EMI Diagnostics The EMI Diagnostic analysis of electrical power systems and large electrical machines is an extension of the equipment, methods and test standards developed since the 1900’s to measure and identify various high frequencies generated by defective substation and transmission line equipment. Since 1980 electromagnetic interference (EMI) measurements have been used to determine the condition of electrical power equipment during normal operation. This is accomplished by the passive measurement of electrical activity in the radio frequency (RF) spectrum from 50 kHz to 100 MHz. Experience from over 8000 tests have shown a great deal of detailed information on equipment conditions can be determined from this measurement. Application has been at fossil, nuclear and hydroelectric utility power plants, petrochemical facilities, air products and steel mills. Motors, generators, cables, switchgear and transformers are very efficient in the conversion and transfer of electrical energy at the power frequency, 50Hz or 60Hz. When defects develop in insulation or conductors, electrical activity will generate radio frequencies at the defect location. This activity can be in the form of arcing, corona, random noise or partial discharges. Each defect will generate a characteristic signature at specific frequencies. This electrical activity can be measured using a split core radio frequency current transformer (RFCT) placed around the equipment safety ground. Data is captured by a special radio frequency selective voltmeter and analyzed by a trained test engineer. Data collection usually requires 20-30 minutes. EMI Diagnostics is a non-intrusive test. No signal or voltage of any kind is injected into the device being tested. No permanent or temporary connections to energized conductors are necessary. There is no safety risk to the equipment or test engineer since no covers are removed to collect data. This is a very important feature for industry wide application. EMI diagnostics has the ability of early detection and classify of numerous electrical equipment anomalies. This is of benefit to reducing maintenance costs, forced outages or forced curtailments and permits better resource planning and budgeting. From the detection of one small problem with a motor before there is a failure a potential savings of several hundred thousand dollars can be realized. Industry and utilities must reduce the number of in service failures. Application of EMI diagnostics is achieving this goal by focusing limited resources to the 5% of equipment tested that need attention and away from the 80 % tested that have no serious deterioration. The use of EMI diagnostics has been successfully applied on motors as low as 480 volts. Equipment operating at voltages of 2 KV and higher are preferred since these devices are usually more critical to operation in industry and generating stations. Equipment operating at higher voltages also develop more problems that can be detected by EMI Diagnostics. Most PDA systems are not sensitive enough at these frequency ranges to detect this activity.

Accumulation of dirt on dry transformer or machine stator windings is a common problem with air cooled equipment. The characteristic EMI pattern is seen as corona or gap discharges with minor contamination and random noise with severe contamination in the RF spectrum from 1 MHz. to 10 MHz. The other widely applied on-line diagnostic the PDA (partial discharge analysis) techniques have difficulty detecting these patterns since most do not evaluate this part of the frequency spectrum. Unlike PDA EMI is a system diagnostic not just a stator winding diagnostic. Motor bearing deterioration is a prevalent problem that EMI detects. In several instances, the EMI pattern has been detected before conventional vibration and temperature monitoring detected a bearing wipe. Broken rotor bars in squirrel cage induction motors is another common deterioration that is easily detected as are loose power cable connections and defective cable splices. Isolated phase bus presently has no available on-line diagnostic other than EMI. Experience has shown this critical system is to be very tolerant of deterioration and defects. Problems found include broken insulators, loose hardware and stray currents. There are several spectacular bus failures each year resulting in millions in lost revenue. The success of equipment maintenance or cleaning other can be verified by a retest. The characteristic patterns will be eliminated with proper maintenance. Likewise improper maintenance can be detected as soon as the equipment is returned to service. Most EMI signals from external sources occur over a wide range of frequencies. In general, the closer you are to the EMI source, the higher in frequency its signal. This process essentially allows one to walk right up to the source and is very helpful in identifying and eliminating problems which were not apparent prior to performing an EMI test, e.g., arcing shaft grounding brushes, shaft oil seal rub and loose lead connections. Summary EMI testing of large motors, generators, isolated phase bus, switchgear, transformers and power cables is safe non invasive cost-effective and worthwhile endeavor for a utility or other industrial organization intent on keeping its machinery operating and avoiding costly forced outages. The testing process is fairly easy and can be performed with the device operating under normal circumstances; no unit outage is needed to connect the monitoring instrumentation necessary to perform an EMI test. As an added benefit, since the testing process examines the entire electrical network, an assessment of more than just the motor or generator is provided. The use of consistent external benchmark signals aids in the analysis process as more of these signals appear in the data of a machine with little or no EMI whereas less of these signals appear in the data of a machine with insulation defects and corresponding higher EMI levels. Rev 1 05-26-2014