Con Edison and Central Hudson Gas & Electric invest in oscilloscopes for their field technicians and troubleshooters.
All electric utilities in New York state test their underground and overhead electric distribution systems for contact voltage. These utilities perform more than 3 million tests each year, making it imperative for them to control costs, increase efficiency and search for improved work methods.
One way that two utilities are boosting the productivity of their field workforces is through an investment in a handheld tool for their technicians. At Con Edison and Central Hudson Gas & Electric, technicians are using field oscilloscopes from Fluke to identify possible phase faults, neutral problems and naturally occurring voltages.
The utilities used to have to go through a process of elimination before they could pinpoint causes of contact voltage or disconnect each potential path of current until the voltage disappeared. By using the oscilloscopes, however, the workers can troubleshoot efficiently. In addition, the utilities can prioritize repairs based on which locations pose threats to public safety.
Equipping Technicians with Tools
Con Edison has deployed more than 300 oscilloscopes to its underground distribution troubleshooters, linemen and mobile testing technicians. Prior to rolling out the tools to the field, the engineers visited all of the work yards to train the users. Through hands-on training, the field workforce learned how to use the tool to differentiate between a phase or neutral condition.
At first, some of the workers misinterpreted the line results. To make it easier for them to read the results accurately and as a quick reference, Con Edison's engineers adhered special stickers to the oscilloscopes. These labels state that if the third harmonic content is more than 10%, it is a neutral fault, and if it is less than 10%, it is a phase fault.
Con Edison mobile detector vehicles survey the streets of New York City five nights a week for contact voltage. If contact voltage is detected, Con Edison technicians cordon and secure the energized structure. A unique trouble ticket is then generated to record the voltage and third harmonic content at the time of detection. Troubleshooters are then dispatched to the field equipped with oscilloscopes to analyze and repair the voltage condition.
Now that Con Edison's field employees can measure energized structures directly and record the results, they are saving a significant amount of time in the field. In fact, some of the troubleshooters have said that the oscilloscope cuts down the time for troubleshooting in half. It also allows them to work more efficiently and wisely.
Central Hudson Gas & Electric also experienced similar results when it rolled out the tools to the field. In the first year alone, the utility saved about $130,000. In subsequent years, the utility has saved $100,000 per year because the technicians and engineers were able to quickly identify and prioritize what needed to be repaired.
The protocol established by the New York State Public Service Commission requires all New York utilities to test for stray and contact voltages yearly. The procedure starts with using a handheld wand to sweep all metal/conductive apparatuses on company-owned facilities. If the wand lights up, they drive a ground rod in the earth. At that point, they use a 500 Ω shunt resistor and a multimeter to measure the voltage value. If the value is greater than 1 Vac, then the source of the voltage must be identified and mitigated if it is due to contact voltage. Team leads can use a field oscilloscope to determine the nature and source of the voltage found during routine testing.
The utility owns 15 Fluke 125 ScopeMeters, which are used by the engineers and the team leads or senior technicians as a tool to validate and identify the sources of voltages found in the field by more than 50 field technicians. If a field technician finds a potential contact voltage, then the team leads can use the ScopeMeter to verify what the technician found. At the end of the work day, the team leads save the wave files and then send them to the engineering department for their review for voltages that are a result of normal operations.
If a voltage discovered in the field is over 4.5 Vac or found to be a contact voltage, then the team lead will safeguard the area until Central Hudson crews can make repairs to the cause of the voltage found or make the area safe through barricades and/or disconnecting the voltage source. Upon arrival at one of these locations, Central Hudson crews will discuss the voltage finding with the team leads and use the ScopeMeter as a diagnostic tool to find the voltage source.
If they have questions while they are using the oscilloscope, the technicians can refer to a training guide, which was written by the engineering department. To ensure that all the team leads know how to use the tool, the utility requires them to become recertified each year. After the initial training, the users find that they catch on quickly because the steps are fairly easy to repeat.
Conducting Field Trials
Con Edison and Central Hudson, in conjunction with the Electric Power Research Institute, have conducted hundreds of field studies to determine the practical use of source identification. The utilities conducted the studies on multiple energized structures including secondary phase faults, neutral faults and induced voltages to validate the use of total harmonic distortion measurement as a diagnostic tool.
For example, during routine contact voltage testing, Con Edison technicians detected 3.5 Vac on a streetlight. Using a handheld oscilloscope, Con Edison engineers determined a neutral fault was causing the contact voltage. The third harmonic content was 20.5%, which indicated a neutral fault. The distorted AC sine wave showed the current had passed through a non-linear load.
Prior to the use of the handheld oscilloscope, the standard procedure for Con Edison troubleshooters was to replace the service to the streetlight. But by using harmonics analysis, troubleshooters can now more accurately diagnose the cause of the problem and the potential solution. In this case, the crew was able to inspect the neutral connection and make repairs, avoiding an expensive service replacement.
Con Edison was also successful using the harmonics analysis and handheld oscilloscopes in Queens, New York. The engineers discovered voltage on a roll-down gate, stair railing, service box cover, cable TV wire and metal curb near the front of a grocery store. The voltage level ranged from 1 Vac to 5 Vac.
Con Edison initiated troubleshooting but was unable to eliminate the voltage on all the objects. Using a handheld oscilloscope, however, Con Edison engineers obtained a high reading of 31.6% third harmonic, indicating a secondary neutral fault.
The engineers instructed the troubleshooters to focus on the neutral conductors. As a result, they found a loose neutral connection in the service box. The cable TV wiring was acting as the system neutral and carrying the return current since it provided the path of least resistance. Since the cable shield was not properly sized to carry the return current and was connected to other neutrals and grounds in the area, all the structures were energized.
In another example, Central Hudson technicians performing routine testing came across a pole near a substation with voltage varying between 2.06 Vac and 3.5 Vac on the down ground. The pole carried a three-phase grounded wye distribution feeder leaving the substation.
Pole grounds nearby also had low voltage levels (less than 2 Vac). The 180-Hz dominant waveform suggested the voltage source was due to either an unbalanced circuit or an induced voltage.
During a field visit by the field engineer and a line crew, investigators determined the three phases were not balanced. During a follow-up visit a few weeks later when the circuits were balanced, the voltage on the down grounds was near zero.
Another high harmonic source of voltage was discovered during routine testing by Central Hudson: pole-mounted capacitors. The testing produced a reading of 1.5 Vac on the ground of a pole where capacitors were in service. The waveform had no clear 60-Hz waveshape. It had high harmonic content in the third, fifth, seventh and ninth harmonics, and a total harmonic distortion of 569%.
When investigators visited the area again, this time when the capacitors were out of service, the voltage readings were near zero. This voltage finding was the result of a normally operating electrical system, as the down ground was being used to dissipate the high harmonic content injected by the capacitors. Since the voltage was due to a normally functioning system and the voltage level would not rise, no remediation action was necessary.
By measuring and analyzing the total harmonic distortion of a voltage waveform and the condition of the location, technicians can quickly determine the probable source of the voltage. Understanding the source allows the technicians to focus their investigation and more quickly identify the component responsible for the elevated voltage. This enables the utility to save time on investigations and avoid making unnecessary repairs that do not address the root cause of the problem. By determining whether the source of contact voltage is on the phase side of the neutral side, a utility also can save money and maintain public safety.
Paul Richardson (firstname.lastname@example.org) is a specialist for Con Edison in New York, New York. He is a part of the team of engineers that manages the Mobile Contact Voltage Program. He has worked for Con Edison for more than seven years and works in the Distribution Engineering, Secondary System Analysis Group.
Sal Martino (email@example.com) is the electric operations engineer for the Newburgh district at Central Hudson Gas & Electric. He has been with the company for five years and works out of New Windsor, New York. He is currently working on committee for IEEE Standard 1695, which will be used as a trial guide for determining and mitigating stray and contact voltages.
Doug Dorr (firstname.lastname@example.org) is a senior project manager for the Electric Power Research Institute. He is responsible for managing all of EPRI's contact voltage research. He has worked for EPRI for 20 years and works in the Advanced Monitoring Applications Group.
Five Facts About Stray and Contact Voltage
Con Edison and Central Hudson may have only used the handheld oscilloscopes in the field since 2009, but they have been working with the Electric Power Research Institute (EPRI) for years on research and development.
For example, Con Edison worked with EPRI eight years ago to investigate a shock complaint at a residence where harmonic neutral currents could not be mitigated. An investigation into another customer shock complaint a year later identified two different sources with the same voltage levels.
Neither investigation could have been resolved with a standard voltmeter. In each case, understanding and diagnosing the source required metering equipment capable of analyzing the harmonic content of the contact voltage waveforms.
Con Edison and Central Hudson have been working with EPRI since 2005 to further advance investigative techniques and source identification. Here is what you need to know about contact voltage and remediation.
During a typical contact voltage investigation, the investigator must remember that the voltage could be caused by many possible factors, including faulted electrical equipment, a neutral conductor voltage drop, induction or the inadvertent energization of a conductive object.
The first step is to understand the source of the energization. When attempting to determine the cause of elevated voltage or a reported shock, it helps to have some theoretical insights into the waveforms and other characteristics of what might be measured from different energization sources.
Through collaborative research, the utilities and EPRI have found that contact voltage rarely has a total harmonic distortion greater than 5% and never more than 10%. This contact voltage results from a variety of conditions including the deterioration of conductors, faulted equipment, the age of the equipment, exposure to the elements and various customer-related issues.
Stray voltage is the result of current traveling through the earth and the neutral return paths, and is an artifact of most normally operating power systems. This voltage is often the result of neutral current traveling though the resistance of the earth and down grounds. In most cases, stray voltage has a total harmonic distortion of greater than 10%. The voltage level is usually stable and not considered hazardous to people or animals.
When the total harmonic distortion is greater than 10% and the voltage is greater than 10 Vac, there could be a power system defect. In these cases, investigators cannot determine the cause simply on the basis of a voltage harmonics measurement. They must examine the circuit to determine whether the voltage is the result of normal power system operations or defective equipment.
Contact voltage is the result of line-side voltage energizing a conductive surface. The voltage level on the surface is a function of the resistance between the energized line and the surface in contact with the phase. The resistance, called fault impedance, has a direct relationship with the voltage appearing on the surface.
Contact voltage events are often the result of damaged insulation. The voltage in these cases can range from low voltage to full-line voltage, which is typically 120 Vac. Because of the limited sensitivity of most commercially available field instruments, harmonics analysis should be performed when the voltage is at least 1 Vac.
Central Hudson Gas & Electric www.cenhud.com
Con Edison www.coned.com