Download Earth System – Current Injection Testing (CIT) Services

www.libertyconsulting.com.au Tel: 0434 005 393 Earth System – Current Injection Testing (CIT) Services Depending on the size of the facility being tested, Liberty Consulting Services (LCS) uses fit for purpose test equipment to accurately measure earth grid impedance, step, touch and transfer voltages, earth potential rise (EPR) contours, current splits in overhead earth wires and underground cable screens, and electrical continuity of earthed structures in high voltage substations, switch‐yards and power plants. Testing of lightning protection earthing systems is also able to be performed accurately by experienced LCS engineers and suitable test equipment. Accurate earth grid system testing Large earthing systems can be difficult to test because they are often bonded to adjacent services and other earthing systems, and have very low impedance – well under 1.0 , often as low as 0.1  or below. High power 50Hz injection has been the “traditional” test method for transmission and distribution earth grid systems. However low current off‐frequency injection has been used widely over the last 20 years, and provides reliable, accurate test results for large, extended earthing systems. In particular, current injection testing of earth systems allows identification of hazardous transferred voltages, which is a major concern for large substations in built up residential or industrial areas. Portable earth testers are designed mainly for small “stand‐alone” earth systems and are not able to accurately measure low grid resistances (< 0.5‐1.0 ). Copyright © LCS 2016 The Standards below outline the latest methods in earth system testing, and highlight the differences using professional grade current injection test equipment, versus portable off the shelf earth testers. Earth System Testing Standards  ENA EG‐0 Power System Earthing Guide (Section 7 – especially 7.2.3)  ENA EG‐1 Substation Earthing Guide (Section 5.2 – especially 5.2.2)  AS/NZS 1768 Lightning protection (Appendix C.10 – especially C10.1)  IEEE Std 80 IEEE Guide for Safety in AC Substation Grounding (Sections 13.3 & 13.4)  IEEE Std 81 IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and Earth Surface Potentials of a Grounding System (Section 7) Fall of Potential (FOP) test method The most prevalent and accepted method to measure the EPR and impedance of an earthing system is the “Fall of Potential Method”. This is done during the CIT. www.libertyconsulting.com.au This is not the same as the simplified three‐point or 61.8% (straight line) earth test recommended in portable earth tester manufacturers’ instructions. The curve below indicates what the FOP test response should look like. The FOP test involves using or setting up a remote electrode system, a minimum distance away from the grid. A test current is injected into this remote electrode location, either via a fixed metallic connection (out of service overhead phase conductor) or a temporarily deployed test cable, or. This will cause, on a smaller scale, an EPR able to be tested. In the latter method above, the remote current injection electrode (RCIE) at the end of the temporarily deployed test cable is established using a number of interconnected driven rods, often wetted down to achieve as low a resistance as possible. While test current is flowing, voltages are recorded at intervals from the grid under test, preferably at 900 to the current test cable. Determining the EPR profile requires graphical analysis of the voltage test points to plot the FOP response. What to measure Often more than one voltage test traverse is required in order to obtain an accurate 360 degree EPR contour around the main grid, especially in built up residential areas. As well as measuring grid impedance and surface voltage contours, step, touch and transfer voltages are tested using voltmeters tuned to the off‐
frequency test current. Test current splits among connected earth paths are able to be measured with Rogowski coils connected to the TVM. Test voltages are scaled up to the actual Phase‐Earth fault level, and represent actual touch voltages. In addition integrity testing of earthing connections, and electrical continuity testing are also conducted. Using CDEGS – MALZ to model FOP test results Test current, frequency and injection circuit The main earth grid and RCIE are modelled in MALZ using the tested soil resistivity profile. The test current is injected at 180 degrees into the RCIE, and at zero degrees into the main grid. A potential profile is then defined to coincide with the voltage test traverse(s), and the computed plot is compared to the measured curve for validity. The test current should be high enough (1‐10A) so that voltage test values are resolved with adequate accuracy. The test current level depends on the size of the grid under test and impedance of the RCIE. How often should testing/inspections be conducted? The test current should be generated “off‐frequency” close to the operating frequency of the power earth system (e.g. 55‐58 Hz). Portable hand‐held tuned voltmeters (TVM) are used to measure voltages generated by the test‐current induced EPR. To ensure earthing systems are compliant with statutory and regulatory requirements, they should be tested when first installed, or when major modifications are made, and again every few years under a preventative maintenance program. Copyright © LCS 2016 www.libertyconsulting.com.au