AU2012202699B2

AU2012202699B2 - Monitoring Long Electric Fences

Info

Publication number: AU2012202699B2
Application number: AU2012202699A
Authority: AU
Inventors: Paul Thompson
Original assignee: Pakton Dev Pty Ltd
Current assignee: PAKTON DEVELOPMENTS Pty Ltd
Priority date: 2011-05-12
Filing date: 2012-05-11
Publication date: 2016-07-28
Anticipated expiration: 2032-05-11
Also published as: AU2012202699A1; ZA201203424B

Abstract

Abstract: In a first aspect of the invention there is provided a method of monitoring a long electric fence from an electrical start of the fence which includes the steps of applying a voltage pulse to the fence, obtaining a measure of the 5 applied pulse voltage, obtaining a measure of current flowing to the fence, processing the voltage measurement and the current measurement to obtain a measurement of real power flow and of reactive power flow to the fence and comparing the real power flow and the reactive power flow measurements to reference values thereby to detect a change in a load on the fence 10 Class: H05C; HO3K Fig: I C-4A

Description

FIELD OF THE INVENTION

[0001] The present invention relates to electric fences and, in particular, to circuits and algorithms for monitoring electric fences.

BACKGROUND OF THE INVENTION

[0002] Electric fences are widely used on farms to restrict the movement of both farm and feral animals. They are also used for security purposes to restrict entry to and to provide distributed perimeter protection for industrial premises.

[0003] Such fences normally include a plurality of electrically insulated posts between which one or more un-insulated wire conductors are strung. The conductors are coupled to an energiser that periodically outputs a high voltage pulse to energise the conductors so that an animal will receive a small electric shock If it comes into contact with the energised conductors.

[0004] Energisers that are used to energise electric fences in farming applications are usually battery or mains powered. Most modern energisers include a discharge capacitor, a capacitor charging circuit for charging the capacitor to a high potential (e.g. several hundred volts), and a capacitor discharging circuit for discharging the capacitor via a step up transformer to produce a very high potential output pulse (e.g. several thousand volts) that is used to energise the fence conductors.

[0005] Energisers come in many sizes from very small to large high powered units. Energisers may be rated in stored joules, output joules or by the number of kilometres of fence wire the manufacturer claims that they can effectively energise. The largest energisers are claimed by their manufacturers to be able to power up to 600km of fence wire.

[0006] For rural fences and non-lethal security electric fences the magnitude and frequency of the electric pulse are restricted by safety standards such as IEC60335.2.76. The limitation is specifically intended to ensure that the shock received from the energiser (and hence the fence) is safe for humans.

[0007] In order to comply with the standard a more powerful commercially available energiser often contains an inbuilt monitoring, also known as feedback, circuits which allows the energiser to increase the output power in order to provide a more consistent high voltage output when the fence has a heavy fence load on it, such as from wet green grass. In the absence of a heavy load a large energiser limits its power output to approximately 5 Joules, enough to power approximately 50km of fence wire.

[0008] When an electric fence is used for perimeter security, the fence is monitored so as to provide an alarm if the fence is breached by intruders. The monitor is either built into the electric fence energiser or provided as a separate appliance.

[0009] Electric fence monitors can be split into two broad categories. Those that monitor the fence from the electrical start, also called the “feed”, and those that monitor the electrical end which is also known as the “return”. Fence return voltage monitoring is the most widely used form for security applications. It is only necessary to monitor the returned pulse for the reduction in peak voltage which will occur with either a short or open circuit at any point between the feed and return. It is however, necessary to wire the fence so that the return is accessible. This is usually achieved by carrying the current from the energiser out on one conductor, looping it at the end of the fence and carrying it back on another conductor to a return terminal on the monitor or security energiser. This means the fence must have at least two live wires.

[0010] Start of fence monitoring requires the monitor to measure the voltage, or current, flowing to the fence. The monitor is inserted between the energiser output and the fence. The main advantage in start of fence monitoring is that there is no requirement to loop the conductors at the end of the fence. As such it can be retro-fitted to any electric fence, even one with only one live wire. Also it can be built into an energiser without the need for an additional return terminal.

SUMMARY OF THE INVENTION

[0011] When a long electric fence is in a good condition, uninterrupted by a short or a heavy load, the fence acts as an un-terminated transmission line.

[0012] When a typical electric fence energiser pulse is applied to a long electric fence the resulting waveform of the current from the energiser to the fence, measured at the start of the fence, will contain reactive components. If the voltage waveform is a positive polarity pulse, the current pulse will start positive (forward current) and then reverse to negative as the pulse voltage of the pulse falls, indicating reverse or reactive current.

[0013] if a heavy load is placed on the fence, even at a distant point, this negative, reactive current peak will diminish. The reflected current is also known as reactive current as a transmission line is a reactive load.

[0014] Simply looking for the peak of each polarity of the current flowing to the fence may not be sufficient to determine if there is a change of load on the fence since many electric fence energiser output pulses are not pure DC pulses. Some energisers even drive an output, connected to the fence, through one complete sinusoidal cycle.

[0015] Since the reactive current occurs out of phase with the applied voltage it is possible to detect a fault by measuring the real power (Watts) being transferred from the energiser to the fence, and by ignoring the reactive power (VARs). This means that a monitoring circuit needs to be able to sample and store the relatively fast waveforms of voltage and current with respect to time.

[0016] Another method of determining that the load has changed on a long fence is to add a high voltage will be between the energiser fence output and the feed to the fence. Since the rectifier stops any reactive power returning to the energiser, the voltage waveform will be significantly affected by a load at an end of the fence. The fence will be charged like a capacitor and will discharge with a discharge curve time constant which is proportional to RC. R is the normal leakage resistance of the fence in parallel with any added load. C is the capacitance of the fence wires with respect to earth, and is proportional to the length of wire which is being energised. A high voltage relay can now be used to periodically add a resistor, of fixed and known value, between the fence and ground. The change in the RC time constant can then be used to calculate the actual fence capacitance by solving the simultaneous equations: T1 - R1 xC1 (equation 1); T2 = (R1 x R2)/(R1 +R2) x C1 (equation 2); where T1 and T2 are the voltage decay time constants, R1 is the fence leakage resistance, C1 is the fence capacitance, and R2 is the added resistance.

[0017] These equations can be solved to find R1 and C1 which, if changed from one calculation to the next, represent a change in the load on connected electric fence. The equations are a useful tool in determining if a security fence has been tampered with. For example, if the fence is cut, C1 will reduce. If the fence is shorted or contact (typically to earth) is made with the fence, R1 will reduce.

[0018] “Long" in relation to an electric fence, as used herein, indicates a fence which displays transmission line characteristics.

[0019] In a first aspect of the invention there is provided a method of monitoring a long electric fence from an electrical start of the fence which includes the steps of applying a voltage pulse to the fence, obtaining a measure of the applied pulse voltage, obtaining a measure of current flowing to the fence, processing the voltage measurement and the current measurement to obtain a measurement of real power flow and of reactive power flow to the fence and comparing the real power flow and the reactive power flow measurements to reference values thereby to detect a change in a load on the fence.

[0020] The invention extends to an apparatus for monitoring a long electric fence from an electrical start which includes a circuit for measuring a voltage of a pulse applied to the fence and for deriving a first signal which is representative of the voltage measurement, a means for measuring a current flowing to the fence and for deriving a second signal which is representative of the current measurement and a processor which processes the first signal and the second signal to obtain a measure of real power flow and a measure of reactive power flow, and which compares these measures to reference values thereby to detect a change in a load on the fence.

[0021] The current measuring means may include a current transformer or a current shunt, connected in series to a conditioning circuit.

[0022] The apparatus may include a display which displays one or more of the following: the measure of real power flow, the measure of reactive power flow, the reference values and the detected change in load on the fence.

[0023] The apparatus may include an alarm which is actuated when a change in the load on the fence, in excess of a predetermined value, is detected. The alarm may be an audible or a visual alarm.

[0024] In a second aspect of the invention, there is provided a method of monitoring a long electric fence from an electrical start of the fence which includes the steps of applying a voltage pulse to the fence, measuring peak voltage of the pulse, measuring a current flow of the pulse, and obtaining a first signal which is dependent on the positive peak current flow and a second signal which is dependent on the negative peak current flow, determining which of the signals corresponds to current flow that is in phase with the voltage pulse, and comparing the peak voltage and either the first signal or the second signal to respective references to derive a measure of a change in the load of the fence.

[0025] The method may include the additional steps of generating a first interrupt signal based on the starting time of the voltage pulse and generating a second interrupt signal based on the starting time of the current flow to which, the starting time of the voltage pulse and the current flow respectively thereby to determine which of the signals corresponds to current flow that is in phase with the voltage pulse.

[0026] The second aspect of the invention extends to an apparatus for monitoring a long electric fence from an electrical start which includes a circuit for measuring the voltage of a voltage pulse applied to the fence and for deriving a first signal which is representative of the voltage measurement, a first comparator which compares the first signal to a first reference and which generates a first interrupt signal which is dependent on the time at which the voltage pulse starts, a means for measuring the current of the current pulse flowing to the fence and for deriving a second signal and a third signal which are respectively proportional to the positive peak current flow and the negative peak current flow of the current pulse to the fence, a second comparator and a third comparator which respectively compare the second signal and the third signal to respective references and which generate a second interrupt signal which is dependent on the time at which the current pulse starts, and a processor which, when interrupted by the first interrupt signal, determines which of the second and third signals corresponds to the current pulse that is in phase to the voltage pulse and which compares the first signal and either the second signal or third signal to respective reference values thereby to determine a change in a load on the fence.

[0027] The invention provides, in a third aspect, an apparatus for monitoring a long electric fence from an electrical start which includes a high voltage rectifier in series between an output of an energiser and a feed to the fence, a circuit for measuring the peak voltage of a voltage pulse applied to the fence by the energiser and for deriving a signal which is representative of the voltage measurement, a comparator which compares the signal to a first reference and which generates an interrupt signal which is dependent on the time at which the voltage pulse starts, a processor which, on receipt of the interrupt signal, determines the time taken for the voltage on the fence to decay to one third of the peak voltage, and which compares this determined time to a reference value thereby to detect a change in a load on the fence.

[0028] According to a fourth aspect of the present invention there is provided a method for a monitoring feedback circuit in an electric fence energiser as per any of the first three aspects using the determination of a change in fence load to modify the amount of energy applied to the output pulse so as to account for the added load and maintain an adequate voltage at the extremities of the fence.

[0029] According to a fifth aspect of the present invention there is provided a method for a monitoring feedback circuit as per any of the first three aspects in a hand held electric fence fault finder. The result of the determination of a load at the end of a long fence can be displayed to the user to help indicate the presence of a fault at a distance from the user.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate typical preferred embodiments of the invention and wherein:

Figure 1 is a block schematic diagram of an apparatus according to the present invention.

Figure 2 is a graphical representation of the voltage and current pulses as they appear at the start of a long fence with and without a short at the end. DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] With reference to Figure 1, a monitor 10, in accordance with the invention, includes an energiser input terminal 12 which is connected to the electric fence energiser (not shown). A fence terminal 14 connects the monitor to the conductors of a fence (not shown) which is to be monitored. An earth terminal 16 connects the monitor to earth.

[0032] With input of a pulse from the energiser to the fence, via the monitor 10, a primary current of the pulse input into primary winding 18 of a transformer 20, connected across terminals 12 and 14, is reduced to a first representative pulse with a secondary current emanating from a secondary winding 22 of the transformer. The first representative pulse is representative, in current waveform, of the primary pulse that energises the fence.

[0033] An amplifier and signal conditioning circuit 24A is connected in series to the secondary winding 22 of the transformer 20 to which the first representative pulse reports. In analogue to digital conversion of the representative pulse, which takes place in circuit 24, current signals, representative of the current waveform of the primary pulse, are generated and input into a micro-controller 26.

[0034] A comparator 27 provides current polarity information of the representative pulse and generates an interrupt signal to the micro-controller which is transmitted to the micro-controller 26, via a conditioning circuit 24B, to enable the micro-controller to determine, when running a suitable algorithm, the start of the energiser pulse and the polarity of the forward current.

[0035] The monitor is also provided with a voltage measurement circuit 28 which provides to the conditioning circuit 24B, a second representative pulse, and from the conditioning circuit, a voltage signal. The second representative pulse and the voltage signal are representative, in voltage waveform, of the primary pulse that energises the fence.

[0036] The voltage measurement circuit 28 includes a resistor 30 and an opto-isolator 32. The light emitted by an emitter 34 of the opto-isolator is proportional to the voltage and, on the opposed side of the opto-isolator, a sensor 36, on detection of the light, generates the second representative pulse.

[0037] The micro-controller receives in the current signals, the voltage signal and, from the comparator, current polarity and the interrupt signal. The microcontroller running the algorithm or program with these inputs, determines against stored reference values which can be previously saved values or preset values, if the fence load has changed. In this way the monitor can determine if the fence has a short or a heavy load, even at a great distance from the monitor.

[0038] The results of the comparison and determination are shown on a display screen 38 and, if it is determined that there is a change fence load, and this change exceeds a predetermined limit, a fault is detected and an audible alarm in the form of a siren 40, is sounded, alternatively or additionally, a visual alarm in the form of a strobe light 42 is actuated.

[0039] Referring to Figure 2, a voltage and current waveform of an energiser pulse, as captured by an oscilloscope using a voltage divider and a current shunt, with respect to time, is illustrated. Figure 2A shows these waveforms for a 10km electric fence with no short circuit. The voltage waveform A is a single DC pulse of positive polarity and 8kV in peak amplitude. The current waveform B has a 20A positive peak and a 6A negative peak, designated X.

[0040] Figure 2B shows the waveforms from the same 10km electric fence with a short circuit at a far end. The peak forward current is unchanged but the reactive negative peak X is almost completely gone.

Claims

1. A method of monitoring a long electric fence from an electrical start of the fence which includes the steps of applying a voltage pulse to the fence, obtaining a measure of the applied pulse voltage, obtaining a measure of current flowing to the fence, processing the voltage measurement and the current measurement to obtain a measurement of real power flow and of reactive power flow to the fence and comparing the real power flow and the reactive power flow measurements to reference values thereby to detect a change in a load on the fence.

2. An apparatus for monitoring a long electric fence from an electrical start which includes a circuit for measuring a voltage of a pulse applied to the fence and for deriving a first signal which is representative of the voltage measurement, a means for measuring a current flowing to the fence and for deriving a second signal which is representative of the current measurement and a processor which processes the first signal and the second signal to obtain a measure of real power flow and a measure of reactive power flow, and which compares these measures to reference values thereby to detect a change in a load on the fence.

3. An apparatus according to claim 2 wherein the current measuring means includes a current transformer or a current shunt, connected in series to a conditioning circuit.

4. An apparatus according to claim 2 or 3 which includes a display which displays one or more of the following: the measure of real power flow, the measure of reactive power flow, the reference values and the detected change in load on the fence.

5. An apparatus according to any one of claims 2 to 4 which includes an alarm which is actuated when a change in the load on the fence, in excess of a predetermined value, is detected.

6. An apparatus according to claim 5 wherein the alarm is an audible or a visual alarm.