While engineering was working on the resolutions for this issue,  both intentional and unintentional Radio Frequency transmissions began affecting other electrical systems and EMI started becoming more problematic. Fortunately, EMI field quietly gained formal recognition in 1933 under a sub-committee of the International   Electrotechnical  Commission  in Paris  France  under the name of International Special Committee on Radio Interference (CISPR).

 

In 1934 CISPR started  subsequently producing  technical publications  covering  measurement and  test  techniques and recommended emission  and immunity  limits.  Although  corona  was  and continues to be an issue, it is not the primary source of interference for radio and TV receivers.  The RFI from  microsparking hardware  on  distribution  lines operating below 150 kV is the major source of EMI for radio and TV. Transmission lines operating above  approximately 150 kV  generate very little microsparking  and  RFI.  Microsparking  and  loose hardware  connections  on  distribution a nd sub transmission  lines  are  the source of the  majority of consumer complaints of electrical interference  to  radios  and  television  receivers.  Modern  digital s systems  are less  prone  to  RFI  problems.  Galvanized metal surfaces      on line hardware corrode when exposed to the weather, and this  corrosion,  being an electrical  insulator when  dry,   effectively insulates energized metal-to-metal hardware and line components from each other with microgaps.

 

Galvanic corrosion commonly forms gaps of approximately  0.00015”  which  allow the ever-present electric  field to spark/arc over and generate  RFI that is  then  propagated  longitudinally  and laterally  along  the line  as a function of  the RF impedance, length of the line, hardware  and  ground paths.  The second  most common cause of microsparking  RFI  is from loose  hardware. Loose hardware usually results from expansion and contraction of wood poles. This action loosens cross arm  braces and through bolts, for example, resulting in floating  washers  and  lock nuts  which  generate  microsparking.  Also,  pole  grounds  are   frequent s sources  of  Microsparking  where expansion  and  contraction  of  the  wood  pole loosens staples holding the ground wire. I have seen staples nearly “eaten” into by long-term microsparking between the ground conductor.

 

Sub transmission lines, especially those operating at 34.5 kV  are notorious generators  of microsparking between the conductors, tie wires and necks of the insulators. In most cases the physical  motion  of the  conductors  due to wind, vibration, ice loading, etc. breaks down the conservatively designed electric field shielding on the necks of  the insulators  allowing microsparking activity to commence. Transmission lines  operating  above  150  kV rarely generate microsparking RFI because  the sheer weight of the conductors and hardware are sufficient to  maintain good  metal-to-metal  conducting  surfaces  between  hardware  components.  Thus,  microgaps  rarely  appear under these conditions and this source of RFI is effectively eliminated.

 

Corona and microsparking are opposite functions of the weather;  Microgaps  are  effectively shorted during wet, humid weather and the sparking/arcing process ceases. Corona, on the other hand, is quite active in wet weather on transmission lines operating approximately 150 kV and above depending on the precipitation rate, temperature, wind speed and direction.

 

There are many more miles of lines operating below 150 kV and adjacent residences are usually  closer: especially in the distribution and sub transmission voltage range, all susceptible to RFI from a  multitude of microsparking sources. The ratio of wet weather to dry weather acts to favor microsparking in the majority of cases. Therefore, it is microsparking and not corona that is the major source of interference along power company rights-of-way.

 

Johnny Johnson of Westinghouse used commercially available RFI/EMI testing equipment in the late 1940’s for the  first application of on-line partial  discharge  detection  in g enerators. Note  that  corona  is  just  a  specialized  form  of  partial discharge (PD), and thus it was natural to use such EMI or ‘RF Noise’ measurement apparatus.

 

In Johnson’s work, the signal for the EMI gear came from the first  use of a high frequency current  transformer  installed  on  the ground connection to the generator’s neutral. In North America, this CT is  often referred to as an RFCT, since it detects radio frequency currents. Johnson found that PD in generators tended to occur at specific frequencies (usually around 1 MHz, the heart of the am (AM) broadcast band), and with experience, he was able to identify a number of generators with loose coils in the slot with the new method.

 

James E. Timperley along with fellow engineers Dave Klinect and Keith Chambers at American Electric Power (AEP) started employing EMI testing around 1980 on generators and motors. His interest in EMSA technology began with the unexpected failure of a 100,000 hp pump-generator and the EPRI funded work by Johnny Johnson at Westinghouse.

 

As fate would have it, his office was located next to the communications group, so they were familiar with EMI problems resulting from defective hardware on the new 765 kV system. The EPRI project made  sense and  collecting  data at the machine  neutral  an  easy fit.

 

The first EMSA  diagnostics  were  performed  on  four  large hydro generators (circa 1980)  at  the Smith Mountain Dam in Virginia. Fellow engineer  Keith Chambers and Jim collected the data and Keith performed  the  analysis.  They  utilized  quasi  peak,   field  intensity and peak  readings  from  10 kHz  to  100  MHz  that  were  collected  by  hand,  no  pen  recorder  was  available  at that  time,  data  collection required  an  entire  week.  They  knew  high  readings  were  bad  and  low  reading good.  But  had no idea of any specific problems. The surprise  was a sister  generator to the one that failed had very little activity, however another machine had high readings. That machine was then scheduled for  a  rotor  pull, slot  deterioration  was  found  and corrected. A follow up test verified maintenance was successful. The value of EMSA technology was truly a game changer from the beginning.

 

The machinery section manager then scheduled other generator tests to help with maintenance planning starting in 1981. First with generators rated 13 kV and then to 4 kV motors since this is where the real condition base maintenance need was in AEP.

 

Several thousand tests were performed after 1980, first on generators, then large motors, bus, cables and transformers. They found the technique  worked  down  to  2300 volt  equipment.  Fellow  engineer  John Allen  was  the  primary  AEP tester for many years. His plant background helped develop a brief report focused on what maintenance was needed.

 

They soon were able to detect several basic patterns such as Partial Discharges,  Corona,  Arcing,  and  Random noise. Each describes a different condition or deterioration. Identifying arcing and random noise are unique  to  EMSA  techniques.  They continue to identify new conditions each year and are now around 70. Each is added to the list only after an inspection confirms the condition and maintenance repairs the damage, followed by a second EMSA test that verifies the pattern is gone.

 

At about the  same time the EMSA technology field was growing, so to was Partial Discharge technology.  This technology first focused on slot   discharges  in  large  hydro  generators,  since  this  was  serious  problem  with  the n ew  “hard”  coils  installed  In  Canadian  hydro generators.   EPRI sponsored research by Ontario Hydro to detect this problem. The spin off was IRIS, headed by Dr. Greg Stone, to develop and market a  high  frequency PD measuring device. With  continued help from EPRI and a built-in customer, IRIS dominated the PD arena quickly.  PD  measurement  has been  applied to  cables,  transformers and SF 6 bus by others. It is a major condition diagnostic in Europe and Asia.

 

The major difference  between the EMSA and PD technologies is that EMSA is based on an international standard, CISPR 16, for measuring “noise” in the radio  frequency spectrum.  The  EMSA  frequency range now scanned is from 50 kHz to 100 MHz. This covers most of the EMI generated  by   equipment  operated  in  air,  hydrogen  or  oil  below  30  kV.  Conditions  like  arcing  connections,   bearing  rub,  winding contamination and   broken  rotor  bars  are  identified  with  EMSA  but  not  PD.  Both  the  video patterns and the audio sound present at specific  radio   frequencies  provides  valuable   information  on  the  condition  present.   Long   unique   to   the   EMSA   technology.   The development  of  an  “EMI Sniffer” with Radar Engineers is a unique EMSA device. Radiated EM energy is measured and will often provide a specific  physical  location  for  many  defects. John Allen often performed motor diagnostics with the sniffer as a quick scan to save time and reduce the number of full EMSA scans needed for motors. Identifying a bearing or seal rub and broken rotor bars in induction motors with the s niffer  was so reliable, a full EMSA scan was not always necessary to provide maintenance recommendations. The Sniffer could also locate system problems such as loose connections at generator or transformer bushings or an open fuse in a metering cabinet.

 

There are several types of high frequency and low frequency PD measuring systems. They do not follow a common detector or bandwidth standard.  The  PD  guide  documents  available  allow  application  of  all  the  various  devices  available.  In  general, only machine stator problems are emphasized. A direct comparison between these systems is not advised.

 

There were several  advances in PD data collection and analysis during the first few decades. Phase resolved comparison of PD spikes and the driving power frequency was a major improvement in defect location identification. Recently capture of the video of activity at specific frequencies  is  being  applied. This  is  the  same  as  EMSA  has  been  doing  for over  40 years. EMSA has continued to evolve. Not only the spectrum  signature  of a  device is  important  but  also the what the patterns look and sound like across the spectrum. In many cases the condition  of  equipment  can  now  be determined  from  the  first test, trending, as historically, required by PD analysis in not necessary for data  analysis.  EMSA  measures and  identifies  external  signals,  PD  does  not.  This  results in identifying equipment with few or only minor problems  where  no maintenance  is  necessary. For 20 years the common feeling was since AEP is the only one doing it, EMSA must not be any good.  Today several  have  developed  internal  EMSA based  condition  base  maintenance.  programs.  The major  advance has been application to identify mechanical rotating shaft related defects, even turbine rubs and internal turbine static discharges (from cold steam ) have  been  identified.  The  EPRI  research  focused  on a specific large STG connection problem, this technique was then applied to hydro generators,  nuclear  plants, motors  and  bus  systems.  Application  then progressed to heavy industry and petrochemical sites. Off-shore platform  equipment  evaluation followed.  One very important advantage EMI/EMSA diagnostics is that this testing is performed on-line, at power, no hot connections are made, no  signals introduced to the system, it is an inherently safe test. Jim retired in 2007 and then brought his technology  to  our  core  group  of  engineers  that  he  started training in 2008 and is still very much involved in the analysis and review process today with AI Advanced Electrical Systems USA.

 

Products

•  Deep Blue RF monitoring system™

• EMSA Testing EMI testing later changed to the more common EPRI term, EMSA (ElectroMagnetic Signature Analysis).

 

Since  2008  the  EMSA  techniques  have  developed into a more comprehensive, non-intrusive method to evaluate the machines’ health. The  technology  has  gotten  more  sophisticated  with  the  size  of  test  equipment  shrinking  down  in  size  significantly and weight. EMI instrumentation  in 1980  filled  a full-size test van. Fast forward to today where artificial intelligence has advanced the techniques to allow for on-line 24/7 monitoring.  AI  Advanced  Electrical  Systems  USA  has  developed the Deep Blue RF monitoring system™ which is a highly complex  Artificial  Intelligence  based  advanced E MSA  diagnostic  online   monitoring  system.  The  Deep  Blue  RF  monitoring  system™ monitors  critical  electrical assets (generators, bus, transformers, cables, motors), around the clock giving you online, real-time condition assessment  of  your  asset’s  health  under  actual  operating  conditions.  Streaming  data is captured by our own advanced high-speed hardware  and  is  analyzed  by  our  proprietary  software  and  is  sent  to  the cloud. The data is parsed and processed using an artificial intelligence engine created specifically for the Deep Blue RF monitoring system™ monitoring system. Only the future will show us what the next evolution in EMSA technology will be…stay tuned!

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Contributors

James E. Timperley,

Keith Chambers,

Peter O. Longo,

Wikipedia