RAPTOR Step and Touch Function

Transcription

1 RAPTOR Step and Touch Function Why to perform Step and Touch Voltage measurements?

2 RAPTOR - Step and Touch function Why to perform Step and Touch Voltage measurements? The latest Raptor software version includes the new Step and Touch function, addressed for ground circuit measurements in Substations, as other functionality provided with the new High Voltage unit, the Raptor HV. In this paper, it is briefly explained what is this function, and how it is performed, and then discuss why it should be done or be included as another routine test in the maintenance and commissioning testing routines of earthing systems. The safety measurements evaluation of grounding systems typically performed are ground impedance measurements, continuity/integrity testing, and soil resistivity measurements. But since it is not always possible to foresee all hazard mechanisms of the over time evolving grounding protection installations, which are extended by the additional requirements and co-existence with other telecommunication tubes and pipelines interferences in the grounds, it becomes essential to measure the outputs of the earthing system in terms of produced voltages and current distributions in case of a fault rather than solely resistance. The Step voltage is the voltage that can be measured between any two ground spots with an adult s step distance from each other in a working electrical environment. This is the voltage that could be encountered by a person with a stepping distance of 1 m, when a fault current occurs in a nearby grounded structure, but without contact of the same, and should be tested and kept under security levels. The Touch voltage is the voltage in which an individual might be exposed to when coming into contact with metallic structures, machine armatures and fences, which normally are not subject to significant voltage, but that could have voltage when a fault current occurs in the electrical substation. April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 1

3 For measuring the step & touch voltage characteristics of the protective earthing installations in substations and other electrical facilities, the Raptor HV is required by using its voltage output in the1000 V range and the measurement capabilities of the Raptor Master unit. An automatic function is included in the Raptor HH control unit, and it requires the use of the optional Step & Touch accessories kit. Typical connections diagrams for step and for touch voltage measurements: Step voltage: The measurement is performed between two ground points at a distance of 1 m as shown on Fig. The 25 kg measuring probes/electrodes simulates the feet. The voltage between the probes is measured by a voltmeter with an internal resistance of 1 kω that simulates the body resistance: Touch voltage: The measurement is performed between an earthed accessible metal part and ground as shown on Fig. The voltage between the probes is measured by a voltmeter with an internal resistance of 1kΩ that simulates the body resistance. Measured step and touch voltages using injected current can be scaled relevant to actual fault levels to determine compliance with relevant standards. As the test current is usually only a small fraction of the highest fault current the measured voltages must be up scaled according to following equation: Vs,t = Vmeasured x (Ifault/I) Vs,t = calculated step or touch voltage in case of fault current Vmeasured= measured voltage during test Ifault = maximal earth current in case of a fault I = injected test current April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 2

4 Implementation in Control Unit HH: This function use the timer as countdown timer (default 20 sec), and have three phases which take place automatically without user intervention (each phase consume a third of the timer): Phase 1. The Raptor-HV inject 0VAC and obtain step & touch voltage generated by currents outside the Raptor, "Vo". Phase 2. The Raptor-HV inject voltage configured in setting, in phase with the line, and get the step & touch voltage generated by the same and those currents outside the Raptor "V +". Phase 3: The Raptor-HV inject voltage configured in setting, in opposite phase to the line, and get the step & touch voltage generated by the same and those currents outside the Raptor "V - ". Vo, V + and V- will go showing in the screen. Finally, if successfully concluded the 3 phases when the timer reach to 0, it is calculated by trigonometry and Vst & tch is displayed at the configured Isc (Isc is shortcircuit current of the grid under test), which is selected in the settings. In this function, the voltage will be injected by the HV output and at the range of 1kV. Another important aspect of this feature is the possibility that the user has to run it at the line frequency or another selected by him, filtering into the result the undesirable frequencies Accessories required (Step & Touch kit): Input switch with 1000 ohms resistance. Cable reel- cable 20m shielded and twisted for the step & touch measurement. Cable reel- cable 50m for injection. 2 grounding electrodes. April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 3

5 Why to invest in Step and Touch test kit? The Raptor System is a multifunctional system, and for those customers who purchase the Raptor for its main applications, such as high current injection, primary testing in general, testing of CTs/VTs (ratio, polarity, burden, knee point), high voltage testing in transformers up to 2000 V, breakers, reclosers, etc they may have also as added functionality if desired just acquiring the accessories kit the chance of using the same system for checking the safe working limits around the substation and/or working area for their staff. From this point of view, since the investment is justified for the main applications, to have also this application is an added value, saving money in purchasing other different set. But at any case, there are other points to consider, mainly thinking in all the investment made in Safety: The grounding of a high-voltage electrical system helps to ensure the safety of personnel. Voltage rises in the neighboring area of electrical systems, caused by a system fault or lightning, can be extremely dangerous. The best assurance that a substation is safe would come from actual field tests of step and touch voltages with a heavy current load on the ground mat. Because of the expense, not all the utilities are likely to make these tests as a routine practice. If, however, large discrepancies between calculated and measured resistance or known anomalies in the ground resistivity throw doubts on the calculated step and touch voltages, then such tests must be considered. This is especially true when the computed values are close to tolerable limits, and further improvement of the ground to provide a larger safety factor would be difficult or costly. Therefore, it is a method that completes the conventional testing routines of soil resistivity, earth grid continuity test, ground impedance testing, etc and at the other hand, portable earth testers are often unsuited to testing extensive earthing systems, and where noise is present in those earthing systems, due to limited test current and inadequate noise rejection. Also, the different standards somehow do not agree at all on the safe limits. IEEE Std states that "total body resistance is usually taken as 1000 Ω for determining body current limits." However, employers should be aware that the impedance of a worker's body can be substantially less than that value. For instance, IEEE Std reports a minimum hand-to-hand resistance of 610 ohms and an internal body resistance of 500 ohms. April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 4

6 There are many ways to Calculate Step Potential of Switch yard and to Calculate Touch Potential of Switch yard (see for example some calculations in this link) but a more certain technique to determine Step and Touch Potential is to measure it directly, i.e to check the Prospective Step/Touch voltage with the Effective (Loaded) Step/touch voltage the voltage across a human body, under fault conditions. Since calculations may differ and also the conditions of the grounding of a high-voltage electrical system that helps to ensure the safety of personnel is changing over time, with the change of the soil resistivity, the humidity, the change of the condition of the ground mat, corrosion, etc...the the main application of this measurement is when the substation is established, in the building process, because although the calculations and design of the ground grid can be right, the commissioning of the same, connections between different groundings and poles, the existence of other installations, and the installation made by workers may results in different values to the calculated, good or bad workmanship, etc so that it is advisable that at the end of the commissioning process to really simulate a fault to ground and measure the possible dangerous voltages in the surroundings of the substation and perform this kind of measurement in some yearly periods after the first measurement in the substation or transformation center. The reason for performing this kind of testing also depends on having a more or lower strict regulation for the safety of personnel. Some countries have a stronger regulation and others not so strict, and someones have a regulation more strict that what it is indicated in the mentioned IEEE standards. April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 5

7 For example, in the Spanish regulation (MIE-RAT 13), to measure the step and touch voltage in principle would only need the following: - provoke an earth fault in the installation and measure voltages. - Measure, using a suitable voltmeter and standard electrodes, the resulting voltages. It is not feasible to cause a defect in the installation to measure it. It is therefore necessary to find alternative methods that allow us to simulate the defect. This simulation is achieved by injecting an alternating current through the ground network that will be measured and considering that the voltages obtained are proportional to the injected current. This method is generally accepted and there is only a slight inconvenience to solve: stray current (also called spurious earth currents ) Indeed, in a ground network of HV facility there are normally stray/spurious currents due to multiple causes such as leakage currents on equipment and insulators, capacitive currents, etc. These currents produce step and touch voltages at the facility, but generally very small value. At the moment a defect occurs in the installation, the voltages caused by stray currents will have absolutely no relevance to those produced by the defect. However, when we make a small injection to simulate the defect, the voltages produced by them can be very significant with respect to which we produce with our injection and even higher than that value. Then, extrapolating the values to the real fault current would produce consequent errors in the results. April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 6

8 To avoid this, our MIE-RAT 13 indicates that a current injection, at least should apply 1 % of the current for which it was designed the installation or use a method that eliminates the effect of the stray currents. Obviously, the higher the injection current, less influence of stray currents. However it is not always easy to reach 1% of the fault current of the network and that can represent several hundred amperes in many big installations. Then the method to achieve the elimination of the effects of stray currents, is achieved by two injections for each measure of the same absolute value but shifted in phase 180º and made three readings of step and touch voltages: Vo: with no current injection V +: The first injection V- : Second injection phase shifted 180 These three measures can readily determine the voltage produced by the injection, eliminating the effects of stray currents, making the geometric calculation. And this is what the RAPTOR CV-05 is making in the automatic function, and providing the final result measured in the electrodes (or test probes) that simulates the human step (the regulation establish these electrodes to have a minimum surface of 200 mm2 and a minimum weight of 25 kg each), in the case of actual fault. And this to be made in a planned grid division of the switchyard, to cover the measuring in the switchyard, fences around the substation (inside and outside the fence is also very important), etc.and it depends if the installation is new and the substation planners decide to make a more or lower detailed control of the installation, and surroundings. There is a lot of documentation available regarding the step and touch voltage, and many different national and international standards, but depending on the regulation to follow, if such regulation exists, or if the company/utility establishes their own regulation, the investment in this kind of equipment is justified to ensure the safety of the personal in the installation. *** END OF THE DOCUMENT *** April 2015 EuroSMC, S.A. RAPTOR Step and Touch Why to perform Step and Touch measurements Pag. 7

9 EUROPEAN OFFICE EUROSMC S. A POLIGONO INDUSTRIAL P-29, C/BURIL 69, 28400, COLLADO VILLALBA, MADRID, SPAIN TEL: USA OFFICE NORAMSMC INC. SOUTH MEMORIAL DRIVE - SUITE 208 TULSA OK USA TEL: sales@noramsmc.com LATIN AMERICA OFFICE EUROSMC S. A. CANTUARIAS 270, OF 604, MIRAFLORES, LIMA, PERU TEL: latinam@eurosmc.com ASIAN OFFICE EUROSMC S. A. UNIT A, 15/F, CHARMHILL CENTRE, 50 HILLWOOD RD, TSIM SHA TSUI, KOWLOON, HONG KONG SAR TEL: asia@eurosmc.com Test Equipment for Substation and Protection Relay Testing PRIMARY INJECTION TEST SETS MULTIFUNTIONAL PRIMARY TEST SYSTEM SECONDARY INJECTION TEST SETS UNIVERSAL RELAY TEST SYSTEM SINGLE/3 PHASE RELAY TESTING AUTOMATION RELAY TESTING SOFTWARE ELECTRICAL MEASUREMENT CIRCUIT BREAKER ANALYZERS PHASE ANGLE METERS DIGITAL MICRO OHMETERS IEC GOOSE SNIFFER TRANSFORMER MAINTENANCE TRANSFORMER TEST EQUIPMENT SUBSTATION TEST SETS MCB TESTING