The LCI2000 portable low current injector operates at 58 Hz as a current source.
Gounding systems installed by transmission and distribution utilities can be difficult to test as they are often extensive and can have very low impedance. The traditional portable ground testing equipment available is designed primarily for relatively small systems and, in many cases, is not able to measure very low resistance. In particular, it cannot test reliably or easily for touch, step or transferred voltages. Grounding systems must be able to perform correctly during the infrequent but serious power-system ground-fault events. Inadequate grounding systems can result in hazardous voltages arising under fault conditions.
To comply with statutory and regulatory requirements, utility grounding systems for existing and new substations should be subjected to testing when installed and again every few years under a maintenance regime. Testing is required to ensure that during a ground fault, the general public and field staff are not exposed to any hazards. Further, telecommunications equipment should not get damaged and voltage hazards should not be transferred onto other facilities or services such as fences, gas pipelines or water pipelines. The magnitude of the ground potential rise (GPR) and the associated voltage hazards are directly linked to the layout, fault currents and impedance of an overall grounding system.
Westpower Ltd., a distribution utility on the West Coast of New Zealand, was faced with the need to undertake grounding compliance testing at six 66-kV and 33-kV substations. It needed to determine the overall grid system impedance and ground fault potentials on security fencing, high-voltage plants, water pipes and metalwork in nearby buildings, as well as to identify the GPR contours to ensure that no dwellings or telecommunications were within the “hot zone,” where GPR is above the allowable threshold. In some cases, existing ground grids can be modeled using appropriate software. However, experience has shown it is almost always cost beneficial to also test the ground grid instead of attempting just to model the grounding system.
Few practical methods exist for comprehensively testing such grounding systems. However, Westpower was well aware of the Mitton Instruments ground grid off-frequency injection test equipment, which had a proven track record of use throughout New Zealand and Australia. For this ground testing project, Westpower staff used the Mitton Instruments ground grid injection test equipment with the assistance of staff from both Westpower and Mitton Instruments.
Ground Testing Equipment
The objective of the test program was to produce, for each substation site, a comprehensive test report that included recommendations for mitigation or ground grid modifications to eliminate any identified hazards. The testing also provided a footprint for future reference.
IEEE Standard 81.2-2012, Guide for Measurement of Impedance and Safety Characteristics of Large, Extended or Interconnected Grounding Systems, includes a background on ground testing and the various test methods that can be applied. The difficulty in ground grid injection testing is to implement a method that does not require a substantial power-system outage to enable the injection of relatively high test currents needed to overcome background electrical noise. One method is to use off-frequency low current injection, whereby a signal is injected into the ground system at a frequency very close to the system frequency. Tuned voltmeters are then used to detect the resulting voltage signals on the grounding system.
The method and equipment are generally suitable for both large and small substations and switchyards (for example, 11 kV to 500 kV) and power stations.
Mitton Instruments has developed specialized equipment to undertake these tests that replicate, on a small scale, the effects of a ground fault on the grounding system. The LCI2000 portable low current injector operates at 58 Hz, and the associated TVM1000 voltmeter is tuned to 58 Hz, with exceptional rejection of the 50-Hz residual noise and any associated harmonics. The LCI2000 operates as a current source and this means no special power-transfer-matching transformers are required.
By using this method, only the test signal is measured, and any fundamental, harmonic or other noise on the grounding system or noise induced in the test cables is rejected. In addition, the unique signature of the 58-Hz signal may be identified easily at significant distances from the substation under test, such as on farm fences, low-voltage grounding systems, substation or power station infrastructure, and third-party equipment such as gas or water pipelines.
Off-frequency injection, together with sensitive measuring equipment, allows identification of the unique and often relatively low signal levels against the much higher background noise levels. The grounding system parameters can be measured without de-energizing the substation. The same test equipment is available for 60-Hz systems where the injection current and tuned voltmeters are set at 52 Hz.
Both new and existing switchyard grounding grids have been tested using this instrumentation. The results confirmed both the ground grid design and the effectiveness of the test equipment. Computer models and test results show close correlation, including predicted touch voltages in and around the sites. However, as mentioned previously, the main benefit of injection testing is to determine the ground grid performance since, in the majority of cases, the actual performance will differ from the design because of the many variables associated with grid design and practical installation.