STATE OF THE ART –ELECTRIC FENCE ENERGIZER
Abstract - This paper introduces fundamentalconcepts of electric fence technology.The available information in this field is practically restricted on industry builders know how and their field experience.Energizer equipment is rounded about many concepts and safety standards and data performance that are discussed in this work. An electric circuit analysis, simulation and prototype of an Electric Fence Energizer Equipment for livestock use are detailed in this paper.
I. Introduction
Nowadays the use of electric fence for control and content livestock are having a large application in almost all countries of the world. Electric Fence was starting to use in the thirties and nowadays is used in all world in little and big farms. Brazil like the major exporter of beef cattle is a great consumer of this technology. Big farms with large areas of control need electric fences energizers of large capacity to keep high voltage in all extension. But not much information about safety of use and project is printed and available for consumers and manufacturers as well electric characteristics. There are in Brazil many manufacturers of this kind of equipment, but these builders are used toutilize empiric rules to design this kind of equipments. This work intends to be a starting point to change this reality involving the academic researchersin the study of this problem. The different parts of the fence are showed infigure 1. The electric fence is arranged by the following parts: Energizer, Wire, Isolation and Ground.
Figure 1: Parts of the Electric Fence
II. Operation:
The current flow on the fence is showed in figure2. When the cattle touch the wire the circuit is closed and the electric impulse current generated by the Energizer flows through the body. In practical experiences is evidenced that the cattle doesn’t transpose the fence for a peak voltage higher than 2 kV.For this voltage the livestock experiments a panic sensation and don’t turn to touch the wire.
Figure 2: Current flow (path) in a fence circuit at the cattle touch moment.
The simplified electric circuit for the fence circuit is showed on figure 3. The resistance of the body of the cattle is assumed in 175 Ω for impulse current [3]. This data is important to preview the voltage applied in the cattle that will depend of the wire, the ground impedance, the distance from the energizer and the conductive characteristics. For human beings the resistance for impulse current is 500 Ω. This data is important to the safety energy limits described in the standard IEC 60335-2-76 [1]. The two bodies resistance 175 Ω and 500 Ω for cattle and human beings respectively are obviously different. As the hide and the skin have a capacitive characteristic, second reference [3], just the internal body resistance for a path of the hand to the foot is considered. In the cattle the path is of the nose to the four legs.
Figure 3: Simplified equivalent electric circuit for current path in the fence.
III. Safety Aspect:
All safety information is important to develop an Electric Fence Energizer circuit. Is very relevant a correct understanding of electric characteristics of this circuit and the produced reaction of the electric shock derived from it. In the table 1 is listed the mainly safety aspects provided by standard IEC 60335-2-76 [1] and by the technical report IEC 60479-2 (chapter 6) [2] need to be consulted for safety limits for a capacitor discharge wave form. There are other two main standards for safety requirement for energizers: UL-69 (USA) [4] and DIN VDE 0131 (Germany) [5].
Table 1: Mainly electric shock safety requirements of IEC 60335-2-76.
Impulse repetition period equal or higher than 1 second.For a 500 Ω load the impulse duration do not must be higher than 100 ms.
The energy of the impulse discharged do not must be higher than 5 J in a 500 Ω load.
For a impulse energy discharged on a 500 Ω load higher than 5 J, the current values needs to be under the limit show in line C2page 43in IEC 60479-2 [2]
IV. Energizer:
The Electric Fence Energizer convert the electricalenergywhich normally comes from the electrical utility, batteries or solar PVs in an electric impulsewith limited energyassociated according to safety limits. The electric circuit is divided in two parts as shown in figure 4:SupplyCircuit and Impulse Generator Circuit.
Figure 4: Electric Fence Energizer Block Diagram.
V. Supply Circuit:
Two supply circuits are proposed and illustrated in figure 5 to exemplify.One is a conventional power supply, 127/220 Vac 50/60 Hz, grid connected (a) and the other one uses a 12 V battery associated with a flyback converterto boost the input voltage (b). Both converters circuits are configured to raisethe output voltage around 400 to 800 Vdc. This DC link charges the storage capacitor C1.
Figure 5: (a) Means supply – voltage duplicator, (b) Battery supply – Fly Back CC-CC Converter.
VI. Impulse Generator Circuit:
The Impulse Generator Circuit consists in a Discharging Impulse Magnetizer circuit (figure 6). This impulse generation circuit is present in many energizers.
Figure 6: Impulse Generator Circuit supplied with a Vdc source.
The transformer T has two main functions to provide electrical isolation and to boost the input voltage this element normally presents a turn ratio around1:10. The secondary of the transformer is connected to the wire of fence and the ground electrodes. The capacitor C1is the energy storage element, to charge this capacitor the circuit has about one second. The switchis usually implemented by a thyristor S that provide the discharge of the capacitor. The resistor R1 limits the current of the supply in the charging of C1 and in the discharging of C1.The operation circuit is divided in two stages: Charge Capacitor Stage (stage 1) and Discharge Capacitor Stage (stage 2).
Stage 1(Figure 7): In a period of 1 second at least, C1 is charged. I1 flows from the supply circuit through C1 and R1, the maximum value of this current is limited by R1.This resistor could be used to adjust the peak voltage on C1. The C1 voltage curve is illustratedon figure 8.
Figure 7: First stage – charge of C1.
Figure 8: Charge voltage curve of C1 – First Stage.
The energy stored in an energizer is expressed by the equation 1:
(1)
Where:
: Stored Energizer Energy (Joule)
: Maximum C1 Voltage (V)
: Capacitance of C1 (Faraday)
Stage 2(Figure 9):In this stage S is closed and C1 is discharged. In the primary of T the voltage of C1 is applied and goes to zero in 100 μS approximated. The voltage is amplified and applied in Rfence. The voltage curve of C1 and the current in the primary of Tare shown in figure 10.
Figure 9: Second stage – discharge of C1 and generation of the impulse magnetizer current.
Figure 10: Discharge of C1, “vC1” is the voltage on the primary of transformer and “is” is the impulse current in the primary of the transformer.
For this simulation was used an ideal transformer with the follow turn ratio (1: 12.7), C1is a 9 μF capacitor, the ESR of this capacitor was adoptedequal to 1 Ω, the current limiter R1was implementedwith a 22 kΩ resistor. Rfence is added as a test load of 500 Ω. This value is the initial electric resistance of a human being. Figure 11 shows the voltage impulse curve in the output of the energizer applied to Rfence. So it is possible to evaluate safety aspect in a human being with measurement of the energy and duration of the discharge. Others test loads (Rfence) are used to simulate field situations like faults on the fence (like loss generated by grass touching the wire that represents a resistance in parallel with the circuit load of figure 3). Rfence = 500 Ω (Low Energy Loss). Rfence = 100 Ω (Considered Energy Loss). Rfence = 50 Ω (High Energy Loss).
The Power Sim 6.0 software was used to simulate the circuit.
Figure 11: Impulse voltage in the Rfence load.
VII. Experimental Results:
A design was realized and a prototype was build in order to obtain a commercial equipment with the following characteristics stored energy 0.7 J and input voltage 220 VRMS for a 5 km extension fence. In figure 12 a picture of an oscilloscope screen is shown with the measured impulse voltage wave form on the output of an Electric Fence Energizer circuit. The capacitor C1 was charged with 400 V the same maximum voltage value. The load (Rfence) for this measurement was 500 Ω.
Figure 12: Voltage wave form in Rfence = 500 Ω prototype circuit.
VIII. Conclusion:
The information presented in this paper remark important safety requirement for energizer circuit design. A state of art was also presented and the main standards were referred and explained. The simulationsshows clearly that this kind of circuit are appropriate to be used as Electric Fence Energizer becauseattendthe standard safety requirements. The impulse wave form generated in the simulation is the same wave form presented in reference [2]. The experimental results are very good and verify the proposed design method that will be presented in the final paper. A commercial version of this prototype is also available.
References:
[1]IEC 60335-2-76 – Safety of household and similar electrical appliances - Part 2-76: Particular requirements for electric fence energizers.
[2]IEC 60479-2 – Effects of current passing through the human body Part 2: Special Aspects
[3]IEC 60479-3 – Effects of current on human beings and livestock – Part 3: Effects of currents passing through the body of livestock
[4]UL69 - Electric-Fence Controllers - Underwriters Laboratories Inc
[5]DIN VDE 0131, Publication date:1984-04 - Construction and operation of electric fence equipment [VDE Specification].