Ultra-sons

meio de diagnóstico

 da

Instalação Eléctrica

Tal como a Termografia com Infravermelhos que converte cores que normalmente não conseguimos "ver" em cores visíveis,as análises de Ultra-sons convertem sons, que normalmente não conseguimos ouvir, em sons AUDÍVEIS.

Def. Ultra-som: Frequência acima da capacidade da audição humana, i.é., superior a ~20kHz.

A análise de Ultra-sons permite encontrar problemas não detectáveis com testes isolados de Termografia, tais como a deterioração de isolamento  .

A análise de Ultra-sons permite:

- Detectar Arco Eléctrico, Falhas de Isolamento e Efeito de Corona, sem uma linha de vista directa, até em ambientes ruidosos.

- Identificar problemas em equipamentos que não podem ser abertos: Condutas de barramentos, equipamentos com óleo, disjuntores, interruptores  etc.

- Os ensaios podem ser realizados em equipamentos de Alta Tensão, à distância e sem acesso directo.

O método mais eficaz de inspecção é combinar a análise de Ultra-sons com a Termografia de Infravermelhos.

  Equipamento usado em diagnosticar possíveis anomalias em equipamentos e condutores eléctricos antes de manutenções.

              ·         Equipamento fiável

              ·         Sem entrar em contacto com os equipamentos em Tensão

              ·         Elucida o técnico e cliente do estado da instalação eléctrica.

 Equipamento muito utilizado em Países com USA

Using Ultrasound to Detect Corona and Tracking

Because corona and tracking problems are occurring in air, it makes sense that the best technique to detect these problems is through airborne ultrasound. Ultrasonic waves are very directional in their movement making it relatively easy to track problems back to their source. Because ultrasound waves are directional, they will rebound off surfaces and can be partially and completely blocked. Using common sense and following the unit’s strongest received signal will usually point the operator to the problem. The operator can also use blocking techniques to filter out competing ultrasound noises, if necessary.

Prior to opening a switchgear cabinet, it should be standard practice to scan ventilation screen openings, the seams around the doors, and the cabinet bolt holes once a few are removed. Typically, advanced cases of corona and tracking will be heard using this “initial” scan technique. However, the interior geometric design of the cabinet may not always allow the signal to reach the ultrasound collector or may only allow a partial and weak signal to be heard. Likewise, mild cases of corona may have a very weak discharge signal that is not heard until the cabinet is open. If you have any question about the safety of opening a switchgear cabinet, do not open it until an outage can be secured.

Other problems that may affect the initial scan are competing ultrasonic noises generated by mechanical vibration from inside switchgear cabinets and hand tools used to open the cabinets. Mechanical vibration signals can sound a lot like electrical discharge signals. By applying light pressure on the sides of cabinets and doors, you can reduce or eliminate a mechanical signal and rule it out as electrical discharge.

Opening switchgear cabinets is a risky business that should only be performed by qualified persons wearing the appropriate arc-flash protection equipment as prescribed by the NFPA 70E guidelines. If at all possible, cabinets should be open while de-energized, and then energized for the inspection. If a strong smell of ozone is detected, do not open the cabinet until it is de-energized. Drafting air into a cabinet with advanced corona and tracking conditions may move conductive air over a grounded object and cause a flash-over.

Teamwork between qualified persons is a must. If you are an “in-house” technician with high-voltage switchgear, consider installing hinges on doors and modifying bolts that can be easily handled while wearing gloves. Doors should be opened very slowly. You should minimize your exposure in front of the open equipment.

Once open, the inspector should slowly scan the interior of the cabinet making sure to cover the entire area. Both front and back compartments should be scanned, if accessible. The ultrasound instrument or any body parts should never break the plane of the cabinet or exceed the approach distance for the given voltage class.

What are We Listening For?

Corona problems will be heard as a continuous buzzing or frying noise. The intensity of this noise will be directly related to the severity of the problem. Tracking problems will sound much like corona problems, but will have pauses and possible drops and builds in intensity.

Once a problem is detected, the problem should be described, photographed, and recorded, if desired. Because of the dangers involved with getting too close to this type of equipment, using a telephoto lens will help become your eyes to get close to problems. Also, a bright flashlight will help illuminate dark and tight areas where corona problems may occur.

Corrective Action

You just received word that corona is present in your switchgear. How bad is it? What corrective action should I take?

The presence of corona and/or tracking in switchgear is a serious problem that should be addressed as soon as possible. The following corrective actions have been suggested by Mark Lautenschlager, PE and Senior Vice-President of Engineering with High Voltage Maintenance Corporation in an article printed by NETA World, Fall 1998.

• Any physical sign of breakdown or injury to conductors, insulators and insulation board should be corrected by repairing or replacing the damaged component
• Fill air gaps with silicon tape, silicon sealant or other corona suppressive compound. Air gaps can also be increased or replaced with porcelain insulators
• Smooth sharp edges, apply corona rings, apply semi-conductive tape or compound, or wrap edges with metallic screening to form round conductive surfaces
• Replace damaged terminations and splices on conductors. Support non-shielded cables from ground
   
   
   
  From: http://irinfo.org/index.html