Condition monitoring of mechanical parameters in rotating machines is established, but it can be worth while to monitor electrical parameters. This also extends to switchgear and control panels. While mechanical failure tends to affect just one item of plant, electrical failure can bring a complete site to a halt.

An increasingly popular monitoring technique is the detection of partial discharges (PDs). It has been known for a long time that many problems start with a PD in alternator windings, motors, transformers, switchgear or on cabling. But it is the development of electronic systems that have increased the popularity and effectiveness of the technique.
PDs are a type of localised discharge and result from transient gaseous ionisation in an insulation system when the voltage stress exceeds a critical value. They occur at poor electrical connections, inside windings and on higher voltage insulators.

High frequency currents

They generate electromagnetic radiation over a very wide spectrum, send high frequency currents along cabling and also generate acoustic waves. These radiate in all directions with a frequency and magnitude dependent on the nature of the discharge. Waves are attenuated as they pass through the plant and, by analysing the relative magnitudes of signals from distributed probes, the discharge location can be calculated.
The Cutler-Hammer Universal Partial Discharge Analyser (UPDA) is one instrument that assesses the condition of MV and HV insulation systems on-line. The biggest challenge when measuring PD's, says the company, is distinguishing between the PD signal and external noise from motor brushes, thyristor firing, control electronics and arcing or sparking anywhere else on the site.
In the UPDA, low-level background noise is rejected by analysing its waveform parameters and filtering out non-PD signals. Periodical noise is automatically detected by processing a large number of power frequency cycles and finding synchronous pulses.
Cross-coupling noise, when a pulse of the same origin is picked up by several sensors, is rejected by comparing amplitudes and phases of related signals. To reject other types of noise, some channels are assigned as noise channels to subtract noise information from the final picture.

Substation monitoring

For factory substations, the On-line Substation Monitor (OSM) from IPEC provides a complete condition monitoring system for switchgear, cables, transformers and generators and is supported by an on-line, continuous, monitoring service (Fig. 3).
Dr Lee Renforth, managing director, says the OSM system is being installed permanently at critical sites where PD activity has been detected. This ensures a build up of reference data so the system can provide an early warning if PD levels increase and show a trend to failure. Renforth believes such systems will become an industry standard in the next five to 10 years as their economic and safety benefits are realised.
In a typical substation, a capacitive coupler would be placed on the circuit breaker panel and an acoustic emission probe adjacent to the cable termination box while a high frequency current transformer would be sited on the cable earth strap.
Ultrasonic emissions are detected by a piezo-electric sensor that responds to frequencies between 100-130kHz. Electromagnetic PD sensors are used to detect high frequency transient earth voltages between 15kHz and 20MHz. These are generated in earth straps and outer panels of cables and circuit breakers and are detected by high frequency current transformers and capacitive couplers. RF sensors capture signals in the 100MHz to 1GHz range.

Infrared imaging

Another technique in condition monitoring is infra-red imaging, which detects hot spots created by shorted windings, high resistance switch contacts and corrosion. It has also been used to detect failed power factor capacitors by comparing the heat emitted from a bank and identifying the cool unit.
Thermal imaging devices such as the PM695 ThermaCAM from FLIR Systems will measure temperature very accurately over a wide angle (Fig. 1). The instrument measures differences as small as 0.07°C so later measurements can be compared precisely. Voice comments can be added to each image for better identification while documentation is handled by FLIR's Reporter software (Fig. 2).
A typical case occurred where a switchbox was running hot. The company could not immediately identify the fault and suspected the machine had gone faulty and was drawing a high current on one or more phases. A substantial amount of production would have been lost if the machine had been taken out of service, or had failed.
But thermography showed a standard pattern of heat on the machine and cleared that. Working back down the cable, a failing electrical connection was diagnosed in the connector box and remaking the joints cured the problem.

Technical developments

The liberalisation of the electricity market has encouraged many companies to generate their own electricity and sell the surplus to the grid. This means switchgear and substations may have to be placed well away from the core of a factory site.
To monitor the condition of switchgear in remote locations, a new concept is being developed by the University of Bath using electromagnetic transient emission. Chye Peck, PhD student, and Dr Philip Moore, senior lecturer in the Electronic and Electrical Engineering department, say that switching transients can be remotely measured at up to 10km by an AM receiver.
One project has found that, for a closing event, the pulse train at switch closure shows a low initial amplitude which gradually increases to a peak at the middle of the pulse and then decays exponentially. When opening, the pulse train appears to exhibit this effect twice, so the type of operation, opening or closing, can be identified by analysing the waveforms.
The pulses also contain unique signatures which can identify the item of switchgear. Further work by the department could reveal the condition of the switchgear.

Low voltage switchgear

Other condition monitoring research has been carried out by Siemens on low voltage switchgear, to assess the condition of contacts. It says one solution could be specially prepared contacts where an insulated wire passes through the contact carrier and into the contact material. When the contact surface wears away, a voltage appears on the signal wire. From the amount of time the contact had been in service, and knowing the thickness of material above and below the insulated wire, it is possible to calculate when the contact should be replaced.
Another option is to monitor the distance between the fixed and moveable contact carriers, which will reduce as the contacts wear. There are sensors which could perform this directly but Siemens says they are not suited to the robust environment in contactors. But the distance can be calculated indirectly by measuring the time delay between triggering the actuator and the parting of the contacts, which will increase as the contact carriers move closer together.

Monitoring services

GE Syprotec specialises in transformer monitoring and management with instrument supply and laboratory services. It says that on-line monitoring of moisture in transformer oil is an essential guide to the condition of transformers.
Its Aquaoil 300 instrument uses a capacitive humidity sensor to check moisture ingress, through breathing vents, for example; abnormal rate of paper degradation; and water content of other insulation.
The company says that water in power equipment is attracted to areas of greater electrical stress and accelerates the deterioration of paper and oil. The company's Hydran 201i system monitors dissolved gases that result from excessive heat and electrical discharges inside transformers.
Around the suppliers, ABB says transformer monitoring systems such as its T-Monitor are becoming increasingly powerful. Where systems are permanently installed, the quality of data improves as time passes.
T-Monitor covers Buchholz gas accumulation; bushing load and leakage current; top and bottom temperatures; fan and pump currents; dissolved gases; and tapchanger position and temperature differential. The system also monitors ambient temperature and humidity.

Instrument range

Adwel International has a rotating machines product group that is specifically targeted to provide condition monitoring of electric motors and generators. It offers several instruments for PD analysis including EL CID, ELectromagnetic Core Imperfection Detector and PDA Premium for high voltage rotating machines. It also offers the PPM-97 corona probe, a hand held instrument with a long probe for accurate location of PDs inside large windings.
Its wedge tightness detector tests stator core wedges of motors and generators. It replaces traditional hand tapping methods with a hand held probe that taps each wedge 10 times a second for three seconds. An accelerometer picks up the reflected vibrations and the data is stored ready to send to a computer for analysis. This creates a map showing wedge position and relative tightness.

High voltage environments

Power Diagnostix in Germany manufactures specialised instruments for high voltage environments. Its principal product is the ICM series of PD detectors and accessories. Its ATT analyser can handle 16 inputs from acoustic sensors using fibre optic cables and connectors.
Norwegian company Transinor sends its products and services to more than 40 countries for on site condition monitoring and diagnosis of electrical transmission and distribution systems. Instruments include the APDA acoustic PD analyser and the AIA portable acoustic insulation analyser.
With electrical condition monitoring equipment becoming more widely available, production engineers now have another set of tools to help them maintain high levels of plant uptime and to reduce operational costs.