WO2010100391A1 - Améliorations apportées aux isolateurs galvaniques - Google Patents

Améliorations apportées aux isolateurs galvaniques Download PDF

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Publication number
WO2010100391A1
WO2010100391A1 PCT/GB2009/000619 GB2009000619W WO2010100391A1 WO 2010100391 A1 WO2010100391 A1 WO 2010100391A1 GB 2009000619 W GB2009000619 W GB 2009000619W WO 2010100391 A1 WO2010100391 A1 WO 2010100391A1
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Prior art keywords
galvanic isolator
current
preset level
current flow
isolator according
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Ceased
Application number
PCT/GB2009/000619
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English (en)
Inventor
Jonathan Robert Shaw
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SHORELINE PRODUCTS Ltd
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SHORELINE PRODUCTS Ltd
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Priority to GB1115342A priority Critical patent/GB2480205A/en
Priority to PCT/GB2009/000619 priority patent/WO2010100391A1/fr
Publication of WO2010100391A1 publication Critical patent/WO2010100391A1/fr
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H7/00—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions
    • H02H7/10—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers
    • H02H7/12—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers
    • H02H7/125—Emergency protective circuit arrangements specially adapted for specific types of electric machines or apparatus or for sectionalised protection of cable or line systems, and effecting automatic switching in the event of an undesired change from normal working conditions for converters; for rectifiers for static converters or rectifiers for rectifiers
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F13/00—Inhibiting corrosion of metals by anodic or cathodic protection
    • C23F13/02—Inhibiting corrosion of metals by anodic or cathodic protection cathodic; Selection of conditions, parameters or procedures for cathodic protection, e.g. of electrical conditions
    • C23F13/04—Controlling or regulating desired parameters

Definitions

  • the present invention relates to galvanic isolators and, more particularly, to a galvanic isolator having means for reducing its power dissipation.
  • the present invention also relates to a method for reducing the power dissipation of a galvanic isolator.
  • Galvanic isolators are used to provide electrical safety earthing of exposed metallic parts associated with marine craft, pipelines, electrical supply distribution transformers and related equipment, or any other exposed metalwork or structure, which may pose a safety hazard to animals or humans under certain unusual conditions such as an electrical supply fault or lightning strike.
  • a galvanic isolator may also be used to provide safety earthing of cathodic protected metallic structures, which are normally isolated from ground and have a small DC voltage deliberately applied thereto, in order to prevent electrochemical corrosion.
  • Galvanic isolators provide electrical isolation of exposed metalwork up to a certain voltage level of direct current (DC), the galvanic isolator threshold voltage, which is deemed sufficient to prevent the flow of electrochemically generated currents, or currents generated by nearby DC electrical installations, whilst providing a low impedance path to alternating or transient currents, such as mains electrical supply current or lightning.
  • the electrical safety earth function is maintained, whilst allowing the exposed metalwork to be isolated from the ground up to a certain DC voltage level, which is sufficient to prevent electrochemical corrosion or other undesirable effects of DC, for instance power transformer core biasing.
  • galvanic isolators have used a parallel configuration of 2 or more electronic semiconductors such as silicon diodes silicon controlled rectifiers or transistors which are automatically switched into a low impedance when the galvanic isolator threshold voltage is exceeded and the safety earth function is required.
  • a galvanic isolator can be installed between the shore safety earth and the craft's safety earth to reduce this problem.
  • the flow of low level DC is reduced to near zero, thereby reducing underwater electrolytic corrosion, whilst maintaining the electrical safety earth function.
  • an example of a prior art galvanic isolator 101 is shown, which comprises a plurality of semiconducting elements, in the example four diodes 104, 105, 106 and 107.
  • the diodes are connected to at least two sources of electric potential having a current flow there between, in the example shore earth 102 and craft earth 103.
  • the galvanic isolator threshold voltage is typically in the range of 1 to 2 Volts.
  • the potential difference generated by electrochemical interaction of submerged dissimilar metals in this situation is generally less than 1 Volt, whereby current flow and therefore electrochemical corrosion, amount to substantially zero.
  • the galvanic isolator In the event of a potential difference between shore earth 102 and craft earth 103 which exceeds the galvanic isolator threshold voltage, such as would occur in the event of a mains power fault, the diodes conduct and current flows.
  • the galvanic isolator therefore maintains a safe potential difference in the range of 1 to 2 Volts between shore earth 102 and craft earth 103, whilst allowing fault current to flow.
  • a worst-case fault comprises two distinct stages.
  • the first stage is characterised by a partial insulation breakdown between live and earth, which is sufficient to pass a medium fault current of up to 150% of the nominal circuit current, but insufficient for triggering any overcurrent protection in the circuit.
  • the first stage of a typical worst-case fault which may last several minutes or hours, may eventually be followed by a second stage characterised by a complete insulation breakdown (a short-circuit), which results in a very -A-
  • overcurrent protection in the circuit may comprise an overcurrent protective device such as an enclosed high breaking current (HBC) fuse or a miniature circuit breaker (MCB), which is connected to the AC mains electrical circuit and which is designed to limit the circuit current to safe levels, apt to prevent heating of the circuit conductors to a level at which insulation or connections would be damaged or compromised.
  • HBC high breaking current
  • MBC miniature circuit breaker
  • a typical overcurrent protective device should pass the nominal circuit current indefinitely, allow an overcurrent of between 125% and 150% of the nominal circuit current to pass for a period of time up to 4 hours, allow an overcurrent of 500% or more of the nominal circuit current to pass for up to 5 seconds, and isolate a very high current surge of 100 times the nominal circuit current within approximately 10 milliseconds.
  • a worst-case fault, as described above may last for several minutes or hours, causing all parts of the circuit, including the galvanic isolator components, to heat up to the limits of maximum safe operating temperature for insulation, connections and reliability.
  • galvanic isolators should be able to withstand mains surges as if overcurrent protection where entirely absent or inoperable and, in such cases, the galvanic isolator must be able to handle a fault current surge equivalent to that required to melt the safety earth conductor itself.
  • the current surge may be close to the maximum prospective current available in the circuit. In a marine craft-shore environment, 5000 Amps RMS for a few tens of milliseconds can be expected in a 16 Amp or 32 Amp shore power circuit. This surge occurs at a time when the galvanic isolator diodes 104, 105, 106 and 107 are least able to withstand high current surges, due to their elevated junction temperature.
  • the invention relates to improvements in the reliability and fault tolerance capacity of a galvanic isolator, under the worst foreseeable operating conditions. Such improvements are achieved through a significant reduction in overall power dissipation, such as by a factor of ten or more, relative to the power dissipation characteristics of prior art designs.
  • a galvanic isolator which comprises a plurality of semiconducting elements, earth connecting means for connecting the plurality of semiconducting elements to at least two sources of electric potential having a current flow there between and control means adapted to switch the plurality of semiconducting elements into a low impedance state when a threshold voltage of the galvanic isolator is exceeded.
  • the galvanic isolator is characterised by further comprising an electronic bypass element adapted to bypass the plurality of semiconducting elements, and an electronic control circuit adapted to monitor the current flowing through the earth connecting means and to switch the electronic bypass element from a high resistance state suitable for a current flow below a first preset level, to a very low resistance state when the current flow increases above the first preset level.
  • the electronic control circuit is preferably further configured to switch the electronic bypass element from the very low resistance state suitable for a current flow above the first preset level, to the high resistance state when the current flow increases above a second preset level higher than the first preset level.
  • the first preset level may be comprised in the range of 20 Amps to 30
  • Amps, and the second preset level may be comprised in the range of 100 Amps to 140 Amps.
  • the semiconducting elements are preferably selected from the group comprising diodes, silicon controlled rectifiers and transistors.
  • the electronic bypass element comprises metal oxide semiconductor field effect transistors (MOSFETs).
  • MOSFETs metal oxide semiconductor field effect transistors
  • the electronic bypass element comprises a pair, or parallel connected pairs, of MOSFETs having body diodes, the body diodes being connected in opposition to prevent current flow when the MOSFETs are switched into a high impedance state.
  • the electronic bypass element comprises two pairs, or parallel connected pairs, of MOSFETs having body diodes, the body diodes being connected to conduct and allow current flow.
  • the body diodes themselves form the plurality of diodes of the galvanic isolator.
  • the galvanic isolator may advantageously further comprise status indicating means adapted to indicate AC fault current flowing or direct current flowing to a user.
  • the status indicating means may for instance be light emitting diodes (LEDs).
  • the galvanic isolator may comprise a current transformer means adapted to power the electronic control circuit.
  • the galvanic isolator may further comprise mains supply connection means adapted to power the electronic control circuit.
  • the galvanic isolator may further comprise an enclosure for housing the galvanic isolator, the enclosure also having an overcurrent device or a residual current device or both an overcurrent device and a residual current device for limiting the prospective fault current available from a mains power supply, thereby limiting the maximum fault current flowing through the galvanic isolator.
  • the galvanic isolator may advantageously further comprise a monitoring element adapted to record events of significant amplitude, such as current surges and lightning strikes.
  • the galvanic isolator may advantageously further comprise surge protection means, the surge protection means comprising a transient voltage suppressor connected in parallel with the galvanic isolator or a series connected inductor, or both a transient voltage suppressor connected in parallel with the galvanic isolator and a series connected inductor.
  • surge protection means comprising a transient voltage suppressor connected in parallel with the galvanic isolator or a series connected inductor, or both a transient voltage suppressor connected in parallel with the galvanic isolator and a series connected inductor.
  • a current transducer of the electronic control circuit is a current transformer.
  • the current transducer is a magnetic field detection device such as a hall-effect device adapted to detect the magnetic field generated by the fault current.
  • the current transducer may be a low value series resistor with a voltage measuring circuit.
  • a galvanic isolator comprising an electronic bypass element comprising an array of metal oxide semiconductor field effect transistors (MOSFETs) having intrinsic body diodes, earth connecting means for connecting the plurality of intrinsic body diodes to at least two sources of electric potential having a current flow there between, and an electronic control circuit adapted to monitor the current flowing through the earth connecting means, and to switch the electronic bypass element from a high resistance state suitable for a current flow below a first preset level, to a very low resistance state when the current flow increases above the first preset level.
  • MOSFETs metal oxide semiconductor field effect transistors
  • the electronic control circuit is preferably further adapted to switch the electronic bypass element from the very low resistance state suitable for a current flow above the first preset level, to the high resistance state when the current flow increases above a second preset level higher than the first preset level.
  • the first preset level may be comprised in the range of 20 Amps to 30 Amps, and the second preset level may be comprised in the range of 100 Amps to 140 Amps.
  • the array is preferably a parallel connected MOSFET array comprising N channel enhancement mode MOSFETs.
  • a method of reducing the power dissipation of a galvanic isolator substantially as recited above comprising the steps of monitoring current flowing through the earth connecting means, and switching the electronic bypass element in dependence on the monitoring, between a high resistance state when the current flows below a first preset level, and a very low resistance state when the current flow increases above the first preset level.
  • the method preferably comprises the further step of switching the electronic bypass element from the very low resistance state to the high resistance state when the current flow increases above a second preset level which is higher than the first preset level.
  • the first preset level may be comprised in the range of 20 Amps to 30 Amps
  • the second preset level may be comprised in the range of 100 Amps to 140 Amps.
  • a craft equipped with a galvanic isolator substantially as recited above, wherein the at least two sources of electric potential respectively comprise a shore source and a craft source.
  • Figure 1 shows a circuit diagram of a prior art galvanic isolator.
  • Figure 2 shows a graph of surge current rating of a diode.
  • Figure 3 shows a circuit diagram of a first embodiment of a galvanic isolator according to the invention.
  • Figure 4 shows a circuit diagram of a second embodiment of a galvanic isolator according to the invention.
  • Figure 5 shows a circuit diagram of a third embodiment of a galvanic isolator according to the invention, having overcurrent protection features.
  • Figure 6 shows a circuit diagram of a fourth embodiment of a galvanic isolator according to the invention, having very high power MOSFETs.
  • Figure 6 shows an oscillograph of the AC voltage drop of a prior art galvanic isolator.
  • Figure 7 shows an oscillograph of the AC voltage drop of a galvanic isolator according to the invention.
  • Figure 2 is a graph of surge overload current for a typical silicon rectifier diode, which illustrates the improvement in surge current handling that may be expected when the starting junction temperature is reduced from 140°C to 45°C. If the temperature is reduced from 140 0 C to 45°C, under a fault current of 6000 Amps peak for example, the duration of surge which can be handled increases from 30 milliseconds to 110 milliseconds.
  • a first embodiment of the invention relates to a galvanic isolator for preventing corrosion of submerged metallic parts of a marine craft, while moored and connected to a land-based source of alternating current (AC) mains electrical power, by reducing the flow of electrochemically generated current.
  • AC alternating current
  • FIG. 3 A block diagram showing the component parts of the galvanic isolator is shown in Figure 3 and comprises a galvanic isolator substantially as illustrated in, and detailed with reference to, Figure 1 , with the addition of an electronic bypass element, which can be automatically switched between a high resistance state and a very low resistance state by an electronic control circuit, which responds to changes in current flowing through the galvanic isolator.
  • the galvanic isolator 301 therefore comprises semiconducting elements, in the example diodes 304, 305, 306 and 307, and further comprises a parallel connected MOSFET array comprising N channel enhancement mode MOSFETs 311 , 312, 313 and 314.
  • the MOSFET array is controlled by an electronic control circuit, which is powered by a power supply 310, and which responds to changes in alternating current flowing between craft earth 303 and shore earth 302.
  • the MOSFET array is made up of one or more pairs of series connected MOSFETS connected source to source.
  • the intrinsic body diode which is inherent in the manufacture of each MOSFET.
  • the cathode of the body diode is the channel itself, while the anode is internally connected to the source terminal.
  • the body diode therefore appears as if it were connected in parallel with the MOSFET from source to drain terminals, with the cathode connected to the drain terminal and the anode connected to the source terminal. It is therefore necessary to use series connected pairs in this way, to prevent current flowing through the body diode when the MOSFET is turned off and the source terminal is more positive than the drain terminal.
  • two series-connected pairs are used in parallel, in order to increase the overall current rating of the array and to reduce the power dissipation, by virtue of the overall lower resistance of the parallel connected paths.
  • more parallel pairs of MOSFETs may be used in parallel in order to increase the current handling and/or reduce the power dissipation.
  • the 'on' resistance of MOSFETs increases with rising temperature, whereby MOSFETS connected in parallel will tend to share current equally due to the thermal feedback effect.
  • Resistors 316, 317, 318 and 319 prevent ringing or oscillation phenomena, due to the parasitic inductance and capacitance around the MOSFETs.
  • the MOSFET array can be switched into a conducting state by applying a positive control voltage between 5 and 15 Volts with respect to the source terminals, to the gate terminals via the resistors 316, 317, 318 and 319.
  • the resistance from source to drain when in the conducting state is less than 1 milliohm for each MOSFET used in this example. If the gate voltage is near zero with respect to the source, then the MOSFET is switched into a high impedance state. When the array is switched into the conducting mode, it appears as a near short-circuit across the galvanic isolator 301 , amounting to approximately 1 milliohm in total in this example.
  • the gate and source terminals of the MOSFETS are kept at the same potential, whereby the MOSFETS are in a high impedance state and this allows the galvanic isolator to function substantially as described above.
  • the MOSFET array is automatically switched into the low-impedance state.
  • the voltage drop across the galvanic isolator is considerably reduced, whereby the dissipated power and consequent heating of the galvanic isolator components are correspondingly reduced.
  • the diodes 304, 305, 306 and 307 are now not conducting any current at all, because the applied voltage is below their threshold voltage for conduction. Their junction temperatures will therefore be close to ambient temperature.
  • the MOSFET array remains in the conducting state as long as the AC fault current flows.
  • the MOSFET array is automatically switched into the high-impedance state and the galvanic isolator returns to normal operation.
  • Capacitor 315 is present in order to provide a low impedance path to low- level AC current, such as that from the AC line filters in connected equipment, and leakage current up to a few milliamps that may occur under normal non- fault circumstances.
  • the low impedance path provided by capacitor 315 prevents low-level AC currents from compromising the effectiveness of the corrosion inhibiting function of the galvanic isolator, by reducing the likelihood that the diodes 304,
  • the power-supply 310 may be derived from an external source, or may be derived from the AC fault current via a current transformer.
  • the MOSFET array may not be able to handle the maximum expected fault current, in which case the control circuit may employ an overcurrent protection function which switches the MOSFET array into the high-impedance state when the current through the galvanic isolator exceeds a pre-set value, for example comprised between 100 Amps and 140 Amps, for instance 140 Amps peak.
  • the current transducer 308 may be a current transformer, a Hall-effect device, or a low value series resistor (approximately 0.5 milliohm) with a voltage measuring circuit.
  • This function can be implemented by using a series inductor which limits the peak current of high speed transients, and a transient voltage suppressor which dissipates the remaining lightning energy and limits the voltage applied to the galvanic isolator to a safe level.
  • the galvanic isolator can be built into an enclosure through which the AC mains power supply to the craft must pass, and which contains a residual current device (RCD) and a current limiting circuit breaker (MCB) for protecting the craft's electrical wiring from overcurrent and earth leakage faults.
  • RCD residual current device
  • MBC current limiting circuit breaker
  • LEDs light emitting diodes
  • a second embodiment of a galvanic isolator is shown, wherein the galvanic isolator 401 again comprises four diodes 304, 305,
  • the current transducer 308 applies a corresponding AC voltage at the input terminals of a bridge-rectifier 420.
  • the rectified DC output of the bridge rectifier 420 is smoothed into steady DC by reservoir capacitor 422.
  • the smoothed DC voltage is applied to voltage regulator 421 , which maintains the DC voltage on its output terminals at 12 Volts or less.
  • Capacitors 423 and 424 provide decoupling and prevent self-oscillation of the voltage regulator 421.
  • the output voltage from the voltage regulator 421 is connected between gate and source connections of the MOSFET array, whereby the MOSFET array will be automatically switched into the conducting mode when the current flowing between craft earth 303 and shore earth 302 is such that the voltage regulator output exceeds approximately 5 Volts, therefore above the gate threshold voltage of the MOSFETs.
  • LED 425 and current limiting resistor 426 provide visual indication that the device is active, i.e. that fault current is flowing.
  • the MOSFET array will remain in the conducting state until the fault current between craft earth 303 and shore earth 302 reduces below a certain value which corresponds to the voltage on the output of the voltage regulator 421 falling below the gate threshold voltage of the MOSFETs.
  • Resistor 451 ensures that the gate-source voltage is close to zero when there is no AC fault current flowing, and ensures the MOSFETs remain in the high-impedance state unless a fault-current is detected.
  • a third embodiment of a galvanic isolator is shown, wherein the galvanic isolator is designed to withstand a long-term fault of 100 Amps RMS for 4 hours at an ambient temperature of 50 0 C, immediately followed by a surge of 5000 Amps RMS for half a second.
  • the diodes 304, 305, 306 and 307 are chosen with characteristics apt to withstand a surge of substantially 5000 Amps for approximately 0.5 seconds from a starting temperature of substantially 5O 0 C.
  • the MOSFET array is designed to reduce power dissipation during the first part of the fault (100 Amps for 4 hours) by short-circuiting the diodes 304, 305, 306 and 307.
  • the MOSFET array uses four pairs of MOSFETS in parallel, however only two pairs are shown in Figure 5 for not obscuring the Figure unnecessarily.
  • the resistance of the array when switched into the conducting state is 0.5 milliohms or less, whereby the array will dissipate only approximately 5 Watts.
  • an overcurrent protection circuit is incorporated in the control circuit, whereby the MOSFET array will be automatically switched off in the event of a high current surge.
  • the overcurrent protection circuit is designed to operate in such a way as to ensure the MOSFET array is always operating within safe limits.
  • the array can handle the continuous 100 Amps RMS. without any significant temperature rise, but cannot handle the 5000 Amps surge for more than 100 microseconds.
  • the protection circuit is designed to turn the MOSFET array off in less than 5 microseconds if the current rises above 140 Amps peak, which keeps the MOSFETs operating within safe limits.
  • the overcurrent protection circuit uses a differential amplifier to measure the voltage drop across the MOSFET array, which is proportional to the current flowing.
  • the galvanic isolator 401 again comprises four diodes 304, 305, 306 and 307, a MOSFET array 311 , 312, 313 and 314 with gate resistors 316, 317, 318 and 319, and a capacitor 315, the apparatus operating substantially as explained with reference to Figures 3 and 4 above.
  • Power for the control circuit is derived from the fault current via current transformer 530, which develops an alternating voltage which is rectified and smoothed by bridge rectifier 554 and reservoir capacitor 555, and regulated at 12 Volts by voltage regulator 556.
  • Varistor 552 and capacitor 553 provide transient voltage suppression along with the inductance and resistance inherent in the current transformer 530.
  • Zener diode 564 is a high power transient suppressor, which protects the voltage regulator 556 from voltages greater than approximately 30 Volts.
  • Capacitor 557 provides decoupling and prevents self-oscillation of the voltage regulator 556.
  • Voltage regulator 562 provides a regulated 5 Volt supply.
  • Capacitor 563 provides decoupling and prevents self-oscillation of the voltage regulator 562.
  • Switched capacitor inverter 570 generates minus 5 Volts for use by the overcurrent sense amplifier 533.
  • the control circuit is activated and the MOSFET array is automatically switched into the low resistance state when the SHUNT-ON signal is generated by zener diode 558, resistors 559, 560 and capacitor 561.
  • Zener diode 558 starts to conduct when the power supply reaches about 9
  • the overcurrent sense amplifier comprises an operational amplifier (op- amp) 533 and gain-setting resistors 529, 530, 531 and 532, which amplify the voltage between point A and point B on the diagram.
  • the resistance between point A and point B is about 0.7 milliohm with the MOSFET array in the low- impedance state. Therefore, a current of plus or minus 140 Amps peak will generate an output from op-amp 533 of approximately plus or minus 1.5 Volts peak.
  • the output of the overcurrent sense amplifier 533 in response to an AC fault current flowing between craft earth 303 and shore earth 302 is a corresponding AC waveform with a magnitude proportional to the current and centred about 0 Volts.
  • the output is AC coupled by capacitor 534, whereby any DC offset errors from the amplifier 533 are eliminated.
  • Op-amps 538 and 540, resistors 535, 536 and 537 and diode 539 are configured as an absolute value circuit, whereby both negative and positive current peaks can be detected by the overcurrent threshold circuit, which consists of a comparator 546 and associated components.
  • the absolute value circuit operates as follows: for positive inputs, the diode 539 is reverse-biased and has no influence on the circuit, op-amp 538 and resistors 535, 536, 537 operate as a simple unity-gain voltage follower; when the input voltage to the absolute value circuit becomes negative, the diode 539 is forward-biased and the output of op-amp 540 holds the non-inverting input of op-amp 538 at 0 Volts, whereby op-amp 538 and resistors 536, 537 now form a simple unity- gain inverting amplifier.
  • the common-mode input range of op-amp 540 must extend below the negative supply of 0 Volts in this case.
  • op-amp 540 must remain at high- impedance within the entire operating range of the input to the absolute value circuit, down to approximately - 2 Volts in this case. Also both op-amps 539, 540 must be capable of driving their outputs to the negative power supply rail (0 Volts).
  • the threshold detection circuit comprises comparator 546, resistors 542, 543 and capacitor 545.
  • Comparator 546 compares the output of the absolute value circuit with a preset threshold voltage derived from the + 5 Volt supply via the divider chain of resistors 542, 543, which is decoupled by capacitor 545.
  • comparator 546 If the maximum design current threshold of 140 Amps peak is exceeded, then this condition will be detected by comparator 546, the output of which will swing to ground if the output of the absolute value circuit exceeds the threshold voltage of 1.5 Volts applied to the non-inverting input of comparator 546.
  • schmitt-trigger inputs and a high speed MOSFET driver i.e. 550 is intended to reduce the turn-on and turn-off time of the MOSFET array, to keep it operating within reliable limits of current and power dissipation.
  • Pull-down resistor 551 ensures that the gate-source voltage is close to zero when there is no AC fault current flowing and therefore no power supply voltage, and ensures the MOSFETs remain in the high-impedance state unless a fault-current is detected.
  • Capacitors 563, 564, 565, 566, 567, 568 and 569 are power supply decoupling capacitors.
  • a fourth embodiment of a galvanic isolator which uses MOSFETs apt to withstand the 5000 Amp surge for 0.5 seconds, and wherein the overcurrent protection circuit is not necessary, because the intrinsic body diodes are able to handle this high current surge and can be used in place of the discrete diodes 304, 305, 306 and 307 used in the previous embodiments.
  • the galvanic isolator 601 comprises four diodes 604, 605, 606 and 607 and a capacitor 315, operating as previously explained in relation to Figures 3, 4 and 5.
  • the diodes 604, 605, 606 and 607 are the intrinsic body diodes of the four MOSFETs 684, 685, 686 and 687.
  • the rectified DC output of the bridge rectifier 1000 is smoothed into steady DC by reservoir capacitor 1010.
  • the smoothed DC voltage is applied to voltage regulator 1070, which maintains the DC voltage on its output terminal at 12 Volts or less.
  • Capacitors 1020 and 1010 provide decoupling and prevent self-oscillation of the voltage regulator 1070.
  • the MOSFET array will remain in the conducting state until the fault current between craft earth 303 and shore earth 302 reduces below a value, which corresponds to the voltage on the output of the voltage regulator 621 falling below the gate threshold voltage of the MOSFETs.
  • Zener diode 1050 starts to conduct when the power supply reaches about 9 Volts, this causes the voltage on capacitor 1060 to rise until after approximately a 2 seconds time delay, it will reach the logic 1 threshold voltage of the MOSFET driver 696.
  • Resistors 1030, 1040 and capacitor 1060 are configured to provide a 2 second delay to avoid triggering the control circuit under transient conditions, such as a lightning strike or high current surges from short-circuit faults. Such transient conditions are expected not to exceed approximately 1 second.
  • the output voltage from the MOSFET driver i.e. 696 forward-biases the diodes 604, 605, 606 and 607, switching the MOSFETs into the conducting mode.
  • Resistors 684, 685, 686 and 687 prevent ringing or oscillation caused by the parasitic inductance and capacitance around the MOSFETs.
  • Resistors 688, 689, 690 and 691 ensure the MOSFETs remain in the high-impedance state when the diodes 604, 605, 606 and 607 are reverse- biased, i.e. when no fault current is flowing.
  • Figures 7 and 8 are oscillographs, which compare the respective AC voltage drop of a prior art galvanic isolator with the much lower voltage drop of the improved galvanic isolator according to the invention, with 100 Amps RMS flowing there through in both cases.
  • the measured power dissipation is reduced from 110 Watts RMS to 6.7 Watts RMS.
  • the fault current handling capacity may be increased in a number of ways.
  • the current handling of the MOSFET array may be increased by using more devices in parallel.
  • the response time of the overcurrent protection circuit may be reduced.
  • the maximum rate of change of current may be reduced with a series connected inductor, whereby the overcurrent protection circuit is given more time to act. Improvements in the reliability and fault tolerance capacity of a galvanic isolator under the worst foreseeable operating conditions are therefore disclosed herein. Such improvements are achieved through a significant reduction in overall power dissipation, such as by a factor of ten or more, relative to power dissipation characteristics of prior art designs.
  • the control circuit monitors the current flowing through the galvanic isolator and, at a predetermined level, automatically switches the bypass element into a very low resistance state, thereby reducing the potential difference across the galvanic isolator and proportionally reducing the power dissipation.
  • medium level fault conditions such as the first phase of a worst-case fault as described above, the rise in temperature caused by the power dissipation within the safety critical components is reduced to substantially zero, whereby the surge handling capability of a galvanic isolator according to the invention is significantly increased, with a corresponding improvement in safety and reliability.
  • control circuit may automatically switch the bypass element into a high resistance state, particularly in the case of a high current surge which might otherwise damage the bypass element.
  • Additional benefits of low power dissipation and reduced temperature rise include improving the reliability of other components and equipment installed within or near the galvanic isolator, reducing the heat danger to nearby personnel and animals, and reducing the need for conventional cooling techniques based on the use of heatsinks, finned or otherwise, which are augmented by natural convection or forced air, thereby reducing the space and volume which the galvanic isolator occupies, as well as its cost and complexity of assembly.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Chemical & Material Sciences (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Emergency Protection Circuit Devices (AREA)

Abstract

La présente invention a trait à un isolateur galvanique qui comprend une pluralité d'éléments semi-conducteurs, un moyen de connexion à la masse permettant de connecter la pluralité d'éléments semi-conducteurs à au moins deux sources de potentiel électrique dotées d'une circulation de courant entre celle—ci et un moyen de commande conçu pour basculer la pluralité d'éléments semi-conducteurs dans un état à basse impédance lorsqu'une tension de seuil de l'isolateur galvanique est dépassée. L'isolateur galvanique comprend en outre un élément de dérivation électronique conçu pour dériver la pluralité d'éléments semi-conducteurs, et un circuit de commande électronique conçu pour surveiller le courant circulant dans le moyen de connexion à la masse et pour basculer l'élément de dérivation électronique d'un état à haute résistance approprié pour une circulation de courant inférieure à un premier niveau préréglé, à un état à très faible résistance lorsque la circulation de courant passe au-dessus du premier niveau préréglé. La présente invention a également trait à un procédé permettant de réduire la dissipation d'énergie d'un isolateur galvanique.
PCT/GB2009/000619 2009-03-05 2009-03-05 Améliorations apportées aux isolateurs galvaniques Ceased WO2010100391A1 (fr)

Priority Applications (2)

Application Number Priority Date Filing Date Title
GB1115342A GB2480205A (en) 2009-03-05 2009-03-05 Improvements in galvanic isolators
PCT/GB2009/000619 WO2010100391A1 (fr) 2009-03-05 2009-03-05 Améliorations apportées aux isolateurs galvaniques

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
PCT/GB2009/000619 WO2010100391A1 (fr) 2009-03-05 2009-03-05 Améliorations apportées aux isolateurs galvaniques

Publications (1)

Publication Number Publication Date
WO2010100391A1 true WO2010100391A1 (fr) 2010-09-10

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PCT/GB2009/000619 Ceased WO2010100391A1 (fr) 2009-03-05 2009-03-05 Améliorations apportées aux isolateurs galvaniques

Country Status (2)

Country Link
GB (1) GB2480205A (fr)
WO (1) WO2010100391A1 (fr)

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114720902A (zh) * 2022-04-07 2022-07-08 国网黑龙江省电力有限公司佳木斯供电公司 变电站用直流电源故障快速隔离系统
EP4459818A3 (fr) * 2023-05-03 2024-11-27 Volvo Penta Corporation Circuit de protection marin

Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5751530A (en) * 1995-08-18 1998-05-12 Dairyland Electrical Industries, Inc. High power DC blocking device for AC and fault current grounding
US5856904A (en) * 1996-11-15 1999-01-05 Dairyland Electrical Industries, Inc. Voltage and current based control and triggering for isolator surge protector

Family Cites Families (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5756904A (en) * 1996-08-30 1998-05-26 Tekscan, Inc. Pressure responsive sensor having controlled scanning speed

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5751530A (en) * 1995-08-18 1998-05-12 Dairyland Electrical Industries, Inc. High power DC blocking device for AC and fault current grounding
US5856904A (en) * 1996-11-15 1999-01-05 Dairyland Electrical Industries, Inc. Voltage and current based control and triggering for isolator surge protector

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN114720902A (zh) * 2022-04-07 2022-07-08 国网黑龙江省电力有限公司佳木斯供电公司 变电站用直流电源故障快速隔离系统
EP4459818A3 (fr) * 2023-05-03 2024-11-27 Volvo Penta Corporation Circuit de protection marin

Also Published As

Publication number Publication date
GB2480205A (en) 2011-11-09
GB201115342D0 (en) 2011-10-19

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