considered
together--have an increased voltage sustaining capability, which
corresponds to the sum of the two rated voltages, but have reduced
capacitance, in accordance with Kirchhoff's voltage law. If the
reduced capacitance is not adequate and larger capacitors (1, 2)
with the required voltage sustaining capability are not available,
further capacitors can be connected in parallel, to increase
overall capacitance. In this example, the intermediate-circuit
voltage is equal to the difference between L.sub.(+) (10) and
L.sub.(-) (11).
[0021] The monitoring circuit (16) is preferably composed of two
series-connected resistors (3, 4), four diodes (5), (6), (7), (8),
a zener diode (15), and a galvanically-insulated output (12). In
this case, the galvanic insulation (9) is realized by a combination
of a light-emitting diode and a light-sensitive transistor, the
transistor including an open collector output (12). Diodes (5) and
(6), and (7) and (8) are connected in series, whereby the cathode
of diodes (5) and (8) is connected to the anode of diodes (6) and
(7). The two diode pairs are then connected in parallel, so that
the cathodes of diodes 6 and 7 are connected with each other, and
the anodes of diodes (5) and (8) are connected with each other. The
connection between diodes (5) and (6) is connected to the capacitor
voltage to be measured. The connection point between diodes (7) and
(8) is connected to the reference voltage.
[0022] The cathode of the zener diode (15) is connected to the
cathodes of diodes (6) and (7), and its anode is connected, with
reverse polarity, to the cathode of the light-emitting diode of the
optical insulating component. The cathode connection of the
insulating component is connected with the anodes of diodes (5) and
(8).
[0023] Since the capacitors (1, 2) and resistors (3, 4) all have a
voltage-dividing function, half--at first approximation--of the
intermediate-circuit voltage is present, in normal operation, at
the two junctions (14) and (13), i.e., the voltage difference
between junctions (14) and (13) is nearly zero. In this state, no
current flows between the two junctions.
[0024] Since both voltages form the same linear function of the
intermediate-circuit voltage, and since only the differential
voltage is relevant, negative effects that would be expected due to
the noise and voltage fluctuations that often occur in the
intermediate-circuit bus are eliminated.
[0025] When a capacitor (2, 1) short circuits, a differential
voltage forms between junctions (13) and (14). When this
differential voltage exceeds a predefined threshold that
corresponds to the sum of two diode voltages (7, 5) or (6, 8) plus
the zener diode (15) breakthrough voltage, current flows. The
circuit is designed so that current always flows through the zener
diode (15) in the same direction, independently of whether the
voltage at junction 13 is greater than or less than the voltage at
junction 14.
[0026] The current resulting from the voltage asymmetry switches
the transistor (9) on, thereby activating the error signal (12).
The strength of the current is limited by the size of the resistor
(3) and/or (4). Since the properties of zener diodes and
light-emitting diodes are dependent on the current intensity, the
components must be designed such that a rapid response by the error
signal (12) is ensured. When a plurality of capacitors (1, 2) is
connected in series, then, to ensure that the individual capacitors
are monitored, the monitoring circuit must be present in plurality,
i.e., an additional monitoring circuit must be installed for each
additional series-connected capacitor. This is shown in FIG. 2,
whereby the 4 capacitors (1) are monitored by 3 monitoring units
(19). One error signal output (12), one reference voltage input
(14) and an associated capacitor voltage input (13) are provided
for each unit. Activation of an error signal (12) is still caused
by a capacitor short circuit (1), whereby the short-circuiting of a
capacitor (1) can trigger one or more error signals (12). All error
signals (12) should be monitored so that, if a short circuit
occurs, it is possible to determine exactly which capacitor (1) has
failed.
REFERENCE NUMERALS
[0027] 1. Capacitor [0028] 2. Capacitor [0029] 3. Capacitor [0030]
4. Capacitor [0031] 5. Diode [0032] 6. Diode [0033] 7. Diode [0034]
8. Diode [0035] 9. Galvanic insulation [0036] 10. Negative
intermediate-circuit voltage [0037] 11. Positive
intermediate-circuit voltage [0038] 12. Error signal [0039] 13.
Junction [0040] 14. Reference voltage measurement point [0041] 15.
Zener diode [0042] 16. Monitoring circuit [0043] 17.
Light-sensitive diode [0044] 18. Light-sensitive transistor [0045]
19. Monitoring unit
* * * * *