Red Phase Instruments Australia Earth Systems Paper

Transcription

1 Red Phase Instruments Australia Earth Systems Paper This document is intended as a guide to Red Phase Instruments IEarth Systems testing products and as a supplement to a visual or power point presentation. The information garnered in this document is assumed to be correct at the time of printing and the author bares no responsibility for obvious omissions or mistakes. Much of the technical documentation, labeling, drawings and or explanations are produced and interpreted by the author however reference is also made to associated papers and material from which some of this information has been sourced and it is with the sincerest of apologies to those whose papers and publications are not fully covered in the reference list but on whom some of this paper may be based.. Patrick Stankovic October 2015

2 WHY PERFORM EARTH TESTING? The integrity of earthed systems which support large electrical infrastructures such as substations, transmission towers and large industrial installations have to be periodically monitored to check if they comply with safety levels established in standards and regulations. This testing is critical. It establishes the ability of an earthed system to divert as much unwanted electrical energy, (brought about by an electrical fault), into the earth and away from equipment and personnel inside and outside of an earthed complex. The most common of these faults is a current fault which can be imposed on an earthed system by a broken high voltage cable, equipment insulation failure, a lightning strike or any other event which allows the flow of high currents away from their intended path and into the earthed system. To establish if an earthed system complies with safety levels, operators rely on equipment which can perform a simulated high power, albeit lower level, of current fault simulation. Appreciable levels of current fault simulation can only be achieved with powerful current injection systems such as the Red Phase Instruments, 2kVA and 8kVA current injection systems. OFF FREQUENCY TESTING Traditional current injection methods required high current to be used at base frequencies of 50Hz or 60 Hz in order to overcome system noise. Red Phase Earth Test equipment generates selectable off frequency injection current that is very near load frequency. This off frequency signal is precisely filtered down to 0.1Hz using a tuneable or frequency selective multimeter which can recover the signal cleanly, noise and interference free. TEST CAPABILITIES To examine the flow of currents and resultant potentials developed though-out an Earthed system during a fault, operators use Red Phase Earth test equipment to measure: EPR, Earth Potential Rise Step Potential Touch Potential Surface Potential distribution Current Branching Soil Resistivity Bonded Earth testing KEY FEATURES Smaller, lighter, safer and faster than conventional high power methods. Tough and durable. Continuous power at 2kVA or 8kVA at up to 40⁰C ambient. Live testing:- tests can be made while an earthed system such as a substation is in service. Immune to system noise High accuracy in voltage, current and impedance measurements. STANDARDS COMPLIANCE Using Red Phase current injection equipment is the truest way to test if earthed systems comply with international standards such as: IEEE IEEE DIN VDE 0101 Cenelec HD637S1

3 EARTH TESTING SET UP The most effective method of measuring the surface potential variations or EPR across an earthed electrical system is to perform a current injection test or CIT. A current injection test comprises two earthed systems, a connecting circuit between them and a current source. The generated current source does not contribute to the EPR, however the return path current generated as a consequence of the current injection will. When performing a CIT the most practical method used to measure EPR is the Fall of Potential Method or FoPM described in the IEEE 81 standard and other sources. The FoPM essentially requires the measure and plot of the V/I = R ratio as a function of the potential probe distance. When the plot begins to level out this is taken to be the grid EPR or impedance. WHICH FALL of POTENTIAL METHOD to use? When selecting which Fall of Potential Method to use the most important factor which must be taken into account is the inductive influence of the injection line on the potential line. Small high resistance earth systems generally don t incur substantive inductive coupling issues and are quite easy to test with small inexpensive, and at times battery operated equipment. The restrictions on the injection and potential cable positions is not so critical. However, when it comes to medium to large low impedance earthing systems, the cable layout, current injection points and choice of FoPM become important factors in correct EPR determination. Hence cable layout planning is very important. An example of inductive influence on layout is shown in Figures 3, 4 and 5 FoPM is a 4 point test method but subsets such as the 3 point test, MER correction method or the Slope method are also effective in some applications. Basic CIT Set Up Figure 1 EPR Plot Figure 2

4 CIT FoPM LAYOUT - Small earth / grounding systems Figure 3 CIT FoPM LAYOUT - Medium to large earthed systems Figure 4 CIT FoPM LAYOUT - Medium to large earthed systems Figure 5 CIT FoPM LAYOUT - Ideal Steup Figure 6

5 STEP & TOUCH Measurements With an established FoPM set up the operator can take a number of step and touch test points in and around an earthed complex such as a substation with a frequency selective multi-meter tuned to the injection frequency. Test points can must be taken at points such as a substation's equipment, its barriers and fence line perimeter. Step Measurement setup SURFACE POTENTIAL DISTRIBUTION Surface potential distribution is a key factor that can reveal the true state of a grounding system. During a FoPM setup operators can measure the generated surface voltages at equidistant points across an earthed system. A plot representing the measured points can easily uncover equi-potential zones and high risk areas. This kind of measurement begins with an operator laying out a grid onto a map of the earthed system where the earthed point of injection is taken as the centre. One end of the frequency selective multimeter is connected to the injection point while the other end is connected to a probe/stake which is inserted into equi-distant points in the grid and consecutive measurements are taken moving away from the centre or point of injection. Main Earth Grid Remote Earth Yellow dots are the potential measurement points Touch measurement setup The surface potential measurements can then be transferred to a voltage against distance graph to help the operator identify which areas of the earthed grid may indicate localized problems. V Poor grounding Localized grounding issue Even grounding distribution Distance Better modelling software can reproduce a 3D potential contour map of the earthed surface uncovering regions of uncertainty in grid integrity which may pose a danger to personnel and equipment.

6 CURRENT BRANCH MEASUREMENTS During a FoPM setup the operator may also perform current branch measurements on current bearing structures to determine the amount of current coming into or leaving a grid or earthed system under test. ELEMENTS OF A RED PHASE INJECTION SYSTEM Solid State Injection unit(s) Frequency selectable current injection instrument with a user interface for selecting injection frequency and current level. Coupling Transformer A multi-tap transformer which isolates the current injection unit from direct ground connection and also provides a corresponding injection selection tap. Frequency Selective Multimeter A multimeter specifically tuned to the current injection frequency selected by the solid state current injection unit.